{"gene":"CR2","run_date":"2026-04-28T17:28:53","timeline":{"discoveries":[{"year":1984,"finding":"CR2 (CD21) was identified as the C3d receptor of human B lymphocytes (Mr 145,000 membrane protein) that binds complement fragments iC3b, C3d,g, and C3d; transfer of CR2 from B cells to protein A-bearing S. aureus particles conferred C3d/iC3b binding activity on those particles.","method":"Monoclonal antibody blocking (HB-5), rosette inhibition assay, immunoadsorption transfer experiment","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 — reconstitution/transfer experiment plus receptor blocking, replicated across two independent 1984 papers","pmids":["6230668","6087328"],"is_preprint":false},{"year":1984,"finding":"CR2 (CD21) is the Epstein-Barr virus receptor of human B lymphocytes; monoclonal antibodies to CR2 block EBV binding, and transfer of CR2 to S. aureus particles confers EBV-binding capacity.","method":"Flow cytometry with anti-CR2 mAbs, EBV binding blockade, immunoadsorption transfer experiment","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 — direct transfer reconstitution experiment, replicated in multiple subsequent studies","pmids":["6087328"],"is_preprint":false},{"year":1985,"finding":"The CR2-binding site on C3 was mapped to an 8.6 kDa CNBr fragment corresponding to residues 1199-1274 of C3, with the CR2-binding site localized to residues 1227-1232.","method":"CNBr cleavage of C3d, peptide binding studies, amino-terminal sequence analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — in vitro biochemical mapping with fragment binding and synthetic peptide competition","pmids":["2408276"],"is_preprint":false},{"year":1985,"finding":"Follicular dendritic cells (FDC) express CR2 (as well as CR1 and CR3) on their entire plasma membrane and cytoplasmic extensions in germinal centers, providing a structural basis for FDC to capture complement-opsonized immune complexes.","method":"Immunohistochemistry (immunoperoxidase) and immunoelectron microscopy with anti-CR2 monoclonal antibodies on frozen lymph node sections","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization by multiple antibodies and ultrastructural EM, single study","pmids":["2411809"],"is_preprint":false},{"year":1986,"finding":"CR2 undergoes phosphorylation in B lymphocytes upon PMA stimulation (via protein kinase C activation), in contrast to CR1 which is only phosphorylated in phagocytic cells, suggesting CR2 (not CR1) mediates PMA-induced regulation of B lymphocyte function.","method":"32PO4 metabolic labeling, SDS-PAGE, autoradiography after PMA stimulation of multiple cell types","journal":"The Journal of experimental medicine","confidence":"Medium","confidence_rationale":"Tier 2 — direct biochemical PTM detection across multiple cell types, single study","pmids":["3484510"],"is_preprint":false},{"year":1987,"finding":"Purified CR2 acts as a cofactor for factor I-mediated cleavage of membrane-bound iC3b (releasing C3c), demonstrating CR2 has regulatory activity in the complement cascade similar to CR1.","method":"In vitro enzyme assay with purified CR2 and factor I, inhibition with anti-CR2 mAb OKB7","journal":"The Journal of experimental medicine","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro reconstitution assay with purified proteins and functional antibody inhibition, single study","pmids":["2437238"],"is_preprint":false},{"year":1987,"finding":"A spontaneously shed 72-kDa proteolytic fragment of CR2 (gp72) retains the C3d-binding site and the OKB7 epitope, indicating the C3d-binding domain resides in the N-terminal portion of the receptor.","method":"Immunoprecipitation, affinity chromatography, flow cytometry, and radioimmune assay comparing intact CR2 and shed gp72","journal":"Complement","confidence":"Medium","confidence_rationale":"Tier 2 — direct biochemical characterization of shed fragment with multiple assays, single study","pmids":["3497773"],"is_preprint":false},{"year":1988,"finding":"CR2 is a complement activator that serves as a major covalent (ester bond) and non-covalent binding site for C3 deposition on Raji cells during alternative pathway activation; purified CR2 alone can activate the alternative complement pathway.","method":"Immunoprecipitation with anti-CR2 and anti-C3 antibodies, hydroxylamine treatment to demonstrate ester linkage, in vitro alternative pathway activation assay with purified CR2","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with purified proteins plus cell-based biochemical experiments showing covalent bond","pmids":["2831273"],"is_preprint":false},{"year":1988,"finding":"CR2 ligands (aggregated/latex-bound C3dg, anti-CR2 mAb OKB7) enhance the transition of PMA-activated B cells from G1 to S phase, whereas monomeric C3dg is inhibitory; the effect is T cell- and monocyte-independent.","method":"Thymidine incorporation assay with purified resting tonsil B cells, various CR2 ligands, and PMA activation","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — clean purified B cell system with multiple ligand types, single study","pmids":["2459218"],"is_preprint":false},{"year":1989,"finding":"The N-terminal region (EDPGFFNVE) of EBV gp350/220 contains the CR2-binding epitope; this peptide binds purified CR2 and CR2-positive cells, blocks EBV binding to CR2, and inhibits EBV-induced B cell proliferation and transformation.","method":"Synthetic peptide binding to purified CR2, competition binding assays, EBV-induced proliferation inhibition assay","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — in vitro binding with purified receptor, functional inhibition with peptide, multiple orthogonal assays","pmids":["2464439"],"is_preprint":false},{"year":1989,"finding":"Soluble recombinant CR2 (rCR2) is a highly extended, flexible molecule composed of ring-like short consensus repeat domains (~24 Å each); gp350/220 binds rCR2 with Kd ~3.2 nM (saturable, univalent), whereas monomeric C3dg binds ~10,000-fold more weakly under physiological conditions.","method":"Gel permeation chromatography, density gradient ultracentrifugation, circular dichroism, high-resolution electron microscopy, quantitative ligand binding studies","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — multiple biophysical methods characterizing structure and quantitative ligand binding in single rigorous study","pmids":["2555366"],"is_preprint":false},{"year":1989,"finding":"Cross-linking of CR2 by mIgM ligation leads to heterologous desensitization of CD21 such that subsequent CR2 ligation fails to mobilize Ca2+; this desensitization reflects uncoupling from G proteins rather than receptor downregulation.","method":"Ca2+ mobilization assay in EBV-transformed B cell lines and peripheral blood B cells, AIF4- control to confirm G protein coupling","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — defined Ca2+ signaling assay with mechanistic interpretation, single study","pmids":["1701054"],"is_preprint":false},{"year":1990,"finding":"Soluble recombinant CR2 substantially inhibits EBV infection of B cells in vitro, demonstrating that CR2 binding is required to initiate (not merely attach) EBV infection.","method":"In vitro EBV infection inhibition assay with soluble recombinant CR2","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 — clean in vitro functional block with soluble receptor, single study","pmids":["2154612"],"is_preprint":false},{"year":1991,"finding":"Recombinant CR2 containing only SCR1-2 retains full ligand-binding activity for both C3dg and EBV gp350/220 and blocks EBV-induced B cell proliferation in vitro and EBV lymphoproliferative disease in SCID mice, localizing the primary ligand-binding domain to the two N-terminal SCRs.","method":"Recombinant protein expression (E. coli and insect cells), ELISA binding assay, in vitro proliferation inhibition, SCID mouse model","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 1-2 — domain mapping with recombinant fragments, in vitro and in vivo functional validation","pmids":["1645784"],"is_preprint":false},{"year":1991,"finding":"Multiple distinct sites in C3 (in C3c and C3d portions as well as the N-terminus of the alpha chain, residues 741-757) interact with CR2, explaining how multivalent C3-opsonized particles support B cell activation through CR2.","method":"Competitive binding assays with synthetic C3 peptides, microsphere-based CR2 binding, monoclonal antibody blocking","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple peptide and antibody competition experiments, single study","pmids":["1834472"],"is_preprint":false},{"year":1992,"finding":"CD21 (CR2) on B cells is a direct ligand for CD23 (FcεRII); CD23-bearing fluorescent liposomes bind specifically to CD21-expressing cells and CD21-transfected BHK-21 cells, and CD21 triggering by anti-CD21 antibody or soluble CD23 enhances IL-4-induced IgE production.","method":"Fluorescent CD23-liposome binding assay, CD21-cDNA transfection, Western blot, IgE production assay from blood mononuclear cells","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — direct binding with fluorescent liposomes, transfection validation, functional IgE assay; multiple methods in a high-impact study","pmids":["1386409"],"is_preprint":false},{"year":1993,"finding":"CR2 (CD21) and CD19 interact through their extracellular and transmembrane domains; CD19 and TAPA-1 interact through extracellular domains; CR2 and CR1/CD35 interact through extracellular domains. Chimeric CD19 lacking the CD21/TAPA-1 interaction retains synergistic Ca2+ signaling and PI3-kinase association with mIgM but loses homotypic aggregation (a TAPA-1 function).","method":"Domain-swap chimeric molecule expression in Daudi B cells, intracellular Ca2+ measurement, PI3-kinase association assay, homotypic aggregation assay","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 1-2 — chimeric protein dissection with multiple functional readouts, single rigorous study","pmids":["7690834"],"is_preprint":false},{"year":1993,"finding":"The CD19/CR2 complex on human B cells contains the src-family kinase Lyn; CD19 is tightly linked to Lyn and CD19 itself serves as a substrate for a serine/threonine kinase present within the complex.","method":"Co-immunoprecipitation of the CR2/CD19 complex followed by kinase assay","journal":"International immunology","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP showing Lyn association and CD19 phosphorylation, single study","pmids":["7690241"],"is_preprint":false},{"year":1993,"finding":"CD21 expressed on basophilic cells mediates CD23 binding and triggers histamine release upon CD23 or anti-CD21 mAb stimulation, demonstrating a functional CD23-CD21 interaction on non-B cells.","method":"CD23-liposome binding assay on KU 812 basophilic cell line, anti-CD21 blocking, mRNA detection, histamine release assay with normal blood basophils","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — receptor binding, functional histamine release, validated in primary basophils, single study","pmids":["7691616"],"is_preprint":false},{"year":1995,"finding":"CD23 recognizes two main epitope regions on CD21: SCR1-2 and SCR5-8; Asn370 and Asn295 within SCR5-8 are critical for CD23-CD21 interaction, and CD23-CD21 engagement preferentially drives IgE production.","method":"Epitope mapping with anti-CD21 antibodies to distinct SCR regions, IgE production assay","journal":"International archives of allergy and immunology","confidence":"Medium","confidence_rationale":"Tier 2 — epitope mapping with defined antibodies and functional IgE assay, single study","pmids":["7542093"],"is_preprint":false},{"year":1995,"finding":"A C3d-binding site on CR2 was characterized by human-mouse CR2 chimeras; an important region in SCR1 (same sequence involved in OKB7 and EBV binding) and a new region in SCR2 both contribute to iC3b binding.","method":"Human-mouse CR2 chimeras, blocking mAb 4E3, human CR2-derived peptides, complement binding assays","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — chimeric receptor domain mapping with multiple reagents, single study","pmids":["7730644"],"is_preprint":false},{"year":1996,"finding":"Cross-linking of CR2 (CD21) by EBV gp350/220 induces IL-6 mRNA and protein in B cells via PKC- and protein tyrosine kinase-dependent pathways, blocked by anti-gp350/220 and anti-CD21 mAbs.","method":"EBV and recombinant gp350/220 stimulation of B cells, cytokine ELISA/mRNA detection, kinase inhibitor studies, anti-CD21 blocking","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 — defined signaling pathway via inhibitors, receptor-specific blockade, single study","pmids":["8523572"],"is_preprint":false},{"year":1998,"finding":"FDC-expressed CR2/CD21 bearing complement C3 fragments as ligand provides a critical co-stimulatory signal for B cell IgG responses; soluble CR2 or CR2 knockout B cells reduce antibody responses 10-1000-fold, and FDC from C3-knockout mice lack co-stimulatory activity.","method":"In vitro antibody response assay with FDC, soluble CR2 blocking, CR2 knockout mouse B cells, C3 knockout mouse FDC","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO models combined with soluble receptor blocking and multiple cell-type reconstitution, single study","pmids":["9794381"],"is_preprint":false},{"year":2000,"finding":"C3dg-conjugated antigen is internalized by CR2 on B cells via coated pits at the bases of microvilli and delivered to multivesicular/multilaminar endocytic compartments containing MHC class II molecules, directly linking CR2 to antigen processing/presentation.","method":"Electron microscopy with C3dg-colloidal gold conjugates, CR2 blocking antibody, immunolabeling for MHC class II","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 — ultrastructural EM with specific inhibition and immunolabeling, direct mechanistic visualization","pmids":["10753842"],"is_preprint":false},{"year":2001,"finding":"Binding of complement-tagged antigens co-ligates BCR and CD19/CD21 complex into plasma membrane lipid rafts, prolonging BCR residency and signaling in rafts and retarding BCR internalization and degradation.","method":"Lipid raft fractionation, BCR internalization assay, signaling assays in B cells with complement-tagged antigens","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 2 — clean biochemical fractionation with functional signaling readouts, single well-controlled study","pmids":["11239449"],"is_preprint":false},{"year":2001,"finding":"CD21/CR2 co-engagement with BCR enhances antigen processing efficiency, producing more rapid and efficient antigenic peptide/MHC class II complexes; this effect operates through CR2's signaling function rather than by direct targeting of antigen for processing.","method":"B cell antigen processing assay with rHEL-C3d3 recombinant protein, peptide/MHC class II complex detection","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — defined functional assay, single study","pmids":["11418645"],"is_preprint":false},{"year":2001,"finding":"The NZM2410 Cr2 allele contains a single nucleotide polymorphism introducing a novel glycosylation site in the C3d binding domain, resulting in higher molecular weight CR2 proteins with reduced C3d ligand binding and receptor-mediated signaling; molecular modeling shows this glycosylation disrupts receptor dimerization.","method":"Sequencing, functional C3d binding assays, cell signaling assays, molecular modeling based on CR2-C3d crystal structure","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 1-2 — direct binding assay, signaling measurement, structural modeling in single study with clear mechanistic explanation","pmids":["11728339"],"is_preprint":false},{"year":2001,"finding":"CR2/CD21 is required for EBV entry into epithelial cells (293 cell line); low-level CD21 surface expression supports EBV infection that is blocked by anti-CD21 antibody, even in the absence of CD19 or CD35.","method":"EBV infection of CD21-expressing 293 cells, anti-CD21 antibody blocking, flow cytometry, EBV gene expression analysis","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 — antibody blocking of infection in defined cell line, single study","pmids":["9971794"],"is_preprint":false},{"year":2001,"finding":"EBV gp350/220 binding to CR2 activates NF-κB and induces IL-6 gene expression in B cells via PKC pathway (for gp350 alone) or multiple signaling pathways (for whole EBV), with EBV inducing longer-lived IL-6 mRNA than gp350 alone.","method":"IL-6 mRNA and protein quantitation, NF-κB band-shift assay, CAT reporter assay, PKC inhibitor, kinase inhibitor studies","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple signaling assays with mechanistic pathway identification, single study","pmids":["11327783"],"is_preprint":false},{"year":2004,"finding":"CD81 (tetraspanin) is required for the CD19/CD21 complex to partition into signaling-active lipid rafts upon BCR co-ligation; in CD81-deficient B cells or cells expressing chimeric CD19 with weak CD81 association, coligated BCR and CD19/CD21 fail to enter lipid rafts and enhanced signaling is lost.","method":"Lipid raft fractionation in CD81 knockout B cells, chimeric CD19 expression, signaling assays","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO combined with chimeric receptor approach and biochemical fractionation, multiple convergent methods","pmids":["14688345"],"is_preprint":false},{"year":2005,"finding":"Mutational analysis of CR2 SCR1-2 identified that mutations at the SCR2-C3d interface (R83A, R83E, G84Y) strongly disrupt C3dg binding, and positively charged residues on SCR1 (R13, R28, R36, K41, K50, K57, K67) also contribute to a direct SCR1-C3d binding site.","method":"Site-directed mutagenesis, K562 cell surface expression of wild-type and mutant CR2, C3dg-biotin tetramer flow cytometry binding assay","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 — systematic site-directed mutagenesis with quantitative binding assay, multiple residues tested","pmids":["15713467"],"is_preprint":false},{"year":2005,"finding":"The three-dimensional NMR structure of the CD23 C-type lectin domain was determined; distinct binding sites for IgE and CD21 were identified on CD23, CD23 can bind both ligands simultaneously, and CD23-IgE interaction can form high-molecular-mass multimeric complexes; none of these interactions require calcium despite the C-type lectin structure.","method":"NMR spectroscopy, chemical shift perturbation mapping for IgE and CD21 binding sites, gel filtration for complex analysis","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 1 — NMR structure with functional binding site mapping, single rigorous study","pmids":["16172256"],"is_preprint":false},{"year":2005,"finding":"CD21/CD19 co-receptor signaling protects activated B cells from CD95 (Fas)-mediated apoptosis by upregulating c-FLIP; Cr2-deficient B cells have reduced c-FLIP levels and are eliminated via CD95 pathway, rescued by Cr2-/- lpr mutation.","method":"Adoptive transfer model, flow cytometric survival analysis, c-FLIP Western blot and mRNA quantitation, in vitro culture with rHEL-C3d3","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis (double KO rescue) plus biochemical mechanism (c-FLIP), multiple methods","pmids":["16116172"],"is_preprint":false},{"year":2005,"finding":"Conjugation of C3d to antigen is sufficient to cause collagen-induced arthritis in the absence of CFA; CD19- and CD21-deficient mice are not susceptible to CIA; adoptive transfer showed CR2 on either B cells or FDCs is sufficient for disease susceptibility.","method":"DBA/1 collagen-induced arthritis model with C3d-conjugated antigen, CD19 and CD21 knockout mice, adoptive transfer experiments","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with multiple adoptive transfer experiments defining cell type specificity","pmids":["16210644"],"is_preprint":false},{"year":2007,"finding":"The gp350 binding site on CR2 maps to a large contiguous surface of SCR1-2 involving multiple positively charged residues (R13, R28, R36, K41, K57, K67, R83) that are complementary to the electronegative CR2-binding site on gp350; both SCR1 and SCR2 make contact with gp350.","method":"Site-directed mutagenesis of CR2 expressed on K562 cells, gp350 flow cytometry binding assay, anti-CR2 mAb epitope competition","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with quantitative binding assay, multiple residues","pmids":["17925391"],"is_preprint":false},{"year":2007,"finding":"Stromal (follicular dendritic cell) CD21/35 expression, but not hematopoietic CD21/35, is responsible for splenic retention and prion replication; reciprocal bone marrow transfers showed protection arose from stromal CD21/35 ablation.","method":"Reciprocal bone marrow adoptive transfer between CR2/35 WT and KO mice, prion infectivity assay, double KO (PrPC−/−;CD21/35−/−) transfer experiments","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal bone marrow transfer with clearly defined cellular compartment attribution","pmids":["17947689"],"is_preprint":false},{"year":2007,"finding":"CD21 (CR2) expressed on myeloma cells mediates adhesion to bone marrow stromal cells via CD23 on stromal cells; anti-CD21 and anti-CD23 antibodies inhibit myeloma-stroma adhesion.","method":"Adhesion inhibition assay with myeloma cell lines and BM stromal cells, anti-CD21 and anti-CD23 monoclonal antibody blocking","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 3 — functional blocking assay without direct binding reconstitution, single study","pmids":["7780154"],"is_preprint":false},{"year":2009,"finding":"Uncoupling CR2 from CD19 (by knockin mutation allowing C3 ligand binding but abolishing CD19 signaling) significantly impairs germinal center B cell survival and secondary antibody titers but preserves some B cell memory, identifying a CD19-independent role for CR2 in B-cell memory.","method":"Knockin mouse (Cr2ΔΔgfp) generating CR2 that binds C3 but does not signal through CD19; GC B cell flow cytometry, antibody titer measurement","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 — precision knockin design to functionally separate two CR2 activities, multiple immunological readouts","pmids":["19706534"],"is_preprint":false},{"year":2009,"finding":"CD21/35 promotes protective humoral immunity to TI-2 antigens through a complement-independent mechanism by negatively regulating CD19 expression and PD-1 induction; reducing CD19 in CD21/35-deficient mice rescues BCR signaling and IgG3 responses.","method":"CD21/35 KO vs. C3 KO vs. C4 KO mice; flow cytometry for CD19, PD-1, and BCR-induced Ca2+ responses; PD-1 blockade experiment","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with multiple KO strains and pharmacological rescue, clearly defines complement-independent pathway","pmids":["19710450"],"is_preprint":false},{"year":2020,"finding":"CD21 (CR2) is required for EBV entry into mature T cells; viral gp350 and CD21 are both necessary for EBV-2 infection of CD3+ T cells, and CRISPR-Cas9 knockout of CD21 in Jurkat cells abolishes EBV entry.","method":"Neutralizing antibody assay, CRISPR-Cas9 CD21 knockout in Jurkat cells, ex vivo infection model","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 1-2 — CRISPR genetic KO combined with antibody blocking; direct functional proof","pmids":["32238579"],"is_preprint":false},{"year":2001,"finding":"The CR2/CD21 intronic silencer requires a CBF1 (RBP-J/Notch signaling component) binding site for lineage-specific transcriptional repression; a 2-bp mutation eliminating CBF1 binding abolishes silencer activity, demonstrating Notch pathway involvement in restricting CR2 expression to mature B cells.","method":"Stable transfection of mutant silencer constructs, transgenic mouse reporter assays, in vitro binding assays","journal":"International immunology","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro binding plus in vivo transgenic validation, single study","pmids":["11312253"],"is_preprint":false}],"current_model":"CR2 (CD21) is a B-lymphocyte and follicular dendritic cell surface glycoprotein whose two N-terminal short consensus repeats (SCR1-2) bind complement C3 fragments (iC3b, C3dg, C3d) via a charged interaction surface, EBV gp350/220 (mediating viral entry into B and T cells), and CD23 (regulating IgE production); on B cells it associates with CD19, CD81, and TAPA-1 to form a coreceptor complex that, upon co-ligation with the BCR by complement-opsonized antigen, recruits Lyn kinase and PI3-kinase, prolongs BCR residency in CD81-dependent lipid rafts, upregulates c-FLIP to protect against Fas-mediated apoptosis, enhances antigen internalization and MHC class II presentation, and regulates CD19 surface density and PD-1 expression to calibrate the threshold for B cell activation and humoral immunity."},"narrative":{"teleology":[{"year":1984,"claim":"Establishing that a single surface glycoprotein mediates both complement C3d fragment binding and EBV attachment on B lymphocytes unified two previously separate receptor activities into one molecule, CR2/CD21.","evidence":"Transfer of CR2 from B cells to S. aureus particles conferred both C3d and EBV binding; anti-CR2 mAbs blocked both interactions","pmids":["6230668","6087328"],"confidence":"High","gaps":["Ligand-binding domain within CR2 not yet localized","No structural information on the receptor"]},{"year":1985,"claim":"Mapping the CR2-binding site on C3 to residues 1199-1274 (critical region 1227-1232) and demonstrating CR2 expression on follicular dendritic cells established the molecular and cellular basis for complement-mediated antigen retention in germinal centers.","evidence":"CNBr fragment binding and peptide competition for C3 mapping; immunohistochemistry and immunoelectron microscopy for FDC localization","pmids":["2408276","2411809"],"confidence":"High","gaps":["Functional significance of FDC-expressed CR2 for antibody responses not yet tested","Whether CR2 binding to C3 is sufficient for B cell activation unknown"]},{"year":1988,"claim":"Demonstrating that CR2 ligands enhance G1-to-S transition in PMA-activated B cells and that CR2 supports covalent C3 deposition and alternative pathway activation established CR2 as both a signaling receptor and a complement amplification platform.","evidence":"Thymidine incorporation with purified B cells and various CR2 ligands; immunoprecipitation showing ester-linked C3 on CR2, alternative pathway activation by purified CR2","pmids":["2831273","2459218"],"confidence":"High","gaps":["Signaling intermediates downstream of CR2 not identified","Physiological relevance of CR2-mediated complement activation unclear"]},{"year":1989,"claim":"Identifying the EBV gp350 N-terminal peptide (EDPGFFNVE) as the CR2-binding epitope and measuring gp350-CR2 affinity (Kd ~3.2 nM, ~10,000-fold stronger than monomeric C3dg) revealed the molecular basis for EBV exploitation of CR2 and suggested multivalent C3d display is needed for physiological signaling.","evidence":"Synthetic peptide binding to purified CR2, competition inhibition of EBV infection; quantitative ligand binding by gel permeation chromatography and ultracentrifugation","pmids":["2464439","2555366"],"confidence":"High","gaps":["Which SCR domains mediate ligand binding not yet resolved","How weak monomeric C3dg binding translates to B cell activation in vivo"]},{"year":1991,"claim":"Localizing all primary ligand-binding activity to SCR1-2 — sufficient to block both C3dg binding and EBV infection in vitro and lymphoproliferative disease in SCID mice — defined the minimal functional unit of CR2.","evidence":"Recombinant SCR1-2 expression in E. coli and insect cells; ELISA binding, proliferation inhibition, SCID mouse EBV lymphoma model","pmids":["1645784"],"confidence":"High","gaps":["Atomic-resolution contacts between SCR1-2 and ligands unknown","Role of remaining SCRs in vivo not addressed"]},{"year":1992,"claim":"Identifying CD23 as a third CR2 ligand that drives IgE production upon CR2 engagement expanded CR2's role beyond complement to IgE class-switch regulation.","evidence":"CD23-bearing fluorescent liposomes bound CD21-transfected BHK cells; anti-CD21/soluble CD23 enhanced IL-4-induced IgE production","pmids":["1386409"],"confidence":"High","gaps":["CD23-binding site on CR2 only partially mapped","Relative contribution of CD23-CR2 vs. CD23-IgE axis to IgE regulation in vivo unknown"]},{"year":1993,"claim":"Dissecting the CD19/CR2/TAPA-1 complex architecture and identifying Lyn kinase association established the coreceptor as a multi-subunit signaling module rather than a simple adhesion receptor.","evidence":"Domain-swap chimeras in Daudi B cells with Ca2+ and PI3-kinase readouts; co-immunoprecipitation showing Lyn in the CR2/CD19 complex","pmids":["7690834","7690241"],"confidence":"High","gaps":["Stoichiometry and assembly order of the complex unknown","Direct contribution of CR2 cytoplasmic tail to signaling not defined"]},{"year":2001,"claim":"Three convergent 2001 studies established that complement-tagged antigen co-ligation prolongs BCR-CD19/CD21 residency in lipid rafts, enhances MHC class II antigen presentation via CR2 signaling, and that a lupus-associated Cr2 polymorphism introduces aberrant glycosylation disrupting C3d binding and receptor dimerization.","evidence":"Lipid raft fractionation and BCR internalization assays; B cell antigen processing with rHEL-C3d3; NZM2410 Cr2 sequencing, binding assays, and molecular modeling","pmids":["11239449","11418645","11728339"],"confidence":"High","gaps":["Whether raft partitioning requires CD81 not yet tested","Structural basis for CR2 dimerization at atomic level unknown","Link between lupus Cr2 variant and disease causation not genetically proven"]},{"year":2004,"claim":"Demonstrating that CD81 is required for CD19/CD21 complex partitioning into lipid rafts resolved how the tetraspanin component enables enhanced BCR signaling upon complement coreceptor engagement.","evidence":"Lipid raft fractionation in CD81-knockout B cells and chimeric CD19 with weak CD81 association","pmids":["14688345"],"confidence":"High","gaps":["Molecular mechanism by which CD81 drives raft association unknown","Whether CD81 affects CR2-independent signaling not addressed"]},{"year":2005,"claim":"Systematic mutagenesis of CR2 SCR1-2 and NMR structural analysis of the CD23 lectin domain resolved the charged interaction surfaces for C3d, gp350, and CD23 binding, showing that C3d and gp350 share overlapping positively charged contact residues while CD23 recognizes both SCR1-2 and SCR5-8 through distinct, calcium-independent sites.","evidence":"Site-directed mutagenesis with flow cytometry binding on K562 cells; NMR chemical shift perturbation mapping of CD23; epitope mapping with anti-CD21 mAbs","pmids":["15713467","16172256","7542093"],"confidence":"High","gaps":["Full co-crystal structure of CR2 with any ligand not available at this point","Whether overlapping binding sites create ligand competition in vivo unknown"]},{"year":2005,"claim":"Showing that CR2/CD19 signaling upregulates c-FLIP to protect activated B cells from Fas-mediated death, with genetic rescue by lpr mutation, established CR2 as a survival signal in the germinal center reaction.","evidence":"Adoptive transfer of Cr2−/− B cells, c-FLIP quantitation, Cr2−/−lpr double mutant rescue","pmids":["16116172"],"confidence":"High","gaps":["Signaling intermediates between CR2/CD19 and c-FLIP transcription not identified","Whether this anti-apoptotic function operates in human GC B cells unknown"]},{"year":2007,"claim":"Reciprocal bone marrow transfers attributing prion splenic retention to stromal (FDC) CD21/35 and mutagenesis confirming gp350-CR2 contact residues extended the functional importance of CR2 beyond classical immunity to infectious disease pathogenesis.","evidence":"Reciprocal BM transfers between CR2/35 WT and KO mice for prion; site-directed mutagenesis of CR2 for gp350 binding","pmids":["17947689","17925391"],"confidence":"High","gaps":["Mechanism by which FDC-CR2 retains prions not elucidated","Whether CR2-prion interaction is direct or complement-mediated not resolved"]},{"year":2009,"claim":"Precision knockin mice and multi-KO epistasis experiments separated CR2's complement-dependent coreceptor signaling from a complement-independent role in regulating CD19 surface density and PD-1 expression, revealing dual mechanisms by which CR2 tunes B cell activation thresholds.","evidence":"Cr2ΔΔgfp knockin allowing C3 binding without CD19 signaling; comparison of CD21/35 KO, C3 KO, and C4 KO mice with PD-1 blockade rescue","pmids":["19706534","19710450"],"confidence":"High","gaps":["How CR2 regulates CD19 surface density mechanistically unknown","Relative contribution of each pathway during natural infection not quantified"]},{"year":2020,"claim":"CRISPR knockout of CD21 in Jurkat T cells established CR2 as the obligate EBV entry receptor on mature T cells, extending CR2's viral receptor function beyond B lymphocytes.","evidence":"CRISPR-Cas9 CD21 KO in Jurkat cells abolished EBV-2 entry; anti-gp350 neutralization confirmed ligand dependence","pmids":["32238579"],"confidence":"High","gaps":["How CR2 expression is regulated on T cells unknown","Post-entry viral lifecycle in T cells not characterized via CR2 engagement"]},{"year":null,"claim":"A high-resolution co-crystal structure of CR2 SCR1-2 simultaneously bound to C3d and gp350 is still needed to resolve whether the overlapping binding surfaces create true competition or allosteric modulation, and the signaling cascade linking CR2/CD19 co-engagement to c-FLIP upregulation and CD19 surface regulation remains molecularly undefined.","evidence":"","pmids":[],"confidence":"Low","gaps":["No ternary complex structure of CR2-C3d-gp350","Signaling intermediates between CR2/CD19 raft entry and c-FLIP induction uncharacterized","Mechanism of complement-independent CD19 regulation by CR2 unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,1,9,11]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,38]},{"term_id":"GO:0001618","term_label":"virus receptor activity","supporting_discovery_ids":[1,12,27,39]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,3,24,29]}],"pathway":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[22,23,24,25,32,33,37,38]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[11,17,21,28,29]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[1,27,35,39]}],"complexes":["CD19/CD21/CD81 (TAPA-1) B cell coreceptor complex"],"partners":["CD19","CD81","LYN","CD23","C3","CR1","EBV GP350/220"],"other_free_text":[]},"mechanistic_narrative":"CR2 (CD21) is a complement receptor on B lymphocytes and follicular dendritic cells that links innate complement opsonization to adaptive humoral immunity by lowering the threshold for B cell activation, enhancing antigen processing, and supporting germinal center survival. Its two N-terminal short consensus repeats (SCR1-2) constitute a positively charged ligand-binding surface that engages complement C3 fragments (iC3b, C3dg, C3d), EBV glycoprotein gp350/220, and CD23, with C3d and gp350 sharing overlapping contact residues including R13, R28, R36, K41, K57, K67, and R83 [PMID:15713467, PMID:17925391]. On B cells, CR2 associates with CD19, CD81, and Lyn kinase to form a coreceptor complex that, upon co-ligation with the BCR by complement-opsonized antigen, prolongs BCR residency in CD81-dependent lipid rafts, activates PI3-kinase signaling, upregulates c-FLIP to protect against Fas-mediated apoptosis, enhances antigen delivery to MHC class II compartments, and regulates CD19 surface density and PD-1 expression to calibrate B cell responsiveness [PMID:11239449, PMID:14688345, PMID:16116172, PMID:10753842, PMID:19710450]. CR2 also serves as the obligate receptor for Epstein-Barr virus entry into B cells and T cells, with soluble recombinant SCR1-2 sufficient to block EBV infection and EBV-driven lymphoproliferation [PMID:1645784, PMID:32238579]."},"prefetch_data":{"uniprot":{"accession":"P20023","full_name":"Complement receptor type 2","aliases":["Complement C3d receptor","Epstein-Barr virus receptor","EBV receptor"],"length_aa":1033,"mass_kda":112.9,"function":"Serves as a receptor for various ligands including complement component CD3d, HNRNPU OR IFNA1 (PubMed:1849076, PubMed:21527715, PubMed:7753047). When C3d is bound to antigens, attaches to C3d on B-cell surface and thereby facilitates the recognition and uptake of antigens by B-cells (PubMed:21527715). This interaction enhances B-cell activation and subsequent immune responses. Forms a complex with several partners on the surface of B-cells including CD19, FCRL5 and CD81, to form the B-cell coreceptor complex that plays a crucial role in B-cell activation and signaling (PubMed:1383329, PubMed:30107486). Also induces specific intracellular signaling separately from the BCR and CD19 by activating the tyrosine kinase SRC, which then phosphorylates nucleolin/NCL and triggers AKT and GSK3 kinase activities in a SYK/CD19-independent manner (PubMed:12938232). Acts as a ligand for CD23 (FcepsilonRII), a low-affinity receptor for IgE, which is expressed on B-cells and other immune cells, and thus participates in the regulation of IgE production (PubMed:1386409) (Microbial infection) Acts as a receptor for Epstein-Barr virus","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/P20023/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CR2","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/CR2","total_profiled":1310},"omim":[{"mim_id":"617951","title":"LEUKODYSTROPHY, HYPOMYELINATING, 15; HLD15","url":"https://www.omim.org/entry/617951"},{"mim_id":"615074","title":"GAND SYNDROME; GAND","url":"https://www.omim.org/entry/615074"},{"mim_id":"614998","title":"GATA ZINC FINGER DOMAIN-CONTAINING PROTEIN 2B; GATAD2B","url":"https://www.omim.org/entry/614998"},{"mim_id":"614997","title":"GATA ZINC FINGER DOMAIN-CONTAINING PROTEIN 2A; GATAD2A","url":"https://www.omim.org/entry/614997"},{"mim_id":"614699","title":"IMMUNODEFICIENCY, COMMON VARIABLE, 7; CVID7","url":"https://www.omim.org/entry/614699"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"lymphoid tissue","ntpm":163.6}],"url":"https://www.proteinatlas.org/search/CR2"},"hgnc":{"alias_symbol":["CD21","C3DR"],"prev_symbol":[]},"alphafold":{"accession":"P20023","domains":[{"cath_id":"2.10.70.10","chopping":"23-149","consensus_level":"medium","plddt":83.6207,"start":23,"end":149},{"cath_id":"2.10.70.10","chopping":"157-213","consensus_level":"high","plddt":79.6718,"start":157,"end":213},{"cath_id":"2.10.70.10","chopping":"225-274","consensus_level":"medium","plddt":82.5012,"start":225,"end":274},{"cath_id":"2.10.70.10","chopping":"276-345","consensus_level":"medium","plddt":77.4917,"start":276,"end":345},{"cath_id":"2.10.70.10","chopping":"351-408","consensus_level":"high","plddt":81.4429,"start":351,"end":408},{"cath_id":"2.10.70.10","chopping":"410-419_436-470","consensus_level":"medium","plddt":82.2533,"start":410,"end":470},{"cath_id":"-","chopping":"473-525","consensus_level":"high","plddt":63.7317,"start":473,"end":525},{"cath_id":"2.10.70.10","chopping":"527-596","consensus_level":"high","plddt":78.2103,"start":527,"end":596},{"cath_id":"-","chopping":"662-717","consensus_level":"medium","plddt":62.08,"start":662,"end":717},{"cath_id":"2.10.70.10","chopping":"789-846","consensus_level":"medium","plddt":79.9178,"start":789,"end":846},{"cath_id":"2.10.70.10","chopping":"861-910","consensus_level":"medium","plddt":80.379,"start":861,"end":910},{"cath_id":"2.10.70.10","chopping":"913-975","consensus_level":"high","plddt":73.3357,"start":913,"end":975}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P20023","model_url":"https://alphafold.ebi.ac.uk/files/AF-P20023-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P20023-F1-predicted_aligned_error_v6.png","plddt_mean":74.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CR2","jax_strain_url":"https://www.jax.org/strain/search?query=CR2"},"sequence":{"accession":"P20023","fasta_url":"https://rest.uniprot.org/uniprotkb/P20023.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P20023/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P20023"}},"corpus_meta":[{"pmid":"6087328","id":"PMC_6087328","title":"Epstein-Barr 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\"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstitution/transfer experiment plus receptor blocking, replicated across two independent 1984 papers\",\n      \"pmids\": [\"6230668\", \"6087328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"CR2 (CD21) is the Epstein-Barr virus receptor of human B lymphocytes; monoclonal antibodies to CR2 block EBV binding, and transfer of CR2 to S. aureus particles confers EBV-binding capacity.\",\n      \"method\": \"Flow cytometry with anti-CR2 mAbs, EBV binding blockade, immunoadsorption transfer experiment\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct transfer reconstitution experiment, replicated in multiple subsequent studies\",\n      \"pmids\": [\"6087328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"The CR2-binding site on C3 was mapped to an 8.6 kDa CNBr fragment corresponding to residues 1199-1274 of C3, with the CR2-binding site localized to residues 1227-1232.\",\n      \"method\": \"CNBr cleavage of C3d, peptide binding studies, amino-terminal sequence analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro biochemical mapping with fragment binding and synthetic peptide competition\",\n      \"pmids\": [\"2408276\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"Follicular dendritic cells (FDC) express CR2 (as well as CR1 and CR3) on their entire plasma membrane and cytoplasmic extensions in germinal centers, providing a structural basis for FDC to capture complement-opsonized immune complexes.\",\n      \"method\": \"Immunohistochemistry (immunoperoxidase) and immunoelectron microscopy with anti-CR2 monoclonal antibodies on frozen lymph node sections\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization by multiple antibodies and ultrastructural EM, single study\",\n      \"pmids\": [\"2411809\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1986,\n      \"finding\": \"CR2 undergoes phosphorylation in B lymphocytes upon PMA stimulation (via protein kinase C activation), in contrast to CR1 which is only phosphorylated in phagocytic cells, suggesting CR2 (not CR1) mediates PMA-induced regulation of B lymphocyte function.\",\n      \"method\": \"32PO4 metabolic labeling, SDS-PAGE, autoradiography after PMA stimulation of multiple cell types\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct biochemical PTM detection across multiple cell types, single study\",\n      \"pmids\": [\"3484510\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"Purified CR2 acts as a cofactor for factor I-mediated cleavage of membrane-bound iC3b (releasing C3c), demonstrating CR2 has regulatory activity in the complement cascade similar to CR1.\",\n      \"method\": \"In vitro enzyme assay with purified CR2 and factor I, inhibition with anti-CR2 mAb OKB7\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution assay with purified proteins and functional antibody inhibition, single study\",\n      \"pmids\": [\"2437238\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"A spontaneously shed 72-kDa proteolytic fragment of CR2 (gp72) retains the C3d-binding site and the OKB7 epitope, indicating the C3d-binding domain resides in the N-terminal portion of the receptor.\",\n      \"method\": \"Immunoprecipitation, affinity chromatography, flow cytometry, and radioimmune assay comparing intact CR2 and shed gp72\",\n      \"journal\": \"Complement\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct biochemical characterization of shed fragment with multiple assays, single study\",\n      \"pmids\": [\"3497773\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"CR2 is a complement activator that serves as a major covalent (ester bond) and non-covalent binding site for C3 deposition on Raji cells during alternative pathway activation; purified CR2 alone can activate the alternative complement pathway.\",\n      \"method\": \"Immunoprecipitation with anti-CR2 and anti-C3 antibodies, hydroxylamine treatment to demonstrate ester linkage, in vitro alternative pathway activation assay with purified CR2\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with purified proteins plus cell-based biochemical experiments showing covalent bond\",\n      \"pmids\": [\"2831273\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"CR2 ligands (aggregated/latex-bound C3dg, anti-CR2 mAb OKB7) enhance the transition of PMA-activated B cells from G1 to S phase, whereas monomeric C3dg is inhibitory; the effect is T cell- and monocyte-independent.\",\n      \"method\": \"Thymidine incorporation assay with purified resting tonsil B cells, various CR2 ligands, and PMA activation\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean purified B cell system with multiple ligand types, single study\",\n      \"pmids\": [\"2459218\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1989,\n      \"finding\": \"The N-terminal region (EDPGFFNVE) of EBV gp350/220 contains the CR2-binding epitope; this peptide binds purified CR2 and CR2-positive cells, blocks EBV binding to CR2, and inhibits EBV-induced B cell proliferation and transformation.\",\n      \"method\": \"Synthetic peptide binding to purified CR2, competition binding assays, EBV-induced proliferation inhibition assay\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro binding with purified receptor, functional inhibition with peptide, multiple orthogonal assays\",\n      \"pmids\": [\"2464439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1989,\n      \"finding\": \"Soluble recombinant CR2 (rCR2) is a highly extended, flexible molecule composed of ring-like short consensus repeat domains (~24 Å each); gp350/220 binds rCR2 with Kd ~3.2 nM (saturable, univalent), whereas monomeric C3dg binds ~10,000-fold more weakly under physiological conditions.\",\n      \"method\": \"Gel permeation chromatography, density gradient ultracentrifugation, circular dichroism, high-resolution electron microscopy, quantitative ligand binding studies\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple biophysical methods characterizing structure and quantitative ligand binding in single rigorous study\",\n      \"pmids\": [\"2555366\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1989,\n      \"finding\": \"Cross-linking of CR2 by mIgM ligation leads to heterologous desensitization of CD21 such that subsequent CR2 ligation fails to mobilize Ca2+; this desensitization reflects uncoupling from G proteins rather than receptor downregulation.\",\n      \"method\": \"Ca2+ mobilization assay in EBV-transformed B cell lines and peripheral blood B cells, AIF4- control to confirm G protein coupling\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined Ca2+ signaling assay with mechanistic interpretation, single study\",\n      \"pmids\": [\"1701054\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"Soluble recombinant CR2 substantially inhibits EBV infection of B cells in vitro, demonstrating that CR2 binding is required to initiate (not merely attach) EBV infection.\",\n      \"method\": \"In vitro EBV infection inhibition assay with soluble recombinant CR2\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean in vitro functional block with soluble receptor, single study\",\n      \"pmids\": [\"2154612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"Recombinant CR2 containing only SCR1-2 retains full ligand-binding activity for both C3dg and EBV gp350/220 and blocks EBV-induced B cell proliferation in vitro and EBV lymphoproliferative disease in SCID mice, localizing the primary ligand-binding domain to the two N-terminal SCRs.\",\n      \"method\": \"Recombinant protein expression (E. coli and insect cells), ELISA binding assay, in vitro proliferation inhibition, SCID mouse model\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — domain mapping with recombinant fragments, in vitro and in vivo functional validation\",\n      \"pmids\": [\"1645784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"Multiple distinct sites in C3 (in C3c and C3d portions as well as the N-terminus of the alpha chain, residues 741-757) interact with CR2, explaining how multivalent C3-opsonized particles support B cell activation through CR2.\",\n      \"method\": \"Competitive binding assays with synthetic C3 peptides, microsphere-based CR2 binding, monoclonal antibody blocking\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple peptide and antibody competition experiments, single study\",\n      \"pmids\": [\"1834472\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"CD21 (CR2) on B cells is a direct ligand for CD23 (FcεRII); CD23-bearing fluorescent liposomes bind specifically to CD21-expressing cells and CD21-transfected BHK-21 cells, and CD21 triggering by anti-CD21 antibody or soluble CD23 enhances IL-4-induced IgE production.\",\n      \"method\": \"Fluorescent CD23-liposome binding assay, CD21-cDNA transfection, Western blot, IgE production assay from blood mononuclear cells\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct binding with fluorescent liposomes, transfection validation, functional IgE assay; multiple methods in a high-impact study\",\n      \"pmids\": [\"1386409\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CR2 (CD21) and CD19 interact through their extracellular and transmembrane domains; CD19 and TAPA-1 interact through extracellular domains; CR2 and CR1/CD35 interact through extracellular domains. Chimeric CD19 lacking the CD21/TAPA-1 interaction retains synergistic Ca2+ signaling and PI3-kinase association with mIgM but loses homotypic aggregation (a TAPA-1 function).\",\n      \"method\": \"Domain-swap chimeric molecule expression in Daudi B cells, intracellular Ca2+ measurement, PI3-kinase association assay, homotypic aggregation assay\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — chimeric protein dissection with multiple functional readouts, single rigorous study\",\n      \"pmids\": [\"7690834\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"The CD19/CR2 complex on human B cells contains the src-family kinase Lyn; CD19 is tightly linked to Lyn and CD19 itself serves as a substrate for a serine/threonine kinase present within the complex.\",\n      \"method\": \"Co-immunoprecipitation of the CR2/CD19 complex followed by kinase assay\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP showing Lyn association and CD19 phosphorylation, single study\",\n      \"pmids\": [\"7690241\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CD21 expressed on basophilic cells mediates CD23 binding and triggers histamine release upon CD23 or anti-CD21 mAb stimulation, demonstrating a functional CD23-CD21 interaction on non-B cells.\",\n      \"method\": \"CD23-liposome binding assay on KU 812 basophilic cell line, anti-CD21 blocking, mRNA detection, histamine release assay with normal blood basophils\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — receptor binding, functional histamine release, validated in primary basophils, single study\",\n      \"pmids\": [\"7691616\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"CD23 recognizes two main epitope regions on CD21: SCR1-2 and SCR5-8; Asn370 and Asn295 within SCR5-8 are critical for CD23-CD21 interaction, and CD23-CD21 engagement preferentially drives IgE production.\",\n      \"method\": \"Epitope mapping with anti-CD21 antibodies to distinct SCR regions, IgE production assay\",\n      \"journal\": \"International archives of allergy and immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epitope mapping with defined antibodies and functional IgE assay, single study\",\n      \"pmids\": [\"7542093\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"A C3d-binding site on CR2 was characterized by human-mouse CR2 chimeras; an important region in SCR1 (same sequence involved in OKB7 and EBV binding) and a new region in SCR2 both contribute to iC3b binding.\",\n      \"method\": \"Human-mouse CR2 chimeras, blocking mAb 4E3, human CR2-derived peptides, complement binding assays\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — chimeric receptor domain mapping with multiple reagents, single study\",\n      \"pmids\": [\"7730644\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Cross-linking of CR2 (CD21) by EBV gp350/220 induces IL-6 mRNA and protein in B cells via PKC- and protein tyrosine kinase-dependent pathways, blocked by anti-gp350/220 and anti-CD21 mAbs.\",\n      \"method\": \"EBV and recombinant gp350/220 stimulation of B cells, cytokine ELISA/mRNA detection, kinase inhibitor studies, anti-CD21 blocking\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined signaling pathway via inhibitors, receptor-specific blockade, single study\",\n      \"pmids\": [\"8523572\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"FDC-expressed CR2/CD21 bearing complement C3 fragments as ligand provides a critical co-stimulatory signal for B cell IgG responses; soluble CR2 or CR2 knockout B cells reduce antibody responses 10-1000-fold, and FDC from C3-knockout mice lack co-stimulatory activity.\",\n      \"method\": \"In vitro antibody response assay with FDC, soluble CR2 blocking, CR2 knockout mouse B cells, C3 knockout mouse FDC\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO models combined with soluble receptor blocking and multiple cell-type reconstitution, single study\",\n      \"pmids\": [\"9794381\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"C3dg-conjugated antigen is internalized by CR2 on B cells via coated pits at the bases of microvilli and delivered to multivesicular/multilaminar endocytic compartments containing MHC class II molecules, directly linking CR2 to antigen processing/presentation.\",\n      \"method\": \"Electron microscopy with C3dg-colloidal gold conjugates, CR2 blocking antibody, immunolabeling for MHC class II\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — ultrastructural EM with specific inhibition and immunolabeling, direct mechanistic visualization\",\n      \"pmids\": [\"10753842\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Binding of complement-tagged antigens co-ligates BCR and CD19/CD21 complex into plasma membrane lipid rafts, prolonging BCR residency and signaling in rafts and retarding BCR internalization and degradation.\",\n      \"method\": \"Lipid raft fractionation, BCR internalization assay, signaling assays in B cells with complement-tagged antigens\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean biochemical fractionation with functional signaling readouts, single well-controlled study\",\n      \"pmids\": [\"11239449\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"CD21/CR2 co-engagement with BCR enhances antigen processing efficiency, producing more rapid and efficient antigenic peptide/MHC class II complexes; this effect operates through CR2's signaling function rather than by direct targeting of antigen for processing.\",\n      \"method\": \"B cell antigen processing assay with rHEL-C3d3 recombinant protein, peptide/MHC class II complex detection\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined functional assay, single study\",\n      \"pmids\": [\"11418645\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The NZM2410 Cr2 allele contains a single nucleotide polymorphism introducing a novel glycosylation site in the C3d binding domain, resulting in higher molecular weight CR2 proteins with reduced C3d ligand binding and receptor-mediated signaling; molecular modeling shows this glycosylation disrupts receptor dimerization.\",\n      \"method\": \"Sequencing, functional C3d binding assays, cell signaling assays, molecular modeling based on CR2-C3d crystal structure\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding assay, signaling measurement, structural modeling in single study with clear mechanistic explanation\",\n      \"pmids\": [\"11728339\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"CR2/CD21 is required for EBV entry into epithelial cells (293 cell line); low-level CD21 surface expression supports EBV infection that is blocked by anti-CD21 antibody, even in the absence of CD19 or CD35.\",\n      \"method\": \"EBV infection of CD21-expressing 293 cells, anti-CD21 antibody blocking, flow cytometry, EBV gene expression analysis\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — antibody blocking of infection in defined cell line, single study\",\n      \"pmids\": [\"9971794\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"EBV gp350/220 binding to CR2 activates NF-κB and induces IL-6 gene expression in B cells via PKC pathway (for gp350 alone) or multiple signaling pathways (for whole EBV), with EBV inducing longer-lived IL-6 mRNA than gp350 alone.\",\n      \"method\": \"IL-6 mRNA and protein quantitation, NF-κB band-shift assay, CAT reporter assay, PKC inhibitor, kinase inhibitor studies\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple signaling assays with mechanistic pathway identification, single study\",\n      \"pmids\": [\"11327783\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"CD81 (tetraspanin) is required for the CD19/CD21 complex to partition into signaling-active lipid rafts upon BCR co-ligation; in CD81-deficient B cells or cells expressing chimeric CD19 with weak CD81 association, coligated BCR and CD19/CD21 fail to enter lipid rafts and enhanced signaling is lost.\",\n      \"method\": \"Lipid raft fractionation in CD81 knockout B cells, chimeric CD19 expression, signaling assays\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO combined with chimeric receptor approach and biochemical fractionation, multiple convergent methods\",\n      \"pmids\": [\"14688345\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Mutational analysis of CR2 SCR1-2 identified that mutations at the SCR2-C3d interface (R83A, R83E, G84Y) strongly disrupt C3dg binding, and positively charged residues on SCR1 (R13, R28, R36, K41, K50, K57, K67) also contribute to a direct SCR1-C3d binding site.\",\n      \"method\": \"Site-directed mutagenesis, K562 cell surface expression of wild-type and mutant CR2, C3dg-biotin tetramer flow cytometry binding assay\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic site-directed mutagenesis with quantitative binding assay, multiple residues tested\",\n      \"pmids\": [\"15713467\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The three-dimensional NMR structure of the CD23 C-type lectin domain was determined; distinct binding sites for IgE and CD21 were identified on CD23, CD23 can bind both ligands simultaneously, and CD23-IgE interaction can form high-molecular-mass multimeric complexes; none of these interactions require calcium despite the C-type lectin structure.\",\n      \"method\": \"NMR spectroscopy, chemical shift perturbation mapping for IgE and CD21 binding sites, gel filtration for complex analysis\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — NMR structure with functional binding site mapping, single rigorous study\",\n      \"pmids\": [\"16172256\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"CD21/CD19 co-receptor signaling protects activated B cells from CD95 (Fas)-mediated apoptosis by upregulating c-FLIP; Cr2-deficient B cells have reduced c-FLIP levels and are eliminated via CD95 pathway, rescued by Cr2-/- lpr mutation.\",\n      \"method\": \"Adoptive transfer model, flow cytometric survival analysis, c-FLIP Western blot and mRNA quantitation, in vitro culture with rHEL-C3d3\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis (double KO rescue) plus biochemical mechanism (c-FLIP), multiple methods\",\n      \"pmids\": [\"16116172\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Conjugation of C3d to antigen is sufficient to cause collagen-induced arthritis in the absence of CFA; CD19- and CD21-deficient mice are not susceptible to CIA; adoptive transfer showed CR2 on either B cells or FDCs is sufficient for disease susceptibility.\",\n      \"method\": \"DBA/1 collagen-induced arthritis model with C3d-conjugated antigen, CD19 and CD21 knockout mice, adoptive transfer experiments\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with multiple adoptive transfer experiments defining cell type specificity\",\n      \"pmids\": [\"16210644\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The gp350 binding site on CR2 maps to a large contiguous surface of SCR1-2 involving multiple positively charged residues (R13, R28, R36, K41, K57, K67, R83) that are complementary to the electronegative CR2-binding site on gp350; both SCR1 and SCR2 make contact with gp350.\",\n      \"method\": \"Site-directed mutagenesis of CR2 expressed on K562 cells, gp350 flow cytometry binding assay, anti-CR2 mAb epitope competition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with quantitative binding assay, multiple residues\",\n      \"pmids\": [\"17925391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Stromal (follicular dendritic cell) CD21/35 expression, but not hematopoietic CD21/35, is responsible for splenic retention and prion replication; reciprocal bone marrow transfers showed protection arose from stromal CD21/35 ablation.\",\n      \"method\": \"Reciprocal bone marrow adoptive transfer between CR2/35 WT and KO mice, prion infectivity assay, double KO (PrPC−/−;CD21/35−/−) transfer experiments\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal bone marrow transfer with clearly defined cellular compartment attribution\",\n      \"pmids\": [\"17947689\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"CD21 (CR2) expressed on myeloma cells mediates adhesion to bone marrow stromal cells via CD23 on stromal cells; anti-CD21 and anti-CD23 antibodies inhibit myeloma-stroma adhesion.\",\n      \"method\": \"Adhesion inhibition assay with myeloma cell lines and BM stromal cells, anti-CD21 and anti-CD23 monoclonal antibody blocking\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — functional blocking assay without direct binding reconstitution, single study\",\n      \"pmids\": [\"7780154\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Uncoupling CR2 from CD19 (by knockin mutation allowing C3 ligand binding but abolishing CD19 signaling) significantly impairs germinal center B cell survival and secondary antibody titers but preserves some B cell memory, identifying a CD19-independent role for CR2 in B-cell memory.\",\n      \"method\": \"Knockin mouse (Cr2ΔΔgfp) generating CR2 that binds C3 but does not signal through CD19; GC B cell flow cytometry, antibody titer measurement\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — precision knockin design to functionally separate two CR2 activities, multiple immunological readouts\",\n      \"pmids\": [\"19706534\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"CD21/35 promotes protective humoral immunity to TI-2 antigens through a complement-independent mechanism by negatively regulating CD19 expression and PD-1 induction; reducing CD19 in CD21/35-deficient mice rescues BCR signaling and IgG3 responses.\",\n      \"method\": \"CD21/35 KO vs. C3 KO vs. C4 KO mice; flow cytometry for CD19, PD-1, and BCR-induced Ca2+ responses; PD-1 blockade experiment\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with multiple KO strains and pharmacological rescue, clearly defines complement-independent pathway\",\n      \"pmids\": [\"19710450\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CD21 (CR2) is required for EBV entry into mature T cells; viral gp350 and CD21 are both necessary for EBV-2 infection of CD3+ T cells, and CRISPR-Cas9 knockout of CD21 in Jurkat cells abolishes EBV entry.\",\n      \"method\": \"Neutralizing antibody assay, CRISPR-Cas9 CD21 knockout in Jurkat cells, ex vivo infection model\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — CRISPR genetic KO combined with antibody blocking; direct functional proof\",\n      \"pmids\": [\"32238579\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The CR2/CD21 intronic silencer requires a CBF1 (RBP-J/Notch signaling component) binding site for lineage-specific transcriptional repression; a 2-bp mutation eliminating CBF1 binding abolishes silencer activity, demonstrating Notch pathway involvement in restricting CR2 expression to mature B cells.\",\n      \"method\": \"Stable transfection of mutant silencer constructs, transgenic mouse reporter assays, in vitro binding assays\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro binding plus in vivo transgenic validation, single study\",\n      \"pmids\": [\"11312253\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CR2 (CD21) is a B-lymphocyte and follicular dendritic cell surface glycoprotein whose two N-terminal short consensus repeats (SCR1-2) bind complement C3 fragments (iC3b, C3dg, C3d) via a charged interaction surface, EBV gp350/220 (mediating viral entry into B and T cells), and CD23 (regulating IgE production); on B cells it associates with CD19, CD81, and TAPA-1 to form a coreceptor complex that, upon co-ligation with the BCR by complement-opsonized antigen, recruits Lyn kinase and PI3-kinase, prolongs BCR residency in CD81-dependent lipid rafts, upregulates c-FLIP to protect against Fas-mediated apoptosis, enhances antigen internalization and MHC class II presentation, and regulates CD19 surface density and PD-1 expression to calibrate the threshold for B cell activation and humoral immunity.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"CR2 (CD21) is a complement receptor on B lymphocytes and follicular dendritic cells that links innate complement opsonization to adaptive humoral immunity by lowering the threshold for B cell activation, enhancing antigen processing, and supporting germinal center survival. Its two N-terminal short consensus repeats (SCR1-2) constitute a positively charged ligand-binding surface that engages complement C3 fragments (iC3b, C3dg, C3d), EBV glycoprotein gp350/220, and CD23, with C3d and gp350 sharing overlapping contact residues including R13, R28, R36, K41, K57, K67, and R83 [PMID:15713467, PMID:17925391]. On B cells, CR2 associates with CD19, CD81, and Lyn kinase to form a coreceptor complex that, upon co-ligation with the BCR by complement-opsonized antigen, prolongs BCR residency in CD81-dependent lipid rafts, activates PI3-kinase signaling, upregulates c-FLIP to protect against Fas-mediated apoptosis, enhances antigen delivery to MHC class II compartments, and regulates CD19 surface density and PD-1 expression to calibrate B cell responsiveness [PMID:11239449, PMID:14688345, PMID:16116172, PMID:10753842, PMID:19710450]. CR2 also serves as the obligate receptor for Epstein-Barr virus entry into B cells and T cells, with soluble recombinant SCR1-2 sufficient to block EBV infection and EBV-driven lymphoproliferation [PMID:1645784, PMID:32238579].\",\n  \"teleology\": [\n    {\n      \"year\": 1984,\n      \"claim\": \"Establishing that a single surface glycoprotein mediates both complement C3d fragment binding and EBV attachment on B lymphocytes unified two previously separate receptor activities into one molecule, CR2/CD21.\",\n      \"evidence\": \"Transfer of CR2 from B cells to S. aureus particles conferred both C3d and EBV binding; anti-CR2 mAbs blocked both interactions\",\n      \"pmids\": [\"6230668\", \"6087328\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ligand-binding domain within CR2 not yet localized\", \"No structural information on the receptor\"]\n    },\n    {\n      \"year\": 1985,\n      \"claim\": \"Mapping the CR2-binding site on C3 to residues 1199-1274 (critical region 1227-1232) and demonstrating CR2 expression on follicular dendritic cells established the molecular and cellular basis for complement-mediated antigen retention in germinal centers.\",\n      \"evidence\": \"CNBr fragment binding and peptide competition for C3 mapping; immunohistochemistry and immunoelectron microscopy for FDC localization\",\n      \"pmids\": [\"2408276\", \"2411809\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional significance of FDC-expressed CR2 for antibody responses not yet tested\", \"Whether CR2 binding to C3 is sufficient for B cell activation unknown\"]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Demonstrating that CR2 ligands enhance G1-to-S transition in PMA-activated B cells and that CR2 supports covalent C3 deposition and alternative pathway activation established CR2 as both a signaling receptor and a complement amplification platform.\",\n      \"evidence\": \"Thymidine incorporation with purified B cells and various CR2 ligands; immunoprecipitation showing ester-linked C3 on CR2, alternative pathway activation by purified CR2\",\n      \"pmids\": [\"2831273\", \"2459218\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling intermediates downstream of CR2 not identified\", \"Physiological relevance of CR2-mediated complement activation unclear\"]\n    },\n    {\n      \"year\": 1989,\n      \"claim\": \"Identifying the EBV gp350 N-terminal peptide (EDPGFFNVE) as the CR2-binding epitope and measuring gp350-CR2 affinity (Kd ~3.2 nM, ~10,000-fold stronger than monomeric C3dg) revealed the molecular basis for EBV exploitation of CR2 and suggested multivalent C3d display is needed for physiological signaling.\",\n      \"evidence\": \"Synthetic peptide binding to purified CR2, competition inhibition of EBV infection; quantitative ligand binding by gel permeation chromatography and ultracentrifugation\",\n      \"pmids\": [\"2464439\", \"2555366\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which SCR domains mediate ligand binding not yet resolved\", \"How weak monomeric C3dg binding translates to B cell activation in vivo\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Localizing all primary ligand-binding activity to SCR1-2 — sufficient to block both C3dg binding and EBV infection in vitro and lymphoproliferative disease in SCID mice — defined the minimal functional unit of CR2.\",\n      \"evidence\": \"Recombinant SCR1-2 expression in E. coli and insect cells; ELISA binding, proliferation inhibition, SCID mouse EBV lymphoma model\",\n      \"pmids\": [\"1645784\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution contacts between SCR1-2 and ligands unknown\", \"Role of remaining SCRs in vivo not addressed\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Identifying CD23 as a third CR2 ligand that drives IgE production upon CR2 engagement expanded CR2's role beyond complement to IgE class-switch regulation.\",\n      \"evidence\": \"CD23-bearing fluorescent liposomes bound CD21-transfected BHK cells; anti-CD21/soluble CD23 enhanced IL-4-induced IgE production\",\n      \"pmids\": [\"1386409\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"CD23-binding site on CR2 only partially mapped\", \"Relative contribution of CD23-CR2 vs. CD23-IgE axis to IgE regulation in vivo unknown\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Dissecting the CD19/CR2/TAPA-1 complex architecture and identifying Lyn kinase association established the coreceptor as a multi-subunit signaling module rather than a simple adhesion receptor.\",\n      \"evidence\": \"Domain-swap chimeras in Daudi B cells with Ca2+ and PI3-kinase readouts; co-immunoprecipitation showing Lyn in the CR2/CD19 complex\",\n      \"pmids\": [\"7690834\", \"7690241\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and assembly order of the complex unknown\", \"Direct contribution of CR2 cytoplasmic tail to signaling not defined\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Three convergent 2001 studies established that complement-tagged antigen co-ligation prolongs BCR-CD19/CD21 residency in lipid rafts, enhances MHC class II antigen presentation via CR2 signaling, and that a lupus-associated Cr2 polymorphism introduces aberrant glycosylation disrupting C3d binding and receptor dimerization.\",\n      \"evidence\": \"Lipid raft fractionation and BCR internalization assays; B cell antigen processing with rHEL-C3d3; NZM2410 Cr2 sequencing, binding assays, and molecular modeling\",\n      \"pmids\": [\"11239449\", \"11418645\", \"11728339\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether raft partitioning requires CD81 not yet tested\", \"Structural basis for CR2 dimerization at atomic level unknown\", \"Link between lupus Cr2 variant and disease causation not genetically proven\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Demonstrating that CD81 is required for CD19/CD21 complex partitioning into lipid rafts resolved how the tetraspanin component enables enhanced BCR signaling upon complement coreceptor engagement.\",\n      \"evidence\": \"Lipid raft fractionation in CD81-knockout B cells and chimeric CD19 with weak CD81 association\",\n      \"pmids\": [\"14688345\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which CD81 drives raft association unknown\", \"Whether CD81 affects CR2-independent signaling not addressed\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Systematic mutagenesis of CR2 SCR1-2 and NMR structural analysis of the CD23 lectin domain resolved the charged interaction surfaces for C3d, gp350, and CD23 binding, showing that C3d and gp350 share overlapping positively charged contact residues while CD23 recognizes both SCR1-2 and SCR5-8 through distinct, calcium-independent sites.\",\n      \"evidence\": \"Site-directed mutagenesis with flow cytometry binding on K562 cells; NMR chemical shift perturbation mapping of CD23; epitope mapping with anti-CD21 mAbs\",\n      \"pmids\": [\"15713467\", \"16172256\", \"7542093\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full co-crystal structure of CR2 with any ligand not available at this point\", \"Whether overlapping binding sites create ligand competition in vivo unknown\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Showing that CR2/CD19 signaling upregulates c-FLIP to protect activated B cells from Fas-mediated death, with genetic rescue by lpr mutation, established CR2 as a survival signal in the germinal center reaction.\",\n      \"evidence\": \"Adoptive transfer of Cr2−/− B cells, c-FLIP quantitation, Cr2−/−lpr double mutant rescue\",\n      \"pmids\": [\"16116172\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling intermediates between CR2/CD19 and c-FLIP transcription not identified\", \"Whether this anti-apoptotic function operates in human GC B cells unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Reciprocal bone marrow transfers attributing prion splenic retention to stromal (FDC) CD21/35 and mutagenesis confirming gp350-CR2 contact residues extended the functional importance of CR2 beyond classical immunity to infectious disease pathogenesis.\",\n      \"evidence\": \"Reciprocal BM transfers between CR2/35 WT and KO mice for prion; site-directed mutagenesis of CR2 for gp350 binding\",\n      \"pmids\": [\"17947689\", \"17925391\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which FDC-CR2 retains prions not elucidated\", \"Whether CR2-prion interaction is direct or complement-mediated not resolved\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Precision knockin mice and multi-KO epistasis experiments separated CR2's complement-dependent coreceptor signaling from a complement-independent role in regulating CD19 surface density and PD-1 expression, revealing dual mechanisms by which CR2 tunes B cell activation thresholds.\",\n      \"evidence\": \"Cr2ΔΔgfp knockin allowing C3 binding without CD19 signaling; comparison of CD21/35 KO, C3 KO, and C4 KO mice with PD-1 blockade rescue\",\n      \"pmids\": [\"19706534\", \"19710450\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How CR2 regulates CD19 surface density mechanistically unknown\", \"Relative contribution of each pathway during natural infection not quantified\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"CRISPR knockout of CD21 in Jurkat T cells established CR2 as the obligate EBV entry receptor on mature T cells, extending CR2's viral receptor function beyond B lymphocytes.\",\n      \"evidence\": \"CRISPR-Cas9 CD21 KO in Jurkat cells abolished EBV-2 entry; anti-gp350 neutralization confirmed ligand dependence\",\n      \"pmids\": [\"32238579\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How CR2 expression is regulated on T cells unknown\", \"Post-entry viral lifecycle in T cells not characterized via CR2 engagement\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A high-resolution co-crystal structure of CR2 SCR1-2 simultaneously bound to C3d and gp350 is still needed to resolve whether the overlapping binding surfaces create true competition or allosteric modulation, and the signaling cascade linking CR2/CD19 co-engagement to c-FLIP upregulation and CD19 surface regulation remains molecularly undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No ternary complex structure of CR2-C3d-gp350\", \"Signaling intermediates between CR2/CD19 raft entry and c-FLIP induction uncharacterized\", \"Mechanism of complement-independent CD19 regulation by CR2 unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 1, 9, 11]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 38]},\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [1, 12, 27, 39]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 3, 24, 29]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [22, 23, 24, 25, 32, 33, 37, 38]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [11, 17, 21, 28, 29]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [1, 27, 35, 39]}\n    ],\n    \"complexes\": [\n      \"CD19/CD21/CD81 (TAPA-1) B cell coreceptor complex\"\n    ],\n    \"partners\": [\n      \"CD19\",\n      \"CD81\",\n      \"LYN\",\n      \"CD23\",\n      \"C3\",\n      \"CR1\",\n      \"EBV gp350/220\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}