A feline thymocyte antigen defined by a monoclonal antibody (FT2) identifies a subpopulation of non-helper cells capable of specific cytotoxicity.

A monoclonal antibody, FT2 (IgG1 kappa) prepared against cat thymocytes, was found to be reactive with an antigenic determinant expressed by approximately 76% of thymocytes, 15% of blood mononuclear cells, 14% of splenocytes, and 1% of bone marrow cells. The FT2-reactive determinant was not expressed on B cells, macrophages, granulocytes, or erythrocytes. Both FT2+ and FT2- populations of peripheral blood mononuclear cells were capable of proliferative responses to the T cell mitogens Con A and PHA. When splenocytes were sensitized to the lymphoblastoid cell line, 79p90, cytotoxic T cells were found in the FT2+ population and were absent from the FT2- population. Conversely, the FT2- population contained the helper T cell activity required for pokeweed mitogen-induced B cell differentiation. Under nonreducing conditions, the FT2 antigen had an apparent m.w. of 71,000. When reduced, subunits of 31,000 and 38,000 apparent m.w. were observed. The data suggest that the FT2 antibody identifies the feline analog of the human T8/Leu-2, murine Ly-2 molecules expressed by cytotoxic/suppressor T cells.