Elevated levels of DLL1 in the TME improves anti-CTLA4 therapy. EO771-R1 and EO771-L1 breast tumors were prepared as described in Fig. 1. (A and B) The tumor growth curves and the survival of EO771-R1 and EO771-L1 tumor-bearing mice without or with anti-CTLA4 therapy. When tumors reached 4 to 5 mm in diameter, mice were randomly assigned to two groups and treated with an anti-CTLA4 antibody or control IgG (5 mg/kg) every 3 d for four doses. (C and D) The tumor growth curves and the survival of EO771-R1 and EO771-L1 tumor-bearing mice without or with CD4+ T cell depletion. When tumors reached 3 to 4 mm in diameter, mice were randomly assigned to two groups and treated with an anti-CD4 antibody or control IgG (200 μg/mouse) on days 6, 8, and 14 post–tumor cell inoculation. The significance was determined by one-way ANOVA. Data are from one experiment representative of two independent experiments with similar results (n = 10 mice per group). **P < 0.01, ***P < 0.001.