(385) Mechanisms by which CFTR may influence SARS-CoV-2 infection

Background: Patients with cystic fibrosis (CF) exhibit chronic and pronounced respiratory damage, and have been considered among those at highest risk for serious harm from SARS-CoV-2. Numerous clinical studies have reported that individuals with CF in North America and Europe – while still highly susceptible to COVID-19 – are often spared from levels of mortality that exceed the general population.

Methods: To understand features that might influence lethality of COVID-19 among patients with CF, we tested potential relationships between CFTR and viral pathogenesis. As one approach to evaluate impact of CFTR on COVID-19 severity, independent sets of blood samples from virally infected individuals were genotyped. Blood was obtained from 424 US patients hospitalized with severe COVID-19 or a much larger European cohort containing 7,147 samples from healthy individuals and 2,587 from patients with severe COVID-19. DNA in both studies was probed for the F508del variant. In other experiments, we investigated the possibility that viral binding and propagation might be altered by absent CFTR. We utilized human bronchial epithelial cell (hBEC) monolayers from individuals without functional CFTR for this purpose. Finally, we examined effects of cystic fibrosis airway secretions and features such as viscosity, pH, and protease/anti-protease imbalance during SARS-CoV-2 infection.

Results: We found no evidence for a relationship between deficient CFTR function (based on carrier status for the severe F508del defect) and clinical outcomes from COVID-19. In addition, viral propagation studies using airway epithelial monolayers (a model that reproduces many aspects of in vivo tissue biology) were not influenced by homozygous absence of CFTR. We show that levels of ACE-2 receptor mRNA appear normal in CF primary epithelium, whereas TMPRSS2 mRNA is variable but decreased (p < 0.0001) in a manner that correlates with viral infectivity (R² = 0.76). Dependence of viral proliferation on features of CF mucosal fluid – including pH (viral replication optimum at pH 7 – 7.5), viscosity (diminished propagation in highly viscous apical media), and protease/anti-protease imbalance were identified as likely contributors to efficiency of SARS-CoV-2 replication and pathogenesis.

Conclusions: These findings using patient data, CF and non-CF primary airway epithelia, and CF airway secretions fail to demonstrate a causal relationship between loss of CFTR and susceptibility to severe COVID-19. Notwithstanding the caveat that addition of virus in small buffer volumes disrupts both airway surface liquid depth and composition, our findings also argue against a role for CFTR during acute infection of airway cells in vitro. On the other hand, chronic disruption of periciliary liquid, diminished pH, altered protease/anti-protease homeostasis, and increased fluid viscosity (sequelae that occur in CF lungs) were implicated as contributors to impaired SARS-CoV-2 propagation. Such studies provide a basis for future work to test relationships between CFTR and severity of COVID-19.

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