Seven murine monoclonal antibodies (mAb) with different binding characteristics for human IgM varied markedly in their ability to induce proliferation of T cell-depleted human splenocytes. Two mAb (HB57 and 5D7) that bound to distinct epitopes on IgM were highly effective initiators of B cell proliferation at very low concentrations, in the presence of a T cell factor source. In the absence of T cell supernatant, both HB57 and 5D7 mAbs produced a markedly reduced degree of stimulation at all concentrations. Two additional anti-IgM mAb (VIIIE11 and Mu53) were distinctive in that, even at high concentrations, only limited proliferation was observed compared with the first group of mAb. This proliferation depended on the presence of T cell supernatant. Competitive-binding studies revealed that the epitope recognized by mAb Mu53 may be identical or very proximate to that recognized by HB57. Three other mAb (1G6, XG9, and P24) induced little or no proliferation. 1G6 bound to a unique epitope on the IgM molecule, whereas XG9 shared a determinant with VIIIE11 mAb. Regulatory influences of Fc receptor binding cannot account for all the diversity in proliferation observed with the individual anti-IgM mAb. Markedly augmented proliferation was obtained when B cells were cultured with certain combinations of anti-IgM mAb in the presence of exogenous T cell supernatant. The proliferation induced in the absence of T cell supernatant by high concentrations of mAb mixtures that included 1G6 approached that observed for the same mixtures in the presence of T cell supernatant. The data suggest that certain signals delivered through membrane IgM can bypass the need for T cell supernatant in the activation of human B lymphocytes.

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