FAS and neoplasm: External suppressive signals (i.e., TGF-β, IL-10, PGE2, soluble FAS, adenosine, ROS) from stromal and suppressive immune cells such as cancer associated fibroblasts (CAFs), myeloid-derived suppressor cells (MDSCs), tumor associated macrophages (TAMs), tumor associated neutrophils, mast cells, and regulatory T cells (Treg) contribute to the initiation of exhaustion of CAR T cells [106].