CD274 and neoplasm: Although this lack of response could be due to the expression of checkpoint ligands in cancer cells and microbiota composition (Sharma et al., 2017; Gopalakrishnan et al., 2018; Routy et al., 2018), recent studies indicate that expression of PD-L1 in leukocytes rather than tumor cells is essential for PD-L1 blockade-mediated tumor regression (Lin et al., 2018; Tang et al., 2018), which emphasizes the contribution of tumor-infiltrating leukocytes to the insufficiency of checkpoint therapies.