{"gene":"FCRL5","run_date":"2026-06-09T23:54:43","timeline":{"discoveries":[{"year":2001,"finding":"FCRL5 (IRTA2) encodes a novel cell surface receptor with an ectodomain containing Ig-like domains homologous to Fc and inhibitory receptor families, selectively expressed in mature B cells (centrocytes, marginal zone B cells, and immunoblasts); chromosome 1q21 translocations deregulate IRTA2 expression in tumor cell lines.","method":"Cloning of chromosomal translocation breakpoints, sequence analysis, expression analysis in B cell lines and tissues","journal":"Immunity","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — structural prediction and expression characterization from cloning, single lab, but foundational identification paper with multiple analyses","pmids":["11290337"],"is_preprint":false},{"year":2001,"finding":"FCRL5 (BXMAS1) is a cell surface receptor whose expression is transcriptionally induced by anti-IgM crosslinking in B cells; the predicted protein contains 6 Ig-like BXMAS1 domains and 2 cytoplasmic ITIM motifs, with expression restricted to B cells.","method":"Gene expression analysis after BCR crosslinking, sequence/domain analysis, cell line and tissue expression profiling","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, primarily sequence/expression characterization with no direct functional assay of the protein","pmids":["11453668"],"is_preprint":false},{"year":2006,"finding":"Epstein-Barr virus nuclear antigen 2 (EBNA2) markedly induces FcRH5 gene expression in a strictly CBF1-dependent manner; EBNA2 targets CBF1 binding sites in the FcRH5 promoter in vivo, as detected by chromatin immunoprecipitation, and induction does not require other viral proteins or de novo protein synthesis.","method":"EBNA2 overexpression in CBF1-deficient and wild-type cells, chromatin immunoprecipitation (ChIP) of FcRH5 promoter CBF1 binding sites","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Moderate — ChIP and genetic complementation (CBF1-deficient cells) with multiple orthogonal methods in single lab","pmids":["16439682"],"is_preprint":false},{"year":2012,"finding":"Human FcRL4 and FcRL5 are bona fide Fc receptors: FcRL5 binds all IgG isotypes with varied efficiency, as demonstrated in cellular binding assays; blocking monoclonal antibodies specific for these interactions were generated.","method":"Cellular binding assays with recombinant FcRL proteins and immunoglobulins; generation of blocking mAbs","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding assay with blocking antibody validation, single lab, two orthogonal approaches","pmids":["22491254"],"is_preprint":false},{"year":2012,"finding":"FcRL5 is internalized upon antibody binding on multiple myeloma/plasma cell surfaces, enabling effective antibody-drug conjugate (ADC) delivery; unconjugated anti-FcRL5 antibody alone was not efficacious whereas ADC formats were effective in vitro and in vivo.","method":"Flow cytometry internalization assay, in vitro cytotoxicity assays, in vivo xenograft mouse models","journal":"Molecular cancer therapeutics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — internalization confirmed by direct assay, functional consequence demonstrated in vitro and in vivo, single lab","pmids":["22807577"],"is_preprint":false},{"year":2013,"finding":"FCRL5 exerts inhibitory function on BCR signaling in marginal zone (MZ) B cells by recruiting the tyrosine phosphatase SHP-1 to a cytoplasmic ITIM; mutagenesis of the SHP-1 docking site revealed a coactivation function orchestrated by independent association of Lyn Src-family kinase with an intracellular ITAM-like sequence. FCRL5 had no influence on BCR signaling in peritoneal B1 B cells, correlating with differential SHP-1 and Lyn activity between MZ and B1 cells.","method":"Mutagenesis of ITIM/ITAM-like motifs, BCR signaling assays, SHP-1 and Lyn kinase activity measurements in MZ vs. B1 B cells","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — site-directed mutagenesis with defined molecular readouts (SHP-1 recruitment, Lyn association, BCR signaling), multiple orthogonal methods, single lab","pmids":["23509253"],"is_preprint":false},{"year":2017,"finding":"Anti-FcRH5/CD3 T cell-dependent bispecific antibody (TDB) triggers T cell receptor activation by inducing FcRH5 target clustering and exclusion of CD45 phosphatase from the immune synapse; the membrane-proximal epitope of FcRH5 is required for efficient synapse formation and myeloma cell killing.","method":"T cell-target cell synapse imaging, CD45 exclusion assay, killing assays with anti-FcRH5/CD3 TDB in human plasma cells and patient-derived myeloma cells, in vivo cynomolgus monkey depletion studies","journal":"Cancer cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — synapse imaging, functional killing assays, and in vivo primate studies across multiple orthogonal methods replicated in human and primate systems","pmids":["28262555"],"is_preprint":false},{"year":2018,"finding":"CD21 and FCRL5 physically associate to form a receptor complex on B cells; triple engagement of FCRL5, CD21, and BCR produces a superior calcium response compared to CD21+BCR co-stimulation alone. FCRL5 inhibits BCR signaling through its ITIMs in the absence of CD21 stimulation, but CD21 co-engagement converts FCRL5 from a negative to a positive co-receptor. Activating signaling molecules CD19, active PLCγ2, and BTK are rapidly recruited to FCRL5 upon engagement.","method":"Co-immunoprecipitation, calcium flux assays, signaling molecule recruitment assays in B cell lines and primary tonsil B cells, flow cytometry","journal":"International immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, calcium flux, and signaling recruitment assays with multiple cell types, single lab","pmids":["30107486"],"is_preprint":false},{"year":2023,"finding":"Upregulation of Fcrl5 in B cells disrupts B cell anergy and causes systemic autoimmunity in mice; Fcrl5 overexpression breaks B cell anergy and facilitates toll-like receptor signaling, demonstrated in B cell-specific Fcrl5 transgenic mice.","method":"B cell-specific Fcrl5 transgenic mouse generation, B cell anergy assays, TLR signaling assays, autoimmune disease model (SLE-like)","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — transgenic mouse model with defined cellular phenotype (broken anergy) and TLR signaling readout, single lab","pmids":["37841260"],"is_preprint":false},{"year":2024,"finding":"Fcrl5 ligation by agonistic antibodies reduces B cell death and enhances proliferation in LPS-stimulated B cells; in the presence of anti-CD40 and IL-5, Fcrl5 ligation suppresses cell death and enhances plasma cell differentiation, thereby promoting humoral immune responses.","method":"In vitro B cell culture with agonistic anti-Fcrl5 antibodies, cell death/proliferation assays, plasma cell differentiation assays, T cell-dependent and -independent antibody response assays in Fcrl5-overexpressing mice","journal":"International immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro ligation assays with defined cellular readouts and in vivo transgenic mouse model, single lab","pmids":["38738271"],"is_preprint":false},{"year":2026,"finding":"Human FcRL5 (but not mouse FcRL5) is a bona fide IgGFc (Fcγ) receptor that uniquely requires two Fcγ molecules in close proximity for robust interaction; cryo-EM reveals FcRL5 engages two Fcγ molecules at a 60° angle, with D1-D2 domains binding the first Fcγ and D3 domain arching over the second Fcγ. FcRL5 can internalize IgG polymers and immune complexes.","method":"Cryo-electron microscopy, binding assays with IgG immune complexes, internalization assays, species comparison (human vs. mouse FcRL5)","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure with domain-level binding mechanism, functional internalization assay, and species ortholog comparison; multiple orthogonal methods in single study","pmids":["41477863"],"is_preprint":false},{"year":2026,"finding":"Crystal structure of the FCRL5-IgG1 Fc complex at 3.4 Å reveals a 1:1 binding stoichiometry (confirmed by native mass spectrometry and SEC); FCRL5 binds IgG1 Fc in a manner completely distinct from classical Fcγ receptors, explaining why most Fc-silencing mutations do not disrupt FCRL5 binding. Selective cross-linking of FCRL5 with BCR in cis using Fc-engineered antibodies inhibits Ca2+ flux in FCRL5-expressing B cells.","method":"X-ray crystallography (3.4 Å), native mass spectrometry, size exclusion chromatography, directed evolution of FCRL5 variant, Ca2+ flux assay in B cells, comparison with FcγRIIb co-ligation","journal":"bioRxiv","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus native MS plus functional Ca2+ signaling assay, multiple orthogonal methods in single rigorous study","pmids":["42079264"],"is_preprint":true}],"current_model":"FCRL5 is a B cell-expressed IgG Fc receptor (binding all IgG isotypes) that structurally engages two Fcγ molecules simultaneously via a unique binding mode distinct from classical Fcγ receptors; it exerts binary (inhibitory or activating) regulation of BCR signaling through ITIM-mediated SHP-1 recruitment or ITAM-like-mediated Lyn association, and its signaling polarity is switched from inhibitory to activating by physical association with CD21 upon immune complex engagement; FcRL5 expression is transcriptionally induced by BCR crosslinking and EBV EBNA2 (via CBF1), and the protein is internalized upon antibody binding, enabling immune complex and IgG polymer uptake."},"narrative":{"mechanistic_narrative":"FCRL5 (IRTA2/BXMAS1) is a B cell-restricted cell-surface receptor that couples engagement of IgG Fc to bidirectional regulation of B cell receptor (BCR) signaling, and its function spans normal humoral immunity, autoimmunity, and B cell malignancy [PMID:11290337, PMID:23509253]. It is a bona fide Fcγ receptor that binds all IgG isotypes [PMID:22491254], engaging Fc through a mode entirely distinct from classical Fcγ receptors—structural work shows it can simultaneously bridge two Fcγ molecules at a 60° angle (D1–D2 binding the first Fcγ, D3 arching over the second), explaining why conventional Fc-silencing mutations fail to abolish binding and enabling uptake of IgG polymers and immune complexes [PMID:41477863, PMID:42079264]. Signaling output is binary: through a cytoplasmic ITIM, FCRL5 recruits the phosphatase SHP-1 to inhibit BCR signaling, whereas an intracellular ITAM-like sequence that associates with the Src-family kinase Lyn confers coactivation, with the net effect dependent on B cell subset [PMID:23509253]. Physical association with CD21 switches this polarity, converting FCRL5 from a negative to a positive co-receptor and driving rapid recruitment of CD19, active PLCγ2, and BTK upon immune-complex engagement [PMID:30107486]. FCRL5 expression is transcriptionally induced by BCR crosslinking [PMID:11453668] and by EBV EBNA2 acting through CBF1 binding sites in the FcRH5 promoter [PMID:16439682]. Functionally, FCRL5 ligation can suppress B cell death and promote proliferation and plasma cell differentiation [PMID:38738271], while its dysregulated overexpression breaks B cell anergy and drives systemic autoimmunity [PMID:37841260]. The receptor is internalized upon antibody binding, a property exploited by antibody-drug conjugates and anti-FcRH5/CD3 bispecific antibodies that kill myeloma cells via synapse formation and CD45 exclusion [PMID:22807577, PMID:28262555].","teleology":[{"year":2001,"claim":"Established FCRL5 as a B cell-restricted surface receptor with an Fc/inhibitory-receptor-like ectodomain and cytoplasmic ITIMs, and linked its deregulation to B cell tumors, defining the central questions of ligand and signaling polarity.","evidence":"Cloning of 1q21 translocation breakpoints, sequence/domain analysis, and expression profiling in B cell subsets and lines; separate BCR-crosslinking expression study","pmids":["11290337","11453668"],"confidence":"Medium","gaps":["No ligand identified at this stage","ITIM/ITAM-like signaling function not tested functionally","Translocation linkage correlative, not causal"]},{"year":2006,"claim":"Defined a viral transcriptional control mechanism for FCRL5, showing EBNA2 induces the gene via CBF1, connecting EBV B cell biology to FcRL5 expression.","evidence":"EBNA2 overexpression in CBF1-deficient and wild-type cells with ChIP of promoter CBF1 sites","pmids":["16439682"],"confidence":"High","gaps":["Does not address consequences of FcRL5 induction for B cell function","Physiological (non-viral) transcriptional regulation not defined"]},{"year":2012,"claim":"Demonstrated FcRL5 is a genuine Fc receptor binding all IgG isotypes, and that antibody binding triggers internalization—answering the long-open ligand question and revealing therapeutic targeting potential.","evidence":"Cellular IgG binding assays with recombinant FcRL5 and blocking mAbs; flow-cytometry internalization plus in vitro/in vivo ADC efficacy","pmids":["22491254","22807577"],"confidence":"Medium","gaps":["Structural basis of IgG binding not resolved","Binding stoichiometry unknown","Signaling outcome of physiological Fc engagement not established"]},{"year":2013,"claim":"Resolved the binary signaling logic of FCRL5, showing ITIM-dependent SHP-1 recruitment inhibits BCR signaling while an ITAM-like motif recruiting Lyn confers coactivation, with output dictated by B cell subset.","evidence":"Site-directed mutagenesis of ITIM/ITAM-like motifs with SHP-1/Lyn activity and BCR signaling readouts in MZ vs B1 B cells","pmids":["23509253"],"confidence":"High","gaps":["What governs the inhibitory-vs-activating switch in vivo not defined","Co-receptor partners not identified at this stage"]},{"year":2017,"claim":"Showed that therapeutic clustering of FcRH5 via a CD3 bispecific drives T cell synapse formation through CD45 exclusion, establishing membrane-proximal epitope requirements for myeloma killing.","evidence":"Synapse imaging, CD45 exclusion assays, killing assays on plasma cells/patient myeloma, and in vivo cynomolgus depletion","pmids":["28262555"],"confidence":"High","gaps":["Concerns therapeutic clustering, not endogenous receptor function","Does not address physiological ligand engagement"]},{"year":2018,"claim":"Identified CD21 as a physical partner that switches FCRL5 from inhibitory to activating, providing the molecular mechanism for context-dependent signaling polarity.","evidence":"Reciprocal Co-IP, calcium flux, and recruitment of CD19/PLCγ2/BTK in B cell lines and primary tonsil B cells","pmids":["30107486"],"confidence":"Medium","gaps":["Structural basis of CD21 association unknown","Whether immune complex serves as the physiological trigger not directly shown","Single lab, not reciprocally validated elsewhere"]},{"year":2023,"claim":"Established that FcRL5 dysregulation has pathological consequences, showing overexpression breaks B cell anergy and enhances TLR signaling to drive systemic autoimmunity.","evidence":"B cell-specific Fcrl5 transgenic mice with anergy and TLR signaling assays in an SLE-like model","pmids":["37841260"],"confidence":"Medium","gaps":["Overexpression model may not reflect endogenous levels","Mechanistic link between FcRL5 and TLR pathway not detailed","Mouse FcRL5 differs from human in Fc binding"]},{"year":2024,"claim":"Demonstrated a pro-survival, pro-differentiation role for FcRL5 ligation, showing it suppresses B cell death and promotes proliferation and plasma cell differentiation to enhance humoral responses.","evidence":"In vitro agonistic anti-Fcrl5 ligation with death/proliferation and plasma-cell differentiation assays, plus antibody responses in Fcrl5-overexpressing mice","pmids":["38738271"],"confidence":"Medium","gaps":["Relies on agonistic antibody and overexpression rather than endogenous engagement","Signaling pathway mediating survival not mapped"]},{"year":2026,"claim":"Solved the structural basis of FcRL5–IgG recognition, revealing a non-canonical Fc-binding mode—including a unique dual-Fcγ engagement geometry—that explains resistance to Fc-silencing mutations and enables immune complex uptake.","evidence":"Cryo-EM of dual-Fcγ engagement and species comparison; X-ray crystallography (3.4 Å), native MS, SEC, and cis BCR cross-linking Ca2+ flux assays","pmids":["41477863","42079264"],"confidence":"High","gaps":["Apparent discrepancy between 1:1 (crystal/MS) and dual-Fcγ (cryo-EM) stoichiometries not reconciled","How structural geometry maps to inhibitory vs activating signaling unresolved","One report is a preprint"]},{"year":null,"claim":"How FcRL5's distinct structural Fc-engagement modes are translated into the inhibitory-versus-activating signaling switch, and what physiological ligand contexts (CD21, immune complexes, subset identity) determine the outcome in vivo, remain to be integrated.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified model linking Fc-binding geometry to ITIM/ITAM-like output","Endogenous physiological trigger in vivo undefined","Human vs mouse functional divergence not fully reconciled"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[5,7]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,7]},{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[4,10]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,3,7]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[4,10]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[5,7,8]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[5,7]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[2]}],"complexes":["FCRL5-CD21 receptor complex"],"partners":["SHP-1","LYN","CD21","CD19","PLCG2","BTK"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q96RD9","full_name":"Fc receptor-like protein 5","aliases":["BXMAS1","Fc receptor homolog 5","FcRH5","Immune receptor translocation-associated protein 2"],"length_aa":977,"mass_kda":106.4,"function":"Plays an important role in B-cell response to antigen that acts both as a negative or positive coreceptor. Inhibits B-cell receptor (BCR) signaling in the absence of CR2 stimulation but engagement with CR2 and the BCR lead to a superior calcium response compared to CR2 and BCR costimulation (PubMed:30107486). May be involved in B-cell development and differentiation in peripheral lymphoid organs and may be useful markers of B-cell stages. May have an immunoregulatory role in marginal zone B-cells. May play a role in fertilization (By similarity)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q96RD9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FCRL5","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/FCRL5","total_profiled":1310},"omim":[{"mim_id":"605877","title":"Fc RECEPTOR-LIKE PROTEIN 5; FCRL5","url":"https://www.omim.org/entry/605877"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"lymphoid tissue","ntpm":34.1}],"url":"https://www.proteinatlas.org/search/FCRL5"},"hgnc":{"alias_symbol":["FCRH5","IRTA2","BXMAS1","CD307e"],"prev_symbol":[]},"alphafold":{"accession":"Q96RD9","domains":[{"cath_id":"2.60.40.10","chopping":"25-187","consensus_level":"medium","plddt":85.7174,"start":25,"end":187},{"cath_id":"2.60.40.10","chopping":"190-279","consensus_level":"high","plddt":84.938,"start":190,"end":279},{"cath_id":"2.60.40.10","chopping":"289-372","consensus_level":"high","plddt":75.0937,"start":289,"end":372},{"cath_id":"2.60.40.10","chopping":"381-465","consensus_level":"high","plddt":84.898,"start":381,"end":465},{"cath_id":"2.60.40.10","chopping":"475-558","consensus_level":"medium","plddt":81.0169,"start":475,"end":558},{"cath_id":"2.60.40.10","chopping":"568-651","consensus_level":"medium","plddt":87.2756,"start":568,"end":651},{"cath_id":"2.60.40.10","chopping":"660-744","consensus_level":"high","plddt":89.1398,"start":660,"end":744},{"cath_id":"2.60.40.10","chopping":"751-836","consensus_level":"high","plddt":84.6505,"start":751,"end":836}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96RD9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96RD9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96RD9-F1-predicted_aligned_error_v6.png","plddt_mean":76.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FCRL5","jax_strain_url":"https://www.jax.org/strain/search?query=FCRL5"},"sequence":{"accession":"Q96RD9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96RD9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96RD9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96RD9"}},"corpus_meta":[{"pmid":"28262555","id":"PMC_28262555","title":"Membrane-Proximal 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IgGs.","date":"2026","source":"Science advances","url":"https://pubmed.ncbi.nlm.nih.gov/41477863","citation_count":2,"is_preprint":false},{"pmid":"38738271","id":"PMC_38738271","title":"Humoral responses are enhanced by facilitating B cell viability by Fcrl5 overexpression in B cells.","date":"2024","source":"International immunology","url":"https://pubmed.ncbi.nlm.nih.gov/38738271","citation_count":1,"is_preprint":false},{"pmid":"38992496","id":"PMC_38992496","title":"Allostimulation leads to emergence of a human B cell population with increased expression of HLA class I antigen presentation-associated molecules and the immunoglobulin receptor FcRL5.","date":"2024","source":"American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons","url":"https://pubmed.ncbi.nlm.nih.gov/38992496","citation_count":1,"is_preprint":false},{"pmid":"39461276","id":"PMC_39461276","title":"Polymorphisms of B-lymphocyte-associated genes CD20 and FCRL5 are associated with susceptibility to autoimmune thyroid diseases.","date":"2024","source":"Human immunology","url":"https://pubmed.ncbi.nlm.nih.gov/39461276","citation_count":1,"is_preprint":false},{"pmid":"38259460","id":"PMC_38259460","title":"Erratum: Upregulated Fcrl5 disrupts B cell anergy and causes autoimmune disease.","date":"2024","source":"Frontiers in immunology","url":"https://pubmed.ncbi.nlm.nih.gov/38259460","citation_count":0,"is_preprint":false},{"pmid":"42079264","id":"PMC_42079264","title":"Structure-Function Analysis of the FCRL5-IgG1 Fc Complex Reveals an Unappreciated Effect of Fc-Silent Antibodies on B cells.","date":"2026","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/42079264","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":16074,"output_tokens":3164,"usd":0.047841,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10720,"output_tokens":3740,"usd":0.07355,"stage2_stop_reason":"end_turn"},"total_usd":0.121391,"stage1_batch_id":"msgbatch_01AsR7g8dGTEiEw9U83bJeWn","stage2_batch_id":"msgbatch_01YV5ZyxtFNUZEjxRcpd9jzb","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2001,\n      \"finding\": \"FCRL5 (IRTA2) encodes a novel cell surface receptor with an ectodomain containing Ig-like domains homologous to Fc and inhibitory receptor families, selectively expressed in mature B cells (centrocytes, marginal zone B cells, and immunoblasts); chromosome 1q21 translocations deregulate IRTA2 expression in tumor cell lines.\",\n      \"method\": \"Cloning of chromosomal translocation breakpoints, sequence analysis, expression analysis in B cell lines and tissues\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — structural prediction and expression characterization from cloning, single lab, but foundational identification paper with multiple analyses\",\n      \"pmids\": [\"11290337\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"FCRL5 (BXMAS1) is a cell surface receptor whose expression is transcriptionally induced by anti-IgM crosslinking in B cells; the predicted protein contains 6 Ig-like BXMAS1 domains and 2 cytoplasmic ITIM motifs, with expression restricted to B cells.\",\n      \"method\": \"Gene expression analysis after BCR crosslinking, sequence/domain analysis, cell line and tissue expression profiling\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, primarily sequence/expression characterization with no direct functional assay of the protein\",\n      \"pmids\": [\"11453668\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Epstein-Barr virus nuclear antigen 2 (EBNA2) markedly induces FcRH5 gene expression in a strictly CBF1-dependent manner; EBNA2 targets CBF1 binding sites in the FcRH5 promoter in vivo, as detected by chromatin immunoprecipitation, and induction does not require other viral proteins or de novo protein synthesis.\",\n      \"method\": \"EBNA2 overexpression in CBF1-deficient and wild-type cells, chromatin immunoprecipitation (ChIP) of FcRH5 promoter CBF1 binding sites\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP and genetic complementation (CBF1-deficient cells) with multiple orthogonal methods in single lab\",\n      \"pmids\": [\"16439682\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Human FcRL4 and FcRL5 are bona fide Fc receptors: FcRL5 binds all IgG isotypes with varied efficiency, as demonstrated in cellular binding assays; blocking monoclonal antibodies specific for these interactions were generated.\",\n      \"method\": \"Cellular binding assays with recombinant FcRL proteins and immunoglobulins; generation of blocking mAbs\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding assay with blocking antibody validation, single lab, two orthogonal approaches\",\n      \"pmids\": [\"22491254\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"FcRL5 is internalized upon antibody binding on multiple myeloma/plasma cell surfaces, enabling effective antibody-drug conjugate (ADC) delivery; unconjugated anti-FcRL5 antibody alone was not efficacious whereas ADC formats were effective in vitro and in vivo.\",\n      \"method\": \"Flow cytometry internalization assay, in vitro cytotoxicity assays, in vivo xenograft mouse models\",\n      \"journal\": \"Molecular cancer therapeutics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — internalization confirmed by direct assay, functional consequence demonstrated in vitro and in vivo, single lab\",\n      \"pmids\": [\"22807577\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"FCRL5 exerts inhibitory function on BCR signaling in marginal zone (MZ) B cells by recruiting the tyrosine phosphatase SHP-1 to a cytoplasmic ITIM; mutagenesis of the SHP-1 docking site revealed a coactivation function orchestrated by independent association of Lyn Src-family kinase with an intracellular ITAM-like sequence. FCRL5 had no influence on BCR signaling in peritoneal B1 B cells, correlating with differential SHP-1 and Lyn activity between MZ and B1 cells.\",\n      \"method\": \"Mutagenesis of ITIM/ITAM-like motifs, BCR signaling assays, SHP-1 and Lyn kinase activity measurements in MZ vs. B1 B cells\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — site-directed mutagenesis with defined molecular readouts (SHP-1 recruitment, Lyn association, BCR signaling), multiple orthogonal methods, single lab\",\n      \"pmids\": [\"23509253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Anti-FcRH5/CD3 T cell-dependent bispecific antibody (TDB) triggers T cell receptor activation by inducing FcRH5 target clustering and exclusion of CD45 phosphatase from the immune synapse; the membrane-proximal epitope of FcRH5 is required for efficient synapse formation and myeloma cell killing.\",\n      \"method\": \"T cell-target cell synapse imaging, CD45 exclusion assay, killing assays with anti-FcRH5/CD3 TDB in human plasma cells and patient-derived myeloma cells, in vivo cynomolgus monkey depletion studies\",\n      \"journal\": \"Cancer cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — synapse imaging, functional killing assays, and in vivo primate studies across multiple orthogonal methods replicated in human and primate systems\",\n      \"pmids\": [\"28262555\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"CD21 and FCRL5 physically associate to form a receptor complex on B cells; triple engagement of FCRL5, CD21, and BCR produces a superior calcium response compared to CD21+BCR co-stimulation alone. FCRL5 inhibits BCR signaling through its ITIMs in the absence of CD21 stimulation, but CD21 co-engagement converts FCRL5 from a negative to a positive co-receptor. Activating signaling molecules CD19, active PLCγ2, and BTK are rapidly recruited to FCRL5 upon engagement.\",\n      \"method\": \"Co-immunoprecipitation, calcium flux assays, signaling molecule recruitment assays in B cell lines and primary tonsil B cells, flow cytometry\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, calcium flux, and signaling recruitment assays with multiple cell types, single lab\",\n      \"pmids\": [\"30107486\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Upregulation of Fcrl5 in B cells disrupts B cell anergy and causes systemic autoimmunity in mice; Fcrl5 overexpression breaks B cell anergy and facilitates toll-like receptor signaling, demonstrated in B cell-specific Fcrl5 transgenic mice.\",\n      \"method\": \"B cell-specific Fcrl5 transgenic mouse generation, B cell anergy assays, TLR signaling assays, autoimmune disease model (SLE-like)\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — transgenic mouse model with defined cellular phenotype (broken anergy) and TLR signaling readout, single lab\",\n      \"pmids\": [\"37841260\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Fcrl5 ligation by agonistic antibodies reduces B cell death and enhances proliferation in LPS-stimulated B cells; in the presence of anti-CD40 and IL-5, Fcrl5 ligation suppresses cell death and enhances plasma cell differentiation, thereby promoting humoral immune responses.\",\n      \"method\": \"In vitro B cell culture with agonistic anti-Fcrl5 antibodies, cell death/proliferation assays, plasma cell differentiation assays, T cell-dependent and -independent antibody response assays in Fcrl5-overexpressing mice\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro ligation assays with defined cellular readouts and in vivo transgenic mouse model, single lab\",\n      \"pmids\": [\"38738271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Human FcRL5 (but not mouse FcRL5) is a bona fide IgGFc (Fcγ) receptor that uniquely requires two Fcγ molecules in close proximity for robust interaction; cryo-EM reveals FcRL5 engages two Fcγ molecules at a 60° angle, with D1-D2 domains binding the first Fcγ and D3 domain arching over the second Fcγ. FcRL5 can internalize IgG polymers and immune complexes.\",\n      \"method\": \"Cryo-electron microscopy, binding assays with IgG immune complexes, internalization assays, species comparison (human vs. mouse FcRL5)\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure with domain-level binding mechanism, functional internalization assay, and species ortholog comparison; multiple orthogonal methods in single study\",\n      \"pmids\": [\"41477863\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Crystal structure of the FCRL5-IgG1 Fc complex at 3.4 Å reveals a 1:1 binding stoichiometry (confirmed by native mass spectrometry and SEC); FCRL5 binds IgG1 Fc in a manner completely distinct from classical Fcγ receptors, explaining why most Fc-silencing mutations do not disrupt FCRL5 binding. Selective cross-linking of FCRL5 with BCR in cis using Fc-engineered antibodies inhibits Ca2+ flux in FCRL5-expressing B cells.\",\n      \"method\": \"X-ray crystallography (3.4 Å), native mass spectrometry, size exclusion chromatography, directed evolution of FCRL5 variant, Ca2+ flux assay in B cells, comparison with FcγRIIb co-ligation\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus native MS plus functional Ca2+ signaling assay, multiple orthogonal methods in single rigorous study\",\n      \"pmids\": [\"42079264\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"FCRL5 is a B cell-expressed IgG Fc receptor (binding all IgG isotypes) that structurally engages two Fcγ molecules simultaneously via a unique binding mode distinct from classical Fcγ receptors; it exerts binary (inhibitory or activating) regulation of BCR signaling through ITIM-mediated SHP-1 recruitment or ITAM-like-mediated Lyn association, and its signaling polarity is switched from inhibitory to activating by physical association with CD21 upon immune complex engagement; FcRL5 expression is transcriptionally induced by BCR crosslinking and EBV EBNA2 (via CBF1), and the protein is internalized upon antibody binding, enabling immune complex and IgG polymer uptake.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"FCRL5 (IRTA2/BXMAS1) is a B cell-restricted cell-surface receptor that couples engagement of IgG Fc to bidirectional regulation of B cell receptor (BCR) signaling, and its function spans normal humoral immunity, autoimmunity, and B cell malignancy [#0, #5]. It is a bona fide Fc\\u03b3 receptor that binds all IgG isotypes [#3], engaging Fc through a mode entirely distinct from classical Fc\\u03b3 receptors\\u2014structural work shows it can simultaneously bridge two Fc\\u03b3 molecules at a 60\\u00b0 angle (D1\\u2013D2 binding the first Fc\\u03b3, D3 arching over the second), explaining why conventional Fc-silencing mutations fail to abolish binding and enabling uptake of IgG polymers and immune complexes [#10, #11]. Signaling output is binary: through a cytoplasmic ITIM, FCRL5 recruits the phosphatase SHP-1 to inhibit BCR signaling, whereas an intracellular ITAM-like sequence that associates with the Src-family kinase Lyn confers coactivation, with the net effect dependent on B cell subset [#5]. Physical association with CD21 switches this polarity, converting FCRL5 from a negative to a positive co-receptor and driving rapid recruitment of CD19, active PLC\\u03b32, and BTK upon immune-complex engagement [#7]. FCRL5 expression is transcriptionally induced by BCR crosslinking [#1] and by EBV EBNA2 acting through CBF1 binding sites in the FcRH5 promoter [#2]. Functionally, FCRL5 ligation can suppress B cell death and promote proliferation and plasma cell differentiation [#9], while its dysregulated overexpression breaks B cell anergy and drives systemic autoimmunity [#8]. The receptor is internalized upon antibody binding, a property exploited by antibody-drug conjugates and anti-FcRH5/CD3 bispecific antibodies that kill myeloma cells via synapse formation and CD45 exclusion [#4, #6].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Established FCRL5 as a B cell-restricted surface receptor with an Fc/inhibitory-receptor-like ectodomain and cytoplasmic ITIMs, and linked its deregulation to B cell tumors, defining the central questions of ligand and signaling polarity.\",\n      \"evidence\": \"Cloning of 1q21 translocation breakpoints, sequence/domain analysis, and expression profiling in B cell subsets and lines; separate BCR-crosslinking expression study\",\n      \"pmids\": [\"11290337\", \"11453668\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No ligand identified at this stage\", \"ITIM/ITAM-like signaling function not tested functionally\", \"Translocation linkage correlative, not causal\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Defined a viral transcriptional control mechanism for FCRL5, showing EBNA2 induces the gene via CBF1, connecting EBV B cell biology to FcRL5 expression.\",\n      \"evidence\": \"EBNA2 overexpression in CBF1-deficient and wild-type cells with ChIP of promoter CBF1 sites\",\n      \"pmids\": [\"16439682\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not address consequences of FcRL5 induction for B cell function\", \"Physiological (non-viral) transcriptional regulation not defined\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstrated FcRL5 is a genuine Fc receptor binding all IgG isotypes, and that antibody binding triggers internalization\\u2014answering the long-open ligand question and revealing therapeutic targeting potential.\",\n      \"evidence\": \"Cellular IgG binding assays with recombinant FcRL5 and blocking mAbs; flow-cytometry internalization plus in vitro/in vivo ADC efficacy\",\n      \"pmids\": [\"22491254\", \"22807577\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis of IgG binding not resolved\", \"Binding stoichiometry unknown\", \"Signaling outcome of physiological Fc engagement not established\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Resolved the binary signaling logic of FCRL5, showing ITIM-dependent SHP-1 recruitment inhibits BCR signaling while an ITAM-like motif recruiting Lyn confers coactivation, with output dictated by B cell subset.\",\n      \"evidence\": \"Site-directed mutagenesis of ITIM/ITAM-like motifs with SHP-1/Lyn activity and BCR signaling readouts in MZ vs B1 B cells\",\n      \"pmids\": [\"23509253\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"What governs the inhibitory-vs-activating switch in vivo not defined\", \"Co-receptor partners not identified at this stage\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Showed that therapeutic clustering of FcRH5 via a CD3 bispecific drives T cell synapse formation through CD45 exclusion, establishing membrane-proximal epitope requirements for myeloma killing.\",\n      \"evidence\": \"Synapse imaging, CD45 exclusion assays, killing assays on plasma cells/patient myeloma, and in vivo cynomolgus depletion\",\n      \"pmids\": [\"28262555\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Concerns therapeutic clustering, not endogenous receptor function\", \"Does not address physiological ligand engagement\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identified CD21 as a physical partner that switches FCRL5 from inhibitory to activating, providing the molecular mechanism for context-dependent signaling polarity.\",\n      \"evidence\": \"Reciprocal Co-IP, calcium flux, and recruitment of CD19/PLC\\u03b32/BTK in B cell lines and primary tonsil B cells\",\n      \"pmids\": [\"30107486\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis of CD21 association unknown\", \"Whether immune complex serves as the physiological trigger not directly shown\", \"Single lab, not reciprocally validated elsewhere\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Established that FcRL5 dysregulation has pathological consequences, showing overexpression breaks B cell anergy and enhances TLR signaling to drive systemic autoimmunity.\",\n      \"evidence\": \"B cell-specific Fcrl5 transgenic mice with anergy and TLR signaling assays in an SLE-like model\",\n      \"pmids\": [\"37841260\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Overexpression model may not reflect endogenous levels\", \"Mechanistic link between FcRL5 and TLR pathway not detailed\", \"Mouse FcRL5 differs from human in Fc binding\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Demonstrated a pro-survival, pro-differentiation role for FcRL5 ligation, showing it suppresses B cell death and promotes proliferation and plasma cell differentiation to enhance humoral responses.\",\n      \"evidence\": \"In vitro agonistic anti-Fcrl5 ligation with death/proliferation and plasma-cell differentiation assays, plus antibody responses in Fcrl5-overexpressing mice\",\n      \"pmids\": [\"38738271\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Relies on agonistic antibody and overexpression rather than endogenous engagement\", \"Signaling pathway mediating survival not mapped\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Solved the structural basis of FcRL5\\u2013IgG recognition, revealing a non-canonical Fc-binding mode\\u2014including a unique dual-Fc\\u03b3 engagement geometry\\u2014that explains resistance to Fc-silencing mutations and enables immune complex uptake.\",\n      \"evidence\": \"Cryo-EM of dual-Fc\\u03b3 engagement and species comparison; X-ray crystallography (3.4 \\u00c5), native MS, SEC, and cis BCR cross-linking Ca2+ flux assays\",\n      \"pmids\": [\"41477863\", \"42079264\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Apparent discrepancy between 1:1 (crystal/MS) and dual-Fc\\u03b3 (cryo-EM) stoichiometries not reconciled\", \"How structural geometry maps to inhibitory vs activating signaling unresolved\", \"One report is a preprint\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How FcRL5's distinct structural Fc-engagement modes are translated into the inhibitory-versus-activating signaling switch, and what physiological ligand contexts (CD21, immune complexes, subset identity) determine the outcome in vivo, remain to be integrated.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model linking Fc-binding geometry to ITIM/ITAM-like output\", \"Endogenous physiological trigger in vivo undefined\", \"Human vs mouse functional divergence not fully reconciled\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0019864\", \"supporting_discovery_ids\": [3, 10, 11]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [5, 7]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 7]},\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [4, 10]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 3, 7]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [4, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [5, 7, 8]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [5, 7]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"complexes\": [\"FCRL5-CD21 receptor complex\"],\n    \"partners\": [\"SHP-1\", \"LYN\", \"CD21\", \"CD19\", \"PLCG2\", \"BTK\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}