NF-κB1 and c-Rel cooperate to promote the survival of TLR4-activated B cells by neutralizing Bim via distinct mechanisms

The nuclear factor-κB (NF-κB) pathway is crucial for the survival of B cells stimulated through Toll-like receptors (TLRs). Here, we show that the heightened death of TLR4-activated nfkb^-/- B cells is the result of a failure of the Tpl₂/MEK/ERK pathway to phosphorylate the proapoptotic BH3-only protein Bim and target it for degradation. ERK inactivation of Bim after TLR4 stimulation is accompanied by an increase in A1/Bim and Bcl-x_L/Bim complexes that we propose represents a c-Rel - dependent mechanism for neutralizing Bim. Together these findings establish that optimal survival of TLR4-activated B cells depends on the NF-κB pathway neutralizing Bim through a combination of Bcl-2 prosurvival protein induction and Tpl2/ERK-dependent Bim phosphorylation and degradation.