Vaccination to prevent T cell subversion can protect against persistent hepacivirus infection

Marmosets as models of infectious diseases

Animal models of infectious disease often serve a crucial purpose in obtaining licensure of therapeutics and medical countermeasures, particularly in situations where human trials are not feasible, i.e., for those diseases that occur infrequently in the human population. The common marmoset (Callithrix jacchus), a Neotropical new-world (platyrrhines) non-human primate, has gained increasing attention as an animal model for a number of diseases given its small size, availability and evolutionary proximity to humans. This review aims to (i) discuss the pros and cons of the common marmoset as an animal model by providing a brief snapshot of how marmosets are currently utilized in biomedical research, (ii) summarize and evaluate relevant aspects of the marmoset immune system to the study of infectious diseases, (iii) provide a historical backdrop, outlining the significance of infectious diseases and the importance of developing reliable animal models to test novel therapeutics, and (iv) provide a summary of infectious diseases for which a marmoset model exists, followed by an in-depth discussion of the marmoset models of two studied bacterial infectious diseases (tularemia and melioidosis) and one viral infectious disease (viral hepatitis C).

Phenotype and fate of liver-resident CD8 T cells during acute and chronic hepacivirus infection

Immune correlates of hepatitis C virus (HCV) clearance and control remain poorly defined due to the lack of an informative animal model. We recently described acute and chronic rodent HCV-like virus (RHV) infections in lab mice. Here, we developed MHC class I and class II tetramers to characterize the serial changes in RHV-specific CD8 and CD4 T cells during acute and chronic infection in C57BL/6J mice. RHV infection induced rapid expansion of T cells targeting viral structural and nonstructural proteins. After virus clearance, the virus-specific T cells transitioned from effectors to long-lived liver-resident memory T cells (TRM). The effector and memory CD8 and CD4 T cells primarily produced Th1 cytokines, IFN-γ, TNF-α, and IL-2, upon ex vivo antigen stimulation, and their phenotype and transcriptome differed significantly between the liver and spleen. Rapid clearance of RHV reinfection coincided with the proliferation of virus-specific CD8 TRM cells in the liver. Chronic RHV infection was associated with the exhaustion of CD8 T cells (Tex) and the development of severe liver diseases. Interestingly, the virus-specific CD8 Tex cells continued proliferation in the liver despite the persistent high-titer viremia and retained partial antiviral functions, as evident from their ability to degranulate and produce IFN-γ upon ex vivo antigen stimulation. Thus, RHV infection in mice provides a unique model to study the function and fate of liver-resident T cells during acute and chronic hepatotropic infection.

Molecular Determinants of Mouse Adaptation of Rat Hepacivirus

The lack of robust immunocompetent animal models for hepatitis C virus (HCV) impedes vaccine development and studies of immune responses. Norway rat hepacivirus (NrHV) infection in rats shares HCV-defining characteristics, including hepatotropism, chronicity, immune responses, and aspects of liver pathology. To exploit genetic variants and research tools, we previously adapted NrHV to prolonged infection in laboratory mice. Through intrahepatic RNA inoculation of molecular clones of the identified variants, we here characterized four mutations in the envelope proteins responsible for mouse adaptation, including one disrupting a glycosylation site. These mutations led to high-titer viremia, similar to that observed in rats. In 4-week-old mice, infection was cleared after around 5 weeks compared to 2 to 3 weeks for nonadapted virus. In contrast, the mutations led to persistent but attenuated infection in rats, and they partially reverted, accompanied by an increase in viremia. Attenuated infection in rat but not mouse hepatoma cells demonstrated that the characterized mutations were indeed mouse adaptive rather than generally adaptive across species and that species determinants and not immune interactions were responsible for attenuation in rats. Unlike persistent NrHV infection in rats, acute resolving infection in mice was not associated with the development of neutralizing antibodies. Finally, infection of scavenger receptor B-I (SR-BI) knockout mice suggested that adaptation to mouse SR-BI was not a primary function of the identified mutations. Rather, the virus may have adapted to lower dependency on SR-BI, thereby potentially surpassing species-specific differences. In conclusion, we identified specific determinants of NrHV mouse adaptation, suggesting species-specific interactions during entry. IMPORTANCE A prophylactic vaccine is required to achieve the World Health Organization's objective for hepatitis C virus elimination as a serious public health threat. However, the lack of robust immunocompetent animal models supporting hepatitis C virus infection impedes vaccine development as well as studies of immune responses and viral evasion. Hepatitis C virus-related hepaciviruses were discovered in a number of animal species and provide useful surrogate infection models. Norway rat hepacivirus is of particular interest, as it enables studies in rats, an immunocompetent and widely used small laboratory animal model. Its adaptation to robust infection also in laboratory mice provides access to a broader set of mouse genetic lines and comprehensive research tools. The presented mouse-adapted infectious clones will be of utility for reverse genetic studies, and the Norway rat hepacivirus mouse model will facilitate studies of hepacivirus infection for in-depth characterization of virus-host interactions, immune responses, and liver pathology.

Studying T Cell Responses to Hepatotropic Viruses in the Liver Microenvironment

T cells play an important role in the clearance of hepatotropic viruses but may also cause liver injury and contribute to disease progression in chronic hepatitis B and C virus infections which affect millions of people worldwide. The liver provides a unique microenvironment of immunological tolerance and hepatic immune regulation can modulate the functional properties of T cell subsets and influence the outcome of a virus infection. Extensive research over the last years has advanced our understanding of hepatic conventional CD4+ and CD8+ T cells and unconventional T cell subsets and their functions in the liver environment during acute and chronic viral infections. The recent development of new small animal models and technological advances should further increase our knowledge of hepatic immunological mechanisms. Here we provide an overview of the existing models to study hepatic T cells and review the current knowledge about the distinct roles of heterogeneous T cell populations during acute and chronic viral hepatitis.

CD8+ T cells contribute to diet-induced memory deficits in aged male rats

We have previously shown that short-term (3-day) high fat diet (HFD) consumption induces a neuroinflammatory response and subsequent impairment of long-term memory in aged, but not young adult, male rats. However, the immune cell phenotypes driving this proinflammatory response are not well understood. Previously, we showed that microglia isolated from young and aged rats fed a HFD express similar levels of priming and proinflammatory transcripts, suggesting that additional factors may drive the exaggerated neuroinflammatory response selectively observed in aged HFD-fed rats. It is established that T cells infiltrate both the young and especially the aged central nervous system (CNS) and contribute to immune surveillance of the parenchyma. Thus, we investigated the modulating role of short-term HFD on T cell presence in the CNS in aged rats using bulk RNA sequencing and flow cytometry. RNA sequencing results indicate that aging and HFD altered the expression of genes and signaling pathways associated with T cell signaling, immune cell trafficking, and neuroinflammation. Moreover, flow cytometry data showed that aging alone increased CD4+ and CD8+ T cell presence in the brain and that CD8+, but not CD4+, T cells were further increased in aged rats fed a HFD. Based on these data, we selectively depleted circulating CD8+ T cells via an intravenous injection of an anti-CD8 antibody in aged rats prior to 3 days of HFD to infer the functional role these cells may be playing in long-term memory and neuroinflammation. Results indicate that peripheral depletion of CD8+ T cells lowered hippocampal cytokine levels and prevented the HFD-induced i) increase in brain CD8+ T cells, ii) memory impairment, and iii) alterations in pre- and post-synaptic structures in the hippocampus and amygdala. Together, these data indicate a substantial role for CD8+ T cells in mediating diet-induced memory impairments in aged male rats.

Neutralization and receptor use of infectious culture-derived rat hepacivirus as a model for HCV

BACKGROUND AND AIMS

Lack of tractable immunocompetent animal models amenable to robust experimental challenge impedes vaccine efforts for HCV. Infection with rodent hepacivirus from Rattus norvegicus (RHV-rn1) in rats shares HCV-defining characteristics, including liver tropism, chronicity, and pathology. RHV in vitro cultivation would facilitate genetic studies on particle production, host factor interactions, and evaluation of antibody neutralization guiding HCV vaccine approaches.

APPROACH AND RESULTS

We report an infectious reverse genetic cell culture system for RHV-rn1 using highly permissive rat hepatoma cells and adaptive mutations in the E2, NS4B, and NS5A viral proteins. Cell culture-derived RHV-rn1 particles (RHVcc) share hallmark biophysical characteristics of HCV and are infectious in mice and rats. Culture adaptive mutations attenuated RHVcc in immunocompetent rats, and the mutations reverted following prolonged infection, but not in severe combined immunodeficiency (SCID) mice, suggesting that adaptive immune pressure is a primary driver of reversion. Accordingly, sera from RHVcc-infected SCID mice or the early acute phase of immunocompetent mice and rats were infectious in culture. We further established an in vitro RHVcc neutralization assay, and observed neutralizing activity of rat sera specifically from the chronic phase of infection. Finally, we found that scavenger receptor class B type I promoted RHV-rn1 entry in vitro and in vivo.

CONCLUSIONS

The RHV-rn1 infectious cell culture system enables studies of humoral immune responses against hepacivirus infection. Moreover, recapitulation of the entire RHV-rn1 infectious cycle in cell culture will facilitate reverse genetic studies and the exploration of tropism and virus-host interactions.

Hepatitis C Vaccination: Where We Are and Where We Need to Be

The hepatitis C virus (HCV) is a common cause of chronic liver disease and liver cancer worldwide. Despite advances in curative therapies for HCV, the incidence of new infections is not decreasing at the expected rate to hit the World Health Organization (WHO) target for the elimination of HCV by 2030. In fact, there are still more new cases of infection in the United States and worldwide than are being cured. The reasons for the rise in new cases include poor access to care and the opioid epidemic. The clinical burden of HCV requires a multimodal approach to eradicating the infection. Vaccination would be an excellent tool to prevent incidence of new infections; however, the genetic diversity of HCV and its ability to generate quasispecies within an infected host make creating a broadly reactive vaccine difficult. Multiple vaccine candidates have been identified, but to date, there has not been a target that has led to a broadly reactive vaccine, though several of the candidates are promising. Additionally, the virus is very difficult to culture and testing candidates in humans or chimpanzees is ethically challenging. Despite the multiple barriers to creating a vaccine, vaccination still represents an important tool in the fight against HCV.

The Bank Vole (Clethrionomys glareolus)—Small Animal Model for Hepacivirus Infection

Many people worldwide suffer from hepatitis C virus (HCV) infection, which is frequently persistent. The lack of efficient vaccines against HCV and the unavailability of or limited compliance with existing antiviral therapies is problematic for health care systems worldwide. Improved small animal models would support further hepacivirus research, including development of vaccines and novel antivirals. The recent discovery of several mammalian hepaciviruses may facilitate such research. In this study, we demonstrated that bank voles (Clethrionomys glareolus) were susceptible to bank vole-associated Hepacivirus F and Hepacivirus J strains, based on the detection of hepaciviral RNA in 52 of 55 experimentally inoculated voles. In contrast, interferon α/β receptor deficient C57/Bl6 mice were resistant to infection with both bank vole hepaciviruses (BvHVs). The highest viral genome loads in infected voles were detected in the liver, and viral RNA was visualized by in situ hybridization in hepatocytes, confirming a marked hepatotropism. Furthermore, liver lesions in infected voles resembled those of HCV infection in humans. In conclusion, infection with both BvHVs in their natural hosts shares striking similarities to HCV infection in humans and may represent promising small animal models for this important human disease.

A Methyltransferase-Defective Vesicular Stomatitis Virus-Based SARS-CoV-2 Vaccine Candidate Provides Complete Protection against SARS-CoV-2 Infection in Hamsters

The current pandemic of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has led to dramatic economic and health burdens. Although the worldwide SARS-CoV-2 vaccination campaign has begun, exploration of other vaccine candidates is needed due to uncertainties with the current approved vaccines, such as durability of protection, cross-protection against variant strains, and costs of long-term production and storage. In this study, we developed a methyltransferase-defective recombinant vesicular stomatitis virus (mtdVSV)-based SARS-CoV-2 vaccine candidate. We generated mtdVSVs expressing SARS-CoV-2 full-length spike (S) protein, S1, or its receptor-binding domain (RBD). All of these recombinant viruses grew to high titers in mammalian cells despite high attenuation in cell culture. The SARS-CoV-2 S protein and its truncations were highly expressed by the mtdVSV vector. These mtdVSV-based vaccine candidates were completely attenuated in both immunocompetent and immunocompromised mice. Among these constructs, mtdVSV-S induced high levels of SARS-CoV-2-specific neutralizing antibodies (NAbs) and Th1-biased T-cell immune responses in mice. In Syrian golden hamsters, the serum levels of SARS-CoV-2-specific NAbs triggered by mtdVSV-S were higher than the levels of NAbs in convalescent plasma from recovered COVID-19 patients. In addition, hamsters immunized with mtdVSV-S were completely protected against SARS-CoV-2 replication in lung and nasal turbinate tissues, cytokine storm, and lung pathology. Collectively, our data demonstrate that mtdVSV expressing SARS-CoV-2 S protein is a safe and highly efficacious vaccine candidate against SARS-CoV-2 infection. IMPORTANCE Viral mRNA cap methyltransferase (MTase) is essential for mRNA stability, protein translation, and innate immune evasion. Thus, viral mRNA cap MTase activity is an excellent target for development of live attenuated or live vectored vaccine candidates. Here, we developed a panel of MTase-defective recombinant vesicular stomatitis virus (mtdVSV)-based SARS-CoV-2 vaccine candidates expressing full-length S, S1, or several versions of the RBD. These mtdVSV-based vaccine candidates grew to high titers in cell culture and were completely attenuated in both immunocompetent and immunocompromised mice. Among these vaccine candidates, mtdVSV-S induces high levels of SARS-CoV-2-specific neutralizing antibodies (Nabs) and Th1-biased immune responses in mice. Syrian golden hamsters immunized with mtdVSV-S triggered SARS-CoV-2-specific NAbs at higher levels than those in convalescent plasma from recovered COVID-19 patients. Furthermore, hamsters immunized with mtdVSV-S were completely protected against SARS-CoV-2 challenge. Thus, mtdVSV is a safe and highly effective vector to deliver SARS-CoV-2 vaccine.

Pathogenesis, MicroRNA-122 Gene-Regulation, and Protective Immune Responses After Acute Equine Hepacivirus Infection

BACKGROUND AND AIMS

Equine hepacivirus (EqHV) is phylogenetically the closest relative of HCV and shares genome organization, hepatotropism, transient or persistent infection outcome, and the ability to cause hepatitis. Thus, EqHV studies are important to understand equine liver disease and further as an outbred surrogate animal model for HCV pathogenesis and protective immune responses. Here, we aimed to characterize the course of EqHV infection and associated protective immune responses.

APPROACH AND RESULTS

Seven horses were experimentally inoculated with EqHV, monitored for 6 months, and rechallenged with the same and, subsequently, a heterologous EqHV. Clearance was the primary outcome (6 of 7) and was associated with subclinical hepatitis characterized by lymphocytic infiltrate and individual hepatocyte necrosis. Seroconversion was delayed and antibody titers waned slowly. Clearance of primary infection conferred nonsterilizing immunity, resulting in shortened duration of viremia after rechallenge. Peripheral blood mononuclear cell responses in horses were minimal, although EqHV-specific T cells were identified. Additionally, an interferon-stimulated gene signature was detected in the liver during EqHV infection, similar to acute HCV in humans. EqHV, as HCV, is stimulated by direct binding of the liver-specific microRNA (miR), miR-122. Interestingly, we found that EqHV infection sequesters enough miR-122 to functionally affect gene regulation in the liver. This RNA-based mechanism thus could have consequences for pathology.

CONCLUSIONS

EqHV infection in horses typically has an acute resolving course, and the protective immune response lasts for at least a year and broadly attenuates subsequent infections. This could have important implications to achieve the primary goal of an HCV vaccine; to prevent chronicity while accepting acute resolving infection after virus exposure.

Mutational escape from cellular immunity in viral hepatitis: variations on a theme

Approx. 320 million individuals worldwide are chronically infected with hepatitis viruses, contributing to viral hepatitis being one of the 10 leading causes of death. Cellular adaptive immunity, namely CD4+ and CD8+ T cells, plays an important role in viral clearance and control. Two main mechanisms, however, may lead to failure of the virus-specific T-cell response: T-cell exhaustion and mutational viral escape. Viral escape has been studied in detail in hepatitis C virus (HCV) infection, where it is thought to affect approx. 50% of virus-specific CD8+ T-cell responses in persistent infection, to influence natural infection outcome and to contribute to failure of preventive vaccination strategies. In hepatitis B virus (HBV) as well as HBV/hepatitis D virus (HDV) co-infection, the impact of viral escape has been studied in detail only recently.

Hepatitis C Virus Vaccine Research: Time to Put Up or Shut Up

Unless urgently needed to prevent a pandemic, the development of a viral vaccine should follow a rigorous scientific approach. Each vaccine candidate should be designed considering the in-depth knowledge of protective immunity, followed by preclinical studies to assess immunogenicity and safety, and lastly, the evaluation of selected vaccines in human clinical trials. The recently concluded first phase II clinical trial of a human hepatitis C virus (HCV) vaccine followed this approach. Still, despite promising preclinical results, it failed to protect against chronic infection, raising grave concerns about our understanding of protective immunity. This setback, combined with the lack of HCV animal models and availability of new highly effective antivirals, has fueled ongoing discussions of using a controlled human infection model (CHIM) to test new HCV vaccine candidates. Before taking on such an approach, however, we must carefully weigh all the ethical and health consequences of human infection in the absence of a complete understanding of HCV immunity and pathogenesis. We know that there are significant gaps in our knowledge of adaptive immunity necessary to prevent chronic HCV infection. This review discusses our current understanding of HCV immunity and the critical gaps that should be filled before embarking upon new HCV vaccine trials. We discuss the importance of T cells, neutralizing antibodies, and HCV genetic diversity. We address if and how the animal HCV-like viruses can be used for conceptualizing effective HCV vaccines and what we have learned so far from these HCV surrogates. Finally, we propose a logical but narrow path forward for HCV vaccine development.

Adenovirus-vectored T cell vaccine for hepacivirus shows reduced effectiveness against a CD8 T cell escape variant in rats

There is an urgent need for a vaccine to prevent chronic infection by hepatitis C virus (HCV) and its many genetic variants. The first human vaccine trial, using recombinant viral vectors that stimulate pan-genotypic T cell responses against HCV non-structural proteins, failed to demonstrate efficacy despite significant preclinical promise. Understanding the factors that govern HCV T cell vaccine success is necessary for design of improved immunization strategies. Using a rat model of chronic rodent hepacivirus (RHV) infection, we assessed the impact of antigenic variation and immune escape upon success of a conceptually analogous RHV T cell vaccine. Naïve Lewis rats were vaccinated with a recombinant human adenovirus expressing RHV non-structural proteins (NS)3-5B and later challenged with a viral variant containing immune escape mutations within major histocompatibility complex (MHC) class I-restricted epitopes (escape virus). Whereas 7 of 11 (64%) rats cleared infection caused by wild-type RHV, only 3 of 12 (25%) were protected against heterologous challenge with escape virus. Uncontrolled replication of escape virus was associated with durable CD8 T cell responses targeting escaped epitopes alone. In contrast, clearance of escape virus correlated with CD4 T cell helper immunity and maintenance of CD8 T cell responses against intact viral epitopes. Interestingly, clearance of wild-type RHV infection after vaccination conferred enhanced protection against secondary challenge with escape virus. These results demonstrate that the efficacy of an RHV T cell vaccine is reduced when challenge virus contains escape mutations within MHC class I-restricted epitopes and that failure to sustain CD8 T cell responses against intact epitopes likely underlies immune failure in this setting. Further investigation of the immune responses that yield protection against diverse RHV challenges in this model may facilitate design of broadly effective HCV vaccines.

The hepatitis C virus is one of the leading causes of chronic liver disease and cancer worldwide. A vaccine is not yet available and the first phase II clinical trial in humans using a T cell-based immunization strategy recently failed to prevent chronic infection in high risk individuals for unclear reasons. In this study we evaluated how immune escape mutations at major histocompatibility complex (MHC) class I-restricted viral epitopes influence the effectiveness of an adenoviral-vectored T cell vaccine in a rat model of chronic HCV-related rodent hepacivirus infection, currently the only animal model available for evaluation of HCV vaccine strategies. We show that vaccine efficacy is markedly diminished when challenge virus contains naturally-acquired escape mutations at dominant MHC class I-restricted viral epitopes that render a subset of vaccine-generated CD8 T cell responses ineffective. We also identify CD4 T cell help as a critical correlate of vaccine success against heterologous virus challenge. Our results have important implications for human vaccination programs that aim to induce broad protective immunity against heterogeneous HCV strains.

Molecular detection and genomic characterization of diverse hepaciviruses in African rodents

Hepatitis C virus (HCV; genus Hepacivirus) represents a major public health problem, infecting about three per cent of the human population. Because no animal reservoir carrying closely related hepaciviruses has been identified, the zoonotic origins of HCV still remain unresolved. Motivated by recent findings of divergent hepaciviruses in rodents and a plausible African origin of HCV genotypes, we have screened a large collection of small mammals samples from seven sub-Saharan African countries. Out of 4,303 samples screened, eighty were found positive for the presence of hepaciviruses in twenty-nine different host species. We, here, report fifty-six novel genomes that considerably increase the diversity of three divergent rodent hepacivirus lineages. Furthermore, we provide strong evidence for hepacivirus co-infections in rodents, which were exclusively found in four sampled species of brush-furred mice. We also detect evidence of recombination within specific host lineages. Our study expands the available hepacivirus genomic data and contributes insights into the relatively deep evolutionary history of these pathogens in rodents. Overall, our results emphasize the importance of rodents as a potential hepacivirus reservoir and as models for investigating HCV infection dynamics.

T-Cell Immunity against the Hepatitis C Virus: A Persistent Research Priority in an Era of Highly Effective Therapy

Approximately 70% of acute hepatitis C virus (HCV) infections become chronic, indicating that the virus is exceptionally well adapted to persist in humans with otherwise normal immune function. Robust, lifelong replication of this small RNA virus does not require a generalized failure of immunity. HCV effectively subverts innate and adaptive host defenses while leaving immunity against other viruses intact. Here, the role of CD4⁺ and CD8⁺ T-cell responses in control of HCV infection and their failure to prevent virus persistence in most individuals are reviewed. Two issues of practical importance remain priorities in an era of highly effective antiviral therapy for chronic hepatitis C. First, the characteristics of successful T-cell responses that promote resolution of HCV infection are considered, as they will underpin development of vaccines that prevent HCV persistence. Second, defects in T-cell immunity that facilitate HCV persistence and whether they are reversed after antiviral cure to provide protection from reinfection are also addressed.

Viral vectored hepatitis C virus vaccines generate pan-genotypic T cell responses to conserved subdominant epitopes

BACKGROUND

Viral genetic variability presents a major challenge to the development of a prophylactic hepatitis C virus (HCV) vaccine. A promising HCV vaccine using chimpanzee adenoviral vectors (ChAd) encoding a genotype (gt) 1b non-structural protein (ChAd-Gt1b-NS) generated high magnitude T cell responses. However, these T cells showed reduced cross-recognition of dominant epitope variants and the vaccine has recently been shown to be ineffective at preventing chronic HCV. To address the challenge of viral diversity, we developed ChAd vaccines encoding HCV genomic sequences that are conserved between all major HCV genotypes and adjuvanted by truncated shark invariant chain (sIi_tr).

METHODS

Age-matched female mice were immunised intramuscularly with ChAd (10⁸ infectious units) encoding gt-1 and -3 (ChAd-Gt1/3) or gt-1 to -6 (ChAd-Gt1-6) conserved segments spanning the HCV proteome, or gt-1b (ChAd-Gt1b-NS control), with immunogenicity assessed 14-days post-vaccination.

RESULTS

Conserved segment vaccines, ChAd-Gt1/3 and ChAd-Gt1-6, generated high-magnitude, broad, and functional CD4⁺ and CD8⁺ T cell responses. Compared to the ChAd-Gt1b-NS vaccine, these vaccines generated significantly greater responses against conserved non-gt-1 antigens, including conserved subdominant epitopes that were not targeted by ChAd-Gt1b-NS. Epitopes targeted by the conserved segment HCV vaccine induced T cells, displayed 96.6% mean sequence homology between all HCV subtypes (100% sequence homology for the majority of genotype-1, -2, -4 sequences and 94% sequence homology for gt-3, -6, -7, and -8) in contrast to 85.1% mean sequence homology for epitopes targeted by ChAd-Gt1b-NS induced T cells. The addition of truncated shark invariant chain (sIi_tr) increased the magnitude, breadth, and cross-reactivity of the T cell response.

CONCLUSIONS

We have demonstrated that genetically adjuvanted ChAd vectored HCV T cell vaccines encoding genetic sequences conserved between genotypes are immunogenic, activating T cells that target subdominant conserved HCV epitopes. These pre-clinical studies support the use of conserved segment HCV T cell vaccines in human clinical trials.

Aloperine inhibits hepatitis C virus entry into cells by disturbing internalisation from endocytosis to the membrane fusion process

Aloperine, a natural alkaloid isolated from the Chinese traditional herb Sophora alopecuroides, is a broad-spectrum antiviral agent with anti-inflammatory activity. Here, we found that aloperine effectively inhibited hepatitis C virus (HCV) propagation in Huh7.5 cells and primary human hepatocytes without cytotoxicity, and it blocked HCV cell-to-cell viral transmission. The antiviral mechanism evidence demonstrated that aloperine inhibits HCV internalisation from endocytosis to the membrane fusion process, and the target may be associated with host factors. Aloperine additively inhibited HCV propagation with direct-acting antivirals (DAAs) and was effective against HCV variants resistant to known DAAs. Therefore, aloperine might be a natural lead compound for the development of innovative antivirals, and the combined use of aloperine with DAAs might contribute to eliminating liver diseases caused by HCV infection.

Animal Models Used in Hepatitis C Virus Research

The narrow range of species permissive to infection by hepatitis C virus (HCV) presents a unique challenge to the development of useful animal models for studying HCV, as well as host immune responses and development of chronic infection and disease. Following earlier studies in chimpanzees, several unique approaches have been pursued to develop useful animal models for research while avoiding the important ethical concerns and costs inherent in research with chimpanzees. Genetically related hepatotropic viruses that infect animals are being used as surrogates for HCV in research studies; chimeras of these surrogate viruses harboring specific regions of the HCV genome are being developed to improve their utility for vaccine testing. Concurrently, genetically humanized mice are being developed and continually advanced using human factors known to be involved in virus entry and replication. Further, xenotransplantation of human hepatocytes into mice allows for the direct study of HCV infection in human liver tissue in a small animal model. The current advances in each of these approaches are discussed in the present review.

Priming of Antiviral CD8 T Cells without Effector Function by a Persistently Replicating Hepatitis C-Like Virus

Immune-competent animal models for the hepatitis C virus (HCV) are nonexistent, impeding studies of host-virus interactions and vaccine development. Experimental infection of laboratory rats with a rodent hepacivirus isolated from Rattus norvegicus (RHV) is a promising surrogate model due to its recapitulation of HCV-like chronicity. However, several aspects of rat RHV infection remain unclear, for instance, how RHV evades host adaptive immunity to establish persistent infection. Here, we analyzed the induction, differentiation, and functionality of RHV-specific CD8 T cell responses that are essential for protection against viral persistence. Virus-specific CD8 T cells targeting dominant and subdominant major histocompatibility complex class I epitopes proliferated considerably in liver after RHV infection. These populations endured long term yet never acquired antiviral effector functions or selected for viral escape mutations. This was accompanied by the persistent upregulation of programmed cell death-1 and absent memory cell formation, consistent with a dysfunctional phenotype. Remarkably, transient suppression of RHV viremia with a direct-acting antiviral led to the priming of CD8 T cells with partial effector function, driving the selection of a viral escape variant. These data demonstrate an intrinsic abnormality within CD8 T cells primed by rat RHV infection, an effect that is governed at least partially by the magnitude of early virus replication. Thus, this model could be useful in investigating mechanisms of CD8 T cell subversion, leading to the persistence of hepatotropic pathogens such as HCV.IMPORTANCE Development of vaccines against hepatitis C virus (HCV), a major cause of cirrhosis and cancer, has been stymied by a lack of animal models. The recent discovery of an HCV-like rodent hepacivirus (RHV) enabled the development of such a model in rats. This platform recapitulates HCV hepatotropism and viral chronicity necessary for vaccine testing. Currently, there are few descriptions of RHV-specific responses and why they fail to prevent persistent infection in this model. Here, we show that RHV-specific CD8 T cells, while induced early at high magnitude, do not develop into functional effectors capable of controlling virus. This defect was partially alleviated by short-term treatment with an HCV antiviral. Thus, like HCV, RHV triggers dysfunction of virus-specific CD8 T cells that are vital for infection resolution. Additional study of this evasion strategy and how to mitigate it could enhance our understanding of hepatotropic viral infections and lead to improved vaccines and therapeutics.

Animal Models of Hepatitis C Virus Infection

Hepatitis C virus (HCV) is an important and underreported infectious disease, causing chronic infection in ∼71 million people worldwide. The limited host range of HCV, which robustly infects only humans and chimpanzees, has made studying this virus in vivo challenging and hampered the development of a desperately needed vaccine. The restrictions and ethical concerns surrounding biomedical research in chimpanzees has made the search for an animal model all the more important. In this review, we discuss different approaches that are being pursued toward creating small animal models for HCV infection. Although efforts to use a nonhuman primate species besides chimpanzees have proven challenging, important advances have been achieved in a variety of humanized mouse models. However, such models still fall short of the overarching goal to have an immunocompetent, inheritably susceptible in vivo platform in which the immunopathology of HCV could be studied and putative vaccines development. Alternatives to overcome this include virus adaptation, such as murine-tropic HCV strains, or the use of related hepaciviruses, of which many have been recently identified. Of the latter, the rodent/rat hepacivirus from Rattus norvegicus species-1 (RHV-rn1) holds promise as a surrogate virus in fully immunocompetent rats that can inform our understanding of the interaction between the immune response and viral outcomes (i.e., clearance vs. persistence). However, further characterization of these animal models is necessary before their use for gaining new insights into the immunopathogenesis of HCV and for conceptualizing HCV vaccines.

Use of an Outbred Rat Hepacivirus Challenge Model for Design and Evaluation of Efficacy of Different Immunization Strategies for Hepatitis C Virus

BACKGROUND AND AIMS

The lack of immunocompetent small animal models for hepatitis C virus (HCV) has greatly hindered the development of effective vaccines. Using rodent hepacivirus (RHV), a homolog of HCV that shares many characteristics of HCV infection, we report the development and application of an RHV outbred rat model for HCV vaccine development.

APPROACH AND RESULTS

Simian adenovirus (ChAdOx1) encoding a genetic immune enhancer (truncated shark class II invariant chain) fused to the nonstructural (NS) proteins NS3-NS5B from RHV (ChAd-NS) was used to vaccinate Sprague-Dawley rats, resulting in high levels of cluster of differentiation 8-positive (CD8+ ) T-cell responses. Following RHV challenge (using 10 or 100 times the minimum infectious dose), 42% of vaccinated rats cleared infection within 6-8 weeks, while all mock vaccinated controls became infected with high-level viremia postchallenge. A single, 7-fold higher dose of ChAd-NS increased efficacy to 67%. Boosting with ChAd-NS or with a plasmid encoding the same NS3-NS5B antigens increased efficacy to 100% and 83%, respectively. A ChAdOx1 vector encoding structural antigens (ChAd-S) was also constructed. ChAd-S alone showed no efficacy. Strikingly, when combined with ChAd-NS, ChAD-S produced 83% efficacy. Protection was associated with a strong CD8+ interferon gamma-positive recall response against NS4. Next-generation sequencing of a putative RHV escape mutant in a vaccinated rat identified mutations in both identified immunodominant CD8+ T-cell epitopes.

CONCLUSIONS

A simian adenovirus vector vaccine strategy is effective at inducing complete protective immunity in the rat RHV model. The RHV Sprague-Dawley rat challenge model enables comparative testing of vaccine platforms and antigens and identification of correlates of protection and thereby provides a small animal experimental framework to guide the development of an effective vaccine for HCV in humans.

Palmitoylation of Hepatitis C Virus NS2 Regulates Its Subcellular Localization and NS2-NS3 Autocleavage

Chronic infection with hepatitis C virus (HCV) is a major cause of severe liver diseases responsible for nearly 400,000 deaths per year. HCV NS2 protein is a multifunctional regulator of HCV replication involved in both viral-genome replication and infectious-virus assembly. However, the underlying mechanism that enables the protein to participate in multiple steps of HCV replication remains unknown. In this study, we discovered that NS2 palmitoylation is the master regulator of its multiple functions, including NS2-mediated self-cleavage and HCV envelope protein recruitment to the virus assembly sites, which in turn promote HCV RNA replication and infectious-particle assembly, respectively. This newly revealed information suggests that NS2 palmitoylation could serve as a promising target to inhibit both HCV RNA replication and virus assembly, representing a new avenue for host-targeting strategies against HCV infection.

Hepatitis C virus (HCV) nonstructural protein 2 (NS2) is a multifunctional protein implicated in both HCV RNA replication and virus particle assembly. NS2-encoded cysteine protease is responsible for autoprocessing of NS2-NS3 precursor, an essential step in HCV RNA replication. NS2 also promotes HCV particle assembly by recruiting envelope protein 2 (E2) to the virus assembly sites located at the detergent-resistant membranes (DRM). However, the fundamental mechanism regulating multiple functions of NS2 remains unclear. In this study, we discovered that NS2 is palmitoylated at the position 113 cysteine residue (NS2/C113) when expressed by itself in cells and during infectious-HCV replication. Blocking NS2 palmitoylation by introducing an NS2/C113S mutation reduced NS2-NS3 autoprocessing and impaired HCV RNA replication. Replication of the NS2/C113S mutant was restored by inserting an encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) between NS2 and NS3 to separate the two proteins independently of NS2-mediated autoprocessing. These results suggest that NS2 palmitoylation is critical for HCV RNA replication by promoting NS2-NS3 autoprocessing. The NS2/C113S mutation also impaired infectious-HCV assembly, DRM localization of NS2 and E2, and colocalization of NS2 with Core and endoplasmic reticulum lipid raft-associated protein 2 (Erlin-2). In conclusion, our study revealed that two major functions of NS2 involved in HCV RNA replication and virus assembly, i.e., NS2-NS3 autoprocessing and E2 recruitment to the DRM, are regulated by palmitoylation at NS2/C113. Since S-palmitoylation is reversible, NS2 palmitoylation likely allows NS2 to fine tune both HCV RNA replication and infectious-particle assembly.

IMPORTANCE Chronic infection with hepatitis C virus (HCV) is a major cause of severe liver diseases responsible for nearly 400,000 deaths per year. HCV NS2 protein is a multifunctional regulator of HCV replication involved in both viral-genome replication and infectious-virus assembly. However, the underlying mechanism that enables the protein to participate in multiple steps of HCV replication remains unknown. In this study, we discovered that NS2 palmitoylation is the master regulator of its multiple functions, including NS2-mediated self-cleavage and HCV envelope protein recruitment to the virus assembly sites, which in turn promote HCV RNA replication and infectious-particle assembly, respectively. This newly revealed information suggests that NS2 palmitoylation could serve as a promising target to inhibit both HCV RNA replication and virus assembly, representing a new avenue for host-targeting strategies against HCV infection.

Dose Effects of Recombinant Adenovirus Immunization in Rodents

Recombinant adenovirus type 5 (rAd) has been used as a vaccine platform against many infectious diseases and has been shown to be an effective vaccine vector. The dose of the vaccine varies significantly from study to study, making it very difficult to compare immune responses and vaccine efficacy. This study determined the immune correlates induced by serial dilutions of rAd vaccines delivered intramuscularly (IM) and intranasally (IN) to mice and rats. When immunized IM, mice had substantially higher antibody responses at the higher vaccine doses, whereas, the IN immunized mice showed a lower response to the higher rAd vaccine doses. Rats did not show dose-dependent antibody responses to increasing vaccine doses. The IM immunized mice and rats also showed significant dose-dependent T cell responses to the rAd vaccine. However, the T cell immunity plateaued in both mice and rats at 10⁹ and 10¹⁰ vp/animal, respectively. Additionally, the highest dose of vaccine in mice and rats did not improve the T cell responses. A final vaccine analysis using a lethal influenza virus challenge showed that despite the differences in the immune responses observed in the mice, the mice had very similar patterns of protection. This indicates that rAd vaccines induced dose-dependent immune responses, especially in IM immunized animals, and that immune correlates are not as predictive of protection as initially thought.

Replicons of a Rodent Hepatitis C Model Virus Permit Selection of Highly Permissive Cells

Animal hepaciviruses represent promising surrogate models for hepatitis C virus (HCV), for which there are no efficient immunocompetent animal models. Experimental infection of laboratory rats with rodent hepacivirus isolated from feral Rattus norvegicus (RHV-rn1) mirrors key aspects of HCV infection in humans, including chronicity, hepatitis, and steatosis. Moreover, RHV has been adapted to infect immunocompetent laboratory mice. RHV in vitro systems have not been developed but would enable detailed studies of the virus life cycle crucial for designing animal experiments to model HCV infection. Here, we established efficient RHV-rn1 selectable subgenomic replicons with and without reporter genes. Rat and mouse liver-derived cells did not readily support the complete RHV life cycle, but replicon-containing cell clones could be selected with and without acquired mutations. Replication was significantly enhanced by mutations in NS4B and NS5A and in cell clones cured of replicon RNA. These mutations increased RHV replication of both mono- and bicistronic constructs, and CpG/UpA-dinucleotide optimization of reporter genes allowed replication. Using the replicon system, we show that the RHV-rn1 NS3-4A protease cleaves a human mitochondrial antiviral signaling protein reporter, providing a sensitive readout for virus replication. RHV-rn1 replication was inhibited by the HCV polymerase inhibitor sofosbuvir and high concentrations of HCV NS5A antivirals but not by NS3 protease inhibitors. The microRNA-122 antagonist miravirsen inhibited RHV-rn1 replication, demonstrating the importance of this HCV host factor for RHV. These novel RHV in vitro systems will be useful for studies of tropism, molecular virology, and characterization of virus-host interactions, thereby providing important complements to in vivo systems.IMPORTANCE A vaccine against hepatitis C virus (HCV) is crucial for global control of this important pathogen, which induces fatal human liver diseases. Vaccine development has been hampered by the lack of immunocompetent animal models. Discovery of rodent hepacivirus (RHV) enabled establishment of novel surrogate animal models. These allow robust infection and reverse genetic and immunization studies of laboratory animals, which develop HCV-like chronicity. Currently, there are no RHV in vitro systems available to study tropism and molecular virology. Here, we established the first culture systems for RHV, recapitulating the intracellular phase of the virus life cycle in vitro These replicon systems enabled identification of replication-enhancing mutations and selection of cells highly permissive to RHV replication, which allow study of virus-host interactions. HCV antivirals targeting NS5A, NS5B, and microRNA-122 efficiently inhibited RHV replication. Hence, several important aspects of HCV replication are shared by the rodent virus system, reinforcing its utility as an HCV model.