Insights into antiviral innate immunity revealed by studying hepatitis C virus

HCV Assembly and Egress via Modifications in Host Lipid Metabolic Systems

Hepatitis C virus (HCV) proliferates by hijacking the host lipid machinery. In vitro replication systems revealed many aspects of the virus life cycle; in particular, viral utilization of host lipid metabolism during HCV proliferation. HCV interacts with lipid droplets (LDs) before starting the process of virus capsid formation at the lipid-rich endoplasmic reticulum (ER) membrane compartment. HCV buds into the ER via lipoprotein assembly and secretion. Exchangeable apolipoproteins, represented by apolipoprotein E (apoE), play pivotal roles in enhancing HCV-specific infectivity. HCV virions are likely to interact with other lipoproteins circulating in blood vessels and incorporate apolipoproteins as well as lipids. This review focuses on virus assembly and egress by briefly describing the recent advances in this area.

HCV and flaviviruses hijack cellular mechanisms for nuclear STAT2 degradation: Up-regulation of PDLIM2 suppresses the innate immune response

Host encounters with viruses lead to an innate immune response that must be rapid and broadly targeted but also tightly regulated to avoid the detrimental effects of unregulated interferon expression. Viral stimulation of host negative regulatory mechanisms is an alternate method of suppressing the host innate immune response. We examined three key mediators of the innate immune response: NF-KB, STAT1 and STAT2 during HCV infection in order to investigate the paradoxical induction of an innate immune response by HCV despite a multitude of mechanisms combating the host response. During infection, we find that all three are repressed only in HCV infected cells but not in uninfected bystander cells, both in vivo in chimeric mouse livers and in cultured Huh7.5 cells after IFNα treatment. We show here that HCV and Flaviviruses suppress the innate immune response by upregulation of PDLIM2, independent of the host interferon response. We show PDLIM2 is an E3 ubiquitin ligase that also acts to stimulate nuclear degradation of STAT2. Interferon dependent relocalization of STAT1/2 to the nucleus leads to PDLIM2 ubiquitination of STAT2 but not STAT1 and the proteasome-dependent degradation of STAT2, predominantly within the nucleus. CRISPR/Cas9 knockout of PDLIM2 results in increased levels of STAT2 following IFNα treatment, retention of STAT2 within the nucleus of HCV infected cells after IFNα stimulation, increased interferon response, and increased resistance to infection by several flaviviruses, indicating that PDLIM2 is a global regulator of the interferon response.

The response of cells to an invading pathogen must be swift and well controlled because of the detrimental effects of chronic inflammation. However, viruses often hijack host control mechanisms. HCV and flaviviruses are known to suppress the innate immune response in cells by a variety of mechanisms. This study clarifies and expands a specific cellular mechanism for global control of the antiviral response after the induction of interferon expression. It shows how several viruses hijack this control mechanism to suppress the innate interferon response.

Ultrastructural and biochemical basis for hepatitis C virus morphogenesis

Chronic infection with HCV is a leading cause of cirrhosis, hepatocellular carcinoma and liver failure. One of the least understood steps in the HCV life cycle is the morphogenesis of new viral particles. HCV infection alters the lipid metabolism and generates a variety of microenvironments in the cell cytoplasm that protect viral proteins and RNA promoting viral replication and assembly. Lipid droplets (LDs) have been proposed to link viral RNA synthesis and virion assembly by physically associating these viral processes. HCV assembly, envelopment, and maturation have been shown to take place at specialized detergent-resistant membranes in the ER, rich in cholesterol and sphingolipids, supporting the synthesis of luminal LDs-containing ApoE. HCV assembly involves a regulated allocation of viral and host factors to viral assembly sites. Then, virus budding takes place through encapsidation of the HCV genome and viral envelopment in the ER. Interaction of ApoE with envelope proteins supports the viral particle acquisition of lipids and maturation. HCV secretion has been suggested to entail the ion channel activity of viral p7, several components of the classical trafficking and autophagy pathways, ESCRT, and exosome-mediated export of viral RNA. Here, we review the most recent advances in virus morphogenesis and the interplay between viral and host factors required for the formation of HCV virions.

Analysis of TLR7, SOCS1 and ISG15 immune genes expression in the peripheral blood of responder and non-responder patients with chronic Hepatitis C

AIM

To evaluate the baseline expression of the immune genes in PBMCs of responder and non-responder patients with chronic Hepatitis C.

BACKGROUND

Although the contribution of peripheral blood mononuclear cell (PBMC) gene expression in treatment outcome of hepatitis C virus (HCV) infection is supposed, it has remained to be distinctly delineated. The baseline expression of the immune genes inside PBMCs may reflect the responsiveness status following IFN treatment.

METHODS

Totally, 22 chronic HCV encompasses 10 responders and 12 non-responsive cases enrolled randomly regarding medical records. The PBMCs from the peripheral blood samples were isolated and then incubated for 6 hours in the culture media. The baseline expression of TLR7, SOCS1 and ISG15 was measured by Real time PCR.

RESULTS

The gene expression pattern in PBMCs of both groups showed a similar trend. The expression of SOCS1 and TLR7 genes showed higher levels in non-responder group (P>0.05). The result of ISG15 showed a higher but non-significant expression in the responder group (P>0.05).

CONCLUSION

The similar pattern of TLR7, SOCS1 and ISG15 expression in the responder and non-responder patients indicated their poor discriminating and predictive value in PBMCs sample.

Interferon (IFN) and Cellular Immune Response Evoked in RNA-Pattern Sensing During Infection with Hepatitis C Virus (HCV)

Hepatitis C virus (HCV) infects hepatocytes but not dendritic cells (DCs), but DCs effectively mature in response to HCV-infected hepatocytes. Using gene-disrupted mice and hydrodynamic injection strategy, we found the MAVS pathway to be crucial for induction of type III interferons (IFNs) in response to HCV in mouse. Human hepatocytes barely express TLR3 under non-infectious states, but frequently express it in HCV infection. Type I and III IFNs are induced upon stimulation with polyI:C, an analog of double-stranded (ds)RNA. Activation of TLR3 and the TICAM-1 pathway, followed by DC-mediated activation of cellular immunity, is augmented during exposure to viral RNA. Although type III IFNs are released from replication-competent human hepatocytes, DC-mediated CTL proliferation and NK cell activation hardly occur in response to the released type III IFNs. Yet, type I IFNs and HCV-infected hepatocytes can induce maturation of DCs in either human or mouse origin. In addition, mouse CD8+ DCs mature in response to HCV-infected hepatocytes unless the TLR3/TICAM-1 pathway is blocked. We found the exosomes containing HCV RNA in the supernatant of the HCV-infected hepatocytes act as a source of TLR3-mediated DC maturation. Here we summarize our view on the mechanism by which DCs mature to induce NK and CTL in a status of HCV infection.