ABSTRACT
1.The use of monoclonal antibodies (mAb) has permitted the identification of T cell surface antigens and the classification of these antigens based upon phenotype and function. Some of these monoclonal antibodies can identify antigens specifically involved in T lymphocyte activation and are also able to induce, under certain conditions, T cell proliferation. 2. We describe a new mAb raised against hamster T cells which binds to a 45-KDa cell surface antigen expressed on 45% of thymic cells and 90% of mature T lymphocytes. This mAb,d esignated X2VA, alone does not cause T cells proliferation, but increases in a synergistic manner T cell proliferation when these cells are cultured in the presence of specific antigens, or used in mixed lymphocyte reactions. 3. When the X2Va mAb is used as single signal for the T cells it induces the production of a T cell growth factor, suggesting that the synergist effect observed during antigen-induced T cell proliferation is mediated by one more cytokines. 4. Our results indicate that the X2Va mAb recognizes an antigen which is expressed during T cell ontogenesis and which is involved in hamster T cell Activation