CTLA4 and neoplasm: Moreover, hypoxia can directly induce the upregulation of inhibitory immune checkpoints ligand (e.g., PD-L1 and cytotoxic T-lymphocyte-associated protein 4, CTLA-4) on dendritic cells (DCs), myeloid-derived suppressor cells (MDSCs), TAMs and tumor cells through HIF-1α [23].