coreceptorsCD4

CD4, also known as T-cell surface glycoprotein CD4, is a transmembrane protein that serves as a co-receptor for the T-cell receptor (TCR). It is primarily expressed on the surface of helper T cells, regulatory T cells, monocytes, macrophages, and dendritic cells. The primary function of CD4 is to assist the TCR in recognizing and binding to antigens presented by major histocompatibility complex (MHC) class II molecules.

The CD4 molecule is composed of four extracellular immunoglobulin-like domains, a transmembrane segment, and a cytoplasmic tail. These extracellular domains are crucial for its interaction with MHC class II molecules. Upon binding of an antigen to the TCR, CD4 interacts with the MHC class II molecule, stabilizing the interaction between the T cell and the antigen-presenting cell. This stabilization enhances the sensitivity of the T cell to the antigen.

Beyond its role in antigen recognition, CD4's cytoplasmic tail plays a vital role in intracellular signaling. It associates with tyrosine kinases, such as Lck, which are essential for initiating the T-cell activation cascade. This signaling pathway ultimately leads to T-cell proliferation, cytokine production, and differentiation.

CD4 is also famously known as the primary receptor for the human immunodeficiency virus (HIV). The envelope protein of HIV, gp120, binds with high affinity to the extracellular domain of CD4. This binding event is the initial step in HIV infection, allowing the virus to enter T helper cells. Consequently, the depletion of CD4+ T cells is a hallmark of AIDS and leads to profound immunodeficiency. The importance of CD4 in immune function and its susceptibility to viral infection have made it a significant focus of research in immunology and virology.