ARG1 and head and neck squamous cell carcinoma: Moreover, neutrophils that display an immunosuppressive phenotype upon stimulation by tumour micro-environmental cues [so called polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC)] have been shown to accumulate in both patient biopsies and murine experimental models of head and neck squamous carcinoma (HNSCC), where they inhibit the tumouricidal functions of natural killer cells by secretion of TGF-β, nitric oxide and arginase-1 [31, 32].