On the hipoxia regulation of CD137 and CD69 expression

Abstract

The tumor microenvironment is characterized to be under severe conditions (nutrient shortage, low pH and oxygen deprivation) that affect not only tumor cells but also stroma and immune infiltrate. Tumor cells have developed mechanism in order to adapt their biology to such extreme circumstances. One of these adaptation strategies is the HIF-1α pathway that is activated under hypoxia (low oxygen availability) promoting angiogenesis, metabolic changes and survival. The projects that have been carried out during this thesis are focused on the influence of hypoxia in the biology of malignant and immune cells in the tumor microenvironment regarding the regulation of two important lymphocyte receptors: CD137 and CD69. Under hypoxic conditions tumor cells promote the alternative splicing of the co-stimulatory receptor CD137 (also known as 4-1BB) resulting in the generation of a functional soluble form of CD137 (sCD137). The secreted sCD137 binds the natural ligand of CD137 (present on APCs) preventing the CD137 ligand-mediated co-stimulation of T-lymphocytes acting as an immune-evasion mechanism. Additionally, CD137 silencing in a colon carcinoma cell line enhances the tumor growth control mediated by CD8+ T lymphocytes. Tumor-infiltrating T lymphocytes (TILs) are also influenced by hypoxia that controls many of the mechanisms that confer T-cells a cytotoxic phenotype. TILs from transplantable and spontaneous mouse tumors express the early T-cell activation marker CD69 more intensely than lymphocytes located in a non-hypoxic tissue such as the spleen. The second study of this thesis points out that hypoxia up-regulates CD69 in T-lymphocytes undergoing TCR-CD3 activation in a HIF-1α dependent manner. Indeed, HIF-1α is able to bind a hypoxia response element (HRE) that we have found in the promoter of the human CD69 gene. In addition, in vivo staining with pimonidazole shows a correlation between hypoxia and the intensity of CD69 expression on T lymphocytes from hypoxic tissues, such as bone marrow and tumor. These studies highlight that hypoxia plays an important role in the interplay between tumors and immune infiltrates in the tumor microenvironment.