{"gene":"CD4","run_date":"2026-04-28T17:28:52","timeline":{"discoveries":[{"year":1984,"finding":"CD4 (T4 antigen) is an essential and specific cell-surface receptor component required for HIV (HTLV-III/LAV) infection; anti-CD4 monoclonal antibodies blocked syncytia formation and pseudotype infection, and productive HIV infection markedly reduced CD4 surface expression, establishing CD4 as the primary viral receptor.","method":"Pseudotype virus infection assay, syncytia formation assay, antibody blocking with 14 anti-CD4 monoclonal antibodies, flow cytometry","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal functional assays with rigorous controls, independently replicated across many subsequent studies","pmids":["6096719"],"is_preprint":false},{"year":1985,"finding":"CD4 (T4) is an integral membrane glycoprotein belonging to the immunoglobulin supergene family, comprising an N-terminal V-like domain, a J-like region, a third extracellular domain, a transmembrane region homologous to MHC class II β-chains, and a highly charged cytoplasmic domain.","method":"cDNA cloning and nucleotide sequencing, deduced protein sequence analysis and structural comparison","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — primary sequence determination with structural domain analysis, foundational paper with >700 citations","pmids":["2990730"],"is_preprint":false},{"year":1986,"finding":"Binding of HIV gp110 (gp120) to the CD4 molecule on T4+ T cell surfaces was directly demonstrated; radioactive surface labeling and co-immunoprecipitation showed that HIV gp110 specifically coprecipitates with the 58K CD4 molecule, and virus binding blocked one CD4 epitope (OKT4A) but not another (OKT4), defining the gp120-binding domain on CD4.","method":"Radioactive surface labeling, co-immunoprecipitation with HIV antibodies and anti-CD4 monoclonal antibodies, blocking and absorption experiments","journal":"Science","confidence":"High","confidence_rationale":"Tier 1-2 — direct biochemical demonstration of CD4-gp110 complex by reciprocal precipitation, replicated widely","pmids":["3001934"],"is_preprint":false},{"year":1986,"finding":"HIV envelope glycoprotein expression induces syncytia and cell death specifically in T4+ cells; syncytium formation requires interaction of envelope-expressing cells with neighboring cells bearing surface CD4 molecules, explaining HIV's specific cytopathic effect on CD4+ cells.","method":"Transfection of HIV env gene, syncytia formation assay in T4+ vs. T4- cell lines, antibody blocking","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — functional genetic approach with defined cellular readout, mechanistic link between CD4-gp120 interaction and cytopathicity","pmids":["3016552"],"is_preprint":false},{"year":1987,"finding":"CD4 directly mediates cell-cell adhesion by binding to MHC class II molecules; CV1 cells expressing recombinant CD4 adhered specifically to class II MHC-bearing B cells, and this adhesion was blocked by anti-class II and anti-CD4 antibodies, demonstrating CD4 as an adhesion molecule independent of T-cell receptor interactions.","method":"SV40-based expression vector, cell adhesion assay, antibody blocking with anti-CD4 and anti-class II MHC antibodies","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — direct functional reconstitution in non-T cells, reciprocal antibody blocking, strong evidence replicated in subsequent studies","pmids":["2823150"],"is_preprint":false},{"year":1987,"finding":"Antigenic stimulation of CD4+ T cell clones causes parallel downregulation of CD4 and CD3 surface expression to ~50% of constitutive levels; PMA-induced downregulation implicates protein kinase C in regulating CD4 expression, suggesting CD4 and CD3 are members of a multimolecular complex mediating T cell activation.","method":"Flow cytometry of T cell clones after antigen stimulation, PMA treatment, staurosporine inhibition","journal":"Journal of Immunology","confidence":"Medium","confidence_rationale":"Tier 3 — functional observation with pharmacological inhibitor, single lab","pmids":["3100638"],"is_preprint":false},{"year":1987,"finding":"A critical region within gp120 (amino acids 397–439) is required for CD4 binding; deletion of 12 amino acids from this region abolished gp120-CD4 interaction, and a single amino acid substitution in this region significantly decreased binding.","method":"Recombinant gp120 production, in vitro mutagenesis (deletion and point mutations), binding assay to recombinant CD4","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with site-directed mutagenesis defining specific gp120 residues for CD4 binding","pmids":["2441877"],"is_preprint":false},{"year":1988,"finding":"CD4 and CD8 T cell surface antigens are physically and functionally associated with the lymphocyte-specific tyrosine kinase p56lck; a large fraction of total cellular lck co-immunoprecipitates with CD4 or CD8, and antibody-mediated cross-linking of CD4 or CD8 specifically modulates p56lck, identifying p56lck as a CD4-associated signaling kinase.","method":"Co-immunoprecipitation with anti-CD4, anti-CD8, and anti-lck antibodies from murine T lymphocytes; antibody cross-linking experiments","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-immunoprecipitation demonstrating physical association, foundational paper with >1400 citations, widely replicated","pmids":["3262426"],"is_preprint":false},{"year":1988,"finding":"A soluble, secreted form of CD4 (sCD4) retains the structural and biological properties of cell-surface CD4, binds to HIV gp110, inhibits virus binding to CD4+ lymphocytes, and potently inhibits HIV infectivity, demonstrating the extracellular domain of CD4 is sufficient for gp120 interaction and viral neutralization.","method":"Recombinant protein expression, binding assay with HIV envelope glycoprotein, viral infectivity inhibition assay","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1-2 — recombinant soluble protein with multiple orthogonal functional assays, >550 citations","pmids":["3257544"],"is_preprint":false},{"year":1991,"finding":"HIV-1 Nef protein downregulates cell-surface CD4 expression via a mechanism independent of serine phosphorylation and protein kinase C; Nef expression in CD4+ lymphocytic lines reduced surface CD4 without altering CD4 RNA or protein steady-state levels, and a CD4 triple serine-to-alanine mutant (non-phosphorylatable, phorbol ester-resistant) was still downregulated by Nef, indicating Nef acts through a distinct pathway.","method":"Retroviral vector expression of Nef, flow cytometry, Western blot, Northern blot, site-directed mutagenesis of CD4 serine residues","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis combined with functional expression system establishing mechanistic distinction from PKC pathway, >700 citations","pmids":["2014052"],"is_preprint":false},{"year":1993,"finding":"CD4 is not strictly required for helper T cell lineage commitment or function; CD4-null mice develop a population of functional CD8- αβ TCR+ T cells that are MHC class II-restricted, produce IFN-γ in response to Leishmania antigens, and control infection, demonstrating CD4 glycoprotein is not absolutely necessary for class II-restricted helper T cell development or effector function.","method":"CD4 gene null mutation in mice, Leishmania infection model, T cell subset analysis, cytokine production assay, MHC restriction assays","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — clean genetic knockout with defined functional and immunological readouts","pmids":["8367726"],"is_preprint":false},{"year":1995,"finding":"Myxoma virus infection triggers CD4 downregulation in rabbit CD4+ T cells via a protein kinase C-independent, lysosomal pathway; early (but not late) viral gene expression was required, NH4Cl (lysosomal inhibitor) blocked CD4 degradation, and p56lck dissociated from CD4 prior to its lysosomal degradation without loss of total p56lck kinase activity.","method":"Flow cytometry, NH4Cl lysosomal inhibitor treatment, staurosporine PKC inhibitor treatment, Western blot, co-immunoprecipitation of p56lck with CD4, kinase activity assay","journal":"Journal of Virology","confidence":"Medium","confidence_rationale":"Tier 2-3 — multiple methods establishing mechanism in a specific viral context, single lab","pmids":["7636966"],"is_preprint":false},{"year":1996,"finding":"A seven-transmembrane G protein-coupled receptor (fusin/CXCR4) serves as a cofactor for HIV-1 fusion and entry together with CD4; recombinant fusin enabled CD4-expressing nonhuman cells to support HIV-1 Env-mediated fusion and infection, and anti-fusin antibodies blocked fusion in normal CD4+ human cells, establishing that HIV-1 entry requires both CD4 and a chemokine receptor co-receptor.","method":"Functional cDNA expression cloning, cell fusion assay, antibody blocking, HIV-1 infection assay in non-permissive cells reconstituted with CD4 and fusin","journal":"Science","confidence":"High","confidence_rationale":"Tier 1-2 — functional reconstitution by co-expression with multiple orthogonal assays, >3400 citations, foundational discovery","pmids":["8629022"],"is_preprint":false},{"year":1996,"finding":"CCR5 and CCR3 function as co-receptors for primary HIV-1 isolates together with CD4; CCR5 expressed with CD4 enabled cell lines resistant to primary HIV-1 to become infected, and CCR3 facilitated infection by a subset of viruses; co-receptor usage depended on the V3 region of gp120, explaining HIV-1 tropism.","method":"Transient transfection of chemokine receptors with CD4 in non-permissive cell lines, HIV-1 infection assays with primary isolates, CCR3 ligand (eotaxin) blocking","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1-2 — functional reconstitution with defined genetic components, widely replicated, >2000 citations","pmids":["8674119"],"is_preprint":false},{"year":1996,"finding":"CD4 binding to gp120 induces a conformational change in gp120 that creates a high-affinity binding site for CCR5; soluble CD4 dramatically increased the affinity of gp120 for CCR5, and a CD4-gp120 complex (but not CD4 alone or gp120 alone at low affinity) specifically interacted with CCR5 and competed with CCR5 natural ligands MIP-1α and MIP-1β.","method":"Competitive binding assay with radiolabeled CCR5 ligands, soluble CD4-gp120 complex formation, antibody neutralization blocking assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — biochemical reconstitution showing CD4-induced conformational activation of gp120 for CCR5 binding, replicated in companion paper","pmids":["8906795"],"is_preprint":false},{"year":1996,"finding":"CD4 binding greatly increases the efficiency of the gp120-CCR5 interaction; competition assays on activated CD4+ T cells and CCR5+/CD4- cells showed CD4 is not absolutely required but greatly enhances gp120 binding to CCR5; neutralizing antibodies to V3 loop and CD4-induced epitopes on gp120 blocked gp120-CCR5 interaction without affecting gp120-CD4 binding.","method":"Competition binding assay using MIP-1β displacement on CD4+ T cells and CD4-negative CCR5+ cells, neutralizing antibody blocking experiments","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — biochemical binding assay with antibody dissection of sequential receptor interactions, independently confirmed in companion paper (PMID:8906795)","pmids":["8906796"],"is_preprint":false},{"year":1998,"finding":"X-ray crystal structure of HIV-1 gp120 core in complex with a two-domain fragment of human CD4 (at 2.5 Å resolution) revealed a cavity-laden CD4-gp120 interface, a conserved chemokine receptor binding site exposed upon CD4 binding, evidence for a CD4-induced conformational change in gp120, and specific mechanisms for immune evasion including glycan shielding.","method":"X-ray crystallography at 2.5 Å resolution of ternary complex (gp120 core + CD4 two-domain fragment + neutralizing antibody Fab)","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — atomic-resolution crystal structure with functional implications, >2400 citations, foundational structural paper","pmids":["9641677"],"is_preprint":false},{"year":1998,"finding":"A conserved gp120 structure adjacent to the V3 loop and containing CD4-induced epitopes is critical for CCR5 binding; gp120 mutants lacking this element failed to bind CCR5 despite retaining CD4 binding, and this element is shared across diverse primate immunodeficiency viruses, defining a conserved CCR5-binding surface on gp120 revealed by CD4 engagement.","method":"Site-directed mutagenesis of gp120, CCR5 binding assays, CD4 binding assays with recombinant mutant proteins","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with defined binding readouts dissecting sequential CD4-dependent CCR5 engagement","pmids":["9632396"],"is_preprint":false}],"current_model":"CD4 is a transmembrane glycoprotein of the immunoglobulin superfamily that functions as the primary receptor for HIV-1 gp120 (binding via its V-like extracellular domain) and as a co-receptor for MHC class II molecules to mediate T cell adhesion and activation; CD4 constitutively associates with the tyrosine kinase p56lck via its cytoplasmic domain to transduce activation signals, is downregulated upon antigen stimulation via PKC-dependent pathways and by HIV-1 Nef via a PKC-independent lysosomal pathway, and sequentially engages HIV-1 gp120 to induce a conformational change that creates a high-affinity binding site for chemokine receptor co-receptors (CCR5 or CXCR4), enabling viral membrane fusion and entry."},"narrative":{"teleology":[{"year":1984,"claim":"Identification of CD4 as the primary cell-surface receptor for HIV answered the fundamental question of how the virus achieves tropism for helper T cells and established the molecular basis of HIV entry.","evidence":"Antibody blocking of syncytia formation and pseudotype infection assays with 14 anti-CD4 monoclonal antibodies","pmids":["6096719"],"confidence":"High","gaps":["Binding site on CD4 for HIV not yet mapped","Whether CD4 alone is sufficient for viral entry was unknown"]},{"year":1985,"claim":"Cloning and sequencing of CD4 cDNA revealed its immunoglobulin superfamily membership and multidomain architecture (V-like, J-like, transmembrane, cytoplasmic), providing the structural framework for understanding its receptor and adhesion functions.","evidence":"cDNA cloning, nucleotide sequencing, and deduced protein domain analysis","pmids":["2990730"],"confidence":"High","gaps":["Three-dimensional structure not yet determined","Function of the cytoplasmic domain unknown"]},{"year":1986,"claim":"Direct biochemical demonstration that HIV gp120 binds to the CD4 molecule, with epitope mapping showing which CD4 epitope is occluded, resolved the molecular identity of the virus–receptor interaction and defined the gp120-binding region on CD4.","evidence":"Radioactive surface labeling and reciprocal co-immunoprecipitation with anti-CD4 and anti-HIV antibodies; parallel work showed gp120-CD4 interaction mediates syncytia and cytopathicity","pmids":["3001934","3016552"],"confidence":"High","gaps":["Precise residues mediating the interaction not yet identified","Whether additional co-receptors are needed for viral entry remained unclear"]},{"year":1987,"claim":"Three concurrent advances established CD4's dual physiological and pathological roles: CD4 mediates MHC class II-dependent cell adhesion independent of TCR, gp120 residues 397–439 are critical for CD4 binding, and antigen stimulation downregulates CD4 via a PKC-dependent pathway linked to a CD3-CD4 multimolecular complex.","evidence":"Reconstitution of CD4-mediated adhesion to class II MHC+ cells in non-T cells; deletion/point mutagenesis of gp120; flow cytometry of T cell clones after antigen/PMA stimulation","pmids":["2823150","2441877","3100638"],"confidence":"High","gaps":["Signaling mechanism downstream of CD4 not defined","Whether CD4 adhesion and TCR signaling are mechanistically coupled was unclear"]},{"year":1988,"claim":"Discovery that CD4 constitutively associates with p56lck via its cytoplasmic domain identified the first intracellular signaling partner of CD4 and explained how CD4 co-engagement amplifies TCR signaling, while soluble CD4 neutralization of HIV proved the extracellular domain alone is sufficient for gp120 binding.","evidence":"Reciprocal co-immunoprecipitation and cross-linking of CD4–p56lck from murine T cells; recombinant soluble CD4 binding and viral inhibition assays","pmids":["3262426","3257544"],"confidence":"High","gaps":["Structural basis of CD4–p56lck interaction not resolved","Whether soluble CD4 could serve as a therapeutic in vivo was untested"]},{"year":1991,"claim":"Demonstration that HIV-1 Nef downregulates CD4 through a PKC-independent mechanism that does not require CD4 cytoplasmic serine phosphorylation established that HIV employs a distinct pathway from physiological CD4 internalization to evade immune recognition.","evidence":"Nef retroviral expression combined with CD4 triple serine-to-alanine mutagenesis, flow cytometry, and Western/Northern blot in lymphocytic lines","pmids":["2014052"],"confidence":"High","gaps":["Endocytic sorting signals and adaptor proteins mediating Nef-induced CD4 degradation not yet identified","Whether Nef directly binds CD4 cytoplasmic tail was unresolved"]},{"year":1993,"claim":"CD4 knockout mice developing functional MHC class II-restricted T cells that control Leishmania infection revealed that CD4 is not absolutely required for helper T cell lineage commitment, redefining CD4 as an enhancer rather than an obligate determinant of class II-restricted immunity.","evidence":"CD4-null mice with Leishmania infection model, T cell subset analysis, cytokine assays, MHC restriction experiments","pmids":["8367726"],"confidence":"High","gaps":["Quantitative contribution of CD4 to thymic positive selection efficiency not determined","Compensatory mechanisms in CD4-null mice not characterized"]},{"year":1995,"claim":"Myxoma virus-induced CD4 downregulation via a lysosomal, PKC-independent pathway with prior dissociation of p56lck provided a second viral system demonstrating that lysosomal targeting of CD4 is a convergent immune evasion strategy.","evidence":"Flow cytometry, NH4Cl lysosomal inhibition, staurosporine PKC inhibition, co-immunoprecipitation of p56lck–CD4, kinase assay","pmids":["7636966"],"confidence":"Medium","gaps":["Whether the myxoma and HIV Nef pathways share sorting machinery was unknown","Viral protein responsible for triggering CD4 degradation not identified"]},{"year":1996,"claim":"The discovery that HIV-1 entry requires chemokine receptor co-receptors (CXCR4 for T-tropic and CCR5 for M-tropic strains) in addition to CD4, and that CD4 binding to gp120 induces a conformational change creating a high-affinity CCR5-binding site, resolved the long-standing puzzle of why CD4 expression alone was insufficient for infection and established the sequential receptor model of HIV entry.","evidence":"Functional reconstitution of HIV fusion/infection by co-expression of CD4 with fusin/CXCR4 or CCR5 in non-permissive cells; competitive binding assays showing CD4-induced enhancement of gp120–CCR5 affinity; antibody blocking of co-receptor engagement","pmids":["8629022","8674119","8906795","8906796"],"confidence":"High","gaps":["Structural basis of the CD4-induced conformational change at atomic resolution not yet available","Kinetics of the sequential CD4→co-receptor engagement in membrane context not measured"]},{"year":1998,"claim":"The 2.5 Å crystal structure of the gp120 core complexed with CD4 revealed the atomic details of the CD4–gp120 interface, the conserved CCR5-binding surface exposed by CD4 engagement, and immune evasion mechanisms including glycan shielding, providing a structural foundation for understanding HIV tropism and vaccine design.","evidence":"X-ray crystallography of ternary gp120 core–CD4 two-domain fragment–Fab complex; complementary mutagenesis defining the conserved CCR5-binding element adjacent to V3","pmids":["9641677","9632396"],"confidence":"High","gaps":["Full-length trimeric Env–CD4 complex structure not available","Dynamics of CD4-induced conformational rearrangement not captured by crystallography"]},{"year":null,"claim":"Key unresolved questions include the structural basis of the CD4 cytoplasmic domain–p56lck interaction at atomic resolution, the precise endocytic sorting machinery that HIV Nef and other viral proteins co-opt to target CD4 for lysosomal degradation, and how CD4 co-engagement quantitatively modulates TCR signaling thresholds during thymic selection and peripheral activation.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No high-resolution structure of CD4 cytoplasmic domain–p56lck complex","Nef-recruited endocytic adaptor complex incompletely defined in the timeline","Quantitative contribution of CD4 to TCR signaling kinetics not measured"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0001618","term_label":"virus receptor activity","supporting_discovery_ids":[0,2,3,12,13]},{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[4]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[7,14]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,4,5]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[9,11]}],"pathway":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[8]}],"complexes":[],"partners":["LCK","CCR5","CXCR4"],"other_free_text":[]},"mechanistic_narrative":"CD4 is a transmembrane glycoprotein of the immunoglobulin superfamily that functions as a co-receptor for MHC class II molecules to mediate T cell adhesion and as the primary receptor for HIV-1, while transducing activation signals through its constitutive association with the tyrosine kinase p56lck [PMID:2823150, PMID:3262426]. Its V-like extracellular domain directly binds HIV-1 gp120, and this interaction induces a conformational change in gp120 that creates a high-affinity binding site for chemokine receptor co-receptors CCR5 or CXCR4, enabling viral membrane fusion and entry [PMID:8906795, PMID:9641677, PMID:8629022, PMID:8674119]. CD4 surface expression is downregulated upon antigen stimulation via a PKC-dependent pathway and independently by HIV-1 Nef through a PKC-independent lysosomal degradation route that proceeds after dissociation of p56lck [PMID:3100638, PMID:2014052, PMID:7636966]. CD4-null mice retain functional MHC class II-restricted helper T cells, demonstrating that CD4 enhances but is not absolutely required for helper lineage commitment or effector function [PMID:8367726]."},"prefetch_data":{"uniprot":{"accession":"P01730","full_name":"T-cell surface glycoprotein CD4","aliases":["T-cell surface antigen T4/Leu-3"],"length_aa":458,"mass_kda":51.1,"function":"Integral membrane glycoprotein that plays an essential role in the immune response and serves multiple functions in responses against both external and internal offenses. In T-cells, functions primarily as a coreceptor for MHC class II molecule:peptide complex. The antigens presented by class II peptides are derived from extracellular proteins while class I peptides are derived from cytosolic proteins. Interacts simultaneously with the T-cell receptor (TCR) and the MHC class II presented by antigen presenting cells (APCs). In turn, recruits the Src kinase LCK to the vicinity of the TCR-CD3 complex. LCK then initiates different intracellular signaling pathways by phosphorylating various substrates ultimately leading to lymphokine production, motility, adhesion and activation of T-helper cells. In other cells such as macrophages or NK cells, plays a role in differentiation/activation, cytokine expression and cell migration in a TCR/LCK-independent pathway. Participates in the development of T-helper cells in the thymus and triggers the differentiation of monocytes into functional mature macrophages (Microbial infection) Primary receptor for human immunodeficiency virus-1 (HIV-1) (PubMed:12089508, PubMed:16331979, PubMed:2214026, PubMed:9641677). Down-regulated by HIV-1 Vpu (PubMed:17346169). Acts as a receptor for Human Herpes virus 7/HHV-7 (PubMed:7909607)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/P01730/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CD4","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/CD4","total_profiled":1310},"omim":[{"mim_id":"621523","title":"SPONDYLOCOSTAL DYSOSTOSIS 7, AUTOSOMAL RECESSIVE; SCDO7","url":"https://www.omim.org/entry/621523"},{"mim_id":"621405","title":"IMMUNODEFICIENCY 134 (EPSTEIN-BARR VIRUS-SPECIFIC); IMD134","url":"https://www.omim.org/entry/621405"},{"mim_id":"621254","title":"IMMUNODEFICIENCY 133 WITH ECTODERMAL DYSPLASIA WITH OR WITHOUT PERIPHERAL NEUROPATHY; IMD133","url":"https://www.omim.org/entry/621254"},{"mim_id":"621235","title":"AUTOIMMUNE DISEASE, MULTISYSTEM, INFANTILE-ONSET, 5; ADMIO5","url":"https://www.omim.org/entry/621235"},{"mim_id":"621234","title":"ICHAD SYNDROME; ICHAD","url":"https://www.omim.org/entry/621234"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Plasma membrane","reliability":"Supported"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"liver","ntpm":143.1},{"tissue":"lymphoid tissue","ntpm":204.2},{"tissue":"parathyroid gland","ntpm":163.6}],"url":"https://www.proteinatlas.org/search/CD4"},"hgnc":{"alias_symbol":["T4","Leu-3"],"prev_symbol":[]},"alphafold":{"accession":"P01730","domains":[{"cath_id":"2.60.40.10","chopping":"26-121","consensus_level":"high","plddt":93.5803,"start":26,"end":121},{"cath_id":"2.60.40.10","chopping":"128-201","consensus_level":"high","plddt":92.4285,"start":128,"end":201},{"cath_id":"2.60.40.10","chopping":"207-315","consensus_level":"medium","plddt":93.1041,"start":207,"end":315},{"cath_id":"2.60.40.10","chopping":"318-385","consensus_level":"medium","plddt":92.0228,"start":318,"end":385}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P01730","model_url":"https://alphafold.ebi.ac.uk/files/AF-P01730-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P01730-F1-predicted_aligned_error_v6.png","plddt_mean":85.25},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CD4","jax_strain_url":"https://www.jax.org/strain/search?query=CD4"},"sequence":{"accession":"P01730","fasta_url":"https://rest.uniprot.org/uniprotkb/P01730.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P01730/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P01730"}},"corpus_meta":[{"pmid":"6096719","id":"PMC_6096719","title":"The CD4 (T4) antigen is an essential component of the receptor for the AIDS retrovirus.","date":"1984","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/6096719","citation_count":3268,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12626685","id":"PMC_12626685","title":"Bacteriophage T4 genome.","date":"2003","source":"Microbiology and molecular biology reviews : MMBR","url":"https://pubmed.ncbi.nlm.nih.gov/12626685","citation_count":622,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3257544","id":"PMC_3257544","title":"A soluble form of CD4 (T4) protein inhibits AIDS virus infection.","date":"1988","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/3257544","citation_count":552,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"28280496","id":"PMC_28280496","title":"CD4 CTL, a Cytotoxic Subset of CD4+ T Cells, Their Differentiation and Function.","date":"2017","source":"Frontiers in 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antibody.","date":"1998","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/9641677","citation_count":2413,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"8674119","id":"PMC_8674119","title":"The beta-chemokine receptors CCR3 and CCR5 facilitate infection by primary HIV-1 isolates.","date":"1996","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/8674119","citation_count":2053,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"12646921","id":"PMC_12646921","title":"Antibody neutralization and escape by HIV-1.","date":"2003","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/12646921","citation_count":2019,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"8674120","id":"PMC_8674120","title":"A dual-tropic primary HIV-1 isolate that uses fusin and the beta-chemokine receptors CKR-5, CKR-3, and CKR-2b as fusion cofactors.","date":"1996","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/8674120","citation_count":1669,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"20616233","id":"PMC_20616233","title":"Rational design of envelope identifies broadly neutralizing human monoclonal antibodies to HIV-1.","date":"2010","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/20616233","citation_count":1511,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"3262426","id":"PMC_3262426","title":"The CD4 and CD8 T cell surface antigens are associated with the internal membrane tyrosine-protein kinase p56lck.","date":"1988","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/3262426","citation_count":1497,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"12477932","id":"PMC_12477932","title":"Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences.","date":"2002","source":"Proceedings of the National Academy of 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networks.","date":"2017","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/28514442","citation_count":1085,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"8906795","id":"PMC_8906795","title":"CD4-induced interaction of primary HIV-1 gp120 glycoproteins with the chemokine receptor CCR-5.","date":"1996","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/8906795","citation_count":1074,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"3001934","id":"PMC_3001934","title":"Binding of HTLV-III/LAV to T4+ T cells by a complex of the 110K viral protein and the T4 molecule.","date":"1986","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/3001934","citation_count":1036,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"20616231","id":"PMC_20616231","title":"Structural basis for broad and potent neutralization of HIV-1 by antibody VRC01.","date":"2010","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/20616231","citation_count":1006,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"8551569","id":"PMC_8551569","title":"Human monoclonal antibody 2G12 defines a distinctive neutralization epitope on the gp120 glycoprotein of human immunodeficiency virus type 1.","date":"1996","source":"Journal of virology","url":"https://pubmed.ncbi.nlm.nih.gov/8551569","citation_count":988,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"8906796","id":"PMC_8906796","title":"CD4-dependent, antibody-sensitive interactions between HIV-1 and its co-receptor CCR-5.","date":"1996","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/8906796","citation_count":974,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"18156204","id":"PMC_18156204","title":"Interleukin-17 production in central nervous system-infiltrating T cells and glial cells is associated with active disease in multiple sclerosis.","date":"2007","source":"The American journal of pathology","url":"https://pubmed.ncbi.nlm.nih.gov/18156204","citation_count":927,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"2823150","id":"PMC_2823150","title":"Interaction between CD4 and class II MHC molecules mediates cell adhesion.","date":"1987","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/2823150","citation_count":879,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"2441877","id":"PMC_2441877","title":"Delineation of a region of the human immunodeficiency virus type 1 gp120 glycoprotein critical for interaction with the CD4 receptor.","date":"1987","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/2441877","citation_count":876,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"24068931","id":"PMC_24068931","title":"A next-generation cleaved, soluble HIV-1 Env trimer, BG505 SOSIP.664 gp140, expresses multiple epitopes for broadly neutralizing but not non-neutralizing antibodies.","date":"2013","source":"PLoS pathogens","url":"https://pubmed.ncbi.nlm.nih.gov/24068931","citation_count":843,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"3016552","id":"PMC_3016552","title":"Role of the HTLV-III/LAV envelope in syncytium formation and cytopathicity.","date":"1986","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/3016552","citation_count":809,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"3257102","id":"PMC_3257102","title":"In vitro mutagenesis identifies a region within the envelope gene of the human immunodeficiency virus that is critical for infectivity.","date":"1988","source":"Journal of virology","url":"https://pubmed.ncbi.nlm.nih.gov/3257102","citation_count":766,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"24179159","id":"PMC_24179159","title":"Crystal structure of a soluble cleaved HIV-1 envelope trimer.","date":"2013","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/24179159","citation_count":729,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"22113616","id":"PMC_22113616","title":"Structure of HIV-1 gp120 V1/V2 domain with broadly neutralizing antibody PG9.","date":"2011","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/22113616","citation_count":726,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"9632396","id":"PMC_9632396","title":"A conserved HIV gp120 glycoprotein structure involved in chemokine receptor binding.","date":"1998","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/9632396","citation_count":726,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"2014052","id":"PMC_2014052","title":"Serine phosphorylation-independent downregulation of cell-surface CD4 by nef.","date":"1991","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/2014052","citation_count":716,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"33961781","id":"PMC_33961781","title":"Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.","date":"2021","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/33961781","citation_count":705,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"2990730","id":"PMC_2990730","title":"The isolation and nucleotide sequence of a cDNA encoding the T cell surface protein T4: a new member of the immunoglobulin gene family.","date":"1985","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/2990730","citation_count":700,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"15616015","id":"PMC_15616015","title":"Defective suppressor function in CD4(+)CD25(+) T-cells from patients with type 1 diabetes.","date":"2005","source":"Diabetes","url":"https://pubmed.ncbi.nlm.nih.gov/15616015","citation_count":679,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"18668044","id":"PMC_18668044","title":"Molecular architecture of native HIV-1 gp120 trimers.","date":"2008","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/18668044","citation_count":671,"is_preprint":false,"source_track":"gene2pubmed"}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":41435,"output_tokens":2571,"usd":0.081435},"stage2":{"model":"claude-opus-4-6","input_tokens":5930,"output_tokens":3099,"usd":0.160688},"total_usd":0.524541,"stage1_batch_id":"msgbatch_011kfMfE81o6CTcyhVUqumuW","stage2_batch_id":"msgbatch_01DTputS6WNkH745x1nVSoBo","note":"batch pricing = 50% of standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":53539,"output_tokens":4676,"usd":0.115378},"round2_rules_fired":"R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":8087,"output_tokens":2837,"usd":0.16704}},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1984,\n      \"finding\": \"CD4 (T4 antigen) is an essential and specific cell-surface receptor component for HIV (HTLV-III/LAV-1); anti-CD4 monoclonal antibodies block syncytia formation and pseudotype infection, and productive HIV infection markedly reduces cell-surface CD4 expression.\",\n      \"method\": \"Pseudotype infection assay, syncytia formation assay, anti-CD4 antibody blocking, flow cytometry\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal functional assays with 14 independent anti-CD4 antibodies; foundational paper with 3268 citations, replicated widely\",\n      \"pmids\": [\"6096719\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"A soluble, secreted form of CD4 (sCD4) retains structural and biological properties of membrane CD4, binds HIV envelope glycoprotein gp110, blocks virus binding to CD4+ lymphocytes, and inhibits HIV infectivity.\",\n      \"method\": \"Recombinant protein expression, binding assay, virus infectivity inhibition assay\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution of receptor-ligand binding with functional inhibition; 552 citations\",\n      \"pmids\": [\"3257544\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"Antigenic stimulation of human CD4+ T cell clones causes ~50% downregulation of cell-surface CD4 expression, paralleling CD3 downregulation, suggesting both are part of a multimolecular activation complex; PMA-induced downregulation implicates protein kinase C in regulating CD4 surface levels.\",\n      \"method\": \"Flow cytometry of human T cell clones after antigen stimulation and PMA treatment\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single-lab flow cytometry study with functional correlation but no direct biochemical mechanism\",\n      \"pmids\": [\"3100638\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CD4-null mice develop MHC class II-restricted, IFN-γ-producing CD8-negative αβ TCR+ T helper cells capable of controlling Leishmania infection, demonstrating that T lymphocyte lineage commitment and helper function can occur independently of CD4 expression.\",\n      \"method\": \"CD4 gene knockout mice, Leishmania infection model, flow cytometry, cytokine assays\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic knockout with specific phenotypic readout and functional rescue; 237 citations\",\n      \"pmids\": [\"8367726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Myxoma virus infection of rabbit CD4+ T lymphocytes causes protein kinase C-independent, lysosome-mediated CD4 downregulation, with dissociation of the CD4-associated tyrosine kinase p56lck from CD4 prior to CD4 degradation; total p56lck kinase activity is unaffected.\",\n      \"method\": \"Flow cytometry, NH4Cl lysosomal inhibition, co-immunoprecipitation of p56lck with CD4, kinase activity assay\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (co-IP, kinase assay, inhibitor studies) in a single-lab study\",\n      \"pmids\": [\"7636966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"TLR7 engagement in human CD4+ T cells induces intracellular calcium flux and activates an NFATc2-dependent anergic gene-expression program, rendering T cells unresponsive; silencing TLR7 in HIV-1-infected CD4+ T cells markedly decreases infection frequency and restores T cell responsiveness.\",\n      \"method\": \"TLR7 ligation, calcium flux assay, gene expression profiling, TLR7 siRNA knockdown, HIV infection assay\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (calcium flux, transcriptomics, siRNA KD, infection assay) establishing pathway placement\",\n      \"pmids\": [\"25401424\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CD4+ T cells expressing choline acetyltransferase (ChAT) synthesize acetylcholine and relay neural signals to regulate blood pressure; mice lacking ChAT in CD4+ cells have elevated arterial blood pressure, and ChAT-overexpressing Jurkat T cells lower blood pressure when infused, acting through increased phosphorylation of endothelial nitric oxide synthase and nitric oxide release.\",\n      \"method\": \"CD4-specific ChAT conditional knockout mice, blood pressure measurement, endothelial co-culture, phospho-eNOS western blot, nitrate/nitrite measurement\",\n      \"journal\": \"Nature biotechnology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific KO with defined physiological phenotype plus mechanistic in vitro rescue; multiple orthogonal methods\",\n      \"pmids\": [\"27617738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Protein kinase D (PKD2/PKD3) is required for CD4+ thymocyte positive selection; PKD is activated in CD4+CD8+ double-positive thymocytes by positively selecting peptides and directly phosphorylates SHP-1; a phosphorylation-defective SHP-1 (S557A) mutant impairs CD4+ T cell generation.\",\n      \"method\": \"T cell-specific PKD2/PKD3 double-knockout mice, phosphoproteomic analysis, SHP-1 mutagenesis, thymocyte development assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic KO plus phosphoproteomic substrate identification plus mutagenesis validation\",\n      \"pmids\": [\"27670070\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"At the human RORC locus in CD4+ T cells, TCR stimulation induces NFAT binding to regulatory elements, recruiting P300/CBP acetyltransferase and promoting permissive histone modifications; NFAT cooperates with NF-κB to activate RORγt transcription, and CRISPR/Cas9 deletion of these elements impairs RORγt expression.\",\n      \"method\": \"CRISPR/Cas9 genomic deletion, chromatin immunoprecipitation, NFAT/NF-κB reporter assays, cyclosporin A inhibition\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — CRISPR deletion plus ChIP plus pharmacological inhibition; multiple orthogonal methods in single study\",\n      \"pmids\": [\"31619674\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Upon activation of human CD4+ T cells, CD74 is upregulated, post-translationally modified by chondroitin sulfate, and traffics to the cell surface, where it forms heterocomplexes with CXCR4 and acts as a functional receptor for MIF, mediating MIF-induced CD4+ T cell migration; MIF treatment induces internalization of CD74/CXCR4 complexes.\",\n      \"method\": \"Flow cytometry, Western blot, proximity ligation assay, 3D live-cell imaging, receptor pathway-specific inhibitors\",\n      \"journal\": \"Cellular and molecular life sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — proximity ligation assay plus functional migration assays plus pharmacological dissection; single-lab study\",\n      \"pmids\": [\"38992165\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"In LEC mutant rats, T cell maturation is specifically arrested at the CD4+CD8+ to CD4+CD8- transition, demonstrating that CD4 single-positive thymocyte development requires a distinct genetic step not shared with CD8 lineage commitment, and is independent of TCR/CD3 or MHC class II expression defects.\",\n      \"method\": \"Flow cytometry of thymic subsets, TCR/CD3 and MHC class II expression analysis in LEC rats\",\n      \"journal\": \"Journal of experimental medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic mutant model with specific developmental phenotype and exclusion of alternative mechanisms\",\n      \"pmids\": [\"2147947\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CD4 is a T lymphocyte co-receptor that binds HIV as an essential entry receptor (blocked by soluble CD4), associates with the tyrosine kinase p56lck (which dissociates during viral-induced lysosomal degradation of CD4), undergoes PKC-dependent and -independent surface downregulation upon antigen or viral stimulation, facilitates MHC class II-restricted T helper cell function (though helper function can occur without CD4 in knockout mice), and participates in diverse signaling contexts including TLR7/NFATc2-mediated anergy, PKD/SHP-1-dependent positive thymic selection, NFAT/NF-κB-driven RORγt transcription, and ChAT-mediated acetylcholine synthesis for blood pressure regulation.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper first, then extract discoveries only from KEEP papers.\n\n**Classification:**\n- Papers about bacteriophage T4 (genome, lysozyme, DNA polymerase, packaging, etc.) → EXCLUDE (symbol collision - T4 bacteriophage, not the human CD4 gene)\n- Papers about human CD4 protein (T cell surface receptor, HIV receptor, MHC class II interaction, p56lck association, etc.) → KEEP\n- Papers about CD4+ T cell biology with mechanistic findings about CD4 protein itself → KEEP\n- Papers about CD4+ T cell subsets/differentiation without mechanistic findings about CD4 protein → EXCLUDE (no mechanistic content about CD4 protein itself)\n\n**KEEP papers:** PMID:6096719, PMID:3257544, PMID:8629022, PMID:9641677, PMID:8674119, PMID:8674120, PMID:3262426, PMID:3001934, PMID:8906795, PMID:8906796, PMID:3016552, PMID:2014052, PMID:2823150, PMID:2441877, PMID:2990730, PMID:3100638, PMID:7636966, PMID:8367726\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1984,\n      \"finding\": \"CD4 (T4 antigen) is an essential and specific cell-surface receptor component required for HIV (HTLV-III/LAV) infection; anti-CD4 monoclonal antibodies blocked syncytia formation and pseudotype infection, and productive HIV infection markedly reduced CD4 surface expression, establishing CD4 as the primary viral receptor.\",\n      \"method\": \"Pseudotype virus infection assay, syncytia formation assay, antibody blocking with 14 anti-CD4 monoclonal antibodies, flow cytometry\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal functional assays with rigorous controls, independently replicated across many subsequent studies\",\n      \"pmids\": [\"6096719\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"CD4 (T4) is an integral membrane glycoprotein belonging to the immunoglobulin supergene family, comprising an N-terminal V-like domain, a J-like region, a third extracellular domain, a transmembrane region homologous to MHC class II β-chains, and a highly charged cytoplasmic domain.\",\n      \"method\": \"cDNA cloning and nucleotide sequencing, deduced protein sequence analysis and structural comparison\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — primary sequence determination with structural domain analysis, foundational paper with >700 citations\",\n      \"pmids\": [\"2990730\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1986,\n      \"finding\": \"Binding of HIV gp110 (gp120) to the CD4 molecule on T4+ T cell surfaces was directly demonstrated; radioactive surface labeling and co-immunoprecipitation showed that HIV gp110 specifically coprecipitates with the 58K CD4 molecule, and virus binding blocked one CD4 epitope (OKT4A) but not another (OKT4), defining the gp120-binding domain on CD4.\",\n      \"method\": \"Radioactive surface labeling, co-immunoprecipitation with HIV antibodies and anti-CD4 monoclonal antibodies, blocking and absorption experiments\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct biochemical demonstration of CD4-gp110 complex by reciprocal precipitation, replicated widely\",\n      \"pmids\": [\"3001934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1986,\n      \"finding\": \"HIV envelope glycoprotein expression induces syncytia and cell death specifically in T4+ cells; syncytium formation requires interaction of envelope-expressing cells with neighboring cells bearing surface CD4 molecules, explaining HIV's specific cytopathic effect on CD4+ cells.\",\n      \"method\": \"Transfection of HIV env gene, syncytia formation assay in T4+ vs. T4- cell lines, antibody blocking\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional genetic approach with defined cellular readout, mechanistic link between CD4-gp120 interaction and cytopathicity\",\n      \"pmids\": [\"3016552\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"CD4 directly mediates cell-cell adhesion by binding to MHC class II molecules; CV1 cells expressing recombinant CD4 adhered specifically to class II MHC-bearing B cells, and this adhesion was blocked by anti-class II and anti-CD4 antibodies, demonstrating CD4 as an adhesion molecule independent of T-cell receptor interactions.\",\n      \"method\": \"SV40-based expression vector, cell adhesion assay, antibody blocking with anti-CD4 and anti-class II MHC antibodies\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct functional reconstitution in non-T cells, reciprocal antibody blocking, strong evidence replicated in subsequent studies\",\n      \"pmids\": [\"2823150\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"Antigenic stimulation of CD4+ T cell clones causes parallel downregulation of CD4 and CD3 surface expression to ~50% of constitutive levels; PMA-induced downregulation implicates protein kinase C in regulating CD4 expression, suggesting CD4 and CD3 are members of a multimolecular complex mediating T cell activation.\",\n      \"method\": \"Flow cytometry of T cell clones after antigen stimulation, PMA treatment, staurosporine inhibition\",\n      \"journal\": \"Journal of Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — functional observation with pharmacological inhibitor, single lab\",\n      \"pmids\": [\"3100638\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"A critical region within gp120 (amino acids 397–439) is required for CD4 binding; deletion of 12 amino acids from this region abolished gp120-CD4 interaction, and a single amino acid substitution in this region significantly decreased binding.\",\n      \"method\": \"Recombinant gp120 production, in vitro mutagenesis (deletion and point mutations), binding assay to recombinant CD4\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with site-directed mutagenesis defining specific gp120 residues for CD4 binding\",\n      \"pmids\": [\"2441877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"CD4 and CD8 T cell surface antigens are physically and functionally associated with the lymphocyte-specific tyrosine kinase p56lck; a large fraction of total cellular lck co-immunoprecipitates with CD4 or CD8, and antibody-mediated cross-linking of CD4 or CD8 specifically modulates p56lck, identifying p56lck as a CD4-associated signaling kinase.\",\n      \"method\": \"Co-immunoprecipitation with anti-CD4, anti-CD8, and anti-lck antibodies from murine T lymphocytes; antibody cross-linking experiments\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-immunoprecipitation demonstrating physical association, foundational paper with >1400 citations, widely replicated\",\n      \"pmids\": [\"3262426\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"A soluble, secreted form of CD4 (sCD4) retains the structural and biological properties of cell-surface CD4, binds to HIV gp110, inhibits virus binding to CD4+ lymphocytes, and potently inhibits HIV infectivity, demonstrating the extracellular domain of CD4 is sufficient for gp120 interaction and viral neutralization.\",\n      \"method\": \"Recombinant protein expression, binding assay with HIV envelope glycoprotein, viral infectivity inhibition assay\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — recombinant soluble protein with multiple orthogonal functional assays, >550 citations\",\n      \"pmids\": [\"3257544\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"HIV-1 Nef protein downregulates cell-surface CD4 expression via a mechanism independent of serine phosphorylation and protein kinase C; Nef expression in CD4+ lymphocytic lines reduced surface CD4 without altering CD4 RNA or protein steady-state levels, and a CD4 triple serine-to-alanine mutant (non-phosphorylatable, phorbol ester-resistant) was still downregulated by Nef, indicating Nef acts through a distinct pathway.\",\n      \"method\": \"Retroviral vector expression of Nef, flow cytometry, Western blot, Northern blot, site-directed mutagenesis of CD4 serine residues\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis combined with functional expression system establishing mechanistic distinction from PKC pathway, >700 citations\",\n      \"pmids\": [\"2014052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"CD4 is not strictly required for helper T cell lineage commitment or function; CD4-null mice develop a population of functional CD8- αβ TCR+ T cells that are MHC class II-restricted, produce IFN-γ in response to Leishmania antigens, and control infection, demonstrating CD4 glycoprotein is not absolutely necessary for class II-restricted helper T cell development or effector function.\",\n      \"method\": \"CD4 gene null mutation in mice, Leishmania infection model, T cell subset analysis, cytokine production assay, MHC restriction assays\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic knockout with defined functional and immunological readouts\",\n      \"pmids\": [\"8367726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Myxoma virus infection triggers CD4 downregulation in rabbit CD4+ T cells via a protein kinase C-independent, lysosomal pathway; early (but not late) viral gene expression was required, NH4Cl (lysosomal inhibitor) blocked CD4 degradation, and p56lck dissociated from CD4 prior to its lysosomal degradation without loss of total p56lck kinase activity.\",\n      \"method\": \"Flow cytometry, NH4Cl lysosomal inhibitor treatment, staurosporine PKC inhibitor treatment, Western blot, co-immunoprecipitation of p56lck with CD4, kinase activity assay\",\n      \"journal\": \"Journal of Virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — multiple methods establishing mechanism in a specific viral context, single lab\",\n      \"pmids\": [\"7636966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"A seven-transmembrane G protein-coupled receptor (fusin/CXCR4) serves as a cofactor for HIV-1 fusion and entry together with CD4; recombinant fusin enabled CD4-expressing nonhuman cells to support HIV-1 Env-mediated fusion and infection, and anti-fusin antibodies blocked fusion in normal CD4+ human cells, establishing that HIV-1 entry requires both CD4 and a chemokine receptor co-receptor.\",\n      \"method\": \"Functional cDNA expression cloning, cell fusion assay, antibody blocking, HIV-1 infection assay in non-permissive cells reconstituted with CD4 and fusin\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — functional reconstitution by co-expression with multiple orthogonal assays, >3400 citations, foundational discovery\",\n      \"pmids\": [\"8629022\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"CCR5 and CCR3 function as co-receptors for primary HIV-1 isolates together with CD4; CCR5 expressed with CD4 enabled cell lines resistant to primary HIV-1 to become infected, and CCR3 facilitated infection by a subset of viruses; co-receptor usage depended on the V3 region of gp120, explaining HIV-1 tropism.\",\n      \"method\": \"Transient transfection of chemokine receptors with CD4 in non-permissive cell lines, HIV-1 infection assays with primary isolates, CCR3 ligand (eotaxin) blocking\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — functional reconstitution with defined genetic components, widely replicated, >2000 citations\",\n      \"pmids\": [\"8674119\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"CD4 binding to gp120 induces a conformational change in gp120 that creates a high-affinity binding site for CCR5; soluble CD4 dramatically increased the affinity of gp120 for CCR5, and a CD4-gp120 complex (but not CD4 alone or gp120 alone at low affinity) specifically interacted with CCR5 and competed with CCR5 natural ligands MIP-1α and MIP-1β.\",\n      \"method\": \"Competitive binding assay with radiolabeled CCR5 ligands, soluble CD4-gp120 complex formation, antibody neutralization blocking assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — biochemical reconstitution showing CD4-induced conformational activation of gp120 for CCR5 binding, replicated in companion paper\",\n      \"pmids\": [\"8906795\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"CD4 binding greatly increases the efficiency of the gp120-CCR5 interaction; competition assays on activated CD4+ T cells and CCR5+/CD4- cells showed CD4 is not absolutely required but greatly enhances gp120 binding to CCR5; neutralizing antibodies to V3 loop and CD4-induced epitopes on gp120 blocked gp120-CCR5 interaction without affecting gp120-CD4 binding.\",\n      \"method\": \"Competition binding assay using MIP-1β displacement on CD4+ T cells and CD4-negative CCR5+ cells, neutralizing antibody blocking experiments\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — biochemical binding assay with antibody dissection of sequential receptor interactions, independently confirmed in companion paper (PMID:8906795)\",\n      \"pmids\": [\"8906796\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"X-ray crystal structure of HIV-1 gp120 core in complex with a two-domain fragment of human CD4 (at 2.5 Å resolution) revealed a cavity-laden CD4-gp120 interface, a conserved chemokine receptor binding site exposed upon CD4 binding, evidence for a CD4-induced conformational change in gp120, and specific mechanisms for immune evasion including glycan shielding.\",\n      \"method\": \"X-ray crystallography at 2.5 Å resolution of ternary complex (gp120 core + CD4 two-domain fragment + neutralizing antibody Fab)\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — atomic-resolution crystal structure with functional implications, >2400 citations, foundational structural paper\",\n      \"pmids\": [\"9641677\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"A conserved gp120 structure adjacent to the V3 loop and containing CD4-induced epitopes is critical for CCR5 binding; gp120 mutants lacking this element failed to bind CCR5 despite retaining CD4 binding, and this element is shared across diverse primate immunodeficiency viruses, defining a conserved CCR5-binding surface on gp120 revealed by CD4 engagement.\",\n      \"method\": \"Site-directed mutagenesis of gp120, CCR5 binding assays, CD4 binding assays with recombinant mutant proteins\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with defined binding readouts dissecting sequential CD4-dependent CCR5 engagement\",\n      \"pmids\": [\"9632396\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CD4 is a transmembrane glycoprotein of the immunoglobulin superfamily that functions as the primary receptor for HIV-1 gp120 (binding via its V-like extracellular domain) and as a co-receptor for MHC class II molecules to mediate T cell adhesion and activation; CD4 constitutively associates with the tyrosine kinase p56lck via its cytoplasmic domain to transduce activation signals, is downregulated upon antigen stimulation via PKC-dependent pathways and by HIV-1 Nef via a PKC-independent lysosomal pathway, and sequentially engages HIV-1 gp120 to induce a conformational change that creates a high-affinity binding site for chemokine receptor co-receptors (CCR5 or CXCR4), enabling viral membrane fusion and entry.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"CD4 is a cell-surface glycoprotein that functions as a co-receptor on T lymphocytes, serving dual roles in adaptive immunity and as the primary entry receptor for HIV. CD4 facilitates MHC class II-restricted T helper cell responses, although CD4-null mice demonstrate that helper lineage commitment and effector function can proceed independently of CD4 expression [PMID:8367726]; during thymic positive selection, CD4 signaling engages protein kinase D, which phosphorylates SHP-1 to promote the CD4 single-positive lineage transition [PMID:27670070, PMID:2147947]. As the essential HIV receptor, CD4 mediates viral binding and entry—functions blocked by anti-CD4 antibodies and soluble CD4 constructs that compete for gp120 engagement—while HIV and other viruses induce CD4 surface downregulation through both PKC-dependent and lysosome-dependent pathways involving dissociation of the associated kinase p56lck [PMID:6096719, PMID:3257544, PMID:7636966]. In activated CD4+ T cells, TCR-triggered NFAT and NF-κB cooperate to drive RORγt transcription for Th17 differentiation [PMID:31619674], TLR7 engagement activates an NFATc2-dependent anergy program that increases susceptibility to HIV-1 infection [PMID:25401424], and a ChAT-expressing CD4+ T cell subset synthesizes acetylcholine to regulate blood pressure via endothelial nitric oxide signaling [PMID:27617738].\",\n  \"teleology\": [\n    {\n      \"year\": 1984,\n      \"claim\": \"Identifying CD4 as the HIV receptor established that a defined T cell surface molecule mediates viral tropism, opening the question of how this interaction could be therapeutically targeted.\",\n      \"evidence\": \"Anti-CD4 monoclonal antibody blocking of syncytia formation and pseudotype HIV infection, with flow cytometric demonstration of CD4 downregulation upon productive infection\",\n      \"pmids\": [\"6096719\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Binding interface between CD4 and HIV envelope not yet mapped\",\n        \"Mechanism of virus-induced CD4 downregulation unknown\",\n        \"Whether CD4 alone is sufficient for viral entry not established\"\n      ]\n    },\n    {\n      \"year\": 1987,\n      \"claim\": \"Demonstrating that antigen stimulation downregulates surface CD4 in parallel with CD3, and that PMA mimics this effect, placed CD4 within a PKC-regulated activation complex rather than treating it as a static receptor.\",\n      \"evidence\": \"Flow cytometry of human CD4+ T cell clones after antigen and PMA stimulation\",\n      \"pmids\": [\"3100638\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Direct PKC phosphorylation of CD4 cytoplasmic domain not shown\",\n        \"Whether CD4 downregulation is required for or incidental to T cell activation unclear\",\n        \"Intracellular trafficking route of internalized CD4 not defined\"\n      ]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Showing that soluble CD4 binds gp120 and blocks HIV infection proved that the extracellular domain alone is sufficient for viral engagement, validating a decoy-receptor therapeutic strategy.\",\n      \"evidence\": \"Recombinant soluble CD4 binding to HIV gp110/gp120 and inhibition of viral infectivity in vitro\",\n      \"pmids\": [\"3257544\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"In vivo efficacy of soluble CD4 not tested\",\n        \"Affinity and stoichiometry of sCD4–gp120 interaction not quantified\",\n        \"Whether sCD4 blocks all HIV strains not addressed\"\n      ]\n    },\n    {\n      \"year\": 1990,\n      \"claim\": \"The LEC rat mutant revealed that the transition from CD4+CD8+ double-positive to CD4 single-positive thymocyte requires a genetically separable step, implicating an unknown factor distinct from TCR or MHC class II in CD4 lineage commitment.\",\n      \"evidence\": \"Flow cytometry of thymic subsets in LEC mutant rats, with exclusion of TCR/CD3 and MHC class II expression defects\",\n      \"pmids\": [\"2147947\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Identity of the LEC mutation not determined\",\n        \"Whether the block is cell-intrinsic to thymocytes not resolved\",\n        \"Relationship to CD4 signaling through p56lck not tested\"\n      ]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"CD4-knockout mice generating functional MHC class II-restricted T helper cells demonstrated that CD4 enhances but is not absolutely required for helper lineage commitment, reframing CD4 as a co-receptor that lowers activation thresholds rather than a lineage-defining molecule.\",\n      \"evidence\": \"CD4 gene knockout mice infected with Leishmania; flow cytometry and IFN-γ cytokine assays identifying CD8−αβTCR+ helper cells\",\n      \"pmids\": [\"8367726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether compensatory mechanisms (e.g., LAG-3) substitute for CD4 not explored\",\n        \"Efficiency of MHC class II-restricted selection in absence of CD4 not quantified\",\n        \"Relevance to human T cell development not established\"\n      ]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Myxoma virus-induced CD4 degradation revealed a PKC-independent, lysosome-dependent pathway of CD4 downregulation in which p56lck dissociates from CD4 before degradation, separating kinase signaling from receptor turnover.\",\n      \"evidence\": \"Co-immunoprecipitation of p56lck with CD4, lysosomal inhibitor (NH4Cl) rescue, kinase activity assays in myxoma virus-infected rabbit T cells\",\n      \"pmids\": [\"7636966\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Viral protein responsible for triggering CD4 internalization not identified\",\n        \"Whether HIV uses the same lysosomal pathway not determined\",\n        \"Ubiquitination status of CD4 during this process not assessed\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Discovering that TLR7 signaling in CD4+ T cells activates NFATc2-dependent anergy and enhances HIV-1 susceptibility linked innate sensing to adaptive T cell dysfunction, explaining how HIV exploits cell-intrinsic pathways.\",\n      \"evidence\": \"TLR7 ligation with calcium flux measurement, transcriptomic profiling, siRNA knockdown of TLR7, and HIV infection frequency quantification in human CD4+ T cells\",\n      \"pmids\": [\"25401424\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether TLR7-induced anergy occurs in vivo during HIV infection not shown\",\n        \"Downstream anergic gene targets beyond NFATc2 not fully characterized\",\n        \"Relationship to CD4 co-receptor signaling per se not dissected\"\n      ]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Two studies in 2016 expanded CD4+ T cell biology beyond classical immunity: PKD2/3-dependent SHP-1 phosphorylation was shown to be essential for CD4 single-positive thymocyte selection, while ChAT-expressing CD4+ T cells were found to synthesize acetylcholine that regulates blood pressure via eNOS activation.\",\n      \"evidence\": \"T cell-specific PKD2/PKD3 double-knockout mice with phosphoproteomics and SHP-1 S557A mutagenesis (thymocyte selection); CD4-specific ChAT conditional knockout mice with blood pressure measurement and endothelial co-culture (neuroimmune regulation)\",\n      \"pmids\": [\"27670070\", \"27617738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How PKD activation is linked upstream to TCR/CD4 co-receptor engagement not defined\",\n        \"Whether ChAT+ CD4+ T cells represent a stable lineage or activation state unknown\",\n        \"Acetylcholine release mechanism from CD4+ T cells not characterized\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"CRISPR deletion of NFAT-responsive elements at the RORC locus in CD4+ T cells established that TCR-triggered NFAT/NF-κB cooperation and P300/CBP-mediated histone acetylation are required for RORγt transcription, providing the chromatin-level mechanism for Th17 lineage specification.\",\n      \"evidence\": \"CRISPR/Cas9 genomic deletion, ChIP for NFAT and histone modifications, NF-κB reporter assays, cyclosporin A inhibition in human CD4+ T cells\",\n      \"pmids\": [\"31619674\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether these regulatory elements are active in all Th17-polarizing conditions not tested\",\n        \"Contribution of additional transcription factors (e.g., STAT3) at these elements not assessed\",\n        \"In vivo relevance in autoimmune Th17 responses not demonstrated\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Demonstrating that activated CD4+ T cells upregulate chondroitin sulfate-modified CD74, which forms surface heterocomplexes with CXCR4 to mediate MIF-driven migration, revealed a new receptor complex on CD4+ T cells linking inflammation to chemotaxis.\",\n      \"evidence\": \"Proximity ligation assay, flow cytometry, 3D live-cell imaging, and receptor-specific pharmacological inhibitors in activated human CD4+ T cells\",\n      \"pmids\": [\"38992165\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Functional significance of chondroitin sulfate modification on CD74 not dissected\",\n        \"Whether CD74/CXCR4 complex formation depends on CD4 co-receptor signaling unknown\",\n        \"In vivo relevance of MIF-CD74-mediated CD4+ T cell migration not established\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include how CD4 co-receptor engagement quantitatively tunes signaling thresholds during thymic selection versus peripheral activation, the structural basis and stoichiometry of CD4–MHC class II interaction in the context of TCR microclusters, and whether non-immune functions of CD4+ T cells (e.g., cholinergic blood pressure regulation) employ CD4 signaling or merely use CD4 as a lineage marker.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No quantitative model of CD4 co-receptor contribution to TCR signal strength\",\n        \"Structural basis of CD4 within the TCR–pMHC-II signaling complex not resolved at atomic level in situ\",\n        \"Whether CD4 itself signals in ChAT+ T cell neuroimmune function or serves only as a lineage identifier\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 2, 4, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [3, 5, 7, 8]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 1, 5]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [5, 7, 8, 9]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"LCK\",\n      \"CXCR4\",\n      \"CD74\",\n      \"SHP1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"CD4 is a transmembrane glycoprotein of the immunoglobulin superfamily that functions as a co-receptor for MHC class II molecules to mediate T cell adhesion and as the primary receptor for HIV-1, while transducing activation signals through its constitutive association with the tyrosine kinase p56lck [PMID:2823150, PMID:3262426]. Its V-like extracellular domain directly binds HIV-1 gp120, and this interaction induces a conformational change in gp120 that creates a high-affinity binding site for chemokine receptor co-receptors CCR5 or CXCR4, enabling viral membrane fusion and entry [PMID:8906795, PMID:9641677, PMID:8629022, PMID:8674119]. CD4 surface expression is downregulated upon antigen stimulation via a PKC-dependent pathway and independently by HIV-1 Nef through a PKC-independent lysosomal degradation route that proceeds after dissociation of p56lck [PMID:3100638, PMID:2014052, PMID:7636966]. CD4-null mice retain functional MHC class II-restricted helper T cells, demonstrating that CD4 enhances but is not absolutely required for helper lineage commitment or effector function [PMID:8367726].\",\n  \"teleology\": [\n    {\n      \"year\": 1984,\n      \"claim\": \"Identification of CD4 as the primary cell-surface receptor for HIV answered the fundamental question of how the virus achieves tropism for helper T cells and established the molecular basis of HIV entry.\",\n      \"evidence\": \"Antibody blocking of syncytia formation and pseudotype infection assays with 14 anti-CD4 monoclonal antibodies\",\n      \"pmids\": [\"6096719\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding site on CD4 for HIV not yet mapped\", \"Whether CD4 alone is sufficient for viral entry was unknown\"]\n    },\n    {\n      \"year\": 1985,\n      \"claim\": \"Cloning and sequencing of CD4 cDNA revealed its immunoglobulin superfamily membership and multidomain architecture (V-like, J-like, transmembrane, cytoplasmic), providing the structural framework for understanding its receptor and adhesion functions.\",\n      \"evidence\": \"cDNA cloning, nucleotide sequencing, and deduced protein domain analysis\",\n      \"pmids\": [\"2990730\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Three-dimensional structure not yet determined\", \"Function of the cytoplasmic domain unknown\"]\n    },\n    {\n      \"year\": 1986,\n      \"claim\": \"Direct biochemical demonstration that HIV gp120 binds to the CD4 molecule, with epitope mapping showing which CD4 epitope is occluded, resolved the molecular identity of the virus–receptor interaction and defined the gp120-binding region on CD4.\",\n      \"evidence\": \"Radioactive surface labeling and reciprocal co-immunoprecipitation with anti-CD4 and anti-HIV antibodies; parallel work showed gp120-CD4 interaction mediates syncytia and cytopathicity\",\n      \"pmids\": [\"3001934\", \"3016552\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise residues mediating the interaction not yet identified\", \"Whether additional co-receptors are needed for viral entry remained unclear\"]\n    },\n    {\n      \"year\": 1987,\n      \"claim\": \"Three concurrent advances established CD4's dual physiological and pathological roles: CD4 mediates MHC class II-dependent cell adhesion independent of TCR, gp120 residues 397–439 are critical for CD4 binding, and antigen stimulation downregulates CD4 via a PKC-dependent pathway linked to a CD3-CD4 multimolecular complex.\",\n      \"evidence\": \"Reconstitution of CD4-mediated adhesion to class II MHC+ cells in non-T cells; deletion/point mutagenesis of gp120; flow cytometry of T cell clones after antigen/PMA stimulation\",\n      \"pmids\": [\"2823150\", \"2441877\", \"3100638\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling mechanism downstream of CD4 not defined\", \"Whether CD4 adhesion and TCR signaling are mechanistically coupled was unclear\"]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Discovery that CD4 constitutively associates with p56lck via its cytoplasmic domain identified the first intracellular signaling partner of CD4 and explained how CD4 co-engagement amplifies TCR signaling, while soluble CD4 neutralization of HIV proved the extracellular domain alone is sufficient for gp120 binding.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation and cross-linking of CD4–p56lck from murine T cells; recombinant soluble CD4 binding and viral inhibition assays\",\n      \"pmids\": [\"3262426\", \"3257544\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of CD4–p56lck interaction not resolved\", \"Whether soluble CD4 could serve as a therapeutic in vivo was untested\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Demonstration that HIV-1 Nef downregulates CD4 through a PKC-independent mechanism that does not require CD4 cytoplasmic serine phosphorylation established that HIV employs a distinct pathway from physiological CD4 internalization to evade immune recognition.\",\n      \"evidence\": \"Nef retroviral expression combined with CD4 triple serine-to-alanine mutagenesis, flow cytometry, and Western/Northern blot in lymphocytic lines\",\n      \"pmids\": [\"2014052\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endocytic sorting signals and adaptor proteins mediating Nef-induced CD4 degradation not yet identified\", \"Whether Nef directly binds CD4 cytoplasmic tail was unresolved\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"CD4 knockout mice developing functional MHC class II-restricted T cells that control Leishmania infection revealed that CD4 is not absolutely required for helper T cell lineage commitment, redefining CD4 as an enhancer rather than an obligate determinant of class II-restricted immunity.\",\n      \"evidence\": \"CD4-null mice with Leishmania infection model, T cell subset analysis, cytokine assays, MHC restriction experiments\",\n      \"pmids\": [\"8367726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative contribution of CD4 to thymic positive selection efficiency not determined\", \"Compensatory mechanisms in CD4-null mice not characterized\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Myxoma virus-induced CD4 downregulation via a lysosomal, PKC-independent pathway with prior dissociation of p56lck provided a second viral system demonstrating that lysosomal targeting of CD4 is a convergent immune evasion strategy.\",\n      \"evidence\": \"Flow cytometry, NH4Cl lysosomal inhibition, staurosporine PKC inhibition, co-immunoprecipitation of p56lck–CD4, kinase assay\",\n      \"pmids\": [\"7636966\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether the myxoma and HIV Nef pathways share sorting machinery was unknown\", \"Viral protein responsible for triggering CD4 degradation not identified\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"The discovery that HIV-1 entry requires chemokine receptor co-receptors (CXCR4 for T-tropic and CCR5 for M-tropic strains) in addition to CD4, and that CD4 binding to gp120 induces a conformational change creating a high-affinity CCR5-binding site, resolved the long-standing puzzle of why CD4 expression alone was insufficient for infection and established the sequential receptor model of HIV entry.\",\n      \"evidence\": \"Functional reconstitution of HIV fusion/infection by co-expression of CD4 with fusin/CXCR4 or CCR5 in non-permissive cells; competitive binding assays showing CD4-induced enhancement of gp120–CCR5 affinity; antibody blocking of co-receptor engagement\",\n      \"pmids\": [\"8629022\", \"8674119\", \"8906795\", \"8906796\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the CD4-induced conformational change at atomic resolution not yet available\", \"Kinetics of the sequential CD4→co-receptor engagement in membrane context not measured\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"The 2.5 Å crystal structure of the gp120 core complexed with CD4 revealed the atomic details of the CD4–gp120 interface, the conserved CCR5-binding surface exposed by CD4 engagement, and immune evasion mechanisms including glycan shielding, providing a structural foundation for understanding HIV tropism and vaccine design.\",\n      \"evidence\": \"X-ray crystallography of ternary gp120 core–CD4 two-domain fragment–Fab complex; complementary mutagenesis defining the conserved CCR5-binding element adjacent to V3\",\n      \"pmids\": [\"9641677\", \"9632396\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length trimeric Env–CD4 complex structure not available\", \"Dynamics of CD4-induced conformational rearrangement not captured by crystallography\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of the CD4 cytoplasmic domain–p56lck interaction at atomic resolution, the precise endocytic sorting machinery that HIV Nef and other viral proteins co-opt to target CD4 for lysosomal degradation, and how CD4 co-engagement quantitatively modulates TCR signaling thresholds during thymic selection and peripheral activation.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No high-resolution structure of CD4 cytoplasmic domain–p56lck complex\", \"Nef-recruited endocytic adaptor complex incompletely defined in the timeline\", \"Quantitative contribution of CD4 to TCR signaling kinetics not measured\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [0, 2, 3, 12, 13]},\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [7, 14]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 4, 5]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [9, 11]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"LCK\",\n      \"CCR5\",\n      \"CXCR4\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}