Studies of antibody-dependent cytotoxicity in a plaque assay: evidence of human monocyte-like effector cells.

Two different methods for evaluating 'in vitro' cytotoxicity against antibody-coated target cells mediated by mononuclear leukocytes were compared. One was a plaque assay for identification of the cytotoxic cell and the other the classical chromium release assay for antibody-dependent cytotoxicity (ADCC). A marked decrease in plaque-forming cells (PFC) was observed in a cell suspension depleted of peroxidase-positive cells and cells with membrane-bound Ig (B lymphocytes) by fractionation on a nylon fiber column. In contrast, the ADCC activity was considerably increased by these depletions. A similar effect was obtained by removal of phagocytic cells with iron. These results, together with the observations after depletion of E-RFC (T lymphocytes) or EA-RFC (Fc-receptor-bearing cells), suggest that the PFC in the assay system used were of monocytic origin and differnet from the cells responsible for the ADCC.