cGAS-STING-mediated self-DNA sensing in COVID-19 immunopathology

Covid-19 is a major public health threat that has already caused more than 200,000 deaths worldwide. Most patients only experience mild-to-moderate upper respiratory tract symptoms, but 10 to 20% progress to severe disease with pneumonia, acute respiratory distress syndrome (ARDS), and systemic endothelitis affecting multiple organs including kidney, heart, brain and skin. Evidence suggests that severe disease is driven by a dysfunctional immune response characterized by excessive inflammation. Accordingly, anti-cytokine therapies have triggered huge excitement for the treatment of Covid-19. However, current data from ongoing trials suggest only limited clinical improvements. Identifying the molecular driver(s) underlying the hyperinflammatory response in Covid-19 is therefore vital to provide and rationally design new and more efficacious therapies. The central hypothesis of our proposal is that aberrant activation of the cGAS-STING pathway - the dominant innate sensing system that recognizes DNA as a danger signal - is a key pathogenic factor in Covid-19-associated immunopathology. We hypothesize that the cytopathic effect of the virus on epithelial and endothelial cells promotes cell death with accrual of extracellular DNA, which is then internalized by infiltrating macrophages, unleashing the fatal inflammatory response via activation of cGAS-STING signaling. Our hypothesis is based on three observations: (i) cGAS-STING is a major sensor of tissue damage and COVID-19 pathology is associated with widespread epithelial and endothelial cell destruction; (ii) bats have evolved a non-functional STING gene, which allows them to withstand tissue stress during flapping flight and may explain their ability to host coronaviruses in the absence of pathology; (iii) COVID-19 associated skin lesions demonstrate transcriptional hallmarks of COVID-19 pathology and resemble skin lesion of SAVI, a monogenic disease caused by activating mutations in STING.Our proposal will test this hypothesis with three aims. In Aim 1 we will determine whether cGAS-STING is activated in COVID-19 pathology. To this end, we will use COVID-19 derived skin lesions, which are common, readily accessible for biopsies, and, most important, demonstrate a striking molecular and phenotypic overlap with inflammatory processes in the lung. Using these clinical samples, we will perform extensive immunophenotyping, transcriptome expression profiling, virus visualization by electron microscopy, and analysis of cGAS-STING activation. In Aim 2 we will dissect the mechanism of cGAS-STING pathway activation by SARS-CoV-2 infection using complementary in vitro cell culture approaches. The goal is to establish the contribution of cell-intrinsic versus cell-extrinsic mechanisms leading to cGAS-STING pathway activation in infected and bystander cells, respectively. In Aim 3 we will pursue proof-of-concept studies on the efficacy of pharmacological inhibition of STING to attenuate SARS-CoV-2 immunopathology using two pre-clinical models of SARS-CoV-induced pneumonia.