Tumor-associated immune-suppressive cells, soluble factors, and immune cell metabolism. Tumor cells are surrounded by several cell subsets including Tregs, TAMs, and MDSCs, which hamper the function of cytotoxic T cells and NK cells. Chemokines play a critical role in recruiting inhibitory cells, whereas soluble factors such as TGF-β, nutrient deprivation, and hypoxia impair the proliferation and function of effector cells.