CD40-mediated signaling in B cells. Balancing cell survival, growth, and death.

Multimerization of CD40 molecules on B cells by binding CD40 ligand on activated T cells initiates signaling events essential for B cell differentiation. In mature B cells, CD40 mediates stimulation and costimulation of cell growth, switch recombination, and transcriptional regulation. CD40-mediated signaling also regulates cell death, rescuing B cells from anti-Ig-induced apoptosis and inducing the expression of the Fas surface molecule. Recent efforts to elucidate the biochemistry of CD40-mediated signaling pathways have identified members of the TRAF protein family that are associated with the cytoplasmic tail of CD40. CD40 cross-linking probably multimerizes TRAF proteins which may act as direct transcriptional regulators. Modulation of protein tyrosine kinase and protein tyrosine phosphatase activity also occurs after CD40-mediated signaling; however, a connection to the TRAF pathway has not been established. Multiple pathways of B cell triggering are probably integrated at the level of transcriptional activation to produce differentiation stage-specific B cell responses.