Inhibitory effect induced by rituximab is mediated by A-SMase activation-derived CER. (Ai) Activation of A-SMase by rituximab in B cells. Daudi (□) and RL (▪) cells were treated with 10 μg/mL rituximab for various time points. After which, A-SMase activity was quantitated as described in “Materials and methods.” Results are representative of 3 independent experiments. (Aii) Mean of peak A-SMase activation obtained from 3 independent experiments, ± SD. ^*P < .01. (B) Inhibition of rituximab-induced A-SMase activation and CER generation by SR33557. Daudi (□) and RL (▪) cells were preincubated 1 hour with 30 μM SR33557 and then treated with 10 μg/mL rituximab for various time points. After which, both A-SMase activity (i) and CER levels (ii) were quantitated as described in “Materials and methods.” Results are mean of peak A-SMase activation and CER obtained from 3 independent experiments, ± SD. ^*P < .01.