B cell–mediated Th-cell suppression is critically dependent on IL-2. (A-C) Th cells were incubated alone (■), with ^lgB25⁺ (▨) or ^smB25⁻ (▤) cells in presence of cCD3 and different concentrations of IL-2 or without IL-2 (med). (A) [³H]TdR incorporation of T cells. Data are expressed as mean plus or minus SEM of triplicates from 1 representative experiment. (B) PKH-26 assay at day 6 (mean ± SEM of 3 independent experiments). (C) Blockade of IL-2Rαβ by addition of Abs directly to culture or by preincubation of ^lgB25⁺ cells (B*) alone; cultures provided with cCD3 and IL-2 (100 U/mL). Mean plus or minus SEM and P value of 5 independent experiments.