Ozone-primed neutrophils promote early steps of tumour cell metastasis to lungs by enhancing their NET production

Background Air pollution, including particulates and gazes such as ozone (O-3), is detrimental for patient's health and has repeatedly been correlated to increased morbidity and mortality in industrialised countries. Although studies have described a link between ambient particulate matter and increased lung cancer morbidity, no direct relation has yet been established between O-3 exposure and metastatic dissemination to lungs. Objectives To outline the mechanisms through which pulmonary O-3 exposure modulates metastasis kinetics in an experimental mouse model of O-3 exposure. Methods Metastatic responses to pulmonary O-3 exposure were assessed using a reliable experimental mouse model of concomitant pulmonary O-3 exposure and tumour cell injection. Roles of neutrophils in O-3-induced lung metastasis were highlighted using blocking anti-Ly6G antibodies; moreover, the implication of neutrophil extracellular traps (NETs) in metastatic processes was evaluated using MRP8cre-Pad4lox/lox mice or by treating mice with DNase I. Results Pulmonary O-3 exposure strongly facilitates the establishment of lung metastasis by (1) Inducing a pulmonary injury and neutrophilic inflammation, (2) Influencing very early steps of metastasis, (3) Priming neutrophils' phenotype to release NETs that favour tumour cell colonisation in lungs. The ability of O-3-primed neutrophils to enhance lung colonisation by tumour cells was proven after their adoptive transfer in Balb/c mice unexposed to O-3. Conclusions Pulmonary neutrophils induced by O-3 promote metastatic dissemination to lungs by producing NETs. These findings open new perspectives to improve treatment and prevention strategies in patients affected by metastatic diseases.