Regulation of effector functions of human K-cells and monocytes by antigen density of the target cells.

Antibody-dependent cytolysis and phagocytosis mediated by human K-cells or monocytes against chicken erythrocytes (ChRBC) were studied. The antigen density of the target cells was varied by coating the cells with different amounts of 3H-labelled dinitrophenyl (DNP) hapten. The degree of antigenicity thus acquired by the target cells was assessed on the basis of their uptake of the isotope. Anti-DNP serum was used to induce lysis or phagocytosis. Below 500 antigenic determinants per ChRBC the target cells were not affected. However, at the density, lysis and/or phagocytosis was seen when the antibody concentration was high (2 X 10(-9) M). With less antibody present (2 X 10(-11) M) only monocyte-mediated phagocytosis was induced. The estimated lowest number of target-cell-bound antibodies required for K-cell-mediated lysis was approximately 50. The corresponding number for monocyte-mediated phagocytosis was approximately 20 IgG per ChRBC. The result suggests that interaction of several Fc receptors on the effector cells with IgG molecules bound to adjacent sites on the target cell membrane is an important factor in the regulation of these antibody-dependent cell-mediated effector functions.