In human monocytes a strong correlation exists between expression of the M3 antigen, Fc-mediated phagocytic activity and failure to participate in extracellular antibody-dependent cytotoxicity.

Human fresh blood monocytes can phagocytize and lyse antibody-coated target cells by contact with membrane Fc receptors. Recently, the monocyte differentiation antigen Leu M3 has been described to be associated with monocyte/macrophage maturation pathway and to be linked to functionally distinct monocyte subsets. In the present study peripheral blood monocytes were separated into M3+ and M3- subsets, and evaluated for their ability to mediate antibody-driven effector functions. A clear cut functional difference could be demonstrated. M3+ monocytes phagocytize antibody-coated sheep red cells but do not carry out contact-mediated extracellular lysis. In contrast, M3- monocytes have exactly the opposite functional features and they mediate cytolysis without exhibiting any phagocytic activity. By using phenotypically defined clones of the U937 histiocytic cell line, the segregation of different lytic abilities and their linkage of the M3 phenotype have been confirmed.