The Chimpanzee Model of Viral Hepatitis: Advances in Understanding the Immune Response and Treatment of Viral Hepatitis

Abstract

Chimpanzees (Pan troglodytes) have contributed to diverse fields of biomedical research due to their close genetic relationship to humans and in many instances due to the lack of any other animal model. This review focuses on the contributions of the chimpanzee model to research on hepatitis viruses where chimpanzees represented the only animal model (hepatitis B and C) or the most appropriate animal model (hepatitis A). Research with chimpanzees led to the development of vaccines for HAV and HBV that are used worldwide to protect hundreds of millions from these diseases and, where fully implemented, have provided immunity for entire generations. More recently, chimpanzee research was instrumental in the development of curative therapies for hepatitis C virus infections. Over a span of 40 years, this research would identify the causative agent of NonA,NonB hepatitis, validate the molecular tools for drug discovery, and provide safety and efficacy data on the therapies that now provide a rapid and complete cure of HCV chronic infections. Several cocktails of antivirals are FDA approved that eliminate the virus following 12 weeks of once-per-day oral therapy. This represents the first cure of a chronic viral disease and, once broadly implemented, will dramatically reduce the occurrence of cirrhosis and liver cancer. The recent contributions of chimpanzees to our current understanding of T cell immunity for HCV, development of novel therapeutics for HBV, and the biology of HAV are reviewed. Finally, a perspective is provided on the events leading to the cessation of the use of chimpanzees in research and the future of the chimpanzees previously used to bring about these amazing breakthroughs in human healthcare.

Keywords: HAV; HBV; HCV; antiviral; chimpanzee; hepatitis; nonhuman primate; vaccine.

Figure 1

Heat Map of Hepatic Gene Expression in HCV Chronic Chimpanzees. The 30 most highly induced genes in the liver of HCV chronically infected chimpanzees are shown in red. The analyses involve total genome microarray analysis from 16 liver samples from HCV-infected chimpanzees in comparison to 6 uninfected chimpanzees. The 30 genes with the greatest decrease in expression in infected liver are shown in green. The genes increased in expression are primarily ISGs as determined by comparison to uninfected animals treated with IFNα. These genes are uniformly expressed at high levels during HCV infection. The genes decreased in expression may reflect some genes specifically regulated by IFN, but the response is variable in different chimpanzees.

Figure 2

TLR7 agonist GS-9620 induces a decrease in HBV DNA in the serum of a chimpanzee. A chimpanzee chronically infected with HBV was treated with oral GS-9620 therapy at 1 mg/kg or 2 mg/kg, with three times per week dosing for four weeks at each level. The line graph illustrates the decline in viremia as determined by quantitative PCR (genomes/ml). A maximum of 2.2 log decrease in viremia was observed, and suppression of HBV persisted for months after discontinuing therapy, suggesting an alteration in the immune response of the chimpanzee to HBV. Figure modified from Gastroenterology 2013;144(7):1508–1517.

Figure 3

Decline in serum HBsAg levels in chimpanzees chronically infected with HBV during therapy with ARC-520. ARC-520 is an RNAi antiviral that targets HBV transcripts. The RNAi triggers the degradation of all HBV transcripts including surface antigen (HBsAg). Four chimpanzees positive (HBeAg+) or negative (HBeAg−) for serum HBV e-antigen were treated with a lead-in period with monotherapy of standard of care nucleoside analogues (days −57 to 0). This therapy had minimal impact on HBsAg but reduced serum HBV DNA levels by multiple logs (data not shown). Initiation of ARC-520 therapy resulted in rapid decline in HBsAg levels, with up to a 2-log decline. ARC-520 provided greater reduction of HBsAg in HBeAg positive chimpanzees compared to HBeAg negative chimpanzees. Figure derived from data in Wooddell et al. RNAi-based treatment of chronically infected patients and chimpanzees implicates integrated hepatitis B virus DNA as a source of HBsAg. Sci Transl Med, In press.

Figure 4

Virologic and immunologic events during acute HAV infection in a chimpanzee inoculated intravenously with wild-type HAV. The bottom panel shows the presence of viral RNA (GE, genome equivalents) in serum (GE/ml), feces (GE/gm), and liver tissue (GE/μg total RNA) in relationship to serum alanine aminotransferase (ALT) activity shown in the shaded zone. The prolonged persistence of intrahepatic HAV RNA is surprising. The panel immediately above shows total anti-HAV antibody (% blocking in a competitive ELISA assay) and IgM anti-HAV (ELISA O.D.) The next two panels show frequencies of HAV-specific CD4+ and CD8+ T cells among peripheral blood mononuclear cells, as determined in an IFN-γ intracellular staining (ICS) assay. CD8+ cells were also quantified on the basis of staining with tetramers targeting epitopes in pX, 2B, and 3D^pol. Note the difference in scale between CD4+ and CD8+ T cell frequencies. The top panel shows type I IFN responses to HAV infection as reflected in minimal and only early serum IFN-α levels detectable by cytokine ELISA, and minimal increases in intrahepatic expression of IFN-stimulated genes IFIT1 and ISG15. pDCs were detected in liver tissue only at 1 week after viral challenge (arrow). Figure reproduced from Curr Opin Virol 2015 Apr; 11:7–13.