ARG1 and neoplasm: Although CAR T cell inactivation may stem from multiple immunosuppressive stimuli, tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) are thought to contribute prominently, since they i) secrete immunosuppressive cytokines (e.g. IL-10 and TGF-β) (3, 4), ii) nitrosylate and inactivate T cell receptors (5, 6), iii) express immune checkpoint receptors (7), iv) promote deposition of a dense extracellular matrix that can impede penetration of immune cells (8, 9), and v) produce immunosuppressive enzymes such as arginase 1, CD39 and 5’-nucleotidase (10–12).