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Authors:CJ Neufeldt et al. 

Journal/ Pre-Print:Biorxiv 

Tags: Cell Biology, Immunology/Immunity, Inflammation, Molecular biology 

Research Highlights 

  • SARS-CoV-2-infected lung epithelial cell lines (Calu-3 and A549-ACE2) induce mRNA expression of pro-inflammatory cytokines (e.g., IL-6 and TNF) but are impaired in IFN-I/III expression. 

  • SARS-CoV-2 infection of A549-ACE2 cells results in activation of NF-kB but not IRF3. The authors suggest that cGAS-STING signalling induces NF-kB, and that IRF3 signalling is blocked by a failure of STING to translocate to the Golgi. 

  • IFN-I pre-treatment of Calu-3 and A549-ACE2 cells reduces their susceptibility to infection.

Summary

In this study, Neufeldt et al. investigate the inflammatory response of lung epithelial cells to SARS-CoV-2 infection. By infecting two cell lines (Calu-3 and A549-ACE2), they find that SARS-CoV-2 drives upregulation of some proinflammatory genes (especially NF-kB driven ones such as IL-6 and TNF) but not a type I/III IFN response. By testing nuclear translocation and phosphorylation, Neufeldt et al. confirm NF-kB signalling activation without IRF3 nuclear translocation. Based on imaging and one pharmacological inhibitor, the authors conclude that cGAS-STING triggers NF-kB activation and further hypothesise that concomitant lack of STING translocation to the Golgi causes NF-kB but not IRF3 signalling. 

Impact for SARS-CoV2/COVID19 research efforts  

Understand the immune response to SARS-CoV2/COVID19  

The study identifies a possible cause of type I/III IFN response dysregulation in COVID19 patients. 

Treat of SARS-CoV2/COVID19 positive individuals 

As other studies before, the data in this preprint support IFN-I therapy to treat COVID19. 

The authors also suggest cGAS-STING as a possible target for the control of excessive inflammation. 

Study Type   

  • In vitro study (mostly, some patient data in Fig 3) 

Strengths and limitations of the paper 

Novelty: The study identifies a possible cause of type I/III IFN response dysregulation in COVID19 patients 

Standing in the field:The study confirms previous findings that the type I/III IFN response is dysregulated following SARS-Cov-2 infection 

as well as showing that pre-treatment of cells with IFN-I reduces infection rates 

Appropriate statistics: yes 

Viral model used:SARS-CoV-2- BavPat1/2020 strain and severe COVID-19 patient samples (fluids) 

Translatability:IFN-I treatment and/or cGAS-STING inhibitors 

Main limitations:  

  • The study does not eliminate the possibility that PRRs other than cGAS-STING activate NF-kB (e.g. TLR3, for which the authors only show overexpression and no KO). Furthermore, using defined synthetic ligands as controls would be helpful for validation. 

  • The authors invoke activation of cGAS and STING in SARS-CoV-2 infected cells from imaging and pharmacological inhibition. Both approaches have limitations. In Fig 6b, STING accumulates in perinuclear regions in infected cells. This is not happening in Fig 6e and 6g. A single inhibitor is used and controls such as dsDNA transfection are missing. KO cells or at least siRNA experiments are desirable. 

  • The effects of SARS-CoV-2 infection on IRF3 inhibition appear overstated in light of the 2-fold effect observed (Fig 5f). In Fig 5i, it is confusing that infected and uninfected cells in the “CoV2 / poly(I:C)” condition have similar rates of nuclear IRF3. This seems at odds with conclusion that the virus inhibits IRF3. 

  • A more systematic comparison between the patient data in Fig 3 and the RNAseq data from cell lines in Fig 2 would be helpful. Do the authors have data from healthy controls to accompany the results in Fig 3?