Treatment of murine CD5^- B cells with anti-lg, but not LPS, induces surface CD5: Two B-cell activation pathways

Antl-lg stimulated murine B cells express high levels of surface CD5 (ly-1) and increased CD44 while maintaining surface IgD, CD23 and J11d. Sorting of CD5^- and CD5⁺ cells demonstrates that anti-lg induces CD5 expression rather than the selective expansion of CD5⁺ cells. Anti Ig plus interieukln-6 (IL-6) induces the CD23, IgD, low ly-5 (B220) (CD45^low), J11d^high phenotype of typical CD5⁺ peritoneal B cells. In contrast, lipopolysaccharide (LPS)-stimulated B cells have high levels of CD44 but decreased surface IgD, CD23 and J11d and no CD5. Thus LPS and anti-lg generate activated cells with differing phenotypes. Induced CD5⁺ cells have increased viability, even in the absence of added exogenous factors, while the viability of CD5^- B cells is dependent on factors such as IL-4. We conclude that conventional CD5^- B cellscan be activated by either of two pathways: one generating CD5⁺ B cells; the other yielding conventional activated cells. We hypothesize that the first path requires slg cross-linking and corresponds to T-independent (type 2) stimulation, while cognate Interaction with helper T cells in the absence of slg cross-linking induces B cells to enter the second path.