• glycoprotein binding • transmembrane receptor activity • extracellular matrix structural constituent • zinc ion binding • coreceptor activity • protein kinase binding • MHC class II protein binding • protein homodimerization activity
• plasma membrane • external side of plasma membrane • integral to membrane • T cell receptor complex
• conjugation with cellular fusion • immune response • cell adhesion • transmembrane receptor protein tyrosine kinase signaling pathway • T cell selection • positive regulation of interleukin-2 biosynthetic process • positive regulation of protein kinase activity • positive regulation of peptidyl-tyrosine phosphorylation • positive regulation of calcium-mediated signaling • positive regulation of T cell activation
RNA expression pattern
More reference expression data
NM_000616 (mRNA) NP_000607 (protein)
NM_013488 (mRNA) NP_038516 (protein)
Chr 12: 6.77 - 6.8 Mb
Chr 6: 124.83 - 124.85 Mb
CD4 (cluster of differentiation 4) is a glycoprotein expressed on the surface of T helper cells, regulatory T cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 (after the OKT4 monoclonal antibody that reacted with it) before being named CD4 in 1984 (ref: Bernard et al, editors. Leukocyte Typing. Springer-Verlag, Heidelberg. 45-48, 1984)
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.
It has four immunoglobulin domains (D1 to D4) that are exposed on the extracellular surface of the cell:
D1 and D3 resemble immunoglobulin variable (IgV) domains.
D2 and D4 resemble immunoglobulin constant (IgC) domains.
CD4 uses its D1 domain to interact with the β2-domain of MHC class II molecules. T cells expressing CD4 molecules (and not CD8) on their surface, therefore, are specific for antigens presented by MHC II and not by MHC class I (they are MHC class II-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.
Role in HIV infection
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 receptorsCCR5 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.
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