PDCD1 and neoplasm: Mechanistically, elevated cholesterol levels in these tumor-infiltrating CD8+ T cells trigger ER stress and activation of XBP1 signaling pathway which, in turn, leads to the expression of immune checkpoint molecules such as PD-1, natural killer cell receptor 2B4, metalloproteinase inhibitor 3 (TIM3) and lymphocyte activation gene 3 protein (LAG3) (63).