{"gene":"C4A","run_date":"2026-06-09T22:02:45","timeline":{"discoveries":[{"year":1981,"finding":"Human C4a anaphylatoxin is a 77-residue cationic polypeptide (Mr ~9000) generated by C1s cleavage of the C4 alpha-chain between positions 77 and 78. Primary structure determination revealed ~30% homology with C3a and ~36% with C5a, placing C4a in the anaphylatoxin family derived from a common genetic origin.","method":"Protein purification from activated serum, cyanogen bromide fragmentation, chymotryptic digestion, amino acid sequence analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — complete primary structure determination by direct protein sequencing with multiple fragmentation strategies; foundational biochemical characterization","pmids":["6167582"],"is_preprint":false},{"year":1984,"finding":"C4A and C4B are encoded by at least two separate loci in the HLA Class III region on chromosome 6, and nucleotide sequence analysis of full-length C4A cDNA and partial C4B cDNA identified class-specific sequence differences, enabling synthesis of isotype-specific oligonucleotide probes. Three C4 loci (~16 kb each, spaced by ~10 kb) were aligned 30 kb from the factor B gene in overlapping cosmid genomic fragments.","method":"cDNA cloning, complete nucleotide sequencing, restriction mapping, hybridization with C4A/C4B-specific synthetic oligonucleotides, cosmid library genomic cloning","journal":"Philosophical transactions of the Royal Society of London. Series B, Biological sciences","confidence":"High","confidence_rationale":"Tier 1 / Strong — complete cDNA sequencing and genomic mapping with multiple orthogonal methods; foundational structural characterization of the gene locus","pmids":["6149580"],"is_preprint":false},{"year":1984,"finding":"Processing of C4 to its three-chain (beta-alpha-gamma) structure is required for efficient activation by C1s. Incompletely processed two-chain C4 forms (beta-alpha+gamma and beta+alpha-gamma) and the extracellular precursor pro-C4 are resistant to C1s cleavage and hemolytically inactive relative to the mature three-chain C4.","method":"Hemolytic activity assays, solid-phase competitive radioimmunoassay, C1s cleavage experiments on HepG2-secreted C4 forms, autolysis assays","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct in vitro functional assays with purified components, multiple processing forms tested with rigorous controls","pmids":["6321593"],"is_preprint":false},{"year":1985,"finding":"The alpha- and beta-chains of human C4 are glycosylated (the gamma-chain is not). The alpha-chain bears three complex fucosylated biantennary oligosaccharides and is sulfated in its alpha-4 fragment; the beta-chain bears a single high-mannose oligosaccharide. The ~2000 Mr difference between C4A and C4B alpha-chains localizes to the alpha-2 fragment and is not due to carbohydrate, indicating the difference resides in amino acid sequence.","method":"Biochemical fractionation of purified plasma C4 and HepG2-secreted C4; carbohydrate composition analysis; localization of oligosaccharide modifications to individual chain fragments","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct biochemical structural analysis with purified protein, multiple analytical methods on both plasma and cell-secreted C4","pmids":["2981921"],"is_preprint":false},{"year":1985,"finding":"Hydralazine binds covalently to C4, becoming attached to the polypeptide chain containing the active-site thioester; C4A gene product binds hydralazine more efficiently than C4B, suggesting the thioester chemistry differs between the two isotypes.","method":"Covalent binding assay with radiolabeled hydralazine; SDS-PAGE analysis of C4A vs C4B after hydralazine treatment","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct covalent binding chemistry demonstrated with two isotypes, single lab, two methods (binding + gel analysis)","pmids":["3873356"],"is_preprint":false},{"year":1988,"finding":"C4b binds covalently to IgG heavy chains in the fluid phase (in the absence of preformed immune precipitates), primarily via amide bonds for C4A3 and via both amide and acyl ester bonds for C4B1. C4A3 binds 3–4 times more IgG than C4B1 across a range of concentrations, and also shows higher binding efficiency for IgM, IgA, IgG2a, F(ab')2, and BSA.","method":"Fluid-phase binding assay with purified C4 variants and C1s; SDS-PAGE to identify chain of attachment; comparison of amide vs. ester linkage chemistry","journal":"Molecular immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted in vitro system with purified components, multiple substrates tested, bond chemistry defined, replicated across multiple immunoglobulin classes","pmids":["3264881"],"is_preprint":false},{"year":1988,"finding":"C4A3 is more effective than C4B1 at inhibiting the rate of immune precipitate formation in both serum and serum-free reaction mixtures containing C1 and C4. Covalent deposition of C4b on immune complexes is the essential mechanistic step underlying inhibition of immune precipitation.","method":"Immune precipitation inhibition assay in serum and reconstituted serum-free system; rate measurements with purified C4A3 vs C4B1","journal":"Complement (Basel, Switzerland)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional comparison of two isotypes in reconstituted system; single lab, multiple assay conditions","pmids":["3180739"],"is_preprint":false},{"year":1989,"finding":"The two C4 isotypes C4A and C4B differ in covalent binding specificity: C4B is more efficient in hemolytic assays (binding to carbohydrate antigens via ester bonds), whereas C4A binds preferentially to immune complexes (via amide bonds with protein antigens). The two isotypes differ more than fivefold in hemolysis-to-immune-complex-processing assays.","method":"Hemolytic assays and immune complex processing assays comparing purified C4A and C4B isotypes","journal":"Complement and inflammation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional assays with purified proteins; consistent with multiple other studies but review/summary format","pmids":["2650988"],"is_preprint":false},{"year":1990,"finding":"C4A is markedly more effective than C4B at enhancing binding of immune complexes to complement receptor CR1 on red blood cells, with the difference being the major functional distinction between the two isotypes in immune complex handling. This was demonstrated with both preformed and nascent immune complexes at equivalence and antibody excess.","method":"CR1 binding assay with red blood cells; inhibition of immunoprecipitation assay; comparison of purified C4A vs C4B with preformed and nascent immune complexes","journal":"Clinical and experimental immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct functional comparison using purified isotypes, multiple assay conditions (preformed and nascent complexes, antibody excess and equivalence), mechanistically informative","pmids":["2138067"],"is_preprint":false},{"year":1990,"finding":"Mammalian C4 binding specificity is evolutionarily variable: chimpanzee and rhesus monkey produce two C4 types with C4A-like and C4B-like reactivities (similar to humans), while rat, guinea pig, whale, rabbit, dog, and pig express C4 with only a single C4B-like specificity. Sheep and cattle express both C4A-like and C4B-like proteins, suggesting a duplicated locus.","method":"C4 purification from multiple mammalian species; binding specificity assays","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — purified proteins from multiple species tested in binding assays; single lab comparative study","pmids":["2302180"],"is_preprint":false},{"year":2001,"finding":"Site-directed mutagenesis of the C4d region revealed that residue D1106 in C4A is responsible for effective amide bond formation with protein/immune complex antigens, whereas H1106 in C4B catalyzes transacylation of the thioester carbonyl to form ester bonds with carbohydrate antigens. Four isotypic residues at positions 1101, 1102, 1105, and 1106 modulate the covalent binding affinities of the thioester carbonyl group.","method":"Site-directed mutagenesis of C4A and C4B at isotypic residues; binding assays to determine bond type (amide vs. ester)","journal":"International immunopharmacology","confidence":"High","confidence_rationale":"Tier 1 / Strong — site-directed mutagenesis with functional validation of specific residues; directly identifies catalytic determinants of C4A vs C4B specificity","pmids":["11367523"],"is_preprint":false},{"year":2002,"finding":"Macrophage-derived C4 produced locally in the spleen is sufficient to restore humoral immunity in C4-deficient mice. Bone marrow-derived macrophages (CD11b+, CD11c−) were identified as the cellular source of C4 synthesis within the spleen, with C4 mRNA identified by in situ hybridization and C4 protein deposits detected in germinal center areas by immunofluorescence.","method":"Bone marrow reconstitution of C4−/− mice; immunization with NP5-KLH and HSV-1 infection; in situ hybridization for C4 mRNA; immunofluorescence for C4 protein; cell-sorting with RT-PCR","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (reconstitution, in situ hybridization, immunofluorescence, cell sorting + RT-PCR) in a clean KO rescue system","pmids":["12421924"],"is_preprint":false},{"year":2002,"finding":"Complete C4 deficiency in a patient was caused by two distinct frameshift mutations: a 2-bp insertion in exon 29 (codon 1213) of the C4A gene causing premature termination, and a single C nucleotide deletion in exon 13 (codon 522) of the C4B gene causing frame-shift and premature termination. The patient expressed no Rodgers (C4A) or Chido (C4B) blood group antigens and had no detectable C4A or C4B protein.","method":"PCR amplification and complete sequencing of all 41 exons of C4A and C4B; multiplex PCR for mutation detection; immunochemical protein quantitation; blood group antigen testing","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — complete genomic sequencing plus protein-level confirmation; direct molecular basis of null alleles established","pmids":["12133986"],"is_preprint":false},{"year":1998,"finding":"Primary human astrocytes constitutively express complement C4 mRNA and protein; expression is enhanced by interferon-gamma and inhibited by interleukin-1β. C4 immunoreactivity was localized to GFAP-positive astrocytes when protein secretion was blocked, identifying astrocytes as an extrahepatic CNS source of C4.","method":"RT-PCR, ELISA, immunocytochemistry, and immunoblot on primary human astrocytes treated with IFN-γ or IL-1β","journal":"Brain research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (RT-PCR, ELISA, immunocytochemistry, immunoblot); single lab, primary human cells","pmids":["9795119"],"is_preprint":false},{"year":2015,"finding":"C4a does not function as an anaphylatoxin; it does not signal through C3a or C5a receptors and has no identified receptor of its own. Prior reports of C4a biological activity were attributable to contamination of C4a preparations with physiologically relevant levels of C3a and/or C5a.","method":"Review of experimental literature; calcium mobilization assays (PMID 9476119) showing C4a up to 3.3 µM has no effect on C3aR-expressing cells or human neutrophils","journal":"Journal of innate immunity","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conclusion supported by direct receptor binding and calcium mobilization experiments (cross-referenced to PMID 9476119); negative finding well-supported","pmids":["25659340"],"is_preprint":false},{"year":1997,"finding":"C4a does not interact with the C3a receptor (C3aR): concentrations up to 3.3 µM of C4a purified by in vitro cleavage of purified C4 had no effect on calcium mobilization in human neutrophils or in RBL-2H3 cells stably expressing cloned human or mouse C3aR. This demonstrates C4a receptor is distinct from C3aR.","method":"Calcium mobilization assay in RBL-2H3 cells expressing cloned human or mouse C3aR; C4a prepared by in vitro cleavage of purified C4 to avoid C5a contamination","journal":"Immunopharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional assay with carefully purified ligand and cloned receptor; negative finding rigorously controlled","pmids":["9476119"],"is_preprint":false},{"year":2019,"finding":"C4 inhibits human adenovirus infection through a C4-dependent but C2/C3-independent mechanism: antibody-mediated classical pathway activation causes rapid C4 activation and deposition of cleaved C4b onto the viral capsid, which inhibits capsid disassembly and prevents endosomal escape. C4-deficient mice exhibit heightened viral burdens. This antiviral function requires C1q antibody engagement but is independent of downstream complement components.","method":"C4-deficient mouse infection model; neutralization assays with purified C4 components; genetic complementation; endosomal escape assays; C2/C3-deficient controls","journal":"Cell host & microbe","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (KO mouse, reconstitution, mechanistic assays for capsid disassembly), genetic controls for downstream components, replicated in vivo and in vitro","pmids":["30926239"],"is_preprint":false},{"year":2020,"finding":"Human C4A binds synapses more efficiently than C4B and rescues visual system synaptic refinement deficits in C4 knockout mice, while C4B does not. Overexpression of C4A (but not C4B) in mice reduces cortical synapse density, increases microglial engulfment of synapses, and alters behavior. C4 knockout mice have normal numbers of cortical synapses, indicating complement is not required for normal developmental synaptic pruning.","method":"Transgenic mouse model expressing human C4A or C4B; visual system synaptic refinement assay; cortical synapse density quantification; microglial engulfment assay; behavioral testing; C4 KO comparison","journal":"Nature neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods in vivo (transgenic rescue, KO, synaptic density, engulfment assays, behavior), isotype-specific comparison, published in high-impact journal","pmids":["33353966"],"is_preprint":false},{"year":2020,"finding":"Human C4A is more protective than C4B against humoral autoimmunity in lupus-prone mice: C4A-expressing 564Igi mice develop fewer germinal centers, fewer autoreactive B cells, fewer autoantibodies, and fewer memory B cells than C4B-expressing mice. The mechanism involves higher C4A efficiency at inducing self-antigen clearance, which promotes follicular exclusion of autoreactive B cells.","method":"Gene-edited mouse strains expressing human C4A or C4B crossed with 564Igi lupus strain; flow cytometry for GC, autoreactive B cells, memory B cells; ELISA for autoantibodies; follicular exclusion assays","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — gene-edited isotype-specific mouse models, multiple orthogonal readouts, mechanistic link to follicular exclusion established","pmids":["33147456"],"is_preprint":false},{"year":1991,"finding":"Possession of C4A null alleles (C4A Q0) is associated with reduced C4 activation as measured by lower C4d split product concentrations, independent of total C4 concentration. This suggests that C4A null alleles predispose to SLE by conditioning poor C4 activation and thereby impairing immune complex clearance via the classical complement pathway.","method":"C4 phenotyping; measurement of C4 concentration and C4d (activation product) by quantitative assay in SLE patients stratified by C4 null allele status","journal":"Annals of the rheumatic diseases","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — functional split product measurement in patient cohort; single lab but mechanistically informative association between null alleles and reduced activation","pmids":["2029208"],"is_preprint":false},{"year":2014,"finding":"Complement anaphylatoxin C4a inhibits C5a-induced neointima formation and macrophage-driven inflammatory responses (TNF-α, IL-6, MCP-1) by suppressing Ca2+-dependent ERK activation in macrophages. C4a does not directly inhibit VSMC migration, proliferation, or VCAM-1 expression; its inhibitory effect on VSMCs is mediated indirectly via conditioned medium from C4a-pretreated macrophages.","method":"Wire-induced arterial injury mouse model; in vitro macrophage treatment with C4a/C5a; conditioned medium transfer experiments; Ca2+ influx and ERK phosphorylation assays; VSMC migration and proliferation assays","journal":"Molecular medicine reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo and in vitro mechanistic experiments identifying Ca2+-ERK pathway; single lab, multiple assays, indirect mechanism via macrophages established","pmids":["24789665"],"is_preprint":false}],"current_model":"Human C4A is a non-enzymatic complement protein whose alpha-chain thioester is cleaved by C1s to generate C4a (a 77-residue peptide with no identified receptor) and C4b, which covalently attaches to immune complexes and pathogens preferentially via amide bonds (determined by isotypic residues D1101/D1106 vs. H1106 in C4B); C4A is markedly more efficient than C4B at enhancing CR1-mediated immune complex binding and clearance, inhibiting immune precipitation, and opsonizing synapses for microglial engulfment, while C4B is more hemolytically active and forms ester bonds with carbohydrate antigens; locally produced macrophage-derived C4A in germinal centers is sufficient for humoral immunity, and excess C4A promotes synaptic loss and follicular exclusion of autoreactive B cells, explaining its dual roles in schizophrenia susceptibility and SLE protection."},"narrative":{"mechanistic_narrative":"C4A is one of two isotypes of the complement component C4, a central effector of the classical and lectin pathways whose alpha-chain thioester is exposed and made reactive by C1s cleavage of mature, fully processed three-chain C4 [PMID:6321593, PMID:3264881]. C1s cleavage also liberates the small C4a fragment, a 77-residue cationic peptide originally placed in the anaphylatoxin family by sequence homology to C3a and C5a [PMID:6167582]; however, C4a has no identified receptor of its own and does not signal through C3aR or C5aR, so earlier reports of anaphylatoxin-like activity reflected C3a/C5a contamination [PMID:25659340, PMID:9476119]. The functional identity of C4A resides in the covalent-binding chemistry of the resulting C4b fragment: the isotypic residue D1106 directs C4A toward amide-bond formation with protein/immune-complex antigens, whereas H1106 in C4B favors transacylation to ester bonds with carbohydrate antigens [PMID:11367523]. This chemistry makes C4A markedly more efficient than C4B at binding immunoglobulin and immune complexes, inhibiting immune precipitation, and enhancing CR1-mediated immune-complex handling on erythrocytes [PMID:3264881, PMID:3180739, PMID:2138067]. C4 deposition is also directly antimicrobial, blocking adenovirus capsid disassembly and endosomal escape independently of downstream C2/C3 [PMID:30926239]. Beyond the liver, C4 is produced locally by splenic macrophages—sufficient to restore humoral immunity in C4-deficient mice—and by CNS astrocytes [PMID:12421924, PMID:9795119]. In vivo, isotype-specific mouse models establish that C4A's superior self-antigen clearance promotes follicular exclusion of autoreactive B cells and protection against lupus-like autoimmunity [PMID:33147456], while excess C4A drives microglial engulfment of synapses and synaptic loss [PMID:33353966]. Consistent with this, C4A null alleles condition poor C4 activation and predispose to SLE [PMID:2029208], and complete C4 deficiency arises from frameshift null mutations in C4A and C4B [PMID:12133986].","teleology":[{"year":1981,"claim":"Establishing the primary structure of C4a defined it as a member of the anaphylatoxin family and located the C1s cleavage site that releases it from the C4 alpha-chain.","evidence":"Protein purification, fragmentation, and direct amino acid sequencing of serum-derived C4a","pmids":["6167582"],"confidence":"High","gaps":["Sequence homology to C3a/C5a did not establish a functional receptor or activity for C4a","Did not address isotypic differences between C4A and C4B"]},{"year":1984,"claim":"Cloning and genomic mapping placed C4A and C4B at separate, tandemly arranged loci in the HLA Class III region and provided isotype-specific sequence probes, the genetic basis for distinguishing the two genes.","evidence":"cDNA sequencing, restriction mapping, and cosmid genomic cloning with isotype-specific oligonucleotides","pmids":["6149580"],"confidence":"High","gaps":["Did not assign the functional consequence of class-specific sequence differences","Copy-number variation at the locus not resolved"]},{"year":1984,"claim":"Demonstrating that only fully processed three-chain C4 is cleaved by C1s and hemolytically active defined a maturation requirement that gates complement activation.","evidence":"Hemolytic and C1s cleavage assays on HepG2-secreted C4 processing intermediates and pro-C4","pmids":["6321593"],"confidence":"High","gaps":["Did not identify the protease(s) responsible for chain processing","No isotype comparison of processing"]},{"year":1985,"claim":"Localizing the C4A/C4B size difference to amino-acid sequence (not glycosylation) and showing differential hydralazine binding pointed to a chemical difference in the thioester region between isotypes.","evidence":"Glycan composition analysis of purified C4 chains; covalent hydralazine binding compared across isotypes","pmids":["2981921","3873356"],"confidence":"Medium","gaps":["The specific residues responsible were not yet identified","Hydralazine result was an indirect probe of thioester chemistry"]},{"year":1988,"claim":"Reconstituted binding assays established that C4A (C4A3) binds immunoglobulin and immune complexes far more efficiently than C4B, preferentially through amide bonds, defining the molecular basis of C4A's immune-complex specialization.","evidence":"Fluid-phase covalent binding and immune-precipitation inhibition assays with purified C4 variants and C1s","pmids":["3264881","3180739"],"confidence":"High","gaps":["Mechanism was demonstrated in vitro; physiological relevance for clearance not yet shown","Residues governing amide vs ester preference not identified"]},{"year":1990,"claim":"Showing C4A is markedly superior at enhancing immune-complex binding to CR1 on erythrocytes identified the dominant functional distinction relevant to immune-complex clearance, and comparative species data placed the C4A/C4B dichotomy in evolutionary context.","evidence":"CR1 binding and immunoprecipitation assays with purified isotypes; comparative C4 binding specificity across mammals","pmids":["2138067","2650988","2302180"],"confidence":"High","gaps":["In vivo clearance kinetics not measured","Receptor-level structural basis of CR1 enhancement not defined"]},{"year":1991,"claim":"Linking C4A null alleles to reduced C4 activation (lower C4d) provided a mechanistic rationale for the C4A-SLE association via impaired immune-complex clearance.","evidence":"C4 phenotyping and C4d split-product quantitation in SLE patients stratified by null-allele status","pmids":["2029208"],"confidence":"Medium","gaps":["Association is correlative within a patient cohort","Causality and copy-number confounding not resolved"]},{"year":1998,"claim":"Identifying constitutive, cytokine-modulated C4 expression in human astrocytes established a CNS-intrinsic source of C4 beyond the liver.","evidence":"RT-PCR, ELISA, immunocytochemistry, and immunoblot on IFN-γ/IL-1β-treated primary human astrocytes","pmids":["9795119"],"confidence":"Medium","gaps":["Single lab; did not distinguish C4A vs C4B expression","Functional role of astrocyte-derived C4 not tested"]},{"year":2001,"claim":"Site-directed mutagenesis pinpointed residue 1106 (D in C4A, H in C4B) as the determinant directing amide- versus ester-bond formation, providing the chemical mechanism behind the two isotypes' divergent target specificities.","evidence":"Mutagenesis of isotypic residues in the C4d region with bond-type binding assays","pmids":["11367523"],"confidence":"High","gaps":["No structural model of the transacylation transition state","Contribution of residues 1101/1102/1105 quantified only as modulators"]},{"year":2002,"claim":"A bone-marrow reconstitution rescue showed that locally synthesized, macrophage-derived C4 in the spleen is sufficient for humoral immunity, decoupling effector function from hepatic C4.","evidence":"Bone-marrow reconstitution of C4−/− mice with immunization/infection challenge, in situ hybridization, immunofluorescence, and cell-sorted RT-PCR","pmids":["12421924"],"confidence":"High","gaps":["Did not separate C4A and C4B contributions","Murine C4 isotype does not directly model human C4A"]},{"year":2002,"claim":"Complete sequencing of a C4-deficient patient defined the molecular basis of C4A and C4B null alleles as distinct frameshift mutations producing absent protein and blood-group antigens.","evidence":"Full exonic sequencing of C4A and C4B, multiplex PCR, protein quantitation, and blood-group antigen typing","pmids":["12133986"],"confidence":"High","gaps":["Single patient; allele spectrum not generalized","Did not address common copy-number null variation"]},{"year":2015,"claim":"Reassessment of C4a established that, despite its anaphylatoxin classification, it has no receptor and prior activities were contamination artifacts, redirecting the field to C4b-mediated effector functions.","evidence":"Literature review supported by calcium-mobilization assays in C3aR-expressing cells using carefully prepared C4a","pmids":["25659340","9476119"],"confidence":"Medium","gaps":["A bona fide C4a receptor, if any, remains unidentified","Negative finding; cannot exclude low-affinity or context-specific signaling"]},{"year":2019,"claim":"Demonstrating that C4b deposition blocks adenovirus capsid disassembly and endosomal escape independently of C2/C3 revealed a direct, downstream-component-independent antiviral function of C4.","evidence":"C4-deficient mouse infection, reconstitution, endosomal-escape assays, and C2/C3-deficient controls","pmids":["30926239"],"confidence":"High","gaps":["C4A vs C4B efficacy not distinguished","Generality across other viruses not established"]},{"year":2020,"claim":"Isotype-specific in vivo models established the divergent disease-relevant roles of human C4A: superior self-antigen clearance drives follicular exclusion of autoreactive B cells (lupus protection), while excess C4A promotes microglial synapse engulfment and synaptic loss.","evidence":"Gene-edited mice expressing human C4A or C4B in lupus (564Igi) and neural synaptic-refinement/overexpression paradigms with flow cytometry, engulfment, density, and behavioral readouts","pmids":["33147456","33353966"],"confidence":"High","gaps":["Molecular tags/receptors mediating synaptic C4A targeting not identified","Direct evidence in human schizophrenia/SLE tissue not provided by these models"]},{"year":null,"claim":"How C4A target selectivity translates into tissue-specific outcomes—the molecular signals marking synapses versus self-antigens for C4A-amide deposition and the receptors that read out that deposition—remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No identified molecular determinant of synapse-versus-immune-complex targeting","No structural model of the C4A thioester transacylation reaction","Human disease causality inferred from mouse isotype models"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[5,6,10]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[6,8]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[5,11,13]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[8,11,16,18]}],"complexes":[],"partners":["C1S","CR1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P0C0L4","full_name":"Complement C4-A","aliases":["Acidic complement C4","C3 and PZP-like alpha-2-macroglobulin domain-containing protein 2"],"length_aa":1744,"mass_kda":192.8,"function":"Precursor of non-enzymatic components of the classical, lectin and GZMK complement pathways, which consist in a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system Non-enzymatic component of C3 and C5 convertases (PubMed:8538770). Generated following cleavage by complement proteases (C1S, MASP2 or GZMK, depending on the complement pathway), it covalently attaches to the surface of pathogens, where it acts as an opsonin that marks the surface of antigens for removal (PubMed:27738201, PubMed:8538770). It then recruits the serine protease complement C2b to form the C3 and C5 convertases, which cleave and activate C3 and C5, respectively, the next components of the complement pathways (PubMed:12878586, PubMed:18204047, PubMed:2387864, PubMed:6906228). Complement C4b-A isotype is responsible for effective binding to form amide bonds with immune aggregates or protein antigens, while complement C4b-B isotype catalyzes the transacylation of the thioester carbonyl group to form ester bonds with carbohydrate antigens (PubMed:8538770) Putative humoral mediator released following cleavage by complement proteases (C1S, MASP2 or GZMK, depending on the complement pathway) (PubMed:6167582). While it is strongly similar to anaphylatoxins, its role is unclear (PubMed:25659340). Was reported to act as a mediator of local inflammatory process; however these effects were probably due to contamination with C3a and/C5a anaphylatoxins in biological assays (PubMed:25659340)","subcellular_location":"Secreted; Cell surface","url":"https://www.uniprot.org/uniprotkb/P0C0L4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/C4A","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":4,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/C4A","total_profiled":1310},"omim":[{"mim_id":"614380","title":"COMPLEMENT COMPONENT 4A DEFICIENCY; C4AD","url":"https://www.omim.org/entry/614380"},{"mim_id":"614379","title":"COMPLEMENT COMPONENT 4B DEFICIENCY; C4BD","url":"https://www.omim.org/entry/614379"},{"mim_id":"614374","title":"BLOOD GROUP, CHIDO/RODGERS SYSTEM","url":"https://www.omim.org/entry/614374"},{"mim_id":"613815","title":"CYTOCHROME P450, FAMILY 21, SUBFAMILY A, POLYPEPTIDE 2; CYP21A2","url":"https://www.omim.org/entry/613815"},{"mim_id":"610379","title":"WEST NILE VIRUS, SUSCEPTIBILITY TO","url":"https://www.omim.org/entry/610379"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"adrenal gland","ntpm":117.5},{"tissue":"liver","ntpm":341.8}],"url":"https://www.proteinatlas.org/search/C4A"},"hgnc":{"alias_symbol":["CPAMD2","C4S","CO4","C4","C4A3","C4A2","C4A4","C4A6","C4B","RG"],"prev_symbol":[]},"alphafold":{"accession":"P0C0L4","domains":[{"cath_id":"2.60.40.1930","chopping":"20-136","consensus_level":"medium","plddt":92.1403,"start":20,"end":136},{"cath_id":"2.60.40.1930","chopping":"141-236_569-613_779-828","consensus_level":"medium","plddt":92.2914,"start":141,"end":828},{"cath_id":"2.60.40.1940","chopping":"242-364","consensus_level":"medium","plddt":88.9685,"start":242,"end":364},{"cath_id":"1.20.91.20","chopping":"691-737","consensus_level":"medium","plddt":72.1636,"start":691,"end":737},{"cath_id":"2.60.40.10","chopping":"833-935","consensus_level":"high","plddt":88.8024,"start":833,"end":935},{"cath_id":"-","chopping":"1221-1233_1259-1319","consensus_level":"medium","plddt":85.9585,"start":1221,"end":1319},{"cath_id":"2.60.40.690","chopping":"1398-1411_1464-1571","consensus_level":"medium","plddt":82.012,"start":1398,"end":1571},{"cath_id":"1.20.58","chopping":"998-1059","consensus_level":"medium","plddt":90.6395,"start":998,"end":1059}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P0C0L4","model_url":"https://alphafold.ebi.ac.uk/files/AF-P0C0L4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P0C0L4-F1-predicted_aligned_error_v6.png","plddt_mean":83.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=C4A","jax_strain_url":"https://www.jax.org/strain/search?query=C4A"},"sequence":{"accession":"P0C0L4","fasta_url":"https://rest.uniprot.org/uniprotkb/P0C0L4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P0C0L4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P0C0L4"}},"corpus_meta":[{"pmid":"33353966","id":"PMC_33353966","title":"Overexpression of schizophrenia susceptibility factor human complement C4A promotes excessive synaptic loss and behavioral changes in mice.","date":"2020","source":"Nature neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/33353966","citation_count":263,"is_preprint":false},{"pmid":"11803016","id":"PMC_11803016","title":"C4-dicarboxylate carriers and sensors in bacteria.","date":"2002","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/11803016","citation_count":213,"is_preprint":false},{"pmid":"23383423","id":"PMC_23383423","title":"International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors.","date":"2013","source":"Pharmacological reviews","url":"https://pubmed.ncbi.nlm.nih.gov/23383423","citation_count":195,"is_preprint":false},{"pmid":"10859342","id":"PMC_10859342","title":"Deficiencies of human complement component C4A and C4B and heterozygosity in length variants of RP-C4-CYP21-TNX (RCCX) modules in caucasians. The load of RCCX genetic diversity on major histocompatibility complex-associated disease.","date":"2000","source":"The Journal of experimental medicine","url":"https://pubmed.ncbi.nlm.nih.gov/10859342","citation_count":155,"is_preprint":false},{"pmid":"11367523","id":"PMC_11367523","title":"Genetic, structural and functional diversities of human complement components C4A and C4B and their mouse homologues, Slp and C4.","date":"2001","source":"International immunopharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/11367523","citation_count":134,"is_preprint":false},{"pmid":"3499152","id":"PMC_3499152","title":"Deletion of C4A genes in patients with systemic lupus erythematosus.","date":"1987","source":"Arthritis and rheumatism","url":"https://pubmed.ncbi.nlm.nih.gov/3499152","citation_count":114,"is_preprint":false},{"pmid":"2546011","id":"PMC_2546011","title":"Analysis of C4-dicarboxylate transport genes in Rhizobium meliloti.","date":"1989","source":"Molecular microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/2546011","citation_count":110,"is_preprint":false},{"pmid":"34335607","id":"PMC_34335607","title":"Complement C4, Infections, and Autoimmune Diseases.","date":"2021","source":"Frontiers in immunology","url":"https://pubmed.ncbi.nlm.nih.gov/34335607","citation_count":103,"is_preprint":false},{"pmid":"24486019","id":"PMC_24486019","title":"Optimal translational termination requires C4 lysyl hydroxylation of eRF1.","date":"2014","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/24486019","citation_count":94,"is_preprint":false},{"pmid":"6606418","id":"PMC_6606418","title":"Complement allotyping in SLE: association with C4A null.","date":"1983","source":"Australian and New Zealand journal of medicine","url":"https://pubmed.ncbi.nlm.nih.gov/6606418","citation_count":85,"is_preprint":false},{"pmid":"12781769","id":"PMC_12781769","title":"Evolution of C4 phosphoenolpyruvate carboxylase.","date":"2003","source":"Archives of biochemistry and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/12781769","citation_count":84,"is_preprint":false},{"pmid":"21414960","id":"PMC_21414960","title":"Functional evolution of C(4) pyruvate, orthophosphate dikinase.","date":"2011","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/21414960","citation_count":81,"is_preprint":false},{"pmid":"6149580","id":"PMC_6149580","title":"Structure and organization of the C4 genes.","date":"1984","source":"Philosophical transactions of the Royal Society of London. Series B, Biological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/6149580","citation_count":74,"is_preprint":false},{"pmid":"6607672","id":"PMC_6607672","title":"Human C4 haplotypes with duplicated C4A or C4B.","date":"1984","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/6607672","citation_count":69,"is_preprint":false},{"pmid":"24799561","id":"PMC_24799561","title":"Deconstructing Kranz anatomy to understand C4 evolution.","date":"2014","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/24799561","citation_count":68,"is_preprint":false},{"pmid":"2568334","id":"PMC_2568334","title":"C4A gene deletion and HLA associations in black Americans with systemic lupus erythematosus.","date":"1989","source":"Immunogenetics","url":"https://pubmed.ncbi.nlm.nih.gov/2568334","citation_count":65,"is_preprint":false},{"pmid":"12226794","id":"PMC_12226794","title":"Genetic sophistication of human complement components C4A and C4B and RP-C4-CYP21-TNX (RCCX) modules in the major histocompatibility complex.","date":"2002","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/12226794","citation_count":63,"is_preprint":false},{"pmid":"30926239","id":"PMC_30926239","title":"Complement C4 Prevents Viral Infection through Capsid Inactivation.","date":"2019","source":"Cell host & microbe","url":"https://pubmed.ncbi.nlm.nih.gov/30926239","citation_count":61,"is_preprint":false},{"pmid":"21680741","id":"PMC_21680741","title":"Stabilization of C4a-hydroperoxyflavin in a two-component flavin-dependent monooxygenase is achieved through interactions at flavin N5 and C4a atoms.","date":"2011","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21680741","citation_count":60,"is_preprint":false},{"pmid":"6167582","id":"PMC_6167582","title":"Complete primary structure of human C4a anaphylatoxin.","date":"1981","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/6167582","citation_count":56,"is_preprint":false},{"pmid":"15012588","id":"PMC_15012588","title":"Structural analysis and tissue localization of human C4.4A: a protein homologue of the urokinase receptor.","date":"2004","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/15012588","citation_count":55,"is_preprint":false},{"pmid":"8341140","id":"PMC_8341140","title":"Clinical expression of systemic lupus erythematosus in patients with C4A deficiency.","date":"1993","source":"Medicine","url":"https://pubmed.ncbi.nlm.nih.gov/8341140","citation_count":55,"is_preprint":false},{"pmid":"16403222","id":"PMC_16403222","title":"Real-time PCR quantification of human complement C4A and C4B genes.","date":"2006","source":"BMC genetics","url":"https://pubmed.ncbi.nlm.nih.gov/16403222","citation_count":55,"is_preprint":false},{"pmid":"19133805","id":"PMC_19133805","title":"Crystallographic, spectroscopic, and computational analysis of a flavin C4a-oxygen adduct in choline oxidase.","date":"2009","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/19133805","citation_count":54,"is_preprint":false},{"pmid":"12175635","id":"PMC_12175635","title":"C4A deficiency and nonresponse to hepatitis B vaccination.","date":"2002","source":"Journal of hepatology","url":"https://pubmed.ncbi.nlm.nih.gov/12175635","citation_count":52,"is_preprint":false},{"pmid":"12895593","id":"PMC_12895593","title":"Leukotriene C(4) synthase.","date":"2003","source":"Prostaglandins, leukotrienes, and essential fatty acids","url":"https://pubmed.ncbi.nlm.nih.gov/12895593","citation_count":47,"is_preprint":false},{"pmid":"2138067","id":"PMC_2138067","title":"Differences between C4A and C4B in the handling of immune complexes: the enhancement of CR1 binding is more important than the inhibition of immunoprecipitation.","date":"1990","source":"Clinical and experimental immunology","url":"https://pubmed.ncbi.nlm.nih.gov/2138067","citation_count":47,"is_preprint":false},{"pmid":"14604014","id":"PMC_14604014","title":"Dancing with complement C4 and the RP-C4-CYP21-TNX (RCCX) modules of the major histocompatibility complex.","date":"2003","source":"Progress in nucleic acid research and molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/14604014","citation_count":46,"is_preprint":false},{"pmid":"25659340","id":"PMC_25659340","title":"C4a: An Anaphylatoxin in Name Only.","date":"2015","source":"Journal of innate immunity","url":"https://pubmed.ncbi.nlm.nih.gov/25659340","citation_count":45,"is_preprint":false},{"pmid":"10893330","id":"PMC_10893330","title":"Biological activities of recombinant chicken leptin C4S analog compared with unmodified leptins.","date":"2000","source":"American journal of physiology. Endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/10893330","citation_count":45,"is_preprint":false},{"pmid":"27059273","id":"PMC_27059273","title":"Walking the C4 pathway: past, present, and future.","date":"2016","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/27059273","citation_count":44,"is_preprint":false},{"pmid":"23720217","id":"PMC_23720217","title":"A recombinant begomovirus resulting from exchange of the C4 gene.","date":"2013","source":"The Journal of general virology","url":"https://pubmed.ncbi.nlm.nih.gov/23720217","citation_count":44,"is_preprint":false},{"pmid":"1696181","id":"PMC_1696181","title":"The c4 repressors of bacteriophages P1 and P7 are antisense RNAs.","date":"1990","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/1696181","citation_count":43,"is_preprint":false},{"pmid":"12133986","id":"PMC_12133986","title":"The molecular basis of complete complement C4A and C4B deficiencies in a systemic lupus erythematosus patient with homozygous C4A and C4B mutant genes.","date":"2002","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/12133986","citation_count":43,"is_preprint":false},{"pmid":"11898391","id":"PMC_11898391","title":"Transport of leukotriene C4 and structurally related conjugates.","date":"2002","source":"Vitamins and hormones","url":"https://pubmed.ncbi.nlm.nih.gov/11898391","citation_count":42,"is_preprint":false},{"pmid":"12374297","id":"PMC_12374297","title":"Maize C4 and non-C4 NADP-dependent malic enzymes are encoded by distinct genes derived from a plastid-localized ancestor.","date":"2002","source":"Plant molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/12374297","citation_count":42,"is_preprint":false},{"pmid":"7451653","id":"PMC_7451653","title":"Genetic analysis of C4 deficiency.","date":"1981","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/7451653","citation_count":41,"is_preprint":false},{"pmid":"32689483","id":"PMC_32689483","title":"C4 mechanisms in aquatic angiosperms: comparisons with terrestrial C4 systems.","date":"2002","source":"Functional plant biology : FPB","url":"https://pubmed.ncbi.nlm.nih.gov/32689483","citation_count":41,"is_preprint":false},{"pmid":"2088664","id":"PMC_2088664","title":"C4 nomenclature statement (1990).","date":"1990","source":"Complement and inflammation","url":"https://pubmed.ncbi.nlm.nih.gov/2088664","citation_count":41,"is_preprint":false},{"pmid":"16879240","id":"PMC_16879240","title":"Immunoglobulins and complement factor C4 in adult rhinosinusitis.","date":"2006","source":"Clinical and experimental immunology","url":"https://pubmed.ncbi.nlm.nih.gov/16879240","citation_count":40,"is_preprint":false},{"pmid":"26493816","id":"PMC_26493816","title":"Gene copy-number variations (CNVs) of complement C4 and C4A deficiency in genetic risk and pathogenesis of juvenile dermatomyositis.","date":"2015","source":"Annals of the rheumatic diseases","url":"https://pubmed.ncbi.nlm.nih.gov/26493816","citation_count":40,"is_preprint":false},{"pmid":"27758680","id":"PMC_27758680","title":"Complement C4A and C4B Gene Copy Number Study in Alzheimer's Disease Patients.","date":"2017","source":"Current Alzheimer research","url":"https://pubmed.ncbi.nlm.nih.gov/27758680","citation_count":39,"is_preprint":false},{"pmid":"12421924","id":"PMC_12421924","title":"Macrophage-derived complement component C4 can restore humoral immunity in C4-deficient mice.","date":"2002","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/12421924","citation_count":39,"is_preprint":false},{"pmid":"27038285","id":"PMC_27038285","title":"Glycine decarboxylase in C3, C4 and C3-C4 intermediate species.","date":"2016","source":"Current opinion in plant biology","url":"https://pubmed.ncbi.nlm.nih.gov/27038285","citation_count":36,"is_preprint":false},{"pmid":"15729693","id":"PMC_15729693","title":"Ly6 family member C4.4A binds laminins 1 and 5, associates with galectin-3 and supports cell migration.","date":"2005","source":"International journal of cancer","url":"https://pubmed.ncbi.nlm.nih.gov/15729693","citation_count":36,"is_preprint":false},{"pmid":"2981921","id":"PMC_2981921","title":"Oligosaccharide structure of human C4.","date":"1985","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/2981921","citation_count":36,"is_preprint":false},{"pmid":"9795119","id":"PMC_9795119","title":"Expression of complement C4 and C9 genes by human astrocytes.","date":"1998","source":"Brain research","url":"https://pubmed.ncbi.nlm.nih.gov/9795119","citation_count":35,"is_preprint":false},{"pmid":"2302180","id":"PMC_2302180","title":"The complement component C4 of mammals.","date":"1990","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/2302180","citation_count":35,"is_preprint":false},{"pmid":"12968987","id":"PMC_12968987","title":"Leukotriene C4 synthase polymorphisms and responsiveness to leukotriene antagonists in asthma.","date":"2003","source":"British journal of clinical pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/12968987","citation_count":34,"is_preprint":false},{"pmid":"17849475","id":"PMC_17849475","title":"Altered expression of the urokinase receptor homologue, C4.4A, in invasive areas of human esophageal squamous cell carcinoma.","date":"2008","source":"International journal of cancer","url":"https://pubmed.ncbi.nlm.nih.gov/17849475","citation_count":34,"is_preprint":false},{"pmid":"17912244","id":"PMC_17912244","title":"C4.4A as a candidate marker in the diagnosis of colorectal cancer.","date":"2007","source":"British journal of cancer","url":"https://pubmed.ncbi.nlm.nih.gov/17912244","citation_count":33,"is_preprint":false},{"pmid":"29050534","id":"PMC_29050534","title":"Low copy numbers of complement C4 and homozygous deficiency of C4A may predispose to severe disease and earlier disease onset in patients with systemic lupus erythematosus.","date":"2017","source":"Lupus","url":"https://pubmed.ncbi.nlm.nih.gov/29050534","citation_count":33,"is_preprint":false},{"pmid":"27055266","id":"PMC_27055266","title":"Finding the genes to build C4 rice.","date":"2016","source":"Current opinion in plant biology","url":"https://pubmed.ncbi.nlm.nih.gov/27055266","citation_count":32,"is_preprint":false},{"pmid":"3264881","id":"PMC_3264881","title":"The fluid-phase binding of human C4 and its genetic variants, C4A3 and C4B1, to immunoglobulins.","date":"1988","source":"Molecular immunology","url":"https://pubmed.ncbi.nlm.nih.gov/3264881","citation_count":29,"is_preprint":false},{"pmid":"34072921","id":"PMC_34072921","title":"C4 Bacterial Volatiles Improve Plant Health.","date":"2021","source":"Pathogens (Basel, Switzerland)","url":"https://pubmed.ncbi.nlm.nih.gov/34072921","citation_count":29,"is_preprint":false},{"pmid":"26800705","id":"PMC_26800705","title":"Low C4, C4A and C4B gene copy numbers are stronger risk factors for juvenile-onset than for adult-onset systemic lupus erythematosus.","date":"2016","source":"Rheumatology (Oxford, England)","url":"https://pubmed.ncbi.nlm.nih.gov/26800705","citation_count":29,"is_preprint":false},{"pmid":"2044237","id":"PMC_2044237","title":"DR3 and nonDR3 associated complement component C4A deficiency in systemic lupus erythematosus.","date":"1991","source":"Clinical immunology and immunopathology","url":"https://pubmed.ncbi.nlm.nih.gov/2044237","citation_count":29,"is_preprint":false},{"pmid":"28205620","id":"PMC_28205620","title":"Association between C4, C4A, and C4B copy number variations and susceptibility to autoimmune diseases: a meta-analysis.","date":"2017","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/28205620","citation_count":28,"is_preprint":false},{"pmid":"32095676","id":"PMC_32095676","title":"Erythritol: Another C4 Platform Chemical in Biomass Refinery.","date":"2020","source":"ACS omega","url":"https://pubmed.ncbi.nlm.nih.gov/32095676","citation_count":27,"is_preprint":false},{"pmid":"18432942","id":"PMC_18432942","title":"Human complement components C4A and C4B genetic diversities: complex genotypes and phenotypes.","date":"2005","source":"Current protocols in immunology","url":"https://pubmed.ncbi.nlm.nih.gov/18432942","citation_count":27,"is_preprint":false},{"pmid":"3873356","id":"PMC_3873356","title":"Hydralazine binds covalently to complement component C4. Different reactivity of C4A and C4B gene products.","date":"1985","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/3873356","citation_count":26,"is_preprint":false},{"pmid":"2650988","id":"PMC_2650988","title":"Two isotypes of human C4, C4A and C4B have different structure and function.","date":"1989","source":"Complement and inflammation","url":"https://pubmed.ncbi.nlm.nih.gov/2650988","citation_count":25,"is_preprint":false},{"pmid":"22404718","id":"PMC_22404718","title":"C4.4A is associated with tumor budding and epithelial-mesenchymal transition of colorectal cancer.","date":"2012","source":"Cancer science","url":"https://pubmed.ncbi.nlm.nih.gov/22404718","citation_count":25,"is_preprint":false},{"pmid":"22737222","id":"PMC_22737222","title":"Copy number analysis of complement C4A, C4B and C4A silencing mutation by real-time quantitative polymerase chain reaction.","date":"2012","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/22737222","citation_count":24,"is_preprint":false},{"pmid":"15489276","id":"PMC_15489276","title":"Untranslated regions from C4 amaranth AhRbcS1 mRNAs confer translational enhancement and preferential bundle sheath cell expression in transgenic C4 Flaveria bidentis.","date":"2004","source":"Plant physiology","url":"https://pubmed.ncbi.nlm.nih.gov/15489276","citation_count":24,"is_preprint":false},{"pmid":"3180739","id":"PMC_3180739","title":"C4-mediated inhibition of immune precipitation and differences in inhibitory action of genetic variants, C4A3 and C4B1.","date":"1988","source":"Complement (Basel, Switzerland)","url":"https://pubmed.ncbi.nlm.nih.gov/3180739","citation_count":23,"is_preprint":false},{"pmid":"35532796","id":"PMC_35532796","title":"Deregulation of complement components C4A and CSMD1 peripheral expression in first-episode psychosis and links to cognitive ability.","date":"2022","source":"European archives of psychiatry and clinical neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/35532796","citation_count":23,"is_preprint":false},{"pmid":"1708323","id":"PMC_1708323","title":"C4 reference typing report.","date":"1990","source":"Complement and inflammation","url":"https://pubmed.ncbi.nlm.nih.gov/1708323","citation_count":23,"is_preprint":false},{"pmid":"10591082","id":"PMC_10591082","title":"The biochemical, molecular, and genomic aspects of leukotriene C4 synthase.","date":"1999","source":"Proceedings of the Association of American Physicians","url":"https://pubmed.ncbi.nlm.nih.gov/10591082","citation_count":23,"is_preprint":false},{"pmid":"29274820","id":"PMC_29274820","title":"Characterization and Stability of Trypanosoma cruzi 24-C4 (Tc24-C4), a Candidate Antigen for a Therapeutic Vaccine Against Chagas Disease.","date":"2017","source":"Journal of pharmaceutical sciences","url":"https://pubmed.ncbi.nlm.nih.gov/29274820","citation_count":22,"is_preprint":false},{"pmid":"33147456","id":"PMC_33147456","title":"Complement C4A Regulates Autoreactive B Cells in Murine Lupus.","date":"2020","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/33147456","citation_count":21,"is_preprint":false},{"pmid":"1545698","id":"PMC_1545698","title":"Neonatal lupus erythematosus syndrome: analysis of C4 allotypes and C4 genes in 18 families.","date":"1992","source":"Medicine","url":"https://pubmed.ncbi.nlm.nih.gov/1545698","citation_count":21,"is_preprint":false},{"pmid":"36171069","id":"PMC_36171069","title":"Low copy numbers of complement C4 and C4A deficiency are risk factors for myositis, its subgroups and autoantibodies.","date":"2022","source":"Annals of the rheumatic diseases","url":"https://pubmed.ncbi.nlm.nih.gov/36171069","citation_count":20,"is_preprint":false},{"pmid":"34456009","id":"PMC_34456009","title":"Genetically Predicted Brain C4A Expression Is Associated With TSPO and Hippocampal Morphology.","date":"2021","source":"Biological psychiatry","url":"https://pubmed.ncbi.nlm.nih.gov/34456009","citation_count":19,"is_preprint":false},{"pmid":"3092184","id":"PMC_3092184","title":"Conservation of sequences adjacent to the telomeric C4A2 repeats of ciliate macronuclear ribosomal RNA gene molecules.","date":"1986","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/3092184","citation_count":19,"is_preprint":false},{"pmid":"27016649","id":"PMC_27016649","title":"Evolution of carbonic anhydrase in C4 plants.","date":"2016","source":"Current opinion in plant biology","url":"https://pubmed.ncbi.nlm.nih.gov/27016649","citation_count":19,"is_preprint":false},{"pmid":"26492168","id":"PMC_26492168","title":"Toward understanding the molecular mechanism of a geminivirus C4 protein.","date":"2015","source":"Plant signaling & behavior","url":"https://pubmed.ncbi.nlm.nih.gov/26492168","citation_count":19,"is_preprint":false},{"pmid":"2029208","id":"PMC_2029208","title":"Influence of C4 null alleles on C4 activation in systemic lupus erythematosus.","date":"1991","source":"Annals of the rheumatic diseases","url":"https://pubmed.ncbi.nlm.nih.gov/2029208","citation_count":18,"is_preprint":false},{"pmid":"21339181","id":"PMC_21339181","title":"Expression of C4.4A, a structural uPAR homolog, reflects squamous epithelial differentiation in the adult mouse and during embryogenesis.","date":"2011","source":"The journal of histochemistry and cytochemistry : official journal of the Histochemistry Society","url":"https://pubmed.ncbi.nlm.nih.gov/21339181","citation_count":18,"is_preprint":false},{"pmid":"30279072","id":"PMC_30279072","title":"Geminivirus C4: Interplaying with Receptor-like Kinases.","date":"2018","source":"Trends in plant science","url":"https://pubmed.ncbi.nlm.nih.gov/30279072","citation_count":17,"is_preprint":false},{"pmid":"21792890","id":"PMC_21792890","title":"Expression of C4.4A in precursor lesions of pulmonary adenocarcinoma and squamous cell carcinoma.","date":"2011","source":"International journal of cancer","url":"https://pubmed.ncbi.nlm.nih.gov/21792890","citation_count":17,"is_preprint":false},{"pmid":"19150565","id":"PMC_19150565","title":"Genetic deficiency of complement isoforms C4A or C4B predicts improved survival of metastatic renal cell carcinoma.","date":"2009","source":"The Journal of urology","url":"https://pubmed.ncbi.nlm.nih.gov/19150565","citation_count":17,"is_preprint":false},{"pmid":"1680806","id":"PMC_1680806","title":"The genomic structure of two ancestral haplotypes carrying C4A duplications.","date":"1991","source":"Immunogenetics","url":"https://pubmed.ncbi.nlm.nih.gov/1680806","citation_count":17,"is_preprint":false},{"pmid":"6321593","id":"PMC_6321593","title":"Functional studies on the secreted form of human C4 (C4s), two incompletely processed two-subunit C4 molecules (beta - alpha + gamma and beta + alpha - gamma), and pro-C4.","date":"1984","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/6321593","citation_count":16,"is_preprint":false},{"pmid":"34108629","id":"PMC_34108629","title":"Adjuvant properties of IFN-γ and GM-CSF in the scFv6.C4 DNA vaccine against CEA-expressing tumors.","date":"2021","source":"Gene therapy","url":"https://pubmed.ncbi.nlm.nih.gov/34108629","citation_count":16,"is_preprint":false},{"pmid":"24789665","id":"PMC_24789665","title":"Complement anaphylatoxin C4a inhibits C5a-induced neointima formation following arterial injury.","date":"2014","source":"Molecular medicine reports","url":"https://pubmed.ncbi.nlm.nih.gov/24789665","citation_count":16,"is_preprint":false},{"pmid":"9476119","id":"PMC_9476119","title":"Evidence that the receptor for C4a is distinct from the C3a receptor.","date":"1997","source":"Immunopharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/9476119","citation_count":16,"is_preprint":false},{"pmid":"9047336","id":"PMC_9047336","title":"DNA synthesis arrest at C4'-modified deoxyribose residues.","date":"1997","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/9047336","citation_count":16,"is_preprint":false},{"pmid":"17971229","id":"PMC_17971229","title":"Analysis of C4 and the C4 binding protein in the MRL/lpr mouse.","date":"2007","source":"Arthritis research & therapy","url":"https://pubmed.ncbi.nlm.nih.gov/17971229","citation_count":15,"is_preprint":false},{"pmid":"37918809","id":"PMC_37918809","title":"Unifying and versatile features of flavin-dependent monooxygenases: Diverse catalysis by a common C4a-(hydro)peroxyflavin.","date":"2023","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/37918809","citation_count":15,"is_preprint":false},{"pmid":"1563101","id":"PMC_1563101","title":"Effects of C4 null alleles and homoduplications on quantitative expression of C4A and C4B.","date":"1992","source":"Clinical and experimental immunology","url":"https://pubmed.ncbi.nlm.nih.gov/1563101","citation_count":15,"is_preprint":false},{"pmid":"37587696","id":"PMC_37587696","title":"Gene duplications facilitate C4-CAM compatibility in common purslane.","date":"2023","source":"Plant physiology","url":"https://pubmed.ncbi.nlm.nih.gov/37587696","citation_count":14,"is_preprint":false},{"pmid":"22420309","id":"PMC_22420309","title":"Leukotriene c4 synthase: upcoming drug target for inflammation.","date":"2012","source":"Current drug targets","url":"https://pubmed.ncbi.nlm.nih.gov/22420309","citation_count":14,"is_preprint":false},{"pmid":"29928053","id":"PMC_29928053","title":"Clinical features of patients with homozygous complement C4A or C4B deficiency.","date":"2018","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/29928053","citation_count":14,"is_preprint":false},{"pmid":"22406254","id":"PMC_22406254","title":"C4A deficiency in children and adolescents with recurrent respiratory infections.","date":"2012","source":"Human immunology","url":"https://pubmed.ncbi.nlm.nih.gov/22406254","citation_count":14,"is_preprint":false},{"pmid":"31387635","id":"PMC_31387635","title":"Impact of C4, C4A and C4B gene copy number variation in the susceptibility, phenotype and progression of systemic lupus erythematosus.","date":"2019","source":"Advances in rheumatology (London, England)","url":"https://pubmed.ncbi.nlm.nih.gov/31387635","citation_count":12,"is_preprint":false},{"pmid":"17278103","id":"PMC_17278103","title":"CEBPbeta, JunD and c-Jun contribute to the transcriptional activation of the metastasis-associated C4.4A gene.","date":"2007","source":"International journal of cancer","url":"https://pubmed.ncbi.nlm.nih.gov/17278103","citation_count":12,"is_preprint":false},{"pmid":"1519366","id":"PMC_1519366","title":"Quantitation of the human component C4: definition of C4 Q0 alleles and C4A duplications.","date":"1992","source":"Vox sanguinis","url":"https://pubmed.ncbi.nlm.nih.gov/1519366","citation_count":12,"is_preprint":false},{"pmid":"28110279","id":"PMC_28110279","title":"Walking the C4 pathway: past, present, and future.","date":"2017","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/28110279","citation_count":11,"is_preprint":false},{"pmid":"34747993","id":"PMC_34747993","title":"Using breeding and quantitative genetics to understand the C4 pathway.","date":"2022","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/34747993","citation_count":11,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":48463,"output_tokens":6126,"usd":0.118639,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":14023,"output_tokens":4276,"usd":0.088508,"stage2_stop_reason":"end_turn"},"total_usd":0.207147,"stage1_batch_id":"msgbatch_0179Z2pxRYBsXi2xADneafRw","stage2_batch_id":"msgbatch_01LaY9ZaVdgjWArcZo9eqQE5","note":"batch pricing = 50% of standard"},"stage1_raw":"Looking through the abstracts, I need to identify papers about the human complement C4A protein (HGNC: C4A, aliases: CPAMD2, C4S, C4, C4A3, C4A2, C4A4, C4A6, RG) and extract mechanistic findings.\n\nI'll classify each paper:\n- Papers about human complement C4A/C4B: 1, 3, 4, 5, 6, 8, 10, 13, 14, 17, 18, 20, 22, 23, 25, 27, 28, 29, 34, 37, 39, 41, 42, 43, 46, 47, 48, 52, 54, 57, 60, 61, 62, 64, 66, 67, 71, 72, 73, 74, 78, 82, 83, 84, 86, 87, 89, 91, 94, 95, 96\n- Papers about C4 photosynthesis, plant biology (symbol collision): 2, 7, 11, 12, 15, 31, 32, 36, 38, 44, 53, 55, 65, 76, 77, 80, 92, 99, 100\n- Papers about C4.4A (a different protein, LYPD3): 21, 45, 50, 51, 63, 79, 81, 97\n- Papers about leukotriene C4 synthase: 26, 35, 49, 69, 93\n- Papers about other \"C4\" symbols: 9 (eRF1 C4 lysyl), 19 (flavin C4a), 24 (flavin C4a), 30 (leptin C4S), 33 (phage c4), 59 (erythritol), 70 (Tc24-C4), 75 (ciliate C4A2 telomeres), 85 (scFv6.C4 vaccine), 88 (C4' deoxyribose), 90 (flavin C4a)\n\nNow extracting mechanistic discoveries from complement C4A papers:\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1981,\n      \"finding\": \"Human C4a anaphylatoxin is a 77-residue cationic polypeptide (Mr ~9000) generated by C1s cleavage of the C4 alpha-chain between positions 77 and 78. Primary structure determination revealed ~30% homology with C3a and ~36% with C5a, placing C4a in the anaphylatoxin family derived from a common genetic origin.\",\n      \"method\": \"Protein purification from activated serum, cyanogen bromide fragmentation, chymotryptic digestion, amino acid sequence analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — complete primary structure determination by direct protein sequencing with multiple fragmentation strategies; foundational biochemical characterization\",\n      \"pmids\": [\"6167582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"C4A and C4B are encoded by at least two separate loci in the HLA Class III region on chromosome 6, and nucleotide sequence analysis of full-length C4A cDNA and partial C4B cDNA identified class-specific sequence differences, enabling synthesis of isotype-specific oligonucleotide probes. Three C4 loci (~16 kb each, spaced by ~10 kb) were aligned 30 kb from the factor B gene in overlapping cosmid genomic fragments.\",\n      \"method\": \"cDNA cloning, complete nucleotide sequencing, restriction mapping, hybridization with C4A/C4B-specific synthetic oligonucleotides, cosmid library genomic cloning\",\n      \"journal\": \"Philosophical transactions of the Royal Society of London. Series B, Biological sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — complete cDNA sequencing and genomic mapping with multiple orthogonal methods; foundational structural characterization of the gene locus\",\n      \"pmids\": [\"6149580\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"Processing of C4 to its three-chain (beta-alpha-gamma) structure is required for efficient activation by C1s. Incompletely processed two-chain C4 forms (beta-alpha+gamma and beta+alpha-gamma) and the extracellular precursor pro-C4 are resistant to C1s cleavage and hemolytically inactive relative to the mature three-chain C4.\",\n      \"method\": \"Hemolytic activity assays, solid-phase competitive radioimmunoassay, C1s cleavage experiments on HepG2-secreted C4 forms, autolysis assays\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct in vitro functional assays with purified components, multiple processing forms tested with rigorous controls\",\n      \"pmids\": [\"6321593\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"The alpha- and beta-chains of human C4 are glycosylated (the gamma-chain is not). The alpha-chain bears three complex fucosylated biantennary oligosaccharides and is sulfated in its alpha-4 fragment; the beta-chain bears a single high-mannose oligosaccharide. The ~2000 Mr difference between C4A and C4B alpha-chains localizes to the alpha-2 fragment and is not due to carbohydrate, indicating the difference resides in amino acid sequence.\",\n      \"method\": \"Biochemical fractionation of purified plasma C4 and HepG2-secreted C4; carbohydrate composition analysis; localization of oligosaccharide modifications to individual chain fragments\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct biochemical structural analysis with purified protein, multiple analytical methods on both plasma and cell-secreted C4\",\n      \"pmids\": [\"2981921\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"Hydralazine binds covalently to C4, becoming attached to the polypeptide chain containing the active-site thioester; C4A gene product binds hydralazine more efficiently than C4B, suggesting the thioester chemistry differs between the two isotypes.\",\n      \"method\": \"Covalent binding assay with radiolabeled hydralazine; SDS-PAGE analysis of C4A vs C4B after hydralazine treatment\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct covalent binding chemistry demonstrated with two isotypes, single lab, two methods (binding + gel analysis)\",\n      \"pmids\": [\"3873356\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"C4b binds covalently to IgG heavy chains in the fluid phase (in the absence of preformed immune precipitates), primarily via amide bonds for C4A3 and via both amide and acyl ester bonds for C4B1. C4A3 binds 3–4 times more IgG than C4B1 across a range of concentrations, and also shows higher binding efficiency for IgM, IgA, IgG2a, F(ab')2, and BSA.\",\n      \"method\": \"Fluid-phase binding assay with purified C4 variants and C1s; SDS-PAGE to identify chain of attachment; comparison of amide vs. ester linkage chemistry\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted in vitro system with purified components, multiple substrates tested, bond chemistry defined, replicated across multiple immunoglobulin classes\",\n      \"pmids\": [\"3264881\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"C4A3 is more effective than C4B1 at inhibiting the rate of immune precipitate formation in both serum and serum-free reaction mixtures containing C1 and C4. Covalent deposition of C4b on immune complexes is the essential mechanistic step underlying inhibition of immune precipitation.\",\n      \"method\": \"Immune precipitation inhibition assay in serum and reconstituted serum-free system; rate measurements with purified C4A3 vs C4B1\",\n      \"journal\": \"Complement (Basel, Switzerland)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional comparison of two isotypes in reconstituted system; single lab, multiple assay conditions\",\n      \"pmids\": [\"3180739\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1989,\n      \"finding\": \"The two C4 isotypes C4A and C4B differ in covalent binding specificity: C4B is more efficient in hemolytic assays (binding to carbohydrate antigens via ester bonds), whereas C4A binds preferentially to immune complexes (via amide bonds with protein antigens). The two isotypes differ more than fivefold in hemolysis-to-immune-complex-processing assays.\",\n      \"method\": \"Hemolytic assays and immune complex processing assays comparing purified C4A and C4B isotypes\",\n      \"journal\": \"Complement and inflammation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional assays with purified proteins; consistent with multiple other studies but review/summary format\",\n      \"pmids\": [\"2650988\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"C4A is markedly more effective than C4B at enhancing binding of immune complexes to complement receptor CR1 on red blood cells, with the difference being the major functional distinction between the two isotypes in immune complex handling. This was demonstrated with both preformed and nascent immune complexes at equivalence and antibody excess.\",\n      \"method\": \"CR1 binding assay with red blood cells; inhibition of immunoprecipitation assay; comparison of purified C4A vs C4B with preformed and nascent immune complexes\",\n      \"journal\": \"Clinical and experimental immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct functional comparison using purified isotypes, multiple assay conditions (preformed and nascent complexes, antibody excess and equivalence), mechanistically informative\",\n      \"pmids\": [\"2138067\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"Mammalian C4 binding specificity is evolutionarily variable: chimpanzee and rhesus monkey produce two C4 types with C4A-like and C4B-like reactivities (similar to humans), while rat, guinea pig, whale, rabbit, dog, and pig express C4 with only a single C4B-like specificity. Sheep and cattle express both C4A-like and C4B-like proteins, suggesting a duplicated locus.\",\n      \"method\": \"C4 purification from multiple mammalian species; binding specificity assays\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — purified proteins from multiple species tested in binding assays; single lab comparative study\",\n      \"pmids\": [\"2302180\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Site-directed mutagenesis of the C4d region revealed that residue D1106 in C4A is responsible for effective amide bond formation with protein/immune complex antigens, whereas H1106 in C4B catalyzes transacylation of the thioester carbonyl to form ester bonds with carbohydrate antigens. Four isotypic residues at positions 1101, 1102, 1105, and 1106 modulate the covalent binding affinities of the thioester carbonyl group.\",\n      \"method\": \"Site-directed mutagenesis of C4A and C4B at isotypic residues; binding assays to determine bond type (amide vs. ester)\",\n      \"journal\": \"International immunopharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — site-directed mutagenesis with functional validation of specific residues; directly identifies catalytic determinants of C4A vs C4B specificity\",\n      \"pmids\": [\"11367523\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Macrophage-derived C4 produced locally in the spleen is sufficient to restore humoral immunity in C4-deficient mice. Bone marrow-derived macrophages (CD11b+, CD11c−) were identified as the cellular source of C4 synthesis within the spleen, with C4 mRNA identified by in situ hybridization and C4 protein deposits detected in germinal center areas by immunofluorescence.\",\n      \"method\": \"Bone marrow reconstitution of C4−/− mice; immunization with NP5-KLH and HSV-1 infection; in situ hybridization for C4 mRNA; immunofluorescence for C4 protein; cell-sorting with RT-PCR\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (reconstitution, in situ hybridization, immunofluorescence, cell sorting + RT-PCR) in a clean KO rescue system\",\n      \"pmids\": [\"12421924\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Complete C4 deficiency in a patient was caused by two distinct frameshift mutations: a 2-bp insertion in exon 29 (codon 1213) of the C4A gene causing premature termination, and a single C nucleotide deletion in exon 13 (codon 522) of the C4B gene causing frame-shift and premature termination. The patient expressed no Rodgers (C4A) or Chido (C4B) blood group antigens and had no detectable C4A or C4B protein.\",\n      \"method\": \"PCR amplification and complete sequencing of all 41 exons of C4A and C4B; multiplex PCR for mutation detection; immunochemical protein quantitation; blood group antigen testing\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — complete genomic sequencing plus protein-level confirmation; direct molecular basis of null alleles established\",\n      \"pmids\": [\"12133986\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Primary human astrocytes constitutively express complement C4 mRNA and protein; expression is enhanced by interferon-gamma and inhibited by interleukin-1β. C4 immunoreactivity was localized to GFAP-positive astrocytes when protein secretion was blocked, identifying astrocytes as an extrahepatic CNS source of C4.\",\n      \"method\": \"RT-PCR, ELISA, immunocytochemistry, and immunoblot on primary human astrocytes treated with IFN-γ or IL-1β\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (RT-PCR, ELISA, immunocytochemistry, immunoblot); single lab, primary human cells\",\n      \"pmids\": [\"9795119\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"C4a does not function as an anaphylatoxin; it does not signal through C3a or C5a receptors and has no identified receptor of its own. Prior reports of C4a biological activity were attributable to contamination of C4a preparations with physiologically relevant levels of C3a and/or C5a.\",\n      \"method\": \"Review of experimental literature; calcium mobilization assays (PMID 9476119) showing C4a up to 3.3 µM has no effect on C3aR-expressing cells or human neutrophils\",\n      \"journal\": \"Journal of innate immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conclusion supported by direct receptor binding and calcium mobilization experiments (cross-referenced to PMID 9476119); negative finding well-supported\",\n      \"pmids\": [\"25659340\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"C4a does not interact with the C3a receptor (C3aR): concentrations up to 3.3 µM of C4a purified by in vitro cleavage of purified C4 had no effect on calcium mobilization in human neutrophils or in RBL-2H3 cells stably expressing cloned human or mouse C3aR. This demonstrates C4a receptor is distinct from C3aR.\",\n      \"method\": \"Calcium mobilization assay in RBL-2H3 cells expressing cloned human or mouse C3aR; C4a prepared by in vitro cleavage of purified C4 to avoid C5a contamination\",\n      \"journal\": \"Immunopharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional assay with carefully purified ligand and cloned receptor; negative finding rigorously controlled\",\n      \"pmids\": [\"9476119\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"C4 inhibits human adenovirus infection through a C4-dependent but C2/C3-independent mechanism: antibody-mediated classical pathway activation causes rapid C4 activation and deposition of cleaved C4b onto the viral capsid, which inhibits capsid disassembly and prevents endosomal escape. C4-deficient mice exhibit heightened viral burdens. This antiviral function requires C1q antibody engagement but is independent of downstream complement components.\",\n      \"method\": \"C4-deficient mouse infection model; neutralization assays with purified C4 components; genetic complementation; endosomal escape assays; C2/C3-deficient controls\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (KO mouse, reconstitution, mechanistic assays for capsid disassembly), genetic controls for downstream components, replicated in vivo and in vitro\",\n      \"pmids\": [\"30926239\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Human C4A binds synapses more efficiently than C4B and rescues visual system synaptic refinement deficits in C4 knockout mice, while C4B does not. Overexpression of C4A (but not C4B) in mice reduces cortical synapse density, increases microglial engulfment of synapses, and alters behavior. C4 knockout mice have normal numbers of cortical synapses, indicating complement is not required for normal developmental synaptic pruning.\",\n      \"method\": \"Transgenic mouse model expressing human C4A or C4B; visual system synaptic refinement assay; cortical synapse density quantification; microglial engulfment assay; behavioral testing; C4 KO comparison\",\n      \"journal\": \"Nature neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods in vivo (transgenic rescue, KO, synaptic density, engulfment assays, behavior), isotype-specific comparison, published in high-impact journal\",\n      \"pmids\": [\"33353966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Human C4A is more protective than C4B against humoral autoimmunity in lupus-prone mice: C4A-expressing 564Igi mice develop fewer germinal centers, fewer autoreactive B cells, fewer autoantibodies, and fewer memory B cells than C4B-expressing mice. The mechanism involves higher C4A efficiency at inducing self-antigen clearance, which promotes follicular exclusion of autoreactive B cells.\",\n      \"method\": \"Gene-edited mouse strains expressing human C4A or C4B crossed with 564Igi lupus strain; flow cytometry for GC, autoreactive B cells, memory B cells; ELISA for autoantibodies; follicular exclusion assays\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — gene-edited isotype-specific mouse models, multiple orthogonal readouts, mechanistic link to follicular exclusion established\",\n      \"pmids\": [\"33147456\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"Possession of C4A null alleles (C4A Q0) is associated with reduced C4 activation as measured by lower C4d split product concentrations, independent of total C4 concentration. This suggests that C4A null alleles predispose to SLE by conditioning poor C4 activation and thereby impairing immune complex clearance via the classical complement pathway.\",\n      \"method\": \"C4 phenotyping; measurement of C4 concentration and C4d (activation product) by quantitative assay in SLE patients stratified by C4 null allele status\",\n      \"journal\": \"Annals of the rheumatic diseases\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — functional split product measurement in patient cohort; single lab but mechanistically informative association between null alleles and reduced activation\",\n      \"pmids\": [\"2029208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Complement anaphylatoxin C4a inhibits C5a-induced neointima formation and macrophage-driven inflammatory responses (TNF-α, IL-6, MCP-1) by suppressing Ca2+-dependent ERK activation in macrophages. C4a does not directly inhibit VSMC migration, proliferation, or VCAM-1 expression; its inhibitory effect on VSMCs is mediated indirectly via conditioned medium from C4a-pretreated macrophages.\",\n      \"method\": \"Wire-induced arterial injury mouse model; in vitro macrophage treatment with C4a/C5a; conditioned medium transfer experiments; Ca2+ influx and ERK phosphorylation assays; VSMC migration and proliferation assays\",\n      \"journal\": \"Molecular medicine reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo and in vitro mechanistic experiments identifying Ca2+-ERK pathway; single lab, multiple assays, indirect mechanism via macrophages established\",\n      \"pmids\": [\"24789665\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Human C4A is a non-enzymatic complement protein whose alpha-chain thioester is cleaved by C1s to generate C4a (a 77-residue peptide with no identified receptor) and C4b, which covalently attaches to immune complexes and pathogens preferentially via amide bonds (determined by isotypic residues D1101/D1106 vs. H1106 in C4B); C4A is markedly more efficient than C4B at enhancing CR1-mediated immune complex binding and clearance, inhibiting immune precipitation, and opsonizing synapses for microglial engulfment, while C4B is more hemolytically active and forms ester bonds with carbohydrate antigens; locally produced macrophage-derived C4A in germinal centers is sufficient for humoral immunity, and excess C4A promotes synaptic loss and follicular exclusion of autoreactive B cells, explaining its dual roles in schizophrenia susceptibility and SLE protection.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"C4A is one of two isotypes of the complement component C4, a central effector of the classical and lectin pathways whose alpha-chain thioester is exposed and made reactive by C1s cleavage of mature, fully processed three-chain C4 [#2, #5]. C1s cleavage also liberates the small C4a fragment, a 77-residue cationic peptide originally placed in the anaphylatoxin family by sequence homology to C3a and C5a [#0]; however, C4a has no identified receptor of its own and does not signal through C3aR or C5aR, so earlier reports of anaphylatoxin-like activity reflected C3a/C5a contamination [#14, #15]. The functional identity of C4A resides in the covalent-binding chemistry of the resulting C4b fragment: the isotypic residue D1106 directs C4A toward amide-bond formation with protein/immune-complex antigens, whereas H1106 in C4B favors transacylation to ester bonds with carbohydrate antigens [#10]. This chemistry makes C4A markedly more efficient than C4B at binding immunoglobulin and immune complexes, inhibiting immune precipitation, and enhancing CR1-mediated immune-complex handling on erythrocytes [#5, #6, #8]. C4 deposition is also directly antimicrobial, blocking adenovirus capsid disassembly and endosomal escape independently of downstream C2/C3 [#16]. Beyond the liver, C4 is produced locally by splenic macrophages—sufficient to restore humoral immunity in C4-deficient mice—and by CNS astrocytes [#11, #13]. In vivo, isotype-specific mouse models establish that C4A's superior self-antigen clearance promotes follicular exclusion of autoreactive B cells and protection against lupus-like autoimmunity [#18], while excess C4A drives microglial engulfment of synapses and synaptic loss [#17]. Consistent with this, C4A null alleles condition poor C4 activation and predispose to SLE [#19], and complete C4 deficiency arises from frameshift null mutations in C4A and C4B [#12].\",\n  \"teleology\": [\n    {\n      \"year\": 1981,\n      \"claim\": \"Establishing the primary structure of C4a defined it as a member of the anaphylatoxin family and located the C1s cleavage site that releases it from the C4 alpha-chain.\",\n      \"evidence\": \"Protein purification, fragmentation, and direct amino acid sequencing of serum-derived C4a\",\n      \"pmids\": [\"6167582\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Sequence homology to C3a/C5a did not establish a functional receptor or activity for C4a\", \"Did not address isotypic differences between C4A and C4B\"]\n    },\n    {\n      \"year\": 1984,\n      \"claim\": \"Cloning and genomic mapping placed C4A and C4B at separate, tandemly arranged loci in the HLA Class III region and provided isotype-specific sequence probes, the genetic basis for distinguishing the two genes.\",\n      \"evidence\": \"cDNA sequencing, restriction mapping, and cosmid genomic cloning with isotype-specific oligonucleotides\",\n      \"pmids\": [\"6149580\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not assign the functional consequence of class-specific sequence differences\", \"Copy-number variation at the locus not resolved\"]\n    },\n    {\n      \"year\": 1984,\n      \"claim\": \"Demonstrating that only fully processed three-chain C4 is cleaved by C1s and hemolytically active defined a maturation requirement that gates complement activation.\",\n      \"evidence\": \"Hemolytic and C1s cleavage assays on HepG2-secreted C4 processing intermediates and pro-C4\",\n      \"pmids\": [\"6321593\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify the protease(s) responsible for chain processing\", \"No isotype comparison of processing\"]\n    },\n    {\n      \"year\": 1985,\n      \"claim\": \"Localizing the C4A/C4B size difference to amino-acid sequence (not glycosylation) and showing differential hydralazine binding pointed to a chemical difference in the thioester region between isotypes.\",\n      \"evidence\": \"Glycan composition analysis of purified C4 chains; covalent hydralazine binding compared across isotypes\",\n      \"pmids\": [\"2981921\", \"3873356\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The specific residues responsible were not yet identified\", \"Hydralazine result was an indirect probe of thioester chemistry\"]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Reconstituted binding assays established that C4A (C4A3) binds immunoglobulin and immune complexes far more efficiently than C4B, preferentially through amide bonds, defining the molecular basis of C4A's immune-complex specialization.\",\n      \"evidence\": \"Fluid-phase covalent binding and immune-precipitation inhibition assays with purified C4 variants and C1s\",\n      \"pmids\": [\"3264881\", \"3180739\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism was demonstrated in vitro; physiological relevance for clearance not yet shown\", \"Residues governing amide vs ester preference not identified\"]\n    },\n    {\n      \"year\": 1990,\n      \"claim\": \"Showing C4A is markedly superior at enhancing immune-complex binding to CR1 on erythrocytes identified the dominant functional distinction relevant to immune-complex clearance, and comparative species data placed the C4A/C4B dichotomy in evolutionary context.\",\n      \"evidence\": \"CR1 binding and immunoprecipitation assays with purified isotypes; comparative C4 binding specificity across mammals\",\n      \"pmids\": [\"2138067\", \"2650988\", \"2302180\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo clearance kinetics not measured\", \"Receptor-level structural basis of CR1 enhancement not defined\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Linking C4A null alleles to reduced C4 activation (lower C4d) provided a mechanistic rationale for the C4A-SLE association via impaired immune-complex clearance.\",\n      \"evidence\": \"C4 phenotyping and C4d split-product quantitation in SLE patients stratified by null-allele status\",\n      \"pmids\": [\"2029208\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Association is correlative within a patient cohort\", \"Causality and copy-number confounding not resolved\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Identifying constitutive, cytokine-modulated C4 expression in human astrocytes established a CNS-intrinsic source of C4 beyond the liver.\",\n      \"evidence\": \"RT-PCR, ELISA, immunocytochemistry, and immunoblot on IFN-γ/IL-1β-treated primary human astrocytes\",\n      \"pmids\": [\"9795119\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab; did not distinguish C4A vs C4B expression\", \"Functional role of astrocyte-derived C4 not tested\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Site-directed mutagenesis pinpointed residue 1106 (D in C4A, H in C4B) as the determinant directing amide- versus ester-bond formation, providing the chemical mechanism behind the two isotypes' divergent target specificities.\",\n      \"evidence\": \"Mutagenesis of isotypic residues in the C4d region with bond-type binding assays\",\n      \"pmids\": [\"11367523\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural model of the transacylation transition state\", \"Contribution of residues 1101/1102/1105 quantified only as modulators\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"A bone-marrow reconstitution rescue showed that locally synthesized, macrophage-derived C4 in the spleen is sufficient for humoral immunity, decoupling effector function from hepatic C4.\",\n      \"evidence\": \"Bone-marrow reconstitution of C4−/− mice with immunization/infection challenge, in situ hybridization, immunofluorescence, and cell-sorted RT-PCR\",\n      \"pmids\": [\"12421924\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not separate C4A and C4B contributions\", \"Murine C4 isotype does not directly model human C4A\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Complete sequencing of a C4-deficient patient defined the molecular basis of C4A and C4B null alleles as distinct frameshift mutations producing absent protein and blood-group antigens.\",\n      \"evidence\": \"Full exonic sequencing of C4A and C4B, multiplex PCR, protein quantitation, and blood-group antigen typing\",\n      \"pmids\": [\"12133986\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single patient; allele spectrum not generalized\", \"Did not address common copy-number null variation\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Reassessment of C4a established that, despite its anaphylatoxin classification, it has no receptor and prior activities were contamination artifacts, redirecting the field to C4b-mediated effector functions.\",\n      \"evidence\": \"Literature review supported by calcium-mobilization assays in C3aR-expressing cells using carefully prepared C4a\",\n      \"pmids\": [\"25659340\", \"9476119\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"A bona fide C4a receptor, if any, remains unidentified\", \"Negative finding; cannot exclude low-affinity or context-specific signaling\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrating that C4b deposition blocks adenovirus capsid disassembly and endosomal escape independently of C2/C3 revealed a direct, downstream-component-independent antiviral function of C4.\",\n      \"evidence\": \"C4-deficient mouse infection, reconstitution, endosomal-escape assays, and C2/C3-deficient controls\",\n      \"pmids\": [\"30926239\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"C4A vs C4B efficacy not distinguished\", \"Generality across other viruses not established\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Isotype-specific in vivo models established the divergent disease-relevant roles of human C4A: superior self-antigen clearance drives follicular exclusion of autoreactive B cells (lupus protection), while excess C4A promotes microglial synapse engulfment and synaptic loss.\",\n      \"evidence\": \"Gene-edited mice expressing human C4A or C4B in lupus (564Igi) and neural synaptic-refinement/overexpression paradigms with flow cytometry, engulfment, density, and behavioral readouts\",\n      \"pmids\": [\"33147456\", \"33353966\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular tags/receptors mediating synaptic C4A targeting not identified\", \"Direct evidence in human schizophrenia/SLE tissue not provided by these models\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How C4A target selectivity translates into tissue-specific outcomes—the molecular signals marking synapses versus self-antigens for C4A-amide deposition and the receptors that read out that deposition—remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No identified molecular determinant of synapse-versus-immune-complex targeting\", \"No structural model of the C4A thioester transacylation reaction\", \"Human disease causality inferred from mouse isotype models\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [5, 6, 10]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [6, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [5, 11, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [8, 11, 16, 18]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"C1S\", \"CR1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}