Abstract
Understanding the dynamics of host innate immune responses against a pathogen marks the first step toward developing intervention strategies against the pathogen. The cytosolic pattern recognition receptor retinoic acid-inducible gene I (RIG-I) has been shown to be the major innate immune sensor for hepatitis E virus (HEV). Here, we show that HEV capsid protein (ORF2), a 660 amino acid long protein, interferes with the RIG-I signaling. Interestingly, only the full length ORF2 protein but not the 112-608 ORF2 protein inhibited RIG-I dependent interferon response. Both synthetic agonist and virus induced RIG-I activation was modulated by ORF2. Interference of interferon response was confirmed by reporter assays involving different interferon inducible promoters, qRT PCR, ELISA, and immunofluorescence microscopy. Neither glycosylation nor dimerization of the ORF2 protein had any effect on the observed inhibition. Further analyses revealed that the ORF2 protein antagonized Toll-like receptor (TLR) pathways as well. ORF2 inhibited signaling by RIG-I and TLR adapters, IPS-1, MyD88, and TRIF but was unable to inhibit activation by ectopically expressed IRF3 suggesting that it may be acting at a site upstream of IRF3 and downstream of adapter proteins. Our data uncover a new mechanism by which HEV may interfere with the host antiviral signaling.
Highlights
Hepatitis E virus (HEV) has emerged as a leading cause of viral hepatitis since its discovery in the 1980s (Purcell and Emerson, 2008)
In order to identify the hepatitis E virus (HEV) protein(s) that might be interfering with the retinoic acid-inducible gene I (RIG-I) signaling, a reporter assay was designed in which RIG-I mediated activation of the IFN-β promoter was quantified by measuring the activity of firefly luciferase, as described earlier (Madhvi et al, 2017)
Plasmids expressing RIG-I and HEV protein domains were transfected in HEK293T cells along with reporter plasmids; firefly luciferase under IFN-β promoter and Renilla luciferase under thymidine kinase promoter
Summary
Hepatitis E virus (HEV) has emerged as a leading cause of viral hepatitis since its discovery in the 1980s (Purcell and Emerson, 2008). The disease is self-limiting in healthy individuals but leads to chronicity and severe liver damage in patients with compromised immune system such as organ transplant patients (Melgaço et al, 2018). Due to its small size, HEV can cross the blood–brain barrier causing neuropathological manifestations (Shi et al, 2016). Other extrahepatic manifestations such as musculoskeletal, hematological, renal, and immunological diseases have been reported (Bazerbachi et al, 2015). A recent report estimated that 70,000 HEV-related deaths occur each year (Melgaço et al, 2018). The case fatality rate is high in pregnant women (25–30%) (Melgaço et al, 2018)
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