Human B-cell differentiation induced by murein

Like "true" polyclonal B-cell activators (PBA) for murine B cells, crude membrane preparations of Klebsiella pneumoniae (Klebs M) and some other enterobacteriaceae stimulate human B cells to mature into immunoglobulin (Ig) secreting cells without significant prior proliferation and in the absence of T cells. To investigate the biochemically defined membrane component with this unique PBA property, we studied lipoprotein and murein isolated from E. coli, since other components (e.g., a variety of lipopolysaccharide (LPS) fragments) failed to imitate Klebs M as a PBA. Mononuclear cells (MNC) and B cell-enriched cell populations from healthy blood donors were stimulated with various doses of lipoprotein and murein and, in comparison, Klebs M and pokeweed mitogen (PWM). Cell cultures exposed to either lipoprotein, murein, or Klebs M failed to incorporate [³H]thymidine significantly after 5 days in culture. In contrast, there was significant DNA synthesis (stimulation index > 3) when PWM was given to the same MNC population. All stimulants, with the exception of lipoprotein, induced B-cell differentiation in MNC cultures, as measured by an ELISA quantitating secreted Ig in the culture supernatants. In cultures with B cell-enriched cell populations, however, only Klebs M and murein were able to induce the production of significant amounts of IgM. Thus, the actual PBA moiety contained in the crude membrane fraction (Klebs M) appears to be associated with murein. It is important to note that murein induced considerably weaker Ig secretion than Klebs M did.