The mononuclear cell in human blood which mediates antibody-dependent cellular cytotoxicity to virus-infected target cells. II. Identification as a K cell.

Studies were carried out to determine whether the mononuclear cell in human blood which mediates antibody-dependent cellular cytotoxicity (ADCC) to herpes simplex virus (HSV)-infected target cells has surface Fc receptors which participate in the reaction. The F (ab')2 fragment of human IgG antibody was inactive both in ADCC and in complement-mediated cytolysis, but retained the capacity to neutralize infectious virus, to agglutinate erythrocytes coated with viral antigens, and to bind to the surface of virus-infected cells. Treatment of sensitized virus-infected target cells with staphylococcus protein A, which has affinity for the Fc epitope of IgG, strongly reduced their susceptibility to lysis by ADCC in a dose-dependent relationship. These findings indicate that the Fc portion of IgG antibody to the virus is necessary for cytotoxicity. Treatment of blood mononuclear cells with either heat-aggregated gamma-globulin or HSV immune complexes inhibited effector cell activity. The presence of "third party" cellular immune complexes also strongly inhibited ADCC. Adsorption of mononuclear cells to plastic surfaces coated with soluble third party immune complexes resulted in a significant reduction in effector cell activity. These findings demonstrate that the ADCC effector cell possesses surface Fc receptors which are utilized in the ADCC reaction. The presence of Fc receptors on the surface of the effector cell indicates that it is a K cell rather than a null cell.