Human IgM anti-IgM cytotoxin for B lymphocytes.

B lymphocytes were shown previously to be killed at 5 degrees C by some autologous and allogeneic human sera. We show here that such cold cytotoxins are directed against immunoglobulins on the surface of B lymphocytes. The activity is partially removed by passing serum through IgG-coupled Sepharose and usually completely removed by passing serum through IgM-coupled Sepharose. Activity is regained from the columns by acid elution and IgM inhibits the cytotoxicity of these eluates. 125I-labeled eluates bind to IgG- and IgM-coupled Sepharose beads and binding is inhibited by IgM and to a lesser degree by IgG thus showing a primarily more avid binding to IgM as compared to IgG. Furthermore, the 125I-labeled cytotoxic eluates recognize the same determinant(s) on IgM and IgG. We suggest tht IgM anti-IgM antiimmunoglobulin may regulate immune reactivity by binding to B-lymphocyte surfaces.