CD80 expression increases proliferative responses of allogeneic T cells to U937 cells. (A) T cells were cocultured with irradiated U937-M3P, U937-CD80, or third-party DCs. Cumulative data of proliferative responses from 8 experiments with 5 different donors with mean ±SE are shown. Stimulator:responder ratios were 1:1 and 1:10 for the leukemic and DCs, respectively. T cells demonstrated up to a 16-fold increase in their proliferative response to U937-CD80 cells compared with the control cells. (B) The increased proliferative response of T cells to CD80⁺ cells seen in (A) could be inhibited by soluble CTLA4.Ig (10 ng/mL), demonstrating this is a CD80-mediated effect. These data are representative of 3 independent experiments.