{"gene":"CR2","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":1984,"finding":"CR2 (CD21) is the Epstein-Barr virus receptor on human B lymphocytes, established by showing that anti-CR2 monoclonal antibodies block EBV binding and that transfer of CR2 from B cells to Staphylococcus aureus particles confers specific EBV binding ability.","method":"Monoclonal antibody blocking assay, receptor transfer to heterologous particles, radiolabeled EBV binding assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — three independent experimental approaches in same paper, subsequently replicated across multiple labs","pmids":["6087328"],"is_preprint":false},{"year":1987,"finding":"CR2 has cofactor activity for Factor I-mediated cleavage of membrane-bound iC3b, releasing C3c, demonstrating functional similarity to CR1 as a complement regulatory protein.","method":"In vitro cofactor activity assay with purified CR2 and Factor I on 125I-labeled iC3b-bearing erythrocytes; inhibition by anti-CR2 mAb OKB7","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro enzymatic assay with purified protein and antibody inhibition control, single lab","pmids":["2437238"],"is_preprint":false},{"year":1988,"finding":"CR2 carries two distinct binding sites: one for EBV and one for C3d, demonstrated using anti-CR2 mAbs that selectively block one ligand but not the other, and anti-idiotypic antibodies that mimic extracellular domains of CR2.","method":"Monoclonal antibody blocking assays, anti-idiotypic antibody production, affinity chromatography","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal blocking with multiple mAbs and anti-idiotypic approach, single lab","pmids":["2842398"],"is_preprint":false},{"year":1988,"finding":"Aggregated and particulate CR2 ligands (C3dg, anti-CR2 mAb OKB7, UV-inactivated EBV) enhance B cell transition from G1 to S phase when combined with phorbol ester activation, while monomeric C3dg is inhibitory; signaling is T cell- and monocyte-independent.","method":"Thymidine incorporation assay, purified resting tonsil B cells, comparison of monomeric vs. aggregated/particulate ligands","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — defined cellular phenotype with multiple ligand forms tested, single lab","pmids":["2459218"],"is_preprint":false},{"year":1989,"finding":"CR2 cDNA transfection into CR2-negative mouse L cells and human K562 cells confers both C3bi/C3d binding (rosette formation) and EBV binding/infection, demonstrating CR2 alone is sufficient to transfer complement receptor and EBV receptor functions.","method":"Stable transfection, flow cytometry, rosette assay with complement-coated erythrocytes, EBV infection and EBNA expression","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 1 / Strong — gain-of-function reconstitution in heterologous cells with multiple functional readouts","pmids":["2473114"],"is_preprint":false},{"year":1990,"finding":"Soluble recombinant CR2 inhibits EBV infection of B cells in vitro, demonstrating that CR2 binding initiates EBV infection rather than merely mediating virus attachment.","method":"In vitro EBV infection inhibition assay with soluble recombinant CR2","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional inhibition with recombinant protein, single lab, single method","pmids":["2154612"],"is_preprint":false},{"year":1990,"finding":"The murine Cr2 gene produces two proteins via alternative splicing: a larger ~190 kDa protein with CR1-like N-terminal domains and a smaller ~145 kDa protein homologous to human CR2, both sharing the same signal sequence.","method":"cDNA cloning, DNA sequence analysis, RNA protection studies, genomic phage mapping","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 1 / Strong — complete sequence analysis with multiple orthogonal molecular methods defining gene structure","pmids":["2139460"],"is_preprint":false},{"year":1991,"finding":"CR2 (CD21) is expressed on 30-40% of normal human peripheral blood T lymphocytes at ~10-fold lower intensity than B cells, immunoprecipitated as a 145 kDa protein, suggesting CR2 may modulate T cell function.","method":"Indirect immunofluorescence with biotinylated anti-CR2 mAb, immunoprecipitation from purified T lymphocytes","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct protein detection by two orthogonal methods (flow cytometry + immunoprecipitation), single lab","pmids":["1703182"],"is_preprint":false},{"year":1992,"finding":"CD21 (CR2) is a ligand for CD23 (FcεRII); fluorescent CD23-liposomes specifically bind CD21 on B cells and CD21-transfected BHK-21 cells, and triggering CD21 with anti-CD21 antibody or soluble CD23 specifically increases IL-4-induced IgE production.","method":"Fluorescent liposome binding assay, CD21 cDNA transfection into BHK-21 cells, anti-CD21/anti-CD23 blocking antibodies, Western blot, IgE production assay from blood mononuclear cells","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution in transfected cells, multiple blocking controls, functional IgE production readout","pmids":["1386409"],"is_preprint":false},{"year":1993,"finding":"Within the CD21/CD19/TAPA-1 complex, CD19 and TAPA-1 interact through their extracellular domains, CD19 and CD21 interact through extracellular and transmembrane domains, and CD21 and CD35 interact through extracellular domains; CD21 is required for TAPA-1-dependent homotypic cellular aggregation but not for CD19-mediated PI3-kinase recruitment or calcium signaling.","method":"Chimeric molecule construction (HLA-A2/CD4 domain swaps), stable transfection in Daudi cells, functional assays for calcium flux, PI3-kinase association, homotypic aggregation","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 1 / Strong — domain mapping via chimeric proteins with multiple functional readouts","pmids":["7690834"],"is_preprint":false},{"year":1993,"finding":"CD21/CD19/TAPA-1 complex amplifies signaling through membrane immunoglobulin and recruits PI3-kinase; CD19 is the signaling subunit coupling to Lyn and PI3-kinase, while CR2 enables nonimmunologic ligation of CD19 via C3 fragments.","method":"Review synthesizing biochemical and genetic data (functional complex characterization)","journal":"Current opinion in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — review of established biochemical findings, no new primary experiment","pmids":["7688513"],"is_preprint":false},{"year":1994,"finding":"CD21 ligands binding to the C3dg/EBV-binding site of CR2 markedly augment B cell activation initiated by antigen receptor ligation via a tyrosine kinase-dependent, c-fos-selective pathway; beads coated with anti-CD21 mAb to the C3dg-binding portion trigger homotypic B cell aggregation.","method":"Latex bead coated with antibodies/ligands, c-fos and c-myc mRNA quantitation, homotypic aggregation assay, tyrosine kinase inhibitor studies with purified small resting B cells","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — defined signaling pathway with epitope-mapped mAbs and kinase inhibitor, single lab","pmids":["7525704"],"is_preprint":false},{"year":1995,"finding":"CR2 binds CD23 at two main epitope regions: SCRs 1-2 and SCRs 5-8, with Asn370 and Asn295 in the SCR 5-8 region being critical for the interaction with the lectin CD23.","method":"Epitope mapping with anti-CD23 antibodies, mutagenesis of CD21 residues","journal":"International archives of allergy and immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — binding site defined but abstract lacks full methodological detail, single lab","pmids":["7542093"],"is_preprint":false},{"year":1995,"finding":"CD21 expressed on myeloma cells mediates adhesion to bone marrow stromal cells via CD23 on stromal cells; antibody blocking of either CD21 or CD23 inhibits this adhesion.","method":"Antibody-blocking adhesion assay, flow cytometry for CD21/CD23 expression, myeloma cell lines and primary stromal cells","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal antibody blocking (anti-CD21 and anti-CD23) with functional adhesion readout","pmids":["7780154"],"is_preprint":false},{"year":1998,"finding":"CR2 (CD21) gene expression is regulated by a B cell- and stage-specific intronic silencer: the 2.5-kb first intron segment silences promoter activity specifically in non-CR2-expressing cells, and this silencer requires nuclear matrix/chromatin interactions in stable (but not transient) transfection.","method":"Transient and stable transfection with reporter gene constructs, transgenic mice","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 1 / Strong — functional dissection by reporter constructs in cell lines and confirmed in transgenic mice","pmids":["9570543"],"is_preprint":false},{"year":1998,"finding":"C3dg binding to CR2 occurs in the recess formed between SCR1 and SCR2; a mAb (FE8) recognizing a discontinuous epitope spanning sequences at the SCR1-SCR2 junction blocks C3dg binding and EBV infection.","method":"Epitope mapping with overlapping peptides on cellulose, mAb inhibition of C3dg binding and EBV infection, 3D homology modeling","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional blocking mAb with peptide epitope mapping, 3D model, single lab","pmids":["9794388"],"is_preprint":false},{"year":1999,"finding":"Low-level surface CD21 expression on epithelial 293 cells supports EBV binding and stable infection; both virus binding and infection are blocked by anti-CD21 antibodies, and CD35 and CD19 are not required.","method":"Anti-CD21 antibody blocking of EBV binding and infection, flow cytometry, EBV infection of 293 cells with selectable marker","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — antibody blocking with defined genetic absence of CD35/CD19, single lab","pmids":["9971794"],"is_preprint":false},{"year":2001,"finding":"CR2 binding to gp350 follows a simple 1:1 kinetics, whereas binding to C3d and iC3b is more complex and involves more than one intramolecular component; iC3b binds CR2 with different kinetics depending on whether it is soluble or surface-immobilized, suggesting an additional binding site in the C3c region of iC3b.","method":"Surface plasmon resonance (SPR) kinetic analysis of CR2 interactions with C3d, iC3b, and gp350/220","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 1 / Moderate — quantitative SPR kinetics with multiple ligand forms, single lab, rigorous biophysical method","pmids":["11466369"],"is_preprint":false},{"year":2001,"finding":"The SCR 1-2 domains of CR2 account for the primary C3dg binding site; the additional SCR domains of full-length CR2 slow both the association and dissociation rates. The SCR1-2:C3d interaction forms a 1:1 complex and is influenced by ionic (charge-dependent) interactions, with the sole His residue near the SCR1-SCR2 linker potentially affecting ligand association.","method":"Recombinant CR2 SCR1-2 and SCR1-15 expression in Pichia pastoris/baculovirus, circular dichroism, surface plasmon resonance, sedimentation equilibrium ultracentrifugation, homology modeling","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple biophysical methods (SPR, CD, sedimentation), recombinant protein, single lab with orthogonal approaches","pmids":["11352728"],"is_preprint":false},{"year":2001,"finding":"An inhibitory anti-CR2 mAb (171) directly contacts the C3d binding site on CR2 SCR1-2 as confirmed by placement of its linear epitope on the CR2-C3d crystal structure, providing solution-phase confirmation of the crystallographic binding interface.","method":"Anti-CR2 mAb generation in Cr2-/- mice, overlapping peptide epitope mapping, CR2-C3d crystal structure-based analysis, ligand binding inhibition assays","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure corroborated by mAb epitope mapping and ligand inhibition assays","pmids":["11698449"],"is_preprint":false},{"year":2001,"finding":"CD21 (CR2) activation on B lymphocyte surface triggers tyrosine phosphorylation of nucleolin (p95) and its interaction with PI3-kinase p85 subunit, SH2 domains of 3BP2 and Grb2 (but not Fyn or Gap), within 2 minutes; this signaling is specific to CR2 activation and not induced by CD19 or BCR activation.","method":"Anti-phosphotyrosine affinity purification, mass spectrometry amino acid analysis, anti-nucleolin antibody co-precipitation, SH2 domain binding assay","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 1 / Moderate — protein identification by mass spectrometry and functional binding assays with multiple SH2 domain controls, single lab","pmids":["11207269"],"is_preprint":false},{"year":2001,"finding":"Binding of complement-tagged antigens causes co-translocation of both BCR and CD19/CD21 complex into plasma membrane lipid rafts, prolonging BCR residency and signaling in rafts compared to BCR cross-linking alone; CD19/CD21 co-ligation also retards BCR internalization and degradation.","method":"Lipid raft fractionation, co-localization assays, BCR internalization and degradation kinetics, complement-tagged antigen stimulation","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 2 / Strong — lipid raft fractionation with BCR signaling kinetics, multiple orthogonal readouts","pmids":["11239449"],"is_preprint":false},{"year":2001,"finding":"A site within the CR2 intronic silencer contains a CBF1 (RBP-J/RBP-Jκ) binding site; a 2-bp mutation eliminating CBF1 binding abolishes silencer activity in vivo, demonstrating that CBF1 (a Notch signaling component) controls CR2 lineage-specific expression.","method":"Site-directed mutagenesis of silencer, stable transfection reporter assay, EMSA (in vitro binding)","journal":"International immunology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis with in vivo functional reporter readout and in vitro binding confirmation","pmids":["11312253"],"is_preprint":false},{"year":2001,"finding":"Murine CD21 gene expression requires both promoter sequences and a 5'-proximal intronic element; the first intron contains an 800-bp 5' inhibitory element active in both B and T cells and an 800-bp 3' element that is inhibitory in T cells but acts as an enhancer in B cells.","method":"Transient transfection with luciferase reporter constructs, minigene analysis","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional dissection of regulatory elements by reporter assay, single lab","pmids":["9144490"],"is_preprint":false},{"year":2001,"finding":"CR2 (CD21) expression is regulated by DNA methylation of the CpG island near the ATG start codon and by histone deacetylation; 5-aza-2'-deoxycytidine (DNA methyltransferase inhibitor) and trichostatin A (histone deacetylase inhibitor) both induce CD21 expression in early B lymphocytes that normally lack it.","method":"Methylation analysis of CpG island in cell lines, 5-aza-2'-deoxycytidine treatment, trichostatin A treatment","journal":"International immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological epigenetic manipulation with defined readout, single lab","pmids":["11312258"],"is_preprint":false},{"year":2003,"finding":"Soluble CD21 (sCD21) shed into human plasma is predominantly a short form lacking exon-11-encoded sequences; shedding involves proteolytic cleavage of only the extracellular portion (C terminus truncated), is induced by PMA+calcium ionophore or BCR stimulation (anti-IgM+anti-CD40), and peripheral blood B cells (not T cells) are the primary source.","method":"Mass spectrometry of purified plasma sCD21, N-terminal sequencing, flow cytometry after stimulation, B/T cell separation experiments","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mass spectrometry identification with cell-source determination and stimulus-defined shedding, single lab","pmids":["12938215"],"is_preprint":false},{"year":2005,"finding":"The NMR structure of the C-type lectin domain of CD23 identifies distinct binding sites for IgE and CD21; CD23 can bind both ligands simultaneously and does not require calcium for either interaction. IgE and CD23 can form high molecular mass multimeric complexes.","method":"NMR solution structure determination, concentration-dependent chemical shift perturbation analysis for binding site mapping, molecular weight analysis","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure with ligand-binding site mapping by chemical shift perturbations, defines molecular basis of CD21-CD23 interaction","pmids":["16172256"],"is_preprint":false},{"year":2005,"finding":"CD21/CD19 coreceptor signaling promotes B cell survival in primary immune responses via upregulation of c-FLIP, protecting against CD95-mediated apoptosis; Cr2-/- B cells show reduced c-FLIP levels and fail to persist in follicles, rescued by the lpr mutation.","method":"Adoptive transfer of Cr2-/- and Cr2+/+ Ig-transgenic B cells, c-FLIP mRNA and protein quantitation, in vitro HEL-C3d3 stimulation, CD95-mediated apoptosis assay","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis (Cr2-/- x lpr double mutant), in vivo adoptive transfer, and in vitro mechanistic follow-up","pmids":["16116172"],"is_preprint":false},{"year":2006,"finding":"CD21 shedding is induced by P2X7 receptor activation (via BzATP, a P2X7R agonist) on peripheral B cells, in addition to BCR stimulation; P2X7R-mediated shedding was confirmed using receptor inhibitors.","method":"BzATP stimulation of peripheral blood lymphocytes, P2X7R inhibitor experiments, flow cytometry for surface CD21","journal":"International immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological receptor activation and inhibition with defined shedding readout, single lab","pmids":["16740600"],"is_preprint":false},{"year":2007,"finding":"The gp350 binding site on CR2 involves a large contiguous surface on SCR1-2 dominated by positively charged residues (Arg-13, Arg-28, Arg-36, Lys-41, Lys-57, Lys-67, Arg-83, and within SCR2 via mAbs 171/1048); both SCR1 and SCR2 make contact with gp350.","method":"Site-directed mutagenesis of CR2 SCR1-2, expression on K562 cells, gp350 binding by flow cytometry, anti-CR2 mAb inhibition","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic site-directed mutagenesis with cell-surface expression and functional binding assay","pmids":["17925391"],"is_preprint":false},{"year":2007,"finding":"Stromal (not hematopoietic) CD21/35 on follicular dendritic cells is required for splenic retention of prion inocula and lymphoid prion colonization; bone marrow chimera experiments showed that protection from prion infection in CD21/35-/- mice is due to loss of stromal CD21/35.","method":"Reciprocal bone marrow adoptive transfer between WT and CD21/35-/- mice, prion challenge, infectivity titration in spleens","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal bone marrow transfer establishes cell-type specific requirement, replicated with double KO controls","pmids":["17947689"],"is_preprint":false},{"year":2008,"finding":"The CR2 binding site on EBV gp350 is defined by a negatively charged surface (Glu-21, Asp-22, Glu-155, Asp-208, Glu-210, Asp-296) and hydrophobic contacts (Tyr-151, Ile-160, Trp-162); the complementary positively charged residues on CR2 SCR1-2 (Arg-13, Arg-28, Arg-36, Lys-41, Lys-57, Lys-67, Arg-83, Arg-89) mediate the charge-dependent interaction.","method":"ELISA with recombinant wild-type and mutant gp350 and CR2 SCR1-2, K562 cell binding assay with PE-conjugated gp350, HADDOCK docking model","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic mutagenesis of both binding partners with functional cell-surface binding assay and structural docking model","pmids":["18786993"],"is_preprint":false},{"year":2009,"finding":"Uncoupling CD21 from CD19 (via knockin of mutant Cr2 that binds C3 but does not signal through CD19) significantly diminishes germinal center B cell survival and secondary antibody titers, but B memory is less impaired than in complete CR deficiency, identifying a CD19-independent role for CR in B cell memory.","method":"Knockin mice expressing Cr2-GFP fusion that binds C3 ligands but fails to signal through CD19, immunization, antibody titer measurement, GC B cell flow cytometry","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — precise genetic uncoupling (knockin), in vivo functional immune readouts, compared with complete KO","pmids":["19706534"],"is_preprint":false},{"year":2010,"finding":"The CR2-binding site on C3d maps to the concave face acidic pocket; mutations to concave face residues significantly impair both CR2 and Staphylococcal Sbi-IV binding, while mutations to the sideface region implicated by the cocrystal structure show no binding defects, casting doubt on the physiologic relevance of the CR2-C3d cocrystal interface.","method":"Surface plasmon resonance with panel of C3dg variant proteins against biosensor-bound CR2 SCR1-2 or Sbi-IV","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic mutagenesis with quantitative SPR, challenges prior structural data","pmids":["20083651"],"is_preprint":false},{"year":2012,"finding":"CR2 binds multiple forms of DNA (bacterial, viral, mammalian) with moderately high affinity; methylated DNA binds with high affinity (Kd ~6 nM) via the first two SCR domains; binding is blocked by an inhibitory anti-CR2 mAb; Cr2-/- mice show specific immune response defects to bacterial DNA.","method":"Surface plasmon resonance, anti-CR2 mAb blocking, truncated SCR domain constructs, immunization of Cr2-/- mice","journal":"Molecular immunology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — quantitative SPR with domain mapping and in vivo functional confirmation in Cr2-/- mice","pmids":["22885687"],"is_preprint":false},{"year":2019,"finding":"DOCK2 deficiency reduces CD21 expression at both mRNA and protein levels through a mechanism involving upregulation of lymphoid enhancer-binding factor 1 (LEF1), leading to downregulation of CD19-mediated BCR signaling and reduced marginal zone B cells.","method":"Genetic models (DOCK2-/-, WASP-/-, LEF1-deficient mice), flow cytometry, mRNA/protein expression analysis, BCR signaling assays","journal":"The Journal of allergy and clinical immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple genetic models with defined mechanistic pathway, single lab","pmids":["31405607"],"is_preprint":false},{"year":2020,"finding":"CD21 is required for EBV type 2 entry into T cells; viral gp350 and CD21 are both necessary for CD3+ T-cell infection, and CRISPR knockout of CD21 in Jurkat T cells abolishes EBV entry.","method":"Neutralizing antibody assay, CRISPR-Cas9 knockout of CD21 in Jurkat cells, flow cytometry for CD21 expression on T cell subsets, ex vivo infection model","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 2 / Strong — CRISPR loss-of-function combined with neutralizing antibody blocking and primary T cell infection model","pmids":["32238579"],"is_preprint":false},{"year":2021,"finding":"Co-engagement of CR2 (CD21) and the BCR on primary human B cells inhibits activation marker expression, cytokine production, proliferation, and antibody production at non-stimulatory concentrations of anti-Ig, while enhancing Ca2+ response; this differs from the positive coreceptor role established in mice.","method":"Co-clustering of BCR and CR2 on primary human B cells, Ca2+ flux assay, activation marker expression by flow cytometry, cytokine/antibody production measurement","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — primary human cell functional assays with multiple readouts, single lab, contradicts established mouse data","pmids":["33868238"],"is_preprint":false},{"year":2018,"finding":"CD21 and FCRL5 physically associate on B cells; triple engagement of FCRL5, CD21, and BCR produces superior calcium response compared to CD21+BCR co-stimulation alone; CD21 co-engagement converts FCRL5 from a negative to a positive co-receptor, recruiting CD19, active PLCγ2, and BTK to FCRL5.","method":"Co-immunoprecipitation, calcium flux assay in cell lines and tonsil B cells, FCRL5 transfection with/without CD21","journal":"International immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP and functional signaling assays, single lab","pmids":["30107486"],"is_preprint":false}],"current_model":"CR2 (CD21) is a multi-ligand complement receptor that binds C3d/iC3b, EBV gp350, CD23, and DNA through its N-terminal SCR1-2 domains; it serves as the EBV receptor on B and T cells, forms a coreceptor complex with CD19/TAPA-1/CD81 that co-translocates with the BCR into lipid rafts to modulate signaling thresholds (enhancing Ca2+ flux but with species-specific effects on proliferation and antibody production), promotes B cell survival via c-FLIP upregulation, facilitates prion retention on follicular dendritic cells, sheds its extracellular domain via protease activity induced by BCR stimulation or P2X7R activation, and is transcriptionally regulated by an intronic silencer requiring CBF1/Notch signaling, DNA methylation, and histone deacetylation."},"narrative":{"mechanistic_narrative":"CR2 (CD21) is a B-lymphocyte multi-ligand complement receptor that links innate complement opsonization to adaptive humoral immunity by lowering the threshold for B cell activation [PMID:2473114, PMID:11239449]. Its N-terminal SCR1-2 domains constitute the primary ligand-binding module, engaging C3d/iC3b complement fragments through a charge-dependent interface in the recess between SCR1 and SCR2 [PMID:9794388, PMID:11352728, PMID:20083651], while a complementary positively charged SCR1-2 surface binds the negatively charged Epstein-Barr virus glycoprotein gp350 [PMID:17925391, PMID:18786993]; the same domains also bind DNA, including methylated DNA with high affinity [PMID:22885687]. CR2 is the EBV receptor: it is sufficient to confer EBV binding and infection on heterologous cells, soluble CR2 neutralizes infection, and CRISPR deletion abolishes EBV type 2 entry into T cells [PMID:6087328, PMID:2473114, PMID:2154612, PMID:32238579]. CR2 also possesses Factor I cofactor activity toward membrane iC3b, and binds CD23 through SCR1-2 and SCR5-8 epitopes, mediating B cell and stromal cell adhesion and modulating IgE production [PMID:2437238, PMID:1386409, PMID:7780154, PMID:16172256]. Functionally, CR2 partitions into a coreceptor complex with CD19 and TAPA-1/CD81 in which CD19 serves as the signaling subunit; co-ligation of CR2-bound complement-tagged antigen with the BCR drives co-translocation into lipid rafts, prolongs BCR signaling, recruits PI3-kinase and tyrosine-phosphorylated nucleolin, and promotes germinal-center B cell survival via c-FLIP upregulation [PMID:7690834, PMID:11207269, PMID:11239449, PMID:16116172, PMID:19706534]. CR2 is transcriptionally restricted to its expressing lineage by an intronic silencer requiring a CBF1/RBP-J (Notch) site, DNA methylation, and histone deacetylation [PMID:9570543, PMID:11312253, PMID:11312258], and its extracellular domain is shed by stimulus-induced proteolysis following BCR or P2X7R activation [PMID:12938215, PMID:16740600]. The coreceptor outcome is species-dependent: co-engagement enhances B cell activation in mice but is inhibitory in primary human B cells despite enhancing Ca2+ flux [PMID:33868238].","teleology":[{"year":1984,"claim":"Established the founding identity of CR2 by showing it is the cellular receptor for Epstein-Barr virus, defining how a complement receptor doubles as a viral entry portal.","evidence":"Anti-CR2 mAb blocking, receptor transfer to S. aureus particles, and radiolabeled EBV binding on human B lymphocytes","pmids":["6087328"],"confidence":"High","gaps":["Did not resolve whether CR2 alone is sufficient or requires accessory molecules","Binding site on CR2 for EBV not mapped"]},{"year":1987,"claim":"Demonstrated CR2 acts as a complement regulatory protein with Factor I cofactor activity, placing it functionally alongside CR1.","evidence":"In vitro cofactor assay with purified CR2 and Factor I on iC3b-bearing erythrocytes with mAb inhibition","pmids":["2437238"],"confidence":"High","gaps":["Physiologic significance of cofactor activity in vivo not established","Does not address signaling role"]},{"year":1988,"claim":"Resolved that CR2 carries separable binding sites for EBV and C3d, indicating multi-ligand recognition is built into distinct epitopes rather than a shared surface.","evidence":"Reciprocal mAb blocking and anti-idiotypic antibodies","pmids":["2842398"],"confidence":"Medium","gaps":["Sites localized only to functional epitopes, not to specific domains/residues","Single lab"]},{"year":1989,"claim":"Proved CR2 alone is sufficient to transfer both complement-receptor and EBV-receptor function, settling whether accessory proteins are required for either activity.","evidence":"Stable cDNA transfection into CR2-negative L cells and K562 with rosette and EBV infection readouts","pmids":["2473114"],"confidence":"High","gaps":["Heterologous cells may lack downstream signaling partners present in B cells","Does not address coreceptor amplification"]},{"year":1990,"claim":"Clarified that CR2 binding initiates productive EBV infection rather than only mediating attachment, and defined murine gene architecture relevant to ortholog studies.","evidence":"Soluble recombinant CR2 infection-inhibition assay; cDNA/genomic cloning of murine Cr2","pmids":["2154612","2139460"],"confidence":"Medium","gaps":["Post-binding entry steps not defined","Single-method infection inhibition"]},{"year":1991,"claim":"Extended CR2 expression beyond B cells to a subset of T lymphocytes, raising the possibility of CR2 function in T cell biology.","evidence":"Immunofluorescence and immunoprecipitation of 145 kDa CR2 from purified peripheral T cells","pmids":["1703182"],"confidence":"Medium","gaps":["Functional consequence in T cells not demonstrated at this stage","Single lab"]},{"year":1992,"claim":"Identified CD23 (FcεRII) as a CR2 ligand and linked CR2 engagement to IL-4-induced IgE production, connecting the receptor to allergic/antibody-class regulation.","evidence":"CD23-liposome binding, CD21 transfection into BHK-21, blocking antibodies, IgE production assay","pmids":["1386409"],"confidence":"High","gaps":["Physiologic relevance of CD23-CR2 axis in vivo not established"]},{"year":1993,"claim":"Defined the molecular architecture and signaling division of labor of the CR2/CD19/TAPA-1 coreceptor complex, establishing CR2 as the ligand-sensing subunit and CD19 as the signal transducer that amplifies BCR signaling.","evidence":"Chimeric domain-swap constructs in Daudi cells with calcium, PI3-kinase, and aggregation readouts; synthesizing review","pmids":["7690834","7688513"],"confidence":"High","gaps":["Exact stoichiometry of the complex not resolved","Quantitative contribution of CR2 to signal amplification not isolated"]},{"year":1994,"claim":"Showed CR2 ligation through the C3dg/EBV site augments antigen-receptor-driven B cell activation via a tyrosine-kinase-dependent, c-fos-selective pathway, linking ligand binding to defined transcriptional and adhesive responses.","evidence":"Bead-coated ligand stimulation of resting B cells with c-fos/c-myc mRNA quantitation, aggregation, kinase inhibitors","pmids":["7525704"],"confidence":"Medium","gaps":["Intermediate signaling components between CR2 and c-fos not identified","Single lab"]},{"year":1995,"claim":"Mapped CD23 binding to two CR2 regions (SCR1-2 and SCR5-8) and connected CR2-CD23 interaction to cell adhesion, including myeloma-stroma interactions.","evidence":"Epitope mapping/mutagenesis of CD21 residues; antibody-blocking adhesion assays with myeloma and stromal cells","pmids":["7542093","7780154"],"confidence":"Medium","gaps":["Methodological detail limited in epitope study","In vivo relevance of myeloma adhesion not tested"]},{"year":1998,"claim":"Localized the C3dg binding site to the SCR1-SCR2 junction recess and defined an intronic silencer governing lineage-restricted CR2 expression, addressing both ligand recognition and transcriptional control.","evidence":"Peptide epitope mapping with blocking mAb FE8 and homology modeling; reporter transfection and transgenic mice for silencer","pmids":["9794388","9570543"],"confidence":"High","gaps":["Trans-acting factors binding the silencer not yet identified at this stage","Atomic-resolution structure of the binding interface not available"]},{"year":1999,"claim":"Demonstrated low-level epithelial CR2 supports EBV binding and infection independently of CD35 and CD19, showing CR2 functions as an autonomous entry receptor across cell types.","evidence":"Anti-CD21 blocking of EBV binding/infection in 293 cells lacking CD35/CD19","pmids":["9971794"],"confidence":"Medium","gaps":["Efficiency relative to B cell infection not benchmarked","Single lab"]},{"year":2001,"claim":"Defined the biophysical and signaling basis of CR2 function: SCR1-2 as the primary charge-dependent C3d-binding module, distinct ligand kinetics, a CR2-specific nucleolin/PI3-kinase signaling branch, lipid-raft co-translocation prolonging BCR signaling, and CBF1/Notch- and epigenetically-controlled lineage-specific transcription.","evidence":"SPR/CD/ultracentrifugation on recombinant SCR constructs; mass-spec identification of phospho-nucleolin and SH2 binding; lipid raft fractionation with BCR kinetics; silencer mutagenesis/EMSA and methylation/HDAC pharmacology; crystal-structure-corroborated mAb mapping","pmids":["11466369","11352728","11207269","11239449","11312253","11312258","9144490","11698449"],"confidence":"High","gaps":["Physiologic CR2-C3d interface later disputed","How nucleolin signaling integrates with CD19 amplification not resolved","Stoichiometry of raft co-translocation not quantified"]},{"year":2003,"claim":"Characterized stimulus-induced proteolytic shedding of the CR2 ectodomain from B cells, identifying a regulated mechanism that generates soluble CD21 and removes surface receptor after activation.","evidence":"Mass spectrometry and N-terminal sequencing of plasma sCD21; stimulation and B/T cell source determination","pmids":["12938215"],"confidence":"High","gaps":["Responsible protease not identified","Functional role of soluble CD21 not established"]},{"year":2005,"claim":"Established a survival function for CR2 coreceptor signaling—c-FLIP upregulation protecting germinal-center B cells from CD95 apoptosis—and defined the molecular basis of the CD21-CD23 interaction structurally.","evidence":"Adoptive transfer of Cr2-/- Ig-transgenic B cells with Cr2 x lpr epistasis and c-FLIP/apoptosis assays; NMR structure of the CD23 C-type lectin domain","pmids":["16116172","16172256"],"confidence":"High","gaps":["Signaling link between CR2/CD19 and c-FLIP transcription not fully mapped","Whether CD23-CR2 contributes to survival not tested"]},{"year":2007,"claim":"Defined the gp350 binding surface on CR2 SCR1-2 as a positively charged contiguous patch and demonstrated that stromal (not hematopoietic) CR2/35 on follicular dendritic cells mediates prion retention, expanding CR2's role to antigen/agent trapping.","evidence":"Systematic SCR1-2 mutagenesis with gp350 binding on K562; reciprocal bone marrow chimeras with prion challenge","pmids":["17925391","17947689"],"confidence":"High","gaps":["Mechanism of FDC retention of prions at molecular level not defined","Whether DNA/complement binding contributes to prion trapping unknown"]},{"year":2008,"claim":"Resolved the complementary electrostatic chemistry of the CR2-gp350 interface, explaining how charge pairing dictates EBV receptor recognition.","evidence":"Mutagenesis of both gp350 and CR2 SCR1-2 with cell-surface binding and HADDOCK docking","pmids":["18786993"],"confidence":"High","gaps":["No co-crystal structure of the CR2-gp350 complex","Affinity contribution of individual residues not ranked"]},{"year":2009,"claim":"Genetically uncoupled CR2 ligand binding from CD19 signaling, revealing a CD19-independent contribution of CR2 to B cell memory while confirming CD19 dependence for germinal-center survival and antibody titers.","evidence":"Knockin Cr2-GFP mice that bind C3 but cannot signal through CD19, with immunization and GC/antibody readouts versus complete KO","pmids":["19706534"],"confidence":"High","gaps":["Molecular basis of the CD19-independent memory function not identified","Whether non-CD19 partners mediate this role unknown"]},{"year":2010,"claim":"Re-mapped the functional CR2-binding site on C3d to the concave-face acidic pocket, challenging the physiologic relevance of the previously reported cocrystal interface.","evidence":"SPR with a panel of C3dg variants against CR2 SCR1-2 and Sbi-IV","pmids":["20083651"],"confidence":"High","gaps":["Discrepancy with crystallographic interface not fully reconciled","Conformational state of C3d governing binding not resolved"]},{"year":2012,"claim":"Identified DNA, especially methylated DNA, as a high-affinity CR2 ligand via SCR1-2, with in vivo immune defects to bacterial DNA, broadening CR2 ligand repertoire beyond complement and viral proteins.","evidence":"SPR with multiple DNA forms, truncated SCR constructs, blocking mAb, and immunization of Cr2-/- mice","pmids":["22885687"],"confidence":"High","gaps":["Mechanism by which DNA binding shapes immune responses not defined","Relationship to complement/EBV binding sites not fully delineated"]},{"year":2018,"claim":"Showed CR2 physically associates with FCRL5 and converts it from a negative to a positive coreceptor, illustrating that CR2 can rewire the signaling output of partner receptors.","evidence":"Co-immunoprecipitation and calcium flux assays in cell lines and tonsil B cells with FCRL5 transfection","pmids":["30107486"],"confidence":"Medium","gaps":["Reciprocal validation and stoichiometry of CR2-FCRL5 association limited","In vivo relevance not tested","Single lab"]},{"year":2019,"claim":"Linked upstream cytoskeletal regulator DOCK2 to CR2 expression via LEF1, connecting B cell developmental signaling to CR2/CD19 coreceptor abundance.","evidence":"Genetic mouse models (DOCK2-/-, LEF1-deficient) with expression and BCR signaling assays","pmids":["31405607"],"confidence":"Medium","gaps":["Direct LEF1 action on CR2 locus not demonstrated","Single lab"]},{"year":2020,"claim":"Established CR2 as the required receptor for EBV type 2 entry into T cells, extending its viral-entry function to the T lineage.","evidence":"CRISPR knockout of CD21 in Jurkat cells, neutralizing antibodies, and primary T cell infection","pmids":["32238579"],"confidence":"High","gaps":["Whether T cell entry uses the same gp350 interface as B cells not confirmed","Co-receptor requirements in T cells not defined"]},{"year":2021,"claim":"Revealed a species divergence in CR2 coreceptor function—co-engagement inhibits human B cell activation despite enhancing Ca2+ flux, contrasting with the positive role established in mice.","evidence":"Co-clustering of BCR and CR2 on primary human B cells with Ca2+, activation, cytokine, and antibody readouts","pmids":["33868238"],"confidence":"Medium","gaps":["Molecular basis for the human-specific inhibitory outcome not defined","Single lab; contradicts mouse data"]},{"year":null,"claim":"How CR2's distinct ligand-binding activities (complement, EBV gp350, CD23, DNA) are integrated to set coreceptor signaling outcomes, and why those outcomes diverge between mouse and human B cells, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified structural model reconciling the disputed C3d interface","Species-specific signaling mechanism unexplained","Protease responsible for ectodomain shedding unidentified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0001618","term_label":"virus receptor activity","supporting_discovery_ids":[0,4,5,16,36]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[9,21,27]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[34]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1]},{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[8,13]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4,7,21]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[21,27,32]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,30,36]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[9,20,21]}],"complexes":["CD21/CD19/TAPA-1 (CD81) coreceptor complex"],"partners":["CD19","CD81","CD23","CD35","FCRL5"],"other_free_text":[]}},"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 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radiolabeled EBV binding assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — three independent experimental approaches in same paper, subsequently replicated across multiple labs\",\n      \"pmids\": [\"6087328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"CR2 has cofactor activity for Factor I-mediated cleavage of membrane-bound iC3b, releasing C3c, demonstrating functional similarity to CR1 as a complement regulatory protein.\",\n      \"method\": \"In vitro cofactor activity assay with purified CR2 and Factor I on 125I-labeled iC3b-bearing erythrocytes; inhibition by anti-CR2 mAb OKB7\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro enzymatic assay with purified protein and antibody inhibition control, single lab\",\n      \"pmids\": [\"2437238\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"CR2 carries two distinct binding sites: one for EBV and one for C3d, demonstrated using anti-CR2 mAbs that selectively block one ligand but not the other, and anti-idiotypic antibodies that mimic extracellular domains of CR2.\",\n      \"method\": \"Monoclonal antibody blocking assays, anti-idiotypic antibody production, affinity chromatography\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal blocking with multiple mAbs and anti-idiotypic approach, single lab\",\n      \"pmids\": [\"2842398\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"Aggregated and particulate CR2 ligands (C3dg, anti-CR2 mAb OKB7, UV-inactivated EBV) enhance B cell transition from G1 to S phase when combined with phorbol ester activation, while monomeric C3dg is inhibitory; signaling is T cell- and monocyte-independent.\",\n      \"method\": \"Thymidine incorporation assay, purified resting tonsil B cells, comparison of monomeric vs. aggregated/particulate ligands\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — defined cellular phenotype with multiple ligand forms tested, single lab\",\n      \"pmids\": [\"2459218\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1989,\n      \"finding\": \"CR2 cDNA transfection into CR2-negative mouse L cells and human K562 cells confers both C3bi/C3d binding (rosette formation) and EBV binding/infection, demonstrating CR2 alone is sufficient to transfer complement receptor and EBV receptor functions.\",\n      \"method\": \"Stable transfection, flow cytometry, rosette assay with complement-coated erythrocytes, EBV infection and EBNA expression\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — gain-of-function reconstitution in heterologous cells with multiple functional readouts\",\n      \"pmids\": [\"2473114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"Soluble recombinant CR2 inhibits EBV infection of B cells in vitro, demonstrating that CR2 binding initiates EBV infection rather than merely mediating virus attachment.\",\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 / Moderate — functional inhibition with recombinant protein, single lab, single method\",\n      \"pmids\": [\"2154612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"The murine Cr2 gene produces two proteins via alternative splicing: a larger ~190 kDa protein with CR1-like N-terminal domains and a smaller ~145 kDa protein homologous to human CR2, both sharing the same signal sequence.\",\n      \"method\": \"cDNA cloning, DNA sequence analysis, RNA protection studies, genomic phage mapping\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — complete sequence analysis with multiple orthogonal molecular methods defining gene structure\",\n      \"pmids\": [\"2139460\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"CR2 (CD21) is expressed on 30-40% of normal human peripheral blood T lymphocytes at ~10-fold lower intensity than B cells, immunoprecipitated as a 145 kDa protein, suggesting CR2 may modulate T cell function.\",\n      \"method\": \"Indirect immunofluorescence with biotinylated anti-CR2 mAb, immunoprecipitation from purified T lymphocytes\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct protein detection by two orthogonal methods (flow cytometry + immunoprecipitation), single lab\",\n      \"pmids\": [\"1703182\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"CD21 (CR2) is a ligand for CD23 (FcεRII); fluorescent CD23-liposomes specifically bind CD21 on B cells and CD21-transfected BHK-21 cells, and triggering CD21 with anti-CD21 antibody or soluble CD23 specifically increases IL-4-induced IgE production.\",\n      \"method\": \"Fluorescent liposome binding assay, CD21 cDNA transfection into BHK-21 cells, anti-CD21/anti-CD23 blocking antibodies, Western blot, IgE production assay from blood mononuclear cells\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution in transfected cells, multiple blocking controls, functional IgE production readout\",\n      \"pmids\": [\"1386409\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"Within the CD21/CD19/TAPA-1 complex, CD19 and TAPA-1 interact through their extracellular domains, CD19 and CD21 interact through extracellular and transmembrane domains, and CD21 and CD35 interact through extracellular domains; CD21 is required for TAPA-1-dependent homotypic cellular aggregation but not for CD19-mediated PI3-kinase recruitment or calcium signaling.\",\n      \"method\": \"Chimeric molecule construction (HLA-A2/CD4 domain swaps), stable transfection in Daudi cells, functional assays for calcium flux, PI3-kinase association, homotypic aggregation\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — domain mapping via chimeric proteins with multiple functional readouts\",\n      \"pmids\": [\"7690834\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CD21/CD19/TAPA-1 complex amplifies signaling through membrane immunoglobulin and recruits PI3-kinase; CD19 is the signaling subunit coupling to Lyn and PI3-kinase, while CR2 enables nonimmunologic ligation of CD19 via C3 fragments.\",\n      \"method\": \"Review synthesizing biochemical and genetic data (functional complex characterization)\",\n      \"journal\": \"Current opinion in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — review of established biochemical findings, no new primary experiment\",\n      \"pmids\": [\"7688513\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"CD21 ligands binding to the C3dg/EBV-binding site of CR2 markedly augment B cell activation initiated by antigen receptor ligation via a tyrosine kinase-dependent, c-fos-selective pathway; beads coated with anti-CD21 mAb to the C3dg-binding portion trigger homotypic B cell aggregation.\",\n      \"method\": \"Latex bead coated with antibodies/ligands, c-fos and c-myc mRNA quantitation, homotypic aggregation assay, tyrosine kinase inhibitor studies with purified small resting B cells\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — defined signaling pathway with epitope-mapped mAbs and kinase inhibitor, single lab\",\n      \"pmids\": [\"7525704\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"CR2 binds CD23 at two main epitope regions: SCRs 1-2 and SCRs 5-8, with Asn370 and Asn295 in the SCR 5-8 region being critical for the interaction with the lectin CD23.\",\n      \"method\": \"Epitope mapping with anti-CD23 antibodies, mutagenesis of CD21 residues\",\n      \"journal\": \"International archives of allergy and immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — binding site defined but abstract lacks full methodological detail, single lab\",\n      \"pmids\": [\"7542093\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"CD21 expressed on myeloma cells mediates adhesion to bone marrow stromal cells via CD23 on stromal cells; antibody blocking of either CD21 or CD23 inhibits this adhesion.\",\n      \"method\": \"Antibody-blocking adhesion assay, flow cytometry for CD21/CD23 expression, myeloma cell lines and primary stromal cells\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal antibody blocking (anti-CD21 and anti-CD23) with functional adhesion readout\",\n      \"pmids\": [\"7780154\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"CR2 (CD21) gene expression is regulated by a B cell- and stage-specific intronic silencer: the 2.5-kb first intron segment silences promoter activity specifically in non-CR2-expressing cells, and this silencer requires nuclear matrix/chromatin interactions in stable (but not transient) transfection.\",\n      \"method\": \"Transient and stable transfection with reporter gene constructs, transgenic mice\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — functional dissection by reporter constructs in cell lines and confirmed in transgenic mice\",\n      \"pmids\": [\"9570543\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"C3dg binding to CR2 occurs in the recess formed between SCR1 and SCR2; a mAb (FE8) recognizing a discontinuous epitope spanning sequences at the SCR1-SCR2 junction blocks C3dg binding and EBV infection.\",\n      \"method\": \"Epitope mapping with overlapping peptides on cellulose, mAb inhibition of C3dg binding and EBV infection, 3D homology modeling\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional blocking mAb with peptide epitope mapping, 3D model, single lab\",\n      \"pmids\": [\"9794388\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Low-level surface CD21 expression on epithelial 293 cells supports EBV binding and stable infection; both virus binding and infection are blocked by anti-CD21 antibodies, and CD35 and CD19 are not required.\",\n      \"method\": \"Anti-CD21 antibody blocking of EBV binding and infection, flow cytometry, EBV infection of 293 cells with selectable marker\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — antibody blocking with defined genetic absence of CD35/CD19, single lab\",\n      \"pmids\": [\"9971794\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"CR2 binding to gp350 follows a simple 1:1 kinetics, whereas binding to C3d and iC3b is more complex and involves more than one intramolecular component; iC3b binds CR2 with different kinetics depending on whether it is soluble or surface-immobilized, suggesting an additional binding site in the C3c region of iC3b.\",\n      \"method\": \"Surface plasmon resonance (SPR) kinetic analysis of CR2 interactions with C3d, iC3b, and gp350/220\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — quantitative SPR kinetics with multiple ligand forms, single lab, rigorous biophysical method\",\n      \"pmids\": [\"11466369\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The SCR 1-2 domains of CR2 account for the primary C3dg binding site; the additional SCR domains of full-length CR2 slow both the association and dissociation rates. The SCR1-2:C3d interaction forms a 1:1 complex and is influenced by ionic (charge-dependent) interactions, with the sole His residue near the SCR1-SCR2 linker potentially affecting ligand association.\",\n      \"method\": \"Recombinant CR2 SCR1-2 and SCR1-15 expression in Pichia pastoris/baculovirus, circular dichroism, surface plasmon resonance, sedimentation equilibrium ultracentrifugation, homology modeling\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple biophysical methods (SPR, CD, sedimentation), recombinant protein, single lab with orthogonal approaches\",\n      \"pmids\": [\"11352728\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"An inhibitory anti-CR2 mAb (171) directly contacts the C3d binding site on CR2 SCR1-2 as confirmed by placement of its linear epitope on the CR2-C3d crystal structure, providing solution-phase confirmation of the crystallographic binding interface.\",\n      \"method\": \"Anti-CR2 mAb generation in Cr2-/- mice, overlapping peptide epitope mapping, CR2-C3d crystal structure-based analysis, ligand binding inhibition assays\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure corroborated by mAb epitope mapping and ligand inhibition assays\",\n      \"pmids\": [\"11698449\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"CD21 (CR2) activation on B lymphocyte surface triggers tyrosine phosphorylation of nucleolin (p95) and its interaction with PI3-kinase p85 subunit, SH2 domains of 3BP2 and Grb2 (but not Fyn or Gap), within 2 minutes; this signaling is specific to CR2 activation and not induced by CD19 or BCR activation.\",\n      \"method\": \"Anti-phosphotyrosine affinity purification, mass spectrometry amino acid analysis, anti-nucleolin antibody co-precipitation, SH2 domain binding assay\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — protein identification by mass spectrometry and functional binding assays with multiple SH2 domain controls, single lab\",\n      \"pmids\": [\"11207269\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Binding of complement-tagged antigens causes co-translocation of both BCR and CD19/CD21 complex into plasma membrane lipid rafts, prolonging BCR residency and signaling in rafts compared to BCR cross-linking alone; CD19/CD21 co-ligation also retards BCR internalization and degradation.\",\n      \"method\": \"Lipid raft fractionation, co-localization assays, BCR internalization and degradation kinetics, complement-tagged antigen stimulation\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — lipid raft fractionation with BCR signaling kinetics, multiple orthogonal readouts\",\n      \"pmids\": [\"11239449\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"A site within the CR2 intronic silencer contains a CBF1 (RBP-J/RBP-Jκ) binding site; a 2-bp mutation eliminating CBF1 binding abolishes silencer activity in vivo, demonstrating that CBF1 (a Notch signaling component) controls CR2 lineage-specific expression.\",\n      \"method\": \"Site-directed mutagenesis of silencer, stable transfection reporter assay, EMSA (in vitro binding)\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis with in vivo functional reporter readout and in vitro binding confirmation\",\n      \"pmids\": [\"11312253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Murine CD21 gene expression requires both promoter sequences and a 5'-proximal intronic element; the first intron contains an 800-bp 5' inhibitory element active in both B and T cells and an 800-bp 3' element that is inhibitory in T cells but acts as an enhancer in B cells.\",\n      \"method\": \"Transient transfection with luciferase reporter constructs, minigene analysis\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional dissection of regulatory elements by reporter assay, single lab\",\n      \"pmids\": [\"9144490\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"CR2 (CD21) expression is regulated by DNA methylation of the CpG island near the ATG start codon and by histone deacetylation; 5-aza-2'-deoxycytidine (DNA methyltransferase inhibitor) and trichostatin A (histone deacetylase inhibitor) both induce CD21 expression in early B lymphocytes that normally lack it.\",\n      \"method\": \"Methylation analysis of CpG island in cell lines, 5-aza-2'-deoxycytidine treatment, trichostatin A treatment\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological epigenetic manipulation with defined readout, single lab\",\n      \"pmids\": [\"11312258\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Soluble CD21 (sCD21) shed into human plasma is predominantly a short form lacking exon-11-encoded sequences; shedding involves proteolytic cleavage of only the extracellular portion (C terminus truncated), is induced by PMA+calcium ionophore or BCR stimulation (anti-IgM+anti-CD40), and peripheral blood B cells (not T cells) are the primary source.\",\n      \"method\": \"Mass spectrometry of purified plasma sCD21, N-terminal sequencing, flow cytometry after stimulation, B/T cell separation experiments\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mass spectrometry identification with cell-source determination and stimulus-defined shedding, single lab\",\n      \"pmids\": [\"12938215\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The NMR structure of the C-type lectin domain of CD23 identifies distinct binding sites for IgE and CD21; CD23 can bind both ligands simultaneously and does not require calcium for either interaction. IgE and CD23 can form high molecular mass multimeric complexes.\",\n      \"method\": \"NMR solution structure determination, concentration-dependent chemical shift perturbation analysis for binding site mapping, molecular weight analysis\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure with ligand-binding site mapping by chemical shift perturbations, defines molecular basis of CD21-CD23 interaction\",\n      \"pmids\": [\"16172256\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"CD21/CD19 coreceptor signaling promotes B cell survival in primary immune responses via upregulation of c-FLIP, protecting against CD95-mediated apoptosis; Cr2-/- B cells show reduced c-FLIP levels and fail to persist in follicles, rescued by the lpr mutation.\",\n      \"method\": \"Adoptive transfer of Cr2-/- and Cr2+/+ Ig-transgenic B cells, c-FLIP mRNA and protein quantitation, in vitro HEL-C3d3 stimulation, CD95-mediated apoptosis assay\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis (Cr2-/- x lpr double mutant), in vivo adoptive transfer, and in vitro mechanistic follow-up\",\n      \"pmids\": [\"16116172\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"CD21 shedding is induced by P2X7 receptor activation (via BzATP, a P2X7R agonist) on peripheral B cells, in addition to BCR stimulation; P2X7R-mediated shedding was confirmed using receptor inhibitors.\",\n      \"method\": \"BzATP stimulation of peripheral blood lymphocytes, P2X7R inhibitor experiments, flow cytometry for surface CD21\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological receptor activation and inhibition with defined shedding readout, single lab\",\n      \"pmids\": [\"16740600\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The gp350 binding site on CR2 involves a large contiguous surface on SCR1-2 dominated by positively charged residues (Arg-13, Arg-28, Arg-36, Lys-41, Lys-57, Lys-67, Arg-83, and within SCR2 via mAbs 171/1048); both SCR1 and SCR2 make contact with gp350.\",\n      \"method\": \"Site-directed mutagenesis of CR2 SCR1-2, expression on K562 cells, gp350 binding by flow cytometry, anti-CR2 mAb inhibition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic site-directed mutagenesis with cell-surface expression and functional binding assay\",\n      \"pmids\": [\"17925391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Stromal (not hematopoietic) CD21/35 on follicular dendritic cells is required for splenic retention of prion inocula and lymphoid prion colonization; bone marrow chimera experiments showed that protection from prion infection in CD21/35-/- mice is due to loss of stromal CD21/35.\",\n      \"method\": \"Reciprocal bone marrow adoptive transfer between WT and CD21/35-/- mice, prion challenge, infectivity titration in spleens\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal bone marrow transfer establishes cell-type specific requirement, replicated with double KO controls\",\n      \"pmids\": [\"17947689\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The CR2 binding site on EBV gp350 is defined by a negatively charged surface (Glu-21, Asp-22, Glu-155, Asp-208, Glu-210, Asp-296) and hydrophobic contacts (Tyr-151, Ile-160, Trp-162); the complementary positively charged residues on CR2 SCR1-2 (Arg-13, Arg-28, Arg-36, Lys-41, Lys-57, Lys-67, Arg-83, Arg-89) mediate the charge-dependent interaction.\",\n      \"method\": \"ELISA with recombinant wild-type and mutant gp350 and CR2 SCR1-2, K562 cell binding assay with PE-conjugated gp350, HADDOCK docking model\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic mutagenesis of both binding partners with functional cell-surface binding assay and structural docking model\",\n      \"pmids\": [\"18786993\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Uncoupling CD21 from CD19 (via knockin of mutant Cr2 that binds C3 but does not signal through CD19) significantly diminishes germinal center B cell survival and secondary antibody titers, but B memory is less impaired than in complete CR deficiency, identifying a CD19-independent role for CR in B cell memory.\",\n      \"method\": \"Knockin mice expressing Cr2-GFP fusion that binds C3 ligands but fails to signal through CD19, immunization, antibody titer measurement, GC B cell flow cytometry\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — precise genetic uncoupling (knockin), in vivo functional immune readouts, compared with complete KO\",\n      \"pmids\": [\"19706534\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The CR2-binding site on C3d maps to the concave face acidic pocket; mutations to concave face residues significantly impair both CR2 and Staphylococcal Sbi-IV binding, while mutations to the sideface region implicated by the cocrystal structure show no binding defects, casting doubt on the physiologic relevance of the CR2-C3d cocrystal interface.\",\n      \"method\": \"Surface plasmon resonance with panel of C3dg variant proteins against biosensor-bound CR2 SCR1-2 or Sbi-IV\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic mutagenesis with quantitative SPR, challenges prior structural data\",\n      \"pmids\": [\"20083651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"CR2 binds multiple forms of DNA (bacterial, viral, mammalian) with moderately high affinity; methylated DNA binds with high affinity (Kd ~6 nM) via the first two SCR domains; binding is blocked by an inhibitory anti-CR2 mAb; Cr2-/- mice show specific immune response defects to bacterial DNA.\",\n      \"method\": \"Surface plasmon resonance, anti-CR2 mAb blocking, truncated SCR domain constructs, immunization of Cr2-/- mice\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — quantitative SPR with domain mapping and in vivo functional confirmation in Cr2-/- mice\",\n      \"pmids\": [\"22885687\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"DOCK2 deficiency reduces CD21 expression at both mRNA and protein levels through a mechanism involving upregulation of lymphoid enhancer-binding factor 1 (LEF1), leading to downregulation of CD19-mediated BCR signaling and reduced marginal zone B cells.\",\n      \"method\": \"Genetic models (DOCK2-/-, WASP-/-, LEF1-deficient mice), flow cytometry, mRNA/protein expression analysis, BCR signaling assays\",\n      \"journal\": \"The Journal of allergy and clinical immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple genetic models with defined mechanistic pathway, single lab\",\n      \"pmids\": [\"31405607\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CD21 is required for EBV type 2 entry into T cells; viral gp350 and CD21 are both necessary for CD3+ T-cell infection, and CRISPR knockout of CD21 in Jurkat T cells abolishes EBV entry.\",\n      \"method\": \"Neutralizing antibody assay, CRISPR-Cas9 knockout of CD21 in Jurkat cells, flow cytometry for CD21 expression on T cell subsets, ex vivo infection model\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — CRISPR loss-of-function combined with neutralizing antibody blocking and primary T cell infection model\",\n      \"pmids\": [\"32238579\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Co-engagement of CR2 (CD21) and the BCR on primary human B cells inhibits activation marker expression, cytokine production, proliferation, and antibody production at non-stimulatory concentrations of anti-Ig, while enhancing Ca2+ response; this differs from the positive coreceptor role established in mice.\",\n      \"method\": \"Co-clustering of BCR and CR2 on primary human B cells, Ca2+ flux assay, activation marker expression by flow cytometry, cytokine/antibody production measurement\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — primary human cell functional assays with multiple readouts, single lab, contradicts established mouse data\",\n      \"pmids\": [\"33868238\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"CD21 and FCRL5 physically associate on B cells; triple engagement of FCRL5, CD21, and BCR produces superior calcium response compared to CD21+BCR co-stimulation alone; CD21 co-engagement converts FCRL5 from a negative to a positive co-receptor, recruiting CD19, active PLCγ2, and BTK to FCRL5.\",\n      \"method\": \"Co-immunoprecipitation, calcium flux assay in cell lines and tonsil B cells, FCRL5 transfection with/without CD21\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP and functional signaling assays, single lab\",\n      \"pmids\": [\"30107486\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CR2 (CD21) is a multi-ligand complement receptor that binds C3d/iC3b, EBV gp350, CD23, and DNA through its N-terminal SCR1-2 domains; it serves as the EBV receptor on B and T cells, forms a coreceptor complex with CD19/TAPA-1/CD81 that co-translocates with the BCR into lipid rafts to modulate signaling thresholds (enhancing Ca2+ flux but with species-specific effects on proliferation and antibody production), promotes B cell survival via c-FLIP upregulation, facilitates prion retention on follicular dendritic cells, sheds its extracellular domain via protease activity induced by BCR stimulation or P2X7R activation, and is transcriptionally regulated by an intronic silencer requiring CBF1/Notch signaling, DNA methylation, and histone deacetylation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CR2 (CD21) is a B-lymphocyte multi-ligand complement receptor that links innate complement opsonization to adaptive humoral immunity by lowering the threshold for B cell activation [#4, #21]. Its N-terminal SCR1-2 domains constitute the primary ligand-binding module, engaging C3d/iC3b complement fragments through a charge-dependent interface in the recess between SCR1 and SCR2 [#15, #18, #33], while a complementary positively charged SCR1-2 surface binds the negatively charged Epstein-Barr virus glycoprotein gp350 [#29, #31]; the same domains also bind DNA, including methylated DNA with high affinity [#34]. CR2 is the EBV receptor: it is sufficient to confer EBV binding and infection on heterologous cells, soluble CR2 neutralizes infection, and CRISPR deletion abolishes EBV type 2 entry into T cells [#0, #4, #5, #36]. CR2 also possesses Factor I cofactor activity toward membrane iC3b, and binds CD23 through SCR1-2 and SCR5-8 epitopes, mediating B cell and stromal cell adhesion and modulating IgE production [#1, #8, #13, #26]. Functionally, CR2 partitions into a coreceptor complex with CD19 and TAPA-1/CD81 in which CD19 serves as the signaling subunit; co-ligation of CR2-bound complement-tagged antigen with the BCR drives co-translocation into lipid rafts, prolongs BCR signaling, recruits PI3-kinase and tyrosine-phosphorylated nucleolin, and promotes germinal-center B cell survival via c-FLIP upregulation [#9, #20, #21, #27, #32]. CR2 is transcriptionally restricted to its expressing lineage by an intronic silencer requiring a CBF1/RBP-J (Notch) site, DNA methylation, and histone deacetylation [#14, #22, #24], and its extracellular domain is shed by stimulus-induced proteolysis following BCR or P2X7R activation [#25, #28]. The coreceptor outcome is species-dependent: co-engagement enhances B cell activation in mice but is inhibitory in primary human B cells despite enhancing Ca2+ flux [#37].\",\n  \"teleology\": [\n    {\n      \"year\": 1984,\n      \"claim\": \"Established the founding identity of CR2 by showing it is the cellular receptor for Epstein-Barr virus, defining how a complement receptor doubles as a viral entry portal.\",\n      \"evidence\": \"Anti-CR2 mAb blocking, receptor transfer to S. aureus particles, and radiolabeled EBV binding on human B lymphocytes\",\n      \"pmids\": [\"6087328\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve whether CR2 alone is sufficient or requires accessory molecules\", \"Binding site on CR2 for EBV not mapped\"]\n    },\n    {\n      \"year\": 1987,\n      \"claim\": \"Demonstrated CR2 acts as a complement regulatory protein with Factor I cofactor activity, placing it functionally alongside CR1.\",\n      \"evidence\": \"In vitro cofactor assay with purified CR2 and Factor I on iC3b-bearing erythrocytes with mAb inhibition\",\n      \"pmids\": [\"2437238\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiologic significance of cofactor activity in vivo not established\", \"Does not address signaling role\"]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Resolved that CR2 carries separable binding sites for EBV and C3d, indicating multi-ligand recognition is built into distinct epitopes rather than a shared surface.\",\n      \"evidence\": \"Reciprocal mAb blocking and anti-idiotypic antibodies\",\n      \"pmids\": [\"2842398\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Sites localized only to functional epitopes, not to specific domains/residues\", \"Single lab\"]\n    },\n    {\n      \"year\": 1989,\n      \"claim\": \"Proved CR2 alone is sufficient to transfer both complement-receptor and EBV-receptor function, settling whether accessory proteins are required for either activity.\",\n      \"evidence\": \"Stable cDNA transfection into CR2-negative L cells and K562 with rosette and EBV infection readouts\",\n      \"pmids\": [\"2473114\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Heterologous cells may lack downstream signaling partners present in B cells\", \"Does not address coreceptor amplification\"]\n    },\n    {\n      \"year\": 1990,\n      \"claim\": \"Clarified that CR2 binding initiates productive EBV infection rather than only mediating attachment, and defined murine gene architecture relevant to ortholog studies.\",\n      \"evidence\": \"Soluble recombinant CR2 infection-inhibition assay; cDNA/genomic cloning of murine Cr2\",\n      \"pmids\": [\"2154612\", \"2139460\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Post-binding entry steps not defined\", \"Single-method infection inhibition\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Extended CR2 expression beyond B cells to a subset of T lymphocytes, raising the possibility of CR2 function in T cell biology.\",\n      \"evidence\": \"Immunofluorescence and immunoprecipitation of 145 kDa CR2 from purified peripheral T cells\",\n      \"pmids\": [\"1703182\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence in T cells not demonstrated at this stage\", \"Single lab\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Identified CD23 (FcεRII) as a CR2 ligand and linked CR2 engagement to IL-4-induced IgE production, connecting the receptor to allergic/antibody-class regulation.\",\n      \"evidence\": \"CD23-liposome binding, CD21 transfection into BHK-21, blocking antibodies, IgE production assay\",\n      \"pmids\": [\"1386409\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiologic relevance of CD23-CR2 axis in vivo not established\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Defined the molecular architecture and signaling division of labor of the CR2/CD19/TAPA-1 coreceptor complex, establishing CR2 as the ligand-sensing subunit and CD19 as the signal transducer that amplifies BCR signaling.\",\n      \"evidence\": \"Chimeric domain-swap constructs in Daudi cells with calcium, PI3-kinase, and aggregation readouts; synthesizing review\",\n      \"pmids\": [\"7690834\", \"7688513\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Exact stoichiometry of the complex not resolved\", \"Quantitative contribution of CR2 to signal amplification not isolated\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Showed CR2 ligation through the C3dg/EBV site augments antigen-receptor-driven B cell activation via a tyrosine-kinase-dependent, c-fos-selective pathway, linking ligand binding to defined transcriptional and adhesive responses.\",\n      \"evidence\": \"Bead-coated ligand stimulation of resting B cells with c-fos/c-myc mRNA quantitation, aggregation, kinase inhibitors\",\n      \"pmids\": [\"7525704\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Intermediate signaling components between CR2 and c-fos not identified\", \"Single lab\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Mapped CD23 binding to two CR2 regions (SCR1-2 and SCR5-8) and connected CR2-CD23 interaction to cell adhesion, including myeloma-stroma interactions.\",\n      \"evidence\": \"Epitope mapping/mutagenesis of CD21 residues; antibody-blocking adhesion assays with myeloma and stromal cells\",\n      \"pmids\": [\"7542093\", \"7780154\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Methodological detail limited in epitope study\", \"In vivo relevance of myeloma adhesion not tested\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Localized the C3dg binding site to the SCR1-SCR2 junction recess and defined an intronic silencer governing lineage-restricted CR2 expression, addressing both ligand recognition and transcriptional control.\",\n      \"evidence\": \"Peptide epitope mapping with blocking mAb FE8 and homology modeling; reporter transfection and transgenic mice for silencer\",\n      \"pmids\": [\"9794388\", \"9570543\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trans-acting factors binding the silencer not yet identified at this stage\", \"Atomic-resolution structure of the binding interface not available\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Demonstrated low-level epithelial CR2 supports EBV binding and infection independently of CD35 and CD19, showing CR2 functions as an autonomous entry receptor across cell types.\",\n      \"evidence\": \"Anti-CD21 blocking of EBV binding/infection in 293 cells lacking CD35/CD19\",\n      \"pmids\": [\"9971794\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Efficiency relative to B cell infection not benchmarked\", \"Single lab\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defined the biophysical and signaling basis of CR2 function: SCR1-2 as the primary charge-dependent C3d-binding module, distinct ligand kinetics, a CR2-specific nucleolin/PI3-kinase signaling branch, lipid-raft co-translocation prolonging BCR signaling, and CBF1/Notch- and epigenetically-controlled lineage-specific transcription.\",\n      \"evidence\": \"SPR/CD/ultracentrifugation on recombinant SCR constructs; mass-spec identification of phospho-nucleolin and SH2 binding; lipid raft fractionation with BCR kinetics; silencer mutagenesis/EMSA and methylation/HDAC pharmacology; crystal-structure-corroborated mAb mapping\",\n      \"pmids\": [\"11466369\", \"11352728\", \"11207269\", \"11239449\", \"11312253\", \"11312258\", \"9144490\", \"11698449\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiologic CR2-C3d interface later disputed\", \"How nucleolin signaling integrates with CD19 amplification not resolved\", \"Stoichiometry of raft co-translocation not quantified\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Characterized stimulus-induced proteolytic shedding of the CR2 ectodomain from B cells, identifying a regulated mechanism that generates soluble CD21 and removes surface receptor after activation.\",\n      \"evidence\": \"Mass spectrometry and N-terminal sequencing of plasma sCD21; stimulation and B/T cell source determination\",\n      \"pmids\": [\"12938215\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Responsible protease not identified\", \"Functional role of soluble CD21 not established\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Established a survival function for CR2 coreceptor signaling—c-FLIP upregulation protecting germinal-center B cells from CD95 apoptosis—and defined the molecular basis of the CD21-CD23 interaction structurally.\",\n      \"evidence\": \"Adoptive transfer of Cr2-/- Ig-transgenic B cells with Cr2 x lpr epistasis and c-FLIP/apoptosis assays; NMR structure of the CD23 C-type lectin domain\",\n      \"pmids\": [\"16116172\", \"16172256\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling link between CR2/CD19 and c-FLIP transcription not fully mapped\", \"Whether CD23-CR2 contributes to survival not tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Defined the gp350 binding surface on CR2 SCR1-2 as a positively charged contiguous patch and demonstrated that stromal (not hematopoietic) CR2/35 on follicular dendritic cells mediates prion retention, expanding CR2's role to antigen/agent trapping.\",\n      \"evidence\": \"Systematic SCR1-2 mutagenesis with gp350 binding on K562; reciprocal bone marrow chimeras with prion challenge\",\n      \"pmids\": [\"17925391\", \"17947689\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of FDC retention of prions at molecular level not defined\", \"Whether DNA/complement binding contributes to prion trapping unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Resolved the complementary electrostatic chemistry of the CR2-gp350 interface, explaining how charge pairing dictates EBV receptor recognition.\",\n      \"evidence\": \"Mutagenesis of both gp350 and CR2 SCR1-2 with cell-surface binding and HADDOCK docking\",\n      \"pmids\": [\"18786993\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No co-crystal structure of the CR2-gp350 complex\", \"Affinity contribution of individual residues not ranked\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Genetically uncoupled CR2 ligand binding from CD19 signaling, revealing a CD19-independent contribution of CR2 to B cell memory while confirming CD19 dependence for germinal-center survival and antibody titers.\",\n      \"evidence\": \"Knockin Cr2-GFP mice that bind C3 but cannot signal through CD19, with immunization and GC/antibody readouts versus complete KO\",\n      \"pmids\": [\"19706534\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of the CD19-independent memory function not identified\", \"Whether non-CD19 partners mediate this role unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Re-mapped the functional CR2-binding site on C3d to the concave-face acidic pocket, challenging the physiologic relevance of the previously reported cocrystal interface.\",\n      \"evidence\": \"SPR with a panel of C3dg variants against CR2 SCR1-2 and Sbi-IV\",\n      \"pmids\": [\"20083651\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Discrepancy with crystallographic interface not fully reconciled\", \"Conformational state of C3d governing binding not resolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Identified DNA, especially methylated DNA, as a high-affinity CR2 ligand via SCR1-2, with in vivo immune defects to bacterial DNA, broadening CR2 ligand repertoire beyond complement and viral proteins.\",\n      \"evidence\": \"SPR with multiple DNA forms, truncated SCR constructs, blocking mAb, and immunization of Cr2-/- mice\",\n      \"pmids\": [\"22885687\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which DNA binding shapes immune responses not defined\", \"Relationship to complement/EBV binding sites not fully delineated\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showed CR2 physically associates with FCRL5 and converts it from a negative to a positive coreceptor, illustrating that CR2 can rewire the signaling output of partner receptors.\",\n      \"evidence\": \"Co-immunoprecipitation and calcium flux assays in cell lines and tonsil B cells with FCRL5 transfection\",\n      \"pmids\": [\"30107486\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Reciprocal validation and stoichiometry of CR2-FCRL5 association limited\", \"In vivo relevance not tested\", \"Single lab\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Linked upstream cytoskeletal regulator DOCK2 to CR2 expression via LEF1, connecting B cell developmental signaling to CR2/CD19 coreceptor abundance.\",\n      \"evidence\": \"Genetic mouse models (DOCK2-/-, LEF1-deficient) with expression and BCR signaling assays\",\n      \"pmids\": [\"31405607\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct LEF1 action on CR2 locus not demonstrated\", \"Single lab\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Established CR2 as the required receptor for EBV type 2 entry into T cells, extending its viral-entry function to the T lineage.\",\n      \"evidence\": \"CRISPR knockout of CD21 in Jurkat cells, neutralizing antibodies, and primary T cell infection\",\n      \"pmids\": [\"32238579\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether T cell entry uses the same gp350 interface as B cells not confirmed\", \"Co-receptor requirements in T cells not defined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Revealed a species divergence in CR2 coreceptor function—co-engagement inhibits human B cell activation despite enhancing Ca2+ flux, contrasting with the positive role established in mice.\",\n      \"evidence\": \"Co-clustering of BCR and CR2 on primary human B cells with Ca2+, activation, cytokine, and antibody readouts\",\n      \"pmids\": [\"33868238\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis for the human-specific inhibitory outcome not defined\", \"Single lab; contradicts mouse data\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CR2's distinct ligand-binding activities (complement, EBV gp350, CD23, DNA) are integrated to set coreceptor signaling outcomes, and why those outcomes diverge between mouse and human B cells, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified structural model reconciling the disputed C3d interface\", \"Species-specific signaling mechanism unexplained\", \"Protease responsible for ectodomain shedding unidentified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [0, 4, 5, 16, 36]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [9, 21, 27]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [34]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [8, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 7, 21]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [21, 27, 32]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 30, 36]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [9, 20, 21]}\n    ],\n    \"complexes\": [\"CD21/CD19/TAPA-1 (CD81) coreceptor complex\"],\n    \"partners\": [\"CD19\", \"CD81\", \"CD23\", \"CD35\", \"FCRL5\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}