COVID19 is caused by SARS-CoV-2, a novel member of the human coronavirus family that includes the closely related SARS-CoV (SARS) and MERS-CoV (MERS) viruses. SARS-CoV-2 viral spike protein binds to human angiotensin converting enzyme-2 (ACE2) on the surface of cells and is then primed by the serine/threonine protease TMPRS22, whereupon the entire complex is internalized by the target cell. ACE2 is expressed by multiple cell types of the body, including lung and gut epithelium (likely the primary sites of initial infection) and vascular endothelial cells (EC) of multiple organs. In addition to the well-described pneumonia-like disease characterized by compromised lung function with subsequent depressed pO2 levels in blood, patients often also show signs of multi-organ involvement, which can include gut, kidney, liver, heart, and brain. Most recently, numerous pediatric patients have been showing signs of Kawasaki disease, a systemic vascular inflammation. Over 30% of COVID19 ICU patients also show signs of thrombosis and 25% suffer venous embolism. Cerebral ischemia, likely due to clot formation, has also been reported. What is not clear is whether this multi-organ involvement is due to secondary infection of these tissues or whether these are all a consequence of systemic hyperinflammation. The sequence of events that could drive systemic hyperinflammation stems from the SARS- CoV-2 mechanism of infection. Angiotensin II (AngII) is an important vasoconstrictor and under normal physiological conditions its level is closely controlled through rapid degradation by ACE2, however, SARS-CoV- 2 entry into cells clears ACE2 from the cell surface, potentially prolonging the action of AngII. Primary consequences of this would be two-fold: prolonged vasoconstriction in the lung (exacerbating poor oxygenation of the blood by the already compromised lungs) and a shift toward a pro-inflammatory state, as it is well established that AngII can drive local vascular inflammation, in large part through the induction of IL-6 in EC and smooth muscle cells (SMC). IL-6 is one of the major drivers of systemic hyperinflammation, and in its most severe form, a so-called ?cytokine storm?. IL-6 is also strongly correlated with thrombosis, likely through upregulation of tissue factor on EC and macrophages, and by downregulation of thrombomodulin on EC. Through the parent award we have generated Vascularized Micro-Organs (VMOs), comprised of perfused human vasculature and a surrounding stroma, and have further developed these into Vascularized Micro-Brains (VMBs) incorporating a Blood-Brain Barrier, and Vascularized Micro-Livers (VMLs). Additional cells, including macrophages and SMC have also been incorporated into the VMO. Using these platforms we will address three hypotheses: 1) That the VMO can be used to assess the ability of convalescent serum, soluble ACE2 or small molecule inhibitors to block entry of a SARS-CoV-2 pseudotyped virus; 2) That sustained AngII expression can trigger a hyperinflammatory response from EC and macrophages; and 3) That hyperinflammation can compromise peripheral and BBB vasculature as well as liver function.
This proposal will seek to develop our Vascularized MicroOrgan technology as a tool for investigating infection mechanisms and potential treatment options for the novel coronavirus. We will test whether antibodies from recovered COVID19 patients, or a soluble form of the cell receptor for the virus can prevent infection. We will also test whether a side effect of viral infection is to induce a ?cytokine storm? that can then damage organs such as the brain and liver ? this will be tested in our Vascularized MicroBrain and Vascularized MicroLiver platforms.
