Cancer-promoting mechanisms of tumor-associated neutrophils

Neutrophils have classically been considered to mount a defensive response against tumor cells, yet recent evidence suggests tumors modulate neutrophil function to support tumor growth and progression.Tumor-associated neutrophils (TANs) are phenotypically distinct from circulating neutrophils in terms of their surface protein composition and cyto/chemokine activity and response. Although TANs have been shown to both promote and inhibit tumor advancement, the preponderant activity augments tumor progression. This review discusses these cancer-promoting molecular pathways, relevant diagnostic studies in patients, and subsequent treatment modalities. The tumor promoting mechanisms of TANs include dampening of CD8+ response via Arginase-1; a neutrophil-secreted neutrophil elastase (NE) upregulation of tumor cellular proliferation pathways; degradation of basement membrane and ECM via NE and MMP-9; upregulation of angiogenesis by VEGF, and HGF; and ICAM-1 dependent tumor intravasation, immune protection in circulation, and extravasation into distant, metastatic tissue beds. Clinicians are constrained in treating TANs systemically as it may induce neutropenia, therefore targeting TANs-mediated tumor progression pathways surgically on a loco-regional level is a viable adjuvant treatment modality.TANs modulate the tumor microenvironment promoting tumor progression. Mechanistic understanding of TANs role in tumor progression will provide unique therapeutic alternatives.