ARG1 and neoplasm: In the tumour sites, MDSCs form the immunosuppressive microenvironment by producing Arg-1, iNOS, IDO, NOX2 and immunosuppressive cytokines [13, 14], and MDSCs express a large amount of PD-L1 through the stimulation of hypoxia-inducible factor 1α (HIF1α) and tumour-derived exosomes and eventually inhibit the activity of T cells [15, 16].