CD4

CD4 is a transmembrane glycoprotein that acts both as a T-cell co-receptor coupling antigen recognition to intracellular signaling and as the essential cellular receptor for HIV (PMID:6096719, PMID:8575197). As an immune receptor, CD4 functions as an adhesion molecule and signaling co-receptor whose co-receptor activity depends on constitutive association of its cytoplasmic tail with the Src-family tyrosine kinase Lck through Lck's unique N-terminal domain, recruiting Lck to the TCR/CD3 complex; Lck catalytic activity is dispensable for this co-receptor function, consistent with an adapter role (PMID:8575197). Genetic ablation establishes that CD4 is not strictly required for helper T-cell lineage commitment, since CD4-null mice generate functional MHC class II-restricted αβ TCR+ T cells (PMID:8367726). CD4 is the primary specific surface receptor for HIV: anti-CD4 antibodies block syncytia formation and viral entry, and a soluble CD4 ectodomain binds the HIV envelope glycoprotein gp120/gp110 and inhibits virus infectivity, although a secondary CD4-independent entry pathway operates in some non-T cells at high virus doses (PMID:6096719, PMID:3257544, PMID:1974766). HIV downregulates surface CD4 through multiple mechanisms—reduced CD4 mRNA, reduced CD4 protein, and intracellular sequestration of CD4 with viral envelope glycoproteins gp120 and the gp150 precursor (PMID:3095925); a parallel virus-driven program (defined in poxvirus-infected T cells) couples CD4 loss to dissociation of p56lck and lysosomal degradation of CD4 (PMID:7636966). Engagement of CD4 together with the coreceptor CXCR4 by gp120 can also trigger a rapid, caspase- and CD95-independent cell death marked by mitochondrial depolarization and chromatin condensation (PMID:9770524).

1. 1984 High

   Established that CD4 is the specific cell-surface receptor required for HIV entry, defining the molecular basis of viral tropism for helper T cells.

   Evidence Pseudotype VSV infection, syncytia formation, and anti-CD4 monoclonal antibody blocking across HIV isolates

   PMID:6096719

   Open questions at the time

   • Did not resolve the post-binding entry steps or coreceptor requirement
   • Mechanism of infection-induced CD4 surface loss not defined here
2. 1986 High

   Resolved how HIV infection depletes surface CD4, showing it occurs through reduced CD4 mRNA, reduced protein, and intracellular complexing of CD4 with viral envelope glycoproteins.

   Evidence Metabolic labeling, co-immunoprecipitation, and Northern blot across T-cell lines

   PMID:3095925

   Open questions at the time

   • Did not establish whether env-CD4 complexing causes degradation or retention
   • Relative contribution of each mechanism not quantified
3. 1988 High

   Showed the CD4 ectodomain alone is sufficient to bind gp120 and neutralize HIV, separating receptor-binding function from the membrane-anchored signaling protein and providing an antiviral strategy.

   Evidence Recombinant soluble CD4 binding assay and viral infectivity inhibition

   PMID:3257544

   Open questions at the time

   • Did not address in vivo efficacy or resistance
   • Structural basis of gp120 binding not defined
4. 1990 Medium

   Demonstrated that while CD4 is the primary entry receptor, a secondary CD4-independent infection pathway exists in non-T cells at high virus doses.

   Evidence Immunoperoxidase staining of infected fibroblastoid cells with anti-CD4 antibody blocking and CD4 mRNA analysis

   PMID:1974766

   Open questions at the time

   • Receptor or mechanism for CD4-independent entry not identified
   • Single-lab observation
5. 1993 High

   Tested whether CD4 is essential for T-cell development, showing helper-lineage commitment and MHC class II-restricted function can proceed without CD4.

   Evidence CD4-null mice with Leishmania challenge and IFN-γ measurement

   PMID:8367726

   Open questions at the time

   • Did not define what compensates for CD4 loss
   • Quantitative effects on T-cell efficiency not fully resolved
6. 1995 Medium

   Linked virus-induced CD4 downregulation to dissociation of p56lck and lysosomal CD4 degradation, mechanistically connecting receptor turnover to signaling decoupling.

   Evidence Flow cytometry, co-immunoprecipitation, kinase assays, and NH4Cl lysosomal inhibition in myxoma virus-infected rabbit T cells

   PMID:7636966

   Open questions at the time

   • Mechanism of Lck dissociation not defined
   • Generalizability to HIV-driven CD4 loss not directly tested
7. 1996 Medium

   Defined the molecular basis of CD4 co-receptor signaling as Lck recruitment via Lck's N-terminal domain, with kinase activity dispensable, framing Lck as an adapter to the TCR/CD3 complex.

   Evidence Biochemical association studies, domain mutagenesis, and T-cell functional assays (review synthesis)

   PMID:8575197

   Open questions at the time

   • Identity of downstream signaling proteins recruited not resolved
   • Abstract/review-level synthesis limits mechanistic granularity
8. 1998 Medium

   Showed CD4 with CXCR4 can transduce a gp120-triggered death signal, revealing a signaling/pathological role beyond viral entry and TCR co-reception.

   Evidence CD4/CXCR4 transfectants with flow cytometry, mitochondrial potential and caspase-inhibitor assays

   PMID:9770524

   Open questions at the time

   • Death effector pathway downstream of CD4/CXCR4 not identified
   • Single-lab transfectant model

• The structural details of how the CD4 cytoplasmic tail engages and releases Lck during signaling versus virus-induced downregulation remain unresolved.
• No structural model of the CD4-Lck interface in the corpus
• Mechanism unifying TCR co-reception and HIV-driven receptor turnover not established