ERK activation protects LPS-activated B cells from apoptosis. (A) wt B cells (> 99% viable at T0) were untreated or stimulated for 24 and 48 hours with LPS in the absence or presence of PD98059. The frequency of apoptotic cells was determined at each time point. The data represent the mean plus or minus SEM from 4 experiments. (B,C) wt or nfkb1^−/− B cells transduced with control or Raf/ER-expressing retroviruses left untreated or stimulated for 48 hours with LPS in the absence or presence of 4-HT. Apoptosis was assessed at 24 and 48 hours. (B) Levels of spontaneous apoptosis. (C) Levels of LPS-induced apoptosis. Both sets of data represent the mean plus or minus SD from 4 separate experiments. (D) Apoptosis in cultures of LPS-treated nfkb1^−/− B cells expressing Raf/ER activated with 4-HT 0, 2, 4, 6, 12, or 24 hours after initiating TLR4 signaling. Results are the mean plus or minus SD from 3 experiments.