TRAF1 is essential for the antiapoptotic activity of p80HT. (A) Immunoblot analysis of p80HT, p52, and TRAF1 expression in WEHI-231 B lymphoma cells infected with retroviruses expressing GFP, p80HT, TRAF1, GFP siRNA (GFPsi), or TRAF1 siRNA (TRAF1si). Levels of α-tubulin are shown as loading control. (B) Anti-IgM-induced apoptosis in WEHI-231 cells overexpressing GFP, p80HT, TRAF1, or p80HT and TRAF1 siRNA (p80HT-TRAF1si), as analyzed by annexin-V staining and trypan blue dye exclusion assay. (C) In vitro survival assays of splenic B cells from the mice of indicated genotypes. Cells were either untreated or treated with 20 μg/mL of lipopolysaccharide. Viability was determined by trypan blue dye exclusion assay. (D) The numbers of total splenic B cells from the mice of indicated genotypes. Splenic lymphocytes were stained with fluorescein-conjugated anti-B220 and analyzed by flow cytometry. Data in panels B-D represent means (± standard deviation) from at least 3 independent experiments or from 3 to 5 mice of each genotype. Statistical analysis was performed using the 2-tailed Student t test; *P < .05; **P < .01.