The cytotoxic effector cells that recognize HLA-D-region determinants and their precursors were characterized using monoclonal antibodies against human T lymphocytes and T-cell subsets. These studies were performed using MLC combinations giving rise to cytotoxic cells specific for both class I (HLA-A, B, C) and class 11 (HLA-D-region) antigens, and then tested against target cells displaying relevant antigens of only one class. Both class I and class II specific CTL (cytotoxic T lymphocytes) were inhibited by treatment with the OKT3 monoclonal antibody and complement, indicating that the effector cells were T lymphocytes. A major portion.of class II specific CTL, and their precursors, were inhibited by OKT4 and complement, while class I specific CTL from the same cultures were not. The T4+T8 - cell subset has previously been associated with helper or inducer functions, but not with cytotoxicity. The present findings indicate that class I and class 11 specific CTL, and their precursors, are different on the basis of the class of target antigen recognized and on the basis of surface phenotype detected by monoclonal antibodies.
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