CD4

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Not to be confused with CD-4 (disambiguation).


Schematic representation of CD4 receptor.
CD4 molecule
Identifiers
Symbol	CD4
HUGO	1678
Entrez	920
OMIM	186940
RefSeq	NM_000616
UniProt	P01730
Other data
Locus	Chr. 12 pter-p12

CD4 (cluster of differentiation 4) is a glycoprotein expressed on the surface of T helper cells, regulatory T cells, monocytes, macrophages, and dendritic cells. On T cells, CD4 is the co-receptor for the T cell receptor (TCR). It amplifies the signal generated by the TCR by recruiting the tyrosine kinase lck that is essential for activating many molecules involved in the signaling cascade of an activated T cell.

Like many cell surface receptors/markers, CD4 is a member of the immunoglobulin superfamily. It has four immunoglobulin domains (D₁ to D₄) that are exposed on the extracellular surface of the cell. D₁ and D₃ of CD4 resemble immunoglobulin variable (IgV) domains, while D₂ and D₄ resemble immunoglobulin constant (IgC) domains. CD4 uses its D₁ domain to interact with the β₂-domain of the MHC class II molecule. T cells expressing CD4 molecules (and not CD8) on their surface, therefore, are specific for antigens presented by class II MHC and not by class I MHC (they are class II MHC-restricted). The short cytoplasmic/intracellular tail (C) of CD4 contains a special sequence of amino acids that allow it to interact with the lck molecule described above.

CD4 is also a primary receptor used by HIV-1 to gain entry into host T cells. The HIV-1 virus attaches to CD4 with a particular protein in its viral envelope known as gp120. The binding to CD4 creates a shift in the conformation of the viral gp120 protein allowing HIV-1 to bind to two other cell surface receptors on the host cell (the chemokine receptors CCR5 and CXCR4). Following another change in shape of a different viral protein (gp41), HIV inserts a fusion peptide into the host T cell that allows the outer membrane of the virus to fuse with the T-cell membrane. HIV infection leads to a progressive reduction in the number of T cells possessing CD4 receptors and, therefore, the CD4 count is used as an indicator to help physicians decide when to begin treatment in HIV-infected patients.