{"gene":"CD5","run_date":"2026-04-28T17:28:52","timeline":{"discoveries":[{"year":2001,"finding":"CD5 functions as a negative regulator of TCR signal transduction during thymocyte development, and the cytoplasmic domain of CD5 is required for its inhibitory function, as demonstrated by the failure of a truncated CD5 transgene (lacking the cytoplasmic tail) to rescue the CD5-/- phenotype.","method":"Transgenic mouse overexpression and CD5-/- genetic knockout with truncated cytoplasmic domain mutant; thymocyte selection assays with TCR transgenic mice","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — clean KO/transgenic rescue with defined phenotypic readout, cytoplasmic domain mutagenesis in vivo","pmids":["11313384"],"is_preprint":false},{"year":1996,"finding":"CD5 mediates a novel inducible cell-surface ligand (CD5L) expressed on activated B and T lymphocytes upon stimulation with anti-CD3/anti-CD28. CD5L binding to CD5Rg (CD5-Ig fusion protein) is trypsin-resistant, pronase-sensitive, and dependent on N-linked glycosylation of CD5; CD5L is distinct from CD72. In vivo, CD5Rg injection abrogated development of antibody-mediated membranous glomerulonephritis in a murine model.","method":"CD5-immunoglobulin fusion protein binding assay, immunoprecipitation, antibody and recombinant protein blocking, in vivo mouse model of glomerulonephritis","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal biochemical methods plus in vivo functional validation in a single study","pmids":["9064341"],"is_preprint":false},{"year":2010,"finding":"CD5 mediates species-specific homophilic interactions through its domain 1 only. Soluble CD5 (purified at neutral pH) binds CD5 on cell surfaces. CD5 monoclonal antibodies that have functional effects block homophilic binding, and inhibiting CD5 domain 1 engagement enhanced antigen-specific T cell responses in vitro, indicating that homophilic CD5-CD5 interaction results in productive inhibitory signaling.","method":"Soluble protein binding assay, domain mutagenesis, antibody blocking, in vitro T cell activation assay","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1/2 — soluble protein reconstitution, mutagenesis, and functional assay in single study","pmids":["20952682"],"is_preprint":false},{"year":2002,"finding":"CD5 and CD6 physically associate at the membrane of lymphoid T cells, co-localize at the immunological synapse, and co-immunoprecipitate from Brij 96 (but not NP-40) lysates. The CD5-CD6 association is independent of the CD5 cytoplasmic region. FRET analysis, co-capping, and co-modulation experiments confirmed in vivo physical association.","method":"Co-immunoprecipitation, FRET, co-capping, co-modulation, immunological synapse imaging in T cell/APC conjugates","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (co-IP, FRET, live-cell imaging) confirming the association","pmids":["12473675"],"is_preprint":false},{"year":1997,"finding":"Thymocyte activation induces association of phosphatidylinositol 3-kinase (PI3K, p85 subunit) and a 120-kDa phosphoprotein (pp120) with CD5. PI3K p85 binds preferentially to tyrosine-phosphorylated Y463 of CD5 via the C-SH2 domain, with bivalent interaction involving the N-SH2 domain at the Y429-Y441 ITAM-like sequence.","method":"Co-immunoprecipitation with pervanadate-stimulated thymocytes, recombinant SH2 domain binding to phosphopeptides, PI3K activity assay","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — biochemical co-IP plus in vitro peptide binding assays with defined phosphotyrosine residues","pmids":["9079809"],"is_preprint":false},{"year":2001,"finding":"CD5 tyrosine phosphorylation occurs at residues Y429 and Y463 in vivo following T cell stimulation with anti-CD3 or pervanadate. This phosphorylation requires Lck kinase activity (absent in Lck-deficient Jurkat cells). In vitro kinase assays with purified Lck and Fyn confirmed these sites as direct substrates.","method":"Tyrosine-mutated CD5 transfectants in Jurkat T cells, in vivo stimulation, in vitro kinase assay with recombinant Lck and Fyn, Lck-deficient Jurkat cells","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — in vitro kinase assay with mutagenesis, confirmed in vivo in Lck-deficient cells","pmids":["11298344"],"is_preprint":false},{"year":1993,"finding":"A serine kinase activity constitutively associates with CD5 immunoprecipitates and is rapidly and transiently increased (within 1–3 min) upon CD3/TCR stimulation, correlating with increased CD5 phosphorylation. Direct CD5 cross-linking also increases the associated kinase activity with different kinetics (peaking at 10 min, sustained for 1 h), suggesting distinct regulatory mechanisms.","method":"Immunoprecipitation with in vitro kinase assay, phosphoamino acid analysis, anti-phosphotyrosine Western blot, cell stimulation with anti-CD3 or anti-CD5","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1/2 — in vitro kinase assay combined with biochemical co-IP, replicated across multiple CD5+ cell types","pmids":["7691949"],"is_preprint":false},{"year":2012,"finding":"CD5 activates casein kinase 2 (CK2) through a specific CK2-binding domain at the end of its cytoplasmic tail. Mice expressing CD5 with a microdeletion selectively unable to interact with CK2 (CD5ΔCK2BD) showed hypoproliferating T cells with enhanced activation-induced cell death, impaired Th2 and Th17 (but not Th1) differentiation, reduced EAE severity, and failed high-dose tolerance induction — phenocopying CD5 KO mice.","method":"Knock-in mouse with selective CD5-CK2 binding domain deletion, T cell proliferation assay, in vivo EAE model, Th differentiation assays","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — domain-specific knock-in mutation with multiple defined phenotypic readouts in vivo and in vitro","pmids":["22904299"],"is_preprint":false},{"year":2014,"finding":"The CD5-CK2 signaling pathway enhances TCR-induced AKT activation, inhibits GSK3, and activates mTOR, promoting Th17 differentiation. In the absence of CD5-CK2 signaling, increased GSK3 activity renders Th17 cells more sensitive to IFN-γ-mediated inhibition, and S6K activity and nuclear translocation of RORγt are attenuated.","method":"CD5ΔCK2BD knock-in mice, AKT/GSK3/mTOR/S6K phosphorylation assays, RORγt nuclear translocation imaging, IFN-γ sensitivity assays, in vitro Th17 differentiation","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — mechanistic pathway dissection with genetic model and multiple signaling readouts","pmids":["24356888"],"is_preprint":false},{"year":2017,"finding":"In CLL cells, CD5 forms a complex with CK2, BLNK, and STAT3 that mediates constitutive phosphorylation of STAT3 on serine 727. siRNA knockdown of CD5 or BLNK, or treatment with CD5-neutralizing antibodies, significantly reduced serine-phosphorylated STAT3 levels. Fractionation studies showed the CK2/CD5/BLNK/STAT3 complex resides in the cytoplasm while phosphorylated STAT3 is shuttled to the nucleus.","method":"Mass spectrometry co-immunoprecipitation, siRNA knockdown, in vitro CK2 kinase assay with recombinant STAT3, confocal microscopy, cell fractionation","journal":"Molecular cancer research","confidence":"High","confidence_rationale":"Tier 1/2 — reciprocal co-IP, in vitro kinase assay, siRNA, neutralizing antibody, and localization studies","pmids":["28130399"],"is_preprint":false},{"year":2011,"finding":"CD5 costimulation drives stable human Th17 development by inducing prolonged STAT3 activation and enhanced RORγt expression, critically dependent on CD5-induced upregulation of IL-23 receptor (IL-23R) expression, providing more durable Th17 commitment than classical CD28 costimulation.","method":"In vitro human naive T cell priming with CD5 or CD28 costimulation, intracellular cytokine staining, STAT3 phosphorylation kinetics, IL-23R expression analysis","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 — clean in vitro functional system with defined molecular readouts (STAT3, RORγt, IL-23R); single lab, multiple methods","pmids":["21926348"],"is_preprint":false},{"year":2002,"finding":"CD5-mediated inhibition of TCR signaling during thymocyte development does not require the CD5 extracellular domain (and thus does not involve ligand binding), as demonstrated by rescue of CD5-/- mice with a chimeric molecule bearing the extracellular domain of human IL-2R p55 (Tac) fused to the CD5 transmembrane and cytoplasmic domains.","method":"Chimeric CD5 transgene in CD5-/- mice, thymocyte development analysis","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with domain-swap transgene rescue in KO mice","pmids":["12115665"],"is_preprint":false},{"year":1999,"finding":"A natural soluble form of human CD5 (nsCD5) is released by proteolytic cleavage of the membrane form upon cell activation (phorbol ester or CD3 stimulation). The nsCD5 is a 52 kDa N-glycosylated molecule present in normal human serum and PBMC supernatants, indistinguishable from a recombinant soluble form composed of the three extracellular SRCR domains.","method":"Protein purification, biochemical characterization, immunoprecipitation, in vitro stimulation with proteolytic cleavage demonstration","journal":"Tissue antigens","confidence":"Medium","confidence_rationale":"Tier 2 — protein purification and biochemical characterization, single lab","pmids":["10488739"],"is_preprint":false},{"year":2004,"finding":"Constitutive CD5 expression in human T cells is driven by Ets transcription factors binding to two conserved Ets-binding sites at positions -239 and -185 upstream of CD5. A DNase I-hypersensitive site at the 5'-flanking region correlates with CD5 expression; a 282-bp region upstream of ATG displays full promoter activity. An inducible DH site 10 kb upstream functions as an enhancer in T and B CD5+ cells.","method":"DNase I hypersensitivity assay, 5'-RACE, luciferase reporter assay, site-directed mutagenesis, EMSA, cotransfection","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1/2 — multiple orthogonal methods including mutagenesis and functional reporter assays","pmids":["15187131"],"is_preprint":false},{"year":2009,"finding":"IL-6 produced by SLE B cells abrogates DNMT1 induction, preventing methylation of the CD5-E1B alternative promoter. Demethylation of CpG islands in the CD5-E1B promoter leads to transcription of the cytoplasm-retained CD5-E1B isoform at the expense of membrane-expressed CD5-E1A, reducing surface CD5 levels and thereby dysregulating BCR signaling in SLE.","method":"Bisulfite sequencing, methylation-sensitive endonuclease assay, anti-IL-6R blocking antibody, DNMT1 expression analysis, BCR engagement experiments","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods linking epigenetic mechanism to CD5 isoform regulation","pmids":["19380809"],"is_preprint":false},{"year":2005,"finding":"RAG1 and RAG2 expression in peripheral B cells outside germinal centers is closely associated with CD5 expression. In vitro activation of CD5-negative naive B cells induces CD5 expression on a subset, and RAG1/RAG2 upregulation occurs only in the cells that become CD5+, suggesting CD5 is associated with receptor revision in activated mature B cells.","method":"Isolation of CD5+ human tonsil B cells, RAG mRNA and protein detection, CD5 induction in vitro, RAG expression correlated with CD5 acquisition by flow cytometry","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 3 — correlation between CD5 induction and RAG expression, single lab with multiple readouts but no direct mechanistic intervention","pmids":["15843554"],"is_preprint":false},{"year":2023,"finding":"CD5 expression by CD1c+ dendritic cells (DCs) is required to generate optimally protective CD5hi T helper and CD8+ T cells. Activating CD5 on DCs enhanced T cell priming and improved survival after immune checkpoint blockade therapy. Low IL-6 concentrations promoted de novo differentiation of CD5+ DCs. Deletion of CD5 from T cells dampened tumor elimination in response to immune checkpoint blockade in vivo.","method":"Human melanoma patient samples, in vivo mouse tumor models with CD5 deletion from T cells, DC differentiation assays with IL-6 manipulation, immune checkpoint blockade treatment","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — genetic deletion with defined functional phenotype, multiple model systems, in vivo tumor models","pmids":["36795805"],"is_preprint":false},{"year":2024,"finding":"CD5 inhibits CAR T cell activation; CRISPR-Cas9 knockout of CD5 enhances CAR T cell cytotoxicity, in vivo expansion, and persistence in multiple hematological and solid cancer models without apparent toxicity. CD5 KO drives increased T cell effector function.","method":"CRISPR-Cas9 knockout, CAR T cell cytotoxicity assays, in vivo mouse tumor models","journal":"Science immunology","confidence":"Medium","confidence_rationale":"Tier 2 — CRISPR KO with defined functional readouts in multiple models; single study","pmids":["39028827"],"is_preprint":false},{"year":1989,"finding":"CD5 on activated human B cells is phosphorylated similarly to CD5 on T cells. IL-4 specifically inhibits CD5 expression on activated B cells at the mRNA level, without affecting other B cell activation antigens (CD25, B5, T9, CD23, CD20), indicating a specific transcriptional or mRNA stability mechanism.","method":"Immunoprecipitation of 125I-labeled cells, Northern blot analysis, flow cytometry, IL-4 treatment","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — protein and mRNA level evidence with specificity controls, single lab","pmids":["2472277"],"is_preprint":false},{"year":2000,"finding":"The human CD5 gene spans ~24.5 kb, consists of at least 11 exons conserved in size, number, and structure with the mouse homologue, maps to chromosome 11q12.2 (82 kb downstream from CD6 in head-to-tail orientation), and encodes a cytoplasmic tailless isoform via alternative splicing.","method":"Genomic cloning, sequencing, FISH mapping, comparative genomics","journal":"Immunogenetics","confidence":"Medium","confidence_rationale":"Tier 2 — direct genomic characterization establishing structural basis for isoform generation","pmids":["11061284"],"is_preprint":false}],"current_model":"CD5 is a scavenger receptor cysteine-rich (SRCR) family glycoprotein that functions primarily as a negative regulator of TCR and BCR signaling through its cytoplasmic tail, which recruits and activates CK2 (regulating AKT/GSK3/mTOR/RORγt axis for Th17 differentiation), associates with PI3K (via phosphorylated Y429/Y441/Y463 targeted by Lck and Fyn), and forms a complex with CK2/BLNK/STAT3 to drive constitutive STAT3 serine phosphorylation in CLL; CD5 also engages in homophilic extracellular interactions through domain 1 that contribute to inhibitory signaling, physically associates with CD6 at the immunological synapse, is released as a soluble form by proteolytic cleavage, and its expression is transcriptionally controlled by Ets factors and epigenetically regulated via IL-6-dependent DNA methylation of an alternative B-cell-specific promoter."},"narrative":{"teleology":[{"year":1989,"claim":"Establishing that CD5 is phosphorylated on B cells similarly to T cells and that its expression is specifically downregulated by IL-4 at the mRNA level revealed that CD5 is subject to lineage-specific transcriptional control beyond T cells.","evidence":"Immunoprecipitation of radiolabeled cells, Northern blot, flow cytometry with IL-4 treatment on activated human B cells","pmids":["2472277"],"confidence":"Medium","gaps":["Single-lab observation; mechanism of IL-4-mediated mRNA reduction (transcription vs. stability) not resolved","IL-4 effect not tested in vivo"]},{"year":1993,"claim":"Identification of a constitutively associated serine kinase activity on CD5 that is rapidly augmented upon TCR stimulation established that CD5 is not merely a passive surface marker but an active signaling scaffold.","evidence":"Immunoprecipitation with in vitro kinase assay, phosphoamino acid analysis upon anti-CD3 or anti-CD5 stimulation","pmids":["7691949"],"confidence":"High","gaps":["Identity of the associated serine kinase not determined at this point","Functional consequence of kinase activation on downstream signaling not tested"]},{"year":1996,"claim":"Discovery of an inducible CD5 ligand (CD5L) on activated lymphocytes that is distinct from CD72 and whose blockade prevented antibody-mediated glomerulonephritis in vivo demonstrated that CD5 participates in receptor–ligand interactions with in vivo immunoregulatory consequences.","evidence":"CD5-Ig fusion protein binding, immunoprecipitation, antibody blocking, murine glomerulonephritis model","pmids":["9064341"],"confidence":"High","gaps":["Molecular identity of CD5L not cloned","Whether CD5L mediates inhibitory or activating signals not resolved"]},{"year":1997,"claim":"Mapping PI3K p85 recruitment to phosphorylated Y463 (C-SH2) and Y429/Y441 (N-SH2) on CD5 defined the first specific cytoplasmic signaling axis for CD5, revealing how it couples to the PI3K pathway upon thymocyte activation.","evidence":"Co-immunoprecipitation from pervanadate-stimulated thymocytes, recombinant SH2 domain binding to synthetic phosphopeptides, PI3K activity assay","pmids":["9079809"],"confidence":"High","gaps":["Functional consequence of PI3K recruitment (survival, proliferation) not directly tested","Kinase(s) responsible for CD5 tyrosine phosphorylation not yet identified"]},{"year":2001,"claim":"Two contemporaneous studies established that the CD5 cytoplasmic tail is essential for its inhibitory function during thymocyte development and that Lck and Fyn directly phosphorylate Y429 and Y463, linking the upstream kinases to the PI3K-recruiting tyrosines.","evidence":"Transgenic rescue of CD5−/− mice with truncated CD5; Lck-deficient Jurkat cells and in vitro kinase assays with recombinant Lck/Fyn","pmids":["11313384","11298344"],"confidence":"High","gaps":["Whether Lck vs. Fyn have non-redundant roles at specific phosphosites in vivo not resolved","Additional cytoplasmic tail effectors beyond PI3K not yet identified"]},{"year":2002,"claim":"Demonstration that a chimeric CD5 lacking its own extracellular domain still rescues inhibitory signaling in CD5−/− mice, combined with identification of a CD5–CD6 physical complex at the immunological synapse, dissociated extracellular ligand engagement from the core inhibitory mechanism and revealed CD5 functions within a multi-receptor signaling unit.","evidence":"Chimeric transgene rescue in CD5−/− mice; co-IP, FRET, co-capping, and synapse imaging for CD5–CD6 association","pmids":["12115665","12473675"],"confidence":"High","gaps":["Functional significance of CD5–CD6 association for signaling output not tested","Whether CD6 modifies CD5 cytoplasmic tail phosphorylation unknown"]},{"year":2004,"claim":"Identification of Ets-binding sites driving the constitutive CD5 promoter and a distal enhancer active in both T and B1 cells established the transcriptional architecture governing lineage-specific CD5 expression.","evidence":"DNase I hypersensitivity, luciferase reporters, site-directed mutagenesis, EMSA in human lymphoid cells","pmids":["15187131"],"confidence":"High","gaps":["Which specific Ets family member(s) are functionally dominant in vivo not determined","Enhancer mechanism in B cells vs. T cells not separately dissected"]},{"year":2009,"claim":"Elucidation of IL-6-driven suppression of DNMT1 leading to demethylation of the CD5-E1B alternative promoter in SLE B cells explained how surface CD5 is downregulated in autoimmunity, linking epigenetic control to BCR signaling dysregulation.","evidence":"Bisulfite sequencing, methylation-sensitive restriction, anti-IL-6R blocking antibody, DNMT1 expression analysis in SLE patient B cells","pmids":["19380809"],"confidence":"High","gaps":["Causal contribution of CD5-E1B isoform to SLE pathogenesis not tested by genetic intervention","Whether therapeutic IL-6R blockade restores CD5 surface levels in patients not shown"]},{"year":2010,"claim":"Demonstration that CD5 engages in species-specific homophilic interactions via domain 1 that deliver inhibitory signals reconciled conflicting data on CD5 ligands and established a direct extracellular mechanism for CD5-mediated immune dampening.","evidence":"Soluble CD5 binding assays, domain mutagenesis, antibody blocking, antigen-specific T cell activation assays","pmids":["20952682"],"confidence":"High","gaps":["Crystal structure of homophilic interaction not available","Relationship between homophilic binding and the previously described CD5L remains unclear"]},{"year":2012,"claim":"Creation of CD5ΔCK2BD knock-in mice identified CK2 as the functionally critical serine kinase recruited to the CD5 cytoplasmic tail, resolving the 1993 observation and showing that CD5–CK2 signaling is specifically required for Th2/Th17 differentiation and tolerance induction.","evidence":"Knock-in mice with selective CK2-binding domain microdeletion, T cell proliferation, EAE, Th differentiation assays","pmids":["22904299"],"confidence":"High","gaps":["CK2 substrates downstream of CD5 not fully catalogued at this stage","Whether CD5–CK2 axis operates identically in B cells not addressed"]},{"year":2014,"claim":"Dissection of the CD5–CK2–AKT–GSK3–mTOR–RORγt signaling cascade provided a complete pathway from receptor to nuclear transcription factor for Th17 commitment, explaining why CD5–CK2 disruption selectively impairs Th17 but not Th1 cells.","evidence":"CD5ΔCK2BD knock-in mice, phosphorylation analysis of AKT/GSK3/mTOR/S6K, RORγt nuclear translocation imaging, IFN-γ sensitivity assays","pmids":["24356888"],"confidence":"High","gaps":["Whether CK2 directly phosphorylates AKT or acts through an intermediary not resolved","Relevance to human Th17 biology not directly shown"]},{"year":2017,"claim":"In CLL, identification of a cytoplasmic CD5/CK2/BLNK/STAT3 complex that constitutively phosphorylates STAT3-S727 revealed a pathological co-option of the CD5–CK2 signaling axis in B-cell malignancy, extending the paradigm from T cells to leukemic B cells.","evidence":"Mass spectrometry co-IP, siRNA knockdown of CD5 and BLNK, in vitro CK2 kinase assay on recombinant STAT3, confocal microscopy and cell fractionation in primary CLL cells","pmids":["28130399"],"confidence":"High","gaps":["Whether CD5/CK2/BLNK/STAT3 complex exists in normal B1 cells not tested","Therapeutic targeting of this complex in CLL not validated"]},{"year":2023,"claim":"Discovery that CD5 expression on CD1c+ dendritic cells is required for optimal T cell priming and that CD5 deletion from T cells impairs checkpoint blockade-mediated tumor elimination expanded CD5 function beyond a purely inhibitory role to one that tunes immune competence in anti-tumor immunity.","evidence":"Human melanoma patient analysis, in vivo mouse tumor models with CD5 conditional deletion, DC differentiation assays, immune checkpoint blockade treatment","pmids":["36795805"],"confidence":"High","gaps":["Mechanism by which DC-expressed CD5 enhances T cell priming not molecularly defined","Whether CD5 on DCs signals through the same CK2 axis as on T cells unknown"]},{"year":2024,"claim":"CRISPR knockout of CD5 in CAR T cells enhanced effector function, expansion, and persistence across multiple tumor models, validating CD5 as a druggable immune checkpoint in adoptive cell therapy.","evidence":"CRISPR-Cas9 KO in human CAR T cells, cytotoxicity assays, in vivo mouse models of hematological and solid tumors","pmids":["39028827"],"confidence":"Medium","gaps":["Single study; long-term safety and autoimmune risk of CD5 deletion not assessed","Molecular basis for enhanced persistence (metabolic reprogramming vs. reduced AICD) not defined"]},{"year":null,"claim":"Key unresolved questions include the structural basis of homophilic CD5–CD5 interaction, the mechanism by which DC-expressed CD5 promotes T cell priming, whether the CD5–CK2 axis operates identically in normal B1 cells and CLL, and whether soluble CD5 has immunomodulatory signaling activity in vivo.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal or cryo-EM structure of CD5 extracellular domains","DC-intrinsic CD5 signaling pathway undefined","In vivo function of soluble CD5 not established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,4,7,9]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,7,8,17]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,2,3,12]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[12]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[9]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,7,8,10,16,17]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[4,5,8,9]}],"complexes":["CD5/CK2 signaling complex","CD5/CK2/BLNK/STAT3 complex","CD5/CD6 membrane complex"],"partners":["CD6","CSNK2A1","PIK3R1","LCK","FYN","BLNK","STAT3"],"other_free_text":[]},"mechanistic_narrative":"CD5 is a scavenger receptor cysteine-rich (SRCR) family glycoprotein that functions as a negative regulator of antigen receptor signaling in T and B lymphocytes, calibrating thymocyte selection thresholds and modulating peripheral immune responses. Its inhibitory function requires the cytoplasmic tail, which recruits casein kinase 2 (CK2) via a dedicated C-terminal binding domain and PI3K (p85 subunit) via Lck/Fyn-phosphorylated tyrosines Y429, Y441, and Y463; the CD5–CK2 axis activates AKT, inhibits GSK3, and promotes mTOR-dependent RORγt nuclear translocation to drive Th17 differentiation, while in CLL B cells a cytoplasmic CD5/CK2/BLNK/STAT3 complex constitutively phosphorylates STAT3 on serine 727 [PMID:11313384, PMID:22904299, PMID:24356888, PMID:9079809, PMID:28130399]. Extracellularly, CD5 engages in species-specific homophilic interactions through domain 1 that contribute to inhibitory signaling and physically associates with CD6 at the immunological synapse, and a soluble form is released by proteolytic cleavage upon activation [PMID:20952682, PMID:12473675, PMID:10488739]. CD5 transcription is controlled by Ets factors at a proximal promoter, while an alternative B-cell-specific promoter (CD5-E1B) is epigenetically regulated by IL-6-dependent suppression of DNMT1-mediated DNA methylation, linking CD5 surface levels to dysregulated BCR signaling in SLE [PMID:15187131, PMID:19380809]."},"prefetch_data":{"uniprot":{"accession":"P06127","full_name":"T-cell surface glycoprotein CD5","aliases":["Lymphocyte antigen T1/Leu-1"],"length_aa":495,"mass_kda":54.6,"function":"Lymphoid-specific receptor expressed by all T-cells and in a subset of B-cells known as B1a cells. Plays a role in the regulation of TCR and BCR signaling, thymocyte selection, T-cell effector differentiation and immune tolerance. Acts by interacting with several ligands expressed on B-cells such as CD5L or CD72 and thereby plays an important role in contact-mediated, T-dependent B-cell activation and in the maintenance of regulatory T and B-cell homeostasis. Functions as a negative regulator of TCR signaling during thymocyte development by associating with several signaling proteins including LCK, CD3Z chain, PI3K or CBL (PubMed:1384049, PubMed:1385158). Mechanistically, co-engagement of CD3 with CD5 enhances phosphorylated CBL recruitment leading to increased VAV1 phosphorylation and degradation (PubMed:23376399). 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immunology","url":"https://pubmed.ncbi.nlm.nih.gov/28505514","citation_count":23,"is_preprint":false},{"pmid":"24356888","id":"PMC_24356888","title":"CD5 enhances Th17-cell differentiation by regulating IFN-γ response and RORγt localization.","date":"2014","source":"European journal of immunology","url":"https://pubmed.ncbi.nlm.nih.gov/24356888","citation_count":22,"is_preprint":false},{"pmid":"28130399","id":"PMC_28130399","title":"Constitutive Phosphorylation of STAT3 by the CK2-BLNK-CD5 Complex.","date":"2017","source":"Molecular cancer research : MCR","url":"https://pubmed.ncbi.nlm.nih.gov/28130399","citation_count":22,"is_preprint":false},{"pmid":"18568640","id":"PMC_18568640","title":"Role of CD5+ B-1 cells in EAE pathogenesis.","date":"2008","source":"Autoimmunity","url":"https://pubmed.ncbi.nlm.nih.gov/18568640","citation_count":22,"is_preprint":false},{"pmid":"24124149","id":"PMC_24124149","title":"Clinicopathologic features of CD5-positive nodal marginal zone lymphoma.","date":"2013","source":"American journal of clinical pathology","url":"https://pubmed.ncbi.nlm.nih.gov/24124149","citation_count":22,"is_preprint":false},{"pmid":"8566084","id":"PMC_8566084","title":"Anti-CD5 extends the proliferative response of human CD5+ B cells activated with anti-IgM and interleukin-2.","date":"1996","source":"European journal of immunology","url":"https://pubmed.ncbi.nlm.nih.gov/8566084","citation_count":22,"is_preprint":false},{"pmid":"29845715","id":"PMC_29845715","title":"Reappraisal of nodal Epstein-Barr Virus-negative cytotoxic T-cell lymphoma: Identification of indolent CD5+ diseases.","date":"2018","source":"Cancer science","url":"https://pubmed.ncbi.nlm.nih.gov/29845715","citation_count":22,"is_preprint":false},{"pmid":"9472686","id":"PMC_9472686","title":"Characterization of porcine CD5 and CD5+ B cells.","date":"1998","source":"Clinical and experimental 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B-CLL.","date":"1999","source":"Leukemia","url":"https://pubmed.ncbi.nlm.nih.gov/10516767","citation_count":20,"is_preprint":false},{"pmid":"10191767","id":"PMC_10191767","title":"CD5- B-cell lymphoproliferative disorders presenting in blood and bone marrow. A clinicopathologic study of 40 patients.","date":"1999","source":"American journal of clinical pathology","url":"https://pubmed.ncbi.nlm.nih.gov/10191767","citation_count":20,"is_preprint":false},{"pmid":"8816596","id":"PMC_8816596","title":"Antibody NCL-CD5 fails to detect neoplastic CD5+ cells in paraffin sections.","date":"1996","source":"American journal of clinical pathology","url":"https://pubmed.ncbi.nlm.nih.gov/8816596","citation_count":20,"is_preprint":false},{"pmid":"15187131","id":"PMC_15187131","title":"Transcriptional regulation of human CD5: important role of Ets transcription factors in CD5 expression in T cells.","date":"2004","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/15187131","citation_count":20,"is_preprint":false},{"pmid":"10587706","id":"PMC_10587706","title":"De novo CD5+ Burkitt lymphoma/leukemia.","date":"1999","source":"American journal of clinical pathology","url":"https://pubmed.ncbi.nlm.nih.gov/10587706","citation_count":19,"is_preprint":false},{"pmid":"11298344","id":"PMC_11298344","title":"Residues Y429 and Y463 of the human CD5 are targeted by protein tyrosine kinases.","date":"2001","source":"European journal of immunology","url":"https://pubmed.ncbi.nlm.nih.gov/11298344","citation_count":19,"is_preprint":false},{"pmid":"8850297","id":"PMC_8850297","title":"Distinctive developmental origins and specificities of the CD5+ B-cell subset.","date":"1996","source":"Seminars in immunology","url":"https://pubmed.ncbi.nlm.nih.gov/8850297","citation_count":19,"is_preprint":false},{"pmid":"7507466","id":"PMC_7507466","title":"CD5+ B cells and the immune system.","date":"1993","source":"Immunology letters","url":"https://pubmed.ncbi.nlm.nih.gov/7507466","citation_count":19,"is_preprint":false},{"pmid":"28514677","id":"PMC_28514677","title":"Human Group 1 Innate Lymphocytes Are Negative for Surface CD3ε but Express CD5.","date":"2017","source":"Immunity","url":"https://pubmed.ncbi.nlm.nih.gov/28514677","citation_count":19,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":44627,"output_tokens":5117,"usd":0.105318},"stage2":{"model":"claude-opus-4-6","input_tokens":8637,"output_tokens":3793,"usd":0.207015},"total_usd":0.312333,"stage1_batch_id":"msgbatch_011kfMfE81o6CTcyhVUqumuW","stage2_batch_id":"msgbatch_01DTputS6WNkH745x1nVSoBo","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2001,\n      \"finding\": \"CD5 functions as a negative regulator of TCR signal transduction during thymocyte development, and the cytoplasmic domain of CD5 is required for its inhibitory function, as demonstrated by the failure of a truncated CD5 transgene (lacking the cytoplasmic tail) to rescue the CD5-/- phenotype.\",\n      \"method\": \"Transgenic mouse overexpression and CD5-/- genetic knockout with truncated cytoplasmic domain mutant; thymocyte selection assays with TCR transgenic mice\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO/transgenic rescue with defined phenotypic readout, cytoplasmic domain mutagenesis in vivo\",\n      \"pmids\": [\"11313384\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"CD5 mediates a novel inducible cell-surface ligand (CD5L) expressed on activated B and T lymphocytes upon stimulation with anti-CD3/anti-CD28. CD5L binding to CD5Rg (CD5-Ig fusion protein) is trypsin-resistant, pronase-sensitive, and dependent on N-linked glycosylation of CD5; CD5L is distinct from CD72. In vivo, CD5Rg injection abrogated development of antibody-mediated membranous glomerulonephritis in a murine model.\",\n      \"method\": \"CD5-immunoglobulin fusion protein binding assay, immunoprecipitation, antibody and recombinant protein blocking, in vivo mouse model of glomerulonephritis\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal biochemical methods plus in vivo functional validation in a single study\",\n      \"pmids\": [\"9064341\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CD5 mediates species-specific homophilic interactions through its domain 1 only. Soluble CD5 (purified at neutral pH) binds CD5 on cell surfaces. CD5 monoclonal antibodies that have functional effects block homophilic binding, and inhibiting CD5 domain 1 engagement enhanced antigen-specific T cell responses in vitro, indicating that homophilic CD5-CD5 interaction results in productive inhibitory signaling.\",\n      \"method\": \"Soluble protein binding assay, domain mutagenesis, antibody blocking, in vitro T cell activation assay\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — soluble protein reconstitution, mutagenesis, and functional assay in single study\",\n      \"pmids\": [\"20952682\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CD5 and CD6 physically associate at the membrane of lymphoid T cells, co-localize at the immunological synapse, and co-immunoprecipitate from Brij 96 (but not NP-40) lysates. The CD5-CD6 association is independent of the CD5 cytoplasmic region. FRET analysis, co-capping, and co-modulation experiments confirmed in vivo physical association.\",\n      \"method\": \"Co-immunoprecipitation, FRET, co-capping, co-modulation, immunological synapse imaging in T cell/APC conjugates\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (co-IP, FRET, live-cell imaging) confirming the association\",\n      \"pmids\": [\"12473675\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Thymocyte activation induces association of phosphatidylinositol 3-kinase (PI3K, p85 subunit) and a 120-kDa phosphoprotein (pp120) with CD5. PI3K p85 binds preferentially to tyrosine-phosphorylated Y463 of CD5 via the C-SH2 domain, with bivalent interaction involving the N-SH2 domain at the Y429-Y441 ITAM-like sequence.\",\n      \"method\": \"Co-immunoprecipitation with pervanadate-stimulated thymocytes, recombinant SH2 domain binding to phosphopeptides, PI3K activity assay\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — biochemical co-IP plus in vitro peptide binding assays with defined phosphotyrosine residues\",\n      \"pmids\": [\"9079809\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"CD5 tyrosine phosphorylation occurs at residues Y429 and Y463 in vivo following T cell stimulation with anti-CD3 or pervanadate. This phosphorylation requires Lck kinase activity (absent in Lck-deficient Jurkat cells). In vitro kinase assays with purified Lck and Fyn confirmed these sites as direct substrates.\",\n      \"method\": \"Tyrosine-mutated CD5 transfectants in Jurkat T cells, in vivo stimulation, in vitro kinase assay with recombinant Lck and Fyn, Lck-deficient Jurkat cells\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro kinase assay with mutagenesis, confirmed in vivo in Lck-deficient cells\",\n      \"pmids\": [\"11298344\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"A serine kinase activity constitutively associates with CD5 immunoprecipitates and is rapidly and transiently increased (within 1–3 min) upon CD3/TCR stimulation, correlating with increased CD5 phosphorylation. Direct CD5 cross-linking also increases the associated kinase activity with different kinetics (peaking at 10 min, sustained for 1 h), suggesting distinct regulatory mechanisms.\",\n      \"method\": \"Immunoprecipitation with in vitro kinase assay, phosphoamino acid analysis, anti-phosphotyrosine Western blot, cell stimulation with anti-CD3 or anti-CD5\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — in vitro kinase assay combined with biochemical co-IP, replicated across multiple CD5+ cell types\",\n      \"pmids\": [\"7691949\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"CD5 activates casein kinase 2 (CK2) through a specific CK2-binding domain at the end of its cytoplasmic tail. Mice expressing CD5 with a microdeletion selectively unable to interact with CK2 (CD5ΔCK2BD) showed hypoproliferating T cells with enhanced activation-induced cell death, impaired Th2 and Th17 (but not Th1) differentiation, reduced EAE severity, and failed high-dose tolerance induction — phenocopying CD5 KO mice.\",\n      \"method\": \"Knock-in mouse with selective CD5-CK2 binding domain deletion, T cell proliferation assay, in vivo EAE model, Th differentiation assays\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain-specific knock-in mutation with multiple defined phenotypic readouts in vivo and in vitro\",\n      \"pmids\": [\"22904299\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The CD5-CK2 signaling pathway enhances TCR-induced AKT activation, inhibits GSK3, and activates mTOR, promoting Th17 differentiation. In the absence of CD5-CK2 signaling, increased GSK3 activity renders Th17 cells more sensitive to IFN-γ-mediated inhibition, and S6K activity and nuclear translocation of RORγt are attenuated.\",\n      \"method\": \"CD5ΔCK2BD knock-in mice, AKT/GSK3/mTOR/S6K phosphorylation assays, RORγt nuclear translocation imaging, IFN-γ sensitivity assays, in vitro Th17 differentiation\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic pathway dissection with genetic model and multiple signaling readouts\",\n      \"pmids\": [\"24356888\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In CLL cells, CD5 forms a complex with CK2, BLNK, and STAT3 that mediates constitutive phosphorylation of STAT3 on serine 727. siRNA knockdown of CD5 or BLNK, or treatment with CD5-neutralizing antibodies, significantly reduced serine-phosphorylated STAT3 levels. Fractionation studies showed the CK2/CD5/BLNK/STAT3 complex resides in the cytoplasm while phosphorylated STAT3 is shuttled to the nucleus.\",\n      \"method\": \"Mass spectrometry co-immunoprecipitation, siRNA knockdown, in vitro CK2 kinase assay with recombinant STAT3, confocal microscopy, cell fractionation\",\n      \"journal\": \"Molecular cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — reciprocal co-IP, in vitro kinase assay, siRNA, neutralizing antibody, and localization studies\",\n      \"pmids\": [\"28130399\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"CD5 costimulation drives stable human Th17 development by inducing prolonged STAT3 activation and enhanced RORγt expression, critically dependent on CD5-induced upregulation of IL-23 receptor (IL-23R) expression, providing more durable Th17 commitment than classical CD28 costimulation.\",\n      \"method\": \"In vitro human naive T cell priming with CD5 or CD28 costimulation, intracellular cytokine staining, STAT3 phosphorylation kinetics, IL-23R expression analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean in vitro functional system with defined molecular readouts (STAT3, RORγt, IL-23R); single lab, multiple methods\",\n      \"pmids\": [\"21926348\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CD5-mediated inhibition of TCR signaling during thymocyte development does not require the CD5 extracellular domain (and thus does not involve ligand binding), as demonstrated by rescue of CD5-/- mice with a chimeric molecule bearing the extracellular domain of human IL-2R p55 (Tac) fused to the CD5 transmembrane and cytoplasmic domains.\",\n      \"method\": \"Chimeric CD5 transgene in CD5-/- mice, thymocyte development analysis\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with domain-swap transgene rescue in KO mice\",\n      \"pmids\": [\"12115665\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"A natural soluble form of human CD5 (nsCD5) is released by proteolytic cleavage of the membrane form upon cell activation (phorbol ester or CD3 stimulation). The nsCD5 is a 52 kDa N-glycosylated molecule present in normal human serum and PBMC supernatants, indistinguishable from a recombinant soluble form composed of the three extracellular SRCR domains.\",\n      \"method\": \"Protein purification, biochemical characterization, immunoprecipitation, in vitro stimulation with proteolytic cleavage demonstration\",\n      \"journal\": \"Tissue antigens\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — protein purification and biochemical characterization, single lab\",\n      \"pmids\": [\"10488739\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Constitutive CD5 expression in human T cells is driven by Ets transcription factors binding to two conserved Ets-binding sites at positions -239 and -185 upstream of CD5. A DNase I-hypersensitive site at the 5'-flanking region correlates with CD5 expression; a 282-bp region upstream of ATG displays full promoter activity. An inducible DH site 10 kb upstream functions as an enhancer in T and B CD5+ cells.\",\n      \"method\": \"DNase I hypersensitivity assay, 5'-RACE, luciferase reporter assay, site-directed mutagenesis, EMSA, cotransfection\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — multiple orthogonal methods including mutagenesis and functional reporter assays\",\n      \"pmids\": [\"15187131\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"IL-6 produced by SLE B cells abrogates DNMT1 induction, preventing methylation of the CD5-E1B alternative promoter. Demethylation of CpG islands in the CD5-E1B promoter leads to transcription of the cytoplasm-retained CD5-E1B isoform at the expense of membrane-expressed CD5-E1A, reducing surface CD5 levels and thereby dysregulating BCR signaling in SLE.\",\n      \"method\": \"Bisulfite sequencing, methylation-sensitive endonuclease assay, anti-IL-6R blocking antibody, DNMT1 expression analysis, BCR engagement experiments\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods linking epigenetic mechanism to CD5 isoform regulation\",\n      \"pmids\": [\"19380809\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"RAG1 and RAG2 expression in peripheral B cells outside germinal centers is closely associated with CD5 expression. In vitro activation of CD5-negative naive B cells induces CD5 expression on a subset, and RAG1/RAG2 upregulation occurs only in the cells that become CD5+, suggesting CD5 is associated with receptor revision in activated mature B cells.\",\n      \"method\": \"Isolation of CD5+ human tonsil B cells, RAG mRNA and protein detection, CD5 induction in vitro, RAG expression correlated with CD5 acquisition by flow cytometry\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — correlation between CD5 induction and RAG expression, single lab with multiple readouts but no direct mechanistic intervention\",\n      \"pmids\": [\"15843554\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CD5 expression by CD1c+ dendritic cells (DCs) is required to generate optimally protective CD5hi T helper and CD8+ T cells. Activating CD5 on DCs enhanced T cell priming and improved survival after immune checkpoint blockade therapy. Low IL-6 concentrations promoted de novo differentiation of CD5+ DCs. Deletion of CD5 from T cells dampened tumor elimination in response to immune checkpoint blockade in vivo.\",\n      \"method\": \"Human melanoma patient samples, in vivo mouse tumor models with CD5 deletion from T cells, DC differentiation assays with IL-6 manipulation, immune checkpoint blockade treatment\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic deletion with defined functional phenotype, multiple model systems, in vivo tumor models\",\n      \"pmids\": [\"36795805\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"CD5 inhibits CAR T cell activation; CRISPR-Cas9 knockout of CD5 enhances CAR T cell cytotoxicity, in vivo expansion, and persistence in multiple hematological and solid cancer models without apparent toxicity. CD5 KO drives increased T cell effector function.\",\n      \"method\": \"CRISPR-Cas9 knockout, CAR T cell cytotoxicity assays, in vivo mouse tumor models\",\n      \"journal\": \"Science immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — CRISPR KO with defined functional readouts in multiple models; single study\",\n      \"pmids\": [\"39028827\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1989,\n      \"finding\": \"CD5 on activated human B cells is phosphorylated similarly to CD5 on T cells. IL-4 specifically inhibits CD5 expression on activated B cells at the mRNA level, without affecting other B cell activation antigens (CD25, B5, T9, CD23, CD20), indicating a specific transcriptional or mRNA stability mechanism.\",\n      \"method\": \"Immunoprecipitation of 125I-labeled cells, Northern blot analysis, flow cytometry, IL-4 treatment\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — protein and mRNA level evidence with specificity controls, single lab\",\n      \"pmids\": [\"2472277\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The human CD5 gene spans ~24.5 kb, consists of at least 11 exons conserved in size, number, and structure with the mouse homologue, maps to chromosome 11q12.2 (82 kb downstream from CD6 in head-to-tail orientation), and encodes a cytoplasmic tailless isoform via alternative splicing.\",\n      \"method\": \"Genomic cloning, sequencing, FISH mapping, comparative genomics\",\n      \"journal\": \"Immunogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct genomic characterization establishing structural basis for isoform generation\",\n      \"pmids\": [\"11061284\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CD5 is a scavenger receptor cysteine-rich (SRCR) family glycoprotein that functions primarily as a negative regulator of TCR and BCR signaling through its cytoplasmic tail, which recruits and activates CK2 (regulating AKT/GSK3/mTOR/RORγt axis for Th17 differentiation), associates with PI3K (via phosphorylated Y429/Y441/Y463 targeted by Lck and Fyn), and forms a complex with CK2/BLNK/STAT3 to drive constitutive STAT3 serine phosphorylation in CLL; CD5 also engages in homophilic extracellular interactions through domain 1 that contribute to inhibitory signaling, physically associates with CD6 at the immunological synapse, is released as a soluble form by proteolytic cleavage, and its expression is transcriptionally controlled by Ets factors and epigenetically regulated via IL-6-dependent DNA methylation of an alternative B-cell-specific promoter.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"CD5 is a scavenger receptor cysteine-rich (SRCR) family glycoprotein that functions as a negative regulator of antigen receptor signaling in T and B lymphocytes, calibrating thymocyte selection thresholds and modulating peripheral immune responses. Its inhibitory function requires the cytoplasmic tail, which recruits casein kinase 2 (CK2) via a dedicated C-terminal binding domain and PI3K (p85 subunit) via Lck/Fyn-phosphorylated tyrosines Y429, Y441, and Y463; the CD5–CK2 axis activates AKT, inhibits GSK3, and promotes mTOR-dependent RORγt nuclear translocation to drive Th17 differentiation, while in CLL B cells a cytoplasmic CD5/CK2/BLNK/STAT3 complex constitutively phosphorylates STAT3 on serine 727 [PMID:11313384, PMID:22904299, PMID:24356888, PMID:9079809, PMID:28130399]. Extracellularly, CD5 engages in species-specific homophilic interactions through domain 1 that contribute to inhibitory signaling and physically associates with CD6 at the immunological synapse, and a soluble form is released by proteolytic cleavage upon activation [PMID:20952682, PMID:12473675, PMID:10488739]. CD5 transcription is controlled by Ets factors at a proximal promoter, while an alternative B-cell-specific promoter (CD5-E1B) is epigenetically regulated by IL-6-dependent suppression of DNMT1-mediated DNA methylation, linking CD5 surface levels to dysregulated BCR signaling in SLE [PMID:15187131, PMID:19380809].\",\n  \"teleology\": [\n    {\n      \"year\": 1989,\n      \"claim\": \"Establishing that CD5 is phosphorylated on B cells similarly to T cells and that its expression is specifically downregulated by IL-4 at the mRNA level revealed that CD5 is subject to lineage-specific transcriptional control beyond T cells.\",\n      \"evidence\": \"Immunoprecipitation of radiolabeled cells, Northern blot, flow cytometry with IL-4 treatment on activated human B cells\",\n      \"pmids\": [\"2472277\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab observation; mechanism of IL-4-mediated mRNA reduction (transcription vs. stability) not resolved\", \"IL-4 effect not tested in vivo\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Identification of a constitutively associated serine kinase activity on CD5 that is rapidly augmented upon TCR stimulation established that CD5 is not merely a passive surface marker but an active signaling scaffold.\",\n      \"evidence\": \"Immunoprecipitation with in vitro kinase assay, phosphoamino acid analysis upon anti-CD3 or anti-CD5 stimulation\",\n      \"pmids\": [\"7691949\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the associated serine kinase not determined at this point\", \"Functional consequence of kinase activation on downstream signaling not tested\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Discovery of an inducible CD5 ligand (CD5L) on activated lymphocytes that is distinct from CD72 and whose blockade prevented antibody-mediated glomerulonephritis in vivo demonstrated that CD5 participates in receptor–ligand interactions with in vivo immunoregulatory consequences.\",\n      \"evidence\": \"CD5-Ig fusion protein binding, immunoprecipitation, antibody blocking, murine glomerulonephritis model\",\n      \"pmids\": [\"9064341\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular identity of CD5L not cloned\", \"Whether CD5L mediates inhibitory or activating signals not resolved\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Mapping PI3K p85 recruitment to phosphorylated Y463 (C-SH2) and Y429/Y441 (N-SH2) on CD5 defined the first specific cytoplasmic signaling axis for CD5, revealing how it couples to the PI3K pathway upon thymocyte activation.\",\n      \"evidence\": \"Co-immunoprecipitation from pervanadate-stimulated thymocytes, recombinant SH2 domain binding to synthetic phosphopeptides, PI3K activity assay\",\n      \"pmids\": [\"9079809\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of PI3K recruitment (survival, proliferation) not directly tested\", \"Kinase(s) responsible for CD5 tyrosine phosphorylation not yet identified\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Two contemporaneous studies established that the CD5 cytoplasmic tail is essential for its inhibitory function during thymocyte development and that Lck and Fyn directly phosphorylate Y429 and Y463, linking the upstream kinases to the PI3K-recruiting tyrosines.\",\n      \"evidence\": \"Transgenic rescue of CD5−/− mice with truncated CD5; Lck-deficient Jurkat cells and in vitro kinase assays with recombinant Lck/Fyn\",\n      \"pmids\": [\"11313384\", \"11298344\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Lck vs. Fyn have non-redundant roles at specific phosphosites in vivo not resolved\", \"Additional cytoplasmic tail effectors beyond PI3K not yet identified\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstration that a chimeric CD5 lacking its own extracellular domain still rescues inhibitory signaling in CD5−/− mice, combined with identification of a CD5–CD6 physical complex at the immunological synapse, dissociated extracellular ligand engagement from the core inhibitory mechanism and revealed CD5 functions within a multi-receptor signaling unit.\",\n      \"evidence\": \"Chimeric transgene rescue in CD5−/− mice; co-IP, FRET, co-capping, and synapse imaging for CD5–CD6 association\",\n      \"pmids\": [\"12115665\", \"12473675\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional significance of CD5–CD6 association for signaling output not tested\", \"Whether CD6 modifies CD5 cytoplasmic tail phosphorylation unknown\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identification of Ets-binding sites driving the constitutive CD5 promoter and a distal enhancer active in both T and B1 cells established the transcriptional architecture governing lineage-specific CD5 expression.\",\n      \"evidence\": \"DNase I hypersensitivity, luciferase reporters, site-directed mutagenesis, EMSA in human lymphoid cells\",\n      \"pmids\": [\"15187131\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which specific Ets family member(s) are functionally dominant in vivo not determined\", \"Enhancer mechanism in B cells vs. T cells not separately dissected\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Elucidation of IL-6-driven suppression of DNMT1 leading to demethylation of the CD5-E1B alternative promoter in SLE B cells explained how surface CD5 is downregulated in autoimmunity, linking epigenetic control to BCR signaling dysregulation.\",\n      \"evidence\": \"Bisulfite sequencing, methylation-sensitive restriction, anti-IL-6R blocking antibody, DNMT1 expression analysis in SLE patient B cells\",\n      \"pmids\": [\"19380809\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Causal contribution of CD5-E1B isoform to SLE pathogenesis not tested by genetic intervention\", \"Whether therapeutic IL-6R blockade restores CD5 surface levels in patients not shown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstration that CD5 engages in species-specific homophilic interactions via domain 1 that deliver inhibitory signals reconciled conflicting data on CD5 ligands and established a direct extracellular mechanism for CD5-mediated immune dampening.\",\n      \"evidence\": \"Soluble CD5 binding assays, domain mutagenesis, antibody blocking, antigen-specific T cell activation assays\",\n      \"pmids\": [\"20952682\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Crystal structure of homophilic interaction not available\", \"Relationship between homophilic binding and the previously described CD5L remains unclear\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Creation of CD5ΔCK2BD knock-in mice identified CK2 as the functionally critical serine kinase recruited to the CD5 cytoplasmic tail, resolving the 1993 observation and showing that CD5–CK2 signaling is specifically required for Th2/Th17 differentiation and tolerance induction.\",\n      \"evidence\": \"Knock-in mice with selective CK2-binding domain microdeletion, T cell proliferation, EAE, Th differentiation assays\",\n      \"pmids\": [\"22904299\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"CK2 substrates downstream of CD5 not fully catalogued at this stage\", \"Whether CD5–CK2 axis operates identically in B cells not addressed\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Dissection of the CD5–CK2–AKT–GSK3–mTOR–RORγt signaling cascade provided a complete pathway from receptor to nuclear transcription factor for Th17 commitment, explaining why CD5–CK2 disruption selectively impairs Th17 but not Th1 cells.\",\n      \"evidence\": \"CD5ΔCK2BD knock-in mice, phosphorylation analysis of AKT/GSK3/mTOR/S6K, RORγt nuclear translocation imaging, IFN-γ sensitivity assays\",\n      \"pmids\": [\"24356888\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CK2 directly phosphorylates AKT or acts through an intermediary not resolved\", \"Relevance to human Th17 biology not directly shown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"In CLL, identification of a cytoplasmic CD5/CK2/BLNK/STAT3 complex that constitutively phosphorylates STAT3-S727 revealed a pathological co-option of the CD5–CK2 signaling axis in B-cell malignancy, extending the paradigm from T cells to leukemic B cells.\",\n      \"evidence\": \"Mass spectrometry co-IP, siRNA knockdown of CD5 and BLNK, in vitro CK2 kinase assay on recombinant STAT3, confocal microscopy and cell fractionation in primary CLL cells\",\n      \"pmids\": [\"28130399\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CD5/CK2/BLNK/STAT3 complex exists in normal B1 cells not tested\", \"Therapeutic targeting of this complex in CLL not validated\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Discovery that CD5 expression on CD1c+ dendritic cells is required for optimal T cell priming and that CD5 deletion from T cells impairs checkpoint blockade-mediated tumor elimination expanded CD5 function beyond a purely inhibitory role to one that tunes immune competence in anti-tumor immunity.\",\n      \"evidence\": \"Human melanoma patient analysis, in vivo mouse tumor models with CD5 conditional deletion, DC differentiation assays, immune checkpoint blockade treatment\",\n      \"pmids\": [\"36795805\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which DC-expressed CD5 enhances T cell priming not molecularly defined\", \"Whether CD5 on DCs signals through the same CK2 axis as on T cells unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"CRISPR knockout of CD5 in CAR T cells enhanced effector function, expansion, and persistence across multiple tumor models, validating CD5 as a druggable immune checkpoint in adoptive cell therapy.\",\n      \"evidence\": \"CRISPR-Cas9 KO in human CAR T cells, cytotoxicity assays, in vivo mouse models of hematological and solid tumors\",\n      \"pmids\": [\"39028827\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single study; long-term safety and autoimmune risk of CD5 deletion not assessed\", \"Molecular basis for enhanced persistence (metabolic reprogramming vs. reduced AICD) not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of homophilic CD5–CD5 interaction, the mechanism by which DC-expressed CD5 promotes T cell priming, whether the CD5–CK2 axis operates identically in normal B1 cells and CLL, and whether soluble CD5 has immunomodulatory signaling activity in vivo.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No crystal or cryo-EM structure of CD5 extracellular domains\", \"DC-intrinsic CD5 signaling pathway undefined\", \"In vivo function of soluble CD5 not established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 4, 7, 9]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 7, 8, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 2, 3, 12]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [12]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 7, 8, 10, 16, 17]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [4, 5, 8, 9]}\n    ],\n    \"complexes\": [\n      \"CD5/CK2 signaling complex\",\n      \"CD5/CK2/BLNK/STAT3 complex\",\n      \"CD5/CD6 membrane complex\"\n    ],\n    \"partners\": [\n      \"CD6\",\n      \"CSNK2A1\",\n      \"PIK3R1\",\n      \"LCK\",\n      \"FYN\",\n      \"BLNK\",\n      \"STAT3\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}