CD5 is a type I transmembrane protein belonging to the scavenger receptor cysteine-enriched (SRCR) family, characterized by the presence of at least one SRCR domain containing 90-110 amino acids. CD5 is expressed in all mature T cells, B-1A subsets of mature B cells, and certain leukemia B cells. It is elevated in regulatory T and B cells (Tregs/Bregs). CD5 expression was also elevated in incompetent T and B cells. Elevated levels of CD5 have also been found in many autoimmune diseases. CD5 binds to T cell receptors (TCR) and negatively regulates T cell activation and differentiation. The expression of CD5 in tumor-infiltrating T lymphocytes was negatively correlated with their antitumor activity. Recently, it has been reported that CD5 binds directly to IL6 and mediates downstream signal transduction. CD5+ B cells promote tumor growth in animal models. Agents targeting CD5 are being actively explored as therapeutic interventions for cancer and other conditions.
