CTLA4 and neoplasm: Administration of IL-15SA/IL-15RαSu-Fc and anti-CTLA-4, but not anti-PD-L1, generated a synergistic anti-tumor response in CT26-bearing mice (Figure 8B), which is consistent with the observation that the efficacy of checkpoint inhibitors, in particular anti-PD-1 and PD-L1, is dependent on the surface expression of inhibitory ligands on tumor cells [45–47].