CD8A and neoplasm: The rationale for investigation of PD-1/PD-L1 targeted immunotherapy in BTC is based on the observation of PD-L1-expressing tumor cells and the presence of tumor-infiltrating CD8 T cells in the tumor microenvironment of BTC, both of which have been recognized as biomarkers of efficacy for PD-1/PD-L1 immunotherapy [20,21].