Stimulation of murine B lymphocytes to IgG synthesis and secretion by the mitogens lipopolysaccharide and lipoprotein and its inhibition by anti‐immunoglobulin antibodies

The two mitogens, lipopolysaccharide (LPS) and lipoprotein (LP), both stimulate to an equal extent murine B lymphocytes to develop IgG‐synthesizing and IgG‐secreting cells. IgG synthesis and secretion was detected either by biosynthetic labeling of stimulated B cells, followed by immunoprecipitation and polyacrylamide gel electrophoresis of labeled Ig, or by the protein A plaque assay detecting all IgG‐secreting cells with IgG‐specific developing antisera. B cells from spleen, lymph node, thoracic duct and fetal liver of normal and T cell‐deficient nu/nu mice are all stimulated by the two mitogens to develop IgG‐secreting cells. Mitogen‐induced IgG secretion, therefore, can occur independently of T cells and of external antigen. Anti‐Ig antibodies inhibit the mitogen‐induced development of Ig‐secreting cells. Antibodies with specificities for μ chains when added before or soon after mitogenic stimulation of small, resting B cells, will inhibit these cells from developing into IgG‐secreting cells. Later in culture, antibodies with specificities for γ chains also become inhibitory for the mitogen‐induced development of IgG‐secreting cells. These results indicate that the precursors of the IgG‐secreting cells carry IgM in the outer surface membrane. They also suggest that during mitogen‐stimulated growth of B cell clones, IgG becomes expressed in the surface membrane in a functional way which allows the inhibition of mitogen‐induced IgG secretion by anti‐γ antibodies.