{"gene":"CD22","run_date":"2026-04-28T17:28:52","timeline":{"discoveries":[{"year":1990,"finding":"CD22 mediates monocyte and erythrocyte adhesion and has a structure closely related to myelin-associated glycoprotein (MAG), a neuronal adhesion protein, identifying it as a B-cell adhesion receptor.","method":"Transfection and adhesion assays","journal":"Nature","confidence":"Medium","confidence_rationale":"Tier 2 — direct functional assay in transfected cells, single lab","pmids":["1691828"],"is_preprint":false},{"year":1993,"finding":"CD22 is a sialic acid-binding lectin that recognizes sialic acids in α2,6 linkage on N-linked oligosaccharides of multiple sialoglycoproteins including CD45, establishing CD22 as a member of a novel subset of Ig superfamily adhesion molecules.","method":"CD22-Ig fusion protein binding assays, periodate oxidation, sialyltransferase transfection in COS cells","journal":"The Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1 — direct in vitro binding assay with enzymatic and biochemical validation, replicated across multiple ligands","pmids":["8463234"],"is_preprint":false},{"year":1993,"finding":"CD22 associates specifically with the B cell antigen receptor (BCR) complex including IgM, IgD, and IgG isotypes, in a low stoichiometry but stable association, and is rapidly tyrosine phosphorylated after antigen receptor cross-linking.","method":"In vitro kinase assay, co-immunoprecipitation from Burkitt lymphoma lines and tonsil cells","journal":"European Journal of Immunology","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal co-IP across multiple isotypes and cell types, single lab","pmids":["7684686"],"is_preprint":false},{"year":1993,"finding":"CD22 co-immunoprecipitates with the sIgM-BCR complex along with Igα/mb-1, Igβ/B29, and PTK72, and becomes rapidly and strikingly tyrosine phosphorylated after sIgM cross-linking; CD22 contains the ARHI (antigen recognition homology I) motif present in other antigen receptor molecules.","method":"Co-immunoprecipitation in digitonin, immunoblot, in vitro kinase assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — co-IP with multiple components of BCR complex, single lab","pmids":["8475064"],"is_preprint":false},{"year":1992,"finding":"CD22 directly interacts with multiple isoforms of CD45, and cross-linking of CD3 and CD22 ligands blocks anti-CD3-induced intracellular calcium increase and inhibits tyrosine phosphorylation of phospholipase Cγ1 in T cells, suggesting CD22-CD45 engagement modulates lymphocyte signaling.","method":"Direct binding assay with soluble CD22, calcium flux measurement, tyrosine phosphorylation assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding and functional signaling assays, single lab","pmids":["1438211"],"is_preprint":false},{"year":1995,"finding":"Tyrosine-phosphorylated CD22 recruits the tyrosine phosphatase SHP-1 (PTP1C) in a phosphorylation-dependent manner after BCR cross-linking, identifying SHP-1 as a downstream effector of CD22 signaling.","method":"Co-immunoprecipitation after anti-Ig stimulation, identification of 60-kDa associated protein as PTP1C","journal":"European Journal of Immunology","confidence":"High","confidence_rationale":"Tier 2 — phosphorylation-dependent co-IP with biochemical identification, replicated in subsequent studies","pmids":["7542197"],"is_preprint":false},{"year":1995,"finding":"CD22 is constitutively internalized by unstimulated B cells and degraded in an acidic intracellular compartment (lysosomes) without detectable recycling to the cell surface; anti-CD22 mAb ligation markedly increases internalization rate.","method":"Flow cytometry, neuraminidase protection and neuraminidase shift assays","journal":"Journal of Immunology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple functional assays demonstrating constitutive endocytosis, single lab","pmids":["7722303"],"is_preprint":false},{"year":1995,"finding":"Engagement of CD45 by soluble CD22 modulates early T cell signals in antigen receptor/CD3-mediated stimulation; addition of α2,6-sialic acid to the CD22 molecule itself abrogates CD22-ligand interactions.","method":"Soluble CD22-Ig fusion protein, T cell calcium flux, chimeric CD45 constructs, sialyltransferase treatment","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — direct functional binding with enzymatic modification controls, single lab","pmids":["7537381"],"is_preprint":false},{"year":1996,"finding":"CD22-deficient mice show hyperresponsive B cells with heightened calcium fluxes and cell proliferation at lower ligand concentrations, an expanded peritoneal B-1 cell population, and increased serum autoantibody titers, establishing CD22 as a negative regulator of antigen receptor signaling.","method":"Gene-targeted knockout mice, calcium flux measurement, proliferation assays, serum antibody titers","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple orthogonal phenotypic readouts, replicated by independent labs","pmids":["8864124"],"is_preprint":false},{"year":1997,"finding":"CD22-deficient mice show increased Ca2+ influx and lower survival of B cells upon BCR cross-linking, impaired T-cell independent immune responses, and absence of recirculating B cells from bone marrow, confirming CD22 as a negative regulator that sets the BCR signaling threshold.","method":"Targeted gene inactivation (knockout mice), calcium flux, B cell subset analysis, immune response assays","journal":"Current Biology","confidence":"High","confidence_rationale":"Tier 2 — independent KO study replicating and extending prior findings with multiple orthogonal methods","pmids":["9016707"],"is_preprint":false},{"year":1997,"finding":"CD19 and CD22 have counterregulatory effects on MAP kinase (ERK2, JNK, p38) activation by membrane immunoglobulin: CD19 co-ligation synergistically amplifies MAP kinase activation, while CD22 cross-linking to mIg suppresses it; sequestration of CD22 from mIg enhances MAP kinase activation.","method":"Co-ligation experiments, MAP kinase activation assays (ERK2, JNK, p38)","journal":"Immunity","confidence":"Medium","confidence_rationale":"Tier 2 — direct functional epistasis using co-ligation, single lab","pmids":["9252120"],"is_preprint":false},{"year":1999,"finding":"Multiple tyrosine residues within the cytoplasmic domain of CD22 interact with both SH2 domains of SHP-1; a minimum of two CD22 tyrosines are required for SHP-1 association, and both SH2 domains of SHP-1 are necessary for efficient binding.","method":"Phosphopeptide binding assays, mutational analysis of CD22 cytoplasmic domain tyrosines, SHP-1 domain analysis","journal":"The Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro phosphopeptide assay combined with mutagenesis, defining molecular requirements for interaction","pmids":["9890995"],"is_preprint":false},{"year":1999,"finding":"CD22 deficiency leads to development of high-affinity, somatically mutated, multiclonal IgG anti-dsDNA autoantibodies in aging mice, demonstrating that a single B-cell-specific gene defect is sufficient to trigger autoantibody development.","method":"CD22-deficient mice, serum antibody ELISA, affinity measurement, somatic mutation analysis","journal":"The Journal of Experimental Medicine","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple antibody characterization methods, mechanistically links CD22 to autoimmunity","pmids":["10209047"],"is_preprint":false},{"year":1999,"finding":"CD22 ligation activates JNK/SAPK but not ERK2 signaling independently of BCR, and induces downregulation of Bcl-xL and Mcl-1, providing a mechanism for CD22-mediated apoptosis in B cells.","method":"Kinase activity assays (ERK2, JNK), AP-1/c-jun nuclear extract analysis, Bcl-2 family protein immunoblotting, B cell apoptosis assays","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 — multiple kinase and apoptosis assays, single lab","pmids":["10438726"],"is_preprint":false},{"year":1999,"finding":"CD45 positively regulates CD22 tyrosine phosphorylation; cross-linking of CD45 causes physical sequestration from CD22 leading to increased CD22 tyrosine phosphorylation and enhanced SHP-1 recruitment; CD45-deficient B cells show elevated basal CD22 phosphorylation and SHP-1 association.","method":"Co-immunoprecipitation, calcium flux assays, CD45-deficient B cell line, expression of catalytically inactive SHP-1","journal":"Journal of Immunology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple methods including dominant-negative approach, single lab","pmids":["10228003"],"is_preprint":false},{"year":2002,"finding":"Sialic acid binding domains of CD22 are required for CD22 to function as a negative regulator; CD22 mutants lacking sialic acid binding activity show reduced tyrosine phosphorylation and SHP-1 association, and greater Ca2+ responses after BCR stimulation.","method":"Site-directed mutagenesis of CD22 sialic acid binding domains, expression in CD22-/- B cell line, calcium flux assays, phosphorylation and co-immunoprecipitation","journal":"The Journal of Experimental Medicine","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis reconstitution in CD22-/- cells with multiple functional readouts","pmids":["11994425"],"is_preprint":false},{"year":2002,"finding":"CD22 binds native CD45 with low affinity (Kd ~130 μM) and very fast kinetics via enthalpically driven lectin-carbohydrate interaction; CD22 does not preferentially bind CD45 over other α2,6-sialylated glycoproteins in terms of intrinsic affinity.","method":"Surface plasmon resonance, thermodynamic (Van't Hoff) analysis","journal":"European Journal of Immunology","confidence":"High","confidence_rationale":"Tier 1 — quantitative biophysical measurement with thermodynamic analysis","pmids":["12115612"],"is_preprint":false},{"year":2003,"finding":"CD22 interacts with AP50 (medium chain of the AP-2 clathrin adaptor complex) via tyrosine-based internalization motifs in its cytoplasmic domain; Tyr843 is the primary binding site for AP50, and either Tyr843 or Tyr863 is sufficient for mAb-mediated internalization.","method":"Yeast two-hybrid analysis, co-immunoprecipitation of α-adaptin, transfectant Jurkat cell internalization assays with wild-type and mutant CD22","journal":"Journal of Immunology","confidence":"High","confidence_rationale":"Tier 1 — yeast two-hybrid with mutagenesis and functional internalization assays","pmids":["12646615"],"is_preprint":false},{"year":2004,"finding":"CD22 ligand binding regulates cell surface CD22 expression, IgM and MHC class II levels, marginal zone B cell maintenance, optimal BCR-induced proliferation, and B cell turnover; however, CD22 negative regulation of calcium mobilization, CD22 phosphorylation, SHP-1 recruitment, and B cell migration do not require CD22 ligand engagement.","method":"Gene-targeted mice expressing ligand-binding-deficient CD22 mutant, flow cytometry, calcium flux, B cell migration assays, co-immunoprecipitation","journal":"Nature Immunology","confidence":"High","confidence_rationale":"Tier 1 — knock-in mutagenesis separating ligand-dependent from ligand-independent functions with multiple orthogonal readouts","pmids":["15378059"],"is_preprint":false},{"year":2005,"finding":"CD22 forms homomultimeric complexes on B cell surfaces by recognizing glycans of neighboring CD22 molecules as cis ligands; CD45, sIgM, and other glycoproteins that bind CD22 in vitro are not important cis ligands in situ.","method":"In situ photoaffinity cross-linking of glycan ligands to CD22, immunoprecipitation","journal":"Nature Chemical Biology","confidence":"High","confidence_rationale":"Tier 1 — novel chemical cross-linking method with direct identification of cis ligands in situ","pmids":["16408005"],"is_preprint":false},{"year":2005,"finding":"Ablation of CD22 in ST6Gal1-deficient mice (lacking the enzyme that generates α2-6-linked sialic acid CD22 ligands) restores BCR signaling; loss of CD22 ligand causes redistribution of BCR to clathrin-rich microdomains containing CD22, linking CD22-ligand interactions to BCR microdomain regulation.","method":"Double knockout mice (Cd22-/- St6gal1-/-), BCR signaling assays, membrane fractionation/microdomain analysis","journal":"Nature Immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis using double knockout, multiple mechanistic readouts","pmids":["16369536"],"is_preprint":false},{"year":2006,"finding":"Anti-CD22 mAbs that block CD22 ligand binding accelerate mature B cell turnover 2-4 fold and inhibit survival of normal and malignant B cells in vivo; effects are independent of complement and FcRs and absent in CD22AA mice expressing ligand-binding-deficient CD22.","method":"In vivo mAb treatment, B cell turnover assays, adoptive transfer of normal and malignant B cells, CD22AA knock-in mice","journal":"Journal of Immunology","confidence":"High","confidence_rationale":"Tier 2 — in vivo genetic and antibody-based epistasis with multiple controls","pmids":["16920943"],"is_preprint":false},{"year":2007,"finding":"Human B lymphocytes express α2-6-sialylated 6-sulfo-N-acetyllactosamine as a preferred high-affinity ligand for CD22; CD22 binding to B cells was abrogated by sulfate metabolism inhibition and by anti-sulfated LacNAc antibody.","method":"Cell binding assays, NaClO3 inhibition, monoclonal antibody blocking, flow cytometry","journal":"The Journal of Biological Chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding inhibition with chemical and antibody tools, single lab","pmids":["17728258"],"is_preprint":false},{"year":2009,"finding":"Sialylated multivalent antigens engage CD22 in trans and inhibit key steps in BCR signaling, revealing that antigens bearing CD22 ligands can suppress B cell activation through trans CD22 interactions.","method":"Sialylated multivalent antigen synthesis, calcium flux assays, BCR signaling readouts","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — synthetic antigen tool with functional BCR signaling readouts, single lab","pmids":["19202057"],"is_preprint":false},{"year":2009,"finding":"The Lyn-CD22-SHP-1 inhibitory pathway becomes operational as B cells mature; Lyn deficiency substantially enhances mature but not immature B cell BCR signaling in a manner similar to CD22 deficiency, indicating this pathway is critical for peripheral B cell tolerance.","method":"Lyn-deficient and CD22-deficient mice, calcium flux and ERK MAPK assays at different B cell developmental stages","journal":"Journal of Immunology","confidence":"Medium","confidence_rationale":"Tier 2 — parallel genetic analysis of Lyn and CD22 KO at multiple developmental stages","pmids":["19380785"],"is_preprint":false},{"year":2010,"finding":"IVIg sialic acid-bearing glycans bind CD22 on B cells, reducing BCR-mediated tyrosine phosphorylation of Lyn and BLNK, upregulating PLCγ2 activation, sustaining ERK1/2 activation, and arresting cell cycle at G1, linking CD22's lectin function to IVIg immunomodulation.","method":"Confocal microscopy of sialylated vs. desialylated IgG binding to CD22, signaling pathway analysis by immunoblot, cell cycle analysis","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding demonstrated with functional signaling cascade characterization, single lab","pmids":["20516366"],"is_preprint":false},{"year":2010,"finding":"IgM on apposing B cells is the major in situ trans ligand of CD22; UV photocross-linking of CD22 to glycoproteins on apposing B cells identified IgM as selectively redistributed to the cell contact site.","method":"UV photocross-linking proteomics, mass spectrometry-based quantitative proteomics, immunochemical analysis of cross-linked products","journal":"Molecular & Cellular Proteomics","confidence":"High","confidence_rationale":"Tier 1 — unbiased proteomics screen followed by in situ photocross-linking validation","pmids":["20172905"],"is_preprint":false},{"year":2010,"finding":"CD22 negatively regulates TLR3, TLR4, and TLR9 signaling in B cells, at least in part by promoting induction of SOCS1 and SOCS3 (suppressors of cytokine signaling); CD22 expression in a CD22-/- B cell line blunts TLR ligand responses and inhibits LPS-induced NF-κB transcription.","method":"CD22-deficient mice, TLR ligand stimulation assays, SOCS1/3 expression analysis, NF-κB reporter assay, CD22 reconstitution in CD22-/- cell line","journal":"Journal of Innate Immunity","confidence":"Medium","confidence_rationale":"Tier 2 — KO mice plus reconstitution experiment with reporter assay, single lab","pmids":["21178327"],"is_preprint":false},{"year":2011,"finding":"CD22 serves as a receptor for soluble IgM (sIgM) due to α2,6-linked sialic acid glycans on sIgM; antigen-sIgM complexes activate CD22 in trans, creating a negative feedback loop for B cell activation analogous to FcγRIIB.","method":"CD22-deficient mice, sIgM binding assays, BCR signaling analysis","journal":"European Journal of Immunology","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding and functional signaling with KO controls, single lab","pmids":["21956693"],"is_preprint":false},{"year":2013,"finding":"Liposomal nanoparticles displaying both antigen and glycan ligands of CD22 (STALs) induce antigen-specific B cell apoptosis through a tolerogenic program; this mechanism requires CD22 engagement and induces robust antigen-specific tolerance to protein antigens in mice.","method":"Liposomal nanoparticle synthesis, in vitro B cell apoptosis assays with mouse and human B cells, in vivo tolerance induction in mice, FVIII hemophilia model","journal":"The Journal of Clinical Investigation","confidence":"High","confidence_rationale":"Tier 1 — reconstituted ligand-receptor system with in vitro and in vivo functional validation","pmids":["23722906"],"is_preprint":false},{"year":2013,"finding":"CD22 cis-ligand binding controls CD22 association to the BCR and is crucial for negative regulation of Ca2+ signaling; mice with mutated CD22 ligand-binding domain show strongly reduced Ca2+ signaling, while mice with mutated CD22 ITIMs show increased Ca2+ responses, increased B cell turnover, and impaired B cell survival.","method":"Knock-in mice with mutated CD22 ligand-binding domain or mutated ITIMs, calcium flux assays, B cell turnover analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — dual knock-in mutagenesis separating ligand-binding from signaling domain functions with multiple readouts","pmids":["23836650"],"is_preprint":false},{"year":2015,"finding":"CD22 is organized into nanodomains and is highly mobile on naïve B cells; its lateral diffusion and nanoscale organization are regulated by CD45 and its lectin domain; mathematical modeling and experiments suggest CD22 provides 'global BCR surveillance' through fast diffusion to maintain inhibitory function despite low numbers of molecules.","method":"Super-resolution microscopy, single-particle tracking, Brownian dynamics simulations, CD45-deficient and CD22 lectin-domain mutant B cells","journal":"The EMBO Journal","confidence":"High","confidence_rationale":"Tier 1 — super-resolution imaging combined with computational modeling and genetic validation","pmids":["26671981"],"is_preprint":false},{"year":2015,"finding":"On germinal center (GC) B cells, loss of high-affinity CD22 ligands unmasks the CD22 binding site relative to naïve B cells, promoting trans ligand recognition; in humans, 6-O-sulfate on GlcNAc is lost on GC B cells; in mice, Neu5Gc is replaced by Neu5Ac on GC B cells.","method":"Glycan binding assays, flow cytometry, mass spectrometry of glycan structures on B cell subsets","journal":"The Journal of Biological Chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — glycan structural analysis combined with CD22 binding studies on sorted B cell subsets, single lab","pmids":["26507663"],"is_preprint":false},{"year":2017,"finding":"Crystal structure of human CD22 at 2.1 Å resolution reveals that α2-6 sialic acid ligand specificity is dictated by a preformed β-hairpin unique among Siglecs; the CD22 ectodomain adopts an extended conformation enabling simultaneous nanocluster formation and trans ligand binding; N-linked glycosylation is critical for epratuzumab engagement at domain 3.","method":"X-ray crystallography (2.1 Å for CD22 alone, 3.1 Å for CD22-epratuzumab complex), structure-function validation","journal":"Nature Communications","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with functional validation including antibody binding epitope delineation","pmids":["28970495"],"is_preprint":false},{"year":2019,"finding":"CD22 is upregulated on aged microglia, mediates the anti-phagocytic effect of α2,6-linked sialic acid, and its inhibition promotes clearance of myelin debris, amyloid-β oligomers, and α-synuclein fibrils; long-term CNS CD22 antibody blockade reprograms microglia towards a homeostatic transcriptional state and improves cognitive function in aged mice.","method":"CRISPR-Cas9 knockout screens, RNA sequencing, in vivo antibody blockade, phagocytosis assays, cognitive behavior testing","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — unbiased CRISPR screen discovery validated with in vivo functional experiments and transcriptomics","pmids":["30944478"],"is_preprint":false},{"year":2021,"finding":"CD22 associates with integrin β7 on B cell surfaces in a sialic acid-dependent manner, recruiting Shp1 to β7 and inhibiting β7 endocytosis, thereby enhancing surface α4β7 display and B cell homing to gut-associated lymphoid tissue; this CD22-Shp1 axis requires both the CD22 sialic acid-binding and Shp1-binding domains.","method":"Co-immunoprecipitation, CD22 KO mice, knock-in mice with mutated CD22 Shp1-binding or carbohydrate-binding domains, β7 endocytosis assays, in vivo homing assays","journal":"Nature Immunology","confidence":"High","confidence_rationale":"Tier 1 — multiple knock-in mutants and KO with in vitro and in vivo functional validation","pmids":["33589816"],"is_preprint":false},{"year":2021,"finding":"Soluble CD22 (sCD22) binds to insulin-like growth factor 2 receptor (IGF2R) on human myeloid cells near mannose 6-phosphate-binding domains, disrupting lysosomal protein trafficking; CD22 blocking antibodies ameliorate lysosome dysfunction in NPC1 mutant iPSC-derived microglia-like cells.","method":"Unbiased genetic and proteomic screens, IGF2R truncation experiments, iPSC-derived microglia functional assays, CD22 blocking antibody treatment","journal":"Science Translational Medicine","confidence":"High","confidence_rationale":"Tier 1 — unbiased screen-based discovery of IGF2R as binding partner, validated with domain truncation and functional rescue","pmids":["34851695"],"is_preprint":false},{"year":2023,"finding":"Notch1 signaling in regulatory T cells (Tregs) induces CD22 expression, leading to Treg destabilization in an mTORC1-dependent manner and promotion of systemic inflammation in MIS-C patients; dominant-negative mutations in Notch1 regulators NUMB/NUMBL cause Notch1 upregulation and downstream CD22 induction.","method":"Genetic analysis of MIS-C patients, Notch1 signaling pathway analysis in Tregs, mTORC1 pathway assessment","journal":"The Journal of Clinical Investigation","confidence":"Medium","confidence_rationale":"Tier 2 — genetic and signaling pathway analysis in patient samples, single lab","pmids":["36282598"],"is_preprint":false}],"current_model":"CD22 (Siglec-2) is a B-cell-restricted transmembrane glycoprotein that functions as a negative co-receptor of the BCR by associating with the BCR complex, becoming tyrosine-phosphorylated by Lyn kinase upon BCR ligation, and recruiting SHP-1 via its ITIM motifs to suppress calcium mobilization and downstream signaling; its inhibitory activity is modulated by cis interactions with α2,6-sialylated glycan ligands (including CD22 homomultimers) that control CD22 nanoscale organization and BCR co-localization, while trans ligand interactions (including sIgM, antigen-bearing sialylated structures, and dendritic cell ligands) can further tune the signaling threshold; CD22 also promotes B cell survival and gut homing via a CD22-Shp1-integrin β7 axis, regulates TLR signaling through SOCS1/3, and on microglia acts as an anti-phagocytic receptor whose sCD22 form binds IGF2R to disrupt lysosomal trafficking."},"narrative":{"teleology":[{"year":1990,"claim":"CD22 was initially identified as a B-cell adhesion molecule related to MAG, establishing it within the immunoglobulin superfamily but leaving its signaling function unknown.","evidence":"Transfection and adhesion assays with CD22-expressing cells","pmids":["1691828"],"confidence":"Medium","gaps":["Adhesion mechanism not linked to any signaling pathway","Ligand identity unknown","No in vivo validation"]},{"year":1993,"claim":"Identification of CD22 as an α2,6-sialic acid-binding lectin and demonstration of its physical association with the BCR complex established the molecular framework for its dual lectin–signaling receptor identity.","evidence":"CD22-Ig fusion binding assays with enzymatic validation; reciprocal co-immunoprecipitation of CD22 with sIgM/IgD/IgG, Igα, Igβ, and PTK72 from B cell lines","pmids":["8463234","7684686","8475064"],"confidence":"High","gaps":["No downstream signaling effector identified","Unclear whether lectin activity is required for BCR association"]},{"year":1995,"claim":"Discovery that tyrosine-phosphorylated CD22 recruits SHP-1 after BCR cross-linking identified the core phosphatase effector of CD22 inhibitory signaling, while constitutive internalization to lysosomes revealed an additional layer of CD22 surface regulation.","evidence":"Phosphorylation-dependent co-IP of SHP-1 with CD22; flow cytometry and neuraminidase protection assays for endocytic trafficking","pmids":["7542197","7722303"],"confidence":"High","gaps":["Specific tyrosine residues mediating SHP-1 binding not mapped","Functional consequence of SHP-1 recruitment not demonstrated in vivo"]},{"year":1996,"claim":"CD22 knockout mice revealed that CD22 is a physiological negative regulator of BCR signaling, resolving ambiguity about whether CD22 was activating or inhibitory in B cells.","evidence":"Gene-targeted CD22-deficient mice with calcium flux, proliferation, B-1 cell expansion, and serum autoantibody measurements","pmids":["8864124","9016707"],"confidence":"High","gaps":["Relative contributions of lectin vs. ITIM domains not separated","Mechanism of autoantibody development not defined"]},{"year":1999,"claim":"Biochemical mapping of CD22–SHP-1 interaction showed that both SH2 domains of SHP-1 engage multiple phosphotyrosines on CD22, while aging CD22-KO mice developed high-affinity anti-dsDNA autoantibodies, linking CD22 loss to lupus-like autoimmunity.","evidence":"Phosphopeptide binding with mutagenesis of CD22 cytoplasmic tyrosines and SHP-1 SH2 domains; serum ELISA and somatic mutation analysis in CD22-KO mice","pmids":["9890995","10209047"],"confidence":"High","gaps":["In vivo disease relevance to human autoimmunity not established","No structural basis for SHP-1–CD22 interaction"]},{"year":2002,"claim":"Knock-in and mutagenesis studies demonstrated that CD22's sialic acid-binding activity is required for its negative regulatory function, linking the lectin and signaling activities mechanistically.","evidence":"Reconstitution of CD22 ligand-binding mutants in CD22-/- B cells with calcium flux and SHP-1 co-IP readouts; SPR measurement of CD22–CD45 affinity (Kd ~130 μM)","pmids":["11994425","12115612"],"confidence":"High","gaps":["Identity of physiological cis ligands unresolved","Contribution of trans vs. cis interactions unclear"]},{"year":2004,"claim":"Gene-targeted mice expressing ligand-binding-deficient CD22 separated lectin-dependent functions (surface expression, MZ B cell maintenance, proliferation) from lectin-independent ones (calcium inhibition, SHP-1 recruitment, migration), revealing that CD22 negative signaling does not absolutely require ligand binding.","evidence":"Knock-in mice with mutated CD22 sialic acid-binding domain, flow cytometry, calcium flux, B cell migration","pmids":["15378059"],"confidence":"High","gaps":["Apparent contradiction with earlier mutagenesis results awaited resolution","Mechanism of ligand-independent ITIM signaling unclear"]},{"year":2005,"claim":"In situ photocross-linking revealed that CD22 homomultimers—not CD45 or other glycoproteins—are the dominant cis ligands, while genetic epistasis in ST6Gal1/CD22 double-KO mice showed that α2,6-sialylation controls BCR microdomain partitioning.","evidence":"Photoaffinity cross-linking proteomics on intact B cells; double-KO mice with membrane fractionation","pmids":["16408005","16369536"],"confidence":"High","gaps":["Structural basis for homomultimer formation unknown","How microdomain redistribution translates to signaling quantitatively unclear"]},{"year":2010,"claim":"Identification of surface IgM on apposing B cells as the major trans ligand of CD22, and demonstration that CD22 dampens TLR signaling via SOCS1/3, expanded CD22's regulatory scope beyond BCR to innate-like pathways.","evidence":"UV photocross-linking proteomics and mass spectrometry for trans ligand identification; CD22-KO mice with TLR stimulation and SOCS expression analysis plus CD22 reconstitution","pmids":["20172905","21178327"],"confidence":"High","gaps":["Molecular mechanism linking CD22 to SOCS induction not defined","Whether trans IgM engagement is inhibitory or activating in physiological contexts unclear"]},{"year":2013,"claim":"Dual knock-in mice separating CD22 ligand-binding from ITIM function resolved earlier contradictions: cis-ligand binding controls CD22–BCR proximity and calcium signaling magnitude, while ITIMs are essential for B cell survival, establishing a two-domain regulatory model.","evidence":"Knock-in mice with mutated CD22 ligand-binding domain or ITIMs, calcium flux and B cell turnover assays","pmids":["23836650"],"confidence":"High","gaps":["Quantitative relationship between nanocluster size and signaling output not established","Role of CD22 in germinal center selection not fully addressed"]},{"year":2015,"claim":"Super-resolution imaging and computational modeling revealed that CD22 nanoclusters are highly mobile, providing 'global BCR surveillance' through rapid lateral diffusion regulated by CD45 and the lectin domain, explaining how low-abundance CD22 can inhibit dispersed BCR clusters.","evidence":"Super-resolution microscopy, single-particle tracking, Brownian dynamics simulations in wild-type, CD45-deficient, and lectin-mutant B cells","pmids":["26671981"],"confidence":"High","gaps":["Whether nanocluster dynamics change during B cell activation not tested","No measurement of CD22–BCR encounter kinetics in live cells during antigen stimulation"]},{"year":2017,"claim":"The 2.1 Å crystal structure of CD22 revealed a preformed β-hairpin unique among Siglecs that dictates α2,6-sialic acid specificity and showed an extended ectodomain compatible with simultaneous nanocluster and trans engagement.","evidence":"X-ray crystallography of CD22 alone and CD22–epratuzumab complex","pmids":["28970495"],"confidence":"High","gaps":["No structure of CD22 in complex with natural glycan ligand at atomic resolution","Ectodomain flexibility in membrane context unknown"]},{"year":2019,"claim":"A CRISPR screen in microglia identified CD22 as an anti-phagocytic receptor outside the B cell lineage; CD22 blockade in aged mice restored microglial clearance of neurotoxic substrates and improved cognition, redefining CD22 as a broader immune checkpoint.","evidence":"Genome-wide CRISPR-Cas9 screen, in vivo anti-CD22 antibody treatment, phagocytosis assays, RNA-seq, cognitive behavioral testing in aged mice","pmids":["30944478"],"confidence":"High","gaps":["Downstream signaling pathway in microglia not fully characterized","Whether SHP-1 mediates anti-phagocytic function in microglia not shown"]},{"year":2021,"claim":"Two new functional axes were defined: CD22 associates with integrin β7 in a sialic acid-dependent manner to recruit SHP-1 and promote gut homing; separately, soluble CD22 binds IGF2R on myeloid cells to disrupt lysosomal trafficking, expanding CD22's roles beyond classical BCR regulation.","evidence":"Co-IP and knock-in mice with in vivo homing assays (β7 axis); unbiased proteomic screen identifying IGF2R, domain truncation, iPSC-derived microglia functional rescue","pmids":["33589816","34851695"],"confidence":"High","gaps":["Whether sCD22–IGF2R interaction is relevant in vivo in aging or neurodegeneration not shown","Mechanism by which CD22 inhibits β7 endocytosis molecularly unclear"]},{"year":null,"claim":"Key unresolved questions include how CD22 nanocluster dynamics are regulated during germinal center responses, whether CD22's microglial anti-phagocytic function operates through SHP-1, the structural basis of the CD22–integrin β7 and sCD22–IGF2R interactions, and how CD22 loss contributes to human autoimmune disease.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structural model of CD22 with natural glycan or integrin β7 partner","SHP-1 requirement in microglial CD22 signaling not tested","Human genetic evidence linking CD22 mutations to autoimmune disease lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[8,9,10,30]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[5,11,35]},{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,2,19,31]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[6]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[36]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[8,9,12,27,29]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[5,10,11,24,30]}],"complexes":["BCR complex (low stoichiometry association)"],"partners":["SHP-1","LYN","CD45","ITGB7","IGF2R","AP2M1","IGM"],"other_free_text":[]},"mechanistic_narrative":"CD22 (Siglec-2) is a B-cell-restricted inhibitory co-receptor and α2,6-sialic acid-binding lectin that sets the threshold for B cell antigen receptor (BCR) signaling and contributes to peripheral B cell tolerance. Upon BCR ligation, CD22 is tyrosine-phosphorylated by Lyn kinase and recruits the phosphatase SHP-1 via its cytoplasmic ITIM motifs, suppressing calcium mobilization, MAP kinase activation, and downstream proliferative responses; CD22-deficient mice exhibit B cell hyperresponsiveness, expanded B-1 cells, and spontaneous autoantibody production [PMID:8864124, PMID:7542197, PMID:9252120, PMID:10209047]. CD22's lectin domain engages α2,6-sialylated glycans in cis—predominantly on neighboring CD22 molecules forming homomultimeric nanoclusters—to regulate its proximity to the BCR, while trans interactions with sialylated ligands such as surface IgM on apposing B cells, sialylated antigens, and soluble IgM provide additional inhibitory feedback [PMID:16408005, PMID:20172905, PMID:23836650, PMID:19202057]. Beyond BCR regulation, CD22 dampens TLR signaling through SOCS1/3 induction, promotes B cell gut homing via a sialic acid-dependent CD22–SHP-1–integrin β7 axis, and on aged microglia functions as an anti-phagocytic receptor whose blockade restores clearance of neurotoxic substrates and improves cognition [PMID:21178327, PMID:33589816, PMID:30944478]."},"prefetch_data":{"uniprot":{"accession":"P20273","full_name":"B-cell receptor CD22","aliases":["B-lymphocyte cell adhesion molecule","BL-CAM","Sialic acid-binding Ig-like lectin 2","Siglec-2","T-cell surface antigen Leu-14"],"length_aa":847,"mass_kda":95.3,"function":"Most highly expressed siglec (sialic acid-binding immunoglobulin-like lectin) on B-cells that plays a role in various aspects of B-cell biology including differentiation, antigen presentation, and trafficking to bone marrow (PubMed:34330755, PubMed:8627166). Binds to alpha 2,6-linked sialic acid residues of surface molecules such as CD22 itself, CD45 and IgM in a cis configuration. Can also bind to ligands on other cells as an adhesion molecule in a trans configuration (PubMed:20172905). Acts as an inhibitory coreceptor on the surface of B-cells and inhibits B-cell receptor induced signaling, characterized by inhibition of the calcium mobilization and cellular activation. Mechanistically, the immunoreceptor tyrosine-based inhibitory motif domain is phosphorylated by the Src kinase LYN, which in turn leads to the recruitment of the protein tyrosine phosphatase 1/PTPN6, leading to the negative regulation of BCR signaling (PubMed:8627166). If this negative signaling from is of sufficient strength, apoptosis of the B-cell can be induced (PubMed:20516366)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/P20273/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CD22","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CD22","total_profiled":1310},"omim":[{"mim_id":"606720","title":"ENDONUCLEASE, POLY(U)-SPECIFIC; ENDOU","url":"https://www.omim.org/entry/606720"},{"mim_id":"606572","title":"NADPH OXIDASE 5; NOX5","url":"https://www.omim.org/entry/606572"},{"mim_id":"604410","title":"SIALIC ACID-BINDING IMMUNOGLOBULIN-LIKE LECTIN 7; SIGLEC7","url":"https://www.omim.org/entry/604410"},{"mim_id":"603823","title":"FREE FATTY ACID RECEPTOR 2; FFAR2","url":"https://www.omim.org/entry/603823"},{"mim_id":"603822","title":"G PROTEIN-COUPLED RECEPTOR 42; GPR42","url":"https://www.omim.org/entry/603822"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"lymphoid tissue","ntpm":215.0},{"tissue":"ovary","ntpm":124.5}],"url":"https://www.proteinatlas.org/search/CD22"},"hgnc":{"alias_symbol":["SIGLEC-2","SIGLEC2"],"prev_symbol":[]},"alphafold":{"accession":"P20273","domains":[{"cath_id":"2.60.40.10","chopping":"23-138","consensus_level":"medium","plddt":84.4143,"start":23,"end":138},{"cath_id":"2.60.40.10","chopping":"145-237","consensus_level":"medium","plddt":92.4862,"start":145,"end":237},{"cath_id":"2.60.40.10","chopping":"244-329","consensus_level":"high","plddt":89.3405,"start":244,"end":329},{"cath_id":"2.60.40.10","chopping":"336-417","consensus_level":"medium","plddt":91.6539,"start":336,"end":417},{"cath_id":"2.60.40,2.60.40","chopping":"424-499","consensus_level":"medium","plddt":87.992,"start":424,"end":499},{"cath_id":"2.60.40.10","chopping":"510-588","consensus_level":"high","plddt":89.7551,"start":510,"end":588},{"cath_id":"2.60.40.10","chopping":"593-682","consensus_level":"high","plddt":90.257,"start":593,"end":682}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P20273","model_url":"https://alphafold.ebi.ac.uk/files/AF-P20273-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P20273-F1-predicted_aligned_error_v6.png","plddt_mean":79.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CD22","jax_strain_url":"https://www.jax.org/strain/search?query=CD22"},"sequence":{"accession":"P20273","fasta_url":"https://rest.uniprot.org/uniprotkb/P20273.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P20273/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P20273"}},"corpus_meta":[{"pmid":"34312556","id":"PMC_34312556","title":"CAR 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\"Nature\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct functional assay in transfected cells, single lab\",\n      \"pmids\": [\"1691828\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CD22 is a sialic acid-binding lectin that recognizes sialic acids in α2,6 linkage on N-linked oligosaccharides of multiple sialoglycoproteins including CD45, establishing CD22 as a member of a novel subset of Ig superfamily adhesion molecules.\",\n      \"method\": \"CD22-Ig fusion protein binding assays, periodate oxidation, sialyltransferase transfection in COS cells\",\n      \"journal\": \"The Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro binding assay with enzymatic and biochemical validation, replicated across multiple ligands\",\n      \"pmids\": [\"8463234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CD22 associates specifically with the B cell antigen receptor (BCR) complex including IgM, IgD, and IgG isotypes, in a low stoichiometry but stable association, and is rapidly tyrosine phosphorylated after antigen receptor cross-linking.\",\n      \"method\": \"In vitro kinase assay, co-immunoprecipitation from Burkitt lymphoma lines and tonsil cells\",\n      \"journal\": \"European Journal of Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP across multiple isotypes and cell types, single lab\",\n      \"pmids\": [\"7684686\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CD22 co-immunoprecipitates with the sIgM-BCR complex along with Igα/mb-1, Igβ/B29, and PTK72, and becomes rapidly and strikingly tyrosine phosphorylated after sIgM cross-linking; CD22 contains the ARHI (antigen recognition homology I) motif present in other antigen receptor molecules.\",\n      \"method\": \"Co-immunoprecipitation in digitonin, immunoblot, in vitro kinase assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — co-IP with multiple components of BCR complex, single lab\",\n      \"pmids\": [\"8475064\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"CD22 directly interacts with multiple isoforms of CD45, and cross-linking of CD3 and CD22 ligands blocks anti-CD3-induced intracellular calcium increase and inhibits tyrosine phosphorylation of phospholipase Cγ1 in T cells, suggesting CD22-CD45 engagement modulates lymphocyte signaling.\",\n      \"method\": \"Direct binding assay with soluble CD22, calcium flux measurement, tyrosine phosphorylation assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding and functional signaling assays, single lab\",\n      \"pmids\": [\"1438211\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Tyrosine-phosphorylated CD22 recruits the tyrosine phosphatase SHP-1 (PTP1C) in a phosphorylation-dependent manner after BCR cross-linking, identifying SHP-1 as a downstream effector of CD22 signaling.\",\n      \"method\": \"Co-immunoprecipitation after anti-Ig stimulation, identification of 60-kDa associated protein as PTP1C\",\n      \"journal\": \"European Journal of Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — phosphorylation-dependent co-IP with biochemical identification, replicated in subsequent studies\",\n      \"pmids\": [\"7542197\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"CD22 is constitutively internalized by unstimulated B cells and degraded in an acidic intracellular compartment (lysosomes) without detectable recycling to the cell surface; anti-CD22 mAb ligation markedly increases internalization rate.\",\n      \"method\": \"Flow cytometry, neuraminidase protection and neuraminidase shift assays\",\n      \"journal\": \"Journal of Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple functional assays demonstrating constitutive endocytosis, single lab\",\n      \"pmids\": [\"7722303\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Engagement of CD45 by soluble CD22 modulates early T cell signals in antigen receptor/CD3-mediated stimulation; addition of α2,6-sialic acid to the CD22 molecule itself abrogates CD22-ligand interactions.\",\n      \"method\": \"Soluble CD22-Ig fusion protein, T cell calcium flux, chimeric CD45 constructs, sialyltransferase treatment\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct functional binding with enzymatic modification controls, single lab\",\n      \"pmids\": [\"7537381\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"CD22-deficient mice show hyperresponsive B cells with heightened calcium fluxes and cell proliferation at lower ligand concentrations, an expanded peritoneal B-1 cell population, and increased serum autoantibody titers, establishing CD22 as a negative regulator of antigen receptor signaling.\",\n      \"method\": \"Gene-targeted knockout mice, calcium flux measurement, proliferation assays, serum antibody titers\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple orthogonal phenotypic readouts, replicated by independent labs\",\n      \"pmids\": [\"8864124\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"CD22-deficient mice show increased Ca2+ influx and lower survival of B cells upon BCR cross-linking, impaired T-cell independent immune responses, and absence of recirculating B cells from bone marrow, confirming CD22 as a negative regulator that sets the BCR signaling threshold.\",\n      \"method\": \"Targeted gene inactivation (knockout mice), calcium flux, B cell subset analysis, immune response assays\",\n      \"journal\": \"Current Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — independent KO study replicating and extending prior findings with multiple orthogonal methods\",\n      \"pmids\": [\"9016707\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"CD19 and CD22 have counterregulatory effects on MAP kinase (ERK2, JNK, p38) activation by membrane immunoglobulin: CD19 co-ligation synergistically amplifies MAP kinase activation, while CD22 cross-linking to mIg suppresses it; sequestration of CD22 from mIg enhances MAP kinase activation.\",\n      \"method\": \"Co-ligation experiments, MAP kinase activation assays (ERK2, JNK, p38)\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct functional epistasis using co-ligation, single lab\",\n      \"pmids\": [\"9252120\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Multiple tyrosine residues within the cytoplasmic domain of CD22 interact with both SH2 domains of SHP-1; a minimum of two CD22 tyrosines are required for SHP-1 association, and both SH2 domains of SHP-1 are necessary for efficient binding.\",\n      \"method\": \"Phosphopeptide binding assays, mutational analysis of CD22 cytoplasmic domain tyrosines, SHP-1 domain analysis\",\n      \"journal\": \"The Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro phosphopeptide assay combined with mutagenesis, defining molecular requirements for interaction\",\n      \"pmids\": [\"9890995\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"CD22 deficiency leads to development of high-affinity, somatically mutated, multiclonal IgG anti-dsDNA autoantibodies in aging mice, demonstrating that a single B-cell-specific gene defect is sufficient to trigger autoantibody development.\",\n      \"method\": \"CD22-deficient mice, serum antibody ELISA, affinity measurement, somatic mutation analysis\",\n      \"journal\": \"The Journal of Experimental Medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple antibody characterization methods, mechanistically links CD22 to autoimmunity\",\n      \"pmids\": [\"10209047\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"CD22 ligation activates JNK/SAPK but not ERK2 signaling independently of BCR, and induces downregulation of Bcl-xL and Mcl-1, providing a mechanism for CD22-mediated apoptosis in B cells.\",\n      \"method\": \"Kinase activity assays (ERK2, JNK), AP-1/c-jun nuclear extract analysis, Bcl-2 family protein immunoblotting, B cell apoptosis assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple kinase and apoptosis assays, single lab\",\n      \"pmids\": [\"10438726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"CD45 positively regulates CD22 tyrosine phosphorylation; cross-linking of CD45 causes physical sequestration from CD22 leading to increased CD22 tyrosine phosphorylation and enhanced SHP-1 recruitment; CD45-deficient B cells show elevated basal CD22 phosphorylation and SHP-1 association.\",\n      \"method\": \"Co-immunoprecipitation, calcium flux assays, CD45-deficient B cell line, expression of catalytically inactive SHP-1\",\n      \"journal\": \"Journal of Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods including dominant-negative approach, single lab\",\n      \"pmids\": [\"10228003\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Sialic acid binding domains of CD22 are required for CD22 to function as a negative regulator; CD22 mutants lacking sialic acid binding activity show reduced tyrosine phosphorylation and SHP-1 association, and greater Ca2+ responses after BCR stimulation.\",\n      \"method\": \"Site-directed mutagenesis of CD22 sialic acid binding domains, expression in CD22-/- B cell line, calcium flux assays, phosphorylation and co-immunoprecipitation\",\n      \"journal\": \"The Journal of Experimental Medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis reconstitution in CD22-/- cells with multiple functional readouts\",\n      \"pmids\": [\"11994425\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CD22 binds native CD45 with low affinity (Kd ~130 μM) and very fast kinetics via enthalpically driven lectin-carbohydrate interaction; CD22 does not preferentially bind CD45 over other α2,6-sialylated glycoproteins in terms of intrinsic affinity.\",\n      \"method\": \"Surface plasmon resonance, thermodynamic (Van't Hoff) analysis\",\n      \"journal\": \"European Journal of Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — quantitative biophysical measurement with thermodynamic analysis\",\n      \"pmids\": [\"12115612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"CD22 interacts with AP50 (medium chain of the AP-2 clathrin adaptor complex) via tyrosine-based internalization motifs in its cytoplasmic domain; Tyr843 is the primary binding site for AP50, and either Tyr843 or Tyr863 is sufficient for mAb-mediated internalization.\",\n      \"method\": \"Yeast two-hybrid analysis, co-immunoprecipitation of α-adaptin, transfectant Jurkat cell internalization assays with wild-type and mutant CD22\",\n      \"journal\": \"Journal of Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — yeast two-hybrid with mutagenesis and functional internalization assays\",\n      \"pmids\": [\"12646615\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"CD22 ligand binding regulates cell surface CD22 expression, IgM and MHC class II levels, marginal zone B cell maintenance, optimal BCR-induced proliferation, and B cell turnover; however, CD22 negative regulation of calcium mobilization, CD22 phosphorylation, SHP-1 recruitment, and B cell migration do not require CD22 ligand engagement.\",\n      \"method\": \"Gene-targeted mice expressing ligand-binding-deficient CD22 mutant, flow cytometry, calcium flux, B cell migration assays, co-immunoprecipitation\",\n      \"journal\": \"Nature Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — knock-in mutagenesis separating ligand-dependent from ligand-independent functions with multiple orthogonal readouts\",\n      \"pmids\": [\"15378059\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"CD22 forms homomultimeric complexes on B cell surfaces by recognizing glycans of neighboring CD22 molecules as cis ligands; CD45, sIgM, and other glycoproteins that bind CD22 in vitro are not important cis ligands in situ.\",\n      \"method\": \"In situ photoaffinity cross-linking of glycan ligands to CD22, immunoprecipitation\",\n      \"journal\": \"Nature Chemical Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — novel chemical cross-linking method with direct identification of cis ligands in situ\",\n      \"pmids\": [\"16408005\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Ablation of CD22 in ST6Gal1-deficient mice (lacking the enzyme that generates α2-6-linked sialic acid CD22 ligands) restores BCR signaling; loss of CD22 ligand causes redistribution of BCR to clathrin-rich microdomains containing CD22, linking CD22-ligand interactions to BCR microdomain regulation.\",\n      \"method\": \"Double knockout mice (Cd22-/- St6gal1-/-), BCR signaling assays, membrane fractionation/microdomain analysis\",\n      \"journal\": \"Nature Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis using double knockout, multiple mechanistic readouts\",\n      \"pmids\": [\"16369536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Anti-CD22 mAbs that block CD22 ligand binding accelerate mature B cell turnover 2-4 fold and inhibit survival of normal and malignant B cells in vivo; effects are independent of complement and FcRs and absent in CD22AA mice expressing ligand-binding-deficient CD22.\",\n      \"method\": \"In vivo mAb treatment, B cell turnover assays, adoptive transfer of normal and malignant B cells, CD22AA knock-in mice\",\n      \"journal\": \"Journal of Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic and antibody-based epistasis with multiple controls\",\n      \"pmids\": [\"16920943\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Human B lymphocytes express α2-6-sialylated 6-sulfo-N-acetyllactosamine as a preferred high-affinity ligand for CD22; CD22 binding to B cells was abrogated by sulfate metabolism inhibition and by anti-sulfated LacNAc antibody.\",\n      \"method\": \"Cell binding assays, NaClO3 inhibition, monoclonal antibody blocking, flow cytometry\",\n      \"journal\": \"The Journal of Biological Chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding inhibition with chemical and antibody tools, single lab\",\n      \"pmids\": [\"17728258\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Sialylated multivalent antigens engage CD22 in trans and inhibit key steps in BCR signaling, revealing that antigens bearing CD22 ligands can suppress B cell activation through trans CD22 interactions.\",\n      \"method\": \"Sialylated multivalent antigen synthesis, calcium flux assays, BCR signaling readouts\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — synthetic antigen tool with functional BCR signaling readouts, single lab\",\n      \"pmids\": [\"19202057\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The Lyn-CD22-SHP-1 inhibitory pathway becomes operational as B cells mature; Lyn deficiency substantially enhances mature but not immature B cell BCR signaling in a manner similar to CD22 deficiency, indicating this pathway is critical for peripheral B cell tolerance.\",\n      \"method\": \"Lyn-deficient and CD22-deficient mice, calcium flux and ERK MAPK assays at different B cell developmental stages\",\n      \"journal\": \"Journal of Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — parallel genetic analysis of Lyn and CD22 KO at multiple developmental stages\",\n      \"pmids\": [\"19380785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"IVIg sialic acid-bearing glycans bind CD22 on B cells, reducing BCR-mediated tyrosine phosphorylation of Lyn and BLNK, upregulating PLCγ2 activation, sustaining ERK1/2 activation, and arresting cell cycle at G1, linking CD22's lectin function to IVIg immunomodulation.\",\n      \"method\": \"Confocal microscopy of sialylated vs. desialylated IgG binding to CD22, signaling pathway analysis by immunoblot, cell cycle analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding demonstrated with functional signaling cascade characterization, single lab\",\n      \"pmids\": [\"20516366\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"IgM on apposing B cells is the major in situ trans ligand of CD22; UV photocross-linking of CD22 to glycoproteins on apposing B cells identified IgM as selectively redistributed to the cell contact site.\",\n      \"method\": \"UV photocross-linking proteomics, mass spectrometry-based quantitative proteomics, immunochemical analysis of cross-linked products\",\n      \"journal\": \"Molecular & Cellular Proteomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — unbiased proteomics screen followed by in situ photocross-linking validation\",\n      \"pmids\": [\"20172905\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CD22 negatively regulates TLR3, TLR4, and TLR9 signaling in B cells, at least in part by promoting induction of SOCS1 and SOCS3 (suppressors of cytokine signaling); CD22 expression in a CD22-/- B cell line blunts TLR ligand responses and inhibits LPS-induced NF-κB transcription.\",\n      \"method\": \"CD22-deficient mice, TLR ligand stimulation assays, SOCS1/3 expression analysis, NF-κB reporter assay, CD22 reconstitution in CD22-/- cell line\",\n      \"journal\": \"Journal of Innate Immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mice plus reconstitution experiment with reporter assay, single lab\",\n      \"pmids\": [\"21178327\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"CD22 serves as a receptor for soluble IgM (sIgM) due to α2,6-linked sialic acid glycans on sIgM; antigen-sIgM complexes activate CD22 in trans, creating a negative feedback loop for B cell activation analogous to FcγRIIB.\",\n      \"method\": \"CD22-deficient mice, sIgM binding assays, BCR signaling analysis\",\n      \"journal\": \"European Journal of Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding and functional signaling with KO controls, single lab\",\n      \"pmids\": [\"21956693\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Liposomal nanoparticles displaying both antigen and glycan ligands of CD22 (STALs) induce antigen-specific B cell apoptosis through a tolerogenic program; this mechanism requires CD22 engagement and induces robust antigen-specific tolerance to protein antigens in mice.\",\n      \"method\": \"Liposomal nanoparticle synthesis, in vitro B cell apoptosis assays with mouse and human B cells, in vivo tolerance induction in mice, FVIII hemophilia model\",\n      \"journal\": \"The Journal of Clinical Investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted ligand-receptor system with in vitro and in vivo functional validation\",\n      \"pmids\": [\"23722906\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"CD22 cis-ligand binding controls CD22 association to the BCR and is crucial for negative regulation of Ca2+ signaling; mice with mutated CD22 ligand-binding domain show strongly reduced Ca2+ signaling, while mice with mutated CD22 ITIMs show increased Ca2+ responses, increased B cell turnover, and impaired B cell survival.\",\n      \"method\": \"Knock-in mice with mutated CD22 ligand-binding domain or mutated ITIMs, calcium flux assays, B cell turnover analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — dual knock-in mutagenesis separating ligand-binding from signaling domain functions with multiple readouts\",\n      \"pmids\": [\"23836650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"CD22 is organized into nanodomains and is highly mobile on naïve B cells; its lateral diffusion and nanoscale organization are regulated by CD45 and its lectin domain; mathematical modeling and experiments suggest CD22 provides 'global BCR surveillance' through fast diffusion to maintain inhibitory function despite low numbers of molecules.\",\n      \"method\": \"Super-resolution microscopy, single-particle tracking, Brownian dynamics simulations, CD45-deficient and CD22 lectin-domain mutant B cells\",\n      \"journal\": \"The EMBO Journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — super-resolution imaging combined with computational modeling and genetic validation\",\n      \"pmids\": [\"26671981\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"On germinal center (GC) B cells, loss of high-affinity CD22 ligands unmasks the CD22 binding site relative to naïve B cells, promoting trans ligand recognition; in humans, 6-O-sulfate on GlcNAc is lost on GC B cells; in mice, Neu5Gc is replaced by Neu5Ac on GC B cells.\",\n      \"method\": \"Glycan binding assays, flow cytometry, mass spectrometry of glycan structures on B cell subsets\",\n      \"journal\": \"The Journal of Biological Chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — glycan structural analysis combined with CD22 binding studies on sorted B cell subsets, single lab\",\n      \"pmids\": [\"26507663\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Crystal structure of human CD22 at 2.1 Å resolution reveals that α2-6 sialic acid ligand specificity is dictated by a preformed β-hairpin unique among Siglecs; the CD22 ectodomain adopts an extended conformation enabling simultaneous nanocluster formation and trans ligand binding; N-linked glycosylation is critical for epratuzumab engagement at domain 3.\",\n      \"method\": \"X-ray crystallography (2.1 Å for CD22 alone, 3.1 Å for CD22-epratuzumab complex), structure-function validation\",\n      \"journal\": \"Nature Communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with functional validation including antibody binding epitope delineation\",\n      \"pmids\": [\"28970495\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CD22 is upregulated on aged microglia, mediates the anti-phagocytic effect of α2,6-linked sialic acid, and its inhibition promotes clearance of myelin debris, amyloid-β oligomers, and α-synuclein fibrils; long-term CNS CD22 antibody blockade reprograms microglia towards a homeostatic transcriptional state and improves cognitive function in aged mice.\",\n      \"method\": \"CRISPR-Cas9 knockout screens, RNA sequencing, in vivo antibody blockade, phagocytosis assays, cognitive behavior testing\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — unbiased CRISPR screen discovery validated with in vivo functional experiments and transcriptomics\",\n      \"pmids\": [\"30944478\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CD22 associates with integrin β7 on B cell surfaces in a sialic acid-dependent manner, recruiting Shp1 to β7 and inhibiting β7 endocytosis, thereby enhancing surface α4β7 display and B cell homing to gut-associated lymphoid tissue; this CD22-Shp1 axis requires both the CD22 sialic acid-binding and Shp1-binding domains.\",\n      \"method\": \"Co-immunoprecipitation, CD22 KO mice, knock-in mice with mutated CD22 Shp1-binding or carbohydrate-binding domains, β7 endocytosis assays, in vivo homing assays\",\n      \"journal\": \"Nature Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple knock-in mutants and KO with in vitro and in vivo functional validation\",\n      \"pmids\": [\"33589816\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Soluble CD22 (sCD22) binds to insulin-like growth factor 2 receptor (IGF2R) on human myeloid cells near mannose 6-phosphate-binding domains, disrupting lysosomal protein trafficking; CD22 blocking antibodies ameliorate lysosome dysfunction in NPC1 mutant iPSC-derived microglia-like cells.\",\n      \"method\": \"Unbiased genetic and proteomic screens, IGF2R truncation experiments, iPSC-derived microglia functional assays, CD22 blocking antibody treatment\",\n      \"journal\": \"Science Translational Medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — unbiased screen-based discovery of IGF2R as binding partner, validated with domain truncation and functional rescue\",\n      \"pmids\": [\"34851695\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Notch1 signaling in regulatory T cells (Tregs) induces CD22 expression, leading to Treg destabilization in an mTORC1-dependent manner and promotion of systemic inflammation in MIS-C patients; dominant-negative mutations in Notch1 regulators NUMB/NUMBL cause Notch1 upregulation and downstream CD22 induction.\",\n      \"method\": \"Genetic analysis of MIS-C patients, Notch1 signaling pathway analysis in Tregs, mTORC1 pathway assessment\",\n      \"journal\": \"The Journal of Clinical Investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic and signaling pathway analysis in patient samples, single lab\",\n      \"pmids\": [\"36282598\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CD22 (Siglec-2) is a B-cell-restricted transmembrane glycoprotein that functions as a negative co-receptor of the BCR by associating with the BCR complex, becoming tyrosine-phosphorylated by Lyn kinase upon BCR ligation, and recruiting SHP-1 via its ITIM motifs to suppress calcium mobilization and downstream signaling; its inhibitory activity is modulated by cis interactions with α2,6-sialylated glycan ligands (including CD22 homomultimers) that control CD22 nanoscale organization and BCR co-localization, while trans ligand interactions (including sIgM, antigen-bearing sialylated structures, and dendritic cell ligands) can further tune the signaling threshold; CD22 also promotes B cell survival and gut homing via a CD22-Shp1-integrin β7 axis, regulates TLR signaling through SOCS1/3, and on microglia acts as an anti-phagocytic receptor whose sCD22 form binds IGF2R to disrupt lysosomal trafficking.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"CD22 (Siglec-2) is a B-cell-restricted inhibitory co-receptor and α2,6-sialic acid-binding lectin that sets the threshold for B cell antigen receptor (BCR) signaling and contributes to peripheral B cell tolerance. Upon BCR ligation, CD22 is tyrosine-phosphorylated by Lyn kinase and recruits the phosphatase SHP-1 via its cytoplasmic ITIM motifs, suppressing calcium mobilization, MAP kinase activation, and downstream proliferative responses; CD22-deficient mice exhibit B cell hyperresponsiveness, expanded B-1 cells, and spontaneous autoantibody production [PMID:8864124, PMID:7542197, PMID:9252120, PMID:10209047]. CD22's lectin domain engages α2,6-sialylated glycans in cis—predominantly on neighboring CD22 molecules forming homomultimeric nanoclusters—to regulate its proximity to the BCR, while trans interactions with sialylated ligands such as surface IgM on apposing B cells, sialylated antigens, and soluble IgM provide additional inhibitory feedback [PMID:16408005, PMID:20172905, PMID:23836650, PMID:19202057]. Beyond BCR regulation, CD22 dampens TLR signaling through SOCS1/3 induction, promotes B cell gut homing via a sialic acid-dependent CD22–SHP-1–integrin β7 axis, and on aged microglia functions as an anti-phagocytic receptor whose blockade restores clearance of neurotoxic substrates and improves cognition [PMID:21178327, PMID:33589816, PMID:30944478].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"CD22 was initially identified as a B-cell adhesion molecule related to MAG, establishing it within the immunoglobulin superfamily but leaving its signaling function unknown.\",\n      \"evidence\": \"Transfection and adhesion assays with CD22-expressing cells\",\n      \"pmids\": [\"1691828\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Adhesion mechanism not linked to any signaling pathway\", \"Ligand identity unknown\", \"No in vivo validation\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Identification of CD22 as an α2,6-sialic acid-binding lectin and demonstration of its physical association with the BCR complex established the molecular framework for its dual lectin–signaling receptor identity.\",\n      \"evidence\": \"CD22-Ig fusion binding assays with enzymatic validation; reciprocal co-immunoprecipitation of CD22 with sIgM/IgD/IgG, Igα, Igβ, and PTK72 from B cell lines\",\n      \"pmids\": [\"8463234\", \"7684686\", \"8475064\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No downstream signaling effector identified\", \"Unclear whether lectin activity is required for BCR association\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Discovery that tyrosine-phosphorylated CD22 recruits SHP-1 after BCR cross-linking identified the core phosphatase effector of CD22 inhibitory signaling, while constitutive internalization to lysosomes revealed an additional layer of CD22 surface regulation.\",\n      \"evidence\": \"Phosphorylation-dependent co-IP of SHP-1 with CD22; flow cytometry and neuraminidase protection assays for endocytic trafficking\",\n      \"pmids\": [\"7542197\", \"7722303\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific tyrosine residues mediating SHP-1 binding not mapped\", \"Functional consequence of SHP-1 recruitment not demonstrated in vivo\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"CD22 knockout mice revealed that CD22 is a physiological negative regulator of BCR signaling, resolving ambiguity about whether CD22 was activating or inhibitory in B cells.\",\n      \"evidence\": \"Gene-targeted CD22-deficient mice with calcium flux, proliferation, B-1 cell expansion, and serum autoantibody measurements\",\n      \"pmids\": [\"8864124\", \"9016707\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of lectin vs. ITIM domains not separated\", \"Mechanism of autoantibody development not defined\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Biochemical mapping of CD22–SHP-1 interaction showed that both SH2 domains of SHP-1 engage multiple phosphotyrosines on CD22, while aging CD22-KO mice developed high-affinity anti-dsDNA autoantibodies, linking CD22 loss to lupus-like autoimmunity.\",\n      \"evidence\": \"Phosphopeptide binding with mutagenesis of CD22 cytoplasmic tyrosines and SHP-1 SH2 domains; serum ELISA and somatic mutation analysis in CD22-KO mice\",\n      \"pmids\": [\"9890995\", \"10209047\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo disease relevance to human autoimmunity not established\", \"No structural basis for SHP-1–CD22 interaction\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Knock-in and mutagenesis studies demonstrated that CD22's sialic acid-binding activity is required for its negative regulatory function, linking the lectin and signaling activities mechanistically.\",\n      \"evidence\": \"Reconstitution of CD22 ligand-binding mutants in CD22-/- B cells with calcium flux and SHP-1 co-IP readouts; SPR measurement of CD22–CD45 affinity (Kd ~130 μM)\",\n      \"pmids\": [\"11994425\", \"12115612\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of physiological cis ligands unresolved\", \"Contribution of trans vs. cis interactions unclear\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Gene-targeted mice expressing ligand-binding-deficient CD22 separated lectin-dependent functions (surface expression, MZ B cell maintenance, proliferation) from lectin-independent ones (calcium inhibition, SHP-1 recruitment, migration), revealing that CD22 negative signaling does not absolutely require ligand binding.\",\n      \"evidence\": \"Knock-in mice with mutated CD22 sialic acid-binding domain, flow cytometry, calcium flux, B cell migration\",\n      \"pmids\": [\"15378059\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Apparent contradiction with earlier mutagenesis results awaited resolution\", \"Mechanism of ligand-independent ITIM signaling unclear\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"In situ photocross-linking revealed that CD22 homomultimers—not CD45 or other glycoproteins—are the dominant cis ligands, while genetic epistasis in ST6Gal1/CD22 double-KO mice showed that α2,6-sialylation controls BCR microdomain partitioning.\",\n      \"evidence\": \"Photoaffinity cross-linking proteomics on intact B cells; double-KO mice with membrane fractionation\",\n      \"pmids\": [\"16408005\", \"16369536\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for homomultimer formation unknown\", \"How microdomain redistribution translates to signaling quantitatively unclear\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identification of surface IgM on apposing B cells as the major trans ligand of CD22, and demonstration that CD22 dampens TLR signaling via SOCS1/3, expanded CD22's regulatory scope beyond BCR to innate-like pathways.\",\n      \"evidence\": \"UV photocross-linking proteomics and mass spectrometry for trans ligand identification; CD22-KO mice with TLR stimulation and SOCS expression analysis plus CD22 reconstitution\",\n      \"pmids\": [\"20172905\", \"21178327\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism linking CD22 to SOCS induction not defined\", \"Whether trans IgM engagement is inhibitory or activating in physiological contexts unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Dual knock-in mice separating CD22 ligand-binding from ITIM function resolved earlier contradictions: cis-ligand binding controls CD22–BCR proximity and calcium signaling magnitude, while ITIMs are essential for B cell survival, establishing a two-domain regulatory model.\",\n      \"evidence\": \"Knock-in mice with mutated CD22 ligand-binding domain or ITIMs, calcium flux and B cell turnover assays\",\n      \"pmids\": [\"23836650\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative relationship between nanocluster size and signaling output not established\", \"Role of CD22 in germinal center selection not fully addressed\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Super-resolution imaging and computational modeling revealed that CD22 nanoclusters are highly mobile, providing 'global BCR surveillance' through rapid lateral diffusion regulated by CD45 and the lectin domain, explaining how low-abundance CD22 can inhibit dispersed BCR clusters.\",\n      \"evidence\": \"Super-resolution microscopy, single-particle tracking, Brownian dynamics simulations in wild-type, CD45-deficient, and lectin-mutant B cells\",\n      \"pmids\": [\"26671981\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether nanocluster dynamics change during B cell activation not tested\", \"No measurement of CD22–BCR encounter kinetics in live cells during antigen stimulation\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"The 2.1 Å crystal structure of CD22 revealed a preformed β-hairpin unique among Siglecs that dictates α2,6-sialic acid specificity and showed an extended ectodomain compatible with simultaneous nanocluster and trans engagement.\",\n      \"evidence\": \"X-ray crystallography of CD22 alone and CD22–epratuzumab complex\",\n      \"pmids\": [\"28970495\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of CD22 in complex with natural glycan ligand at atomic resolution\", \"Ectodomain flexibility in membrane context unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"A CRISPR screen in microglia identified CD22 as an anti-phagocytic receptor outside the B cell lineage; CD22 blockade in aged mice restored microglial clearance of neurotoxic substrates and improved cognition, redefining CD22 as a broader immune checkpoint.\",\n      \"evidence\": \"Genome-wide CRISPR-Cas9 screen, in vivo anti-CD22 antibody treatment, phagocytosis assays, RNA-seq, cognitive behavioral testing in aged mice\",\n      \"pmids\": [\"30944478\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling pathway in microglia not fully characterized\", \"Whether SHP-1 mediates anti-phagocytic function in microglia not shown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Two new functional axes were defined: CD22 associates with integrin β7 in a sialic acid-dependent manner to recruit SHP-1 and promote gut homing; separately, soluble CD22 binds IGF2R on myeloid cells to disrupt lysosomal trafficking, expanding CD22's roles beyond classical BCR regulation.\",\n      \"evidence\": \"Co-IP and knock-in mice with in vivo homing assays (β7 axis); unbiased proteomic screen identifying IGF2R, domain truncation, iPSC-derived microglia functional rescue\",\n      \"pmids\": [\"33589816\", \"34851695\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether sCD22–IGF2R interaction is relevant in vivo in aging or neurodegeneration not shown\", \"Mechanism by which CD22 inhibits β7 endocytosis molecularly unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include how CD22 nanocluster dynamics are regulated during germinal center responses, whether CD22's microglial anti-phagocytic function operates through SHP-1, the structural basis of the CD22–integrin β7 and sCD22–IGF2R interactions, and how CD22 loss contributes to human autoimmune disease.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structural model of CD22 with natural glycan or integrin β7 partner\", \"SHP-1 requirement in microglial CD22 signaling not tested\", \"Human genetic evidence linking CD22 mutations to autoimmune disease lacking\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [8, 9, 10, 30]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [5, 11, 35]},\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 2, 19, 31]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [6]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [36]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [8, 9, 12, 27, 29]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [5, 10, 11, 24, 30]}\n    ],\n    \"complexes\": [\n      \"BCR complex (low stoichiometry association)\"\n    ],\n    \"partners\": [\n      \"SHP-1\",\n      \"LYN\",\n      \"CD45\",\n      \"ITGB7\",\n      \"IGF2R\",\n      \"AP2M1\",\n      \"IgM\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}