The U-Rich Untranslated Region of the Hepatitis E Virus Induces Differential Type I and Type III Interferon Responses in a Host Cell-Dependent Manner
Heffron, Connie L.
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Hepatitis E virus (HEV), a single-strand positive-sense RNA virus, is an understudied but important human pathogen. The virus can establish infection at a number of host tissues, including the small intestine and liver, causing acute and chronic hepatitis E as well as certain neurological disorders. The retinoic acid-inducible gene I (RIG-I) pathway is essential to induce the interferon (IFN) response during HEV infection. However, the pathogen-associated motif patterns (PAMPs) in the HEV genome that are recognized by RIG-I remain unknown. In this study, we first identified that HEV RNA PAMPs derived from the 3' untranslated region (UTR) of the HEV genome induced higher levels of IFN mRNA, interferon regulatory factor-3 (IRF3) phosphorylation, and nuclear translocation than the 5' UTR of HEV. We revealed that the U-rich region in the 3' UTR of the HEV genome acts as a potent RIG-I PAMP, while the presence of poly(A) tail in the 3' UTR further increases the potency. We further demonstrated that HEV UTR PAMPs induce differential type I and type III IFN responses in a cell type-dependent fashion. Predominant type III IFN response was observed in the liver tissues of pigs experimentally infected with HEV as well as in HEV RNA PAMP-induced human hepatocytes in vitro. In contrast, HEV RNA PAMPs induced a predominant type I IFN response in swine enterocytes. Taken together, the results from this study indicated that the IFN response during HEV infection depends both on viral RNA motifs and host target cell types. The results have important implications in understanding the mechanism of HEV pathogenesis. IMPORTANCE Hepatitis E virus (HEV) is an important human pathogen causing both acute and chronic viral hepatitis E infection. Currently, the mechanisms of HEV replication and pathogenesis remain poorly understood. The innate immune response acts as the first line of defense during viral infection. The retinoic acid-inducible gene I (RIG-I)-mediated interferon (IFN) response has been implicated in establishing antiviral response during HEV infection, although the HEV RNA motifs that are recognized by RIG-I are unknown. This study identified that the U-rich region in the 3' untranslated region (UTR) of the HEV genome acts as a potent RIG-I agonist compared to the HEV 5' UTR. We further revealed that the HEV RNA pathogen-associated motif patterns (PAMPs) induced a differential IFN response in a cell type-dependent manner: a predominantly type III IFN response in hepatocytes, and a predominantly type I IFN response in enterocytes. These data demonstrate the complexity by which both host and viral factors influence the IFN response during HEV infection.