{"gene":"CD28","run_date":"2026-06-09T22:57:17","timeline":{"discoveries":[{"year":1993,"finding":"CD28 ligation by B7 induces tyrosine phosphorylation of specific substrates including phospholipase C gamma 1, and triggers both calcium-dependent and calcium-independent signaling cascades. CTLA-4, structurally related to CD28, binds B7 with approximately 20-fold higher affinity than CD28.","method":"Biochemical assays, tyrosine phosphorylation studies, binding affinity measurements","journal":"Annual review of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct biochemical assays measuring phosphorylation substrates and binding affinities, single review summarizing multiple experimental findings","pmids":["8386518"],"is_preprint":false},{"year":1993,"finding":"CD28-deficient mice have impaired T cell responses to lectins, fail to produce IL-2 upon lectin stimulation, show significantly decreased IL-2 receptor alpha expression, and have defective immunoglobulin class switching; however, cytotoxic T cells can still be induced, demonstrating that CD28 is not required for all T cell responses in vivo.","method":"CD28 knockout mouse model, T cell functional assays, immunoglobulin measurement","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout with multiple defined cellular phenotypes, replicated across multiple assays in a single rigorous study","pmids":["7688139"],"is_preprint":false},{"year":1996,"finding":"CD28 costimulation up-regulates Bcl-xL expression to enhance T cell survival during activation; this survival signal is independent of Fas expression and is mediated through ICE protease-dependent apoptosis pathways when absent. CD28 costimulation did not alter Bcl-2 or Fas expression.","method":"Wild-type vs CD28-deficient mice, CTLA4Ig treatment, western blot for Bcl-xL/Bcl-2/Fas, ICE protease inhibitors, apoptosis morphology assessment","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic KO combined with pharmacological blockade (CTLA4Ig), multiple orthogonal survival assays, explicit negative results for Bcl-2/Fas","pmids":["8752911"],"is_preprint":false},{"year":1996,"finding":"CD28 is required for germinal center formation in T-dependent immune responses; CD28-deficient mice fail to support proliferative expansion of B cells in follicles, preventing germinal center formation and somatic hypermutation.","method":"CD28 knockout mouse model, immunohistology of lymphoid tissue, B cell response analysis after antigen challenge","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic KO with specific cellular phenotype (absence of germinal centers and somatic mutations) established by histological and molecular analysis","pmids":["8648099"],"is_preprint":false},{"year":2000,"finding":"Cbl-b suppresses TCR-mediated Vav (guanine nucleotide exchange factor for Rac1/Rho/CDC42) activation downstream of the TCR, and this suppression underlies the CD28 dependence of T cell activation. Cbl-b-null T cells do not require CD28 for IL-2 production; Cbl-b(-/-) mutation fully restores T-cell-dependent antibody responses in CD28-/- mice, without affecting ZAP-70, Lck, Ras/MAPK, PLCgamma-1, or Ca2+ mobilization.","method":"Cbl-b knockout mice crossed with CD28 knockout mice, IL-2 production assays, western blot for signaling intermediates, Vav activation assay","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — double genetic KO epistasis, multiple orthogonal signaling assays, specific mechanistic placement of Cbl-b upstream of Vav in CD28-dependent pathway","pmids":["10646609"],"is_preprint":false},{"year":2001,"finding":"A single point mutation in CD28 at tyrosine 170 (mouse)/173 (human) — the YMNM motif — uncouples SH2-mediated interactions (including PI3K and Grb2 recruitment) from CD28 signaling. This mutant CD28 cannot upregulate Bcl-xL, rendering T cells susceptible to radiation-induced death, but still prevents anergy and promotes proliferation, IL-2 secretion, and B cell help.","method":"Transgenic knockin mice expressing Y170F CD28 mutant, Bcl-xL western blot, T cell survival assays, proliferation/IL-2 measurements","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — site-directed mutagenesis (knockin), multiple functional readouts separating two distinct CD28 signaling outputs (survival vs. proliferation)","pmids":["11276203"],"is_preprint":false},{"year":2002,"finding":"CD28 costimulation, acting through PI3K and Akt, is required for T cells to increase glycolytic rate in response to activation. CD28 induces glucose uptake and glycolysis in excess of what is needed to maintain ATP/ADP ratios or macromolecular synthesis, suggesting it anticipates future energetic demands.","method":"CD28-deficient mice, PI3K/Akt inhibitors, glucose uptake assays, glycolysis measurements, ATP/ADP measurements","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic KO combined with pharmacological inhibition of PI3K/Akt, multiple metabolic assays providing mechanistic pathway placement","pmids":["12121659"],"is_preprint":false},{"year":2001,"finding":"Mouse dendritic cells are induced by soluble CD28 to express IL-6 and IFN-gamma via B7-1 (CD80) and B7-2 (CD86) and p38 MAPK signaling — demonstrating bidirectional (reverse) signaling from CD28 through B7 molecules into dendritic cells.","method":"Soluble CD28 stimulation of mouse DCs, cytokine measurement, p38 MAPK inhibitor, B7-1/B7-2 blocking experiments, in vivo tumor challenge","journal":"Nature immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple in vitro and in vivo assays in a single lab study, but reverse signaling concept from a single lab","pmids":["15467723"],"is_preprint":false},{"year":2005,"finding":"Crystal structure of soluble CD28 in complex with a Fab fragment reveals that CD28 is a monovalent homodimer whose structural features redefine evolutionary relationships with CTLA-4, antigen receptors, and adhesion molecules, and account for distinct ligand-binding and stoichiometric properties compared to CTLA-4. Cryo-EM comparisons of CD28 with mitogenic vs non-mitogenic antibody complexes place new constraints on models of receptor triggering.","method":"X-ray crystallography of soluble CD28-Fab complex, cryo-electron microscopy","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure determination with cryo-EM validation, directly establishing structural basis for CD28/CTLA-4 functional differences","pmids":["15696168"],"is_preprint":false},{"year":1998,"finding":"CD28 affects the earliest TCR signaling events: inhibiting CD28/B7 interaction results in impaired TCR-induced tyrosine phosphorylation of the zeta chain and ZAP-70, and strongly diminished downstream signaling including Ca2+/calcineurin, ERK/MAPK, and JNK pathways, under physiological conditions of antigen presentation.","method":"Jurkat cells and primary TCR-transgenic T cells stimulated with antigen/superantigen on B7-expressing APCs, anti-CD28 blocking, western blot for phosphorylation of TCR zeta, ZAP-70, downstream signaling","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — physiological conditions with multiple signaling readouts, single lab with two cell systems (Jurkat and primary T cells)","pmids":["9692882"],"is_preprint":false},{"year":2000,"finding":"CD28 can activate PI3K and cooperate with adapters Vav and SLP-76 to independently induce IL-2 and IL-4 transcription in the absence of TCR ligation, providing a molecular basis for in trans costimulation.","method":"Biochemical signaling assays, IL-2/IL-4 reporter assays, CD28 mutants","journal":"Immunological reviews","confidence":"Low","confidence_rationale":"Tier 3 / Weak — signaling assays described in a review context without full primary experimental detail visible in the abstract","pmids":["12670393"],"is_preprint":false},{"year":2000,"finding":"The YMNM motif of CD28 cytoplasmic domain, when mutated at position N191 (retaining PI3K binding), abolishes IL-2 promoter activation, whereas Y189F (disrupting PI3K binding) only reduces it, suggesting that PI3K associated with the YMNM motif acts as a negative mediator of CD28-mediated IL-2 gene transcription.","method":"CD28 point and deletion mutants expressed in Jurkat cells, IL-2 promoter reporter assays, PI3K pharmacological inhibitors, constitutively active PI3K overexpression","journal":"Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — site-directed mutagenesis with reporter assays and pharmacological/genetic PI3K manipulation, single lab","pmids":["11113113"],"is_preprint":false},{"year":2004,"finding":"CD28 accumulates at the immunological synapse before c-SMAC formation (in the immature synapse), co-localizing with TCR, and exerts effects on initiation and stabilization of the synapse within seconds of TCR-mediated calcium rise, regulating downstream T cell proliferation.","method":"Live imaging of T cell–APC interactions, CD28-deficient and reconstituted murine cells, single-cell calcium recording, synapse morphology analysis","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — live imaging with genetic reconstitution, single-cell functional readouts, single lab","pmids":["15128767"],"is_preprint":false},{"year":2009,"finding":"CD28 forms TCR-CD28 microclusters in cooperation with TCRs at the immunological synapse, associates with PKCtheta, and after mature IS formation, CD28 microclusters accumulate at a specific subregion of the IS where they continuously assemble with kinases (not TCRs) to generate sustained T cell signaling.","method":"Dynamic imaging of immunological synapse, fluorescent protein-tagged molecules, microcluster analysis","journal":"Immunological reviews","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — dynamic imaging with defined molecular co-localization at IS subregions, described in a review of prior published imaging experiments","pmids":["19426213"],"is_preprint":false},{"year":2011,"finding":"B7-H2 (ICOSL) acts as a ligand for CD28 and CTLA-4 in humans (but not mice) through distinctive binding domains from B7-1/B7-2. B7-H2-CD28 interaction costimulates human T cell primary and memory responses, inducing Bcl-xL, downregulating p27(kip1), and triggering ERK and AKT kinase-dependent signaling, similar to B7-1/B7-2.","method":"Receptor array assay, co-stimulation functional assays with human T cells, western blot for Bcl-xL, p27(kip1), phospho-ERK, phospho-AKT","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 2 / Strong — receptor array identification plus multiple functional and signaling readouts in primary human T cells, single lab with multiple orthogonal methods","pmids":["21530327"],"is_preprint":false},{"year":2013,"finding":"CD28 has an obligate cell-intrinsic function in Foxp3+ Tregs postmaturation: Treg-specific CD28 knockout mice develop severe autoimmunity despite normal Treg numbers, with dampened CTLA-4, PD-1, and CCR6 expression and pronounced proliferative/survival disadvantage in Tregs under competitive conditions.","method":"Treg-specific conditional CD28 knockout mice (Cre-lox), autoimmunity phenotyping, flow cytometry for CTLA-4/PD-1/CCR6, competitive reconstitution experiments","journal":"Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional cell-type-specific KO with complementation experiment, multiple molecular and cellular phenotypes","pmids":["23281398"],"is_preprint":false},{"year":2014,"finding":"CD28 and CD3 provide complementary functions in T cell traction force generation: force generation is associated with CD3/TCR complex engagement, while CD28 engagement increases traction forces associated with CD3 through the PI3K signaling pathway rather than through direct cell-surface coupling. Force generation is concentrated at the cell periphery and associated with molecular complexes containing phosphorylated Pyk2.","method":"Traction force microscopy using micrometer-scale elastomer pillar arrays, primary human and mouse T cells, PI3K inhibitors, Pyk2 immunofluorescence, TCR-specific T cells on peptide-loaded MHC pillars","journal":"PNAS","confidence":"High","confidence_rationale":"Tier 2 / Strong — traction force microscopy with pharmacological dissection of PI3K pathway, primary cells from multiple species with defined genetic TCR","pmids":["24469820"],"is_preprint":false},{"year":2013,"finding":"Functional convergence of CD28 costimulation and TCR signaling requires spatial co-proximity within the immunological synapse: micrometer-scale separation of CD3 and CD28 ligands curtails T cell activation. Active Lck is concentrated to regions of CD3 engagement with low diffusion rates, suggesting costimulation is controlled by a reaction-diffusion balance of active Lck transport to CD28 vs. its deactivation. Disruption of actin cytoskeleton increases Lck mobility and rescues activation by spatially separated CD3/CD28.","method":"Micropatterned surfaces with spatially controlled ligand presentation, primary human and mouse CD4+ T cells, Lck phosphorylation imaging, actin cytoskeleton disruption, FRAP-like mobility measurements","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — engineered micropatterned surfaces with pharmacological and genetic manipulations, multiple orthogonal readouts, two species","pmids":["24379441"],"is_preprint":false},{"year":2017,"finding":"CD28 transmembrane signaling is regulated by two counteractive factors: acidic phospholipids sequester CD28 signaling motifs within the membrane to limit basal signaling, while TCR-induced local Ca2+ increase directly disrupts the CD28-lipid interaction, releasing signaling motifs. TCR, Ca2+, and CD28 form a dual-positive-feedback circuit that amplifies T cell signaling and antigen sensitivity.","method":"NMR spectroscopy of CD28 transmembrane domain with lipids, Ca2+ measurement in T cells, mutagenesis of CD28 charged residues, functional T cell signaling assays","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structural analysis combined with functional mutagenesis and live-cell Ca2+ measurements establishing the mechanistic basis of transmembrane regulation","pmids":["29058713"],"is_preprint":false},{"year":2017,"finding":"CD28 transiently promotes expression of carnitine palmitoyltransferase 1a (Cpt1a) before the first cell division upon T cell activation, facilitating mitochondrial fatty acid oxidation (FAO), mitochondrial elongation, and enhanced spare respiratory capacity (SRC). Without CD28, microRNA-33 (a target of TXNIP) attenuates Cpt1a expression, resulting in metabolically compromised cells. This early CD28-dependent mitochondrial priming enables T cells to remodel cristae, develop SRC, and rapidly produce cytokines upon restimulation — features of protective memory T cells.","method":"CD28-deficient T cells, miR-33 manipulation, Cpt1a overexpression/knockdown, mitochondrial morphology imaging (electron microscopy), metabolic flux assays (oxygen consumption, FAO), in vivo memory T cell functional assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal methods (genetic KO, miRNA manipulation, metabolic assays, structural mitochondrial imaging, in vivo memory responses) in a single rigorous study","pmids":["28919076"],"is_preprint":false},{"year":2014,"finding":"CD28 pro-survival signaling in multiple myeloma cells is dependent on downstream activation of PI3K/Akt, inactivation of transcription factor FoxO3a, and decreased expression of pro-apoptotic Bim. Blocking CD28-CD80/CD86 interaction abrogates DC-mediated protection of MM cells against chemotherapy-induced death in vitro and in vivo.","method":"PI3K/Akt inhibitors, FoxO3a/Bim western blots, CD28 crosslinking with agonistic antibodies or DCs, in vivo Vk*MYC murine myeloma model with CD28 blockade, tumor burden measurement","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple pharmacological inhibitors targeting defined pathway components, in vitro and in vivo validation, mechanistic pathway placement","pmids":["24782505"],"is_preprint":false},{"year":2016,"finding":"The CTLA4-CD28 gene fusion product (extracellular CTLA4 domain fused to CD28 cytoplasmic domain) transforms inhibitory signals into stimulatory signals for T-cell activation, enhancing proliferation and AKT and ERK phosphorylation when ectopically expressed in Jurkat and H9 cells.","method":"RNA sequencing to identify fusion, RT-PCR/Sanger sequencing in 115 T-cell lymphoma samples, ectopic expression of fusion in Jurkat/H9 cells, proliferation assays, western blot for phospho-AKT and phospho-ERK","journal":"Haematologica","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ectopic expression with multiple signaling readouts, but in cell lines rather than primary cells, single lab","pmids":["26819049"],"is_preprint":false},{"year":2020,"finding":"CD28 transduces pro-survival signaling specifically in long-lived plasma cells (LLPCs) through differential SLP76 expression. CD28 signaling in LLPCs increases glucose uptake, mitochondrial mass/respiration, and reactive oxygen species (ROS) production. CD28-mediated mitochondrial respiration, NF-κB activation, and survival are ROS-dependent. IRF4 (a NF-κB target) is upregulated by CD28 in LLPCs and is required for CD28-mediated metabolic and survival program.","method":"CD28 activation in LLPCs vs SLPCs, SLP76 expression analysis, glucose uptake assays, mitochondrial mass/respiration measurements, ROS detection, NF-κB reporter/western blot, IRF4 knockdown, survival assays","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal metabolic and molecular assays with genetic knockdown, mechanistic pathway from CD28 through SLP76→ROS→NF-κB→IRF4 established","pmids":["32579940"],"is_preprint":false},{"year":2021,"finding":"CARs containing a CD28 transmembrane domain (TMD) form heterodimers with endogenous CD28 in human T cells; this dimerization is dependent on polar amino acids in the CD28-TMD and is distinct from CD8-TMD-containing CARs which do not form such heterodimers. CD28-TMD heterodimers respond to anti-CD28 stimulation for CAR-dependent proliferation but do not respond to CD80/CD86 stimulation and reduce CD28 cell-surface expression.","method":"Co-immunoprecipitation, anti-CD28 stimulation-dependent proliferation assays, surface CD28 expression by flow cytometry, mutagenesis of polar residues in CD28-TMD","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP with functional validation and mutagenesis, single lab with multiple assays","pmids":["33833759"],"is_preprint":false},{"year":2023,"finding":"CD8+ T cells display B7 ligands (CD80/CD86) on their surface that interact with CD28 in cis at membrane invaginations of the immunological synapse. This cis-B7:CD28 interaction is driven by PI3K and sorting-nexin-9 (SNX9)-mediated membrane remodeling. cis-B7:CD28 signaling occurs through PKCθ and promotes CD8+ T cell survival, migration, and cytokine production. Loss of T cell-intrinsic cis-B7:CD28 interactions decreases intratumoral T cells and accelerates tumor growth in mouse models.","method":"Live imaging of IS membrane invaginations, PI3K/SNX9 genetic/pharmacological manipulation, cis-B7 knockout on T cells, PKCθ signaling readouts, in vivo mouse tumor models","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (live imaging, genetic KO of pathway components, in vivo tumor models), mechanistic pathway (PI3K→SNX9→membrane remodeling→cis-B7:CD28→PKCθ) established","pmids":["37160118"],"is_preprint":false},{"year":2016,"finding":"Bacterial superantigens hyperinduce inflammatory cytokines by directly engaging both CD28 and its coligand B7-2 (CD86) at their respective homodimer interfaces — sites remote from where B7-2 and CD28 normally interact — thereby potently enhancing the avidity of the B7-2:CD28 interaction and forcing excessive costimulatory signaling.","method":"Binding studies with CD28 and B7-2 homodimer interface peptides, cytokine production assays, murine lethal superantigen challenge with mimotope peptides","journal":"PNAS","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — binding domain mapping with functional cytokine assays and in vivo protection data, single lab","pmids":["27708164"],"is_preprint":false},{"year":2024,"finding":"In CAR-NK cells, CD28 costimulatory domain linked to CD3ζ creates a signaling platform that recruits LCK and ZAP70, initiating a kinase cascade that enhances CAR-NK cell antitumor efficacy and long-term cytotoxicity; this is a non-canonical role since CD28 is not inherently present in mature NK cells.","method":"Co-immunoprecipitation of LCK/ZAP70 with CD28-CD3ζ CAR construct, in vitro cytotoxicity assays, multiple xenograft tumor models in vivo","journal":"Cancer discovery","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP establishing kinase recruitment plus in vitro and in vivo functional validation, single lab","pmids":["38900051"],"is_preprint":false},{"year":2001,"finding":"TNF-alpha directly inhibits CD28 gene transcription by reducing DNA-protein complex formation at two sequence motifs (site alpha and beta) of the CD28 minimal promoter, including the transcriptional initiator sequences, resulting in reversible downregulation of CD28 surface expression.","method":"Reporter gene bioassays with CD28 minimal promoter, in vitro transcription assays, EMSA (DNA-protein binding), flow cytometry for CD28 surface expression","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro transcription reconstitution plus promoter reporter assays and EMSA, single lab with multiple complementary methods","pmids":["11544310"],"is_preprint":false},{"year":2009,"finding":"CD28 co-stimulation suppresses Th17 differentiation through an IL-2 and IFN-gamma-dependent mechanism; blockade of CD28/B7 interactions facilitates both murine and human Th17 cell differentiation, and this suppression is not mediated through increased Foxp3+ T cells.","method":"Anti-CD28 antibody, CTLA4-Ig blockade, cytokine measurement (IL-17, IL-2, IFN-gamma, IL-10), flow cytometry for Foxp3+ cells, DC co-culture experiments","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple cytokine blockade experiments with human and murine cells, single lab, mechanistic pathway identified through cytokine dependency","pmids":["19333372"],"is_preprint":false},{"year":2013,"finding":"CD28-activated Tec family kinase ITK specifically licenses autoreactive T cells to traffic from lymphoid organs into peripheral tissues to mount destructive immune responses; loss of ITK in CTLA-4-deficient mice does not block T cell activation/proliferation but prevents tissue infiltration and fatal autoimmunity.","method":"Itk(-/-); Ctla4(-/-) double knockout mice, T cell localization by tissue histology, T cell activation/proliferation assays, ITK pharmacological inhibitors, diabetogenic T cell transfer model","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — double genetic KO epistasis with pharmacological inhibitor validation, specific mechanistic dissociation of activation from trafficking, in vivo disease model","pmids":["24270545"],"is_preprint":false},{"year":2022,"finding":"CARMIL2 is required for canonical NF-κB activation (but not AP-1 or NFAT) in T cells stimulated via CD28; CARMIL2-deficient patients phenocopy CD28-deficient patients in showing defective CD4+ and CD8+ memory T cells and TREG counts, but additionally show NK cell and memory B cell deficiencies, indicating CARMIL2 governs immunological pathways beyond CD28.","method":"Patient-derived cells from 89 CARMIL2-deficient patients, NF-κB/AP-1/NFAT reporter assays upon CD28 stimulation, flow cytometry for lymphocyte subsets, somatic reversion analysis","journal":"Journal of experimental medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — large patient cohort with functional signaling assays, pathway placement established by comparison of CARMIL2 and CD28 deficiencies","pmids":["36515678"],"is_preprint":false}],"current_model":"CD28 is a homodimeric costimulatory receptor on T cells that, upon engagement by B7-1 (CD80), B7-2 (CD86), or B7-H2 (in humans) on APCs, recruits PI3K and Akt via its YMNM cytoplasmic motif and cooperates with Vav/SLP-76 to amplify TCR signaling, drive IL-2 production, upregulate Bcl-xL for survival, promote glycolytic and mitochondrial metabolic reprogramming (including Cpt1a-driven FAO essential for memory formation), regulate germinal center formation and Treg maintenance through cell-intrinsic signals, and can also signal in cis when T cell-expressed B7 ligands engage CD28 at immunological synapse invaginations via a PI3K/SNX9/PKCθ pathway; its transmembrane domain anchors lipid-sequestered signaling motifs that are released by TCR-induced Ca2+ influx, forming a dual positive-feedback amplification circuit with the TCR."},"narrative":{"mechanistic_narrative":"CD28 is the principal costimulatory receptor of T cells, a homodimer that lowers the threshold for activation by acting at the earliest steps of TCR signaling—its engagement is required for normal tyrosine phosphorylation of the TCR zeta chain and ZAP-70 and for downstream Ca2+/calcineurin, ERK/MAPK and JNK signaling [PMID:9692882], and CD28-deficient mice show defective IL-2 production, IL-2 receptor alpha expression, and immunoglobulin class switching while retaining cytotoxic responses [PMID:7688139]. The receptor accumulates at the immature immunological synapse together with the TCR, where it forms TCR-CD28 microclusters that associate with PKCθ and sustain signaling at a discrete synapse subregion [PMID:15128767, PMID:19426213]; productive costimulation requires spatial co-proximity of CD3 and CD28 ligands, governed by actin-constrained transport of active Lck [PMID:24379441]. Costimulatory output is gated at the membrane itself: acidic phospholipids sequester the CD28 cytoplasmic signaling motifs, and TCR-induced Ca2+ disrupts this lipid interaction to release them, creating a dual positive-feedback amplification circuit with the TCR [PMID:29058713]. Signal transduction proceeds through the YMNM motif at tyrosine 170/173, which recruits SH2-domain partners including PI3K and Grb2; mutation of this tyrosine uncouples Bcl-xL-dependent survival from the proliferative and IL-2-promoting outputs of CD28 [PMID:11276203], and CD28 cooperates with Vav and SLP-76, with Cbl-b suppression of Vav activation explaining the CD28-dependence of T cell activation [PMID:10646609, PMID:12670393]. Through PI3K/Akt, CD28 drives metabolic reprogramming—increasing glycolysis and glucose uptake beyond immediate energetic need [PMID:12121659] and transiently inducing Cpt1a to prime mitochondrial fatty acid oxidation and spare respiratory capacity essential for protective memory [PMID:28919076]. At the organismal level CD28 is required for germinal center formation and somatic hypermutation [PMID:8648099], exerts a cell-intrinsic function maintaining Foxp3+ Treg fitness and identity [PMID:23281398], and suppresses Th17 differentiation via IL-2/IFN-γ [PMID:19333372]. Human CD28 is engaged not only by B7-1/B7-2 but also by B7-H2 (ICOSL), which costimulates primary and memory responses through Bcl-xL induction and ERK/AKT signaling [PMID:21530327]. Crystallographic analysis defines CD28 as a monovalent homodimer distinct in ligand-binding and stoichiometry from the higher-affinity inhibitory relative CTLA-4 [PMID:8386518, PMID:15696168].","teleology":[{"year":1993,"claim":"Established CD28 as a costimulatory receptor whose B7 engagement triggers tyrosine phosphorylation cascades, and positioned it against the structurally related, higher-affinity CTLA-4.","evidence":"Biochemical phosphorylation and binding-affinity assays summarized from primary studies","pmids":["8386518"],"confidence":"Medium","gaps":["Did not map the specific cytoplasmic motifs responsible for partner recruitment","Affinity comparison did not resolve functional consequences of CTLA-4 vs CD28 binding"]},{"year":1993,"claim":"Genetic loss-of-function showed CD28 is required for IL-2 production, IL-2Rα expression and class switching but dispensable for cytotoxic T cell induction, defining the scope and limits of its in vivo role.","evidence":"CD28 knockout mice with T cell functional and immunoglobulin assays","pmids":["7688139"],"confidence":"High","gaps":["Did not identify the molecular signals downstream of CD28 underlying these phenotypes","Could not separate proliferation from survival contributions"]},{"year":1996,"claim":"Identified Bcl-xL upregulation as a discrete CD28 survival output independent of Bcl-2 and Fas, explaining how costimulation protects activated T cells from apoptosis.","evidence":"CD28-KO mice plus CTLA4Ig blockade with Bcl-xL/Bcl-2/Fas western blots and ICE-protease readouts","pmids":["8752911"],"confidence":"High","gaps":["Did not link Bcl-xL induction to a specific cytoplasmic signaling motif","Mechanism connecting CD28 to Bcl-xL transcription unresolved at this stage"]},{"year":1996,"claim":"Demonstrated CD28 is required for germinal center formation and somatic hypermutation, defining its role in T-dependent humoral responses.","evidence":"CD28-KO mice with lymphoid histology and B cell response analysis","pmids":["8648099"],"confidence":"High","gaps":["Did not distinguish T-cell-intrinsic from indirect contributions to GC formation","Downstream signaling requirements not addressed"]},{"year":1998,"claim":"Showed CD28 acts on the earliest TCR signaling events under physiological antigen presentation, placing costimulation upstream rather than parallel to proximal TCR signaling.","evidence":"Antigen/superantigen stimulation on B7+ APCs with anti-CD28 blockade and phospho-blots in Jurkat and primary T cells","pmids":["9692882"],"confidence":"Medium","gaps":["Single-lab two-cell-system study","Did not define the biochemical bridge from CD28 to TCR zeta/ZAP-70 phosphorylation"]},{"year":2000,"claim":"Epistasis placed Cbl-b suppression of Vav activation as the basis for CD28-dependence, showing CD28 functions to relieve a negative checkpoint on TCR signaling.","evidence":"Cbl-b x CD28 double-KO mice with IL-2, Vav activation and signaling-intermediate assays","pmids":["10646609"],"confidence":"High","gaps":["Did not establish direct CD28 engagement of Vav/Cbl-b","Left open how CD28 itself amplifies Vav signaling"]},{"year":2000,"claim":"Linked CD28 to PI3K, Vav and SLP-76 cooperation for IL-2/IL-4 transcription, providing a molecular basis for in-trans costimulation, and probed whether YMNM-associated PI3K is positive or negative for IL-2.","evidence":"CD28 point/deletion mutants with IL-2 promoter reporters and PI3K manipulation in Jurkat cells (with review summary)","pmids":["12670393","11113113"],"confidence":"Medium","gaps":["Reporter-based and partly review-derived; in vivo relevance of negative PI3K role unresolved","Conflicting positive vs negative roles for YMNM-associated PI3K not reconciled"]},{"year":2001,"claim":"Knockin mutation of the YMNM tyrosine separated the SH2-dependent survival arm (Bcl-xL induction) from the proliferative/IL-2/anergy-prevention arm, demonstrating CD28 has genetically separable signaling outputs.","evidence":"Y170F CD28 knockin mice with survival, proliferation and IL-2 readouts","pmids":["11276203"],"confidence":"High","gaps":["Did not identify which SH2 partner mediates the survival arm in vivo","Mechanism of the tyrosine-independent proliferative arm unresolved"]},{"year":2001,"claim":"Identified transcriptional control of CD28 itself, showing TNF-α represses CD28 promoter activity to reversibly downregulate surface CD28.","evidence":"CD28 minimal-promoter reporters, in vitro transcription, EMSA and surface flow cytometry","pmids":["11544310"],"confidence":"Medium","gaps":["TNF-α-responsive transcription factors at sites α/β not identified","Physiological contexts of CD28 downregulation not defined"]},{"year":2002,"claim":"Defined CD28-driven metabolic reprogramming, showing PI3K/Akt-dependent glycolytic upregulation that exceeds immediate energetic demand.","evidence":"CD28-KO mice, PI3K/Akt inhibitors and glucose/glycolysis/ATP-ADP assays","pmids":["12121659"],"confidence":"High","gaps":["Did not address mitochondrial or fatty-acid metabolism","Link between metabolic reprogramming and effector/memory fate not established"]},{"year":2005,"claim":"Crystal and cryo-EM structures defined CD28 as a monovalent homodimer with binding and stoichiometric properties distinct from CTLA-4, constraining models of receptor triggering.","evidence":"X-ray crystallography of CD28-Fab and cryo-EM of antibody complexes","pmids":["15696168"],"confidence":"High","gaps":["Structure of ligand-bound CD28 signaling complex not resolved here","Mechanistic basis of mitogenic vs non-mitogenic triggering inferred, not directly shown"]},{"year":2004,"claim":"Live imaging placed CD28 at the immature immunological synapse co-localized with the TCR, acting within seconds of the Ca2+ rise to stabilize the synapse.","evidence":"Live T cell-APC imaging with CD28-KO/reconstituted cells and single-cell Ca2+ recording","pmids":["15128767"],"confidence":"Medium","gaps":["Single-lab imaging study","Molecular machinery anchoring CD28 to the immature synapse not defined"]},{"year":2009,"claim":"Resolved CD28 spatiotemporal signaling into TCR-CD28 microclusters that associate with PKCθ and sustain kinase-driven signaling at a synapse subregion after mature synapse formation; separately showed CD28 suppresses Th17 differentiation via IL-2/IFN-γ.","evidence":"Dynamic synapse imaging of tagged molecules; cytokine-blockade differentiation assays in human and mouse cells","pmids":["19426213","19333372"],"confidence":"Medium","gaps":["Microcluster work described in review of prior imaging","Th17 suppression mechanism downstream of IL-2/IFN-γ not molecularly defined"]},{"year":2011,"claim":"Established B7-H2 (ICOSL) as a human-specific CD28 ligand that costimulates primary and memory T cells, broadening the CD28 ligand repertoire.","evidence":"Receptor array plus human T cell costimulation and signaling readouts (Bcl-xL, p27, phospho-ERK/AKT)","pmids":["21530327"],"confidence":"High","gaps":["Relative physiological contribution of B7-H2 vs B7-1/B7-2 not quantified","Mouse-human divergence mechanism not explored"]},{"year":2013,"claim":"Conditional knockouts and epistasis defined cell-intrinsic CD28 functions: maintenance of Foxp3+ Treg fitness and identity, and ITK-dependent licensing of autoreactive T cell tissue trafficking.","evidence":"Treg-specific CD28 cKO with competitive reconstitution; Itk/Ctla4 double-KO with tissue histology and diabetogenic transfer","pmids":["23281398","24270545"],"confidence":"High","gaps":["Treg CD28 signaling effectors maintaining CTLA-4/PD-1/CCR6 not pinpointed","Direct link from CD28 to ITK activation not biochemically resolved"]},{"year":2013,"claim":"Micropatterning showed costimulation requires CD3-CD28 spatial co-proximity, controlled by actin-constrained transport of active Lck, defining a physical basis for functional convergence.","evidence":"Spatially controlled ligand micropatterns, Lck phospho-imaging, actin disruption and mobility measurements in primary T cells","pmids":["24379441"],"confidence":"High","gaps":["Reaction-diffusion model not validated in physiological APC contacts","Identity of CD28-proximal Lck substrates not defined"]},{"year":2014,"claim":"Showed CD28 augments CD3-associated traction forces through PI3K and links to phospho-Pyk2 complexes; separately identified CD28/PI3K/Akt/FoxO3a/Bim pro-survival signaling co-opted by multiple myeloma cells.","evidence":"Traction force microscopy with PI3K inhibitors; PI3K/Akt inhibitors, FoxO3a/Bim blots and in vivo Vk*MYC myeloma model with CD28 blockade","pmids":["24469820","24782505"],"confidence":"High","gaps":["Mechanical signaling role in physiological T cell activation not fully defined","Therapeutic implications of myeloma CD28 dependence beyond mouse model unestablished"]},{"year":2016,"claim":"Defined disease-relevant CD28 mechanisms: superantigen direct engagement of CD28/B7-2 homodimer interfaces to force excessive costimulation, and a CTLA4-CD28 fusion that converts inhibitory signals to stimulatory ones.","evidence":"Homodimer-interface peptide binding with cytokine and lethal challenge assays; RNA-seq fusion identification and ectopic expression with phospho-AKT/ERK readouts","pmids":["27708164","26819049"],"confidence":"Medium","gaps":["Superantigen mechanism mapped via peptides, not full complex structures","Fusion oncogenicity shown in cell lines, not primary lymphoma cells"]},{"year":2017,"claim":"Two structural/mechanistic studies defined membrane-level gating of CD28: phospholipid sequestration of signaling motifs released by TCR-induced Ca2+ as a dual positive-feedback circuit, and transient Cpt1a-driven mitochondrial priming enabling protective memory.","evidence":"NMR of CD28 TMD with lipids plus Ca2+ and mutagenesis assays; CD28-KO with miR-33 manipulation, Cpt1a perturbation, EM, metabolic flux and in vivo memory assays","pmids":["29058713","28919076"],"confidence":"High","gaps":["Quantitative thresholds of the Ca2+/lipid switch in vivo not defined","Upstream regulators coupling CD28 to Cpt1a/miR-33 not fully mapped"]},{"year":2020,"claim":"Extended CD28 pro-survival signaling beyond T cells to long-lived plasma cells, defining an SLP76-dependent ROS/NF-κB/IRF4 metabolic survival program.","evidence":"CD28 activation in LLPCs vs SLPCs with metabolic, ROS, NF-κB and IRF4-knockdown survival assays","pmids":["32579940"],"confidence":"High","gaps":["How differential SLP76 expression rewires CD28 output not mechanistically resolved","Relationship to T cell CD28 metabolic programs not directly compared"]},{"year":2021,"claim":"Showed CD28-transmembrane-domain CARs heterodimerize with endogenous CD28 via polar TMD residues, altering CAR responsiveness and reducing CD28 surface expression.","evidence":"Co-IP, TMD mutagenesis, anti-CD28-dependent proliferation and surface CD28 flow cytometry","pmids":["33833759"],"confidence":"Medium","gaps":["Single-lab co-IP study","Functional consequences of endogenous CD28 sequestration in vivo not assessed"]},{"year":2023,"claim":"Identified a cis-B7:CD28 mechanism in which CD8+ T cells display B7 ligands engaging CD28 at synapse invaginations via PI3K/SNX9 membrane remodeling and PKCθ, promoting survival, migration and antitumor function.","evidence":"Live IS-invagination imaging, PI3K/SNX9 manipulation, cis-B7 T cell KO, PKCθ readouts and in vivo tumor models","pmids":["37160118"],"confidence":"High","gaps":["Relative contribution of cis vs trans B7:CD28 signaling not quantified","Generality across T cell subsets beyond CD8+ not established"]},{"year":2024,"claim":"Demonstrated the CD28 costimulatory domain functions non-canonically in CAR-NK cells by recruiting LCK and ZAP70 to a CD28-CD3ζ platform that enhances antitumor cytotoxicity.","evidence":"Co-IP of LCK/ZAP70 with CD28-CD3ζ construct, in vitro cytotoxicity and xenograft models","pmids":["38900051"],"confidence":"Medium","gaps":["Single-lab co-IP; direct enzymatic activation kinetics not measured","Whether endogenous NK signaling machinery is co-opted vs ectopically supplied unresolved"]},{"year":null,"claim":"How CD28's separable signaling arms (survival vs proliferation vs metabolic priming) are wired to distinct downstream effectors, and how trans, cis, and reverse signaling modes are integrated at the synapse, remain incompletely defined.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified map linking individual cytoplasmic motifs to each functional output","Stoichiometry and structure of the ligand-engaged CD28 signaling complex unresolved","Quantitative balance of cis vs trans vs reverse signaling in physiological responses unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,1,9,14]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,6,18]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[18]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[8,12,13,18,27]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[1,3,15,28]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[5,6,9,18]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[6,19,22]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[2,5,20]}],"complexes":["TCR-CD28 microcluster","immunological synapse"],"partners":["CD80","CD86","ICOSLG","PIK3","VAV1","LCP2","PRKCQ","CBLB"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P10747","full_name":"T-cell-specific surface glycoprotein CD28","aliases":["TP44"],"length_aa":220,"mass_kda":25.1,"function":"Receptor that plays a role in T-cell activation, proliferation, survival and the maintenance of immune homeostasis (PubMed:1650475, PubMed:7568038). Functions not only as an amplifier of TCR signals but delivers unique signals that control intracellular biochemical events that alter the gene expression program of T-cells (PubMed:24665965). Stimulation upon engagement of its cognate ligands CD80 or CD86 increases proliferation and expression of various cytokines in particular IL2 production in both CD4(+) and CD8(+) T-cell subsets (PubMed:12196291, PubMed:1650475, PubMed:35397202). Mechanistically, ligation induces recruitment of protein kinase C-theta/PRKCQ and GRB2 leading to NF-kappa-B activation via both PI3K/Akt-dependent and -independent pathways (PubMed:21964608, PubMed:24665965, PubMed:7568038). In conjunction with TCR/CD3 ligation and CD40L costimulation, enhances the production of IL4 and IL10 in T-cells (PubMed:8617933) Enhances CD40L-mediated activation of NF-kappa-B and kinases MAPK8 and PAK2 in T-cells (PubMed:15067037)","subcellular_location":"Cell surface","url":"https://www.uniprot.org/uniprotkb/P10747/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CD28","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/CD28","total_profiled":1310},"omim":[{"mim_id":"621017","title":"ZINC FINGER PROTEIN 334; ZNF334","url":"https://www.omim.org/entry/621017"},{"mim_id":"620901","title":"IMMUNODEFICIENCY 123 WITH HPV-RELATED VERRUCOSIS; IMD123","url":"https://www.omim.org/entry/620901"},{"mim_id":"620678","title":"RAS AND RAB INTERACTOR-LIKE PROTEIN; RINL","url":"https://www.omim.org/entry/620678"},{"mim_id":"620430","title":"AUTOIMMUNE DISEASE, MULTISYSTEM, INFANTILE-ONSET, 3; ADMIO3","url":"https://www.omim.org/entry/620430"},{"mim_id":"619164","title":"IMMUNODEFICIENCY 76; IMD76","url":"https://www.omim.org/entry/619164"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Plasma membrane","reliability":"Supported"}],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"lymphoid tissue","ntpm":28.7},{"tissue":"placenta","ntpm":13.9}],"url":"https://www.proteinatlas.org/search/CD28"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"P10747","domains":[{"cath_id":"2.60.40.10","chopping":"22-134","consensus_level":"high","plddt":93.3325,"start":22,"end":134},{"cath_id":"1.20.5","chopping":"151-181","consensus_level":"medium","plddt":89.8632,"start":151,"end":181}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P10747","model_url":"https://alphafold.ebi.ac.uk/files/AF-P10747-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P10747-F1-predicted_aligned_error_v6.png","plddt_mean":81.31},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CD28","jax_strain_url":"https://www.jax.org/strain/search?query=CD28"},"sequence":{"accession":"P10747","fasta_url":"https://rest.uniprot.org/uniprotkb/P10747.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P10747/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P10747"}},"corpus_meta":[{"pmid":"8717514","id":"PMC_8717514","title":"CD28/B7 system of T cell costimulation.","date":"1996","source":"Annual review of immunology","url":"https://pubmed.ncbi.nlm.nih.gov/8717514","citation_count":2086,"is_preprint":false},{"pmid":"11910893","id":"PMC_11910893","title":"The B7-CD28 superfamily.","date":"2002","source":"Nature reviews. 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CTLA-4, structurally related to CD28, binds B7 with approximately 20-fold higher affinity than CD28.\",\n      \"method\": \"Biochemical assays, tyrosine phosphorylation studies, binding affinity measurements\",\n      \"journal\": \"Annual review of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct biochemical assays measuring phosphorylation substrates and binding affinities, single review summarizing multiple experimental findings\",\n      \"pmids\": [\"8386518\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CD28-deficient mice have impaired T cell responses to lectins, fail to produce IL-2 upon lectin stimulation, show significantly decreased IL-2 receptor alpha expression, and have defective immunoglobulin class switching; however, cytotoxic T cells can still be induced, demonstrating that CD28 is not required for all T cell responses in vivo.\",\n      \"method\": \"CD28 knockout mouse model, T cell functional assays, immunoglobulin measurement\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout with multiple defined cellular phenotypes, replicated across multiple assays in a single rigorous study\",\n      \"pmids\": [\"7688139\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"CD28 costimulation up-regulates Bcl-xL expression to enhance T cell survival during activation; this survival signal is independent of Fas expression and is mediated through ICE protease-dependent apoptosis pathways when absent. CD28 costimulation did not alter Bcl-2 or Fas expression.\",\n      \"method\": \"Wild-type vs CD28-deficient mice, CTLA4Ig treatment, western blot for Bcl-xL/Bcl-2/Fas, ICE protease inhibitors, apoptosis morphology assessment\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic KO combined with pharmacological blockade (CTLA4Ig), multiple orthogonal survival assays, explicit negative results for Bcl-2/Fas\",\n      \"pmids\": [\"8752911\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"CD28 is required for germinal center formation in T-dependent immune responses; CD28-deficient mice fail to support proliferative expansion of B cells in follicles, preventing germinal center formation and somatic hypermutation.\",\n      \"method\": \"CD28 knockout mouse model, immunohistology of lymphoid tissue, B cell response analysis after antigen challenge\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic KO with specific cellular phenotype (absence of germinal centers and somatic mutations) established by histological and molecular analysis\",\n      \"pmids\": [\"8648099\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Cbl-b suppresses TCR-mediated Vav (guanine nucleotide exchange factor for Rac1/Rho/CDC42) activation downstream of the TCR, and this suppression underlies the CD28 dependence of T cell activation. Cbl-b-null T cells do not require CD28 for IL-2 production; Cbl-b(-/-) mutation fully restores T-cell-dependent antibody responses in CD28-/- mice, without affecting ZAP-70, Lck, Ras/MAPK, PLCgamma-1, or Ca2+ mobilization.\",\n      \"method\": \"Cbl-b knockout mice crossed with CD28 knockout mice, IL-2 production assays, western blot for signaling intermediates, Vav activation assay\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — double genetic KO epistasis, multiple orthogonal signaling assays, specific mechanistic placement of Cbl-b upstream of Vav in CD28-dependent pathway\",\n      \"pmids\": [\"10646609\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"A single point mutation in CD28 at tyrosine 170 (mouse)/173 (human) — the YMNM motif — uncouples SH2-mediated interactions (including PI3K and Grb2 recruitment) from CD28 signaling. This mutant CD28 cannot upregulate Bcl-xL, rendering T cells susceptible to radiation-induced death, but still prevents anergy and promotes proliferation, IL-2 secretion, and B cell help.\",\n      \"method\": \"Transgenic knockin mice expressing Y170F CD28 mutant, Bcl-xL western blot, T cell survival assays, proliferation/IL-2 measurements\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — site-directed mutagenesis (knockin), multiple functional readouts separating two distinct CD28 signaling outputs (survival vs. proliferation)\",\n      \"pmids\": [\"11276203\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CD28 costimulation, acting through PI3K and Akt, is required for T cells to increase glycolytic rate in response to activation. CD28 induces glucose uptake and glycolysis in excess of what is needed to maintain ATP/ADP ratios or macromolecular synthesis, suggesting it anticipates future energetic demands.\",\n      \"method\": \"CD28-deficient mice, PI3K/Akt inhibitors, glucose uptake assays, glycolysis measurements, ATP/ADP measurements\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic KO combined with pharmacological inhibition of PI3K/Akt, multiple metabolic assays providing mechanistic pathway placement\",\n      \"pmids\": [\"12121659\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Mouse dendritic cells are induced by soluble CD28 to express IL-6 and IFN-gamma via B7-1 (CD80) and B7-2 (CD86) and p38 MAPK signaling — demonstrating bidirectional (reverse) signaling from CD28 through B7 molecules into dendritic cells.\",\n      \"method\": \"Soluble CD28 stimulation of mouse DCs, cytokine measurement, p38 MAPK inhibitor, B7-1/B7-2 blocking experiments, in vivo tumor challenge\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple in vitro and in vivo assays in a single lab study, but reverse signaling concept from a single lab\",\n      \"pmids\": [\"15467723\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Crystal structure of soluble CD28 in complex with a Fab fragment reveals that CD28 is a monovalent homodimer whose structural features redefine evolutionary relationships with CTLA-4, antigen receptors, and adhesion molecules, and account for distinct ligand-binding and stoichiometric properties compared to CTLA-4. Cryo-EM comparisons of CD28 with mitogenic vs non-mitogenic antibody complexes place new constraints on models of receptor triggering.\",\n      \"method\": \"X-ray crystallography of soluble CD28-Fab complex, cryo-electron microscopy\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure determination with cryo-EM validation, directly establishing structural basis for CD28/CTLA-4 functional differences\",\n      \"pmids\": [\"15696168\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"CD28 affects the earliest TCR signaling events: inhibiting CD28/B7 interaction results in impaired TCR-induced tyrosine phosphorylation of the zeta chain and ZAP-70, and strongly diminished downstream signaling including Ca2+/calcineurin, ERK/MAPK, and JNK pathways, under physiological conditions of antigen presentation.\",\n      \"method\": \"Jurkat cells and primary TCR-transgenic T cells stimulated with antigen/superantigen on B7-expressing APCs, anti-CD28 blocking, western blot for phosphorylation of TCR zeta, ZAP-70, downstream signaling\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — physiological conditions with multiple signaling readouts, single lab with two cell systems (Jurkat and primary T cells)\",\n      \"pmids\": [\"9692882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"CD28 can activate PI3K and cooperate with adapters Vav and SLP-76 to independently induce IL-2 and IL-4 transcription in the absence of TCR ligation, providing a molecular basis for in trans costimulation.\",\n      \"method\": \"Biochemical signaling assays, IL-2/IL-4 reporter assays, CD28 mutants\",\n      \"journal\": \"Immunological reviews\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — signaling assays described in a review context without full primary experimental detail visible in the abstract\",\n      \"pmids\": [\"12670393\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The YMNM motif of CD28 cytoplasmic domain, when mutated at position N191 (retaining PI3K binding), abolishes IL-2 promoter activation, whereas Y189F (disrupting PI3K binding) only reduces it, suggesting that PI3K associated with the YMNM motif acts as a negative mediator of CD28-mediated IL-2 gene transcription.\",\n      \"method\": \"CD28 point and deletion mutants expressed in Jurkat cells, IL-2 promoter reporter assays, PI3K pharmacological inhibitors, constitutively active PI3K overexpression\",\n      \"journal\": \"Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — site-directed mutagenesis with reporter assays and pharmacological/genetic PI3K manipulation, single lab\",\n      \"pmids\": [\"11113113\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"CD28 accumulates at the immunological synapse before c-SMAC formation (in the immature synapse), co-localizing with TCR, and exerts effects on initiation and stabilization of the synapse within seconds of TCR-mediated calcium rise, regulating downstream T cell proliferation.\",\n      \"method\": \"Live imaging of T cell–APC interactions, CD28-deficient and reconstituted murine cells, single-cell calcium recording, synapse morphology analysis\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — live imaging with genetic reconstitution, single-cell functional readouts, single lab\",\n      \"pmids\": [\"15128767\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"CD28 forms TCR-CD28 microclusters in cooperation with TCRs at the immunological synapse, associates with PKCtheta, and after mature IS formation, CD28 microclusters accumulate at a specific subregion of the IS where they continuously assemble with kinases (not TCRs) to generate sustained T cell signaling.\",\n      \"method\": \"Dynamic imaging of immunological synapse, fluorescent protein-tagged molecules, microcluster analysis\",\n      \"journal\": \"Immunological reviews\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — dynamic imaging with defined molecular co-localization at IS subregions, described in a review of prior published imaging experiments\",\n      \"pmids\": [\"19426213\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"B7-H2 (ICOSL) acts as a ligand for CD28 and CTLA-4 in humans (but not mice) through distinctive binding domains from B7-1/B7-2. B7-H2-CD28 interaction costimulates human T cell primary and memory responses, inducing Bcl-xL, downregulating p27(kip1), and triggering ERK and AKT kinase-dependent signaling, similar to B7-1/B7-2.\",\n      \"method\": \"Receptor array assay, co-stimulation functional assays with human T cells, western blot for Bcl-xL, p27(kip1), phospho-ERK, phospho-AKT\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — receptor array identification plus multiple functional and signaling readouts in primary human T cells, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"21530327\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"CD28 has an obligate cell-intrinsic function in Foxp3+ Tregs postmaturation: Treg-specific CD28 knockout mice develop severe autoimmunity despite normal Treg numbers, with dampened CTLA-4, PD-1, and CCR6 expression and pronounced proliferative/survival disadvantage in Tregs under competitive conditions.\",\n      \"method\": \"Treg-specific conditional CD28 knockout mice (Cre-lox), autoimmunity phenotyping, flow cytometry for CTLA-4/PD-1/CCR6, competitive reconstitution experiments\",\n      \"journal\": \"Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional cell-type-specific KO with complementation experiment, multiple molecular and cellular phenotypes\",\n      \"pmids\": [\"23281398\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CD28 and CD3 provide complementary functions in T cell traction force generation: force generation is associated with CD3/TCR complex engagement, while CD28 engagement increases traction forces associated with CD3 through the PI3K signaling pathway rather than through direct cell-surface coupling. Force generation is concentrated at the cell periphery and associated with molecular complexes containing phosphorylated Pyk2.\",\n      \"method\": \"Traction force microscopy using micrometer-scale elastomer pillar arrays, primary human and mouse T cells, PI3K inhibitors, Pyk2 immunofluorescence, TCR-specific T cells on peptide-loaded MHC pillars\",\n      \"journal\": \"PNAS\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — traction force microscopy with pharmacological dissection of PI3K pathway, primary cells from multiple species with defined genetic TCR\",\n      \"pmids\": [\"24469820\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Functional convergence of CD28 costimulation and TCR signaling requires spatial co-proximity within the immunological synapse: micrometer-scale separation of CD3 and CD28 ligands curtails T cell activation. Active Lck is concentrated to regions of CD3 engagement with low diffusion rates, suggesting costimulation is controlled by a reaction-diffusion balance of active Lck transport to CD28 vs. its deactivation. Disruption of actin cytoskeleton increases Lck mobility and rescues activation by spatially separated CD3/CD28.\",\n      \"method\": \"Micropatterned surfaces with spatially controlled ligand presentation, primary human and mouse CD4+ T cells, Lck phosphorylation imaging, actin cytoskeleton disruption, FRAP-like mobility measurements\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — engineered micropatterned surfaces with pharmacological and genetic manipulations, multiple orthogonal readouts, two species\",\n      \"pmids\": [\"24379441\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"CD28 transmembrane signaling is regulated by two counteractive factors: acidic phospholipids sequester CD28 signaling motifs within the membrane to limit basal signaling, while TCR-induced local Ca2+ increase directly disrupts the CD28-lipid interaction, releasing signaling motifs. TCR, Ca2+, and CD28 form a dual-positive-feedback circuit that amplifies T cell signaling and antigen sensitivity.\",\n      \"method\": \"NMR spectroscopy of CD28 transmembrane domain with lipids, Ca2+ measurement in T cells, mutagenesis of CD28 charged residues, functional T cell signaling assays\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structural analysis combined with functional mutagenesis and live-cell Ca2+ measurements establishing the mechanistic basis of transmembrane regulation\",\n      \"pmids\": [\"29058713\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"CD28 transiently promotes expression of carnitine palmitoyltransferase 1a (Cpt1a) before the first cell division upon T cell activation, facilitating mitochondrial fatty acid oxidation (FAO), mitochondrial elongation, and enhanced spare respiratory capacity (SRC). Without CD28, microRNA-33 (a target of TXNIP) attenuates Cpt1a expression, resulting in metabolically compromised cells. This early CD28-dependent mitochondrial priming enables T cells to remodel cristae, develop SRC, and rapidly produce cytokines upon restimulation — features of protective memory T cells.\",\n      \"method\": \"CD28-deficient T cells, miR-33 manipulation, Cpt1a overexpression/knockdown, mitochondrial morphology imaging (electron microscopy), metabolic flux assays (oxygen consumption, FAO), in vivo memory T cell functional assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal methods (genetic KO, miRNA manipulation, metabolic assays, structural mitochondrial imaging, in vivo memory responses) in a single rigorous study\",\n      \"pmids\": [\"28919076\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CD28 pro-survival signaling in multiple myeloma cells is dependent on downstream activation of PI3K/Akt, inactivation of transcription factor FoxO3a, and decreased expression of pro-apoptotic Bim. Blocking CD28-CD80/CD86 interaction abrogates DC-mediated protection of MM cells against chemotherapy-induced death in vitro and in vivo.\",\n      \"method\": \"PI3K/Akt inhibitors, FoxO3a/Bim western blots, CD28 crosslinking with agonistic antibodies or DCs, in vivo Vk*MYC murine myeloma model with CD28 blockade, tumor burden measurement\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple pharmacological inhibitors targeting defined pathway components, in vitro and in vivo validation, mechanistic pathway placement\",\n      \"pmids\": [\"24782505\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The CTLA4-CD28 gene fusion product (extracellular CTLA4 domain fused to CD28 cytoplasmic domain) transforms inhibitory signals into stimulatory signals for T-cell activation, enhancing proliferation and AKT and ERK phosphorylation when ectopically expressed in Jurkat and H9 cells.\",\n      \"method\": \"RNA sequencing to identify fusion, RT-PCR/Sanger sequencing in 115 T-cell lymphoma samples, ectopic expression of fusion in Jurkat/H9 cells, proliferation assays, western blot for phospho-AKT and phospho-ERK\",\n      \"journal\": \"Haematologica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ectopic expression with multiple signaling readouts, but in cell lines rather than primary cells, single lab\",\n      \"pmids\": [\"26819049\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CD28 transduces pro-survival signaling specifically in long-lived plasma cells (LLPCs) through differential SLP76 expression. CD28 signaling in LLPCs increases glucose uptake, mitochondrial mass/respiration, and reactive oxygen species (ROS) production. CD28-mediated mitochondrial respiration, NF-κB activation, and survival are ROS-dependent. IRF4 (a NF-κB target) is upregulated by CD28 in LLPCs and is required for CD28-mediated metabolic and survival program.\",\n      \"method\": \"CD28 activation in LLPCs vs SLPCs, SLP76 expression analysis, glucose uptake assays, mitochondrial mass/respiration measurements, ROS detection, NF-κB reporter/western blot, IRF4 knockdown, survival assays\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal metabolic and molecular assays with genetic knockdown, mechanistic pathway from CD28 through SLP76→ROS→NF-κB→IRF4 established\",\n      \"pmids\": [\"32579940\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CARs containing a CD28 transmembrane domain (TMD) form heterodimers with endogenous CD28 in human T cells; this dimerization is dependent on polar amino acids in the CD28-TMD and is distinct from CD8-TMD-containing CARs which do not form such heterodimers. CD28-TMD heterodimers respond to anti-CD28 stimulation for CAR-dependent proliferation but do not respond to CD80/CD86 stimulation and reduce CD28 cell-surface expression.\",\n      \"method\": \"Co-immunoprecipitation, anti-CD28 stimulation-dependent proliferation assays, surface CD28 expression by flow cytometry, mutagenesis of polar residues in CD28-TMD\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP with functional validation and mutagenesis, single lab with multiple assays\",\n      \"pmids\": [\"33833759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CD8+ T cells display B7 ligands (CD80/CD86) on their surface that interact with CD28 in cis at membrane invaginations of the immunological synapse. This cis-B7:CD28 interaction is driven by PI3K and sorting-nexin-9 (SNX9)-mediated membrane remodeling. cis-B7:CD28 signaling occurs through PKCθ and promotes CD8+ T cell survival, migration, and cytokine production. Loss of T cell-intrinsic cis-B7:CD28 interactions decreases intratumoral T cells and accelerates tumor growth in mouse models.\",\n      \"method\": \"Live imaging of IS membrane invaginations, PI3K/SNX9 genetic/pharmacological manipulation, cis-B7 knockout on T cells, PKCθ signaling readouts, in vivo mouse tumor models\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (live imaging, genetic KO of pathway components, in vivo tumor models), mechanistic pathway (PI3K→SNX9→membrane remodeling→cis-B7:CD28→PKCθ) established\",\n      \"pmids\": [\"37160118\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Bacterial superantigens hyperinduce inflammatory cytokines by directly engaging both CD28 and its coligand B7-2 (CD86) at their respective homodimer interfaces — sites remote from where B7-2 and CD28 normally interact — thereby potently enhancing the avidity of the B7-2:CD28 interaction and forcing excessive costimulatory signaling.\",\n      \"method\": \"Binding studies with CD28 and B7-2 homodimer interface peptides, cytokine production assays, murine lethal superantigen challenge with mimotope peptides\",\n      \"journal\": \"PNAS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — binding domain mapping with functional cytokine assays and in vivo protection data, single lab\",\n      \"pmids\": [\"27708164\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In CAR-NK cells, CD28 costimulatory domain linked to CD3ζ creates a signaling platform that recruits LCK and ZAP70, initiating a kinase cascade that enhances CAR-NK cell antitumor efficacy and long-term cytotoxicity; this is a non-canonical role since CD28 is not inherently present in mature NK cells.\",\n      \"method\": \"Co-immunoprecipitation of LCK/ZAP70 with CD28-CD3ζ CAR construct, in vitro cytotoxicity assays, multiple xenograft tumor models in vivo\",\n      \"journal\": \"Cancer discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP establishing kinase recruitment plus in vitro and in vivo functional validation, single lab\",\n      \"pmids\": [\"38900051\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"TNF-alpha directly inhibits CD28 gene transcription by reducing DNA-protein complex formation at two sequence motifs (site alpha and beta) of the CD28 minimal promoter, including the transcriptional initiator sequences, resulting in reversible downregulation of CD28 surface expression.\",\n      \"method\": \"Reporter gene bioassays with CD28 minimal promoter, in vitro transcription assays, EMSA (DNA-protein binding), flow cytometry for CD28 surface expression\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro transcription reconstitution plus promoter reporter assays and EMSA, single lab with multiple complementary methods\",\n      \"pmids\": [\"11544310\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"CD28 co-stimulation suppresses Th17 differentiation through an IL-2 and IFN-gamma-dependent mechanism; blockade of CD28/B7 interactions facilitates both murine and human Th17 cell differentiation, and this suppression is not mediated through increased Foxp3+ T cells.\",\n      \"method\": \"Anti-CD28 antibody, CTLA4-Ig blockade, cytokine measurement (IL-17, IL-2, IFN-gamma, IL-10), flow cytometry for Foxp3+ cells, DC co-culture experiments\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple cytokine blockade experiments with human and murine cells, single lab, mechanistic pathway identified through cytokine dependency\",\n      \"pmids\": [\"19333372\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"CD28-activated Tec family kinase ITK specifically licenses autoreactive T cells to traffic from lymphoid organs into peripheral tissues to mount destructive immune responses; loss of ITK in CTLA-4-deficient mice does not block T cell activation/proliferation but prevents tissue infiltration and fatal autoimmunity.\",\n      \"method\": \"Itk(-/-); Ctla4(-/-) double knockout mice, T cell localization by tissue histology, T cell activation/proliferation assays, ITK pharmacological inhibitors, diabetogenic T cell transfer model\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — double genetic KO epistasis with pharmacological inhibitor validation, specific mechanistic dissociation of activation from trafficking, in vivo disease model\",\n      \"pmids\": [\"24270545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CARMIL2 is required for canonical NF-κB activation (but not AP-1 or NFAT) in T cells stimulated via CD28; CARMIL2-deficient patients phenocopy CD28-deficient patients in showing defective CD4+ and CD8+ memory T cells and TREG counts, but additionally show NK cell and memory B cell deficiencies, indicating CARMIL2 governs immunological pathways beyond CD28.\",\n      \"method\": \"Patient-derived cells from 89 CARMIL2-deficient patients, NF-κB/AP-1/NFAT reporter assays upon CD28 stimulation, flow cytometry for lymphocyte subsets, somatic reversion analysis\",\n      \"journal\": \"Journal of experimental medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — large patient cohort with functional signaling assays, pathway placement established by comparison of CARMIL2 and CD28 deficiencies\",\n      \"pmids\": [\"36515678\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CD28 is a homodimeric costimulatory receptor on T cells that, upon engagement by B7-1 (CD80), B7-2 (CD86), or B7-H2 (in humans) on APCs, recruits PI3K and Akt via its YMNM cytoplasmic motif and cooperates with Vav/SLP-76 to amplify TCR signaling, drive IL-2 production, upregulate Bcl-xL for survival, promote glycolytic and mitochondrial metabolic reprogramming (including Cpt1a-driven FAO essential for memory formation), regulate germinal center formation and Treg maintenance through cell-intrinsic signals, and can also signal in cis when T cell-expressed B7 ligands engage CD28 at immunological synapse invaginations via a PI3K/SNX9/PKCθ pathway; its transmembrane domain anchors lipid-sequestered signaling motifs that are released by TCR-induced Ca2+ influx, forming a dual positive-feedback amplification circuit with the TCR.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CD28 is the principal costimulatory receptor of T cells, a homodimer that lowers the threshold for activation by acting at the earliest steps of TCR signaling—its engagement is required for normal tyrosine phosphorylation of the TCR zeta chain and ZAP-70 and for downstream Ca2+/calcineurin, ERK/MAPK and JNK signaling [#9], and CD28-deficient mice show defective IL-2 production, IL-2 receptor alpha expression, and immunoglobulin class switching while retaining cytotoxic responses [#1]. The receptor accumulates at the immature immunological synapse together with the TCR, where it forms TCR-CD28 microclusters that associate with PKCθ and sustain signaling at a discrete synapse subregion [#12, #13]; productive costimulation requires spatial co-proximity of CD3 and CD28 ligands, governed by actin-constrained transport of active Lck [#17]. Costimulatory output is gated at the membrane itself: acidic phospholipids sequester the CD28 cytoplasmic signaling motifs, and TCR-induced Ca2+ disrupts this lipid interaction to release them, creating a dual positive-feedback amplification circuit with the TCR [#18]. Signal transduction proceeds through the YMNM motif at tyrosine 170/173, which recruits SH2-domain partners including PI3K and Grb2; mutation of this tyrosine uncouples Bcl-xL-dependent survival from the proliferative and IL-2-promoting outputs of CD28 [#5], and CD28 cooperates with Vav and SLP-76, with Cbl-b suppression of Vav activation explaining the CD28-dependence of T cell activation [#4, #10]. Through PI3K/Akt, CD28 drives metabolic reprogramming—increasing glycolysis and glucose uptake beyond immediate energetic need [#6] and transiently inducing Cpt1a to prime mitochondrial fatty acid oxidation and spare respiratory capacity essential for protective memory [#19]. At the organismal level CD28 is required for germinal center formation and somatic hypermutation [#3], exerts a cell-intrinsic function maintaining Foxp3+ Treg fitness and identity [#15], and suppresses Th17 differentiation via IL-2/IFN-γ [#28]. Human CD28 is engaged not only by B7-1/B7-2 but also by B7-H2 (ICOSL), which costimulates primary and memory responses through Bcl-xL induction and ERK/AKT signaling [#14]. Crystallographic analysis defines CD28 as a monovalent homodimer distinct in ligand-binding and stoichiometry from the higher-affinity inhibitory relative CTLA-4 [#0, #8].\",\n  \"teleology\": [\n    {\n      \"year\": 1993,\n      \"claim\": \"Established CD28 as a costimulatory receptor whose B7 engagement triggers tyrosine phosphorylation cascades, and positioned it against the structurally related, higher-affinity CTLA-4.\",\n      \"evidence\": \"Biochemical phosphorylation and binding-affinity assays summarized from primary studies\",\n      \"pmids\": [\"8386518\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not map the specific cytoplasmic motifs responsible for partner recruitment\", \"Affinity comparison did not resolve functional consequences of CTLA-4 vs CD28 binding\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Genetic loss-of-function showed CD28 is required for IL-2 production, IL-2Rα expression and class switching but dispensable for cytotoxic T cell induction, defining the scope and limits of its in vivo role.\",\n      \"evidence\": \"CD28 knockout mice with T cell functional and immunoglobulin assays\",\n      \"pmids\": [\"7688139\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify the molecular signals downstream of CD28 underlying these phenotypes\", \"Could not separate proliferation from survival contributions\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Identified Bcl-xL upregulation as a discrete CD28 survival output independent of Bcl-2 and Fas, explaining how costimulation protects activated T cells from apoptosis.\",\n      \"evidence\": \"CD28-KO mice plus CTLA4Ig blockade with Bcl-xL/Bcl-2/Fas western blots and ICE-protease readouts\",\n      \"pmids\": [\"8752911\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not link Bcl-xL induction to a specific cytoplasmic signaling motif\", \"Mechanism connecting CD28 to Bcl-xL transcription unresolved at this stage\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Demonstrated CD28 is required for germinal center formation and somatic hypermutation, defining its role in T-dependent humoral responses.\",\n      \"evidence\": \"CD28-KO mice with lymphoid histology and B cell response analysis\",\n      \"pmids\": [\"8648099\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not distinguish T-cell-intrinsic from indirect contributions to GC formation\", \"Downstream signaling requirements not addressed\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Showed CD28 acts on the earliest TCR signaling events under physiological antigen presentation, placing costimulation upstream rather than parallel to proximal TCR signaling.\",\n      \"evidence\": \"Antigen/superantigen stimulation on B7+ APCs with anti-CD28 blockade and phospho-blots in Jurkat and primary T cells\",\n      \"pmids\": [\"9692882\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab two-cell-system study\", \"Did not define the biochemical bridge from CD28 to TCR zeta/ZAP-70 phosphorylation\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Epistasis placed Cbl-b suppression of Vav activation as the basis for CD28-dependence, showing CD28 functions to relieve a negative checkpoint on TCR signaling.\",\n      \"evidence\": \"Cbl-b x CD28 double-KO mice with IL-2, Vav activation and signaling-intermediate assays\",\n      \"pmids\": [\"10646609\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish direct CD28 engagement of Vav/Cbl-b\", \"Left open how CD28 itself amplifies Vav signaling\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Linked CD28 to PI3K, Vav and SLP-76 cooperation for IL-2/IL-4 transcription, providing a molecular basis for in-trans costimulation, and probed whether YMNM-associated PI3K is positive or negative for IL-2.\",\n      \"evidence\": \"CD28 point/deletion mutants with IL-2 promoter reporters and PI3K manipulation in Jurkat cells (with review summary)\",\n      \"pmids\": [\"12670393\", \"11113113\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Reporter-based and partly review-derived; in vivo relevance of negative PI3K role unresolved\", \"Conflicting positive vs negative roles for YMNM-associated PI3K not reconciled\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Knockin mutation of the YMNM tyrosine separated the SH2-dependent survival arm (Bcl-xL induction) from the proliferative/IL-2/anergy-prevention arm, demonstrating CD28 has genetically separable signaling outputs.\",\n      \"evidence\": \"Y170F CD28 knockin mice with survival, proliferation and IL-2 readouts\",\n      \"pmids\": [\"11276203\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify which SH2 partner mediates the survival arm in vivo\", \"Mechanism of the tyrosine-independent proliferative arm unresolved\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identified transcriptional control of CD28 itself, showing TNF-α represses CD28 promoter activity to reversibly downregulate surface CD28.\",\n      \"evidence\": \"CD28 minimal-promoter reporters, in vitro transcription, EMSA and surface flow cytometry\",\n      \"pmids\": [\"11544310\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"TNF-α-responsive transcription factors at sites α/β not identified\", \"Physiological contexts of CD28 downregulation not defined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Defined CD28-driven metabolic reprogramming, showing PI3K/Akt-dependent glycolytic upregulation that exceeds immediate energetic demand.\",\n      \"evidence\": \"CD28-KO mice, PI3K/Akt inhibitors and glucose/glycolysis/ATP-ADP assays\",\n      \"pmids\": [\"12121659\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address mitochondrial or fatty-acid metabolism\", \"Link between metabolic reprogramming and effector/memory fate not established\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Crystal and cryo-EM structures defined CD28 as a monovalent homodimer with binding and stoichiometric properties distinct from CTLA-4, constraining models of receptor triggering.\",\n      \"evidence\": \"X-ray crystallography of CD28-Fab and cryo-EM of antibody complexes\",\n      \"pmids\": [\"15696168\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of ligand-bound CD28 signaling complex not resolved here\", \"Mechanistic basis of mitogenic vs non-mitogenic triggering inferred, not directly shown\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Live imaging placed CD28 at the immature immunological synapse co-localized with the TCR, acting within seconds of the Ca2+ rise to stabilize the synapse.\",\n      \"evidence\": \"Live T cell-APC imaging with CD28-KO/reconstituted cells and single-cell Ca2+ recording\",\n      \"pmids\": [\"15128767\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab imaging study\", \"Molecular machinery anchoring CD28 to the immature synapse not defined\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Resolved CD28 spatiotemporal signaling into TCR-CD28 microclusters that associate with PKCθ and sustain kinase-driven signaling at a synapse subregion after mature synapse formation; separately showed CD28 suppresses Th17 differentiation via IL-2/IFN-γ.\",\n      \"evidence\": \"Dynamic synapse imaging of tagged molecules; cytokine-blockade differentiation assays in human and mouse cells\",\n      \"pmids\": [\"19426213\", \"19333372\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Microcluster work described in review of prior imaging\", \"Th17 suppression mechanism downstream of IL-2/IFN-γ not molecularly defined\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Established B7-H2 (ICOSL) as a human-specific CD28 ligand that costimulates primary and memory T cells, broadening the CD28 ligand repertoire.\",\n      \"evidence\": \"Receptor array plus human T cell costimulation and signaling readouts (Bcl-xL, p27, phospho-ERK/AKT)\",\n      \"pmids\": [\"21530327\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative physiological contribution of B7-H2 vs B7-1/B7-2 not quantified\", \"Mouse-human divergence mechanism not explored\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Conditional knockouts and epistasis defined cell-intrinsic CD28 functions: maintenance of Foxp3+ Treg fitness and identity, and ITK-dependent licensing of autoreactive T cell tissue trafficking.\",\n      \"evidence\": \"Treg-specific CD28 cKO with competitive reconstitution; Itk/Ctla4 double-KO with tissue histology and diabetogenic transfer\",\n      \"pmids\": [\"23281398\", \"24270545\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Treg CD28 signaling effectors maintaining CTLA-4/PD-1/CCR6 not pinpointed\", \"Direct link from CD28 to ITK activation not biochemically resolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Micropatterning showed costimulation requires CD3-CD28 spatial co-proximity, controlled by actin-constrained transport of active Lck, defining a physical basis for functional convergence.\",\n      \"evidence\": \"Spatially controlled ligand micropatterns, Lck phospho-imaging, actin disruption and mobility measurements in primary T cells\",\n      \"pmids\": [\"24379441\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Reaction-diffusion model not validated in physiological APC contacts\", \"Identity of CD28-proximal Lck substrates not defined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showed CD28 augments CD3-associated traction forces through PI3K and links to phospho-Pyk2 complexes; separately identified CD28/PI3K/Akt/FoxO3a/Bim pro-survival signaling co-opted by multiple myeloma cells.\",\n      \"evidence\": \"Traction force microscopy with PI3K inhibitors; PI3K/Akt inhibitors, FoxO3a/Bim blots and in vivo Vk*MYC myeloma model with CD28 blockade\",\n      \"pmids\": [\"24469820\", \"24782505\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanical signaling role in physiological T cell activation not fully defined\", \"Therapeutic implications of myeloma CD28 dependence beyond mouse model unestablished\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Defined disease-relevant CD28 mechanisms: superantigen direct engagement of CD28/B7-2 homodimer interfaces to force excessive costimulation, and a CTLA4-CD28 fusion that converts inhibitory signals to stimulatory ones.\",\n      \"evidence\": \"Homodimer-interface peptide binding with cytokine and lethal challenge assays; RNA-seq fusion identification and ectopic expression with phospho-AKT/ERK readouts\",\n      \"pmids\": [\"27708164\", \"26819049\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Superantigen mechanism mapped via peptides, not full complex structures\", \"Fusion oncogenicity shown in cell lines, not primary lymphoma cells\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Two structural/mechanistic studies defined membrane-level gating of CD28: phospholipid sequestration of signaling motifs released by TCR-induced Ca2+ as a dual positive-feedback circuit, and transient Cpt1a-driven mitochondrial priming enabling protective memory.\",\n      \"evidence\": \"NMR of CD28 TMD with lipids plus Ca2+ and mutagenesis assays; CD28-KO with miR-33 manipulation, Cpt1a perturbation, EM, metabolic flux and in vivo memory assays\",\n      \"pmids\": [\"29058713\", \"28919076\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative thresholds of the Ca2+/lipid switch in vivo not defined\", \"Upstream regulators coupling CD28 to Cpt1a/miR-33 not fully mapped\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Extended CD28 pro-survival signaling beyond T cells to long-lived plasma cells, defining an SLP76-dependent ROS/NF-κB/IRF4 metabolic survival program.\",\n      \"evidence\": \"CD28 activation in LLPCs vs SLPCs with metabolic, ROS, NF-κB and IRF4-knockdown survival assays\",\n      \"pmids\": [\"32579940\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How differential SLP76 expression rewires CD28 output not mechanistically resolved\", \"Relationship to T cell CD28 metabolic programs not directly compared\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Showed CD28-transmembrane-domain CARs heterodimerize with endogenous CD28 via polar TMD residues, altering CAR responsiveness and reducing CD28 surface expression.\",\n      \"evidence\": \"Co-IP, TMD mutagenesis, anti-CD28-dependent proliferation and surface CD28 flow cytometry\",\n      \"pmids\": [\"33833759\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab co-IP study\", \"Functional consequences of endogenous CD28 sequestration in vivo not assessed\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified a cis-B7:CD28 mechanism in which CD8+ T cells display B7 ligands engaging CD28 at synapse invaginations via PI3K/SNX9 membrane remodeling and PKCθ, promoting survival, migration and antitumor function.\",\n      \"evidence\": \"Live IS-invagination imaging, PI3K/SNX9 manipulation, cis-B7 T cell KO, PKCθ readouts and in vivo tumor models\",\n      \"pmids\": [\"37160118\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contribution of cis vs trans B7:CD28 signaling not quantified\", \"Generality across T cell subsets beyond CD8+ not established\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Demonstrated the CD28 costimulatory domain functions non-canonically in CAR-NK cells by recruiting LCK and ZAP70 to a CD28-CD3ζ platform that enhances antitumor cytotoxicity.\",\n      \"evidence\": \"Co-IP of LCK/ZAP70 with CD28-CD3ζ construct, in vitro cytotoxicity and xenograft models\",\n      \"pmids\": [\"38900051\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab co-IP; direct enzymatic activation kinetics not measured\", \"Whether endogenous NK signaling machinery is co-opted vs ectopically supplied unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CD28's separable signaling arms (survival vs proliferation vs metabolic priming) are wired to distinct downstream effectors, and how trans, cis, and reverse signaling modes are integrated at the synapse, remain incompletely defined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified map linking individual cytoplasmic motifs to each functional output\", \"Stoichiometry and structure of the ligand-engaged CD28 signaling complex unresolved\", \"Quantitative balance of cis vs trans vs reverse signaling in physiological responses unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 1, 9, 14]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 6, 18]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [18]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [8, 12, 13, 18, 27]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [1, 3, 15, 28]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [5, 6, 9, 18]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [6, 19, 22]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [2, 5, 20]}\n    ],\n    \"complexes\": [\"TCR-CD28 microcluster\", \"immunological synapse\"],\n    \"partners\": [\"CD80\", \"CD86\", \"ICOSLG\", \"PIK3\", \"VAV1\", \"LCP2\", \"PRKCQ\", \"CBLB\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}