A natural IgM antibody does inhibit polyclonal and antigen-specific IgM but not IgG B-cell responses

Since a B-cell growth-inhibitory natural IgM antibody was identified in the culture supernatants of LPS-stimulated murine splenic B lymphocytes [11], attempts have been made to define other possible functional role(s) of this antibody. Here we show that this regulatory IgM is able to inhibit not only the proliferation of splenic B cells, but also their IgM secretion during LPS-induced polyclonal, as well as antigen (FITC-KLH)-specific antibody responses. In contrast, IgG1 production of hapten (FITC)-specific B cells neither during restimulation with LPS nor in the presence of carrier-specific T lymphocytes in vitro was affected by regulatory IgM. Therefore, whereas newly emerging naive B cells are highly susceptible, IgG-secreting B cells appear to be completely resistant to inactivation by the regulatory IgM autoantibody.