HAVCR2 and neoplasm: Hypoxia is also inclined to trigger the expression levels of immune checkpoint molecules including PD-L1 on tumor cells as well as PD-L1, T cell immunoglobulin and mucin-domain containing-3 (TIM-3) and CTLA-4 on tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs) (66, 67).