Role of IgM in human monocyte-mediated target cell destruction in vitro

The presence on human monocytes of surface receptors binding IgM molecules (Fc mu receptors) and the role of IgM antibodies in monocytic effector functions were investigated. IgM antibodies were first purified by one passage through anti-Fc gamma immunosorbent. Rosette techniques in which either the indicator erythrocytes or the monocytes had been pretreated with IgM antibodies did not reveal the presence of Fc mu receptors on monocytes. Phagocytosis and cytolysis of bovine erythrocytes were, however, regularly observed in the presence of high concentrations of one IgM preparation. Inhibition experiments with human IgG or IgM indicated that the phagocytic and cytolytic effects obtained with this IgM preparation were due to contaminating IgG antibodies. This was confirmed by further immunosorbent purification of the IgM antibody preparation. Lymphocytes carrying Fc micro receptors did not transmit any effector cell activity to co-cultivated autologous monocytes as a result of interaction with IgM. It was concluded that IgM does not mediate antibody-dependent target cell destruction by human monocytes that seem to lack Fc micro receptors.