Identification of surface IgM as the target antigen of cold lymphocytotoxins.

IgG and IgM were stripped from the surface of human B lymphocytes using heterologous anti-IgG or IgM antisera. The percent SIg (surface Ig) determined by fluorescence for SIgM was reduced from 55% to 20%, or a reduction of 64%. Upon stripping SIgM, the B lymphocytes were no longer killed by seven different cold cytotoxic antibodies obtained from kidney transplant patients. We conclude that SIgM is the target of cold cytotoxins. These cold cytotoxic antibodies are postulated to be immunoregulatory antibodies. They are less specific than anti-idiotypic antibodies.