Impact of hepatitis E virus variants on human health

Abstract

Hepatitis E virus (HEV) is a major cause of viral hepatitis globally. Disease manifestations of hepatitis E range from subclinical infections to severe hepatitis. Hepatitis E in immunocompromised persons is often complicated by persistent infection that can lead to fibrosis and cirrhosis if left untreated. Conventionally, human hepatitis E is believed to be exclusively caused by genotypes within species A (HEV-A) of the Orthohepevirus genus of the family Hepeviridae. HEV-A genotypes 1 & 2 are transmitted from person-to-person in areas with poor sanitation while HEV-A swine genotypes 3 & 4 are acquired from undercooked pork products in developed regions. Orthohepevirus C (HEV-C) is a HEV species that circulates in rodents and ferrets. HEV-C is highly divergent from HEV-A. HEV-C genotype 1 (HEV-C1), also known as rat hepatitis E, is an ubiquitous pathogen of street rats across the world. Due to limited sequence identity with HEV-A and inability to infect non-human primates in experimental settings, HEV-C1 was hitherto considered incapable of infecting humans. Conventional molecular assays for hepatitis E completely miss HEV-C1 infections due to sequence divergence. In this thesis, HEV-C1 is definitively proven to be a novel cause of hepatitis in humans. A dedicated surveillance system using HEV-C1 nucleic acid amplification tests found frequent human infections across Hong Kong over several years of observation. HEV-C1 infected patients were frequently elderly and/or immunocompromised. Infections were concentrated in districts of the city with active HEV-C1 epizootics in the rat population. Evidence for transfusion-transmitted HEV-C1 infection is also presented illustrating its potential as a blood-borne pathogen. The clinical features of HEV-C1 and HEV-A infections were compared. Rat hepatitis E fully recapitulates the clinical features of HEV-A infection. Immunocompetent individuals usually suffered a mild self-limiting hepatitis, but some also demonstrated severe and relapsing hepatitis. Immunocompromised individuals routinely progressed to persistent infection and chronic hepatitis despite reduction in immunosuppression. Meningoencephalitis was documented as a possible extrahepatic manifestation of HEV-C1 infection. Oral ribavirin was usually effective at achieving sustained virological response in persistently infected individuals, but refractory disease was observed. Using a multimodal assessment, HEV-A and HEV-C1 were shown to be antigenically highly diverse. As commercial hepatitis E antigen and antibody assays are based on HEV-A, they were frequently insensitive for the diagnosis of HEV-C1 infection. A parallel immunoblot system using bespoke HEV-A and HEV-C1 peptides as capture antigens was able to differentiate HEV-A and HEV-C1 antibody signatures in infected patient sera with high accuracy. Furthermore, the same peptides formed virus like particles (VLPs) enabling them to be deployed as immunogenic vaccines. Vaccination with HEV-A VLPs was unable to protect experimental rats from a HEV-C1 virus challenge while the HEV-C1 VLP conferred sterilizing immunity against a heterotypic HEV-C1 strain challenge. This confirmed that HEV-A exposure is not cross-protective against HEV-C1 infection. Evidence of an anamnestic response to subsequent HEV-C1 infection in HEV-A vaccinated animals is presented. Overall, rat hepatitis E is a novel cause of human hepatitis, which represents a paradigm shift in the assumption that only HEV-A variants cause hepatitis E infection.