Lymphocyte activation antigens. I. A monoclonal antibody, anti-Act I, defines a new late lymphocyte activation antigen.

Lymphocyte activation entails a sequence of events identified by analyzing the time course of expression of various distinctive cell surface molecules on lymphocytes that appear early (before initiation of DNA synthesis), parallel with DNA synthesis and cellular proliferation, or late (after peak proliferation). In this study we present identification of a novel late lymphocyte activation antigen, Act I, utilizing a murine monoclonal antibody. Anti-Act I was identified in a fusion of NS1 with BALB/c spleen cells immunized with a human tetanus toxoid-reactive T lymphoblast line. Flow cytometry analysis shows that Act I antigen is present in markedly greater amounts on activated T and B lymphocytes than on resting, small peripheral blood lymphocytes. Act I expression by these lymphocytes is promoted by PHA, tetanus toxoid, or alloantigens and lags behind maximal thymidine incorporation by 1 to 2 days. Thymocytes can be triggered to express Act I antigen during maturation induced by PHA/T cell growth factor stimulation. In vitro, anti-Act I does not affect antigen- or lectin-stimulated T cell proliferation, T cell-mediated lymphocytotoxicity, or T cell growth factor-induced proliferation of T lymphoblasts. By immunoperoxidase analysis, Act I antigen is restricted to lymphoid tissue, staining many lymphocytes in the paracortex, germinal centers, and mantle zones of lymph nodes, tonsil, and spleen. By immunoprecipitation, the Act I antigen is a single band of 63,000 m.w. on reduced or nonreduced SDS gels. The distribution, size, time course, and functional correlates indicate that Act I is different from other known T cell activation markers detected with anti-Tac, OKT9, B3/25, OKT10, 4F2, CBL1, and anti-Ia-like antibodies. Although the function of Act I is still undetermined, it may serve as a useful marker of a late stage of activation in vitro and in vivo.