Hepatitis delta: immunopathogenesis and clinical challenges

Hepatitis delta is caused by infection with the hepatitis D virus (HDV) and is considered to be the most severe form of viral hepatitis in humans. Hepatitis delta occurs only in hepatitis B virus (HBV) surface antigen (HBsAg)-positive individuals as HDV is a defective RNA virus which requires the HBsAg for complete replication and transmission. Eight different HDV genotypes have been described with specific geographic distributions and distinct clinical courses. HDV/HBV co-infection can be associated with complex and dynamic viral dominance patterns. While HDV is frequently the dominating virus not only in HBV/HDV co-infection, but also in HBV/HCV/HDV triple-infected patients, the fluctuating courses of HDV and HBV viremia can be observed in other patients. Chronic HDV infection leads to more severe liver disease than HBV monoinfection with accelerated fibrosis progression, earlier hepatic decompensation and an increased risk for the development of hepatocellular carcinoma. However and in contrast to HCV infection, hepatic decompensation, rather than development of liver cancer, is the first clinical endpoint that develops during the course of infection. So far, only interferon-alpha treatment has proven antiviral activity against HDV in humans and has been linked to improved long-term outcome. Recent studies on the use of pegylated interferon showed a sustained virological response concerning HDV-RNA in about one quarter of the patients. HDV-specific immune responses might be associated with the response to treatment. Novel alternative treatment options, including prenylation inhibitors, are still awaiting clinical development for delta hepatitis. So far, only few studies have investigated immune responses against HDV and HBV in humans.