Activated B cells mediate inhibition of Th-cell proliferation. (A) Effect of differently activated B cells on CD4⁺ Th-cell proliferation: Equal amounts of B cells were added unstimulated, or after prestimulation for 3 days with SAC or αIg to Th cells and cCD3⁺ IL-2. [3H]TdR incorporation was measured after 4 days of culture. Mean plus or minus SEM of 3 representative experiments. (B) Separation of SAC-activated B cells into 2 major populations, according to cell size distribution and activation status. Shown is 1 representative sort gate for separation of small CD25⁻ (R2 and R4 and not R6) and large CD25⁺ (R3 and R5 and not R6) B cells after prestimulation for 3 days