Hypoxia plays an extensive role in the development of the tumor microenvironment (TME), particularly in mediating immunosuppression. Respiratory hyperoxia therapy has the potential to improve the effects of conventional cancer therapies via molecular mechanisms mediating antitumor immunity. Here, we investigated whether hyperoxia therapy can restore tumor immunity and inhibit lung metastases in a mouse model of triple-negative breast cancer (TNBC) by treating a 4T1 mammary carcinoma mouse model with normoxia (21% oxygen) or hyperoxia (60% oxygen) therapy, after tumor development. Using flow cytometry analysis, we observed significant organ-specific expansion of myeloid-derived suppressor cells (MDSCs) and protein expression upregulation of the programmed death-ligand 1 (PD-L1) in the hypoxic TME of 4T1 tumor-bearing mice maintained under normoxia conditions, with the TME converting to a T-cell immune-suppressive state as early as the premetastatic phase. Markedly, hyperoxia treatments ameliorated hypoxia levels in the lung TME and decreased the proportion of MDSCs and the expression of PD-L1 in both the primary tumor and in the metastatic lung, when compared to animals treated with respiratory normoxia therapy. In addition, the number of lung metastatic nodes fell from 90 per lung in the normoxic treated group to 13 per lung in the hyperoxic treated group (P < 0.05), with the latter having limited hyperoxia effects on primary tumor growth (mammary glands). Notably, hyperoxia therapy was characterized by the differential recruitment of CD4+ and CD8+ T-cells. Thus, our study confirms that hyperoxia therapy may be used to overcome TME immunosuppression and control the extend of lung metastases in TNBC. Importantly, changes in immunosuppressive MDSCs frequency and PD-L1 expression levels may serve as biomarkers of hypoxia levels in cancer affected tissues that can benefit from hyperoxia treatments.
Keywords: 4T1 mammary carcinoma; Respiratory hyperoxia therapy; T-cell immune-suppression; hypoxia; lung metastasis; myeloid-derived suppressor cells; programmed death-ligand 1; tumor immunity; tumor microenvironment.