Evidence by reactivity with hybridoma antibodies for a probable myeloid origin of peripheral blood cells active in natural cytotoxicity and antibody-dependent cell-mediated cytotoxicity.

Lymphocytes with Fc receptors (FcR) for IgG active in natural cytotoxicity and antibody-dependent cellular cytotoxicity were separated into sheep erythrocyte rosetting (E+) and nonrosetting (E-) fractions, and examined for reactivity with the OK panel of hybridoma-produced monoclonal antibodies. Few cells in either the E+ FcR+ or the E- FcR+ fraction reacted with seven antibodies used to define T cells in various stages of differentiation (OK3, OKT4, OKT5, OKT6, OKT8, OKT9, OKT10). Neither fraction expressed an Ia-like antigen (detected by OKI1), but both were highly reactive with OKM1, an antibody that reacts with monocytes and granulocytes. Incubation of these cytotoxic effector cells with OKM1 plus complement abolished all cytotoxic reactivity, but incubation with a pan-T cell antibody (OKT3) plus complement had no significant effect. These cells were not monocyte precursors, because they could not be induced in vitro to develop macrophage characteristics. The data indicate that most cytotoxic effector cells in natural cytotoxicity and antibody-dependent cellular cytotoxicity are not in the T cell lineage, but have a myeloid origin.