Toso specific monoclonal antibody A38 blocks survival signaling pathways. (A) Anti–human Toso mAb A38 enhances CD95-mediated apoptosis. Human primary T cells were preincubated for 60 minutes with 2 μg/mL of anti–human Toso mAb A36, A38 or control rat IgG. Cells were then stimulated for 16 hours with 100 ng/mL CD95L and apoptosis was assessed by annexin V staining. Representative flow cytometric histograms from cells treated with control IgG (gray filled), anti–human Toso mAb A36 (blue) or anti–human Toso mAb A38 (red) are shown on the left. Relative % of apoptosis is quantified on the right. Data are representative of 4 experiments. Statistical analysis: Student t test (**P < .01). (B) Anti–human Toso mAb A38 inhibits TNFα-mediated Erk-phosphorylation. Human primary T cells were preincubated for 60 minutes with 2 μg/mL human Toso mAb A36, A38 or control rat IgG. Cells were then stimulated for 10 minutes with 100 ng/mL TNFα. Erk-phosphorylation was examined by intracellular flow cytometric analysis using a phospho-Erk specific antibody. Representative flow cytometric histograms from cells treated with control IgG (gray filled), anti–human Toso mAb A36 (blue) or anti–human Toso mAb A38 (red) are shown on the left. Relative % of Erk-phosphorylation is quantified on the right. Data are representative of 3 experiments. Statistical analysis: Student t test (**P < .05).