Innate Immunity to Enteric Hepatitis Viruses

Abstract

Although hepatitis A virus (HAV) and hepatitis E virus (HEV) are both positive-strand RNA viruses that replicate in the cytoplasm of hepatocytes, there are important differences in the ways they induce and counteract host innate immune responses. HAV is remarkably stealthy because of its ability to evade and disrupt innate signaling pathways that lead to interferon production. In contrast, HEV does not block interferon production. Instead, it persists in the presence of an interferon response. These differences may provide insight into HEV persistence in immunocompromised patients, an emerging health problem in developed countries.

Figure 1.

Innate immune signaling pathways leading to interferon (IFN) production in hepatocytes. The cytosolic retinoic acid activated gene I (RIG-I)-like helicases, melanoma differentiation-associated protein 5 (MDA5) and RIG-I, and endosomal Toll-like receptor (TLR)3 are important pathogen recognition sensors within hepatocytes, signaling downstream through the adaptors mitochondria-associated antiviral protein (MAVS) and TIR-domain-containing adapter-inducing IFN-β (TRIF), respectively, inducing phosphorylation, dimerization, and, ultimately, nuclear translocation of the critical transcription factor IRF3 along with activation of nuclear factor (NF)-κB, resulting in transcription of type I (IFN-α/β) and type III (IFN-λ) IFN genes. IL, Interleukin. (From Lemon 2010; adapted, with permission, from The American Society for Biochemistry and Molecular Biology © 2010.)

Figure 2.

Intrahepatic interferon (IFN)-stimulated gene expression in a chimpanzee with experimental hepatitis A virus (HAV) infection. (A) Intrahepatic IFN-stimulated gene (ISG) messenger RNA (mRNA) expression, shown as fold change from baseline in Affymetrix GeneChip assays, in a chimpanzee (4x0395) infected experimentally with HAV. With the exception of CXCL10 (IP10) and ISG20, both of which are dually regulated by IFN-α/β and IFN-γ, low-level ISG induction is restricted to the first weeks of infection, and subsides before peak HAV RNA abundance in the liver. (From Lanford et al. 2011; adapted, with permission, from the National Academy of Sciences © 2011.) (B) Early course of HAV infection in chimpanzee 4x0395, showing alanine aminotransferase (ALT) elevation and IFN and cytokine detection in serum samples.

Figure 3.

Comparative analysis of intrahepatic interferon (IFN)-stimulated genes (ISGs) expression resulting from infection with hepatitis C virus (HCV) versus hepatitis A virus (HAV) or hepatitis E virus (HEV) in chimpanzees. Columns reflect the average maximum-fold change for each of the indicated ISGs from baseline during the course of acute infection, as determined in two independent studies, one of HAV and the other of HEV. Data from HAV-infected animals are from Lanford et al. (2011), whereas HEV-infected animal data are from Yu et al. (2010). Contemporaneous data from chimpanzees with experimental HCV infection are shown for comparison from each of these two studies, both of which used Affymetrix GeneChips.

Figure 4.

Hepatitis A virus (HAV) infection actively blocks virus-induced interferon (IFN) responses and degrades mitochondria-associated antiviral protein (MAVS). (A) Confocal fluorescence microscopic images of IFN-regulatory factor (IRF)3 expression in a Huh7 cell line containing a replicating subgenomic HAV RNA (HAV-Bla cells) or its cured derivative lacking HAV RNA (Bla-C cells), following infection with Sendai virus (SenV), a potent retinoic acid activated gene I (RIG-I) agonist, or mock infection (Mock). IRF3 is predominantly nuclear in localization in Bla-C cells, indicating its activation, whereas it remains cytoplasmic in HAV-Bla cells because of disruption of RIG-I/MAVS signaling. (Panel A from Yang et al. 2007; adapted, with permission, from the National Academy of Sciences © 2007.) (B) Confocal microscopic images showing MAVS and viral antigens in human liver-derived HepG2 cells infected with either HAV (top panels) or HEV (lower panels). MAVS expression is lost because of 3C^pro cleavage in HAV-infected cells, whereas MAVS abundance is not altered in HEV-infected cells. Nuclei are counterstained with 4′, 6-diamidino-2-phenylindole (DAPI). (Panel B from Yin et al. 2017; adapted under the terms of the Creative Commons Attribution License, which permits unrestricted use.)

Figure 5.

Hepatitis A virus (HAV) proteases degrade key innate immune signaling proteins: mitochondria-associated antiviral protein (MAVS), TIR-domain-containing adapter-inducing interferon (IFN) (TRIF), and nuclear factor (NF)-κB essential modulator (NEMO) (IKKγ). Schematic showing how HAV polyprotein processing intermediates with 3C^pro catalytic activity cleave MAVS (3ABC), TRIF (3CD), and NEMO also known as IKKγ (mature 3C^pro), thereby disrupting signaling leading from pathogen-associated molecular pattern (PAMP) sensors to the critical transcription factors IFN-regulatory factor (IRF)3 and NF-κB that drive type I and III IFN expression.

Figure 6.

Contrasting strategies for disrupting interferon (IFN)-mediated responses in hepatitis A virus (HAV) versus hepatitis E virus (HEV) infections. (A) In HAV-infected cells, viral double-stranded RNA (dsRNA) replication intermediates are sensed by cytosolic retinoic acid activated gene I (RIG-I)-like RNA (RLRs) (RIG-I and melanoma differentiation-associated protein 5 [MDA5]) as well as endosomal Toll-like receptor (TLR)3. However, the adaptor proteins mitochondria-associated antiviral protein (MAVS) and TIR-domain-containing adapter-inducing IFN-β (TRIF), and the regulatory subunit of the IκB kinase complex, NF-κB essential modulator (NEMO), are degraded by viral proteinases (Fig. 5), disrupting signals extending from RLRs and TLR3 such that little or no activated IFN-regulatory factor (IRF)3 and nuclear factor (NF)-κB cells reach responsive promoters in the nucleus, and very little or no IFNs are produced. (B) In contrast, in HEV-infected cells, HEV RNA is recognized by RIG-I and MDA5, and possibly by TLR3 as well. Signaling pathways may be impeded by open reading frame (ORF)1 papain-like cysteine protease (PCP) domain antagonism of RIG-I activation and X domain inhibition of IRF3 phosphorylation, but they are not disrupted and substantial amounts of IFNs (predominantly IFN-λ) are produced. Secreted IFNs bind to their receptors on the surface of infected cells and induce IFN-stimulated gene (ISG) expression via the Janus tyrosine kinase/signal transducers and activators of transcription (JAK/STAT) pathway in both an autocrine and paracrine fashion. Despite this activation of JAK/STAT signaling, HEV replication persists.