Antibodies specific for Ig idiotype, but not isotype, can substitute for antigen to induce IgM secretion by a B cell clone

Cells of the mouse B cell clone, CH12.LX, secrete IgM when cultured with nominal antigen (sheep erythrocytes, SRBC) and mAbs which bind their membrane E^k molecules. To determine whether anti-lg antibodies can substitute for antigen In the Induction of IgM secretion by CH12.LX, the B cells were cultured with antl-E^k mAbs and antl-IgM or antl-idiotype antibodies. Anti-IgM antibodies were capable of cross-linking the membrane IgM of CH12.LX, and Inhibited mitogen-induced differentiation of the B cells. However, anti-IgM could not substitute for SRBC In delivering a major histocompatibility complex-restricted differentlative signal. In contrast, either polyclonal or monoclonal antibodies specific for the CH12.LX Ig idlotype were fully capable of substituting for antigen in the induction of IgM secretion by CH12.LX. The binding of anti-IgM antibodies did not prevent anti-Idiotype antibodies from delivering a differentiative signal. Thus, binding of ligand to different parts of the membrane Ig molecule can result in the delivery of different biological signals to the B cell.