ARG1 and neoplasm: The main barriers that CAR-T cells need to overcome once in the tumor include: (i) suppression by immunoregulatory cells, including myeloid-derived suppressor cells (MDSC), tumor associated macrophages and neutrophils, and regulatory T cells; (ii) presence of an array of immunosuppressive molecules, such as IL-10, TGF-β, PD-L1, IDO and arginase-1, and (iii) microenvironment factors, such as hypoxia, low pH and nutritional depletion.