Elucidating the mechanisms involved in Hepatitis Delta virus (HDV) pathogenesis

Abstract

Hepatitis Delta virus (HDV) infection is associated with the most severe form of viral hepatitis.

Despite this, little is known about the molecular mechanism involved in HDV-induced liver pathology and the limited availability of small animal models hamper the development of specific therapies to ameliorate liver damage. We have recently developed an HDV mouse model based on the delivery of HDV and hepatitis B virus (HBV) replication-competent genomes using a hepatotropic adenoassociated virus (AAV). This model mimics several important characteristics of the human disease and led us to the identification of TNF-α as a critical factor involved in HDV-induced pathology.

The initial part of this thesis was focused on the identification of the main cell population in charge of TNF-α production. By RNA-Fluorescent In Situ Hybridization (FISH), we identified macrophages as the main cell population responsible for TNF-α production, representing 78% of TNF-α-positive cells, while HDV-infected hepatocytes and non-identified cells represent 6% and 16% of TNF-α-producing cells, respectively.

In addition, the second aim of this research project was to identify the main viral factors that are involved in HDV-induced liver damage. For that, we generated different HDV mutants by Site-Directed Mutagenesis (SDM) to avoid Hepatitis Delta Antigens (HDAgs) expression, their post-translational modifications, and to alter their subcellular localization in the infected cell.

First, HDV mutants were characterized in Huh7 cells to analyse the levels of viral replication, HDAgs expression, and the ability to produce HDV infective particles. Then, AAV8 vectors were constructed carrying the HDV wild-type (WT) or HDV mutated genomes to characterize their effect in terms of liver damage and liver inflammation. On the one hand, we found that the lack of L-HDAg exacerbates liver damage, which was associated with an overexpression of S- HDAg and with an increase in viral replication. On the other hand, the lack of isoprenylation of L-HDAg at Cys-211 resulted in lower levels of this isoform, leading to a favored expression of S- HDAg over L-HDAg that was also associated with more severe liver damage. Finally, the analysis of HDV mutants with alterations in HDAgs subcellular localization led us to associate the accumulation of HDAgs in the cell nucleus with a more acute liver injury.

In conclusion, our data showed that both host and viral factors are involved in HDV-induced pathology. On one side, TNF-α, produced by macrophages that become activated during HBV/HDV coinfection. On the other side, an increment in the S-HDAg/L-HDAg ratio and the accumulation of HDAgs in the infected cell is associated with a more severe liver damage.