It has become clear that the type 1 interferon responses to SARS-CoV-2 have a great deal to do with the outcome of infection. Patients with benign courses of infection yield high levels of expression of interferon-α (IFNα), while critical cases of COVID-19 show low levels of its expression. Paradoxically, in severe disease, expression of INF receptors is elevated, while expression of INF sensitive genes is depressed, suggesting that things are awry downstream from the receptors. It is not clear if this is because of the activity of virally encoded gene products or the lack of adequate INF-α or both. Variants of genes involved in INF activity have been identified in severe cases of Covid-19 patients. And the virus seems to have multiple proteins that are able to antagonize aspects of INF signaling. We will consider these points separately.
Coronavirus infections in general appear to involve perturbations in INF activities. A good overview of the role of INF in coronavirus infections shows some of the involved signaling pathways(1). Innate immunity is abnormal in severe disease. Early expression of INF-α appears to correlate with a favorable outcome in controlling SARS-CoV-2 infection, whereas INF-β expression appears to be negligible(2). While mild infections are correlated with continued high levels of expression of INF-α, severe disease is marked by low levels of INF-α expression(2). In contrast, inflammatory cytokine expression, as evidenced by IL-6 and TNFα, is high. Although INF-α and inflammatory cytokines are both part of innate immunity, the former is regulated by the transcription factors IRF3 and 7, while the latter are regulated by NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells), a protein complex controlling transcription of DNA, cytokine production and cell survival. This suggests that NF-κB signaling is relatively unperturbed by the virus. Thus, its IFN perturbation could be important for antagonizing host defense system.
Some clues as to which factors involved in regulating INF activities may be affected in Covid-19 have been learned by whole exome sequencing of patients with severe disease genomes, particularly those that lack obvious risk factors. In light of this, it is instructive to look at a study of two pairs of young adult brothers, one pair from the Netherlands and the other from West Africa, who had severe Covid-19 (one died)(3). The pairs each had different loss-of-function mutations in TLR7, an intracellular receptor for viral RNA. Primary mononuclear cells showed downregulation of downstream INF signaling, including that of IRF7. The genes are X-linked; this means that men who inherit the defective gene are at greater risk than females, who would need to be homozygous for risk. While loss of function mutations in TLR7 are rare, so is severe Covid-19 in young people, and the congruency of these two rare phenomena is telling. Another study showed loss of function variants in 3.5% (out of 659) of severely ill patients of 13 genes involved in TLR3 and IRF7-dependent INF activities(4). Cells with IRF7 variants produced no type 1 IFN when infected in vitro with SARS-CoV-2.
There is also a report that some patients with severe Covid-19 have neutralizing autoantibodies to IFN-α, -ω, or both(5). These were present in 101 of 987 patients with life threatening disease but in none of the 663 patients with mild or asymptomatic infection. The autoantibodies existed prior to infection. Oddly, most of the patients with autoantibodies were male, whereas generally women have a higher incidence of autoimmunity. This suggests an X-linked genetic basis.
The above studies (and others for which space constraints preclude detailed consideration) point to a failure of INF-related activities as a critical risk factor for the development of severe Covid-19 and suggest that INF represents a potentially serious block to replication of SARS-CoV-2. We might then ask how the virus deals with this problem.
It appears that the virus encodes a number proteins that potentially antagonize cellular signaling pathways involved in INF activity. As a convenient reference to the proteins encoded by SARS-CoV2, we commend you to a website maintained by the Zhang lab at the University of Michigan (https://zhanglab.ccmb.med.umich.edu/COVID-19/). Several studies have shown an effect on IFN activity in vitro(6-10), but these should be interpreted with caution as they use artificially high levels of viral gene expression. In fact, it is not clear at what levels these proteins are expressed in vivo. Moreover, these studies do not completely agree on which are the most relevant viral proteins.
Yuen et al.(10) transfected expression constructs for 27 different genes for their ability to inhibit RIG-I-dependent induction of an IFN-β promoter. RIG-I (retinoic acid-inducible gene I) is a cytosolic pattern recognition receptor (PRR) responsible for the type 1 interferon (IFN1) response. They found that four viral proteins (nsp13, 14 and 15 and orf6) were strongly inhibitory. However, the papain-like protease (PLpro) of SARS-CoV-2 was not an inhibitor, even though the SARS-CoV-1 PLpro is known to inhibit type I IFN by inhibiting IRF3 phosphorylation, thus blocking downstream Interferon induction. This suggests interference with activities downstream from RIG-I. Orf6 also inhibited IFN α-2, β, and λ secretion induced by Sendai virus infection. Using similar methodology, Lei et al.(7) found similar activities with nsp1, 3, 12, 13 and 14, ORFs 3 and 6, and the membrane (M) protein leading to RIG I or MDA5 activation. In contrast, ORF6 overexpression inhibited nuclear translocation of STAT1, which is required for IFN-stimulated gene expression. Li et al.(8) looked at the ability of selected viral proteins to inhibit Sendai virus-induced IFN activity. ORF 6 strongly inhibited the activity of IFN-β, interferon-sensitive response element (ISRE), and NF-κB promoters, as assessed by reporter gene activity. ORF8 and the nucleocapsid protein (N) showed less dramatic inhibitory activity. ISRE related activity was inhibited less markedly by ORF6 when cells were stimulated by IFN-β. From the above studies, there appears to be general agreement that ORF6 is a critical component of IFN antagonism by SARS-CoV-2 infection, with the caveat that the studies all involve overexpression.
Liu et al.(9) show that PLpro has IFN antagonistic activity, but by a different mechanism than those reported above. ISG15 is a multi-function protein that serves as a cytokine but also as a ubiquitin-like intracellular protein conjugate. ISGylation activates the viral RNA sensor MDA5, which is involved in IFN activity. MDA5 activity is antagonized by removal of ISG moieties by PLpro, thus inhibiting IFN expression.
Konno et al.(6) reported that ORF3b, a 22 amino acid protein that is truncated by introduction of a stop codon, antagonizes IFN activity. This is contradictory to SARS-COV-1 homolog. In spite of its small size, ORF3b suppressed Sendai virus-mediated stimulation of the IFNβ1 promoter more strongly than did the larger size of SARS-CoV-1 protein. Interestingly, there are naturally occurring SARS-CoV2 variants in which the full length ORF3b is restored; these are even more strongly inhibitory than is the truncated version.
The above presents a complicated picture that nonetheless points to the importance of type 1 INF in the outcome of Covid-19. A wide variety of data point to defects in the host IFN response that are correlative with severe disease. According to in vitro studies of the viral proteins (with the caveats inherent from the use of overexpression systems), the virus mounts multifactorial defenses against IFN antiviral activities, with ORF6 and PLpro (and perhaps ORF3b) being prime candidates. Hopefully, future in vivo studies will bring more clarity. Overall these studies point to type 1 IFN-signaling as a potential important therapeutic target.
- A. Park, A. Iwasaki, Type I and Type III Interferons – Induction, Signaling, Evasion, and Application to Combat COVID-19. Cell Host Microbe 27, 870-878 (2020).
- J. Hadjadj et al., Impaired type I interferon activity and inflammatory responses in severe COVID-19 patients. Science 369, 718-724 (2020).
- C. I. van der Made et al., Presence of Genetic Variants Among Young Men With Severe COVID-19. JAMA 324, 663-673 (2020).
- Q. Zhang et al., Inborn errors of type I IFN immunity in patients with life-threatening COVID-19. Science, (2020).
- P. Bastard et al., Auto-antibodies against type I IFNs in patients with life-threatening COVID-19. Science, (2020).
- Y. Konno et al., SARS-CoV-2 ORF3b Is a Potent Interferon Antagonist Whose Activity Is Increased by a Naturally Occurring Elongation Variant. Cell Rep 32, 108185 (2020).
- X. Lei et al., Activation and evasion of type I interferon responses by SARS-CoV-2. Nat Commun 11, 3810 (2020).
- J. Y. Li et al., The ORF6, ORF8 and nucleocapsid proteins of SARS-CoV-2 inhibit type I interferon signaling pathway. Virus Res 286, 198074 (2020).
- G. Liu et al., ISG15-dependent Activation of the RNA Sensor MDA5 and its Antagonism by the SARS-CoV-2 papain-like protease. bioRxiv, (2020).
- C. K. Yuen et al., SARS-CoV-2 nsp13, nsp14, nsp15 and orf6 function as potent interferon antagonists. Emerg Microbes Infect 9, 1418-1428 (2020).