The B-cell surface protein CD72/Lyb-2 is the ligand for CDS

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Abstract
THE glycoprotein CD5 is expressed on the surface membrane of all mature T cells1 and a small proportion of B lymphocytes1,2. Its exact role in immune interactions is still unknown. Studies3–10 indicate that CD5 functions both in mice and humans as a receptor, delivering co-stimulatory signals to T cells in a manner similar to CD2 (ref. 11) and CD28 (ref. 12). Anti-CDS antibodies stimulate both T-cell proliferation mediated by CD3 in association with the T-cell receptor and secretion of interleukin-2 and expression of its receptor, as well as inducing an increase in intracellular Ca2+concentration (refs 5–10). To identify the ligand for CD5 we purified the human CD5 protein, labelled it with biotin and used it as a probe. Here we report that CD5 specifically interacts with the cell-surface protein CD72 exclusive to B cells. This interaction is blocked by anti-CD72 antibodies, but not by any other anti-B-cell antibodies. Moreover, non-B cells (mouse L-cell fibroblasts and human Jurkat T cells) expressing a transfected human CD72 complementary DNA could bind to the CD5–biotin conjugate. The results demonstrate that the B-cell surface protein CD72 (Lyb-2 in mice) is the ligand for CD5.