Effect of CTLA-4 blockade on the response of tumor-bearing mice to early immunization. Mice with or without established tumors received anti-HA TCR+ transgenic T cells on day 0 and were treated with anti-CTLA-4 antibodies or a similar amount of hamster IgG antibody on days −1, 0, +1 or were left untreated. Half the mice in each group were immunized with vacc-HA on day +2, and all mice were analyzed 6 days later. The responses of mice with a small tumor burden (tumor given day −9) are displayed on the Left and those with a large tumor burden (day −18) on the Right. (A) T cells from unimmunized mice (solid bars), vacc-HA-immunized mice (interrupted cross-hatched bars), and mice treated with anti-CTLA-4 antibodies plus vacc-HA (hatched bars) were analyzed by FACScan for CD4 versus anti-HA TCR. Values represent mean + SE of percentage of T cells expressing the clonotypic TCR for three mice per group. Proliferation of T cells in response to HA peptide in vitro (B) as well as IFN-γ production (C) were also measured. Values are the mean + SE of triplicate cultures from three mice in each group.