CD5

CD5, also known as T-cell antigen CD5, is a cell surface glycoprotein that serves as a useful marker in immunology. It is expressed on the majority of mature T lymphocytes and on a subset of B lymphocytes, notably B-1a cells and some memory B cells. The protein is a member of the scavenger receptor cysteine-rich (SRCR) superfamily and is encoded by the CD5 gene. Its extracellular region comprises SRCR domains, and it has a single transmembrane domain followed by a cytoplasmic tail.

Functionally, CD5 acts as a negative regulator of antigen receptor signaling. When the T-cell receptor (TCR) is engaged, CD5 becomes phosphorylated on tyrosine residues in its cytoplasmic tail and can recruit phosphatases such as SHP-1, dampening downstream signaling, including calcium flux and T-cell activation. This modulation helps set activation thresholds and maintain self-tolerance. In B cells, CD5 marks a specialized subset (B-1a) and may play a role in modulating B-cell receptor signaling, though its exact functions in B cells are less clearly defined.

Clinical significance attributed to CD5 largely comes from its use in immunophenotyping. CD5 is commonly used to distinguish certain lymphoid malignancies: most cases of chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL) are CD5-positive, while normal mature B cells are typically CD5-negative. In T-cell malignancies, CD5 is usually expressed on mature T cells and can aid in lineage assignment. Aberrant CD5 expression patterns can contribute to diagnostic and prognostic assessments and may influence therapeutic considerations in some contexts.

Genetic regulation and expression of CD5 are developmentally controlled, with upregulation during thymic maturation and varying levels in peripheral lymphoid tissues. Ongoing research continues to refine understanding of its ligands and broader physiological roles.