Identification of phenotypically distinct subpopulations of cytotoxic T lymphocytes

Two distinct subpopulations of cytotoxic T lymphocytes (CTL) were identified on the basis of their expression of a marker recognized by a monoclonal antibody H9/25. Conventional bulk mixed lymphocyte reactions (MLR) against either major histocompatibility antigens or minor histocompatibility antigens yield cytotoxic T cells which carry the H9/25 marker (H9/25+). Removal of H9/25+ spleen cells before MLR culture did not prevent the generation of CTL. However, the CTL had the phenotype H9/25-. These results imply that two distinct subpopulations of CTL exist, H9/25+ and H9/25-. Under normal conditions the H9/25+ CTL subpopulation dominates. Removal of H9/25+ spleen cells from the responder spleen cell population before MLR stimulation allows the expression of the otherwise silent H9/25- subpopulation of CTL. The data suggest that H9/25+ spleen cells regulate the expression of CTL subpopulations in bulk MLR cultures.