Phenotypic Convergence and Divergence of Surface Immunoglobulin and CD40 Signals

Both anti‐CD40 antibodies and anti‐immunoglobulin (Ig) coupled to Sepharose induced proliferation of resting B cells and suppressed lipopolysaccharide (LPS)‐induced B‐cell differentiation to immunoglobulin secretion at comparable levels determined with the plaque‐forming assay and Ig RNA steady state levels. Anti‐CD40 antibodies also increased the proliferation of B cells stimulated by T helper cells in vitro while suppressing their differentiation to Ig secretion. Further, B cells preactivated by anti‐Ig, anti‐CD40 or a combination of the two mitogens could be restimulated by anti‐CD40 but not by anti‐Ig antibodies. Phenotypic divergence of Ig and CD40 signals regarding surface expression of activation markers was observed. Restimulation of anti‐Ig‐ or anti‐CD40‐prestimulated cells with anti‐Ig induced apoptosis whereas apoptosis could be inhibited when cells were recultivated with anti‐CD40.