Lack of evidence for IgM-induced ADCC: studies with monoclonal and polyclonal antibodies

Different kinds of IgM antibodies were tested for their activity in antibody-dependent cellular cytotoxicity (ADCC): firstly an anti-benzylpenicilloyl (BPO) IgM antibody from immune rabbit serum purified by affinity, ion exchange, and molecular-sieving chromatography, secondly two monoclonal rat anti-BPO IgM antibodies and thirdly a human antidextran antibody prepared from a patient showing restriction of anti-dextran antibodies to the IgM class. Human lymphocytes or purified monocytes served as effector cells. While the two monoclonal rat and the human IgM antibodies showed no ADCC-mediating capacity, ADCC was induced by the rabbit anti-BPO IgM antibody when high antibody concentrations were used. This activity was abolished by further purification using an anti-rabbit IgG (Fc) immunosorbent. The initially observed activity was shown to be likely due to traces of aggregated anti-BPO IgG, which cannot be detected by the methods commonly used. Preincubation of lymphocytes for 24 hr increased the number of EA (IgM)] rosette forming cells but failed to induce IgM-mediated ADCC. Furthermore, evidence for amplification of low-dose IgG-ADCC by IgM could not be found.