Monoclonal antibodies specific for murine IgM. II. Activation of B lymphocytes by monoclonal antibodies specific for the four constant domains of IgM.

Seventeen monoclonal antibodies specific for IgM and one kappa light chain-specific antibody were used to test the effect of immunoglobulin (Ig)-specific antibodies on B cell activation. In soluble form, either alone or together with T cell-derived growth and maturation factors, none of the antibodies stimulated resting B cells to divide or secrete Ig. The soluble antibodies inhibited lipopolysaccharide-induced B cell activation. The inhibitory effect of the antibodies was independent of their Fc part. When immobilized, the same antibodies could activate B cells to proliferate and together with T cell-derived maturation factors to mature to plasma cells. Occupation by immobilized antibody of determinants on any of the four constant region domains and on the light chain of surface Ig can lead to the stimulation of B cells.