A role for CD5 in cognate interactions between T cells and B cells, and identification of a novel ligand for CD5

CD5 is a glycoprotein expressed at a high level on the surface of mature T lymphocytes. Studies with CD5 mAb and CD5-deficient mice have shown that the CD5 molecules have a significant role in T cell growth response. However, the precise role of CD5 in immune cell interactions is still unclear. The present study provides evidence that CD5 plays a direct role in providing growth signals during the contact-dependent activation and proliferation of splenic B cells. An anti-CD5 mAb inhibited T(h)1- and T(h)2-type cell-induced B cell proliferation. CD5-Ig, a chimeric fusion protein, induced proliferation of resting B cells. Flow cytometric analyses using CD5-Ig and mAb to CD72 demonstrated that CD5 bound to a ligand (CD5L), and this binding was not blocked by a variety of anti-CD72 mAb. Also, CD5-Ig did not bind to CD72⁺-transfected cells. Immunoprecipitation of surface labeled B cell molecules with CD5-Ig showed that CD5L was composed of 77-80 and 38-40 kDa polypeptide chains, distinct from CD72. CD5L was expressed on activated splenic B cells, but not T cells, whereas its expression was constitutive on peritoneal B cells and on B lymphoma cell lines.