LPS augments human B-cell differentiation by direct stimulation of PWM-responsive B cells

Bacterial lipopolysaccharide (LPS) augments production of IgM and IgG by two- to seven-fold in cultures of peripheral blood lymphocytes (PBL) stimulated by pokeweed mitogen (PWM), but only if monocytes are rigorously depleted. When PBL were separated into adherent cell (AC), B-cell-enriched, and T-cell-enriched fractions, pulsed with LPS, and recombined in culture with PWM, increased generation of plasma cells was seen only in cultures containing LPS-treated B cells. This effect of LPS appears to be independent of soluble factors. Supernatants from LPS-stimulated B cells or AC did not consistently increase PWM responses when cultured with fresh B cells in the presence of polymyxin B. Furthermore, pulsing of B cells with purified interleukin 1 from two different commercial sources failed to augment PWM-induced differentiation. When B cells were depleted of surface IgD (sIgD)-bearing cells by panning, no effect on LPS-mediated augmentation of PWM-driven differentiation was seen. B cells were also fractionated by rosetting with mouse erythrocytes. Treatment of BMR+ cells with LPS did not induce them to respond to PWM, while treatment of BMR- cells with LPS augmented generation of plasma cells. These results indicate that LPS acts directly to augment differentiation of PWM-responsive B cells, rather than recruiting sIgD+, BMR+ cells to become PWM responsive.