ARG1 and neoplasm: Another molecule that is believed to contribute to free radical production in MDSCs is myeloperoxidase (MPO), an enzyme that is highly abundant in neutrophils and PMN-MDSCs, inducing cytotoxicity during the respiratory burst.67 Splenic PMN-MDSCs from tumour-bearing mice displayed an increased activity of MPO, ARG-1 and ROS producing enzymes, which correlated with their ability to suppress antigen-specific T cell responses in vitro.67 In this setting, PMN-MDSCs expressed high levels of CD115 and CD244 compared to splenic neutrophils from wild-type controls.