Immunoglobulin Variable (IgV) domain of cytotoxic T lymphocyte-associated antigen 4 (CTLA-4)
The members here are composed of the variable(v)-type immunoglobulin (Ig) domain found in cytotoxic T lymphocyte-associated antigen 4 (CTLA-4). CTLA-4 is involved in the regulation of T cell response, acting as an inhibitor of intracellular signaling. CTLA-4 is similar to CD28, a T cell co-receptor protein that recognizes the B7 proteins (CD80 and CD86). CD28 binding of the B7 proteins occurs after the presentation of antigen to the T cell receptor (TCR) via the peptide-MHC complex on the surface of an antigen presenting cell (APC). CTLA-4 also binds the B7 molecules with a higher affinity than does CD28. The B7/CTLA-4 interaction generates inhibitory signals down-regulating the response, and may prevent T cell activation by weak TCR signals. CD28 and CTLA-4 then elicit opposing signals in the regulation of T cell responsiveness and homeostasis. T cell activation leads to increased CTLA-4 gene expression and trafficking of CTLA-4 protein to the cell surface. CTLA-4 is not detected on the T-cell surface until 24 hours after activation. Covalent dimerization of CTLA-4 has been shown to be required for its high binding avidity, although each CTLA-4 monomer contains a binding site for CD80 and CD86.