Fibroblast growth factor-9 expression in airway epithelial cells amplifies the type I interferon response and alters influenza A virus pathogenesis

Influenza A virus (IAV) preferentially infects conducting airway and alveolar epithelial cells in the lung. The outcome of these infections is impacted by the host response, including the production of various cytokines, chemokines, and growth factors. Fibroblast growth factor-9 (FGF9) is required for lung development, can display antiviral activity in vitro, and is upregulated in asymptomatic patients during early IAV infection. We therefore hypothesized that FGF9 would protect the lungs from respiratory virus infection and evaluated IAV pathogenesis in mice that overexpress FGF9 in club cells in the conducting airway epithelium (FGF9-OE mice). However, we found that FGF9-OE mice were highly susceptible to IAV and Sendai virus infection compared to control mice. FGF9-OE mice displayed elevated and persistent viral loads, increased expression of cytokines and chemokines, and increased numbers of infiltrating immune cells as early as 1 day post-infection (dpi). Gene expression analysis showed an elevated type I interferon (IFN) signature in the conducting airway epithelium and analysis of IAV tropism uncovered a dramatic shift in infection from the conducting airway epithelium to the alveolar epithelium in FGF9-OE lungs. These results demonstrate that FGF9 signaling primes the conducting airway epithelium to rapidly induce a localized IFN and proinflammatory cytokine response during viral infection. Although this response protects the airway epithelial cells from IAV infection, it allows for early and enhanced infection of the alveolar epithelium, ultimately leading to increased morbidity and mortality. Our study illuminates a novel role for FGF9 in regulating respiratory virus infection and pathogenesis.

Influenza viruses are respiratory viruses that cause significant morbidity and mortality worldwide. In the lungs, influenza A virus primarily infects epithelial cells that line the conducting airways and alveoli. Fibroblast growth factor-9 (FGF9) is a growth factor that has been shown to have antiviral activity and is upregulated during early IAV infection in asymptomatic patients, leading us to hypothesize that FGF9 would protect the lung epithelium from IAV infection. However, mice that express and secrete FGF9 from club cells in the conducting airway had more severe respiratory virus infection and a hyperactive inflammatory immune response as early as 1 day post-infection. Analysis of the FGF9-expressing airway epithelial cells found an elevated antiviral and inflammatory interferon signature, which protected these cells from severe IAV infection. However, heightened infection of alveolar cells resulted in excessive inflammation in the alveoli, resulting in more severe disease and death. Our study identifies a novel antiviral and inflammatory role for FGFs in the lung airway epithelium and confirms that early and robust IAV infection of alveolar cells results in more severe disease.

6-Thioguanine blocks SARS-CoV-2 replication by inhibition of PLpro protease activities

A recently emerged betacoronavirus, SARS-CoV-2, has led to a global health crisis that calls for the identification of effective therapeutics for COVID-19 disease. Coronavirus papain-like protease (PLpro) is an attractive drug target as it is essential for viral polyprotein cleavage and for deconjugation of ISG15, an antiviral ubiquitin-like protein. We show here that 6-Thioguanine (6-TG) inhibits SARS-CoV-2 PLpro-catalyzed viral polyprotein cleavage and ISG15 deconjugation in cells and inhibits replication of SARS-CoV-2 in Vero-E6 cells and Calu3 cells at submicromolar levels. As a well-characterized FDA-approved orally delivered drug, 6-TG represents a promising therapeutic for COVID-19 and other emerging coronaviruses.

ONE SENTENCE SUMMARY

A repurposed drug that targets an essential enzymatic activity of SARS-CoV-2 represents a promising COVID-19 therapeutic.

Intracellular C3 Protects Human Airway Epithelial Cells from Stress-associated Cell Death

The complement system provides host defense against pathogens and environmental stress. C3, the central component of complement, is present in the blood and increases in BAL fluid after injury. We recently discovered that C3 is taken up by certain cell types and cleaved intracellularly to C3a and C3b. C3a is required for CD4⁺ T-cell survival. These observations made us question whether complement operates at environmental interfaces, particularly in the respiratory tract. We found that airway epithelial cells (AECs, represented by both primary human tracheobronchial cells and BEAS-2B [cell line]) cultured in C3-free media were unique from other cell types in that they contained large intracellular stores of de novo synthesized C3. A fraction of this protein reduced ("storage form") but the remainder did not, consistent with it being pro-C3 ("precursor form"). These two forms of intracellular C3 were absent in CRISPR knockout-induced C3-deficient AECs and decreased with the use of C3 siRNA, indicating endogenous generation. Proinflammatory cytokine exposure increased both stored and secreted forms of C3. Furthermore, AECs took up C3 from exogenous sources, which mitigated stress-associated cell death (e.g., from oxidative stress or starvation). C3 stores were notably increased within AECs in lung tissues from individuals with different end-stage lung diseases. Thus, at-risk cells furnish C3 through biosynthesis and/or uptake to increase locally available C3 during inflammation, while intracellularly, these stores protect against certain inducers of cell death. These results establish the relevance of intracellular C3 to airway epithelial biology and suggest novel pathways for complement-mediated host protection in the airway.

ISG15 in antiviral immunity and beyond

The host response to viral infection includes the induction of type I interferons and the subsequent upregulation of hundreds of interferon-stimulated genes. Ubiquitin-like protein ISG15 is an interferon-induced protein that has been implicated as a central player in the host antiviral response. Over the past 15 years, efforts to understand how ISG15 protects the host during infection have revealed that its actions are diverse and pathogen-dependent. In this Review, we describe new insights into how ISG15 directly inhibits viral replication and discuss the recent finding that ISG15 modulates the host damage and repair response, immune response and other host signalling pathways. We also explore the viral immune-evasion strategies that counteract the actions of ISG15. These findings are integrated with a discussion of the recent identification of ISG15-deficient individuals and a cellular receptor for ISG15 that provides new insights into how ISG15 shapes the host response to viral infection.

Ubiquitin-like protein ISG15 is an interferon-induced protein that has been implicated as a central player in the host antiviral response. In this Review, Perng and Lenschow provide new insights into how ISG15 restricts and shapes the host response to viral infection and the viral immune-evasion strategies that counteract ISG15.

Human cytomegalovirus pUL79 is an elongation factor of RNA polymerase II for viral gene transcription

In this study, we have identified a unique mechanism in which human cytomegalovirus (HCMV) protein pUL79 acts as an elongation factor to direct cellular RNA polymerase II for viral transcription during late times of infection. We and others previously reported that pUL79 and its homologues are required for viral transcript accumulation after viral DNA synthesis. We hypothesized that pUL79 represented a unique mechanism to regulate viral transcription at late times during HCMV infection. To test this hypothesis, we analyzed the proteome associated with pUL79 during virus infection by mass spectrometry. We identified both cellular transcriptional factors, including multiple RNA polymerase II (RNAP II) subunits, and novel viral transactivators, including pUL87 and pUL95, as protein binding partners of pUL79. Co-immunoprecipitation (co-IP) followed by immunoblot analysis confirmed the pUL79-RNAP II interaction, and this interaction was independent of any other viral proteins. Using a recombinant HCMV virus where pUL79 protein is conditionally regulated by a protein destabilization domain ddFKBP, we showed that this interaction did not alter the total levels of RNAP II or its recruitment to viral late promoters. Furthermore, pUL79 did not alter the phosphorylation profiles of the RNAP II C-terminal domain, which was critical for transcriptional regulation. Rather, a nuclear run-on assay indicated that, in the absence of pUL79, RNAP II failed to elongate and stalled on the viral DNA. pUL79-dependent RNAP II elongation was required for transcription from all three kinetic classes of viral genes (i.e. immediate-early, early, and late) at late times during virus infection. In contrast, host gene transcription during HCMV infection was independent of pUL79. In summary, we have identified a novel viral mechanism by which pUL79, and potentially other viral factors, regulates the rate of RNAP II transcription machinery on viral transcription during late stages of HCMV infection.

In this study, we report a novel mechanism used by human cytomegalovirus (HCMV) to regulate the elongation rate of RNA polymerase II (RNAP II) to facilitate viral transcription during late stages of infection. Recently, we and others have identified several viral factors that regulate gene expression during late infection. These factors are functionally conserved among beta- and gamma- herpesviruses, suggesting a unique transcriptional regulation shared by viruses of these two subfamilies. However, the mechanism remains elusive. Here we show that HCMV pUL79, one of these factors, interacts with RNAP II as well as other viral factors involved in late gene expression. We have started to elucidate the nature of the pUL79-RNAP II interaction, finding that pUL79 does not alter the protein levels of RNAP II or its recruitment to viral promoters. However, during late times of infection, pUL79 helps RNAP II efficiently elongate along the viral DNA template to transcribe HCMV genes. Host genes are not regulated by this pUL79-mediated mechanism. Therefore, our study discovers a previously uncharacterized mechanism where RNAP II activity is modulated by viral factor pUL79, and potentially other viral factors as well, for coordinated viral transcription.