Background.Although signaling via class I MHC molecules has been shown to suppress T-cell responses, the mechanisms by which these effects are mediated have not been delineated. Studies were conducted to examine the possibility that 5H7 (a murine anti-human mAb specific for the α3 domain of human class I MHC) induces programmed cell death (PCD).Materials.Normal human T cells and lymphocyte tumor lines were used. PCD was assessed by viable cell recovery (VCR) with trypan blue, ethidium bromide/acridine orange staining, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling techniques.Results.5H7 induced growth inhibition of lymphocyte tumor cell lines as assessed by [3H]thymidine incorporation. 5H7 also induced marked reductions in VCR of lymphocyte tumors and normal human T and B cells, with the most dramatic reductions occurring in B cells and B cell-derived tumors (JY, BeVD, and 521). Reductions in tumor growth observed with 5H7 monoclonal antibody, however, were not observed with other anti-human class I MHC monoclonal antibodies, TP2599 (α3 domain specific) and W6/32 (α2/α3 domain specific). Cells treated with 5H7 demonstrated typical features of PCD including cytoplasmic vacuolization, DNA condensation, and apoptotic body formation. Reduction in VCR was augmented by anti-CD3 monoclonal antibody and was not reversed by exogenous interleukin 2. Induction of PCD was not observed with soluble 5H7 F(ab)′2fragments alone, but cross-linking of 5H7 F(ab)′2fragments by F(ab)′2fragments of human Ig-absorbed goat anti-mouse Ig partially restored PCD induction, indicating that anti-class I MHC monoclonal antibody can induce PCD in an FcR-independent system and that monoclonal antibody cross-linking is necessary for PCD induction.Conclusions.These studies provide the first evidence that class I MHC molecules mediate PCD in human T and B cells.