{"gene":"CTLA4","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":1995,"finding":"CTLA-4 delivers inhibitory signals opposing CD28 costimulation: cross-linking CTLA-4 together with TCR and CD28 strongly inhibits T cell proliferation and IL-2 secretion, and B7-2 on freshly explanted T cells can partially inhibit T cell proliferation via CTLA-4 interactions.","method":"Anti-CTLA-4 antibody cross-linking assay using purified T cells; comparison of proliferation and IL-2 secretion with and without CTLA-4 engagement","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean functional assay with purified T cells, reciprocal antibody blocking, foundational paper replicated widely","pmids":["7543139"],"is_preprint":false},{"year":1992,"finding":"CTLA-4 is coexpressed with CD28 on activated T lymphocytes and cooperatively regulates T cell adhesion and activation by B7; anti-CTLA-4 mAbs cooperate with anti-CD28 mAbs to inhibit T cell adhesion to B7 and block proliferation in mixed lymphocyte culture.","method":"Monoclonal antibody development; flow cytometry; co-immobilized antibody stimulation assays; mixed lymphocyte culture","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal functional assays, foundational paper replicated by many subsequent studies","pmids":["1334116"],"is_preprint":false},{"year":1996,"finding":"CTLA-4 ligation blocks CD28-dependent IL-2 production, IL-2 receptor expression, and cell cycle progression of activated T cells; the primary effect is not induction of apoptosis; addition of exogenous IL-2 restores IL-2 receptor expression and T cell proliferation, indicating CTLA-4 does not block IL-2 responsiveness.","method":"Anti-CTLA-4 mAb ligation assay; cytokine ELISA; flow cytometry cell cycle analysis; exogenous IL-2 rescue experiment","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal readouts, rescue experiment, widely replicated","pmids":["8676075"],"is_preprint":false},{"year":1993,"finding":"CTLA-4 expression is induced on activated T cells (not detectable on resting T cells), is regulated by PKC-dependent signaling, and exists as a ~41-43 kDa protein that migrates primarily as a monomer at the cell surface rather than as a disulfide-bonded homodimer or CD28 heterodimer.","method":"Anti-CTLA-4 rabbit antiserum; [35S]methionine metabolic labeling; 125I surface labeling; SDS-PAGE; Northern blot; PKC inhibitor experiments","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical characterization with multiple methods in a single lab","pmids":["8397258"],"is_preprint":false},{"year":1993,"finding":"B7-2 (a second ligand distinct from B7-1/CD80) can bind CTLA-4 and provide costimulatory signals; CTLA4-Ig fusion protein blocks both B7 and B7-2 interactions, indicating CTLA-4 engages two distinct B7-family ligands on antigen-presenting cells.","method":"CTLA4-Ig blocking experiments; anti-B7 mAb blocking; T cell proliferation assays; B cell stimulation with LPS","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — functional blocking with CTLA4-Ig and mAbs, replicated by many subsequent studies","pmids":["7504292"],"is_preprint":false},{"year":1996,"finding":"CTLA-4 expression is transcriptionally induced upon T cell activation; 335 bp of upstream CTLA-4 sequence are sufficient to control inducibility; both positive and negative response elements modulate transcriptional regulation; expression is T cell-specific.","method":"Nuclear run-off assay; Northern blot; reporter gene analysis; non-T cell Northern blot analysis","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple molecular biology methods in a single lab establishing transcriptional control","pmids":["8666782"],"is_preprint":false},{"year":1996,"finding":"In vivo blockade of CTLA-4 with antibodies results in rejection of preestablished tumors and induces immunity to secondary tumor exposure, demonstrating CTLA-4 negatively regulates antitumor immune responses in vivo.","method":"In vivo anti-CTLA-4 antibody administration in tumor-bearing mice; rechallenge with tumor cells","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo loss-of-function with defined phenotypic readout, foundational paper replicated extensively","pmids":["8596936"],"is_preprint":false},{"year":2002,"finding":"CTLA-4 regulates cell cycle progression during primary immune responses: in the absence of CTLA-4, T cells show increased cycling in S and G2/M phases, increased IL-2 production, and delayed re-expression of the cell cycle inhibitor p27kip1, indicating CTLA-4 enforces the G1-to-S phase transition checkpoint.","method":"CTLA-4 knockout vs wild-type T cell comparison; flow cytometry cell cycle analysis; cyclin/CDK protein analysis; IL-2 mRNA and protein assays","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout with multiple orthogonal molecular readouts","pmids":["11807776"],"is_preprint":false},{"year":2008,"finding":"Treg-specific CTLA-4 deficiency results in spontaneous systemic lymphoproliferation, fatal autoimmune disease, and impairs Treg-mediated downregulation of CD80 and CD86 expression on dendritic cells, establishing that CTLA-4 on Tregs suppresses immune responses by controlling APC costimulatory ligand expression.","method":"Conditional CTLA-4 knockout in Foxp3+ Tregs; in vivo and in vitro Treg suppression assays; flow cytometry for CD80/CD86 on DCs","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional genetic knockout with multiple in vivo and in vitro mechanistic readouts, replicated","pmids":["18845758"],"is_preprint":false},{"year":2008,"finding":"CTLA-4 directly inhibits osteoclast formation dose-dependently in vitro (in the absence of T cells), inhibiting both RANKL- and TNF-mediated osteoclastogenesis, and also inhibits TNF-induced osteoclast formation in a non-T cell-dependent arthritis model in vivo.","method":"In vitro osteoclastogenesis assay with CTLA-4 protein; TNF-induced arthritis mouse model; histological analysis of bone erosion","journal":"Annals of the rheumatic diseases","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro reconstitution and in vivo model, single lab","pmids":["18203760"],"is_preprint":false},{"year":2008,"finding":"CTLA-4 surface expression is primarily regulated by restricted trafficking: the majority of CTLA-4 is intracellular, and several signaling molecules including TRIM, PLD, ARF-1, and TIRC7 are involved in transport of CTLA-4 to the cell surface; minor changes in surface expression levels have major effects on T cell activation outcome.","method":"Subcellular fractionation; co-immunoprecipitation with TRIM, PLD, ARF-1; flow cytometry surface expression analysis","journal":"Trends in immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — review synthesizing multiple co-IP and localization studies from multiple labs","pmids":["18468488"],"is_preprint":false},{"year":2009,"finding":"CTLA-4 inhibitory signaling in Treg cells controls T cell proliferation and confers resistance against activation-induced cell death (AICD) mediated by CD95/CD95L-caspase pathway; blockade of CTLA-4 signaling in Tregs reduces their suppressive capacity. Transfer of CTLA-4-competent Tregs into CTLA-4-deficient mice significantly prolongs their survival.","method":"In vitro CTLA-4 blockade on Treg cells; cell cycle analysis; apoptosis assay; caspase activation assay; adoptive transfer into CTLA-4-deficient mice","journal":"Arthritis and rheumatism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional assays and in vivo transfer experiment, single lab","pmids":["19116935"],"is_preprint":false},{"year":2010,"finding":"CTLA-4 inhibits T cell function by activating the ubiquitin ligase Itch: CTLA-4-mediated signaling leads to dephosphorylation/activation of Itch and enhanced ubiquitination of the Itch target molecule JunB; Itch knockdown abolishes CTLA-4-mediated inhibition of IFN-γ and IL-4 mRNA accumulation.","method":"Itch knockdown (siRNA/shRNA); phosphorylation assay; ubiquitination assay; cytokine mRNA analysis; comparison of CTLA-4- and Itch-deficient mice phenotypes","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — knockdown rescue, biochemical pathway analysis, single lab","pmids":["20417562"],"is_preprint":false},{"year":2002,"finding":"CTLA-4 is constitutively expressed by human monocytes (surface ~3%, intracellular ~20%), and ligation of CTLA-4 in monocytes suppresses proliferation, upregulation of CD86/CD54/HLA-DR/HLA-DQ, and inhibits AP-1 and NF-κB activation.","method":"Flow cytometry; cDNA sequencing; IFN-γ/PMA stimulation; anti-CTLA-4 mAb ligation; AP-1/NF-κB reporter assay in U937 cells","journal":"Scandinavian journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional readouts in primary cells and cell line, single lab","pmids":["11841692"],"is_preprint":false},{"year":2002,"finding":"cAMP and calcium ionophore (ionomycin) independently induce upregulation of CD152 (CTLA-4) in resting human CD4+ T cells; cAMP-induced upregulation is cyclosporin A-resistant whereas ionomycin-induced upregulation is cyclosporin A-sensitive (calcineurin-dependent); the upregulated CTLA-4 is functional and prevents NF-κB activation upon anti-CD3/CD28 stimulation.","method":"Flow cytometry; RT-PCR; cyclosporin A inhibition; NF-κB reporter assay; anti-CD3/CD28 stimulation","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological dissection of two pathways, functional validation, single lab","pmids":["12444128"],"is_preprint":false},{"year":2014,"finding":"CTLA-4 surface expression is regulated by multiple trafficking mechanisms involving TRIM and LAX (immune cell-specific adapters), Rab8 GTPase, ARF-1, phospholipase D, and the AP1/2 clathrin adaptor complex, which collectively control CTLA-4 transport to the cell surface and its residency.","method":"Co-immunoprecipitation; dominant-negative/constitutively active GTPase expression; surface expression assays; internalization assays","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — synthesis of multiple co-IP and localization studies from multiple labs, review article","pmids":["25538704"],"is_preprint":false},{"year":2021,"finding":"CTLA-4 recycling to the cell surface is regulated by LRBA and Rab11: CTLA-4 distributes across Rab5, Rab7, and Rab11 compartments; dominant-negative Rab5 increases surface CTLA-4; constitutively active Rab11 increases and dominant-negative Rab11 decreases surface CTLA-4; LRBA deficiency impairs CTLA-4 recycling and increases its degradation, and LRBA acts upstream of Rab11.","method":"Dominant-negative and constitutively active Rab GTPase expression; LRBA knockdown/knockout; flow cytometry surface expression; colocalization microscopy; degradation assays in HeLa and Jurkat cells","journal":"Immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal genetic and cell biology methods, epistasis between LRBA and Rab11 established","pmids":["33960403"],"is_preprint":false},{"year":2019,"finding":"Ipilimumab and TremeIgG1 (irAE-prone antibodies) rapidly direct cell surface CTLA-4 to lysosomal degradation, whereas non-irAE-prone antibodies allow CTLA-4 to recycle to the cell surface via the LRBA-dependent mechanism. Disrupting CTLA-4 recycling (via LRBA knockout) causes robust CTLA-4 downregulation by all anti-CTLA-4 antibodies and confers toxicity. Introducing pH-sensitive tyrosine-to-histidine mutations in TremeIgG1 prevents lysosomal CTLA-4 downregulation and attenuates irAE while improving antitumor efficacy.","method":"LRBA knockout cells; antibody endocytosis and recycling assays; lysosomal tracking; pH-sensitive antibody mutagenesis; in vivo tumor models; intratumor Treg depletion assays","journal":"Cell research","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — mutagenesis, KO rescue, in vivo validation, multiple orthogonal methods","pmids":["31267017"],"is_preprint":false},{"year":2021,"finding":"CTLA-4 blockade destabilizes tumor-infiltrating Treg cells toward IFNγ- and TNF-producing cells in glycolysis-defective tumors; this effect depends on Treg cell glycolysis and CD28 signaling, establishing that glucose availability in the tumor microenvironment gates the effect of CTLA-4 blockade on Treg stability.","method":"Glycolysis-defective tumor mouse models; anti-CTLA-4 antibody treatment; metabolic analysis; in vitro mimicry of glycolytic tumor microenvironment; flow cytometry for Treg phenotype; CD28 blocking experiments","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple in vivo and in vitro models, epistasis with CD28 signaling and glucose metabolism established","pmids":["33588426"],"is_preprint":false},{"year":2022,"finding":"CTLA-4 performs transendocytosis of both CD80 and CD86, but the fates differ: in the presence of CD80, CTLA-4 remains ligand-bound, is ubiquitylated, and is trafficked to late endosomes/lysosomes for degradation; in the presence of CD86, CTLA-4 detaches in a pH-dependent manner and recycles to the cell surface for further transendocytosis. Clinically relevant autoimmune-associated mutations selectively disrupt CD86 transendocytosis by affecting either CTLA-4 recycling or CD86 binding, identifying CD86 as the key target for CTLA-4 immune regulation.","method":"Transendocytosis assays; pH-dependent dissociation assays; ubiquitylation assays; endosomal trafficking/colocalization studies; functional analysis of clinical CTLA-4 mutations","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — reconstitution-level mechanistic dissection, multiple orthogonal methods, clinical mutation validation","pmids":["35999394"],"is_preprint":false},{"year":1999,"finding":"Interaction of CTLA-4 with CD80 or CD86 on human T cells inhibits T cell activation (cytokine production, proliferation, and cytotoxic responses) in polyclonal and alloantigen-specific assays; CTLA-4-mediated inhibition operates independently of CD28 occupancy.","method":"Anti-CTLA-4 mAb blocking (soluble and Fab fragments); T cell proliferation assays; cytokine ELISA; cytotoxicity assays","journal":"Immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional readouts and Fab controls, single lab","pmids":["10583602"],"is_preprint":false},{"year":2000,"finding":"Selective CTLA-4 ligation using a membrane-bound single-chain antibody (scFv) on artificial APCs reduces T cell proliferation and IL-2 production and inhibits tyrosine phosphorylation of proximal TCR signaling components; this inhibitory effect requires coexpression of TCR and CTLA-4 ligands on the same surface, demonstrating that CTLA-4 modifies the proximal TCR signal in cis.","method":"Surface-linked scFv to CTLA-4; co-expression of anti-CD3ε and anti-CD28 scFvs; proliferation assays; IL-2 ELISA; tyrosine phosphorylation assay; TCR transgenic T cells","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — engineered APC system with biochemical readout, epistatic requirement for cis engagement demonstrated, single lab","pmids":["10779742"],"is_preprint":false},{"year":2021,"finding":"CTLA-4 expression on B-1a B cells (not only T cells) is critical for immune tolerance: selective deletion of CTLA-4 from B cells causes spontaneous autoantibodies, T follicular helper cell expansion, germinal center formation, and autoimmune pathology; CTLA-4-deficient B-1a cells upregulate epigenetic/transcriptional activation, show increased self-replenishment, internalize surface IgM, differentiate into APCs, and upon transfer induce germinal centers in normal recipients.","method":"B cell-specific CTLA-4 conditional knockout; flow cytometry; autoantibody detection; adoptive transfer; transcriptomic/epigenomic analysis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional knockout with multiple mechanistic readouts and transfer experiments establishing non-T cell function","pmids":["33483505"],"is_preprint":false},{"year":2023,"finding":"The alternatively spliced soluble CTLA-4 isoform (sCTLA-4) suppresses CD8+ T cells in vitro and accelerates tumor growth and metastasis in vivo; sCTLA-4 expression by tumor cells restrains intratumoral CD8+ T cells in a non-cytotoxic state; isoform-specific anti-sCTLA-4 antibody blockade reverses this restraint and enhances CD8+ T cell cytolytic potential.","method":"In vitro CD8+ T cell suppression assay; syngeneic mouse tumor growth and metastasis models; isoform-specific antibody blockade; flow cytometry immune infiltrate analysis","journal":"Molecular therapy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro and in vivo functional evidence with isoform-specific tools, single lab","pmids":["38053333"],"is_preprint":false},{"year":2014,"finding":"Soluble CTLA-4 (sCTLA-4) is generated by alternatively spliced mRNA lacking the transmembrane-encoding exon 3; the sCTLA-4 protein is secreted at low levels by activated primary human CD4+ T cells; transmembrane CTLA-4 (Tm-CTLA-4) is unexpectedly associated with microvesicles produced by activated T cells.","method":"Novel isoform-specific mAbs and polyclonal Abs; ELISA; flow cytometry; microvesicle isolation; activated CD4+ T cell cultures","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — isoform-specific antibody tools, biochemical characterization, single lab","pmids":["24928993"],"is_preprint":false},{"year":2018,"finding":"CTLA-4 expressed on mesenchymal stem/stromal cells (MSCs) mediates their immunosuppressive function; under hypoxic conditions, the secreted isoform (sCTLA-4) is the most abundant; immunosuppression by MSCs is mediated mainly by sCTLA-4 secretion.","method":"CTLA-4 isoform expression analysis; functional immunosuppression assays with CTLA-4 blockade; hypoxia experiments","journal":"International journal of molecular sciences","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, limited mechanistic detail in abstract, no mutagenesis or reconstitution","pmids":["30087255"],"is_preprint":false},{"year":2023,"finding":"Fusion of CTLA-4 cytoplasmic tail (CCT) to CAR constructs progressively lowers CAR surface expression via constant endocytosis, recycling, and degradation; this reduces CAR-mediated trogocytosis and tumor antigen loss, enhances CAR-T persistence and central memory phenotype, and improves antitumor efficacy in a relapsed leukemia model.","method":"CAR-T cell engineering with monomeric/duplex/triplex CCT fusions; surface expression assays; trogocytosis assay; in vivo leukemia model; scRNA-seq; flow cytometry","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — systematic structure-function engineering, multiple orthogonal readouts including in vivo efficacy and scRNA-seq","pmids":["37500885"],"is_preprint":false}],"current_model":"CTLA-4 is an inducible inhibitory co-receptor on activated T cells (and Tregs) that competes with CD28 for shared B7 ligands (CD80/CD86); it suppresses T cell activation by blocking IL-2 production and cell cycle progression via proximal TCR signal attenuation, activating the ubiquitin ligase Itch, and removing CD80/CD86 from antigen-presenting cells through transendocytosis (preferentially targeting CD86 for recycling-dependent repeated rounds and CD80 for lysosomal degradation); its surface expression is tightly regulated by intracellular trafficking through Rab5/Rab7/Rab11 compartments and by LRBA-dependent recycling, and its function extends to Tregs (where CTLA-4 is required for suppressive activity and Treg-mediated APC modulation), B-1a B cells (where it maintains self-tolerance), and monocytes/MSCs (where it mediates immunosuppression via secreted isoforms)."},"narrative":{"mechanistic_narrative":"CTLA-4 is an inducible inhibitory co-receptor that opposes CD28 costimulation to enforce peripheral T cell tolerance and restrain immune activation [PMID:7543139, PMID:1334116]. Its expression is restricted to activated T cells through transcriptional induction following T cell activation, with PKC-dependent and cAMP/calcineurin-dependent inputs, and is largely absent from resting cells [PMID:8397258, PMID:8666782, PMID:12444128]. CTLA-4 engages the B7-family ligands CD80 and CD86 on antigen-presenting cells, and ligation blocks CD28-dependent IL-2 production, IL-2 receptor expression, and cell cycle progression, enforcing the G1-to-S checkpoint via delayed p27kip1 re-expression rather than inducing apoptosis [PMID:8676075, PMID:7504292, PMID:11807776, PMID:10583602]. Mechanistically, CTLA-4 attenuates proximal TCR signaling in cis when its ligands and the TCR are engaged on the same surface [PMID:10779742], and it transmits inhibitory signals by activating the ubiquitin ligase Itch to enhance ubiquitination of JunB and suppress cytokine mRNA accumulation [PMID:20417562]. A central effector mechanism is transendocytosis: CTLA-4 physically removes CD80 and CD86 from APCs, with CD80-bound CTLA-4 routed to lysosomal degradation while CD86-bound CTLA-4 detaches in a pH-dependent manner and recycles for further rounds, identifying CD86 as the key regulatory target [PMID:35999394]. CTLA-4 surface levels are tightly governed by intracellular trafficking through Rab5/Rab7/Rab11 compartments and LRBA-dependent recycling, where LRBA acts upstream of Rab11 to prevent CTLA-4 degradation [PMID:33960403]. This function is essential in regulatory T cells, where CTLA-4 deficiency causes fatal lymphoproliferative autoimmunity and impairs Treg-mediated downregulation of CD80/CD86 on dendritic cells [PMID:18845758], and extends beyond T cells to B-1a B cells, where it maintains self-tolerance [PMID:33483505], and to soluble/secreted isoforms that suppress CD8+ T cells and APC activation [PMID:38053333, PMID:24928993]. In vivo, CTLA-4 negatively regulates antitumor immunity, and its blockade drives tumor rejection, providing the basis for checkpoint immunotherapy [PMID:8596936, PMID:33588426].","teleology":[{"year":1992,"claim":"Established that CTLA-4 is co-expressed with CD28 on activated T cells and cooperates to regulate B7-driven adhesion and activation, placing it in the costimulatory axis.","evidence":"Monoclonal antibody development, flow cytometry, and co-immobilized antibody stimulation in mixed lymphocyte culture","pmids":["1334116"],"confidence":"High","gaps":["Did not distinguish inhibitory from activating function","No mechanism of signal transduction defined"]},{"year":1993,"claim":"Defined CTLA-4 as an activation-induced cell-surface monomeric glycoprotein under PKC-dependent and transcriptional control, explaining its inducible expression pattern.","evidence":"Metabolic and surface labeling, SDS-PAGE, Northern blot, PKC inhibitor and reporter assays; identification of B7-2 as a second ligand","pmids":["8397258","7504292","8666782"],"confidence":"Medium","gaps":["Promoter elements only coarsely mapped","Transcription factors not identified"]},{"year":1996,"claim":"Demonstrated that CTLA-4 delivers a genuine inhibitory signal opposing CD28, blocking IL-2 production and proliferation without inducing apoptosis, defining its functional role as a brake.","evidence":"Anti-CTLA-4 cross-linking and ligation assays, cytokine ELISA, cell cycle analysis, exogenous IL-2 rescue, and in vivo tumor blockade in mice","pmids":["7543139","8676075","8596936"],"confidence":"High","gaps":["Molecular basis of inhibition not resolved","Did not distinguish cell-intrinsic from cell-extrinsic effects"]},{"year":2000,"claim":"Showed that CTLA-4 inhibits proximal TCR signaling in cis, requiring co-engagement of TCR and CTLA-4 ligands on the same surface, defining the spatial logic of its inhibitory action.","evidence":"Surface-linked scFv on artificial APCs, tyrosine phosphorylation assays, and TCR transgenic T cells; CD28-independent inhibition via CD80/CD86 ligation","pmids":["10779742","10583602"],"confidence":"Medium","gaps":["Specific phosphatases or signaling intermediates not identified","Single-lab engineered system"]},{"year":2002,"claim":"Established that CTLA-4 enforces the G1-to-S cell cycle checkpoint via p27kip1, and revealed expression and function in monocytes, broadening its regulatory scope.","evidence":"CTLA-4 knockout T cell cell-cycle analysis, cyclin/CDK and IL-2 readouts; flow cytometry and reporter assays in human monocytes and U937 cells","pmids":["11807776","11841692","12444128"],"confidence":"High","gaps":["Link between surface CTLA-4 and p27kip1 regulation not mechanistically connected","Monocyte function based on single-lab data"]},{"year":2008,"claim":"Genetically established CTLA-4 as essential for Treg suppressive function and APC ligand downregulation, explaining the fatal autoimmunity of CTLA-4 deficiency.","evidence":"Foxp3-conditional CTLA-4 knockout with in vivo/in vitro suppression assays and CD80/CD86 flow cytometry on dendritic cells; osteoclastogenesis assays","pmids":["18845758","18203760"],"confidence":"High","gaps":["Molecular mechanism of CD80/CD86 removal not yet defined here","Cell-intrinsic versus Treg-extrinsic contributions not fully separated"]},{"year":2010,"claim":"Identified the ubiquitin ligase Itch as a downstream effector of CTLA-4 inhibitory signaling, providing a biochemical pathway from CTLA-4 to cytokine suppression.","evidence":"Itch knockdown rescue, phosphorylation and JunB ubiquitination assays, cytokine mRNA analysis, and comparison of CTLA-4- and Itch-deficient mice","pmids":["20417562"],"confidence":"Medium","gaps":["Upstream signal connecting CTLA-4 engagement to Itch activation not defined","Single-lab pathway"]},{"year":2014,"claim":"Resolved that CTLA-4 surface availability is dominated by intracellular trafficking, identifying adaptors and GTPases controlling its transport and residency.","evidence":"Co-immunoprecipitation, dominant-negative/constitutively active GTPase expression, and surface/internalization assays (TRIM, LAX, Rab8, ARF-1, PLD, AP1/2); soluble isoform characterization","pmids":["25538704","18468488","24928993"],"confidence":"Medium","gaps":["Trafficking steps synthesized from review-level data","Hierarchy among trafficking factors not established"]},{"year":2021,"claim":"Established the LRBA–Rab11 recycling axis as the controller of CTLA-4 surface expression and stability, and extended CTLA-4 tolerance function to B-1a B cells.","evidence":"Dominant-negative/constitutively active Rab GTPases, LRBA knockdown/knockout with colocalization and degradation assays in HeLa/Jurkat; B cell-conditional knockout with autoantibody and transfer experiments; tumor Treg destabilization linked to glycolysis and CD28","pmids":["33960403","33483505","33588426"],"confidence":"High","gaps":["Molecular details of LRBA action upstream of Rab11 not fully resolved","B-1a-specific tolerance mechanism distinct from T cell role not fully delineated"]},{"year":2022,"claim":"Provided reconstitution-level dissection of transendocytosis, showing divergent fates for CD80- versus CD86-bound CTLA-4 and identifying CD86 as the key regulatory target, with clinical mutations selectively disrupting CD86 capture.","evidence":"Transendocytosis, pH-dependent dissociation, ubiquitylation, and endosomal trafficking assays with functional analysis of clinical CTLA-4 mutations; antibody-driven lysosomal targeting and pH-sensitive antibody engineering","pmids":["35999394","31267017"],"confidence":"High","gaps":["Ubiquitin ligase mediating CD80-bound CTLA-4 degradation not identified","Quantitative contribution of transendocytosis to in vivo suppression not measured"]},{"year":2023,"claim":"Demonstrated functional roles for soluble CTLA-4 isoforms in suppressing CD8+ T cells and showed that the CTLA-4 cytoplasmic tail can be engineered to improve CAR-T persistence, translating trafficking biology into therapeutic design.","evidence":"Isoform-specific antibody blockade in tumor models, CD8 suppression assays; CAR-CCT fusion engineering with trogocytosis, persistence, scRNA-seq, and in vivo leukemia readouts; MSC immunosuppression","pmids":["38053333","37500885","30087255"],"confidence":"Medium","gaps":["Receptor/target of secreted sCTLA-4 not identified","MSC sCTLA-4 mechanism based on low-confidence single-lab data"]},{"year":null,"claim":"The identity of the ubiquitin ligase and signaling intermediates that route CD80-bound CTLA-4 to degradation, and the molecular target of secreted sCTLA-4, remain undefined.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No defined receptor for soluble CTLA-4 isoforms","Mechanism converting CTLA-4 ligand binding into intracellular inhibitory signaling incompletely mapped"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,2,7,20]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,21]},{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[19]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[3,16,19]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[16,19]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[17,19]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[10,16]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,8,22]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[16,19]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[7]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[21,12]}],"complexes":[],"partners":["CD80","CD86","CD28","LRBA","ITCH","RAB11"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P16410","full_name":"Cytotoxic T-lymphocyte protein 4","aliases":["Cytotoxic T-lymphocyte-associated antigen 4","CTLA-4"],"length_aa":223,"mass_kda":24.7,"function":"Inhibitory receptor acting as a major negative regulator of T-cell responses (PubMed:11279501, PubMed:11279502, PubMed:16551244, PubMed:1714933, PubMed:18641304, PubMed:28484017). Acts as a decoy receptor: the affinity of CTLA4 for its natural B7 family ligands, CD80 and CD86, is considerably stronger than the affinity of their cognate stimulatory coreceptor CD28 (PubMed:11279501, PubMed:11279502, PubMed:16551244, PubMed:1714933, PubMed:28484017)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/P16410/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CTLA4","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CTLA4","total_profiled":1310},"omim":[{"mim_id":"619573","title":"IMMUNODEFICIENCY 87 AND AUTOIMMUNITY; IMD87","url":"https://www.omim.org/entry/619573"},{"mim_id":"616100","title":"IMMUNE DYSREGULATION WITH AUTOIMMUNITY, IMMUNODEFICIENCY, AND LYMPHOPROLIFERATION; 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Supplement = Journal international du cancer. Supplement","url":"https://pubmed.ncbi.nlm.nih.gov/1330947","citation_count":36,"is_preprint":false},{"pmid":"30087255","id":"PMC_30087255","title":"CTLA-4 Mediates Inhibitory Function of Mesenchymal Stem/Stromal Cells.","date":"2018","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/30087255","citation_count":36,"is_preprint":false},{"pmid":"23551370","id":"PMC_23551370","title":"Pruritus to anticancer agents targeting the EGFR, BRAF, and CTLA-4.","date":"2013","source":"Dermatologic therapy","url":"https://pubmed.ncbi.nlm.nih.gov/23551370","citation_count":35,"is_preprint":false},{"pmid":"24928993","id":"PMC_24928993","title":"Investigation of soluble and transmembrane CTLA-4 isoforms in serum and microvesicles.","date":"2014","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/24928993","citation_count":34,"is_preprint":false},{"pmid":"20417562","id":"PMC_20417562","title":"CTLA-4 (CD152) inhibits T cell function by activating the ubiquitin ligase Itch.","date":"2010","source":"Molecular immunology","url":"https://pubmed.ncbi.nlm.nih.gov/20417562","citation_count":33,"is_preprint":false},{"pmid":"34148159","id":"PMC_34148159","title":"CTLA-4 Expression and Its Clinical Significance in Breast Cancer.","date":"2021","source":"Archivum immunologiae et therapiae experimentalis","url":"https://pubmed.ncbi.nlm.nih.gov/34148159","citation_count":31,"is_preprint":false},{"pmid":"33021155","id":"PMC_33021155","title":"FoxP3, CTLA-4, and IDO in Canine Melanocytic Tumors.","date":"2020","source":"Veterinary pathology","url":"https://pubmed.ncbi.nlm.nih.gov/33021155","citation_count":31,"is_preprint":false},{"pmid":"29409002","id":"PMC_29409002","title":"PTPN22 and CTLA-4 Polymorphisms Are Associated With Polyglandular Autoimmunity.","date":"2018","source":"The Journal of clinical endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/29409002","citation_count":31,"is_preprint":false},{"pmid":"28544686","id":"PMC_28544686","title":"Regulatory T cells and CTLA-4 in idiopathic nephrotic syndrome.","date":"2017","source":"Pediatrics international : official journal of the Japan Pediatric Society","url":"https://pubmed.ncbi.nlm.nih.gov/28544686","citation_count":31,"is_preprint":false},{"pmid":"37441075","id":"PMC_37441075","title":"Bispecific antibodies targeting CTLA-4: game-changer troopers in cancer immunotherapy.","date":"2023","source":"Frontiers in immunology","url":"https://pubmed.ncbi.nlm.nih.gov/37441075","citation_count":30,"is_preprint":false},{"pmid":"11266089","id":"PMC_11266089","title":"Expression and functional significance of CTLA-4, a negative regulator of T cell activation.","date":"2001","source":"Archivum immunologiae et therapiae experimentalis","url":"https://pubmed.ncbi.nlm.nih.gov/11266089","citation_count":30,"is_preprint":false},{"pmid":"25387592","id":"PMC_25387592","title":"Braking bad: novel mechanisms of CTLA-4 inhibition of T cell responses.","date":"2014","source":"American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons","url":"https://pubmed.ncbi.nlm.nih.gov/25387592","citation_count":29,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":43676,"output_tokens":6759,"usd":0.116207,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":15805,"output_tokens":4380,"usd":0.094262,"stage2_stop_reason":"end_turn"},"total_usd":0.210469,"stage1_batch_id":"msgbatch_01ACAxJQabDsht39JXeWG22d","stage2_batch_id":"msgbatch_01QEU22A9U5sbPTgsMzTmvSf","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1995,\n      \"finding\": \"CTLA-4 delivers inhibitory signals opposing CD28 costimulation: cross-linking CTLA-4 together with TCR and CD28 strongly inhibits T cell proliferation and IL-2 secretion, and B7-2 on freshly explanted T cells can partially inhibit T cell proliferation via CTLA-4 interactions.\",\n      \"method\": \"Anti-CTLA-4 antibody cross-linking assay using purified T cells; comparison of proliferation and IL-2 secretion with and without CTLA-4 engagement\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean functional assay with purified T cells, reciprocal antibody blocking, foundational paper replicated widely\",\n      \"pmids\": [\"7543139\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"CTLA-4 is coexpressed with CD28 on activated T lymphocytes and cooperatively regulates T cell adhesion and activation by B7; anti-CTLA-4 mAbs cooperate with anti-CD28 mAbs to inhibit T cell adhesion to B7 and block proliferation in mixed lymphocyte culture.\",\n      \"method\": \"Monoclonal antibody development; flow cytometry; co-immobilized antibody stimulation assays; mixed lymphocyte culture\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal functional assays, foundational paper replicated by many subsequent studies\",\n      \"pmids\": [\"1334116\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"CTLA-4 ligation blocks CD28-dependent IL-2 production, IL-2 receptor expression, and cell cycle progression of activated T cells; the primary effect is not induction of apoptosis; addition of exogenous IL-2 restores IL-2 receptor expression and T cell proliferation, indicating CTLA-4 does not block IL-2 responsiveness.\",\n      \"method\": \"Anti-CTLA-4 mAb ligation assay; cytokine ELISA; flow cytometry cell cycle analysis; exogenous IL-2 rescue experiment\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal readouts, rescue experiment, widely replicated\",\n      \"pmids\": [\"8676075\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CTLA-4 expression is induced on activated T cells (not detectable on resting T cells), is regulated by PKC-dependent signaling, and exists as a ~41-43 kDa protein that migrates primarily as a monomer at the cell surface rather than as a disulfide-bonded homodimer or CD28 heterodimer.\",\n      \"method\": \"Anti-CTLA-4 rabbit antiserum; [35S]methionine metabolic labeling; 125I surface labeling; SDS-PAGE; Northern blot; PKC inhibitor experiments\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical characterization with multiple methods in a single lab\",\n      \"pmids\": [\"8397258\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"B7-2 (a second ligand distinct from B7-1/CD80) can bind CTLA-4 and provide costimulatory signals; CTLA4-Ig fusion protein blocks both B7 and B7-2 interactions, indicating CTLA-4 engages two distinct B7-family ligands on antigen-presenting cells.\",\n      \"method\": \"CTLA4-Ig blocking experiments; anti-B7 mAb blocking; T cell proliferation assays; B cell stimulation with LPS\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — functional blocking with CTLA4-Ig and mAbs, replicated by many subsequent studies\",\n      \"pmids\": [\"7504292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"CTLA-4 expression is transcriptionally induced upon T cell activation; 335 bp of upstream CTLA-4 sequence are sufficient to control inducibility; both positive and negative response elements modulate transcriptional regulation; expression is T cell-specific.\",\n      \"method\": \"Nuclear run-off assay; Northern blot; reporter gene analysis; non-T cell Northern blot analysis\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple molecular biology methods in a single lab establishing transcriptional control\",\n      \"pmids\": [\"8666782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"In vivo blockade of CTLA-4 with antibodies results in rejection of preestablished tumors and induces immunity to secondary tumor exposure, demonstrating CTLA-4 negatively regulates antitumor immune responses in vivo.\",\n      \"method\": \"In vivo anti-CTLA-4 antibody administration in tumor-bearing mice; rechallenge with tumor cells\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo loss-of-function with defined phenotypic readout, foundational paper replicated extensively\",\n      \"pmids\": [\"8596936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CTLA-4 regulates cell cycle progression during primary immune responses: in the absence of CTLA-4, T cells show increased cycling in S and G2/M phases, increased IL-2 production, and delayed re-expression of the cell cycle inhibitor p27kip1, indicating CTLA-4 enforces the G1-to-S phase transition checkpoint.\",\n      \"method\": \"CTLA-4 knockout vs wild-type T cell comparison; flow cytometry cell cycle analysis; cyclin/CDK protein analysis; IL-2 mRNA and protein assays\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout with multiple orthogonal molecular readouts\",\n      \"pmids\": [\"11807776\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Treg-specific CTLA-4 deficiency results in spontaneous systemic lymphoproliferation, fatal autoimmune disease, and impairs Treg-mediated downregulation of CD80 and CD86 expression on dendritic cells, establishing that CTLA-4 on Tregs suppresses immune responses by controlling APC costimulatory ligand expression.\",\n      \"method\": \"Conditional CTLA-4 knockout in Foxp3+ Tregs; in vivo and in vitro Treg suppression assays; flow cytometry for CD80/CD86 on DCs\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional genetic knockout with multiple in vivo and in vitro mechanistic readouts, replicated\",\n      \"pmids\": [\"18845758\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"CTLA-4 directly inhibits osteoclast formation dose-dependently in vitro (in the absence of T cells), inhibiting both RANKL- and TNF-mediated osteoclastogenesis, and also inhibits TNF-induced osteoclast formation in a non-T cell-dependent arthritis model in vivo.\",\n      \"method\": \"In vitro osteoclastogenesis assay with CTLA-4 protein; TNF-induced arthritis mouse model; histological analysis of bone erosion\",\n      \"journal\": \"Annals of the rheumatic diseases\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro reconstitution and in vivo model, single lab\",\n      \"pmids\": [\"18203760\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"CTLA-4 surface expression is primarily regulated by restricted trafficking: the majority of CTLA-4 is intracellular, and several signaling molecules including TRIM, PLD, ARF-1, and TIRC7 are involved in transport of CTLA-4 to the cell surface; minor changes in surface expression levels have major effects on T cell activation outcome.\",\n      \"method\": \"Subcellular fractionation; co-immunoprecipitation with TRIM, PLD, ARF-1; flow cytometry surface expression analysis\",\n      \"journal\": \"Trends in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — review synthesizing multiple co-IP and localization studies from multiple labs\",\n      \"pmids\": [\"18468488\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"CTLA-4 inhibitory signaling in Treg cells controls T cell proliferation and confers resistance against activation-induced cell death (AICD) mediated by CD95/CD95L-caspase pathway; blockade of CTLA-4 signaling in Tregs reduces their suppressive capacity. Transfer of CTLA-4-competent Tregs into CTLA-4-deficient mice significantly prolongs their survival.\",\n      \"method\": \"In vitro CTLA-4 blockade on Treg cells; cell cycle analysis; apoptosis assay; caspase activation assay; adoptive transfer into CTLA-4-deficient mice\",\n      \"journal\": \"Arthritis and rheumatism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional assays and in vivo transfer experiment, single lab\",\n      \"pmids\": [\"19116935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CTLA-4 inhibits T cell function by activating the ubiquitin ligase Itch: CTLA-4-mediated signaling leads to dephosphorylation/activation of Itch and enhanced ubiquitination of the Itch target molecule JunB; Itch knockdown abolishes CTLA-4-mediated inhibition of IFN-γ and IL-4 mRNA accumulation.\",\n      \"method\": \"Itch knockdown (siRNA/shRNA); phosphorylation assay; ubiquitination assay; cytokine mRNA analysis; comparison of CTLA-4- and Itch-deficient mice phenotypes\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knockdown rescue, biochemical pathway analysis, single lab\",\n      \"pmids\": [\"20417562\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CTLA-4 is constitutively expressed by human monocytes (surface ~3%, intracellular ~20%), and ligation of CTLA-4 in monocytes suppresses proliferation, upregulation of CD86/CD54/HLA-DR/HLA-DQ, and inhibits AP-1 and NF-κB activation.\",\n      \"method\": \"Flow cytometry; cDNA sequencing; IFN-γ/PMA stimulation; anti-CTLA-4 mAb ligation; AP-1/NF-κB reporter assay in U937 cells\",\n      \"journal\": \"Scandinavian journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional readouts in primary cells and cell line, single lab\",\n      \"pmids\": [\"11841692\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"cAMP and calcium ionophore (ionomycin) independently induce upregulation of CD152 (CTLA-4) in resting human CD4+ T cells; cAMP-induced upregulation is cyclosporin A-resistant whereas ionomycin-induced upregulation is cyclosporin A-sensitive (calcineurin-dependent); the upregulated CTLA-4 is functional and prevents NF-κB activation upon anti-CD3/CD28 stimulation.\",\n      \"method\": \"Flow cytometry; RT-PCR; cyclosporin A inhibition; NF-κB reporter assay; anti-CD3/CD28 stimulation\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological dissection of two pathways, functional validation, single lab\",\n      \"pmids\": [\"12444128\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CTLA-4 surface expression is regulated by multiple trafficking mechanisms involving TRIM and LAX (immune cell-specific adapters), Rab8 GTPase, ARF-1, phospholipase D, and the AP1/2 clathrin adaptor complex, which collectively control CTLA-4 transport to the cell surface and its residency.\",\n      \"method\": \"Co-immunoprecipitation; dominant-negative/constitutively active GTPase expression; surface expression assays; internalization assays\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — synthesis of multiple co-IP and localization studies from multiple labs, review article\",\n      \"pmids\": [\"25538704\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CTLA-4 recycling to the cell surface is regulated by LRBA and Rab11: CTLA-4 distributes across Rab5, Rab7, and Rab11 compartments; dominant-negative Rab5 increases surface CTLA-4; constitutively active Rab11 increases and dominant-negative Rab11 decreases surface CTLA-4; LRBA deficiency impairs CTLA-4 recycling and increases its degradation, and LRBA acts upstream of Rab11.\",\n      \"method\": \"Dominant-negative and constitutively active Rab GTPase expression; LRBA knockdown/knockout; flow cytometry surface expression; colocalization microscopy; degradation assays in HeLa and Jurkat cells\",\n      \"journal\": \"Immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal genetic and cell biology methods, epistasis between LRBA and Rab11 established\",\n      \"pmids\": [\"33960403\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Ipilimumab and TremeIgG1 (irAE-prone antibodies) rapidly direct cell surface CTLA-4 to lysosomal degradation, whereas non-irAE-prone antibodies allow CTLA-4 to recycle to the cell surface via the LRBA-dependent mechanism. Disrupting CTLA-4 recycling (via LRBA knockout) causes robust CTLA-4 downregulation by all anti-CTLA-4 antibodies and confers toxicity. Introducing pH-sensitive tyrosine-to-histidine mutations in TremeIgG1 prevents lysosomal CTLA-4 downregulation and attenuates irAE while improving antitumor efficacy.\",\n      \"method\": \"LRBA knockout cells; antibody endocytosis and recycling assays; lysosomal tracking; pH-sensitive antibody mutagenesis; in vivo tumor models; intratumor Treg depletion assays\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — mutagenesis, KO rescue, in vivo validation, multiple orthogonal methods\",\n      \"pmids\": [\"31267017\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CTLA-4 blockade destabilizes tumor-infiltrating Treg cells toward IFNγ- and TNF-producing cells in glycolysis-defective tumors; this effect depends on Treg cell glycolysis and CD28 signaling, establishing that glucose availability in the tumor microenvironment gates the effect of CTLA-4 blockade on Treg stability.\",\n      \"method\": \"Glycolysis-defective tumor mouse models; anti-CTLA-4 antibody treatment; metabolic analysis; in vitro mimicry of glycolytic tumor microenvironment; flow cytometry for Treg phenotype; CD28 blocking experiments\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple in vivo and in vitro models, epistasis with CD28 signaling and glucose metabolism established\",\n      \"pmids\": [\"33588426\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CTLA-4 performs transendocytosis of both CD80 and CD86, but the fates differ: in the presence of CD80, CTLA-4 remains ligand-bound, is ubiquitylated, and is trafficked to late endosomes/lysosomes for degradation; in the presence of CD86, CTLA-4 detaches in a pH-dependent manner and recycles to the cell surface for further transendocytosis. Clinically relevant autoimmune-associated mutations selectively disrupt CD86 transendocytosis by affecting either CTLA-4 recycling or CD86 binding, identifying CD86 as the key target for CTLA-4 immune regulation.\",\n      \"method\": \"Transendocytosis assays; pH-dependent dissociation assays; ubiquitylation assays; endosomal trafficking/colocalization studies; functional analysis of clinical CTLA-4 mutations\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — reconstitution-level mechanistic dissection, multiple orthogonal methods, clinical mutation validation\",\n      \"pmids\": [\"35999394\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Interaction of CTLA-4 with CD80 or CD86 on human T cells inhibits T cell activation (cytokine production, proliferation, and cytotoxic responses) in polyclonal and alloantigen-specific assays; CTLA-4-mediated inhibition operates independently of CD28 occupancy.\",\n      \"method\": \"Anti-CTLA-4 mAb blocking (soluble and Fab fragments); T cell proliferation assays; cytokine ELISA; cytotoxicity assays\",\n      \"journal\": \"Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional readouts and Fab controls, single lab\",\n      \"pmids\": [\"10583602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Selective CTLA-4 ligation using a membrane-bound single-chain antibody (scFv) on artificial APCs reduces T cell proliferation and IL-2 production and inhibits tyrosine phosphorylation of proximal TCR signaling components; this inhibitory effect requires coexpression of TCR and CTLA-4 ligands on the same surface, demonstrating that CTLA-4 modifies the proximal TCR signal in cis.\",\n      \"method\": \"Surface-linked scFv to CTLA-4; co-expression of anti-CD3ε and anti-CD28 scFvs; proliferation assays; IL-2 ELISA; tyrosine phosphorylation assay; TCR transgenic T cells\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — engineered APC system with biochemical readout, epistatic requirement for cis engagement demonstrated, single lab\",\n      \"pmids\": [\"10779742\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CTLA-4 expression on B-1a B cells (not only T cells) is critical for immune tolerance: selective deletion of CTLA-4 from B cells causes spontaneous autoantibodies, T follicular helper cell expansion, germinal center formation, and autoimmune pathology; CTLA-4-deficient B-1a cells upregulate epigenetic/transcriptional activation, show increased self-replenishment, internalize surface IgM, differentiate into APCs, and upon transfer induce germinal centers in normal recipients.\",\n      \"method\": \"B cell-specific CTLA-4 conditional knockout; flow cytometry; autoantibody detection; adoptive transfer; transcriptomic/epigenomic analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional knockout with multiple mechanistic readouts and transfer experiments establishing non-T cell function\",\n      \"pmids\": [\"33483505\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The alternatively spliced soluble CTLA-4 isoform (sCTLA-4) suppresses CD8+ T cells in vitro and accelerates tumor growth and metastasis in vivo; sCTLA-4 expression by tumor cells restrains intratumoral CD8+ T cells in a non-cytotoxic state; isoform-specific anti-sCTLA-4 antibody blockade reverses this restraint and enhances CD8+ T cell cytolytic potential.\",\n      \"method\": \"In vitro CD8+ T cell suppression assay; syngeneic mouse tumor growth and metastasis models; isoform-specific antibody blockade; flow cytometry immune infiltrate analysis\",\n      \"journal\": \"Molecular therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro and in vivo functional evidence with isoform-specific tools, single lab\",\n      \"pmids\": [\"38053333\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Soluble CTLA-4 (sCTLA-4) is generated by alternatively spliced mRNA lacking the transmembrane-encoding exon 3; the sCTLA-4 protein is secreted at low levels by activated primary human CD4+ T cells; transmembrane CTLA-4 (Tm-CTLA-4) is unexpectedly associated with microvesicles produced by activated T cells.\",\n      \"method\": \"Novel isoform-specific mAbs and polyclonal Abs; ELISA; flow cytometry; microvesicle isolation; activated CD4+ T cell cultures\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — isoform-specific antibody tools, biochemical characterization, single lab\",\n      \"pmids\": [\"24928993\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"CTLA-4 expressed on mesenchymal stem/stromal cells (MSCs) mediates their immunosuppressive function; under hypoxic conditions, the secreted isoform (sCTLA-4) is the most abundant; immunosuppression by MSCs is mediated mainly by sCTLA-4 secretion.\",\n      \"method\": \"CTLA-4 isoform expression analysis; functional immunosuppression assays with CTLA-4 blockade; hypoxia experiments\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, limited mechanistic detail in abstract, no mutagenesis or reconstitution\",\n      \"pmids\": [\"30087255\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Fusion of CTLA-4 cytoplasmic tail (CCT) to CAR constructs progressively lowers CAR surface expression via constant endocytosis, recycling, and degradation; this reduces CAR-mediated trogocytosis and tumor antigen loss, enhances CAR-T persistence and central memory phenotype, and improves antitumor efficacy in a relapsed leukemia model.\",\n      \"method\": \"CAR-T cell engineering with monomeric/duplex/triplex CCT fusions; surface expression assays; trogocytosis assay; in vivo leukemia model; scRNA-seq; flow cytometry\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — systematic structure-function engineering, multiple orthogonal readouts including in vivo efficacy and scRNA-seq\",\n      \"pmids\": [\"37500885\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CTLA-4 is an inducible inhibitory co-receptor on activated T cells (and Tregs) that competes with CD28 for shared B7 ligands (CD80/CD86); it suppresses T cell activation by blocking IL-2 production and cell cycle progression via proximal TCR signal attenuation, activating the ubiquitin ligase Itch, and removing CD80/CD86 from antigen-presenting cells through transendocytosis (preferentially targeting CD86 for recycling-dependent repeated rounds and CD80 for lysosomal degradation); its surface expression is tightly regulated by intracellular trafficking through Rab5/Rab7/Rab11 compartments and by LRBA-dependent recycling, and its function extends to Tregs (where CTLA-4 is required for suppressive activity and Treg-mediated APC modulation), B-1a B cells (where it maintains self-tolerance), and monocytes/MSCs (where it mediates immunosuppression via secreted isoforms).\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CTLA-4 is an inducible inhibitory co-receptor that opposes CD28 costimulation to enforce peripheral T cell tolerance and restrain immune activation [#0, #1]. Its expression is restricted to activated T cells through transcriptional induction following T cell activation, with PKC-dependent and cAMP/calcineurin-dependent inputs, and is largely absent from resting cells [#3, #5, #14]. CTLA-4 engages the B7-family ligands CD80 and CD86 on antigen-presenting cells, and ligation blocks CD28-dependent IL-2 production, IL-2 receptor expression, and cell cycle progression, enforcing the G1-to-S checkpoint via delayed p27kip1 re-expression rather than inducing apoptosis [#2, #4, #7, #20]. Mechanistically, CTLA-4 attenuates proximal TCR signaling in cis when its ligands and the TCR are engaged on the same surface [#21], and it transmits inhibitory signals by activating the ubiquitin ligase Itch to enhance ubiquitination of JunB and suppress cytokine mRNA accumulation [#12]. A central effector mechanism is transendocytosis: CTLA-4 physically removes CD80 and CD86 from APCs, with CD80-bound CTLA-4 routed to lysosomal degradation while CD86-bound CTLA-4 detaches in a pH-dependent manner and recycles for further rounds, identifying CD86 as the key regulatory target [#19]. CTLA-4 surface levels are tightly governed by intracellular trafficking through Rab5/Rab7/Rab11 compartments and LRBA-dependent recycling, where LRBA acts upstream of Rab11 to prevent CTLA-4 degradation [#16]. This function is essential in regulatory T cells, where CTLA-4 deficiency causes fatal lymphoproliferative autoimmunity and impairs Treg-mediated downregulation of CD80/CD86 on dendritic cells [#8], and extends beyond T cells to B-1a B cells, where it maintains self-tolerance [#22], and to soluble/secreted isoforms that suppress CD8+ T cells and APC activation [#23, #24]. In vivo, CTLA-4 negatively regulates antitumor immunity, and its blockade drives tumor rejection, providing the basis for checkpoint immunotherapy [#6, #18].\",\n  \"teleology\": [\n    {\n      \"year\": 1992,\n      \"claim\": \"Established that CTLA-4 is co-expressed with CD28 on activated T cells and cooperates to regulate B7-driven adhesion and activation, placing it in the costimulatory axis.\",\n      \"evidence\": \"Monoclonal antibody development, flow cytometry, and co-immobilized antibody stimulation in mixed lymphocyte culture\",\n      \"pmids\": [\"1334116\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not distinguish inhibitory from activating function\", \"No mechanism of signal transduction defined\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Defined CTLA-4 as an activation-induced cell-surface monomeric glycoprotein under PKC-dependent and transcriptional control, explaining its inducible expression pattern.\",\n      \"evidence\": \"Metabolic and surface labeling, SDS-PAGE, Northern blot, PKC inhibitor and reporter assays; identification of B7-2 as a second ligand\",\n      \"pmids\": [\"8397258\", \"7504292\", \"8666782\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Promoter elements only coarsely mapped\", \"Transcription factors not identified\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Demonstrated that CTLA-4 delivers a genuine inhibitory signal opposing CD28, blocking IL-2 production and proliferation without inducing apoptosis, defining its functional role as a brake.\",\n      \"evidence\": \"Anti-CTLA-4 cross-linking and ligation assays, cytokine ELISA, cell cycle analysis, exogenous IL-2 rescue, and in vivo tumor blockade in mice\",\n      \"pmids\": [\"7543139\", \"8676075\", \"8596936\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of inhibition not resolved\", \"Did not distinguish cell-intrinsic from cell-extrinsic effects\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Showed that CTLA-4 inhibits proximal TCR signaling in cis, requiring co-engagement of TCR and CTLA-4 ligands on the same surface, defining the spatial logic of its inhibitory action.\",\n      \"evidence\": \"Surface-linked scFv on artificial APCs, tyrosine phosphorylation assays, and TCR transgenic T cells; CD28-independent inhibition via CD80/CD86 ligation\",\n      \"pmids\": [\"10779742\", \"10583602\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific phosphatases or signaling intermediates not identified\", \"Single-lab engineered system\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Established that CTLA-4 enforces the G1-to-S cell cycle checkpoint via p27kip1, and revealed expression and function in monocytes, broadening its regulatory scope.\",\n      \"evidence\": \"CTLA-4 knockout T cell cell-cycle analysis, cyclin/CDK and IL-2 readouts; flow cytometry and reporter assays in human monocytes and U937 cells\",\n      \"pmids\": [\"11807776\", \"11841692\", \"12444128\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Link between surface CTLA-4 and p27kip1 regulation not mechanistically connected\", \"Monocyte function based on single-lab data\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Genetically established CTLA-4 as essential for Treg suppressive function and APC ligand downregulation, explaining the fatal autoimmunity of CTLA-4 deficiency.\",\n      \"evidence\": \"Foxp3-conditional CTLA-4 knockout with in vivo/in vitro suppression assays and CD80/CD86 flow cytometry on dendritic cells; osteoclastogenesis assays\",\n      \"pmids\": [\"18845758\", \"18203760\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of CD80/CD86 removal not yet defined here\", \"Cell-intrinsic versus Treg-extrinsic contributions not fully separated\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identified the ubiquitin ligase Itch as a downstream effector of CTLA-4 inhibitory signaling, providing a biochemical pathway from CTLA-4 to cytokine suppression.\",\n      \"evidence\": \"Itch knockdown rescue, phosphorylation and JunB ubiquitination assays, cytokine mRNA analysis, and comparison of CTLA-4- and Itch-deficient mice\",\n      \"pmids\": [\"20417562\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Upstream signal connecting CTLA-4 engagement to Itch activation not defined\", \"Single-lab pathway\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Resolved that CTLA-4 surface availability is dominated by intracellular trafficking, identifying adaptors and GTPases controlling its transport and residency.\",\n      \"evidence\": \"Co-immunoprecipitation, dominant-negative/constitutively active GTPase expression, and surface/internalization assays (TRIM, LAX, Rab8, ARF-1, PLD, AP1/2); soluble isoform characterization\",\n      \"pmids\": [\"25538704\", \"18468488\", \"24928993\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Trafficking steps synthesized from review-level data\", \"Hierarchy among trafficking factors not established\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Established the LRBA–Rab11 recycling axis as the controller of CTLA-4 surface expression and stability, and extended CTLA-4 tolerance function to B-1a B cells.\",\n      \"evidence\": \"Dominant-negative/constitutively active Rab GTPases, LRBA knockdown/knockout with colocalization and degradation assays in HeLa/Jurkat; B cell-conditional knockout with autoantibody and transfer experiments; tumor Treg destabilization linked to glycolysis and CD28\",\n      \"pmids\": [\"33960403\", \"33483505\", \"33588426\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular details of LRBA action upstream of Rab11 not fully resolved\", \"B-1a-specific tolerance mechanism distinct from T cell role not fully delineated\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Provided reconstitution-level dissection of transendocytosis, showing divergent fates for CD80- versus CD86-bound CTLA-4 and identifying CD86 as the key regulatory target, with clinical mutations selectively disrupting CD86 capture.\",\n      \"evidence\": \"Transendocytosis, pH-dependent dissociation, ubiquitylation, and endosomal trafficking assays with functional analysis of clinical CTLA-4 mutations; antibody-driven lysosomal targeting and pH-sensitive antibody engineering\",\n      \"pmids\": [\"35999394\", \"31267017\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ubiquitin ligase mediating CD80-bound CTLA-4 degradation not identified\", \"Quantitative contribution of transendocytosis to in vivo suppression not measured\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstrated functional roles for soluble CTLA-4 isoforms in suppressing CD8+ T cells and showed that the CTLA-4 cytoplasmic tail can be engineered to improve CAR-T persistence, translating trafficking biology into therapeutic design.\",\n      \"evidence\": \"Isoform-specific antibody blockade in tumor models, CD8 suppression assays; CAR-CCT fusion engineering with trogocytosis, persistence, scRNA-seq, and in vivo leukemia readouts; MSC immunosuppression\",\n      \"pmids\": [\"38053333\", \"37500885\", \"30087255\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor/target of secreted sCTLA-4 not identified\", \"MSC sCTLA-4 mechanism based on low-confidence single-lab data\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The identity of the ubiquitin ligase and signaling intermediates that route CD80-bound CTLA-4 to degradation, and the molecular target of secreted sCTLA-4, remain undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No defined receptor for soluble CTLA-4 isoforms\", \"Mechanism converting CTLA-4 ligand binding into intracellular inhibitory signaling incompletely mapped\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 2, 7, 20]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 21]},\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [19]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [3, 16, 19]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [16, 19]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [17, 19]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [10, 16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 8, 22]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [16, 19]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [7]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [21, 12]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"CD80\", \"CD86\", \"CD28\", \"LRBA\", \"ITCH\", \"RAB11\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":8,"faith_pct":87.5}}