Anti-CD38 stimulation induces proliferation of transitional 2 B cells. Splenocytes from wild-type or CD38^−/− mice were stained for B220, CD21, and CD24. The cells were gated on B220-positive cells, and each population was purified according to its expression of CD21 and CD24. The purity of each subpopulation (T1, T2, or M) was more than 90%. (A) In 96-well plates, triplicates of each subset from wild-type mice were stimulated as indicated. (B) T1 B cells from wild-type or CD38^−/− mice were preincubated 30 minutes with 8-Br-cADPR or only medium. The cells, without washing, were then stimulated with rat-IgG2a (□), rat anti–mouse CD38 (rat IgG2a, ■), F(ab′)2 anti–mouse IgM (▒), or LPS (▨). (C) T2 B cell subsets from wild-type or CD38^−/− mice were stimulated as in panel B except that LPS was not included. The plates were incubated 48 hours at 37°C. Each well was pulsed with 1 μCi of [³H] thymidine 8 hours before being harvested. Results are expressed as mean plus or minus SD from 3 independent experiments (P < .01 as indicated, Student t test).