T-cell inhibition requires cell contact and constant presence of activated B cells. (A) PKH assay of Th cells stimulated alone or together with SAC-activated B cells, or separated by cell culture insert (T/Insert/B-SAC) after 3 and 6 days. (B) Surface expression of CD71, CD69, and CD25 on gated CD4⁺ T cells after 24-hour stimulation with cCD3/IL-2 in presence or absence of ^lgB25⁺ cells. (C) T cells prestimulated alone (T-prestim) or with ^lgB25⁺ cells (T-prestimB) for 48 hours and restimulated with IL-2 or IL-2+ ^lgB25⁺ cells. Data are expressed as mean plus or minus SEM of triplicates from 1 representative experiment. (D) Coculture of PKH-26⁺ responder T cells (Tresps) with Tprestim or with TprestimB for 24 or 48 hours. Tresps were used fresh () or prestimulated for 48 hours (■). PKH-26 was measured after 3 days.