ABSTRACT
Background: The innate immune system is a powerful anti-viral defense mechanism, which includes the interferon (IFN) system and autophagy. Thus, successful pathogens like SARS-CoV-2 need to counteract or evade these defenses to establish an infection. However, due to its ongoing, worldwide spread in the human population SARS-CoV-2 is evolving and in the meantime four variants with selection advantages (variants of concern) emerged. Methods: Using expression constructs for 29 SARS-CoV-2 proteins we evaluated the impact of individual viral proteins on induction of cytokines (IFNA4, IFNB1, IRF3-signalling, NF-κB-signaling) and cytokine signaling (IFNα2, IFNβ, IFNγ, IFNa;1, IL-1α, TNFα) in luciferase reporter assays, validated by endogenous transcription factor phosphorylation analysis. We assessed the influence of SARS-CoV-2 proteins on autophagy using a flow cytometry-based system. Underlying molecular mechanisms were investigated on an endogenous level using Western blot, confocal fluorescence microscopy, and flow cytometry. In addition, we examined the susceptibility of SARS-CoV-2 including all variants of concern towards type-I,-II, and-III interferons. Results: To understand how SARS-CoV-2 efficiently manipulates the host's innate immune defenses, we systematically analyzed the impact of SARS-CoV-2 encoded proteins on induction of various IFNs and pro-inflammatory cytokines, IFN signaling, and autophagy. Our results reveal the range of innate immune antagonists encoded by SARS-CoV-2 and we characterized selected molecular mechanisms employed by Nsp1 and Nsp14 to downregulate the IFN system or ORF3a and ORF7a to prevent autophagic degradation. Interestingly, our assays show that variants of concern of SARS-CoV-2 remain sensitive to type-II interferon signaling but show increased resistance towards type-I and/or type-III interferons. Conclusion: SARS-CoV-2 has evolved to counteract innate immunity using several synergistic approaches but remains relatively sensitive to type-II and-III interferons. However, emerged variants of concern remain sensitive overall but are less susceptible towards IFNα2/β and IFNa;1 than early SARS-CoV-2 isolates.
ABSTRACT
PURPOSE: This literature review summarizes the main immunological characteristics of type III interferons (IFN) and highlights the clinically relevant aspects and future therapeutic perspectives for these inflammatory molecules. SOURCE: Relevant articles in PubMed MEDLINE from the first publication (2003) until 2020. N=101 articles were included in this review. PRINCIPAL FINDINGS: Type III IFNs represent a relatively newly described inflammatory cytokine family. Although they induce substantially similar signalling to the well-known type I IFNs, significant functional differences make these molecules remarkable. Type III IFNs have extensive biological effects, contributing to the pathogenesis of several diseases and also offering new diagnostic and therapeutic approaches: 1) their potent anti-viral properties make them promising therapeutics against viral hepatitis and even against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is causing the current coronavirus disease 2019 (COVID-19) pandemic; 2) imbalances in the IFN-λs contribute to several forms of chronic inflammation (e.g., systemic and organ-specific autoimmune diseases) and potentially predict disease progression and therapeutic response to biologic therapies; and 3) the antitumor properties of the type III IFNs open up new therapeutic perspectives against malignant diseases. CONCLUSION: Over the last 18 years, researchers have gathered extensive information about the presence and role of these versatile inflammatory cytokines in human diseases, but further research is needed to clarify the mechanistic background of those observations. Better understanding of their biological activities will permit us to use type III IFNs more efficiently in new diagnostic approaches and individualized therapies, consequently improving patient care.