CD80 expression by tumor cells increases the frequency of precursor T cells activated. (A) The data show calculated HTLp frequencies (log10 scale) to U937-M3P and U937-CD80 cells for 5 individuals as described in Materials and Methods. Precursor frequencies were measured by a limiting dilution assay. The frequencies were increased up to 5-fold by CD80 expression and correlated with the proliferation observed in bulk cultures. (B) This bar chart represents the mean ± SE ³H-thymidine incorporation by the IL-2–dependent cell line, CTLL-2, for the wells of the bottom dilution from the 5 experiments shown in (A). The values from wells scored negative or positive are shown, when either U937-M3P (□) or U937-CD80 (▪) cells were used. ³H-thymidine incorporation of the negative wells was similar in both U937-M3P and U937-CD80 assays, demonstrating a comparable baseline level. Mean CTLL-2 proliferation in the positive wells was 78% greater in the presence of U937-CD80 cells. This suggests that, after activation, precursors stimulated by U937-CD80 cells produce more IL-2.