Inhibition of CD40 + IL-4–mediated cell proliferation by all-trans and 13-cis RA. Proliferation of PBMC (A) and purified peripheral B cells (B) induced by anti-CD40 + IL-4 (1 μg/mL and 5 ng/mL) and anti-CD40 + IL-4 + all-trans RA or 13-cis RA (10⁻¹² to 10⁻⁶ mol/L). Cells (10⁶/mL) were incubated for 3 days, and thymidine (0.5 μCi) was added for the last 16 hours of culture to each well. Each experiment was done in triplicate, the cpm values of unstimulated cells were <500 cpm and ranged after anti-CD40 + IL-4 stimulation from 20,000 to 30,000 cpm for PBMC and 10,000 to 20,000 for the B cells. Anti-CD40 + IL-4 stimulation was set at 100% and mean values of four experiments + standard error of mean (SEM) are shown.