Tumor-defective antigen presentation and costimulation, resistance to cytolysis, and induction of immune exhaustion. Tumor cells can escape immune recognition by downregulating MHC class I molecules and costimulatory molecules and by developing defective antigen processing. Tumor cells can also resist the cytolytic effects of immune cells by overexpressing antiapoptotic molecules or inhibitory ligands. PD-L1 expression by tumor cells causes T-cell exhaustion. APM, antigen-presenting machinery.