{"gene":"B2M","run_date":"2026-06-09T22:02:44","timeline":{"discoveries":[{"year":1991,"finding":"Transfection of B2M into HLA class I-negative melanoma cells (FO-1) restored surface HLA class I antigen expression and significantly reduced susceptibility to NK cell-mediated lysis; anti-HLA class I antibodies restored NK lysis of transfected cells, establishing that B2M-dependent HLA class I complex formation directly modulates NK cell killing.","method":"B2M gene transfection into melanoma cells, NK cell cytotoxicity assays, anti-HLA class I monoclonal antibody blocking","journal":"The Journal of Clinical Investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct loss-of-function/gain-of-function with defined phenotypic readout, antibody blocking confirmation, replicated across human and mouse B2m constructs in same study","pmids":["1905328"],"is_preprint":false},{"year":2017,"finding":"Somatic mutations in B2M in lung cancer impair correct formation of the HLA class I complex, abolishing surface HLA-I expression; restitution of B2M in B2M-deficient lung cancer cells upregulated targets of IFNα/IFNγ signaling, placing B2M upstream of interferon-responsive antigen presentation.","method":"Whole-exome sequencing, RNA sequencing, B2M re-expression in cancer cells, gene expression microarrays, immunohistochemistry for HLA-I and B2M","journal":"Clinical Cancer Research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — B2M reconstitution with gene expression readout in cancer cells plus mutation analysis, single lab with two orthogonal methods","pmids":["28302866"],"is_preprint":false},{"year":2021,"finding":"Cardiomyocytes express and secrete elevated levels of B2M in response to ischemic injury, and secreted B2M activates cardiac fibroblasts in a paracrine manner, identifying B2M as an intercellular signaling molecule in cardiac remodeling.","method":"Single-cell transcriptomics, follow-up experiments confirming B2M secretion by cardiomyocytes and paracrine fibroblast activation","journal":"Communications Biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — single-cell transcriptomics plus functional follow-up experiments confirming paracrine signaling, single lab","pmids":["33514846"],"is_preprint":false},{"year":2022,"finding":"B2M (β2-microglobulin) was identified as a host factor required for Vaccinia virus infection; B2M knockout reduced VV infection efficiency by 54–91% without affecting virus binding, but impairing virus internalization and early gene expression, placing B2M at the virus entry/internalization step.","method":"Genome-wide CRISPR screen, B2M knockout in HeLa and HAP1 cells, virus binding and internalization assays, early gene expression assay","journal":"PLoS Pathogens","confidence":"High","confidence_rationale":"Tier 2 / Strong — genome-wide CRISPR screen hit validated by KO in two cell lines with orthogonal mechanistic assays (binding vs internalization vs gene expression), single paper with multiple methods","pmids":["36574441"],"is_preprint":false},{"year":2024,"finding":"Plasma and skeletal muscle B2M levels are elevated in PH-HFpEF; exogenous B2M promotes migration and proliferation of pulmonary arterial vascular endothelial and smooth muscle cells; B2m deletion in mice reduced pulmonary vascular remodeling, lowered pulmonary hypertension, and attenuated right ventricular hypertrophy, establishing a direct pathogenic role for secreted B2M in pulmonary vascular proliferative remodeling.","method":"Plasma proteomics, in vitro cell culture with exogenous B2M, in vivo B2m knockout mouse model (high-fat diet PH-HFpEF), human cohort validation","journal":"Arteriosclerosis, Thrombosis, and Vascular Biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vitro gain-of-function with exogenous B2M, in vivo KO phenotype, human cohort validation across multiple orthogonal methods in single study","pmids":["38813697"],"is_preprint":false},{"year":2021,"finding":"B2M knockout in murine tumor models (MC38, YUMMER2.1, B16) results in loss of surface MHC class I; in B2M-KO tumors, anti-tumor immune responses are mediated by CD4+ T cells and NK cells rather than CD8+ T cells, demonstrating that B2M-dependent MHC class I presentation is specifically required for CD8+ T cell-mediated tumor killing.","method":"B2M gene knockout in murine tumor cell lines, anti-PD-1 therapy, immune cell depletion experiments, analysis of human melanoma biopsies for B2M LOH","journal":"Cancer Immunology Research","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic KO across three tumor models with immune cell depletion epistasis, replicated in multiple model systems","pmids":["37801341"],"is_preprint":false},{"year":2022,"finding":"B2M knockout in DLBCL cells significantly downregulated both PD-L1 mRNA and protein, abolished IFN-γ-induced PD-L1 upregulation, and impaired MHC-I complex assembly with reduced TAP1/2 expression; B2M-deficient cells failed to activate CD8+ T cells, establishing that intact B2M/MHC-I functionality is a prerequisite for PD-L1 expression and antigen presentation.","method":"B2M knockout cell line, qRT-PCR, western blot, flow cytometry, PBMC co-culture cytotoxicity assays, IFN-γ stimulation, MHC-I inhibitor experiments","journal":"Open Life Sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO cell model with multiple orthogonal readouts (mRNA, protein, flow cytometry, functional co-culture), single lab","pmids":["42046553"],"is_preprint":false},{"year":2022,"finding":"Cell-free synthesis of B2M and CD1E proteins followed by Surface Plasmon Resonance confirmed direct physical interaction between B2M and CD1E, a glycolipid antigen-presenting molecule, identifying B2M as a binding partner of CD1E.","method":"Cell-free protein synthesis (E. coli CFPS), Ni2+ affinity purification, Surface Plasmon Resonance (SPR)","journal":"Protein Expression and Purification","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro reconstitution with SPR measurement of direct binding, but single lab and no mutagenesis or functional validation beyond binding measurement","pmids":["36460227"],"is_preprint":false},{"year":1998,"finding":"In common carp, temperature-dependent downregulation of surface MHC class I expression at 6°C results from insufficient B2m gene transcription, while MHC class I alpha chain (Cyca-UA) mRNA remains present at normal levels, demonstrating that B2M availability is the rate-limiting determinant of surface MHC class I assembly.","method":"Polyclonal antibody staining, flow cytometry of peripheral blood leucocytes at different temperatures, RT-PCR for Cyca-B2m and Cyca-UA mRNA","journal":"Developmental Immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ortholog study in carp with temperature-shift experiment, comparing B2m vs alpha-chain mRNA levels with surface protein readout, single lab two orthogonal methods","pmids":["9814583"],"is_preprint":false},{"year":2000,"finding":"MHC-determined individual odor identity (odortype) requires functional B2M: mice homozygous for B2m disruption (unable to express surface MHC class I) are distinguishable by scent from otherwise genetically identical B2m-intact mice, and this odortype disparity appears at 9–12 days of gestational age when MHC is first expressed.","method":"B2m gene disruption in mice, olfactory discrimination behavioral assays, developmental timing analysis","journal":"Immunogenetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — B2m KO mouse with defined behavioral phenotype, single lab","pmids":["10912502"],"is_preprint":false},{"year":1987,"finding":"The first intron of the B2m gene contains sequences highly susceptible to site-specific recombination: spontaneous deletions at the B2mb allele in a murine cell line map to a breakpoint within 39 bp of a first-intron sequence, and independent clones show indistinguishable deletions, indicating site-specific genetic rearrangement drives B2m loss-of-function mutations.","method":"Immunoselection of spontaneous mutants, Southern blot analysis with oligonucleotide probes mapping deletion breakpoints","journal":"Journal of Immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — 20 independent mutant clones with Southern blot breakpoint mapping, single lab","pmids":["3543125"],"is_preprint":false},{"year":2021,"finding":"B2M knockout in NK-like YT cells removes surface MHC class I; YT-VAV1+-B2M-/- cells show increased cytotoxic activity against primary glioblastoma cultures compared to wild-type, demonstrating that B2M-mediated MHC class I expression restrains NK cell cytotoxicity.","method":"CRISPR/Cas9 B2M knockout, flow cytometry validation, cell impedance-based cytotoxicity assay against primary glioblastoma monolayers","journal":"Molekuliarnaia biologiia","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, functional assay in NK cell line model with limited mechanistic detail in abstract","pmids":["36165021"],"is_preprint":false},{"year":2021,"finding":"Constitutive expression of mutant B2M-HLA-E (mBE) and B2M-HLA-G (mBG) fusion proteins in B2M-knockout CAR-T cells restored protection against allogeneic NK cell-mediated lysis, demonstrating that B2M-HLA fusion proteins functionally reconstitute the NK-inhibitory signal lost upon B2M deletion.","method":"Gene editing to generate B2M-KO CAR-T cells, lentiviral expression of B2M-HLA fusion proteins, NK cell cytotoxicity assays, allogeneic T cell recognition assays","journal":"European Journal of Immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional reconstitution experiment with B2M-HLA fusion proteins in KO background, multiple cell assay readouts, single lab","pmids":["34323289"],"is_preprint":false},{"year":2016,"finding":"TALEN-mediated biallelic disruption of B2m in pigs resulted in complete absence of B2M protein across tissues from all three germ layers and absence of surface MHC class I on lymphocytes; skin grafts from B2M-deficient pigs showed prolonged survival on xenogeneic wounds compared to wild-type littermates, establishing that B2M is required for surface MHC class I expression and donor MHC class I drives xenograft rejection.","method":"TALEN-mediated B2m knockout in porcine zygotes, Western blot/flow cytometry for B2M and MHC I expression, xenogeneic skin graft survival assay","journal":"Scientific Reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo KO pig model with protein expression validation and functional graft survival assay, single lab","pmids":["27982048"],"is_preprint":false},{"year":1992,"finding":"In the H-2-deficient T-cell lymphoma derivative LR.4, B2m genes were found to be severely damaged by Southern blot analysis, and B2m mRNA was absent, while class I H-2 genes were extensively methylated; together these two independent mechanisms eliminated MHC class I surface expression, demonstrating that B2M is required for surface class I complex expression.","method":"Southern blot of genomic B2m DNA, RT-PCR/Northern blot for mRNA, complement-mediated cytotoxicity for surface antigen detection, anti-H-2 antibody staining","journal":"European Journal of Immunogenetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — correlation of B2m gene damage with loss of surface MHC-I in a natural tumor variant, two orthogonal mechanisms examined, single lab","pmids":["1627535"],"is_preprint":false},{"year":2025,"finding":"B2M knockdown in dental pulp stem cells (DPSCs) impaired IFN-γ-inducible HLA class I surface expression, reduced leukocyte-mediated cytotoxicity, and significantly inhibited T cell proliferation, without affecting stemness or differentiation capacity, establishing a direct mechanistic role for B2M in regulating HLA-I-dependent immunogenicity.","method":"Lentiviral shRNA knockdown of B2M, western blot, immunofluorescence, flow cytometry for HLA-I, leukocyte cytotoxicity assay, T cell proliferation assay","journal":"Stem Cell Research & Therapy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KD with multiple orthogonal validation methods and functional immune assays, single lab","pmids":["39538338"],"is_preprint":false},{"year":2025,"finding":"B2M knockout in iPSC-derived NK (iNK) cells did not impair acquisition of NKG2A or functional potency, demonstrating that HLA-E interactions mediated by B2M in cis or trans are not required for education-independent functional maturation of iNK cells.","method":"CRISPR B2M knockout in iNK cells, transcriptional profiling, functional cytotoxicity assays, NKG2A surface expression analysis","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 2 / Weak — preprint, single lab, mechanistic conclusion (negative result on education) from KO plus functional assay","pmids":[],"is_preprint":true},{"year":2025,"finding":"B2M knockout in iPSC-derived cardiac organoids (combined with HLA-E/HLA-G knock-in) eliminated surface HLA class I expression; these hypoimmunogenic organoids resisted T cell and NK cell cytotoxicity in vitro, restored contractile function in infarcted rat hearts in vivo, and showed superior graft retention in humanized mice, establishing B2M as the key determinant of HLA class I-mediated cardiac graft rejection.","method":"CRISPR-Cas9 B2M knockout, HLA-E/G knock-in, T and NK cell cytotoxicity assays, rat myocardial infarction model, humanized mouse engraftment","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 2 / Weak — preprint, rigorous multi-method study but not yet peer-reviewed","pmids":[],"is_preprint":true}],"current_model":"B2M (β2-microglobulin) is the obligate light chain of MHC class I heterotrimers: it associates with HLA heavy chains to enable surface expression, peptide presentation to CD8+ T cells, and NK cell inhibition; secreted B2M additionally acts as a paracrine signaling molecule (activating cardiac fibroblasts after ischemia and promoting pulmonary vascular smooth muscle/endothelial cell proliferation), and functions as a host factor required for Vaccinia virus internalization and early gene expression; loss of B2M abolishes surface MHC class I, shifting anti-tumor immunity to CD4+ T cells and NK cells, downregulates PD-L1 expression and IFN-γ responsiveness, and is the rate-limiting determinant of surface MHC class I assembly across vertebrate species."},"narrative":{"mechanistic_narrative":"B2M (β2-microglobulin) is the obligate light chain required for assembly and surface display of MHC class I complexes, and its availability is the rate-limiting determinant of surface class I expression: in common carp, surface MHC class I falls when B2m transcription is insufficient even while alpha-chain mRNA persists [PMID:9814583], and across diverse systems loss of B2M—whether by spontaneous intronic recombination, gene damage, knockout, or knockdown—abolishes surface class I in human, mouse, and porcine cells and tissues [PMID:3543125, PMID:27982048, PMID:1627535, PMID:39538338]. Through this assembly role, B2M governs two opposing arms of immune surveillance: B2M-dependent class I presents peptide to CD8+ T cells, and its loss in tumors redirects anti-tumor immunity to CD4+ T cells and NK cells [PMID:37801341]; simultaneously, surface class I delivers an inhibitory signal that restrains NK killing, so restoring B2M-dependent HLA complexes reduces NK lysis while its loss enhances it [PMID:1905328, PMID:36165021]. B2M-HLA-E/HLA-G fusion proteins reconstitute the NK-inhibitory signal lost on B2M deletion, exploited to engineer hypoimmunogenic cells and grafts [PMID:34323289]. B2M function lies upstream of interferon-responsive antigen presentation and immune checkpoint biology: its loss downregulates PD-L1, abolishes IFN-γ-induced PD-L1 and HLA-I upregulation, and reduces TAP1/2 expression [PMID:28302866, PMID:42046553]. Beyond its intracellular chaperone role, secreted B2M acts as a paracrine signaling molecule, activating cardiac fibroblasts after ischemic injury [PMID:33514846] and driving pulmonary vascular endothelial and smooth muscle proliferation in pulmonary hypertension, where B2m deletion attenuates vascular remodeling [PMID:38813697]. B2M also serves as a host factor required for Vaccinia virus internalization and early gene expression [PMID:36574441] and physically binds the glycolipid-presenting molecule CD1E [PMID:36460227].","teleology":[{"year":1987,"claim":"Established a genetic mechanism for spontaneous B2M loss-of-function, showing that intrinsic sequence features predispose the locus to inactivation.","evidence":"Immunoselection of spontaneous murine mutants with Southern blot breakpoint mapping of first-intron deletions","pmids":["3543125"],"confidence":"Medium","gaps":["Mechanism of site-specific recombination not resolved","Relevance to human B2M inactivation not addressed"]},{"year":1991,"claim":"Demonstrated that B2M-dependent HLA class I assembly directly controls NK cell killing, establishing B2M as a determinant of the NK-inhibitory signal.","evidence":"B2M transfection into HLA-negative melanoma cells with NK cytotoxicity assays and anti-HLA antibody blocking","pmids":["1905328"],"confidence":"High","gaps":["Did not define the inhibitory receptor engaged","Single tumor cell context"]},{"year":1992,"claim":"Confirmed in a natural tumor variant that B2M is required for surface class I, with gene damage and alpha-chain methylation independently abolishing expression.","evidence":"Southern/Northern blot and cytotoxicity in the H-2-deficient lymphoma LR.4","pmids":["1627535"],"confidence":"Medium","gaps":["Correlative rather than reconstitution evidence","Single cell line"]},{"year":1998,"claim":"Identified B2M availability as the rate-limiting determinant of surface MHC class I assembly, distinguishing it from alpha-chain expression.","evidence":"Temperature-shift flow cytometry and RT-PCR for B2m vs alpha-chain mRNA in carp leucocytes","pmids":["9814583"],"confidence":"Medium","gaps":["Ortholog system; quantitative stoichiometry in mammals not addressed","Transcriptional control mechanism unresolved"]},{"year":2000,"claim":"Linked B2M-dependent MHC class I expression to organismal phenotypes beyond immunity, showing it shapes individual odortype during development.","evidence":"B2m disruption mice with olfactory discrimination assays and developmental timing","pmids":["10912502"],"confidence":"Medium","gaps":["Molecular basis linking MHC to odor not defined","Single behavioral readout"]},{"year":2016,"claim":"Validated B2M as obligate for surface class I in a large-animal in vivo model and demonstrated its role in driving xenograft rejection.","evidence":"TALEN B2m knockout pigs with protein/flow validation and xenogeneic skin graft survival","pmids":["27982048"],"confidence":"Medium","gaps":["Did not dissect NK-mediated rejection of the class I-negative grafts","Single lab"]},{"year":2017,"claim":"Placed B2M upstream of interferon-responsive antigen presentation, connecting somatic B2M mutation to loss of HLA-I and altered IFN signaling in cancer.","evidence":"Exome/RNA sequencing and B2M re-expression with expression profiling in lung cancer cells","pmids":["28302866"],"confidence":"Medium","gaps":["Causal mechanism linking B2M to IFN target induction unresolved","Single lab"]},{"year":2021,"claim":"Defined the divergent immune consequences of B2M loss—abolition of CD8+ T cell killing with compensatory CD4+/NK responses, and relief of NK inhibition.","evidence":"B2M knockout across three murine tumor models with immune cell depletion epistasis and anti-PD-1; CRISPR KO in NK-like YT cells against glioblastoma","pmids":["37801341","36165021"],"confidence":"High","gaps":["NK-line study (idx 11) is Low confidence with limited mechanistic detail","Receptor-level basis of compensatory immunity not mapped"]},{"year":2021,"claim":"Identified secreted B2M as a paracrine signaling molecule in cardiac remodeling, expanding its role beyond intracellular chaperoning.","evidence":"Single-cell transcriptomics with secretion and fibroblast-activation follow-up after ischemic injury","pmids":["33514846"],"confidence":"Medium","gaps":["Receptor mediating fibroblast activation unknown","Single lab"]},{"year":2021,"claim":"Showed that B2M-HLA-E/HLA-G fusion proteins can reconstitute the NK-inhibitory signal in B2M-null cells, providing a strategy to engineer hypoimmunogenic cells.","evidence":"B2M-KO CAR-T cells with lentiviral fusion-protein expression and NK/T cytotoxicity assays","pmids":["34323289"],"confidence":"Medium","gaps":["Did not compare to native B2M-HLA stoichiometry","Single lab"]},{"year":2022,"claim":"Established B2M/MHC-I integrity as a prerequisite for PD-L1 expression and IFN-γ-induced antigen presentation, coupling B2M to checkpoint biology.","evidence":"B2M knockout in DLBCL cells with qRT-PCR, western, flow, IFN-γ stimulation and PBMC co-culture","pmids":["42046553"],"confidence":"Medium","gaps":["Mechanism coupling B2M to PD-L1 transcription not defined","Single lab"]},{"year":2022,"claim":"Revealed a non-immune role for B2M as a host factor required for Vaccinia virus internalization and early gene expression.","evidence":"Genome-wide CRISPR screen with B2M KO in HeLa and HAP1 and binding/internalization/early-gene assays","pmids":["36574441"],"confidence":"High","gaps":["Molecular role at the internalization step undefined","Direct virus-B2M interaction not shown"]},{"year":2022,"claim":"Demonstrated a direct physical interaction between B2M and the glycolipid-presenting molecule CD1E, identifying a binding partner beyond classical HLA heavy chains.","evidence":"Cell-free protein synthesis and surface plasmon resonance","pmids":["36460227"],"confidence":"Medium","gaps":["No mutagenesis or functional validation of the interaction","Physiological relevance not established"]},{"year":2024,"claim":"Established secreted B2M as directly pathogenic in pulmonary vascular proliferative remodeling, with B2m deletion protecting against pulmonary hypertension.","evidence":"Plasma proteomics, exogenous B2M on vascular cells, B2m knockout PH-HFpEF mice, human cohort validation","pmids":["38813697"],"confidence":"High","gaps":["Receptor mediating vascular cell proliferation unknown","Signaling pathway downstream of secreted B2M undefined"]},{"year":2025,"claim":"Confirmed in stem cell systems that B2M governs IFN-γ-inducible HLA-I immunogenicity without affecting cell identity, supporting B2M editing for hypoimmunogenic grafts.","evidence":"shRNA knockdown in dental pulp stem cells with immune assays; CRISPR KO in iPSC-derived cardiac organoids and NK cells (preprints)","pmids":["39538338"],"confidence":"Medium","gaps":["Organoid and iNK studies (idx 16, 17) are unpeer-reviewed preprints","Long-term graft outcomes not established"]},{"year":null,"claim":"The receptors and downstream signaling pathways mediating secreted B2M's paracrine effects in cardiac and pulmonary vascular tissue remain unidentified.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No receptor for extracellular B2M defined","Mechanism linking B2M to fibroblast and vascular cell proliferation unknown","Whether secreted B2M acts as monomer or in complex unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,8,13,14]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[2,4]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,5,13]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[2,4]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,5,6,15]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[1,4,5]}],"complexes":["MHC class I complex"],"partners":["HLA-E","HLA-G","CD1E"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P61769","full_name":"Beta-2-microglobulin","aliases":[],"length_aa":119,"mass_kda":13.7,"function":"Component of the class I major histocompatibility complex (MHC). Involved in the presentation of peptide antigens to the immune system. Exogenously applied M.tuberculosis EsxA or EsxA-EsxB (or EsxA expressed in host) binds B2M and decreases its export to the cell surface (total protein levels do not change), probably leading to defects in class I antigen presentation (PubMed:25356553)","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/P61769/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/B2M","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/B2M","total_profiled":1310},"omim":[{"mim_id":"620817","title":"MHC CLASS II DEFICIENCY 4; MHC2D4","url":"https://www.omim.org/entry/620817"},{"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":"620659","title":"AMYLOIDOSIS, HEREDITARY SYSTEMIC 6; AMYLD6","url":"https://www.omim.org/entry/620659"},{"mim_id":"618884","title":"PROTEINURIA, CHRONIC BENIGN; PROCHOB","url":"https://www.omim.org/entry/618884"},{"mim_id":"613609","title":"HOMEOSTATIC IRON REGULATOR; HFE","url":"https://www.omim.org/entry/613609"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Golgi apparatus","reliability":"Enhanced"},{"location":"Plasma membrane","reliability":"Enhanced"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/B2M"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"P61769","domains":[{"cath_id":"2.60.40.10","chopping":"20-113","consensus_level":"high","plddt":97.3717,"start":20,"end":113}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P61769","model_url":"https://alphafold.ebi.ac.uk/files/AF-P61769-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P61769-F1-predicted_aligned_error_v6.png","plddt_mean":94.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=B2M","jax_strain_url":"https://www.jax.org/strain/search?query=B2M"},"sequence":{"accession":"P61769","fasta_url":"https://rest.uniprot.org/uniprotkb/P61769.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P61769/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P61769"}},"corpus_meta":[{"pmid":"12357350","id":"PMC_12357350","title":"The essential roles of the chemokine SDF-1 and its 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modulates NK cell killing.\",\n      \"method\": \"B2M gene transfection into melanoma cells, NK cell cytotoxicity assays, anti-HLA class I monoclonal antibody blocking\",\n      \"journal\": \"The Journal of Clinical Investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct loss-of-function/gain-of-function with defined phenotypic readout, antibody blocking confirmation, replicated across human and mouse B2m constructs in same study\",\n      \"pmids\": [\"1905328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Somatic mutations in B2M in lung cancer impair correct formation of the HLA class I complex, abolishing surface HLA-I expression; restitution of B2M in B2M-deficient lung cancer cells upregulated targets of IFNα/IFNγ signaling, placing B2M upstream of interferon-responsive antigen presentation.\",\n      \"method\": \"Whole-exome sequencing, RNA sequencing, B2M re-expression in cancer cells, gene expression microarrays, immunohistochemistry for HLA-I and B2M\",\n      \"journal\": \"Clinical Cancer Research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — B2M reconstitution with gene expression readout in cancer cells plus mutation analysis, single lab with two orthogonal methods\",\n      \"pmids\": [\"28302866\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cardiomyocytes express and secrete elevated levels of B2M in response to ischemic injury, and secreted B2M activates cardiac fibroblasts in a paracrine manner, identifying B2M as an intercellular signaling molecule in cardiac remodeling.\",\n      \"method\": \"Single-cell transcriptomics, follow-up experiments confirming B2M secretion by cardiomyocytes and paracrine fibroblast activation\",\n      \"journal\": \"Communications Biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — single-cell transcriptomics plus functional follow-up experiments confirming paracrine signaling, single lab\",\n      \"pmids\": [\"33514846\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"B2M (β2-microglobulin) was identified as a host factor required for Vaccinia virus infection; B2M knockout reduced VV infection efficiency by 54–91% without affecting virus binding, but impairing virus internalization and early gene expression, placing B2M at the virus entry/internalization step.\",\n      \"method\": \"Genome-wide CRISPR screen, B2M knockout in HeLa and HAP1 cells, virus binding and internalization assays, early gene expression assay\",\n      \"journal\": \"PLoS Pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genome-wide CRISPR screen hit validated by KO in two cell lines with orthogonal mechanistic assays (binding vs internalization vs gene expression), single paper with multiple methods\",\n      \"pmids\": [\"36574441\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Plasma and skeletal muscle B2M levels are elevated in PH-HFpEF; exogenous B2M promotes migration and proliferation of pulmonary arterial vascular endothelial and smooth muscle cells; B2m deletion in mice reduced pulmonary vascular remodeling, lowered pulmonary hypertension, and attenuated right ventricular hypertrophy, establishing a direct pathogenic role for secreted B2M in pulmonary vascular proliferative remodeling.\",\n      \"method\": \"Plasma proteomics, in vitro cell culture with exogenous B2M, in vivo B2m knockout mouse model (high-fat diet PH-HFpEF), human cohort validation\",\n      \"journal\": \"Arteriosclerosis, Thrombosis, and Vascular Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vitro gain-of-function with exogenous B2M, in vivo KO phenotype, human cohort validation across multiple orthogonal methods in single study\",\n      \"pmids\": [\"38813697\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"B2M knockout in murine tumor models (MC38, YUMMER2.1, B16) results in loss of surface MHC class I; in B2M-KO tumors, anti-tumor immune responses are mediated by CD4+ T cells and NK cells rather than CD8+ T cells, demonstrating that B2M-dependent MHC class I presentation is specifically required for CD8+ T cell-mediated tumor killing.\",\n      \"method\": \"B2M gene knockout in murine tumor cell lines, anti-PD-1 therapy, immune cell depletion experiments, analysis of human melanoma biopsies for B2M LOH\",\n      \"journal\": \"Cancer Immunology Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic KO across three tumor models with immune cell depletion epistasis, replicated in multiple model systems\",\n      \"pmids\": [\"37801341\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"B2M knockout in DLBCL cells significantly downregulated both PD-L1 mRNA and protein, abolished IFN-γ-induced PD-L1 upregulation, and impaired MHC-I complex assembly with reduced TAP1/2 expression; B2M-deficient cells failed to activate CD8+ T cells, establishing that intact B2M/MHC-I functionality is a prerequisite for PD-L1 expression and antigen presentation.\",\n      \"method\": \"B2M knockout cell line, qRT-PCR, western blot, flow cytometry, PBMC co-culture cytotoxicity assays, IFN-γ stimulation, MHC-I inhibitor experiments\",\n      \"journal\": \"Open Life Sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO cell model with multiple orthogonal readouts (mRNA, protein, flow cytometry, functional co-culture), single lab\",\n      \"pmids\": [\"42046553\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cell-free synthesis of B2M and CD1E proteins followed by Surface Plasmon Resonance confirmed direct physical interaction between B2M and CD1E, a glycolipid antigen-presenting molecule, identifying B2M as a binding partner of CD1E.\",\n      \"method\": \"Cell-free protein synthesis (E. coli CFPS), Ni2+ affinity purification, Surface Plasmon Resonance (SPR)\",\n      \"journal\": \"Protein Expression and Purification\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro reconstitution with SPR measurement of direct binding, but single lab and no mutagenesis or functional validation beyond binding measurement\",\n      \"pmids\": [\"36460227\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"In common carp, temperature-dependent downregulation of surface MHC class I expression at 6°C results from insufficient B2m gene transcription, while MHC class I alpha chain (Cyca-UA) mRNA remains present at normal levels, demonstrating that B2M availability is the rate-limiting determinant of surface MHC class I assembly.\",\n      \"method\": \"Polyclonal antibody staining, flow cytometry of peripheral blood leucocytes at different temperatures, RT-PCR for Cyca-B2m and Cyca-UA mRNA\",\n      \"journal\": \"Developmental Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ortholog study in carp with temperature-shift experiment, comparing B2m vs alpha-chain mRNA levels with surface protein readout, single lab two orthogonal methods\",\n      \"pmids\": [\"9814583\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"MHC-determined individual odor identity (odortype) requires functional B2M: mice homozygous for B2m disruption (unable to express surface MHC class I) are distinguishable by scent from otherwise genetically identical B2m-intact mice, and this odortype disparity appears at 9–12 days of gestational age when MHC is first expressed.\",\n      \"method\": \"B2m gene disruption in mice, olfactory discrimination behavioral assays, developmental timing analysis\",\n      \"journal\": \"Immunogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — B2m KO mouse with defined behavioral phenotype, single lab\",\n      \"pmids\": [\"10912502\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"The first intron of the B2m gene contains sequences highly susceptible to site-specific recombination: spontaneous deletions at the B2mb allele in a murine cell line map to a breakpoint within 39 bp of a first-intron sequence, and independent clones show indistinguishable deletions, indicating site-specific genetic rearrangement drives B2m loss-of-function mutations.\",\n      \"method\": \"Immunoselection of spontaneous mutants, Southern blot analysis with oligonucleotide probes mapping deletion breakpoints\",\n      \"journal\": \"Journal of Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — 20 independent mutant clones with Southern blot breakpoint mapping, single lab\",\n      \"pmids\": [\"3543125\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"B2M knockout in NK-like YT cells removes surface MHC class I; YT-VAV1+-B2M-/- cells show increased cytotoxic activity against primary glioblastoma cultures compared to wild-type, demonstrating that B2M-mediated MHC class I expression restrains NK cell cytotoxicity.\",\n      \"method\": \"CRISPR/Cas9 B2M knockout, flow cytometry validation, cell impedance-based cytotoxicity assay against primary glioblastoma monolayers\",\n      \"journal\": \"Molekuliarnaia biologiia\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, functional assay in NK cell line model with limited mechanistic detail in abstract\",\n      \"pmids\": [\"36165021\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Constitutive expression of mutant B2M-HLA-E (mBE) and B2M-HLA-G (mBG) fusion proteins in B2M-knockout CAR-T cells restored protection against allogeneic NK cell-mediated lysis, demonstrating that B2M-HLA fusion proteins functionally reconstitute the NK-inhibitory signal lost upon B2M deletion.\",\n      \"method\": \"Gene editing to generate B2M-KO CAR-T cells, lentiviral expression of B2M-HLA fusion proteins, NK cell cytotoxicity assays, allogeneic T cell recognition assays\",\n      \"journal\": \"European Journal of Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional reconstitution experiment with B2M-HLA fusion proteins in KO background, multiple cell assay readouts, single lab\",\n      \"pmids\": [\"34323289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"TALEN-mediated biallelic disruption of B2m in pigs resulted in complete absence of B2M protein across tissues from all three germ layers and absence of surface MHC class I on lymphocytes; skin grafts from B2M-deficient pigs showed prolonged survival on xenogeneic wounds compared to wild-type littermates, establishing that B2M is required for surface MHC class I expression and donor MHC class I drives xenograft rejection.\",\n      \"method\": \"TALEN-mediated B2m knockout in porcine zygotes, Western blot/flow cytometry for B2M and MHC I expression, xenogeneic skin graft survival assay\",\n      \"journal\": \"Scientific Reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo KO pig model with protein expression validation and functional graft survival assay, single lab\",\n      \"pmids\": [\"27982048\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"In the H-2-deficient T-cell lymphoma derivative LR.4, B2m genes were found to be severely damaged by Southern blot analysis, and B2m mRNA was absent, while class I H-2 genes were extensively methylated; together these two independent mechanisms eliminated MHC class I surface expression, demonstrating that B2M is required for surface class I complex expression.\",\n      \"method\": \"Southern blot of genomic B2m DNA, RT-PCR/Northern blot for mRNA, complement-mediated cytotoxicity for surface antigen detection, anti-H-2 antibody staining\",\n      \"journal\": \"European Journal of Immunogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — correlation of B2m gene damage with loss of surface MHC-I in a natural tumor variant, two orthogonal mechanisms examined, single lab\",\n      \"pmids\": [\"1627535\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"B2M knockdown in dental pulp stem cells (DPSCs) impaired IFN-γ-inducible HLA class I surface expression, reduced leukocyte-mediated cytotoxicity, and significantly inhibited T cell proliferation, without affecting stemness or differentiation capacity, establishing a direct mechanistic role for B2M in regulating HLA-I-dependent immunogenicity.\",\n      \"method\": \"Lentiviral shRNA knockdown of B2M, western blot, immunofluorescence, flow cytometry for HLA-I, leukocyte cytotoxicity assay, T cell proliferation assay\",\n      \"journal\": \"Stem Cell Research & Therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KD with multiple orthogonal validation methods and functional immune assays, single lab\",\n      \"pmids\": [\"39538338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"B2M knockout in iPSC-derived NK (iNK) cells did not impair acquisition of NKG2A or functional potency, demonstrating that HLA-E interactions mediated by B2M in cis or trans are not required for education-independent functional maturation of iNK cells.\",\n      \"method\": \"CRISPR B2M knockout in iNK cells, transcriptional profiling, functional cytotoxicity assays, NKG2A surface expression analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 2 / Weak — preprint, single lab, mechanistic conclusion (negative result on education) from KO plus functional assay\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"B2M knockout in iPSC-derived cardiac organoids (combined with HLA-E/HLA-G knock-in) eliminated surface HLA class I expression; these hypoimmunogenic organoids resisted T cell and NK cell cytotoxicity in vitro, restored contractile function in infarcted rat hearts in vivo, and showed superior graft retention in humanized mice, establishing B2M as the key determinant of HLA class I-mediated cardiac graft rejection.\",\n      \"method\": \"CRISPR-Cas9 B2M knockout, HLA-E/G knock-in, T and NK cell cytotoxicity assays, rat myocardial infarction model, humanized mouse engraftment\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 2 / Weak — preprint, rigorous multi-method study but not yet peer-reviewed\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"B2M (β2-microglobulin) is the obligate light chain of MHC class I heterotrimers: it associates with HLA heavy chains to enable surface expression, peptide presentation to CD8+ T cells, and NK cell inhibition; secreted B2M additionally acts as a paracrine signaling molecule (activating cardiac fibroblasts after ischemia and promoting pulmonary vascular smooth muscle/endothelial cell proliferation), and functions as a host factor required for Vaccinia virus internalization and early gene expression; loss of B2M abolishes surface MHC class I, shifting anti-tumor immunity to CD4+ T cells and NK cells, downregulates PD-L1 expression and IFN-γ responsiveness, and is the rate-limiting determinant of surface MHC class I assembly across vertebrate species.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"B2M (β2-microglobulin) is the obligate light chain required for assembly and surface display of MHC class I complexes, and its availability is the rate-limiting determinant of surface class I expression: in common carp, surface MHC class I falls when B2m transcription is insufficient even while alpha-chain mRNA persists [#8], and across diverse systems loss of B2M—whether by spontaneous intronic recombination, gene damage, knockout, or knockdown—abolishes surface class I in human, mouse, and porcine cells and tissues [#10, #13, #14, #15]. Through this assembly role, B2M governs two opposing arms of immune surveillance: B2M-dependent class I presents peptide to CD8+ T cells, and its loss in tumors redirects anti-tumor immunity to CD4+ T cells and NK cells [#5]; simultaneously, surface class I delivers an inhibitory signal that restrains NK killing, so restoring B2M-dependent HLA complexes reduces NK lysis while its loss enhances it [#0, #11]. B2M-HLA-E/HLA-G fusion proteins reconstitute the NK-inhibitory signal lost on B2M deletion, exploited to engineer hypoimmunogenic cells and grafts [#12]. B2M function lies upstream of interferon-responsive antigen presentation and immune checkpoint biology: its loss downregulates PD-L1, abolishes IFN-γ-induced PD-L1 and HLA-I upregulation, and reduces TAP1/2 expression [#1, #6]. Beyond its intracellular chaperone role, secreted B2M acts as a paracrine signaling molecule, activating cardiac fibroblasts after ischemic injury [#2] and driving pulmonary vascular endothelial and smooth muscle proliferation in pulmonary hypertension, where B2m deletion attenuates vascular remodeling [#4]. B2M also serves as a host factor required for Vaccinia virus internalization and early gene expression [#3] and physically binds the glycolipid-presenting molecule CD1E [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 1987,\n      \"claim\": \"Established a genetic mechanism for spontaneous B2M loss-of-function, showing that intrinsic sequence features predispose the locus to inactivation.\",\n      \"evidence\": \"Immunoselection of spontaneous murine mutants with Southern blot breakpoint mapping of first-intron deletions\",\n      \"pmids\": [\"3543125\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of site-specific recombination not resolved\", \"Relevance to human B2M inactivation not addressed\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Demonstrated that B2M-dependent HLA class I assembly directly controls NK cell killing, establishing B2M as a determinant of the NK-inhibitory signal.\",\n      \"evidence\": \"B2M transfection into HLA-negative melanoma cells with NK cytotoxicity assays and anti-HLA antibody blocking\",\n      \"pmids\": [\"1905328\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define the inhibitory receptor engaged\", \"Single tumor cell context\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Confirmed in a natural tumor variant that B2M is required for surface class I, with gene damage and alpha-chain methylation independently abolishing expression.\",\n      \"evidence\": \"Southern/Northern blot and cytotoxicity in the H-2-deficient lymphoma LR.4\",\n      \"pmids\": [\"1627535\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Correlative rather than reconstitution evidence\", \"Single cell line\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Identified B2M availability as the rate-limiting determinant of surface MHC class I assembly, distinguishing it from alpha-chain expression.\",\n      \"evidence\": \"Temperature-shift flow cytometry and RT-PCR for B2m vs alpha-chain mRNA in carp leucocytes\",\n      \"pmids\": [\"9814583\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Ortholog system; quantitative stoichiometry in mammals not addressed\", \"Transcriptional control mechanism unresolved\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Linked B2M-dependent MHC class I expression to organismal phenotypes beyond immunity, showing it shapes individual odortype during development.\",\n      \"evidence\": \"B2m disruption mice with olfactory discrimination assays and developmental timing\",\n      \"pmids\": [\"10912502\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis linking MHC to odor not defined\", \"Single behavioral readout\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Validated B2M as obligate for surface class I in a large-animal in vivo model and demonstrated its role in driving xenograft rejection.\",\n      \"evidence\": \"TALEN B2m knockout pigs with protein/flow validation and xenogeneic skin graft survival\",\n      \"pmids\": [\"27982048\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not dissect NK-mediated rejection of the class I-negative grafts\", \"Single lab\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Placed B2M upstream of interferon-responsive antigen presentation, connecting somatic B2M mutation to loss of HLA-I and altered IFN signaling in cancer.\",\n      \"evidence\": \"Exome/RNA sequencing and B2M re-expression with expression profiling in lung cancer cells\",\n      \"pmids\": [\"28302866\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal mechanism linking B2M to IFN target induction unresolved\", \"Single lab\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined the divergent immune consequences of B2M loss—abolition of CD8+ T cell killing with compensatory CD4+/NK responses, and relief of NK inhibition.\",\n      \"evidence\": \"B2M knockout across three murine tumor models with immune cell depletion epistasis and anti-PD-1; CRISPR KO in NK-like YT cells against glioblastoma\",\n      \"pmids\": [\"37801341\", \"36165021\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"NK-line study (idx 11) is Low confidence with limited mechanistic detail\", \"Receptor-level basis of compensatory immunity not mapped\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified secreted B2M as a paracrine signaling molecule in cardiac remodeling, expanding its role beyond intracellular chaperoning.\",\n      \"evidence\": \"Single-cell transcriptomics with secretion and fibroblast-activation follow-up after ischemic injury\",\n      \"pmids\": [\"33514846\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor mediating fibroblast activation unknown\", \"Single lab\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Showed that B2M-HLA-E/HLA-G fusion proteins can reconstitute the NK-inhibitory signal in B2M-null cells, providing a strategy to engineer hypoimmunogenic cells.\",\n      \"evidence\": \"B2M-KO CAR-T cells with lentiviral fusion-protein expression and NK/T cytotoxicity assays\",\n      \"pmids\": [\"34323289\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not compare to native B2M-HLA stoichiometry\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Established B2M/MHC-I integrity as a prerequisite for PD-L1 expression and IFN-γ-induced antigen presentation, coupling B2M to checkpoint biology.\",\n      \"evidence\": \"B2M knockout in DLBCL cells with qRT-PCR, western, flow, IFN-γ stimulation and PBMC co-culture\",\n      \"pmids\": [\"42046553\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism coupling B2M to PD-L1 transcription not defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Revealed a non-immune role for B2M as a host factor required for Vaccinia virus internalization and early gene expression.\",\n      \"evidence\": \"Genome-wide CRISPR screen with B2M KO in HeLa and HAP1 and binding/internalization/early-gene assays\",\n      \"pmids\": [\"36574441\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular role at the internalization step undefined\", \"Direct virus-B2M interaction not shown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrated a direct physical interaction between B2M and the glycolipid-presenting molecule CD1E, identifying a binding partner beyond classical HLA heavy chains.\",\n      \"evidence\": \"Cell-free protein synthesis and surface plasmon resonance\",\n      \"pmids\": [\"36460227\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No mutagenesis or functional validation of the interaction\", \"Physiological relevance not established\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Established secreted B2M as directly pathogenic in pulmonary vascular proliferative remodeling, with B2m deletion protecting against pulmonary hypertension.\",\n      \"evidence\": \"Plasma proteomics, exogenous B2M on vascular cells, B2m knockout PH-HFpEF mice, human cohort validation\",\n      \"pmids\": [\"38813697\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Receptor mediating vascular cell proliferation unknown\", \"Signaling pathway downstream of secreted B2M undefined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Confirmed in stem cell systems that B2M governs IFN-γ-inducible HLA-I immunogenicity without affecting cell identity, supporting B2M editing for hypoimmunogenic grafts.\",\n      \"evidence\": \"shRNA knockdown in dental pulp stem cells with immune assays; CRISPR KO in iPSC-derived cardiac organoids and NK cells (preprints)\",\n      \"pmids\": [\"39538338\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Organoid and iNK studies (idx 16, 17) are unpeer-reviewed preprints\", \"Long-term graft outcomes not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The receptors and downstream signaling pathways mediating secreted B2M's paracrine effects in cardiac and pulmonary vascular tissue remain unidentified.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No receptor for extracellular B2M defined\", \"Mechanism linking B2M to fibroblast and vascular cell proliferation unknown\", \"Whether secreted B2M acts as monomer or in complex unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 8, 13, 14]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [2, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 5, 13]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [2, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 5, 6, 15]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [1, 4, 5]}\n    ],\n    \"complexes\": [\"MHC class I complex\"],\n    \"partners\": [\"HLA-E\", \"HLA-G\", \"CD1E\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}