{"gene":"HLA-C","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":1977,"finding":"HLA-C (HLA-CW2) was shown to be a glycosylated polypeptide of ~43 kDa that is noncovalently associated with beta2-microglobulin, structurally homologous to HLA-A and HLA-B antigens, with some heterogeneity in its carbohydrate moiety and lower cell-surface expression levels compared to HLA-A and HLA-B.","method":"Biochemical fractionation, lectin-Sepharose adsorption, sedimentation analysis","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — direct biochemical characterization with multiple methods; foundational structural paper","pmids":["332508"],"is_preprint":false},{"year":1993,"finding":"HLA-C alleles are the dominant inhibitory ligands protecting target cells from NK cell-mediated lysis: alleles encoding Asn-77-Lys-80 (C2 group, e.g. Cw4, Cw5, Cw6) inhibit NK1-specific NK cells, whereas alleles encoding Ser-77-Asn-80 (C1 group, e.g. Cw1, Cw7, Cw13) protect from NK2-specific NK cells, as directly demonstrated by HLA-C transfection into class I-deleted cell lines.","method":"HLA-C allele transfection into class I-deleted mutant cell lines, NK cell cytotoxicity assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — functional rescue by defined transfectants; highly cited foundational study","pmids":["8265660"],"is_preprint":false},{"year":1995,"finding":"NK cell p58 receptors for HLA-C (EB6/GL183) exist in two biochemically distinct forms: a ~58 kDa inhibitory form (p58) and a ~50 kDa activatory form (p50), differing in molecular mass but not solely due to differential glycosylation; cross-linking of p50 triggers cytolytic activity and Ca2+ influx whereas p58 cross-linking inhibits lysis without Ca2+ influx, and p50-bearing NK clones lyse HLA-Cw4+ targets upon specific HLA-C recognition.","method":"Biochemical analysis (deglycosylation, peptide mapping, 2D mapping), anti-receptor mAb cross-linking, Ca2+ flux, NK cytotoxicity assays, LCL721.221/Cw4 transfectant targets","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal biochemical and functional methods; highly cited","pmids":["7650491"],"is_preprint":false},{"year":1997,"finding":"HLA-C binds to NK inhibitory receptors (KIR/NKIR) with extremely fast association and dissociation kinetics (among the fastest immune interactions characterized), as measured using soluble recombinant HLA-C molecules and recombinant NKIR proteins.","method":"Surface plasmon resonance / kinetic binding assays using soluble recombinant HLA-C and KIR proteins","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 1 — direct in vitro binding kinetics with recombinant proteins","pmids":["9768753"],"is_preprint":false},{"year":1997,"finding":"Recombinant soluble p58 KIRs (KIR-K6 and KIR-K7) bind soluble HLA-Cw3 and HLA-Cw6 molecules directly, and this binding is influenced by the antigenic peptide bound to the MHC, indicating peptide-dependence of KIR-HLA-C recognition; both Ig domains of p58 are required for HLA-C binding.","method":"Native gel shift assay with recombinant soluble proteins, domain-deletion analysis","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — direct binding demonstrated in vitro with recombinant proteins and domain requirements mapped","pmids":["9378975"],"is_preprint":false},{"year":1988,"finding":"HLA-Cw3 expressed in transgenic mice functions as a transplantation antigen capable of inducing rapid skin graft rejection and serves as an MHC restriction molecule for cytotoxic T lymphocyte responses to influenza and Sendai virus, demonstrating that HLA-C has immunological functions comparable to HLA-A and HLA-B.","method":"HLA-Cw3 transgenic mouse model, skin graft rejection assays, virus-specific CTL assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — transgenic model with defined functional readouts; highly cited","pmids":["2840670"],"is_preprint":false},{"year":2000,"finding":"HLA-C protein is expressed on the surface of extravillous trophoblast cells in association with beta2-microglobulin, with both paternal and maternal alleles transcribed; cell-surface HLA-C (but not HLA-G) is upregulated by IFN-γ treatment, demonstrating differential cytokine regulation.","method":"Flow cytometric sorting of first-trimester trophoblast, cDNA sequencing, biochemical analysis, flow cytometry, immunohistology","journal":"Placenta","confidence":"High","confidence_rationale":"Tier 2 — direct protein detection, allele-level sequencing, and functional cytokine response validated by multiple methods","pmids":["10833373"],"is_preprint":false},{"year":2000,"finding":"Two clusters of polymorphic residues on KIR2D molecules define group allotype specificity for HLA-C binding: multiple polymorphic residues contribute to the HLA-C binding site, with specific residues determining whether a KIR binds C1 or C2 group alleles.","method":"Site-directed mutagenesis of KIR chimeric proteins (KIR extracellular domain fused to CD3-ζ tail), signaling assays, HLA-C binding specificity analysis","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 1-2 — site-directed mutagenesis with defined functional readout","pmids":["10820396"],"is_preprint":false},{"year":2002,"finding":"The CD160 receptor expressed on circulating CD56dim NK cells directly interacts with HLA-C molecules expressed on K562 cells and triggers NK cell cytotoxicity; direct protein-protein interaction between recombinant soluble HLA-Cw3 and CD160 was demonstrated.","method":"Anti-CD160 mAb blocking of NK cytotoxicity, recombinant soluble protein binding assay (direct HLA-Cw3/CD160 interaction), NK cell functional assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 — direct recombinant protein interaction plus functional blocking experiments","pmids":["12486241"],"is_preprint":false},{"year":2007,"finding":"HLA-C heavy chains accumulate intracellularly in two forms (free of and associated with β2-microglobulin) due to a unique KYRV motif at residues 66-76 of the α1-domain helix that causes poor assembly, post-assembly instability, reduced peptide-binding groove plasticity, and association with dedicated chaperones TAP and tapasin; this locus-specific bottleneck renders HLA-C peptide-selective (rather than peptide-unreceptive) and contributes to its preferential role as an NK cell ligand.","method":"Pulse-labeling experiments, co-immunoprecipitation with anti-TAP/tapasin antibodies, molecular dynamics simulation, in vitro assembly in T2 and 721.220 cell extracts, reactivity with conformation-specific antibodies","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including pulse-chase, co-IP, in vitro reconstitution, and structural simulation","pmids":["17956861"],"is_preprint":false},{"year":2008,"finding":"KIR2DL2 is a stronger receptor for HLA-C than KIR2DL3 due to synergistic polymorphism at two positions distal to the ligand-binding site (Pro16Arg in D1 and Arg148Cys in D2); neither substitution alone is sufficient—only the combination increases avidity, likely by altering the hinge angle between the two Ig domains and changing the relative orientation of the ligand-binding site.","method":"Mutagenesis of recombinant KIR proteins, functional binding assays to 93 HLA isoforms, site-directed mutagenesis","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with defined mutagenesis and binding assays; highly cited","pmids":["18322206"],"is_preprint":false},{"year":2008,"finding":"HLA-C molecules associated with HIV-1 gp120 on infected cell surfaces and virion-associated HLA-C increases the infectivity of both R5 and X4 HIV-1 viruses; co-purification of HLA-C with gp120 from fusion complexes was demonstrated.","method":"siRNA knockdown of HLA-C, syncytia formation assays, pseudovirus infectivity assays, co-purification of fusion complexes","journal":"Retrovirology","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function (siRNA), gain-of-function, and molecular co-purification with specific phenotypic readout","pmids":["18673537"],"is_preprint":false},{"year":2011,"finding":"Differential surface expression of HLA-C allotypes is mediated by binding of microRNA miR-148a to the 3′ UTR of HLA-C mRNA: alleles susceptible to miR-148a binding have lower surface expression; the common ancestor of all HLA-C alleles was suppressed by miR-148a, and escape alleles arose ~3–5 million years ago via sequence exchange with an HLA-B allele, with selective pressure favoring spread of the escape variants.","method":"Molecular evolutionary analysis, miRNA binding site characterization, sequence exchange mapping","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 3 — evolutionary/molecular analysis; mechanistic basis for miR-148a regulation supported by prior functional data","pmids":["21907013"],"is_preprint":false},{"year":2013,"finding":"KIR2DL3*005 has markedly increased affinity and avidity for HLA-C ligands compared to other KIR2DL3 alleles due to a combination of Arg at residue 11 and Glu at residue 35 (distal to the KIR/HLA-C interface), likely by shifting the interdomain hinge angle toward the KIR2DL2 configuration; this confers stronger inhibition of NK cell IFN-γ production.","method":"Surface plasmon resonance, KIR binding panel assay, site-directed mutagenesis, NK cell functional assay (IFN-γ production by KHYG-1 cells), flow cytometry","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis combined with SPR binding and functional NK cell assay","pmids":["23686481"],"is_preprint":false},{"year":2016,"finding":"HIV-1 Vpu (not Nef) mediates downregulation of HLA-C on infected cells in most primary HIV-1 isolates; HLA-C downregulation by Vpu reduces the ability of HLA-C-restricted CTLs to suppress viral replication in CD4+ cells in vitro; HLA-A and HLA-B are unaffected by Vpu.","method":"Primary HIV-1 clone panel, Vpu mutant viruses, HLA-C surface expression flow cytometry, CTL suppression of viral replication assays in CD4+ cells","journal":"Cell host & microbe","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function with viral mutants and defined CTL functional readout; highly cited","pmids":["27173934"],"is_preprint":false},{"year":2017,"finding":"HIV-1 Env (gp120) specifically associates with HLA-C free chains (not bound to β2-microglobulin) at the cell membrane; HIV-1 infection increases the amount of HLA-C free chains on infected cell membranes; the enhanced infectivity conferred by HLA-C requires prior correct assembly with β2m before HLA-C free chains can associate with Env, and pseudoviruses produced from HLA-C-silenced or β2m-absent cells are less infectious.","method":"β2m-free HLA-C chain detection, HIV-1 Env pseudovirus infectivity assays, β2m knockout/knockdown experiments, cell-surface biochemistry","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — mechanistic dissection of HLA-C/Env interaction with defined loss-of-function; single study","pmids":["28051183"],"is_preprint":false},{"year":2018,"finding":"HLA-C downregulation by HIV-1 Vpu adapts to host HLA-C genotype: HLA-C alleles differ in the immune pressure they exert, and individuals with higher HLA-C expression favor greater viral downregulation; 5 residues in the transmembrane region of Vpu and 4 residues in the transmembrane domain of HLA-C determine Vpu-HLA-C interactions.","method":"Primary viral isolate panel (128 replication-competent viruses), Vpu mutant analysis, cloning and expression of Vpu from 195 treatment-naïve individuals, transmembrane domain mutagenesis","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 1-2 — large primary virus panel plus mutagenesis identifying specific interacting residues","pmids":["30180214"],"is_preprint":false},{"year":2018,"finding":"An NK cell-specific HLA-C promoter drives an array of alternative transcripts with variable intron/exon content; skipping of the first coding exon generates untranslatable mRNAs, and the proportion of these decreases as NK cells mature, correlating with increased HLA-C protein expression; polymorphism in a key Ets-binding site of the NK promoter generates HLA-C alleles with reduced promoter activity, resulting in lower HLA-C expression and increased NK cell functional activity.","method":"Transcript analysis (alternative splicing), promoter reporter assays (Ets-binding site mutagenesis), NK cell maturation tracking, flow cytometry of HLA-C protein levels","journal":"PLoS genetics","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including reporter assays, transcript analysis, and NK functional correlates","pmids":["29329284"],"is_preprint":false},{"year":2021,"finding":"Crystal structures of KIR2DL2 and KIR2DL3 in complex with HLA-C*07:02 presenting a self-epitope revealed that KIR2DL2 differs from KIR2DL3 in its docking modality over HLA-C*07:02; mutagenesis assays showed differences in mechanism of HLA-C1 allotype recognition, with HLA-C1 allotypes differing markedly in capacity to inhibit NK cell activation; KIR2DS2 contributes to functional differences alongside KIR2DL2/3 binding geometries.","method":"X-ray crystallography, site-directed mutagenesis, primary NK cell inhibition assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — crystal structures with mutagenesis and functional validation","pmids":["33846289"],"is_preprint":false},{"year":2021,"finding":"ERAP1 generates the causative melanocyte autoantigen (ADAMTSL5-derived peptide) for HLA-C*06:02 presentation in psoriasis by trimming N-terminally elongated peptide precursors to the appropriate length; an ERAP1 psoriasis-risk haplotype produces the autoantigen more efficiently, increases HLA-C surface expression, and enhances stimulation of psoriatic TCR by melanocytes; HLA-C surface expression decreases significantly more than overall HLA class I upon ERAP1 knockout, demonstrating a specific dependency of HLA-C on ERAP1.","method":"Genetically modified cell lines with ERAP1 risk/protective haplotypes, ERAP1 knockout, TCR activation assay with autoreactive psoriatic TCR, flow cytometry of HLA-C surface expression","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple specific readouts (TCR activation, peptide generation, surface expression) and haplotype comparison","pmids":["34580106"],"is_preprint":false},{"year":2011,"finding":"HLA-Cw*0602 allele-specific regulatory variants virtually abolish the HLA-C transcriptional response to TNF-α (rs2524094) and IFN-γ (rs10657191), as demonstrated by reporter assays and validated in primary keratinocytes; HLA-Cw*0602 transcripts are not upregulated in psoriatic skin lesions despite elevated TNF-α, indicating allele-specific cytokine unresponsiveness.","method":"Promoter reporter assays, primary keratinocyte cytokine treatment, qRT-PCR in psoriatic skin","journal":"The Journal of investigative dermatology","confidence":"Medium","confidence_rationale":"Tier 2 — reporter assay plus primary cell validation; single study","pmids":["22113476"],"is_preprint":false},{"year":1996,"finding":"HIV-1 Nef stimulates endocytosis and intracellular accumulation of surface MHC class I molecules (including HLA-A and HLA-B) in lymphoid, monocytic and epithelial cells; MHC-I synthesis and ER/cis-Golgi transport are unaffected, but surface molecules are rapidly internalized into endosomal vesicles and degraded. (Note: original Nef paper does not distinguish HLA-C from HLA-A/B, but subsequent work established HLA-C is NOT efficiently downregulated by Nef—see Vpu entries above.)","method":"Nef expression in multiple cell lines, pulse-chase analysis, immunofluorescence/endosomal fractionation","journal":"Nature medicine","confidence":"Medium","confidence_rationale":"Tier 2 — mechanistic dissection of endocytosis; foundational; directly relevant to HLA-C immune evasion context","pmids":["8612235"],"is_preprint":false},{"year":1996,"finding":"Assembly of MHC class I heavy chain-β2m dimers in the ER requires calreticulin (which binds class I-β2m dimers) and the novel glycoprotein tapasin (which bridges class I-β2m-calreticulin complexes to TAP), establishing the peptide-loading complex; this mechanism applies to HLA-C as one of the class I molecules assembled via this pathway.","method":"Co-immunoprecipitation, biochemical fractionation, characterization of .220 (tapasin-deficient) mutant cells","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 1-2 — foundational co-IP study of MHC class I assembly machinery directly relevant to HLA-C assembly; replicated extensively","pmids":["8769474"],"is_preprint":false},{"year":2022,"finding":"HLA-C interacts with PCSK9 via an R-X-E motif in the HLA-C transmembrane/intracellular region engaging the M2 subdomain of PCSK9's CHRD; modeling and mutagenesis identified Glu567 and Arg549 as critical M2 residues binding HLA-C, and these same residues are required for PCSK9-induced LDLR degradation, suggesting HLA-C (or a similar MHC-I member) may act as 'protein X' escorting the PCSK9-LDLR complex to degradation compartments.","method":"X-ray crystallography of CHRD-nanobody complex (2.2 Å), atomic modeling, site-directed mutagenesis, bio-layer interferometry, SAXS, immunocytochemistry, deep mutational scanning","journal":"Molecular metabolism","confidence":"Medium","confidence_rationale":"Tier 1 — structural and mutagenesis data; HLA-C interaction is modeled/hypothesized rather than directly demonstrated biochemically","pmids":["36566984"],"is_preprint":false}],"current_model":"HLA-C is a classical MHC class I glycoprotein (~43 kDa heavy chain non-covalently associated with β2-microglobulin) that assembles in the ER via the calreticulin/tapasin/TAP peptide-loading complex, with a locus-specific KYRV motif causing a biosynthetic bottleneck that renders it peptide-selective and preferentially expressed at lower surface levels than HLA-A/B; cell-surface HLA-C presents peptides to CD8+ CTLs and serves as the dominant inhibitory and activating ligand for KIR2D-family NK cell receptors (with C1/C2 epitope dimorphism at position 77/80 determining KIR specificity), is expressed by extravillous trophoblast to regulate uterine NK cells during pregnancy, is transcriptionally regulated by an NK-specific promoter and by miR-148a binding to its 3′ UTR, is selectively downregulated by HIV-1 Vpu (but not Nef) via transmembrane domain interactions to evade HLA-C-restricted CTLs, and associates with HIV-1 gp120/Env to enhance viral infectivity; additionally, ERAP1 specifically trims peptides for HLA-C*06:02 presentation in psoriasis autoimmunity, and HLA-C may interact with PCSK9 to facilitate LDLR degradation."},"narrative":{"teleology":[{"year":1977,"claim":"Establishing that HLA-C is a distinct ~43 kDa glycoprotein associated with β2-microglobulin resolved its structural identity as a bona fide MHC class I molecule expressed at lower surface levels than HLA-A/B.","evidence":"Biochemical fractionation, lectin-Sepharose, and sedimentation analysis of HLA-CW2","pmids":["332508"],"confidence":"High","gaps":["No peptide-binding characterization","Mechanism of low surface expression unknown"]},{"year":1988,"claim":"Demonstrating that HLA-Cw3 functioned as a transplantation antigen and CTL restriction element in transgenic mice established that HLA-C is not a pseudogene but has immunological function comparable to HLA-A/B.","evidence":"HLA-Cw3 transgenic mice, skin graft rejection, virus-specific CTL assays","pmids":["2840670"],"confidence":"High","gaps":["NK cell ligand role not yet recognized","No endogenous peptide repertoire defined"]},{"year":1993,"claim":"Identifying HLA-C alleles as the dominant inhibitory ligands for NK cells, with C1 versus C2 group specificity defined by residues 77/80, established the central role of HLA-C in innate immune surveillance.","evidence":"HLA-C allele transfection into class I-deleted cell lines, NK cytotoxicity assays","pmids":["8265660"],"confidence":"High","gaps":["Receptor identity (KIR) not yet molecularly defined at this point","Structural basis of C1/C2 recognition unknown"]},{"year":1995,"claim":"Resolving that HLA-C-recognizing p58 receptors exist as biochemically distinct inhibitory (~58 kDa) and activating (~50 kDa) isoforms explained how a single ligand system can generate opposing NK cell signals.","evidence":"Receptor cross-linking, Ca²⁺ flux, deglycosylation and peptide mapping, NK cytotoxicity with HLA-Cw4 transfectants","pmids":["7650491"],"confidence":"High","gaps":["Structural basis of activating versus inhibitory signaling not defined","Full KIR allelic diversity not yet explored"]},{"year":1996,"claim":"Characterizing the calreticulin–tapasin–TAP peptide-loading complex as essential for MHC class I assembly provided the ER chaperone framework within which HLA-C-specific assembly bottlenecks would later be understood.","evidence":"Co-immunoprecipitation in tapasin-deficient .220 mutant cells","pmids":["8769474"],"confidence":"High","gaps":["HLA-C-specific assembly behavior not distinguished from HLA-A/B at this point"]},{"year":1997,"claim":"Measuring KIR–HLA-C binding kinetics by SPR revealed unusually fast on/off rates, and showing peptide-dependence of KIR–HLA-C recognition established that the bound peptide contributes to the interaction interface.","evidence":"Surface plasmon resonance with recombinant soluble KIR and HLA-C proteins; native gel shift with domain-deletion mutants","pmids":["9768753","9378975"],"confidence":"High","gaps":["Atomic-level structural details of the KIR–HLA-C interface unresolved","Range of peptides modulating binding not systematically explored"]},{"year":2000,"claim":"Detecting HLA-C protein on extravillous trophoblast with biallelic expression and IFN-γ responsiveness established HLA-C as a key MHC molecule at the maternal–fetal interface, distinct from HLA-G in its regulation.","evidence":"Flow cytometric sorting of first-trimester trophoblast, cDNA sequencing, IFN-γ treatment","pmids":["10833373"],"confidence":"High","gaps":["Functional consequences for uterine NK cell tolerance not directly tested","Relative importance of HLA-C versus HLA-G for pregnancy outcome unknown"]},{"year":2000,"claim":"Mapping two clusters of polymorphic residues on KIR2D that determine C1 versus C2 group specificity provided the first molecular basis for allotype-specific NK cell inhibition patterns.","evidence":"Site-directed mutagenesis of KIR chimeric proteins with CD3-ζ signaling assays","pmids":["10820396"],"confidence":"High","gaps":["Crystal structures of KIR–HLA-C complexes not yet available"]},{"year":2002,"claim":"Demonstrating direct interaction between CD160 and HLA-C broadened the receptor repertoire for HLA-C beyond KIR family members and showed CD160 engagement triggers NK cytotoxicity.","evidence":"Anti-CD160 blocking antibody, recombinant soluble HLA-Cw3/CD160 binding assay","pmids":["12486241"],"confidence":"High","gaps":["Physiological relevance relative to KIR not established","Binding affinity not quantified"]},{"year":2007,"claim":"Identifying the KYRV motif as causing a locus-specific biosynthetic bottleneck explained why HLA-C is retained intracellularly as both β2m-associated and free forms and is peptide-selective rather than peptide-unreceptive.","evidence":"Pulse-chase, co-IP with TAP/tapasin, molecular dynamics, in vitro assembly in T2 and 721.220 extracts","pmids":["17956861"],"confidence":"High","gaps":["Whether the KYRV bottleneck specifically selects peptide repertoires important for KIR recognition was untested","Structural basis of KYRV effect on groove plasticity not crystallographically confirmed"]},{"year":2008,"claim":"Showing that HIV-1 gp120 associates with HLA-C on infected cells and that virion-incorporated HLA-C enhances infectivity revealed an unexpected viral exploitation of this molecule.","evidence":"siRNA knockdown, syncytia formation, pseudovirus infectivity, co-purification from fusion complexes","pmids":["18673537"],"confidence":"High","gaps":["Whether the HLA-C/gp120 interaction is direct or bridged by other factors unclear","Relevance to in vivo transmission not tested"]},{"year":2008,"claim":"Synergistic polymorphism at positions distal to the ligand-binding site (Pro16Arg in D1 and Arg148Cys in D2) explained the stronger HLA-C binding of KIR2DL2 relative to KIR2DL3 by altering the interdomain hinge angle.","evidence":"Mutagenesis of recombinant KIR, binding panel across 93 HLA isoforms","pmids":["18322206"],"confidence":"High","gaps":["Structural confirmation of hinge angle change awaited crystal structures"]},{"year":2011,"claim":"Identifying miR-148a targeting of the HLA-C 3′ UTR as a determinant of allotype-specific surface expression, with evolutionary escape via HLA-B sequence exchange ~3–5 Mya, established post-transcriptional regulation as a major axis of HLA-C expression variation.","evidence":"Molecular evolutionary analysis, miRNA binding site characterization","pmids":["21907013"],"confidence":"Medium","gaps":["Functional validation of miR-148a effect on protein levels in primary cells limited","Contribution of miR-148a relative to promoter-level regulation not quantified"]},{"year":2016,"claim":"Demonstrating that HIV-1 Vpu—not Nef—selectively downregulates HLA-C via transmembrane domain contacts, reducing HLA-C-restricted CTL responses, resolved the specific viral immune evasion mechanism targeting this locus.","evidence":"Primary HIV-1 isolate panel, Vpu mutant viruses, CTL suppression assays in CD4+ cells","pmids":["27173934"],"confidence":"High","gaps":["Molecular mechanism of Vpu-mediated HLA-C degradation (proteasomal vs. lysosomal) not resolved"]},{"year":2017,"claim":"Showing that HIV-1 Env specifically associates with β2m-free HLA-C chains—requiring prior correct assembly—clarified the conformational state of HLA-C exploited by HIV for enhanced infectivity.","evidence":"β2m-free HLA-C chain detection, β2m knockout, pseudovirus infectivity assays","pmids":["28051183"],"confidence":"Medium","gaps":["Direct binding interface between Env and HLA-C free chains not structurally resolved","Single-study finding"]},{"year":2018,"claim":"Mapping an NK cell-specific HLA-C promoter with Ets-binding site polymorphisms and maturation-dependent alternative splicing revealed a dedicated transcriptional program controlling HLA-C expression in NK cells.","evidence":"Promoter reporter assays, transcript/splicing analysis, NK maturation tracking, flow cytometry","pmids":["29329284"],"confidence":"High","gaps":["Transcription factors binding the NK-specific promoter not fully identified","Interplay between NK promoter and miR-148a regulation unexplored"]},{"year":2018,"claim":"Identifying 5 Vpu and 4 HLA-C transmembrane residues governing their interaction, and showing that Vpu-mediated downregulation adapts to host HLA-C genotype, defined the co-evolutionary interface between virus and host.","evidence":"128 replication-competent primary viral isolates, Vpu from 195 treatment-naïve individuals, transmembrane mutagenesis","pmids":["30180214"],"confidence":"High","gaps":["Whether Vpu targets HLA-C to ER-associated degradation or endolysosomal pathway unresolved"]},{"year":2021,"claim":"Crystal structures of KIR2DL2 and KIR2DL3 with HLA-C*07:02 revealed distinct docking geometries over the same ligand, providing the atomic-level explanation for differential NK cell inhibition by C1 allotypes.","evidence":"X-ray crystallography, site-directed mutagenesis, primary NK cell inhibition assays","pmids":["33846289"],"confidence":"High","gaps":["Structures with C2-group HLA-C allotypes not yet available","Dynamic aspects of the interaction not captured"]},{"year":2021,"claim":"Demonstrating that ERAP1 specifically trims N-extended precursors to generate the HLA-C*06:02-restricted ADAMTSL5 autoantigen in psoriasis, with risk haplotype-dependent efficiency, established a direct mechanistic link between peptide processing and autoimmune disease.","evidence":"ERAP1 risk/protective haplotype cell lines, ERAP1 knockout, autoreactive TCR activation assay","pmids":["34580106"],"confidence":"High","gaps":["Full peptide repertoire presented by HLA-C*06:02 under different ERAP1 haplotypes not catalogued","In vivo validation in patient-derived cells limited"]},{"year":2022,"claim":"Structural modeling and mutagenesis suggested HLA-C interacts with PCSK9 via an R-X-E motif engaging the CHRD M2 subdomain, potentially serving as 'protein X' that escorts PCSK9–LDLR complexes to degradation, linking MHC class I to lipid metabolism.","evidence":"X-ray crystallography of CHRD, atomic modeling, site-directed mutagenesis, bio-layer interferometry, SAXS","pmids":["36566984"],"confidence":"Medium","gaps":["Direct HLA-C–PCSK9 biochemical interaction not demonstrated with purified proteins","Functional impact on LDLR degradation via HLA-C not shown in loss-of-function experiment","Specificity for HLA-C versus other MHC-I molecules not tested"]},{"year":null,"claim":"Key open questions include the full structural basis of KIR recognition of C2-group HLA-C allotypes, the relative contributions of promoter, miR-148a, and ERAP1 regulation to HLA-C surface expression in different cell types, the precise degradation pathway used by Vpu for HLA-C downregulation, and whether HLA-C genuinely functions as a PCSK9 co-receptor for LDLR degradation in vivo.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal structure of KIR with C2-group HLA-C","Integrated quantitative model of HLA-C expression regulation lacking","Vpu-mediated HLA-C degradation pathway mechanistically unresolved","PCSK9–HLA-C interaction requires direct biochemical demonstration"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,9]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[1,2,3,4]},{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[1,6]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,6,14,15]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[9,22]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[11,15]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[1,2,5,14,19]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[9,22]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[14,16,19]}],"complexes":["MHC class I peptide-loading complex (calreticulin–tapasin–TAP)","HLA-C–β2-microglobulin heterodimer"],"partners":["B2M","KIR2DL2","KIR2DL3","TAPBP","TAP1","CD160","CALR","ERAP1"],"other_free_text":[]},"mechanistic_narrative":"HLA-C is a classical MHC class I glycoprotein that presents intracellular peptides to CD8+ cytotoxic T lymphocytes and serves as the principal ligand for KIR2D-family receptors on NK cells, with a C1/C2 epitope dimorphism at positions 77/80 determining KIR specificity and governing NK cell inhibition or activation [PMID:8265660, PMID:7650491, PMID:33846289]. Assembly in the ER proceeds through the calreticulin–tapasin–TAP peptide-loading complex, but a locus-specific KYRV motif at residues 66–76 creates a biosynthetic bottleneck that renders HLA-C peptide-selective and contributes to its characteristically low surface expression; ERAP1-mediated peptide trimming is specifically required for efficient HLA-C surface presentation, with an ERAP1 psoriasis-risk haplotype enhancing generation of the HLA-C*06:02-restricted ADAMTSL5 autoantigen [PMID:17956861, PMID:34580106, PMID:8769474]. Cell-surface HLA-C expression is regulated transcriptionally by an NK cell-specific promoter with an Ets-binding polymorphism and post-transcriptionally by miR-148a targeting the 3′ UTR, and is selectively downregulated during HIV-1 infection by Vpu (but not Nef) through transmembrane domain interactions that impair HLA-C-restricted CTL responses [PMID:29329284, PMID:21907013, PMID:30180214]. HLA-C is also expressed on extravillous trophoblast where it regulates uterine NK cell activity, and associates with HIV-1 gp120/Env as β2m-free chains on infected cells to enhance viral infectivity [PMID:10833373, PMID:18673537, PMID:28051183]."},"prefetch_data":{"uniprot":{"accession":"P10321","full_name":"HLA class I histocompatibility antigen, C alpha chain","aliases":["HLA-Cw","Human leukocyte antigen C"],"length_aa":366,"mass_kda":40.6,"function":"Antigen-presenting major histocompatibility complex class I (MHCI) molecule with an important role in reproduction and antiviral immunity (PubMed:11172028, PubMed:20104487, PubMed:20439706, PubMed:20972337, PubMed:24091323, PubMed:28649982, PubMed:29312307). In complex with B2M/beta 2 microglobulin displays a restricted repertoire of self and viral peptides and acts as a dominant ligand for inhibitory and activating killer immunoglobulin receptors (KIRs) expressed on NK cells (PubMed:16141329). In an allogeneic setting, such as during pregnancy, mediates interaction of extravillous trophoblasts with KIR on uterine NK cells and regulate trophoblast invasion necessary for placentation and overall fetal growth (PubMed:20972337, PubMed:24091323). During viral infection, may present viral peptides with low affinity for KIRs, impeding KIR-mediated inhibition through peptide antagonism and favoring lysis of infected cells (PubMed:20439706). Presents a restricted repertoire of viral peptides on antigen-presenting cells for recognition by alpha-beta T cell receptor (TCR) on HLA-C-restricted CD8-positive T cells, guiding antigen-specific T cell immune response to eliminate infected cells, particularly in chronic viral infection settings such as HIV-1 or CMV infection (PubMed:11172028, PubMed:20104487, PubMed:28649982). Both the peptide and the MHC molecule are recognized by TCR, the peptide is responsible for the fine specificity of antigen recognition and MHC residues account for the MHC restriction of T cells (By similarity). Typically presents intracellular peptide antigens of 9 amino acids that arise from cytosolic proteolysis via proteasome. Can bind different peptides containing allele-specific binding motifs, which are mainly defined by anchor residues at position 2 and 9. Preferentially displays peptides having a restricted repertoire of hydrophobic or aromatic amino acids (Phe, Ile, Leu, Met, Val and Tyr) at the C-terminal anchor (PubMed:25311805, PubMed:8265661) ALLELE C*01:02: The peptide-bound form interacts with KIR2DL2 and KIR2DL3 inhibitory receptors on NK cells. The low affinity peptides compete with the high affinity peptides impeding KIR-mediated inhibition and favoring lysis of infected cells (PubMed:20439706). Presents to CD8-positive T cells a CMV epitope derived from UL83/pp65 (RCPEMISVL), an immediate-early antigen necessary for initiating viral replication (PubMed:12947002) ALLELE C*04:01: Presents a conserved HIV-1 epitope derived from env (SFNCGGEFF) to memory CD8-positive T cells, eliciting very strong IFNG responses (PubMed:20104487). Presents CMV epitope derived from UL83/pp65 (QYDPVAALF) to CD8-positive T cells, triggering T cell cytotoxic response (PubMed:12947002) ALLELE C*05:01: Presents HIV-1 epitope derived from rev (SAEPVPLQL) to CD8-positive T cells, triggering T cell cytotoxic response ALLELE C*06:02: In trophoblasts, interacts with KIR2DS2 on uterine NK cells and triggers NK cell activation, including secretion of cytokines such as GMCSF that enhances trophoblast migration ALLELE C*07:02: Plays an important role in the control of chronic CMV infection. Presents immunodominant CMV epitopes derived from IE1 (LSEFCRVL and CRVLCCYVL) and UL28 (FRCPRRFCF), both antigens synthesized during immediate-early period of viral replication. Elicits a strong anti-viral CD8-positive T cell immune response that increases markedly with age ALLELE C*08:01: Presents viral epitopes derived from CMV UL83 (VVCAHELVC) and IAV M1 (GILGFVFTL), triggering CD8-positive T cell cytotoxic response ALLELE C*12:02: Presents CMV epitope derived from UL83 (VAFTSHEHF) to CD8-positive T cells ALLELE C*15:02: Presents CMV epitope derived from UL83 CC (VVCAHELVC) to CD8-positive T cells, triggering T cell cytotoxic response","subcellular_location":"Cell membrane; Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/P10321/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/HLA-C","classification":"Common Essential","n_dependent_lines":471,"n_total_lines":1208,"dependency_fraction":0.38990066225165565},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/HLA-C","total_profiled":1310},"omim":[{"mim_id":"620778","title":"KILLER CELL IMMUNOGLOBULIN-LIKE RECEPTOR, THREE DOMAINS, SHORT CYTOPLASMIC TAIL, 1; KIR3DS1","url":"https://www.omim.org/entry/620778"},{"mim_id":"618690","title":"PSORIASIS SUSCEPTIBILITY 1 CANDIDATE GENE 3, NONCODING; PSORS1C3","url":"https://www.omim.org/entry/618690"},{"mim_id":"614070","title":"PSORIASIS 13, SUSCEPTIBILITY TO; PSORS13","url":"https://www.omim.org/entry/614070"},{"mim_id":"613787","title":"MICRO RNA 148B; MIR148B","url":"https://www.omim.org/entry/613787"},{"mim_id":"613786","title":"MICRO RNA 148A; MIR148A","url":"https://www.omim.org/entry/613786"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"lymphoid tissue","ntpm":1377.9}],"url":"https://www.proteinatlas.org/search/HLA-C"},"hgnc":{"alias_symbol":[],"prev_symbol":["HLA-JY3","D6S204","PSORS1"]},"alphafold":{"accession":"P10321","domains":[{"cath_id":"3.30.500.10","chopping":"26-203","consensus_level":"high","plddt":97.3451,"start":26,"end":203},{"cath_id":"2.60.40.10","chopping":"209-296","consensus_level":"high","plddt":96.5055,"start":209,"end":296}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P10321","model_url":"https://alphafold.ebi.ac.uk/files/AF-P10321-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P10321-F1-predicted_aligned_error_v6.png","plddt_mean":86.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=HLA-C","jax_strain_url":"https://www.jax.org/strain/search?query=HLA-C"},"sequence":{"accession":"P10321","fasta_url":"https://rest.uniprot.org/uniprotkb/P10321.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P10321/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P10321"}},"corpus_meta":[{"pmid":"7650491","id":"PMC_7650491","title":"Existence 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foundational study\",\n      \"pmids\": [\"332508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"HLA-Cw3 functions as a transplantation antigen and as an MHC restriction molecule for cytotoxic T lymphocytes (CTL) recognizing influenza and Sendai virus antigens, demonstrating immunological functions comparable to HLA-A and HLA-B.\",\n      \"method\": \"Transgenic mouse model (HLA-Cw3 transgenic C57BL/6 mice), skin graft rejection assay, virus-specific CTL assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean transgenic KO/KI model with defined functional readouts (graft rejection and CTL restriction)\",\n      \"pmids\": [\"2840670\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"HLA-C alleles are the dominant inhibitory ligands for NK cell allorecognition: alleles encoding Asn-77/Lys-80 (C2 group: Cw4, Cw5, Cw6) inhibit NK1-specific lysis, while alleles encoding Ser-77/Asn-80 (C1 group: Cw1, Cw7, Cw13) protect targets from NK2-specific lysis, with specificity determined by the dimorphism at positions 77–80.\",\n      \"method\": \"HLA-C transfection into MHC class I-deleted mutant cell lines, NK cell lysis assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — transfection plus functional NK lysis assay, highly cited foundational paper\",\n      \"pmids\": [\"8265660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"NK cells express both inhibitory (p58, ~58 kDa) and activatory (p50, ~50 kDa) forms of the EB6 receptor for HLA-C Cw4 alleles; the two forms differ in molecular mass (not glycosylation) and in signaling: p58 inhibits cytolytic activity without Ca2+ flux, while p50 cross-linking triggers cytolysis and intracellular Ca2+ increases upon HLA-Cw4 recognition.\",\n      \"method\": \"Biochemical analysis (deglycosylation, proteolytic peptide mapping, 2D mapping), NK clone functional assays (cytolysis, Ca2+ flux), mAb cross-linking, HLA-C transfected target cells\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal biochemical and functional methods in a highly cited foundational paper\",\n      \"pmids\": [\"7650491\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"p58 KIR molecules bind directly to recombinant soluble HLA-C molecules (Cw3 and Cw6); binding requires both Ig domains of the p58 KIR and is influenced by the peptide bound in the MHC groove, suggesting peptide-dependent recognition.\",\n      \"method\": \"Native gel shift assay with recombinant soluble p58 KIRs and soluble HLA-C; domain deletion constructs\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding assay with recombinant proteins and domain analysis, single lab\",\n      \"pmids\": [\"9378975\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"HLA-C binds to KIR (NK inhibitory receptor) molecules with extremely fast association and dissociation rates (among the fastest immune system protein–protein interactions), measured using soluble recombinant HLA-C and KIR proteins.\",\n      \"method\": \"Surface plasmon resonance / binding kinetics with recombinant soluble HLA-C and NKIR proteins\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with recombinant proteins, quantitative kinetic measurements\",\n      \"pmids\": [\"9768753\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"KIR residues at two polymorphic clusters define the group allotype specificity of HLA-C binding: site-directed mutagenesis showed that multiple polymorphic residues contribute to the KIR HLA-C binding site, with two clusters being primary determinants of C1 vs. C2 specificity.\",\n      \"method\": \"Site-directed mutagenesis of KIR2D; chimeric KIR-CD3ζ transfectants; in vitro signaling assay measuring HLA-C specificity\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with functional signaling assay\",\n      \"pmids\": [\"10820396\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"HLA-C protein is expressed on the surface of extravillous trophoblast cells in association with beta2-microglobulin; both maternal and paternal alleles are transcribed; HLA-C (but not HLA-G) expression is upregulated by IFN-γ treatment, and trophoblast HLA-C can interact with KIR on decidual NK cells.\",\n      \"method\": \"Flow cytometric sorting and cDNA sequencing, biochemical analysis, flow cytometry (surface expression), immunohistology, IFN-γ treatment\",\n      \"journal\": \"Placenta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (sequencing, biochemistry, flow cytometry, immunohistology) establishing surface expression and regulation\",\n      \"pmids\": [\"10833373\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CD160 receptor expressed on circulating CD56dim NK cells directly interacts with HLA-C molecules (demonstrated with recombinant soluble HLA-Cw3 and CD160 proteins), and this HLA-C/CD160 engagement triggers NK cytotoxic activity against HLA-C-expressing K562 target cells; CD158b inhibitory KIR partially interferes with this pathway.\",\n      \"method\": \"Recombinant soluble protein binding assay, NK cytotoxicity assay, antibody blocking experiments\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct protein–protein interaction with recombinant molecules plus functional NK lysis assay\",\n      \"pmids\": [\"12486241\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"A single locus-specific biosynthetic bottleneck renders HLA-C peptide-selective: the KYRV motif at residues 66–76 of the α1-domain helix causes HLA-C heavy chains to accumulate as free (unassembled) chains and post-assembly unstable complexes in the ER; KYRV alleles associate with TAP and tapasin chaperones and show reduced plasticity/unfolding in the peptide-binding groove; in vitro assembly experiments showed HLA-Cw1 retains the ability to form a peptide-receptive interface without functional tapasin.\",\n      \"method\": \"Pulse-labeling experiments, co-immunoprecipitation, antibodies to linear epitopes, molecular dynamics simulation, in vitro assembly in T2/721.220 mutant cell extracts\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution in vitro, co-IP, molecular dynamics, multiple orthogonal approaches in one study\",\n      \"pmids\": [\"17956861\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"KIR2DL2 is a stronger receptor for HLA-C than KIR2DL3 due to synergistic polymorphism at two positions distal to the ligand-binding site (Pro16→Arg in D1 and Arg148→Cys in D2); neither residue alone suffices, and their juxtaposition near the D1–D2 hinge alters the relative domain orientation and binding avidity.\",\n      \"method\": \"Binding assays to 93 HLA allotypes, site-directed mutagenesis of KIR2DL2/3, functional NK inhibition assays\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis plus quantitative binding panel, replicated with multiple allotypes\",\n      \"pmids\": [\"18322206\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Differential HLA-C surface expression is controlled by binding of microRNA miR-148a to the 3' UTR of HLA-C mRNA: alleles that lack miR-148a binding sites (due to sequence exchange from an HLA-B allele occurring ~3–5 million years ago) escape miR-148a-mediated downregulation and are expressed at higher levels; lower expression alleles associate with higher HIV-1 viral load.\",\n      \"method\": \"Molecular evolutionary analysis, sequence exchange detection, functional miRNA binding site characterization\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic identification of miRNA regulatory element with evolutionary and functional validation\",\n      \"pmids\": [\"21907013\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Reporter assays identified two regulatory variants in the HLA-C promoter region that abolish TNF-α (rs2524094) and IFN-γ (rs10657191) responsiveness specifically in HLA-Cw*0602 and related alleles; this was validated in primary keratinocytes, showing allele-specific differences in cytokine-driven HLA-C expression.\",\n      \"method\": \"Reporter gene assays, primary keratinocyte treatment with TNF-α/IFN-γ, quantitative PCR\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional reporter assays validated in primary cells with two orthogonal cytokines\",\n      \"pmids\": [\"22113476\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"KIR2DL3*005 allele binds HLA-C with higher affinity and avidity than KIR2DL3*001 due to the combination of Arg at residue 11 and Glu at residue 35 (not individually sufficient); this results in greater inhibition of NK cell IFN-γ production. Molecular modeling suggests altered interdomain hinge angle as the mechanism.\",\n      \"method\": \"Surface plasmon resonance, KIR binding to HLA allotype panel, flow cytometry, site-directed mutagenesis, NK functional assays (IFN-γ inhibition)\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — SPR binding kinetics plus mutagenesis plus NK functional assay\",\n      \"pmids\": [\"23686481\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"HIV-1 Vpu mediates downregulation of HLA-C on infected cells (but not HLA-A or HLA-B), reducing HLA-C-restricted CTL suppression of viral replication; this was demonstrated for most primary HIV-1 clones including transmitted founder viruses (unlike laboratory-adapted NL4-3). Five transmembrane residues in Vpu and four in the HLA-C transmembrane domain determine this interaction.\",\n      \"method\": \"Primary HIV-1 clone infection assays, siRNA knockdown, CTL suppression assays, Vpu mutagenesis, flow cytometry\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple primary isolates, mutagenesis identifying interacting residues, functional CTL assay, replicated across isolates\",\n      \"pmids\": [\"27173934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"HLA-C surface expression levels and HIV-1 Vpu-mediated HLA-C downregulation adapt to host HLA-C genotype; 5 Vpu transmembrane residues and 4 HLA-C transmembrane domain residues were identified as determining Vpu–HLA-C interactions.\",\n      \"method\": \"Primary HIV-1 viral isolates from 195 individuals, Vpu mutagenesis, flow cytometry, clonal viral analysis\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis plus large-scale primary virus analysis identifying specific interaction residues\",\n      \"pmids\": [\"30180214\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"An NK cell-specific HLA-C promoter (NK-Pro) drives alternative transcripts including untranslatable mRNAs (generated by skipping of the first coding exon via SNPs at splice sites); the proportion of untranslatable HLA-C mRNA decreases as NK cells mature, correlating with increased HLA-C protein expression; polymorphism in an Ets-binding site of the NK promoter reduces HLA-C expression and increases NK functional activity.\",\n      \"method\": \"Transcriptomic analysis, promoter reporter assays, flow cytometry, NK cell differentiation assays, allele-specific analysis\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (promoter assays, transcriptomics, flow cytometry) demonstrating a novel NK-specific regulatory mechanism\",\n      \"pmids\": [\"29329284\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ERAP1 generates the melanocyte autoantigen ADAMTSL5 by N-terminal trimming of elongated peptide precursors, producing optimal-length peptides for presentation by HLA-C*06:02 to psoriatic CD8+ T cells; an ERAP1 risk haplotype produces the autoantigen more efficiently and increases HLA-C surface expression and psoriatic TCR stimulation by melanocytes; HLA-C surface expression decreases significantly more than overall HLA class I upon ERAP1 knockout.\",\n      \"method\": \"Genetically modified cell lines, TCR activation assay with psoriatic autoreactive TCR, ERAP1 knockout, ERAP1 haplotype comparison, flow cytometry\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — functional reconstitution with KO cells and autoreactive TCR assay, multiple orthogonal methods\",\n      \"pmids\": [\"34580106\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Crystal structures of KIR2DL2 and KIR2DL3 in complex with HLA-C*07:02 (presenting a self-epitope) reveal that KIR2DL2 and KIR2DL3 adopt different docking geometries over HLA-C1, with structural plasticity correlating with allotype recognition differences; mutagenesis confirmed distinct HLA-C1 allotype recognition mechanisms for the two receptors.\",\n      \"method\": \"X-ray crystallography, site-directed mutagenesis, primary NK cell inhibition assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures plus mutagenesis plus functional NK assays in one study\",\n      \"pmids\": [\"33846289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"HLA-C (via its gp120-associated form on virions) increases HIV-1 infectivity: cells co-expressing HLA-C and HIV-1 gp120/gp41 fuse more rapidly and produce larger syncytia; pseudoviruses from HLA-C-silenced cells are less infectious; HLA-C was co-purified with gp120 in fusion complexes; the effect requires prior assembly of HLA-C with beta2-microglobulin.\",\n      \"method\": \"siRNA silencing of HLA-C, cell fusion assays, pseudovirus infectivity assays, co-immunoprecipitation/co-purification of HLA-C with gp120\",\n      \"journal\": \"Retrovirology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — siRNA KD plus co-purification plus pseudovirus infectivity, multiple orthogonal approaches\",\n      \"pmids\": [\"18673537\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"HIV-1 Env (gp120/gp41) specifically associates with HLA-C free chains (not bound to β2-microglobulin) at the cell membrane; HIV-1 infection increases the amount of HLA-C free chains on infected cells; the enhanced infectivity conferred by HLA-C requires HLA-C molecules that have been correctly assembled with β2m prior to the interaction with Env.\",\n      \"method\": \"Flow cytometry, siRNA knockdown of β2m, β2m-negative cell lines, pseudovirus infectivity assays, co-immunoprecipitation\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — single lab but multiple orthogonal methods (KD, KO cells, functional infectivity, co-IP)\",\n      \"pmids\": [\"28051183\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PCSK9 M2 subdomain (of the CHRD) interacts with HLA-C via an R-X-E motif; Glu567 and Arg549 in M2 are critical for both HLA-C binding and PCSK9-induced LDLR degradation, suggesting HLA-C (or a similar MHC-I family member) may serve as 'protein X' that escorts the PCSK9-LDLR complex to degradation compartments.\",\n      \"method\": \"X-ray crystallography (CHRD-nanobody complex at 2.2 Å), site-directed mutagenesis, bio-layer interferometry, SAXS, deep mutational scanning, hepatic cell line expression/LDLR degradation assays\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure and mutagenesis establish binding residues, but HLA-C as 'protein X' remains a hypothesis supported by modeling rather than direct reconstitution\",\n      \"pmids\": [\"36566984\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"KIR2DS4 activating receptor triggers trogocytosis-mediated CCR7 uptake by KIR2DS4+ NKG2A+ NK cell clones upon recognition of HLA-Cw4 and HLA-Cw6 alleles on target cells, and can trigger cytotoxicity against HLA-C-transfected cells; NKG2A co-expression can partially override KIR2DS4 activation.\",\n      \"method\": \"NK cell clone functional assays, cytotoxicity against HLA-C transfected cells, trogocytosis measurement, antibody blocking\",\n      \"journal\": \"Journal of immunology research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional NK assays with HLA-C transfected targets, single lab\",\n      \"pmids\": [\"25961063\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"HLA-C is a classical MHC class I molecule that assembles with beta2-microglobulin and peptides in the ER (with assembly bottlenecked by the KYRV motif requiring TAP/tapasin), is expressed at lower surface levels than HLA-A/B through regulation by an NK-specific promoter generating untranslatable transcripts and by miR-148a binding to the 3' UTR, presents intracellular peptides (processed by ERAP1) to CD8+ CTLs and to CD4+ T cells, serves as the dominant ligand for KIR2DL1/2/3 inhibitory and KIR2DS1/2/4/5 activating receptors on NK cells (with C1/C2 dimorphism at position 80 governing specificity), triggers NK cytotoxicity via CD160 engagement, is selectively downregulated on HIV-infected cells by viral Vpu targeting transmembrane residues, associates with HIV-1 gp120 to enhance viral infectivity, and is the exclusive classical MHC-I molecule expressed by extravillous trophoblast where KIR/HLA-C allorecognition by uterine NK cells regulates placentation.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1977,\n      \"finding\": \"HLA-C (HLA-CW2) was shown to be a glycosylated polypeptide of ~43 kDa that is noncovalently associated with beta2-microglobulin, structurally homologous to HLA-A and HLA-B antigens, with some heterogeneity in its carbohydrate moiety and lower cell-surface expression levels compared to HLA-A and HLA-B.\",\n      \"method\": \"Biochemical fractionation, lectin-Sepharose adsorption, sedimentation analysis\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct biochemical characterization with multiple methods; foundational structural paper\",\n      \"pmids\": [\"332508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"HLA-C alleles are the dominant inhibitory ligands protecting target cells from NK cell-mediated lysis: alleles encoding Asn-77-Lys-80 (C2 group, e.g. Cw4, Cw5, Cw6) inhibit NK1-specific NK cells, whereas alleles encoding Ser-77-Asn-80 (C1 group, e.g. Cw1, Cw7, Cw13) protect from NK2-specific NK cells, as directly demonstrated by HLA-C transfection into class I-deleted cell lines.\",\n      \"method\": \"HLA-C allele transfection into class I-deleted mutant cell lines, NK cell cytotoxicity assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional rescue by defined transfectants; highly cited foundational study\",\n      \"pmids\": [\"8265660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"NK cell p58 receptors for HLA-C (EB6/GL183) exist in two biochemically distinct forms: a ~58 kDa inhibitory form (p58) and a ~50 kDa activatory form (p50), differing in molecular mass but not solely due to differential glycosylation; cross-linking of p50 triggers cytolytic activity and Ca2+ influx whereas p58 cross-linking inhibits lysis without Ca2+ influx, and p50-bearing NK clones lyse HLA-Cw4+ targets upon specific HLA-C recognition.\",\n      \"method\": \"Biochemical analysis (deglycosylation, peptide mapping, 2D mapping), anti-receptor mAb cross-linking, Ca2+ flux, NK cytotoxicity assays, LCL721.221/Cw4 transfectant targets\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal biochemical and functional methods; highly cited\",\n      \"pmids\": [\"7650491\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"HLA-C binds to NK inhibitory receptors (KIR/NKIR) with extremely fast association and dissociation kinetics (among the fastest immune interactions characterized), as measured using soluble recombinant HLA-C molecules and recombinant NKIR proteins.\",\n      \"method\": \"Surface plasmon resonance / kinetic binding assays using soluble recombinant HLA-C and KIR proteins\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro binding kinetics with recombinant proteins\",\n      \"pmids\": [\"9768753\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Recombinant soluble p58 KIRs (KIR-K6 and KIR-K7) bind soluble HLA-Cw3 and HLA-Cw6 molecules directly, and this binding is influenced by the antigenic peptide bound to the MHC, indicating peptide-dependence of KIR-HLA-C recognition; both Ig domains of p58 are required for HLA-C binding.\",\n      \"method\": \"Native gel shift assay with recombinant soluble proteins, domain-deletion analysis\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct binding demonstrated in vitro with recombinant proteins and domain requirements mapped\",\n      \"pmids\": [\"9378975\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"HLA-Cw3 expressed in transgenic mice functions as a transplantation antigen capable of inducing rapid skin graft rejection and serves as an MHC restriction molecule for cytotoxic T lymphocyte responses to influenza and Sendai virus, demonstrating that HLA-C has immunological functions comparable to HLA-A and HLA-B.\",\n      \"method\": \"HLA-Cw3 transgenic mouse model, skin graft rejection assays, virus-specific CTL assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — transgenic model with defined functional readouts; highly cited\",\n      \"pmids\": [\"2840670\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"HLA-C protein is expressed on the surface of extravillous trophoblast cells in association with beta2-microglobulin, with both paternal and maternal alleles transcribed; cell-surface HLA-C (but not HLA-G) is upregulated by IFN-γ treatment, demonstrating differential cytokine regulation.\",\n      \"method\": \"Flow cytometric sorting of first-trimester trophoblast, cDNA sequencing, biochemical analysis, flow cytometry, immunohistology\",\n      \"journal\": \"Placenta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct protein detection, allele-level sequencing, and functional cytokine response validated by multiple methods\",\n      \"pmids\": [\"10833373\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Two clusters of polymorphic residues on KIR2D molecules define group allotype specificity for HLA-C binding: multiple polymorphic residues contribute to the HLA-C binding site, with specific residues determining whether a KIR binds C1 or C2 group alleles.\",\n      \"method\": \"Site-directed mutagenesis of KIR chimeric proteins (KIR extracellular domain fused to CD3-ζ tail), signaling assays, HLA-C binding specificity analysis\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — site-directed mutagenesis with defined functional readout\",\n      \"pmids\": [\"10820396\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The CD160 receptor expressed on circulating CD56dim NK cells directly interacts with HLA-C molecules expressed on K562 cells and triggers NK cell cytotoxicity; direct protein-protein interaction between recombinant soluble HLA-Cw3 and CD160 was demonstrated.\",\n      \"method\": \"Anti-CD160 mAb blocking of NK cytotoxicity, recombinant soluble protein binding assay (direct HLA-Cw3/CD160 interaction), NK cell functional assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct recombinant protein interaction plus functional blocking experiments\",\n      \"pmids\": [\"12486241\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"HLA-C heavy chains accumulate intracellularly in two forms (free of and associated with β2-microglobulin) due to a unique KYRV motif at residues 66-76 of the α1-domain helix that causes poor assembly, post-assembly instability, reduced peptide-binding groove plasticity, and association with dedicated chaperones TAP and tapasin; this locus-specific bottleneck renders HLA-C peptide-selective (rather than peptide-unreceptive) and contributes to its preferential role as an NK cell ligand.\",\n      \"method\": \"Pulse-labeling experiments, co-immunoprecipitation with anti-TAP/tapasin antibodies, molecular dynamics simulation, in vitro assembly in T2 and 721.220 cell extracts, reactivity with conformation-specific antibodies\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including pulse-chase, co-IP, in vitro reconstitution, and structural simulation\",\n      \"pmids\": [\"17956861\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"KIR2DL2 is a stronger receptor for HLA-C than KIR2DL3 due to synergistic polymorphism at two positions distal to the ligand-binding site (Pro16Arg in D1 and Arg148Cys in D2); neither substitution alone is sufficient—only the combination increases avidity, likely by altering the hinge angle between the two Ig domains and changing the relative orientation of the ligand-binding site.\",\n      \"method\": \"Mutagenesis of recombinant KIR proteins, functional binding assays to 93 HLA isoforms, site-directed mutagenesis\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with defined mutagenesis and binding assays; highly cited\",\n      \"pmids\": [\"18322206\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"HLA-C molecules associated with HIV-1 gp120 on infected cell surfaces and virion-associated HLA-C increases the infectivity of both R5 and X4 HIV-1 viruses; co-purification of HLA-C with gp120 from fusion complexes was demonstrated.\",\n      \"method\": \"siRNA knockdown of HLA-C, syncytia formation assays, pseudovirus infectivity assays, co-purification of fusion complexes\",\n      \"journal\": \"Retrovirology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function (siRNA), gain-of-function, and molecular co-purification with specific phenotypic readout\",\n      \"pmids\": [\"18673537\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Differential surface expression of HLA-C allotypes is mediated by binding of microRNA miR-148a to the 3′ UTR of HLA-C mRNA: alleles susceptible to miR-148a binding have lower surface expression; the common ancestor of all HLA-C alleles was suppressed by miR-148a, and escape alleles arose ~3–5 million years ago via sequence exchange with an HLA-B allele, with selective pressure favoring spread of the escape variants.\",\n      \"method\": \"Molecular evolutionary analysis, miRNA binding site characterization, sequence exchange mapping\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — evolutionary/molecular analysis; mechanistic basis for miR-148a regulation supported by prior functional data\",\n      \"pmids\": [\"21907013\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"KIR2DL3*005 has markedly increased affinity and avidity for HLA-C ligands compared to other KIR2DL3 alleles due to a combination of Arg at residue 11 and Glu at residue 35 (distal to the KIR/HLA-C interface), likely by shifting the interdomain hinge angle toward the KIR2DL2 configuration; this confers stronger inhibition of NK cell IFN-γ production.\",\n      \"method\": \"Surface plasmon resonance, KIR binding panel assay, site-directed mutagenesis, NK cell functional assay (IFN-γ production by KHYG-1 cells), flow cytometry\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis combined with SPR binding and functional NK cell assay\",\n      \"pmids\": [\"23686481\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"HIV-1 Vpu (not Nef) mediates downregulation of HLA-C on infected cells in most primary HIV-1 isolates; HLA-C downregulation by Vpu reduces the ability of HLA-C-restricted CTLs to suppress viral replication in CD4+ cells in vitro; HLA-A and HLA-B are unaffected by Vpu.\",\n      \"method\": \"Primary HIV-1 clone panel, Vpu mutant viruses, HLA-C surface expression flow cytometry, CTL suppression of viral replication assays in CD4+ cells\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with viral mutants and defined CTL functional readout; highly cited\",\n      \"pmids\": [\"27173934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"HIV-1 Env (gp120) specifically associates with HLA-C free chains (not bound to β2-microglobulin) at the cell membrane; HIV-1 infection increases the amount of HLA-C free chains on infected cell membranes; the enhanced infectivity conferred by HLA-C requires prior correct assembly with β2m before HLA-C free chains can associate with Env, and pseudoviruses produced from HLA-C-silenced or β2m-absent cells are less infectious.\",\n      \"method\": \"β2m-free HLA-C chain detection, HIV-1 Env pseudovirus infectivity assays, β2m knockout/knockdown experiments, cell-surface biochemistry\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic dissection of HLA-C/Env interaction with defined loss-of-function; single study\",\n      \"pmids\": [\"28051183\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"HLA-C downregulation by HIV-1 Vpu adapts to host HLA-C genotype: HLA-C alleles differ in the immune pressure they exert, and individuals with higher HLA-C expression favor greater viral downregulation; 5 residues in the transmembrane region of Vpu and 4 residues in the transmembrane domain of HLA-C determine Vpu-HLA-C interactions.\",\n      \"method\": \"Primary viral isolate panel (128 replication-competent viruses), Vpu mutant analysis, cloning and expression of Vpu from 195 treatment-naïve individuals, transmembrane domain mutagenesis\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — large primary virus panel plus mutagenesis identifying specific interacting residues\",\n      \"pmids\": [\"30180214\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"An NK cell-specific HLA-C promoter drives an array of alternative transcripts with variable intron/exon content; skipping of the first coding exon generates untranslatable mRNAs, and the proportion of these decreases as NK cells mature, correlating with increased HLA-C protein expression; polymorphism in a key Ets-binding site of the NK promoter generates HLA-C alleles with reduced promoter activity, resulting in lower HLA-C expression and increased NK cell functional activity.\",\n      \"method\": \"Transcript analysis (alternative splicing), promoter reporter assays (Ets-binding site mutagenesis), NK cell maturation tracking, flow cytometry of HLA-C protein levels\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including reporter assays, transcript analysis, and NK functional correlates\",\n      \"pmids\": [\"29329284\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Crystal structures of KIR2DL2 and KIR2DL3 in complex with HLA-C*07:02 presenting a self-epitope revealed that KIR2DL2 differs from KIR2DL3 in its docking modality over HLA-C*07:02; mutagenesis assays showed differences in mechanism of HLA-C1 allotype recognition, with HLA-C1 allotypes differing markedly in capacity to inhibit NK cell activation; KIR2DS2 contributes to functional differences alongside KIR2DL2/3 binding geometries.\",\n      \"method\": \"X-ray crystallography, site-directed mutagenesis, primary NK cell inhibition assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures with mutagenesis and functional validation\",\n      \"pmids\": [\"33846289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ERAP1 generates the causative melanocyte autoantigen (ADAMTSL5-derived peptide) for HLA-C*06:02 presentation in psoriasis by trimming N-terminally elongated peptide precursors to the appropriate length; an ERAP1 psoriasis-risk haplotype produces the autoantigen more efficiently, increases HLA-C surface expression, and enhances stimulation of psoriatic TCR by melanocytes; HLA-C surface expression decreases significantly more than overall HLA class I upon ERAP1 knockout, demonstrating a specific dependency of HLA-C on ERAP1.\",\n      \"method\": \"Genetically modified cell lines with ERAP1 risk/protective haplotypes, ERAP1 knockout, TCR activation assay with autoreactive psoriatic TCR, flow cytometry of HLA-C surface expression\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple specific readouts (TCR activation, peptide generation, surface expression) and haplotype comparison\",\n      \"pmids\": [\"34580106\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"HLA-Cw*0602 allele-specific regulatory variants virtually abolish the HLA-C transcriptional response to TNF-α (rs2524094) and IFN-γ (rs10657191), as demonstrated by reporter assays and validated in primary keratinocytes; HLA-Cw*0602 transcripts are not upregulated in psoriatic skin lesions despite elevated TNF-α, indicating allele-specific cytokine unresponsiveness.\",\n      \"method\": \"Promoter reporter assays, primary keratinocyte cytokine treatment, qRT-PCR in psoriatic skin\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reporter assay plus primary cell validation; single study\",\n      \"pmids\": [\"22113476\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"HIV-1 Nef stimulates endocytosis and intracellular accumulation of surface MHC class I molecules (including HLA-A and HLA-B) in lymphoid, monocytic and epithelial cells; MHC-I synthesis and ER/cis-Golgi transport are unaffected, but surface molecules are rapidly internalized into endosomal vesicles and degraded. (Note: original Nef paper does not distinguish HLA-C from HLA-A/B, but subsequent work established HLA-C is NOT efficiently downregulated by Nef—see Vpu entries above.)\",\n      \"method\": \"Nef expression in multiple cell lines, pulse-chase analysis, immunofluorescence/endosomal fractionation\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic dissection of endocytosis; foundational; directly relevant to HLA-C immune evasion context\",\n      \"pmids\": [\"8612235\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Assembly of MHC class I heavy chain-β2m dimers in the ER requires calreticulin (which binds class I-β2m dimers) and the novel glycoprotein tapasin (which bridges class I-β2m-calreticulin complexes to TAP), establishing the peptide-loading complex; this mechanism applies to HLA-C as one of the class I molecules assembled via this pathway.\",\n      \"method\": \"Co-immunoprecipitation, biochemical fractionation, characterization of .220 (tapasin-deficient) mutant cells\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — foundational co-IP study of MHC class I assembly machinery directly relevant to HLA-C assembly; replicated extensively\",\n      \"pmids\": [\"8769474\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"HLA-C interacts with PCSK9 via an R-X-E motif in the HLA-C transmembrane/intracellular region engaging the M2 subdomain of PCSK9's CHRD; modeling and mutagenesis identified Glu567 and Arg549 as critical M2 residues binding HLA-C, and these same residues are required for PCSK9-induced LDLR degradation, suggesting HLA-C (or a similar MHC-I member) may act as 'protein X' escorting the PCSK9-LDLR complex to degradation compartments.\",\n      \"method\": \"X-ray crystallography of CHRD-nanobody complex (2.2 Å), atomic modeling, site-directed mutagenesis, bio-layer interferometry, SAXS, immunocytochemistry, deep mutational scanning\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — structural and mutagenesis data; HLA-C interaction is modeled/hypothesized rather than directly demonstrated biochemically\",\n      \"pmids\": [\"36566984\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"HLA-C is a classical MHC class I glycoprotein (~43 kDa heavy chain non-covalently associated with β2-microglobulin) that assembles in the ER via the calreticulin/tapasin/TAP peptide-loading complex, with a locus-specific KYRV motif causing a biosynthetic bottleneck that renders it peptide-selective and preferentially expressed at lower surface levels than HLA-A/B; cell-surface HLA-C presents peptides to CD8+ CTLs and serves as the dominant inhibitory and activating ligand for KIR2D-family NK cell receptors (with C1/C2 epitope dimorphism at position 77/80 determining KIR specificity), is expressed by extravillous trophoblast to regulate uterine NK cells during pregnancy, is transcriptionally regulated by an NK-specific promoter and by miR-148a binding to its 3′ UTR, is selectively downregulated by HIV-1 Vpu (but not Nef) via transmembrane domain interactions to evade HLA-C-restricted CTLs, and associates with HIV-1 gp120/Env to enhance viral infectivity; additionally, ERAP1 specifically trims peptides for HLA-C*06:02 presentation in psoriasis autoimmunity, and HLA-C may interact with PCSK9 to facilitate LDLR degradation.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"HLA-C is a classical MHC class I molecule that assembles with β2-microglobulin and peptide in the endoplasmic reticulum, presents intracellular peptides to CD8⁺ cytotoxic T lymphocytes, and serves as the dominant ligand for KIR2D inhibitory and activating receptors on NK cells. A C1/C2 dimorphism at positions 77–80 of the α1 helix governs specificity for KIR2DL2/3 versus KIR2DL1, with KIR binding exhibiting extremely fast kinetics and structural plasticity in docking geometry that differs between KIR allotypes [PMID:8265660, PMID:9768753, PMID:33846289]. HLA-C surface expression is characteristically lower than HLA-A/B, controlled by multiple mechanisms including a locus-specific KYRV biosynthetic bottleneck that restricts peptide loading, miR-148a–mediated translational repression, and an NK cell–specific alternative promoter generating untranslatable transcripts [PMID:17956861, PMID:21907013, PMID:29329284]. HIV-1 Vpu selectively downregulates HLA-C via transmembrane domain interactions to evade HLA-C–restricted CTL responses, while virion-associated HLA-C enhances HIV-1 infectivity through association with gp120 [PMID:27173934, PMID:18673537].\",\n  \"teleology\": [\n    {\n      \"year\": 1977,\n      \"claim\": \"Establishing that HLA-C is a bona fide β2-microglobulin-associated MHC class I glycoprotein resolved its molecular identity and explained its lower surface expression relative to HLA-A/B.\",\n      \"evidence\": \"Biochemical fractionation, sedimentation analysis, and lectin chromatography of immunoprecipitated HLA-Cw2\",\n      \"pmids\": [\"332508\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism underlying low expression was unknown\", \"Peptide-binding function not yet demonstrated\"]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Demonstrating that HLA-C restricts virus-specific CTL responses and mediates graft rejection established it as a functional antigen-presenting molecule, not merely a serologically weak locus.\",\n      \"evidence\": \"HLA-Cw3 transgenic mice: skin graft rejection and influenza/Sendai virus-specific CTL assays\",\n      \"pmids\": [\"2840670\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"NK cell ligand function not yet known\", \"Endogenous peptide repertoire undefined\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Identifying HLA-C as the dominant NK cell inhibitory ligand and defining the C1/C2 dimorphism at positions 77–80 as the specificity determinant established the central role of HLA-C in NK cell education and tolerance.\",\n      \"evidence\": \"HLA-C allele transfection into MHC class I–deleted cells, NK clone lysis assays\",\n      \"pmids\": [\"8265660\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of KIR–HLA-C binding interface unresolved\", \"Activating KIR recognition not yet characterized\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Distinguishing inhibitory (p58) and activatory (p50) KIR isoforms that both recognize HLA-C revealed that the same ligand can trigger opposing NK cell outcomes depending on receptor isoform.\",\n      \"evidence\": \"Biochemical characterization and Ca²⁺ flux/cytolysis assays in NK clones with HLA-Cw4 transfected targets\",\n      \"pmids\": [\"7650491\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling pathways downstream of activating KIR–HLA-C unresolved\", \"Structural basis of isoform discrimination unknown\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Showing that KIR binding to HLA-C requires both Ig domains and is peptide-dependent established that the MHC-bound peptide contributes to NK recognition specificity.\",\n      \"evidence\": \"Native gel shift with recombinant soluble KIR (domain deletion constructs) and HLA-C\",\n      \"pmids\": [\"9378975\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Crystal structure of KIR–HLA-C complex not yet available\", \"Extent of peptide selectivity across allotypes undefined\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Quantifying the ultra-fast association/dissociation kinetics of KIR–HLA-C binding explained how NK cells can rapidly scan targets and integrate inhibitory signals during transient contacts.\",\n      \"evidence\": \"Surface plasmon resonance with recombinant soluble HLA-C and KIR proteins\",\n      \"pmids\": [\"9768753\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relationship between fast kinetics and in vivo NK education unclear\", \"Kinetics of activating KIR–HLA-C not measured\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Mapping polymorphic KIR residue clusters that govern C1/C2 specificity, and demonstrating HLA-C expression on extravillous trophoblast, established the molecular logic of KIR–HLA-C allotype discrimination and its relevance to placentation.\",\n      \"evidence\": \"KIR mutagenesis with chimeric signaling assays [PMID:10820396]; flow cytometry, immunohistology, and IFN-γ regulation in primary trophoblast [PMID:10833373]\",\n      \"pmids\": [\"10820396\", \"10833373\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequences of KIR–HLA-C interaction in uterine NK cells for placental remodeling not demonstrated\", \"Paternal HLA-C allorecognition outcome unclear\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identifying CD160 as a second activating receptor for HLA-C on NK cells expanded the receptor repertoire beyond KIR and showed that HLA-C can trigger cytotoxicity through a non-KIR pathway.\",\n      \"evidence\": \"Recombinant soluble HLA-Cw3–CD160 binding plus NK cytotoxicity and antibody blocking assays\",\n      \"pmids\": [\"12486241\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"CD160 signaling pathway upon HLA-C engagement undefined\", \"Relative contribution of CD160 vs. KIR in vivo unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Discovering that the KYRV motif creates a locus-specific biosynthetic bottleneck requiring TAP/tapasin for efficient peptide loading explained why HLA-C surface expression is intrinsically lower than HLA-A/B.\",\n      \"evidence\": \"Pulse-labeling, co-IP with TAP/tapasin, in vitro assembly in mutant cell extracts, molecular dynamics\",\n      \"pmids\": [\"17956861\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether KYRV restriction shapes the HLA-C peptide repertoire differently from HLA-A/B not established\", \"Relative contribution of KYRV vs. transcriptional regulation to low expression not quantified\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Demonstrating that HLA-C associates with HIV-1 gp120 on virions and enhances viral infectivity, and that synergistic KIR polymorphisms modulate HLA-C binding strength, linked HLA-C biology to HIV pathogenesis at both the innate and viral levels.\",\n      \"evidence\": \"siRNA silencing, co-IP of HLA-C with gp120, pseudovirus infectivity [PMID:18673537]; KIR2DL2/3 mutagenesis and binding to 93 HLA allotypes [PMID:18322206]\",\n      \"pmids\": [\"18673537\", \"18322206\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of HLA-C–gp120 interaction unknown\", \"Whether gp120 association is unique to HLA-C among MHC-I unclear\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Identifying miR-148a targeting of the HLA-C 3ʹ UTR and allele-specific promoter variants responsive to TNF-α/IFN-γ revealed post-transcriptional and transcriptional layers controlling differential HLA-C expression with consequences for HIV control and psoriasis.\",\n      \"evidence\": \"Evolutionary and functional miRNA binding site analysis [PMID:21907013]; reporter assays and primary keratinocyte cytokine treatment [PMID:22113476]\",\n      \"pmids\": [\"21907013\", \"22113476\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How miR-148a and promoter polymorphism interact quantitatively to set allele-specific expression unclear\", \"Whether miR-148a regulation is NK cell–specific not determined\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Showing that HIV-1 Vpu selectively downregulates HLA-C via specific transmembrane domain contacts to evade CTL killing identified a viral immune evasion mechanism operating exclusively on this MHC-I locus.\",\n      \"evidence\": \"Primary HIV-1 isolate infection, Vpu mutagenesis identifying 5 TM residues, CTL suppression assays\",\n      \"pmids\": [\"27173934\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of Vpu-mediated HLA-C degradation (proteasomal vs. lysosomal) not defined\", \"Whether Vpu targets HLA-C free chains or assembled complexes unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Discovery of an NK cell–specific alternative promoter generating untranslatable HLA-C mRNAs that decrease during NK maturation added a developmental regulatory layer explaining how HLA-C expression is tuned during NK cell education.\",\n      \"evidence\": \"Transcriptomic analysis, promoter reporter assays, NK differentiation time-course, flow cytometry\",\n      \"pmids\": [\"29329284\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Transcription factors driving NK-Pro activity beyond Ets not fully characterized\", \"Whether NK-Pro operates in other immune cell subsets unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Structural determination of KIR2DL2 and KIR2DL3 complexes with HLA-C*07:02 revealed distinct docking geometries over the same ligand, providing atomic-level explanation for differential allotype recognition and NK inhibition strength, while ERAP1-dependent peptide trimming was shown to control HLA-C*06:02 surface expression and autoantigen presentation in psoriasis.\",\n      \"evidence\": \"X-ray crystallography with mutagenesis and NK functional assays [PMID:33846289]; ERAP1 KO cells and psoriatic TCR activation assay [PMID:34580106]\",\n      \"pmids\": [\"33846289\", \"34580106\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How peptide repertoire differences between ERAP1 haplotypes globally alter HLA-C immunopeptidome unknown\", \"Crystal structures of KIR–HLA-C2 group complexes still lacking\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the multiple layers of HLA-C expression regulation (KYRV bottleneck, miR-148a, NK-Pro, cytokine-responsive promoter variants) are integrated quantitatively to set cell-type-specific and allele-specific surface levels, and how these influence NK education thresholds and disease susceptibility in vivo, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No integrated quantitative model of HLA-C expression regulation exists\", \"In vivo consequences of allele-specific expression for NK education thresholds not directly measured\", \"Structural basis of Vpu–HLA-C transmembrane interaction not determined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1, 9]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [2, 3, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 7, 14, 16]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [9]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [19, 20]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [1, 2, 3, 14, 17]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [9]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [14, 15, 19]}\n    ],\n    \"complexes\": [\n      \"MHC class I peptide-loading complex (TAP/tapasin)\"\n    ],\n    \"partners\": [\n      \"B2M\",\n      \"KIR2DL1\",\n      \"KIR2DL2\",\n      \"KIR2DL3\",\n      \"KIR2DS4\",\n      \"CD160\",\n      \"ERAP1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"HLA-C is a classical MHC class I glycoprotein that presents intracellular peptides to CD8+ cytotoxic T lymphocytes and serves as the principal ligand for KIR2D-family receptors on NK cells, with a C1/C2 epitope dimorphism at positions 77/80 determining KIR specificity and governing NK cell inhibition or activation [PMID:8265660, PMID:7650491, PMID:33846289]. Assembly in the ER proceeds through the calreticulin–tapasin–TAP peptide-loading complex, but a locus-specific KYRV motif at residues 66–76 creates a biosynthetic bottleneck that renders HLA-C peptide-selective and contributes to its characteristically low surface expression; ERAP1-mediated peptide trimming is specifically required for efficient HLA-C surface presentation, with an ERAP1 psoriasis-risk haplotype enhancing generation of the HLA-C*06:02-restricted ADAMTSL5 autoantigen [PMID:17956861, PMID:34580106, PMID:8769474]. Cell-surface HLA-C expression is regulated transcriptionally by an NK cell-specific promoter with an Ets-binding polymorphism and post-transcriptionally by miR-148a targeting the 3′ UTR, and is selectively downregulated during HIV-1 infection by Vpu (but not Nef) through transmembrane domain interactions that impair HLA-C-restricted CTL responses [PMID:29329284, PMID:21907013, PMID:30180214]. HLA-C is also expressed on extravillous trophoblast where it regulates uterine NK cell activity, and associates with HIV-1 gp120/Env as β2m-free chains on infected cells to enhance viral infectivity [PMID:10833373, PMID:18673537, PMID:28051183].\",\n  \"teleology\": [\n    {\n      \"year\": 1977,\n      \"claim\": \"Establishing that HLA-C is a distinct ~43 kDa glycoprotein associated with β2-microglobulin resolved its structural identity as a bona fide MHC class I molecule expressed at lower surface levels than HLA-A/B.\",\n      \"evidence\": \"Biochemical fractionation, lectin-Sepharose, and sedimentation analysis of HLA-CW2\",\n      \"pmids\": [\"332508\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No peptide-binding characterization\", \"Mechanism of low surface expression unknown\"]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Demonstrating that HLA-Cw3 functioned as a transplantation antigen and CTL restriction element in transgenic mice established that HLA-C is not a pseudogene but has immunological function comparable to HLA-A/B.\",\n      \"evidence\": \"HLA-Cw3 transgenic mice, skin graft rejection, virus-specific CTL assays\",\n      \"pmids\": [\"2840670\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"NK cell ligand role not yet recognized\", \"No endogenous peptide repertoire defined\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Identifying HLA-C alleles as the dominant inhibitory ligands for NK cells, with C1 versus C2 group specificity defined by residues 77/80, established the central role of HLA-C in innate immune surveillance.\",\n      \"evidence\": \"HLA-C allele transfection into class I-deleted cell lines, NK cytotoxicity assays\",\n      \"pmids\": [\"8265660\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Receptor identity (KIR) not yet molecularly defined at this point\", \"Structural basis of C1/C2 recognition unknown\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Resolving that HLA-C-recognizing p58 receptors exist as biochemically distinct inhibitory (~58 kDa) and activating (~50 kDa) isoforms explained how a single ligand system can generate opposing NK cell signals.\",\n      \"evidence\": \"Receptor cross-linking, Ca²⁺ flux, deglycosylation and peptide mapping, NK cytotoxicity with HLA-Cw4 transfectants\",\n      \"pmids\": [\"7650491\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of activating versus inhibitory signaling not defined\", \"Full KIR allelic diversity not yet explored\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Characterizing the calreticulin–tapasin–TAP peptide-loading complex as essential for MHC class I assembly provided the ER chaperone framework within which HLA-C-specific assembly bottlenecks would later be understood.\",\n      \"evidence\": \"Co-immunoprecipitation in tapasin-deficient .220 mutant cells\",\n      \"pmids\": [\"8769474\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"HLA-C-specific assembly behavior not distinguished from HLA-A/B at this point\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Measuring KIR–HLA-C binding kinetics by SPR revealed unusually fast on/off rates, and showing peptide-dependence of KIR–HLA-C recognition established that the bound peptide contributes to the interaction interface.\",\n      \"evidence\": \"Surface plasmon resonance with recombinant soluble KIR and HLA-C proteins; native gel shift with domain-deletion mutants\",\n      \"pmids\": [\"9768753\", \"9378975\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-level structural details of the KIR–HLA-C interface unresolved\", \"Range of peptides modulating binding not systematically explored\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Detecting HLA-C protein on extravillous trophoblast with biallelic expression and IFN-γ responsiveness established HLA-C as a key MHC molecule at the maternal–fetal interface, distinct from HLA-G in its regulation.\",\n      \"evidence\": \"Flow cytometric sorting of first-trimester trophoblast, cDNA sequencing, IFN-γ treatment\",\n      \"pmids\": [\"10833373\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequences for uterine NK cell tolerance not directly tested\", \"Relative importance of HLA-C versus HLA-G for pregnancy outcome unknown\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Mapping two clusters of polymorphic residues on KIR2D that determine C1 versus C2 group specificity provided the first molecular basis for allotype-specific NK cell inhibition patterns.\",\n      \"evidence\": \"Site-directed mutagenesis of KIR chimeric proteins with CD3-ζ signaling assays\",\n      \"pmids\": [\"10820396\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Crystal structures of KIR–HLA-C complexes not yet available\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstrating direct interaction between CD160 and HLA-C broadened the receptor repertoire for HLA-C beyond KIR family members and showed CD160 engagement triggers NK cytotoxicity.\",\n      \"evidence\": \"Anti-CD160 blocking antibody, recombinant soluble HLA-Cw3/CD160 binding assay\",\n      \"pmids\": [\"12486241\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance relative to KIR not established\", \"Binding affinity not quantified\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Identifying the KYRV motif as causing a locus-specific biosynthetic bottleneck explained why HLA-C is retained intracellularly as both β2m-associated and free forms and is peptide-selective rather than peptide-unreceptive.\",\n      \"evidence\": \"Pulse-chase, co-IP with TAP/tapasin, molecular dynamics, in vitro assembly in T2 and 721.220 extracts\",\n      \"pmids\": [\"17956861\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the KYRV bottleneck specifically selects peptide repertoires important for KIR recognition was untested\", \"Structural basis of KYRV effect on groove plasticity not crystallographically confirmed\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Showing that HIV-1 gp120 associates with HLA-C on infected cells and that virion-incorporated HLA-C enhances infectivity revealed an unexpected viral exploitation of this molecule.\",\n      \"evidence\": \"siRNA knockdown, syncytia formation, pseudovirus infectivity, co-purification from fusion complexes\",\n      \"pmids\": [\"18673537\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the HLA-C/gp120 interaction is direct or bridged by other factors unclear\", \"Relevance to in vivo transmission not tested\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Synergistic polymorphism at positions distal to the ligand-binding site (Pro16Arg in D1 and Arg148Cys in D2) explained the stronger HLA-C binding of KIR2DL2 relative to KIR2DL3 by altering the interdomain hinge angle.\",\n      \"evidence\": \"Mutagenesis of recombinant KIR, binding panel across 93 HLA isoforms\",\n      \"pmids\": [\"18322206\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural confirmation of hinge angle change awaited crystal structures\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Identifying miR-148a targeting of the HLA-C 3′ UTR as a determinant of allotype-specific surface expression, with evolutionary escape via HLA-B sequence exchange ~3–5 Mya, established post-transcriptional regulation as a major axis of HLA-C expression variation.\",\n      \"evidence\": \"Molecular evolutionary analysis, miRNA binding site characterization\",\n      \"pmids\": [\"21907013\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional validation of miR-148a effect on protein levels in primary cells limited\", \"Contribution of miR-148a relative to promoter-level regulation not quantified\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Demonstrating that HIV-1 Vpu—not Nef—selectively downregulates HLA-C via transmembrane domain contacts, reducing HLA-C-restricted CTL responses, resolved the specific viral immune evasion mechanism targeting this locus.\",\n      \"evidence\": \"Primary HIV-1 isolate panel, Vpu mutant viruses, CTL suppression assays in CD4+ cells\",\n      \"pmids\": [\"27173934\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of Vpu-mediated HLA-C degradation (proteasomal vs. lysosomal) not resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Showing that HIV-1 Env specifically associates with β2m-free HLA-C chains—requiring prior correct assembly—clarified the conformational state of HLA-C exploited by HIV for enhanced infectivity.\",\n      \"evidence\": \"β2m-free HLA-C chain detection, β2m knockout, pseudovirus infectivity assays\",\n      \"pmids\": [\"28051183\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct binding interface between Env and HLA-C free chains not structurally resolved\", \"Single-study finding\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Mapping an NK cell-specific HLA-C promoter with Ets-binding site polymorphisms and maturation-dependent alternative splicing revealed a dedicated transcriptional program controlling HLA-C expression in NK cells.\",\n      \"evidence\": \"Promoter reporter assays, transcript/splicing analysis, NK maturation tracking, flow cytometry\",\n      \"pmids\": [\"29329284\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Transcription factors binding the NK-specific promoter not fully identified\", \"Interplay between NK promoter and miR-148a regulation unexplored\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identifying 5 Vpu and 4 HLA-C transmembrane residues governing their interaction, and showing that Vpu-mediated downregulation adapts to host HLA-C genotype, defined the co-evolutionary interface between virus and host.\",\n      \"evidence\": \"128 replication-competent primary viral isolates, Vpu from 195 treatment-naïve individuals, transmembrane mutagenesis\",\n      \"pmids\": [\"30180214\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Vpu targets HLA-C to ER-associated degradation or endolysosomal pathway unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Crystal structures of KIR2DL2 and KIR2DL3 with HLA-C*07:02 revealed distinct docking geometries over the same ligand, providing the atomic-level explanation for differential NK cell inhibition by C1 allotypes.\",\n      \"evidence\": \"X-ray crystallography, site-directed mutagenesis, primary NK cell inhibition assays\",\n      \"pmids\": [\"33846289\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structures with C2-group HLA-C allotypes not yet available\", \"Dynamic aspects of the interaction not captured\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstrating that ERAP1 specifically trims N-extended precursors to generate the HLA-C*06:02-restricted ADAMTSL5 autoantigen in psoriasis, with risk haplotype-dependent efficiency, established a direct mechanistic link between peptide processing and autoimmune disease.\",\n      \"evidence\": \"ERAP1 risk/protective haplotype cell lines, ERAP1 knockout, autoreactive TCR activation assay\",\n      \"pmids\": [\"34580106\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full peptide repertoire presented by HLA-C*06:02 under different ERAP1 haplotypes not catalogued\", \"In vivo validation in patient-derived cells limited\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Structural modeling and mutagenesis suggested HLA-C interacts with PCSK9 via an R-X-E motif engaging the CHRD M2 subdomain, potentially serving as 'protein X' that escorts PCSK9–LDLR complexes to degradation, linking MHC class I to lipid metabolism.\",\n      \"evidence\": \"X-ray crystallography of CHRD, atomic modeling, site-directed mutagenesis, bio-layer interferometry, SAXS\",\n      \"pmids\": [\"36566984\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct HLA-C–PCSK9 biochemical interaction not demonstrated with purified proteins\", \"Functional impact on LDLR degradation via HLA-C not shown in loss-of-function experiment\", \"Specificity for HLA-C versus other MHC-I molecules not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the full structural basis of KIR recognition of C2-group HLA-C allotypes, the relative contributions of promoter, miR-148a, and ERAP1 regulation to HLA-C surface expression in different cell types, the precise degradation pathway used by Vpu for HLA-C downregulation, and whether HLA-C genuinely functions as a PCSK9 co-receptor for LDLR degradation in vivo.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No crystal structure of KIR with C2-group HLA-C\", \"Integrated quantitative model of HLA-C expression regulation lacking\", \"Vpu-mediated HLA-C degradation pathway mechanistically unresolved\", \"PCSK9–HLA-C interaction requires direct biochemical demonstration\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 9]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [1, 2, 3, 4]},\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [1, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 6, 14, 15]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [9, 22]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [11, 15]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [1, 2, 5, 14, 19]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [9, 22]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [14, 16, 19]}\n    ],\n    \"complexes\": [\n      \"MHC class I peptide-loading complex (calreticulin–tapasin–TAP)\",\n      \"HLA-C–β2-microglobulin heterodimer\"\n    ],\n    \"partners\": [\n      \"B2M\",\n      \"KIR2DL2\",\n      \"KIR2DL3\",\n      \"TAPBP\",\n      \"TAP1\",\n      \"CD160\",\n      \"CALR\",\n      \"ERAP1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}