Hypoxia: a key player in antitumor immune response. A Review in the Theme: Cellular Responses to Hypoxia

Abstract

-The tumor microenvironment is a complex system, playing an important role in tumor development and progression. Besides cellular stromal components, extracellular matrix fibers, cytokines, and other metabolic mediators are also involved. In this review we outline the potential role of hypoxia, a major feature of most solid tumors, within the tumor microenvironment and how it contributes to immune resistance and immune suppression/tolerance and can be detrimental to antitumor effector cell functions. We also outline how hypoxic stress influences immunosuppressive pathways involving macrophages, myeloid-derived suppressor cells, T regulatory cells, and immune checkpoints and how it may confer tumor resistance. Finally, we discuss how microenvironmental hypoxia poses both obstacles and opportunities for new therapeutic immune interventions.hypoxia; hypoxia-inducible factor; tumor microenvironment; myeloid cells; lymphoid cells; immune suppression; cancer stem cells; programmed death-ligand 1; epithelial-mesenchymal transition; circulating tumor cells; autophagy and antitumor immune response IT HAS BECOME INCREASINGLY apparent that malignant cells exist in a complex cellular and extracellular microenvironment that plays key roles in the initiation and maintenance of the malignant phenotype. Among the microenvironmental factors that play a dominant role in neoplasia, hypoxia is believed to be one of the most relevant in the neoplastic response of tumor cells. It is widely appreciated that the majority of malignancies create a hostile hypoxic microenvironment that can hamper cellmediated immunity and dampen the efficacy of the immune response. Poorly vascularized and hypoxic zones inside solid tumors contribute to immune tolerance of tumor cells by impeding the homing of immunocompetent cells into tumors and inhibiting their antitumor efficacy. Several regulatory mechanisms can occur concurrently within the hypoxic tumor microenvironment, resulting in multiple redundant levels of immune cell plasticity and suppression, tumor plasticity, and functional heterogeneity. Hypoxic stress clearly plays a crucial role in tumor promotion and immune escape by controlling angiogenesis and favoring immune suppression and tumor resistance. Like other therapeutic attempts, immunotherapy is heavily hampered by the morphologically aberrant tumor microvasculature, preventing migration of immune effector cells into established tumors. Many strategies are emerging for changing the immunosuppressive nature of the tumor to a microenvironment able to support antitumor immunity. The identification of ways to induce a permissive and less hostile tumor microenvironment to avoid tumor resistance and immune suppression is of major interest and pertinence. Hypoxia as an Integral Component of the Tumor MicroenvironmentAll life on Earth depends on O 2 , and all physiological and pathological processes of a living cell rely principally on O 2 (103). Hypoxia is characterized by lack of O 2 , and hypoxic tissues are inadequately oxygenated (107). A p...

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