Tumor Hypoxia: A Key Determinant of Microenvironment Hostility and a Major Checkpoint during the Antitumor Response

Recent antitumor immunotherapies such as monoclonal antibodies targeting immune checkpoints have led to outstanding results in several cancers. However, despite the favorable outcomes for responding patients, the response rate remains relatively low. This is in part due to the influence of the tumor microenvironment (TME) in protecting the tumor from the antitumor immune response and facilitating immune escape. Tumor hypoxia is one of the most important features of the TME, exerting an adverse effect on tumor aggressiveness and patient prognosis. Hypoxic stress interferes with immune plasticity and promotes tumor heterogeneity and progression. Cellular adaptation to hypoxia is primarily mediated by a family of transcriptional regulators, hypoxia-inducible factor (HIF). Apart from hypoxia, the HIF pathway is modulated in a hypoxia-independent manner. HIF-1α stabilization and activity are regulated by epigenetic changes and mutations. Strong evidence indicates that tumor hypoxia controls malignant and metastatic phenotype of cancer cells and therefore presents a unique therapeutic challenge in the treatment of solid malignancies. An alluring alternative strategy to reinvigorate anticancer immune responses comes from the emerging field of TME and its associated pathways. Targeting hypoxia or its associated pathways may therefore offer new options in the design of innovative cancer immunotherapy approaches. In this article, we briefly review the potential of hypoxic stress on tumor plasticity and stromal reactivity as well as the possible targeting of hypoxia-induced pathways to increase immunotherapy efficiency.