{"gene":"KLRC2","run_date":"2026-06-10T02:59:49","timeline":{"discoveries":[{"year":1997,"finding":"NKG2-C delivers an activating transmembrane signal in NK cells: chimeric NKG2-C/NKR-P1C receptors stimulated lytic activity and calcium mobilization when cross-linked, and the inhibitory tyrosine phosphatase SHP-1 did NOT associate with the NKG2-C chimeric receptor (in contrast to the inhibitory NKG2-A chimeric receptor).","method":"Chimeric receptor transfection in RNK-16 rat NK cells; cytolytic assays; calcium mobilization assays; immunoprecipitation","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct functional reconstitution with chimeric receptors, multiple orthogonal assays (cytotoxicity, calcium flux, co-IP), clear mechanistic contrast with inhibitory NKG2-A","pmids":["9103421"],"is_preprint":false},{"year":1998,"finding":"CD94/NKG2C forms a non-covalent complex with the ITAM-bearing adapter DAP12; charged residues in the transmembrane domains of both DAP12 and NKG2C are required for this interaction, and DAP12 is necessary for efficient surface expression of CD94/NKG2C.","method":"Co-immunoprecipitation; cell-surface expression analysis with transmembrane domain mutants","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct biochemical reconstitution with mutagenesis identifying critical transmembrane residues, replicated in subsequent studies","pmids":["9655483"],"is_preprint":false},{"year":1998,"finding":"The 39-kDa protein (Kp39) co-precipitated with CD94 from NKG2A-negative NK cells is covalently associated with CD94 and is the product of the NKG2-C gene; co-transfection of CD94 and NKG2-C in COS7 cells confirmed the identity of Kp39 as NKG2-C, and engagement of the complex by a specific mAb triggered redirected cytotoxicity.","method":"Immunoprecipitation; COS7 co-transfection; peptide mapping; RT-PCR on NK clones; redirected killing assay","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — biochemical reconstitution in transfected cells plus functional cytotoxicity assay, multiple orthogonal methods in single study","pmids":["9485212"],"is_preprint":false},{"year":1999,"finding":"Both CD94/NKG2-A and CD94/NKG2-C bind HLA-E in a peptide-dependent manner; CD94/NKG2-A has higher binding affinity for HLA-E than CD94/NKG2-C, and both receptors show very fast association and dissociation kinetics. The magnitude of NK cell response correlates directly with the binding affinity of peptide-HLA-E complexes for the receptors.","method":"Surface plasmon resonance (kinetic binding analysis) using soluble recombinant HLA-E and recombinant soluble receptor proteins assembled with defined peptides","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — quantitative in vitro binding assay with recombinant proteins and multiple peptide variants, replicated across conditions","pmids":["10428963"],"is_preprint":false},{"year":1999,"finding":"Mouse CD94/NKG2C and CD94/NKG2E bind the MHC class Ib molecule Qa-1(b); a blocking anti-NKG2 monoclonal antibody demonstrated that CD94/NKG2 molecules are the only Qa-1(b) receptors on mouse NK cells.","method":"Cloning and expression; binding assay; anti-NKG2 blocking antibody","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct binding demonstration with receptor cloning and specific antibody blockade, first direct evidence for exclusive Qa-1(b) recognition","pmids":["10601355"],"is_preprint":false},{"year":2000,"finding":"Engagement of CD94/NKG2C on an NK clone induces MAPK (ERK) phosphorylation; the MEK inhibitor PD098059 blocks CD94-dependent TNF-α production and cytotoxicity. In RBL cells transfected with CD94/NKG2C/DAP12, receptor cross-linking induces calcium mobilization, serotonin release, and MAPK phosphorylation.","method":"Pharmacological inhibition (MEK inhibitor); Western blot for phospho-MAPK; calcium mobilization; serotonin release assay in transfected RBL cells","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution in heterologous cell line plus pharmacological dissection in primary NK clone, multiple orthogonal readouts","pmids":["11069065"],"is_preprint":false},{"year":2004,"finding":"CD94/NKG2A and CD94/NKG2C recognize an overlapping but partly distinct set of residues on the top of HLA-E (α1/α2 domain); two HLA-E mutations (D69A and H155A) selectively abrogated binding to CD94/NKG2A without substantially affecting CD94/NKG2C binding, demonstrating differential epitope usage.","method":"Alanine-scanning mutagenesis of HLA-E; binding assay with soluble CD94/NKG2A and CD94/NKG2C","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — systematic alanine-scan across HLA-E surface with two purified receptor complexes tested in parallel","pmids":["14971033"],"is_preprint":false},{"year":2005,"finding":"CD94/NKG2C is expressed on a subset of CD8+ T cells (and rare TCRγδ+ and CD4+ cells) and is coupled to DAP12 in these T cells; selective engagement of CD94/NKG2C triggers cytotoxicity, cytokine production, IL-2Rα upregulation, and proliferation in CD8+ T cell clones independently of TCR.","method":"Co-immunoprecipitation of DAP12 with anti-CD94; co-culture with HLA-E transfectants; mAb engagement; proliferation assays; cytokine assays; cytotoxicity assays","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP, functional assays with multiple T cell clones, HLA-E transfectant target cells","pmids":["15940674"],"is_preprint":false},{"year":2005,"finding":"Expansion of CD94/NKG2C+ NK cells in response to HCMV-infected fibroblasts requires interaction of CD94/NKG2C with the infected cells (blocked by anti-CD94 mAb) and requires IL-15 for proliferation; this expansion is independent of HCMV genes UL16, UL18, and UL40 but is impaired by deletion of the US2-11 gene region.","method":"Co-culture with HCMV-infected fibroblasts and deletion mutant viruses; anti-CD94 antibody blockade; IL-15 supplementation; flow cytometry","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — antibody blockade of receptor, viral gene deletion panel, IL-15 requirement established experimentally","pmids":["16384928"],"is_preprint":false},{"year":2009,"finding":"IL-12 induces transient expression of the inhibitory CD94/NKG2A receptor on CD94/NKG2C+ NK cells; this inducible NKG2A inhibits the cytolytic activity of NKG2C+ NK clones upon engagement by HLA-E. Anti-IL-12 antibody prevented this NKG2A upregulation in HCMV-infected DC co-cultures.","method":"In vitro NK cell stimulation; anti-IL-12 mAb neutralization; flow cytometry; cytolytic assays with HLA-E transfectants; NK clones","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — antibody neutralization and functional readout, single lab, two orthogonal methods","pmids":["19124726"],"is_preprint":false},{"year":2012,"finding":"NMR-based structure refinement of the DAP12-NKG2C transmembrane helix complex in explicit membranes reveals that five functionally required interfacial residues (two Asp and two Thr in DAP12 dimer; one Lys in NKG2C) form a stable network of salt bridges and hydrogen bonds; the arrangement is energetically consistent with experimental observations.","method":"NMR observable-constrained molecular dynamics simulation in explicit micelles and bilayers","journal":"Biophysical journal","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — computational refinement of published NMR structure, single study, no mutagenesis validation reported in this paper","pmids":["22500771"],"is_preprint":false},{"year":2014,"finding":"IL-12 produced by CD14+ inflammatory monocytes, combined with HLA-E/CD94-NKG2C engagement, is required for expansion of NKG2C+ NK cells in response to HCMV infection; IL-12 neutralization substantially reduced CD25 upregulation and NKG2C+ subset expansion; blockade of CD94/NKG2C or silencing of HLA-E greatly impaired NKG2C+ NK cell expansion.","method":"Co-culture system with HCMV-infected fibroblasts and monocytes; IL-12 neutralization; anti-CD94/NKG2C blocking antibody; HLA-E siRNA silencing; flow cytometry","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple genetic and antibody perturbations (IL-12 neutralization, receptor blockade, HLA-E knockdown) in same study, orthogonal methods","pmids":["25384219"],"is_preprint":false},{"year":2013,"finding":"NKG2C zygosity quantitatively influences surface CD94/NKG2C receptor levels, NKG2C+ NK cell numbers, calcium influx, degranulation, and IL-15-dependent proliferation in response to NKG2C engagement; NKG2C+/+ individuals show greater responses than hemizygous NKG2C+/del individuals.","method":"Flow cytometry; intracellular calcium measurement; degranulation assay (CD107a); IL-15-driven proliferation assay in cohort stratified by NKG2C copy number","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional assays in gene-dose cohort, single lab","pmids":["24030638"],"is_preprint":false},{"year":2015,"finding":"HLA-E regulates NKG2C+ NK cell degranulation in a peptide-restricted manner: peptide-stabilized HLA-E activates NKG2A-NKG2C+ NK cells only with a restricted subset of peptides (CMV-I, A80, B13, and especially HLA-G-derived peptide), and HLA-E/G peptide complex triggers NKG2C receptor internalization (reversible with bafilomycin).","method":"Peptide-loading of target cells; CD107a degranulation assay; flow cytometric measurement of NKG2C surface expression; bafilomycin rescue","journal":"Human immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional assay with pharmacological rescue, single lab, multiple peptides tested","pmids":["26382247"],"is_preprint":false},{"year":2016,"finding":"In NKG2C-deficient (homozygous deletion) humans, adaptive NK cells still expand and display characteristic adaptive features (terminal differentiation, functional reprogramming, IFN-γ promoter epigenetic remodeling); these cells express high levels of CD2, which synergistically enhances ERK and S6RP phosphorylation after CD16 ligation, demonstrating that CD2 provides co-stimulatory 'signal 2' for antibody-driven adaptive NK cell responses independent of NKG2C.","method":"NK cell repertoire analysis in 60 NKG2C-/- donors; phospho-flow cytometry (ERK, S6RP); ADCC functional assays; epigenetic analysis of IFN-γ promoter","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — natural human genetic KO cohort (n=60), multiple orthogonal assays including signaling phosphorylation, epigenetics, and function","pmids":["27117418"],"is_preprint":false},{"year":2010,"finding":"The CD94/NKG2C complex, associated with KARAP/DAP12, is fully functional in γδ T cells (Vδ2 subset), triggering IFN-γ production, proliferation, and cytolytic activity; when NKG2A and NKG2C are co-expressed on the same cell, the inhibitory signal from NKG2A prevails over NKG2C-mediated activation.","method":"Flow cytometry; IFN-γ ELISA; proliferation assay; cytolytic assays; functional comparison in NKG2A/NKG2C double-positive cells","journal":"Journal of leukocyte biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional assays in primary γδ T cells, NKG2A dominance demonstrated in double-positive cells, single lab","pmids":["20952657"],"is_preprint":false},{"year":2017,"finding":"CD94/NKG2C activation by HCMV-infected cells is stimulus-dependent on the virus strain: infection with laboratory strains (AD169, Towne) failed to activate a Jurkat-NKG2C reporter system despite preserving surface HLA-E, whereas clinical isolates and the endotheliotropic TB40/E strain triggered reporter activation; the TB40/E-driven response was not blocked by anti-NKG2C or anti-HLA-E antibodies, suggesting a non-canonical or additional ligand.","method":"Jurkat reporter cell line expressing CD94/NKG2C/DAP12 with NFAT/AP1-luciferase reporter; HCMV infection with multiple strains; antibody blockade of NKG2C and HLA-E","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reconstituted reporter system with antibody blockade, multiple viral strains compared, single lab","pmids":["29114247"],"is_preprint":false},{"year":2019,"finding":"NKG2C+ adaptive NK cells expressing HLA-DR can process and present HCMV antigen to autologous CD4+ T cells in an HLA-DR-dependent manner, activating polyfunctional effector-memory CD4+ T cells (CD107a+, IFN-γ+, TNF-α+) with cytotoxic potential; HCMV complexed with specific antibodies upregulates surface HLA-DR on NK cells via CD16.","method":"Purified NK cell-T cell co-culture; HLA-DR blocking antibody; flow cytometry for CD107a, IFN-γ, TNF-α; HLA-DR upregulation after antibody-complexed antigen loading","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — antigen presentation established by HLA-DR blocking and functional T cell readout, single lab","pmids":["31001281"],"is_preprint":false},{"year":2022,"finding":"In liver sinusoidal CD56hiCD161-CD8+ T cells, NKG2C mediates NK-like effector functions (degranulation and cytotoxicity) in a TCR-independent manner; these cells express CD94, KIRs, and NKG2C, and respond to NKG2C ligation without TCR stimulation.","method":"CITE-seq with TCR-seq; flow cytometry; functional assays with NKG2C ligation in absence of TCR stimulation; liver perfusate from transplant donors","journal":"Journal of hepatology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — TCR-independent NKG2C function demonstrated in primary human liver cells, single lab, orthogonal transcriptomics + functional validation","pmids":["35644434"],"is_preprint":false},{"year":1995,"finding":"NKG2-C is expressed as a 36-kDa glycoprotein (protein backbone ~26 kDa) on NK cells; a recombinant soluble NKG2-C specifically binds K562 and RPMI 8866 cells but not other hematopoietic cell lines; this binding is lost when K562 cells differentiate with phorbol ester/Ca2+ ionophore, concurrent with loss of susceptibility to NK killing.","method":"In vitro translation; recombinant expression; immunoprecipitation; binding assay with soluble NKG2-C on multiple cell lines; differentiation of K562 with phorbol ester","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding demonstrated with soluble recombinant receptor and functional correlation, single lab","pmids":["7589093"],"is_preprint":false}],"current_model":"KLRC2 (NKG2C) encodes a C-type lectin-type II transmembrane protein that heterodimerizes covalently with CD94 and non-covalently associates with the ITAM-bearing adapter DAP12 via charged transmembrane residues; this CD94/NKG2C–DAP12 complex functions as an activating receptor on NK cells and subsets of CD8+ T and γδ T cells that recognizes HLA-E loaded with defined signal-peptide-derived nonamers, with lower binding affinity than the inhibitory CD94/NKG2A counterpart, and transduces activation through DAP12-dependent MAPK/ERK and calcium mobilization pathways to trigger cytotoxicity and cytokine production; HCMV infection drives persistent clonal expansion of NKG2C+ NK cells in an IL-12-, IL-15-, and HLA-E-dependent manner, generating memory-like adaptive NK cells capable of antigen presentation to CD4+ T cells."},"narrative":{"mechanistic_narrative":"KLRC2 (NKG2C) encodes a C-type lectin-like type II transmembrane receptor that pairs with CD94 to form an activating NK-cell receptor recognizing the non-classical MHC class Ib molecule HLA-E (and its murine homolog Qa-1b) in a peptide-dependent manner [PMID:9485212, PMID:10428963, PMID:10601355]. Unlike its inhibitory counterpart CD94/NKG2A, CD94/NKG2C does not recruit the phosphatase SHP-1 but instead associates non-covalently with the ITAM-bearing adapter DAP12 through reciprocal charged transmembrane residues, an interaction also required for efficient surface expression of the complex [PMID:9103421, PMID:9655483]. Receptor engagement signals through DAP12 to drive MAPK/ERK phosphorylation and calcium mobilization, triggering cytotoxicity and cytokine production [PMID:11069065]; CD94/NKG2A and CD94/NKG2C engage overlapping but distinct epitopes on the HLA-E α1/α2 surface, with NKG2C binding at lower affinity and the response magnitude scaling with peptide-HLA-E binding strength [PMID:10428963, PMID:14971033]. Beyond NK cells, functional DAP12-coupled CD94/NKG2C operates on subsets of CD8+ T and Vδ2 γδ T cells, mediating TCR-independent effector activity, with NKG2A signaling dominating when both receptors are co-expressed [PMID:15940674, PMID:20952657, PMID:35644434]. HCMV infection drives clonal expansion of NKG2C+ NK cells in a manner requiring CD94/NKG2C–HLA-E engagement plus IL-12 and IL-15, generating adaptive memory-like NK cells; these adaptive features can also arise in NKG2C-deficient individuals through CD2-dependent co-stimulation [PMID:16384928, PMID:25384219, PMID:27117418].","teleology":[{"year":1995,"claim":"Established that NKG2-C is a cell-surface glycoprotein capable of binding specific target cells, linking receptor expression to NK target recognition.","evidence":"Recombinant soluble NKG2-C binding assays across hematopoietic cell lines with phorbol-ester differentiation of K562","pmids":["7589093"],"confidence":"Medium","gaps":["Ligand identity unknown at this stage","No signaling mechanism defined"]},{"year":1997,"claim":"Distinguished NKG2-C from inhibitory NKG2-A by showing it delivers an activating rather than inhibitory signal, settling its functional polarity.","evidence":"Chimeric NKG2-C/NKR-P1C receptor transfection in rat NK cells with cytotoxicity, calcium flux, and SHP-1 co-IP","pmids":["9103421"],"confidence":"High","gaps":["Adapter mediating activation not yet identified","Native ligand not defined"]},{"year":1998,"claim":"Identified the molecular basis of activation by showing CD94/NKG2C associates with the ITAM adapter DAP12 via transmembrane charged residues, explaining how an activating signal is transduced.","evidence":"Co-immunoprecipitation and transmembrane-domain mutagenesis; COS7 co-transfection and redirected killing","pmids":["9655483","9485212"],"confidence":"High","gaps":["Downstream signaling cascade not yet mapped","Affinity/kinetics of ligand binding unmeasured"]},{"year":1999,"claim":"Defined HLA-E (and Qa-1b in mouse) as the peptide-dependent ligand and quantified that NKG2C binds at lower affinity than NKG2A, establishing the ligand recognition logic.","evidence":"Surface plasmon resonance with recombinant peptide-loaded HLA-E; mouse receptor cloning and antibody blockade","pmids":["10428963","10601355"],"confidence":"High","gaps":["Structural basis of differential affinity not resolved","Physiologic peptide repertoire incompletely defined"]},{"year":2000,"claim":"Mapped the activating signal to the MAPK/ERK and calcium pathways, connecting DAP12 engagement to effector output.","evidence":"MEK inhibitor blockade, phospho-MAPK Western blot, calcium and serotonin release in transfected RBL cells","pmids":["11069065"],"confidence":"High","gaps":["Proximal kinases linking DAP12 to ERK not identified here"]},{"year":2004,"claim":"Showed NKG2A and NKG2C use overlapping but partly distinct HLA-E epitopes, explaining how the two receptors discriminate the same ligand.","evidence":"Alanine-scanning mutagenesis of HLA-E with soluble receptor binding assays","pmids":["14971033"],"confidence":"High","gaps":["No co-crystal structure of NKG2C–HLA-E"]},{"year":2005,"claim":"Extended functional CD94/NKG2C–DAP12 signaling to CD8+ T cells, showing TCR-independent activation of cytotoxicity and proliferation.","evidence":"Reciprocal DAP12 co-IP, HLA-E transfectant co-culture, and functional assays in CD8+ T cell clones","pmids":["15940674"],"confidence":"High","gaps":["In vivo relevance of T-cell NKG2C not established"]},{"year":2005,"claim":"Identified the determinants of HCMV-driven NKG2C+ NK cell expansion, requiring receptor engagement of infected cells and IL-15.","evidence":"Co-culture with HCMV-infected fibroblasts, viral deletion mutants, anti-CD94 blockade, IL-15 supplementation","pmids":["16384928"],"confidence":"High","gaps":["Specific viral/host ligand driving expansion not pinpointed","Role of US2-11 region mechanistically unresolved"]},{"year":2009,"claim":"Revealed a regulatory loop whereby IL-12 induces inhibitory NKG2A on NKG2C+ cells, dampening their cytolytic activity.","evidence":"In vitro stimulation, anti-IL-12 neutralization, cytolytic assays with HLA-E transfectants","pmids":["19124726"],"confidence":"Medium","gaps":["Single lab","Durability and in vivo significance of induced NKG2A unclear"]},{"year":2010,"claim":"Demonstrated functional CD94/NKG2C–DAP12 in Vδ2 γδ T cells and established NKG2A dominance over NKG2C in co-expressing cells.","evidence":"Functional assays (IFN-γ, proliferation, cytolysis) in primary γδ T cells","pmids":["20952657"],"confidence":"Medium","gaps":["Single lab","Mechanism of NKG2A dominance over NKG2C not resolved"]},{"year":2012,"claim":"Refined the atomic architecture of the DAP12–NKG2C transmembrane interface, defining the salt-bridge/hydrogen-bond network underlying complex assembly.","evidence":"NMR-constrained molecular dynamics in explicit membranes","pmids":["22500771"],"confidence":"Medium","gaps":["Computational refinement without new mutagenesis validation in this study"]},{"year":2013,"claim":"Showed NKG2C gene dosage quantitatively tunes receptor levels and effector responses, linking genotype to functional output.","evidence":"Functional assays (calcium, CD107a, IL-15 proliferation) in NKG2C copy-number-stratified cohort","pmids":["24030638"],"confidence":"Medium","gaps":["Single lab","Mechanism connecting zygosity to per-cell response not dissected"]},{"year":2014,"claim":"Defined IL-12 from inflammatory monocytes plus HLA-E/NKG2C engagement as jointly required for HCMV-driven NKG2C+ expansion, integrating innate cytokine and receptor signals.","evidence":"Co-culture with infected fibroblasts and monocytes, IL-12 neutralization, receptor blockade, HLA-E siRNA","pmids":["25384219"],"confidence":"High","gaps":["Transcriptional program downstream of these signals not mapped"]},{"year":2015,"claim":"Showed HLA-E peptide identity governs NKG2C+ degranulation and that engagement triggers receptor internalization, revealing peptide-restricted activation control.","evidence":"Peptide-loaded targets, CD107a assay, surface NKG2C measurement with bafilomycin rescue","pmids":["26382247"],"confidence":"Medium","gaps":["Single lab","In vivo relevance of HLA-G-derived peptide restriction unclear"]},{"year":2016,"claim":"Demonstrated that adaptive NK features can develop independent of NKG2C, with CD2 providing co-stimulatory signal 2 for antibody-driven responses.","evidence":"Repertoire and phospho-flow (ERK, S6RP), ADCC, and IFN-γ epigenetics in 60 NKG2C-/- donors","pmids":["27117418"],"confidence":"High","gaps":["Relative contribution of NKG2C versus CD2 in NKG2C-sufficient individuals unresolved"]},{"year":2017,"claim":"Showed HCMV strain dependence of NKG2C activation and evidence for a non-canonical ligand beyond HLA-E.","evidence":"Jurkat-NKG2C/DAP12 NFAT/AP1-luciferase reporter with multiple viral strains and antibody blockade","pmids":["29114247"],"confidence":"Medium","gaps":["Identity of the antibody-resistant ligand unknown","Single lab"]},{"year":2019,"claim":"Revealed an unexpected antigen-presenting function of HLA-DR+ NKG2C+ adaptive NK cells, activating CD4+ T cells against HCMV antigen.","evidence":"NK–T cell co-culture, HLA-DR blockade, cytokine/CD107a flow cytometry","pmids":["31001281"],"confidence":"Medium","gaps":["Single lab","In vivo contribution of NK antigen presentation unproven"]},{"year":2022,"claim":"Extended TCR-independent NKG2C effector function to tissue-resident liver CD8+ T cells, broadening the receptor's role in tissue immunity.","evidence":"CITE-seq with TCR-seq and NKG2C ligation functional assays in liver perfusate cells","pmids":["35644434"],"confidence":"Medium","gaps":["Single lab","Physiologic ligand engaging these cells in liver not defined"]},{"year":null,"claim":"The identity of the non-canonical, antibody-resistant ligand engaged by CD94/NKG2C on certain HCMV-infected cells remains unresolved, as does the structural basis of NKG2C versus NKG2A discrimination of peptide-HLA-E.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No co-crystal structure of NKG2C bound to HLA-E","Non-HLA-E ligand uncharacterized","Mechanism of viral strain-specific activation unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,5,7]},{"term_id":"GO:0001618","term_label":"virus receptor activity","supporting_discovery_ids":[2,3]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[1]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,2,19]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,7,8]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[5,1]}],"complexes":["CD94/NKG2C-DAP12 receptor complex"],"partners":["CD94","DAP12","HLA-E"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P26717","full_name":"NKG2-C type II integral membrane protein","aliases":["CD159 antigen-like family member C","NK cell receptor C","NKG2-C-activating NK receptor"],"length_aa":231,"mass_kda":26.2,"function":"Immune activating receptor involved in self-nonself discrimination. In complex with KLRD1 on cytotoxic lymphocyte subsets, recognizes non-classical major histocompatibility (MHC) class Ib HLA-E loaded with signal sequence-derived peptides from non-classical MHC class Ib HLA-G molecules, likely playing a role in the generation and effector functions of adaptive natural killer (NK) cells and in maternal-fetal tolerance during pregnancy (PubMed:30134159, PubMed:37264229, PubMed:9754572). Regulates the effector functions of terminally differentiated cytotoxic lymphocyte subsets, and in particular may play a role in adaptive NK cell response to viral infection (PubMed:20952657, PubMed:21825173). Upon HLA-E-peptide binding, transmits intracellular signals via the adapter protein TYROBP/DAP12, triggering the phosphorylation of proximal signaling molecules and cell activation (PubMed:15940674, PubMed:9655483)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/P26717/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/KLRC2","classification":"Not Classified","n_dependent_lines":3,"n_total_lines":1165,"dependency_fraction":0.002575107296137339},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/KLRC2","total_profiled":1310},"omim":[{"mim_id":"606782","title":"C-TYPE LECTIN DOMAIN FAMILY 1, MEMBER A; CLEC1A","url":"https://www.omim.org/entry/606782"},{"mim_id":"602894","title":"KILLER CELL LECTIN-LIKE RECEPTOR, SUBFAMILY D, MEMBER 1; KLRD1","url":"https://www.omim.org/entry/602894"},{"mim_id":"602891","title":"KILLER CELL LECTIN-LIKE RECEPTOR, SUBFAMILY C, MEMBER 2; KLRC2","url":"https://www.omim.org/entry/602891"},{"mim_id":"161555","title":"KILLER CELL LECTIN-LIKE RECEPTOR, SUBFAMILY C, MEMBER 1; KLRC1","url":"https://www.omim.org/entry/161555"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"brain","ntpm":9.0},{"tissue":"lymphoid 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immunology","url":"https://pubmed.ncbi.nlm.nih.gov/35401541","citation_count":18,"is_preprint":false},{"pmid":"34990406","id":"PMC_34990406","title":"Donor NKG2C homozygosity contributes to CMV clearance after haploidentical transplantation.","date":"2022","source":"JCI insight","url":"https://pubmed.ncbi.nlm.nih.gov/34990406","citation_count":16,"is_preprint":false},{"pmid":"27312142","id":"PMC_27312142","title":"Differential frequency of NKG2C/KLRC2 deletion in distinct African populations and susceptibility to Trachoma: a new method for imputation of KLRC2 genotypes from SNP genotyping data.","date":"2016","source":"Human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/27312142","citation_count":16,"is_preprint":false},{"pmid":"25510434","id":"PMC_25510434","title":"Functional anti-CD94/NKG2A and anti-CD94/NKG2C autoantibodies in patients with systemic lupus erythematosus.","date":"2015","source":"Arthritis & rheumatology (Hoboken, 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dermatology","url":"https://pubmed.ncbi.nlm.nih.gov/24279916","citation_count":14,"is_preprint":false},{"pmid":"11069065","id":"PMC_11069065","title":"Mitogen-activated protein kinase activity is involved in effector functions triggered by the CD94/NKG2-C NK receptor specific for HLA-E.","date":"2000","source":"European journal of immunology","url":"https://pubmed.ncbi.nlm.nih.gov/11069065","citation_count":13,"is_preprint":false},{"pmid":"32131843","id":"PMC_32131843","title":"Adaptive NKG2C+ natural killer cells are related to exacerbations and nutritional abnormalities in COPD patients.","date":"2020","source":"Respiratory research","url":"https://pubmed.ncbi.nlm.nih.gov/32131843","citation_count":13,"is_preprint":false},{"pmid":"34076484","id":"PMC_34076484","title":"Influence of NKG2C Genotypes on HIV Susceptibility and Viral Load Set Point.","date":"2021","source":"Journal of virology","url":"https://pubmed.ncbi.nlm.nih.gov/34076484","citation_count":12,"is_preprint":false},{"pmid":"9212363","id":"PMC_9212363","title":"The CD94/NKG2 C-type lectin receptor complex: involvement in NK cell-mediated recognition of HLA class I molecules.","date":"1997","source":"Immunologic research","url":"https://pubmed.ncbi.nlm.nih.gov/9212363","citation_count":12,"is_preprint":false},{"pmid":"30461721","id":"PMC_30461721","title":"Enrichment of Cytomegalovirus-induced NKG2C+ Natural Killer Cells in the Lung Allograft.","date":"2019","source":"Transplantation","url":"https://pubmed.ncbi.nlm.nih.gov/30461721","citation_count":12,"is_preprint":false},{"pmid":"30018803","id":"PMC_30018803","title":"Expression analysis of HLA-E and NKG2A and NKG2C receptors points at a role for natural killer function in ankylosing spondylitis.","date":"2018","source":"RMD open","url":"https://pubmed.ncbi.nlm.nih.gov/30018803","citation_count":12,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":52861,"output_tokens":5301,"usd":0.119049,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":13585,"output_tokens":4687,"usd":0.09255,"stage2_stop_reason":"end_turn"},"total_usd":0.211599,"stage1_batch_id":"msgbatch_01BbTCMyrRPyhtEq6b1MdwCN","stage2_batch_id":"msgbatch_011dEed5bbxYvTAaQWKJevxu","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1997,\n      \"finding\": \"NKG2-C delivers an activating transmembrane signal in NK cells: chimeric NKG2-C/NKR-P1C receptors stimulated lytic activity and calcium mobilization when cross-linked, and the inhibitory tyrosine phosphatase SHP-1 did NOT associate with the NKG2-C chimeric receptor (in contrast to the inhibitory NKG2-A chimeric receptor).\",\n      \"method\": \"Chimeric receptor transfection in RNK-16 rat NK cells; cytolytic assays; calcium mobilization assays; immunoprecipitation\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct functional reconstitution with chimeric receptors, multiple orthogonal assays (cytotoxicity, calcium flux, co-IP), clear mechanistic contrast with inhibitory NKG2-A\",\n      \"pmids\": [\"9103421\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"CD94/NKG2C forms a non-covalent complex with the ITAM-bearing adapter DAP12; charged residues in the transmembrane domains of both DAP12 and NKG2C are required for this interaction, and DAP12 is necessary for efficient surface expression of CD94/NKG2C.\",\n      \"method\": \"Co-immunoprecipitation; cell-surface expression analysis with transmembrane domain mutants\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct biochemical reconstitution with mutagenesis identifying critical transmembrane residues, replicated in subsequent studies\",\n      \"pmids\": [\"9655483\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The 39-kDa protein (Kp39) co-precipitated with CD94 from NKG2A-negative NK cells is covalently associated with CD94 and is the product of the NKG2-C gene; co-transfection of CD94 and NKG2-C in COS7 cells confirmed the identity of Kp39 as NKG2-C, and engagement of the complex by a specific mAb triggered redirected cytotoxicity.\",\n      \"method\": \"Immunoprecipitation; COS7 co-transfection; peptide mapping; RT-PCR on NK clones; redirected killing assay\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — biochemical reconstitution in transfected cells plus functional cytotoxicity assay, multiple orthogonal methods in single study\",\n      \"pmids\": [\"9485212\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Both CD94/NKG2-A and CD94/NKG2-C bind HLA-E in a peptide-dependent manner; CD94/NKG2-A has higher binding affinity for HLA-E than CD94/NKG2-C, and both receptors show very fast association and dissociation kinetics. The magnitude of NK cell response correlates directly with the binding affinity of peptide-HLA-E complexes for the receptors.\",\n      \"method\": \"Surface plasmon resonance (kinetic binding analysis) using soluble recombinant HLA-E and recombinant soluble receptor proteins assembled with defined peptides\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — quantitative in vitro binding assay with recombinant proteins and multiple peptide variants, replicated across conditions\",\n      \"pmids\": [\"10428963\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Mouse CD94/NKG2C and CD94/NKG2E bind the MHC class Ib molecule Qa-1(b); a blocking anti-NKG2 monoclonal antibody demonstrated that CD94/NKG2 molecules are the only Qa-1(b) receptors on mouse NK cells.\",\n      \"method\": \"Cloning and expression; binding assay; anti-NKG2 blocking antibody\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct binding demonstration with receptor cloning and specific antibody blockade, first direct evidence for exclusive Qa-1(b) recognition\",\n      \"pmids\": [\"10601355\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Engagement of CD94/NKG2C on an NK clone induces MAPK (ERK) phosphorylation; the MEK inhibitor PD098059 blocks CD94-dependent TNF-α production and cytotoxicity. In RBL cells transfected with CD94/NKG2C/DAP12, receptor cross-linking induces calcium mobilization, serotonin release, and MAPK phosphorylation.\",\n      \"method\": \"Pharmacological inhibition (MEK inhibitor); Western blot for phospho-MAPK; calcium mobilization; serotonin release assay in transfected RBL cells\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution in heterologous cell line plus pharmacological dissection in primary NK clone, multiple orthogonal readouts\",\n      \"pmids\": [\"11069065\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"CD94/NKG2A and CD94/NKG2C recognize an overlapping but partly distinct set of residues on the top of HLA-E (α1/α2 domain); two HLA-E mutations (D69A and H155A) selectively abrogated binding to CD94/NKG2A without substantially affecting CD94/NKG2C binding, demonstrating differential epitope usage.\",\n      \"method\": \"Alanine-scanning mutagenesis of HLA-E; binding assay with soluble CD94/NKG2A and CD94/NKG2C\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — systematic alanine-scan across HLA-E surface with two purified receptor complexes tested in parallel\",\n      \"pmids\": [\"14971033\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"CD94/NKG2C is expressed on a subset of CD8+ T cells (and rare TCRγδ+ and CD4+ cells) and is coupled to DAP12 in these T cells; selective engagement of CD94/NKG2C triggers cytotoxicity, cytokine production, IL-2Rα upregulation, and proliferation in CD8+ T cell clones independently of TCR.\",\n      \"method\": \"Co-immunoprecipitation of DAP12 with anti-CD94; co-culture with HLA-E transfectants; mAb engagement; proliferation assays; cytokine assays; cytotoxicity assays\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP, functional assays with multiple T cell clones, HLA-E transfectant target cells\",\n      \"pmids\": [\"15940674\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Expansion of CD94/NKG2C+ NK cells in response to HCMV-infected fibroblasts requires interaction of CD94/NKG2C with the infected cells (blocked by anti-CD94 mAb) and requires IL-15 for proliferation; this expansion is independent of HCMV genes UL16, UL18, and UL40 but is impaired by deletion of the US2-11 gene region.\",\n      \"method\": \"Co-culture with HCMV-infected fibroblasts and deletion mutant viruses; anti-CD94 antibody blockade; IL-15 supplementation; flow cytometry\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — antibody blockade of receptor, viral gene deletion panel, IL-15 requirement established experimentally\",\n      \"pmids\": [\"16384928\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"IL-12 induces transient expression of the inhibitory CD94/NKG2A receptor on CD94/NKG2C+ NK cells; this inducible NKG2A inhibits the cytolytic activity of NKG2C+ NK clones upon engagement by HLA-E. Anti-IL-12 antibody prevented this NKG2A upregulation in HCMV-infected DC co-cultures.\",\n      \"method\": \"In vitro NK cell stimulation; anti-IL-12 mAb neutralization; flow cytometry; cytolytic assays with HLA-E transfectants; NK clones\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — antibody neutralization and functional readout, single lab, two orthogonal methods\",\n      \"pmids\": [\"19124726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"NMR-based structure refinement of the DAP12-NKG2C transmembrane helix complex in explicit membranes reveals that five functionally required interfacial residues (two Asp and two Thr in DAP12 dimer; one Lys in NKG2C) form a stable network of salt bridges and hydrogen bonds; the arrangement is energetically consistent with experimental observations.\",\n      \"method\": \"NMR observable-constrained molecular dynamics simulation in explicit micelles and bilayers\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — computational refinement of published NMR structure, single study, no mutagenesis validation reported in this paper\",\n      \"pmids\": [\"22500771\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"IL-12 produced by CD14+ inflammatory monocytes, combined with HLA-E/CD94-NKG2C engagement, is required for expansion of NKG2C+ NK cells in response to HCMV infection; IL-12 neutralization substantially reduced CD25 upregulation and NKG2C+ subset expansion; blockade of CD94/NKG2C or silencing of HLA-E greatly impaired NKG2C+ NK cell expansion.\",\n      \"method\": \"Co-culture system with HCMV-infected fibroblasts and monocytes; IL-12 neutralization; anti-CD94/NKG2C blocking antibody; HLA-E siRNA silencing; flow cytometry\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple genetic and antibody perturbations (IL-12 neutralization, receptor blockade, HLA-E knockdown) in same study, orthogonal methods\",\n      \"pmids\": [\"25384219\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"NKG2C zygosity quantitatively influences surface CD94/NKG2C receptor levels, NKG2C+ NK cell numbers, calcium influx, degranulation, and IL-15-dependent proliferation in response to NKG2C engagement; NKG2C+/+ individuals show greater responses than hemizygous NKG2C+/del individuals.\",\n      \"method\": \"Flow cytometry; intracellular calcium measurement; degranulation assay (CD107a); IL-15-driven proliferation assay in cohort stratified by NKG2C copy number\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional assays in gene-dose cohort, single lab\",\n      \"pmids\": [\"24030638\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"HLA-E regulates NKG2C+ NK cell degranulation in a peptide-restricted manner: peptide-stabilized HLA-E activates NKG2A-NKG2C+ NK cells only with a restricted subset of peptides (CMV-I, A80, B13, and especially HLA-G-derived peptide), and HLA-E/G peptide complex triggers NKG2C receptor internalization (reversible with bafilomycin).\",\n      \"method\": \"Peptide-loading of target cells; CD107a degranulation assay; flow cytometric measurement of NKG2C surface expression; bafilomycin rescue\",\n      \"journal\": \"Human immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional assay with pharmacological rescue, single lab, multiple peptides tested\",\n      \"pmids\": [\"26382247\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In NKG2C-deficient (homozygous deletion) humans, adaptive NK cells still expand and display characteristic adaptive features (terminal differentiation, functional reprogramming, IFN-γ promoter epigenetic remodeling); these cells express high levels of CD2, which synergistically enhances ERK and S6RP phosphorylation after CD16 ligation, demonstrating that CD2 provides co-stimulatory 'signal 2' for antibody-driven adaptive NK cell responses independent of NKG2C.\",\n      \"method\": \"NK cell repertoire analysis in 60 NKG2C-/- donors; phospho-flow cytometry (ERK, S6RP); ADCC functional assays; epigenetic analysis of IFN-γ promoter\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — natural human genetic KO cohort (n=60), multiple orthogonal assays including signaling phosphorylation, epigenetics, and function\",\n      \"pmids\": [\"27117418\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The CD94/NKG2C complex, associated with KARAP/DAP12, is fully functional in γδ T cells (Vδ2 subset), triggering IFN-γ production, proliferation, and cytolytic activity; when NKG2A and NKG2C are co-expressed on the same cell, the inhibitory signal from NKG2A prevails over NKG2C-mediated activation.\",\n      \"method\": \"Flow cytometry; IFN-γ ELISA; proliferation assay; cytolytic assays; functional comparison in NKG2A/NKG2C double-positive cells\",\n      \"journal\": \"Journal of leukocyte biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional assays in primary γδ T cells, NKG2A dominance demonstrated in double-positive cells, single lab\",\n      \"pmids\": [\"20952657\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"CD94/NKG2C activation by HCMV-infected cells is stimulus-dependent on the virus strain: infection with laboratory strains (AD169, Towne) failed to activate a Jurkat-NKG2C reporter system despite preserving surface HLA-E, whereas clinical isolates and the endotheliotropic TB40/E strain triggered reporter activation; the TB40/E-driven response was not blocked by anti-NKG2C or anti-HLA-E antibodies, suggesting a non-canonical or additional ligand.\",\n      \"method\": \"Jurkat reporter cell line expressing CD94/NKG2C/DAP12 with NFAT/AP1-luciferase reporter; HCMV infection with multiple strains; antibody blockade of NKG2C and HLA-E\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reconstituted reporter system with antibody blockade, multiple viral strains compared, single lab\",\n      \"pmids\": [\"29114247\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"NKG2C+ adaptive NK cells expressing HLA-DR can process and present HCMV antigen to autologous CD4+ T cells in an HLA-DR-dependent manner, activating polyfunctional effector-memory CD4+ T cells (CD107a+, IFN-γ+, TNF-α+) with cytotoxic potential; HCMV complexed with specific antibodies upregulates surface HLA-DR on NK cells via CD16.\",\n      \"method\": \"Purified NK cell-T cell co-culture; HLA-DR blocking antibody; flow cytometry for CD107a, IFN-γ, TNF-α; HLA-DR upregulation after antibody-complexed antigen loading\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — antigen presentation established by HLA-DR blocking and functional T cell readout, single lab\",\n      \"pmids\": [\"31001281\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In liver sinusoidal CD56hiCD161-CD8+ T cells, NKG2C mediates NK-like effector functions (degranulation and cytotoxicity) in a TCR-independent manner; these cells express CD94, KIRs, and NKG2C, and respond to NKG2C ligation without TCR stimulation.\",\n      \"method\": \"CITE-seq with TCR-seq; flow cytometry; functional assays with NKG2C ligation in absence of TCR stimulation; liver perfusate from transplant donors\",\n      \"journal\": \"Journal of hepatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — TCR-independent NKG2C function demonstrated in primary human liver cells, single lab, orthogonal transcriptomics + functional validation\",\n      \"pmids\": [\"35644434\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"NKG2-C is expressed as a 36-kDa glycoprotein (protein backbone ~26 kDa) on NK cells; a recombinant soluble NKG2-C specifically binds K562 and RPMI 8866 cells but not other hematopoietic cell lines; this binding is lost when K562 cells differentiate with phorbol ester/Ca2+ ionophore, concurrent with loss of susceptibility to NK killing.\",\n      \"method\": \"In vitro translation; recombinant expression; immunoprecipitation; binding assay with soluble NKG2-C on multiple cell lines; differentiation of K562 with phorbol ester\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding demonstrated with soluble recombinant receptor and functional correlation, single lab\",\n      \"pmids\": [\"7589093\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"KLRC2 (NKG2C) encodes a C-type lectin-type II transmembrane protein that heterodimerizes covalently with CD94 and non-covalently associates with the ITAM-bearing adapter DAP12 via charged transmembrane residues; this CD94/NKG2C–DAP12 complex functions as an activating receptor on NK cells and subsets of CD8+ T and γδ T cells that recognizes HLA-E loaded with defined signal-peptide-derived nonamers, with lower binding affinity than the inhibitory CD94/NKG2A counterpart, and transduces activation through DAP12-dependent MAPK/ERK and calcium mobilization pathways to trigger cytotoxicity and cytokine production; HCMV infection drives persistent clonal expansion of NKG2C+ NK cells in an IL-12-, IL-15-, and HLA-E-dependent manner, generating memory-like adaptive NK cells capable of antigen presentation to CD4+ T cells.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"KLRC2 (NKG2C) encodes a C-type lectin-like type II transmembrane receptor that pairs with CD94 to form an activating NK-cell receptor recognizing the non-classical MHC class Ib molecule HLA-E (and its murine homolog Qa-1b) in a peptide-dependent manner [#2, #3, #4]. Unlike its inhibitory counterpart CD94/NKG2A, CD94/NKG2C does not recruit the phosphatase SHP-1 but instead associates non-covalently with the ITAM-bearing adapter DAP12 through reciprocal charged transmembrane residues, an interaction also required for efficient surface expression of the complex [#0, #1]. Receptor engagement signals through DAP12 to drive MAPK/ERK phosphorylation and calcium mobilization, triggering cytotoxicity and cytokine production [#5]; CD94/NKG2A and CD94/NKG2C engage overlapping but distinct epitopes on the HLA-E α1/α2 surface, with NKG2C binding at lower affinity and the response magnitude scaling with peptide-HLA-E binding strength [#3, #6]. Beyond NK cells, functional DAP12-coupled CD94/NKG2C operates on subsets of CD8+ T and Vδ2 γδ T cells, mediating TCR-independent effector activity, with NKG2A signaling dominating when both receptors are co-expressed [#7, #15, #18]. HCMV infection drives clonal expansion of NKG2C+ NK cells in a manner requiring CD94/NKG2C–HLA-E engagement plus IL-12 and IL-15, generating adaptive memory-like NK cells; these adaptive features can also arise in NKG2C-deficient individuals through CD2-dependent co-stimulation [#8, #11, #14].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Established that NKG2-C is a cell-surface glycoprotein capable of binding specific target cells, linking receptor expression to NK target recognition.\",\n      \"evidence\": \"Recombinant soluble NKG2-C binding assays across hematopoietic cell lines with phorbol-ester differentiation of K562\",\n      \"pmids\": [\"7589093\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Ligand identity unknown at this stage\", \"No signaling mechanism defined\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Distinguished NKG2-C from inhibitory NKG2-A by showing it delivers an activating rather than inhibitory signal, settling its functional polarity.\",\n      \"evidence\": \"Chimeric NKG2-C/NKR-P1C receptor transfection in rat NK cells with cytotoxicity, calcium flux, and SHP-1 co-IP\",\n      \"pmids\": [\"9103421\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Adapter mediating activation not yet identified\", \"Native ligand not defined\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Identified the molecular basis of activation by showing CD94/NKG2C associates with the ITAM adapter DAP12 via transmembrane charged residues, explaining how an activating signal is transduced.\",\n      \"evidence\": \"Co-immunoprecipitation and transmembrane-domain mutagenesis; COS7 co-transfection and redirected killing\",\n      \"pmids\": [\"9655483\", \"9485212\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling cascade not yet mapped\", \"Affinity/kinetics of ligand binding unmeasured\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Defined HLA-E (and Qa-1b in mouse) as the peptide-dependent ligand and quantified that NKG2C binds at lower affinity than NKG2A, establishing the ligand recognition logic.\",\n      \"evidence\": \"Surface plasmon resonance with recombinant peptide-loaded HLA-E; mouse receptor cloning and antibody blockade\",\n      \"pmids\": [\"10428963\", \"10601355\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of differential affinity not resolved\", \"Physiologic peptide repertoire incompletely defined\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Mapped the activating signal to the MAPK/ERK and calcium pathways, connecting DAP12 engagement to effector output.\",\n      \"evidence\": \"MEK inhibitor blockade, phospho-MAPK Western blot, calcium and serotonin release in transfected RBL cells\",\n      \"pmids\": [\"11069065\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Proximal kinases linking DAP12 to ERK not identified here\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Showed NKG2A and NKG2C use overlapping but partly distinct HLA-E epitopes, explaining how the two receptors discriminate the same ligand.\",\n      \"evidence\": \"Alanine-scanning mutagenesis of HLA-E with soluble receptor binding assays\",\n      \"pmids\": [\"14971033\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No co-crystal structure of NKG2C–HLA-E\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Extended functional CD94/NKG2C–DAP12 signaling to CD8+ T cells, showing TCR-independent activation of cytotoxicity and proliferation.\",\n      \"evidence\": \"Reciprocal DAP12 co-IP, HLA-E transfectant co-culture, and functional assays in CD8+ T cell clones\",\n      \"pmids\": [\"15940674\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of T-cell NKG2C not established\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identified the determinants of HCMV-driven NKG2C+ NK cell expansion, requiring receptor engagement of infected cells and IL-15.\",\n      \"evidence\": \"Co-culture with HCMV-infected fibroblasts, viral deletion mutants, anti-CD94 blockade, IL-15 supplementation\",\n      \"pmids\": [\"16384928\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific viral/host ligand driving expansion not pinpointed\", \"Role of US2-11 region mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Revealed a regulatory loop whereby IL-12 induces inhibitory NKG2A on NKG2C+ cells, dampening their cytolytic activity.\",\n      \"evidence\": \"In vitro stimulation, anti-IL-12 neutralization, cytolytic assays with HLA-E transfectants\",\n      \"pmids\": [\"19124726\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Durability and in vivo significance of induced NKG2A unclear\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstrated functional CD94/NKG2C–DAP12 in Vδ2 γδ T cells and established NKG2A dominance over NKG2C in co-expressing cells.\",\n      \"evidence\": \"Functional assays (IFN-γ, proliferation, cytolysis) in primary γδ T cells\",\n      \"pmids\": [\"20952657\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Mechanism of NKG2A dominance over NKG2C not resolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Refined the atomic architecture of the DAP12–NKG2C transmembrane interface, defining the salt-bridge/hydrogen-bond network underlying complex assembly.\",\n      \"evidence\": \"NMR-constrained molecular dynamics in explicit membranes\",\n      \"pmids\": [\"22500771\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Computational refinement without new mutagenesis validation in this study\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Showed NKG2C gene dosage quantitatively tunes receptor levels and effector responses, linking genotype to functional output.\",\n      \"evidence\": \"Functional assays (calcium, CD107a, IL-15 proliferation) in NKG2C copy-number-stratified cohort\",\n      \"pmids\": [\"24030638\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Mechanism connecting zygosity to per-cell response not dissected\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defined IL-12 from inflammatory monocytes plus HLA-E/NKG2C engagement as jointly required for HCMV-driven NKG2C+ expansion, integrating innate cytokine and receptor signals.\",\n      \"evidence\": \"Co-culture with infected fibroblasts and monocytes, IL-12 neutralization, receptor blockade, HLA-E siRNA\",\n      \"pmids\": [\"25384219\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Transcriptional program downstream of these signals not mapped\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Showed HLA-E peptide identity governs NKG2C+ degranulation and that engagement triggers receptor internalization, revealing peptide-restricted activation control.\",\n      \"evidence\": \"Peptide-loaded targets, CD107a assay, surface NKG2C measurement with bafilomycin rescue\",\n      \"pmids\": [\"26382247\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"In vivo relevance of HLA-G-derived peptide restriction unclear\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Demonstrated that adaptive NK features can develop independent of NKG2C, with CD2 providing co-stimulatory signal 2 for antibody-driven responses.\",\n      \"evidence\": \"Repertoire and phospho-flow (ERK, S6RP), ADCC, and IFN-γ epigenetics in 60 NKG2C-/- donors\",\n      \"pmids\": [\"27117418\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contribution of NKG2C versus CD2 in NKG2C-sufficient individuals unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Showed HCMV strain dependence of NKG2C activation and evidence for a non-canonical ligand beyond HLA-E.\",\n      \"evidence\": \"Jurkat-NKG2C/DAP12 NFAT/AP1-luciferase reporter with multiple viral strains and antibody blockade\",\n      \"pmids\": [\"29114247\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the antibody-resistant ligand unknown\", \"Single lab\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Revealed an unexpected antigen-presenting function of HLA-DR+ NKG2C+ adaptive NK cells, activating CD4+ T cells against HCMV antigen.\",\n      \"evidence\": \"NK–T cell co-culture, HLA-DR blockade, cytokine/CD107a flow cytometry\",\n      \"pmids\": [\"31001281\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"In vivo contribution of NK antigen presentation unproven\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Extended TCR-independent NKG2C effector function to tissue-resident liver CD8+ T cells, broadening the receptor's role in tissue immunity.\",\n      \"evidence\": \"CITE-seq with TCR-seq and NKG2C ligation functional assays in liver perfusate cells\",\n      \"pmids\": [\"35644434\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Physiologic ligand engaging these cells in liver not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The identity of the non-canonical, antibody-resistant ligand engaged by CD94/NKG2C on certain HCMV-infected cells remains unresolved, as does the structural basis of NKG2C versus NKG2A discrimination of peptide-HLA-E.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No co-crystal structure of NKG2C bound to HLA-E\", \"Non-HLA-E ligand uncharacterized\", \"Mechanism of viral strain-specific activation unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 5, 7]},\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 2, 19]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 7, 8]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [5, 1]}\n    ],\n    \"complexes\": [\n      \"CD94/NKG2C-DAP12 receptor complex\"\n    ],\n    \"partners\": [\n      \"CD94\",\n      \"DAP12\",\n      \"HLA-E\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}