IDO1 and cholangiocarcinoma: We speculate that the future clinical trials in CCA could include IDO inhibitors alone or in the combination with anti-PD-1, as well as other anti-TGF-β antibodies, which were tested in preclinical settings and demonstrated an unleashed anti-tumor CTL response upon TGF-β blockade, rendering tumors more susceptible to anti-PD-(L)1 [85] and leading to tumor regression.