Enhancement of murine T cell I-J expression by limited proteolysis.

I-J-encoded structures on peripheral T cells and thymocytes appear normally to be blocked or shielded by material that is susceptible to proteolysis. Limited proteolysis with trypsin, papain, pronase, or chymotrypsin increased the number of peripheral T cells and thymocytes lysed by anti-I-Jk serum and complement. Proteolysis did not induce I-Jk expression on B cells or on negative strain T cells. Increased lysis was enzyme concentration and time dependent and was not due to increased susceptibility of protease-treated cells to lysis by antibody plus complement; proteolysis rendered T cells and thymocytes less susceptible to lysis by anti-H-2Kk, anti-H-2Dd, and anti-Lyt-2 antibodies. Absorption experiments showed that I-Jk determinant density was increased in the protease-treated T cell population. The I-Jk determinants detected are proteins or glycoproteins; extended proteolysis removed these molecules from the T cell surface. Treatment of T cells or thymocytes with activated macrophage culture supernatant containing proteolytic activity produced a small but reproducible increase in I-Jk expression. Proteolysis of lymphocyte membranes, possibly mediated by macrophages, may have a role in cellular differentiation and immune activation.