PDCD1 and neoplasm: We hypothesized that combination therapy with UDCA and either anti-PD-1 or anti-PD-L1 could overcome tumor immunosuppression in two ways, thereby significantly enhancing antitumor immunity: 1) protecting effector T cells from suppression by inhibiting Treg cell differentiation and activation and 2) preventing PD-1 signaling in effector T cells to suppress their activation (Supplementary Fig. 9a).