The CD5 gene, located on chromosome 11q12.2, encodes a type I transmembrane glycoprotein belonging to the scavenger receptor cysteine-rich superfamily. CD5, also known as Leu-1 or Ly-1 in mice, functions primarily as a co-receptor that modulates T-cell receptor and B-cell receptor signaling. Through its immunoreceptor tyrosine-based inhibitory motifs in the cytoplasmic tail, CD5 fine-tunes activation thresholds in both T cells and a subset of B1a cells, thereby maintaining immune homeostasis by preventing excessive or sustained lymphocyte activation.
The role of CD5 extends beyond basic immune regulation. Its constitutive expression on T cells and B1a cells, with inducible upregulation on certain B cell subsets, underscores its systemic importance in balancing activation and tolerance. Dysregulation or aberrant expression of CD5 is implicated in several pathological conditions. For instance, in chronic lymphocytic leukemia and mantle cell lymphoma, CD5 is a defining surface marker and contributes to malignant cell survival by attenuating apoptosis and modulating tumor-immune interactions. Furthermore, CD5 expression has been increasingly recognized as a negative regulator of chimeric antigen receptor-T cell persistence and function, positioning it as a promising therapeutic target for improving adoptive cell therapies.