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Abstract

Changes in viral host cell tropism by e.g. changing receptor usage can lead to altered disease manifestations, requiring different treatment modalities. Hepatitis Delta virus (HDV), a small hepatotropic RNA virus, is known to depend on hepatitis B virus (HBV) envelope proteins for envelopment and spread. However, as HDV possesses the ability to replicate in non-hepatic cells and since envelopment of HDV was recently proposed to be mediated by non-HBV viruses, there might be an increased potential for altered host cell tropism of HDV due to differential envelope protein usage. This would have major clinical implications, as currently only HBV positive carriers are screened for HDV and specific therapeutic options only target HBV-dependent spread. Viruses that were proposed to mediate envelopment of HDV include hepatitis C virus (HCV) and vesicular stomatitis virus (VSV). As HDV is known for its liver tropism, I focused primarily on hepatitis C virus (HCV) as potential HBV-independent helper and aimed to evaluate the following questions: a) Does HDV make use of HCV envelope proteins? b) Can HDV be rendered susceptible for alternative helper usage by modification of HBV specificity determinants in HDV? c) What is the zoonotic potential of HDV via utilisation of non-human hepadnaviral helper viruses? I made use of different human cell culture systems based on RNA transfection or viral infection. Thereby, I was able to show efficient co-replication and co-infection of HDV and HCV in the same cell. However, production of HCV enveloped HDV particles was not observed. Based on plasmid transfection, I showed that HDV preferentially uses HBV with little, if any, envelopment by HCV or VSV. Based on sequence comparison to an animal Delta-like agent putatively using HBV-independent spread, I hypothesised that determinants specifically targeting the HBV envelope might hinder HDV from using alternative viral envelopes. However, eliminating HBV specificity determinants from HDV did not render HDV competent to use non-HBV envelopes. As the HDV-HBV interaction is mediated by a tryptophan-rich domain on HBV envelope proteins, I examined HCV and VSV envelope proteins for the presence of similar determinants. However, algorithm-based protein structure prediction did not detect such determinants, which is consistent with the almost exclusive envelopment of HDV by HBV. Last, I investigated various non-human hepadnaviruses for their capacity to provide helper function for HDV. To this end, I produced non-infectious self-assembling particles exclusively consisting of envelope proteins and preliminarily investigated secretion of HDV RNA mediated by these particles. Taken together, the data presented in this study support the current view that HDV most likely utilises exclusively HBV as helper virus, further arguing for co-evolution of these two viruses.