Role of IgM in Human Monocyte‐Mediated Target Cell Destruction in Vitro

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Abstract
The presence on human monocytes of surface receptors binding IgM molecules (Fcμ receptors) and the role of IgM antibodies in monocylic effector functions were investigated. IgM antibodies were first purified by one passage through anti-Feγ immunosorbent. Rosette techniques in which either the indicator crythrocyles or the monocytes had been pretreatcd with IgM antibodies did not reveal the presence of Fcμ receptors on monocytes. Phagocytosis and cytolysis of bovine erythrocytcs were, however, regularly observed in the presence of high concentrations of one IgM preparation. Inhibition experiments with human IgG or IgM indicated lhat 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 Fcμ 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 Fcμ receptors.