RIG-I Resists Hypoxia-Induced Immunosuppression and Dedifferentiation

Christina Engel, Grethe Brügmann, Silke Lambing, Larissa H Mühlenbeck, Samira Marx, Christian Hagen, Dorottya Horváth, Marion Goldeck, Janos Ludwig, Anna-Maria Herzner, Jan W Drijfhout, Daniela Wenzel, Christoph Coch, Thomas Tüting, Martin Schlee, Veit Hornung, Gunther Hartmann, Jasper G Van den Boorn

A hypoxic tumor microenvironment is linked to poor prognosis. It promotes tumor cell dedifferentiation and metastasis and desensitizes tumor cells to type-I interferon (IFN), chemotherapy, and irradiation. The cytoplasmic immunoreceptor retinoic acid-inducible gene-I (RIG-I) is ubiquitously expressed in tumor cells and upon activation by 5'-triphosphate RNA (3pRNA) drives the induction of type I IFN and immunogenic cell death. Here, we analyzed the impact of hypoxia on the expression of RIG-I in various human and murine tumor and nonmalignant cell types and further investigated its function in hypoxic murine melanoma. 3pRNA-inducible RIG-I-expression was reduced in hypoxic melanoma cells compared to normoxic controls, a phenomenon that depended on the hypoxia-associated transcription factor HIF1α. Still, RIG-I functionality was conserved in hypoxic melanoma cells, whereas responsiveness to recombinant type-I IFN was abolished, due to hypoxia-induced loss of type I IFN receptor expression. Likewise, RIG-I activation in hypoxic melanoma cells, but not exposure to recombinant IFNα, provoked melanocyte antigen-specific CD8+ T cell and NK cell attack. Scavenging of hypoxia-induced reactive oxygen species by vitamin C restored the inducible expression of RIG-I under hypoxia in vitro, boosted in vitro anti-melanoma NK and CD8+ T cell attack and augmented 3pRNA antitumor efficacy in vivo. These results demonstrate that RIG-I remains operational under hypoxia and that RIG-I function is largely insensitive to lower cell surface expression of the IFNα-receptor. RIG-I function could be fortified under hypoxia by the combined use of 3pRNA with antioxidants.