Relevance of individual CD5 extracellular domains on antibody recognition, glycosylation and co-mitogenic signalling.

CD5 is a type I glycoprotein which modulates T- and B-cell receptor-mediated signals and is expressed by thymocytes, mature T cells and a subset of mature B cells. The extracellular region of CD5 is composed of three scavenger receptor cysteine-rich domains (D1, D2, D3) for which only limited functional and structural data are available. Using cell transfectants expressing ectodomain-deficient CD5 molecules or CD5 immunoglobulin fusion proteins, we analysed individual CD5 domains with respect to monoclonal antibody binding specificity, glycosylation, and co-mitogenic signalling. Our results show the presence of N-linked oligosaccharides on D1 and D2, but not on D3. D1, the most amino-terminal domain, is predicted to be the most appropriately placed domain for an interaction with a ligand. This domain is recognised by a large panel of well characterised CD5 mAbs, reflecting its higher immunogenicity. In an attempt to develop mAbs with specificity for the more conserved membrane-proximal domains, we generated a unique mAb, named 83-C4, whose binding mapped to D3. Co-stimulatory studies revealed no significant differences between anti-D1 and anti-D3 mAbs. The high interspecies conservation of D3 implies a conserved role of this domain in CD5 function and the 83-C4 mAb promises to be a valuable tool in exploring this.