Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Mediates Transforming Growth Factor Beta (TGF-b) Hijacking and Immune Dysregulation Through A Novel Gain of Function Mutation in Its Nonstructural 15 (NSP15) Protein

ABSTRACT

Rationale The COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has killed millions via the acute respiratory distress syndrome (ARDS). The early immune suppression of SARS-CoV-2 then subsequent inflammation suggests an unusual ability to cause immune dysregulation. Host transforming growth factor beta (TGF-β) is an immunesuppressing and profibrotic cytokine frequently “hijacked” by microbes to evade immune detection. We discovered a KRFK domain (a potent activating motif for latent TGF-β) in the SARS-CoV-2 nonstructural 15 (NSP15) protein. We hypothesized that this NSP15 protein causes immune dysregulation by activation of latent TGF-β and subsequent activation of immunosuppressive Tregulatory (Treg) cells, and that substantial TGF-β is present in the lungs of COVID-19 ARDS patients. Methods: We evaluated TGF-β1 concentrations in endotracheal aspirates (ETA) of 27 COVID-19 ARDS patients by ELISA. We produced recombinant SARS-CoV-2 NSP15 protein in E. coli and tested its ability to activate latent TGF-β1 using in vitro assays. TGF-β inhibitors were assessed for their ability to block effects. We obtained blood mononuclear cells from healthy subjects and isolated Tregs to assess their activation state via intracellular smad-2 phosphorylation (pSMAD2) using flow cytometry. Results: The KRFK domain was present in all SARS-CoV-2 variants. High concentrations of both active and total TGF-β1 were detected in ETA of COVID-19 ARDS patients (150 +/- 34 pg/ml active;1,819 +/- 304 pg/ml total) in a range previously shown to affect T cell function. NSP15 at 2.4 nM increased activation of latent TGF-β 12-fold (P < .001 vs. vehicle), compared to an 11% activation with the positive control thrombospondin-1 (TSP1;10 nM) (Figure). TGF-β receptor inhibitors blocked NSP15 effects on latent TGF-β activation and intracellular TGF-β1 signaling in a bioassay by over 95% (p<.01). At tested concentrations (25, 50, 100 nM) NSP15 increased Treg pSMAD2 levels via activation of latent TGF-β1, exceeding levels seen in Tregs stimulated with 400 pM of active TGF-β1 (+ control) (pSMAD2 + cells vehicle 1.1%, active TGF-β1 43%, NSP15/latent TGF-β1 49-56%). Conclusions: High concentrations of active and total TGF-β1 are present in the lungs of COVID-19 ARDS patients, suggesting SARS-CoV-2 uses host TGF-β hijacking as a mechanism for immune evasion. The NSP15 protein of SARSCoV- 2 potently activates latent TGF-β, leading to Treg activation. TGF-β inhibitors are potent inhibitors of these NSP15 effects. A strategy to block NSP15-mediated effects with TGF-β inhibitors is an innovative therapy worthy of testing in animal models of COVID-19.