CD4 Protein – T-cell surface glycoprotein CD4(CD4)

General information on CD4 protein

CD4 protein also known as cluster of differentiation 4 is a glycoprotein located on the surface of different immune cells such as dendritic cells, macrophages, monocytes and T helper cells. The protein is encoded by CD4 gene in humans. CD4 protein is an essential part of immune response. The suppression CD4 protein levels, for example in immune suppression prior to transplant or in the case of untreated HIV infection, leaves the organism vulnerable to a wide range of infections.
CD4 protein is a member of immunoglobulin superfamily. The latter is characterized by surface proteins or soluble proteins that are involved in the recognition, adhesion and binding processes of cells. CD4 protein contains four immunoglobulin domains, respectively D1, D2, D3 and D4 that are located on the surface of the cells. On one hand, D1 and D3 resemble immunoglobulin variable domains also known as IgV domain. On the other hand, D2 and D4 resemble the immunoglobulin constant domains also known as IgC. CD4 protein interacts with MHC class II molecules via its D1 domain. APC cells include macrophages, Langerhans cells, B cells and dendritic cells. The intracellular tail of CD4 consists of a special sequence of amino acids that interact with Lck tyrosine kinase. The latter is a 56 kDa proteins found in lymphocytes.
CD4 protein is a co-receptor of the T cell receptor (TCR) whose main function is to bind to MHCII of APC cells. However, the TCR complex and CD4 protein bind to different region of MHCII, respectively α1/β1 and β2. The close proximity between CD4 and TCR complex results in the tyrosine-phosphorylation of ITAM (Immunoreceptor tyrosine activation motifs) complex by Lck tyrosine kinase. The latter is considered to be essential for the activation of many molecular components of the signaling cascade of T cells. According to the signal, there may be different T helper cells results.