Human natural killer cells, activated lymphocyte killer cells, and monocytes possess similar cytotoxic mechanisms.

The relationship between the killing mechanisms of human natural killer (NK) cells, mitogen- and mixed-lymphocyte-culture-induced activated lymphocyte killer (ALK) cells, and monocytes was investigated with a monoclonal antibody. The IgG2 antibody 9.1C3 was prepared from mice immunized with purified human large granular lymphocytes and selected from clones that inhibited NK cell killing. The 9.1C3 antibody bound to all mononuclear cells but not to granulocytes or K562 cells, and it selectively blocked killing of K562 targets by both NK and ALK cells without affecting the binding of effector to target cells. The antibody blocked killing when present from time zero and it still inhibited partially even when added 1 hr after initiation of the lytic reaction. Killing of Epstein-Barr virus-transformed B lymphoblasts by classical cytotoxic T lymphocytes was not inhibited. Of interest, 9.1C3 did block the killing of K562 target cells by cultured peripheral blood monocytes. Other monoclonal antibodies that bound to monocytes did not block killing, and a nonspecific effect of the antibody on monocytes was excluded. These data suggest that NK cells, ALK cells, and monocytes can kill tumor cell targets by using similar lytic mechanisms.