Immune Responses During Resolution of Acute Hepatitis B in Woodchucks and Their Modulation for Treatment of Chronic Hepatitis B
Understanding the innate and adaptive immune responses during resolution of hepatitis B virus (HBV) infection is important for exploring new treatment options for chronic hepatitis B (CHB). Studying acute hepatitis B (AHB) in humans is challenging since the early weeks after infection are mostly asymptomatic. Infection of woodchucks with woodchuck hepatitis virus (WHV) resembles HBV infection in humans. WHV-infected adult woodchucks showed an increase and subsequent decline in WHV replication, suggesting AHB resolution. The intrahepatic increase in IFN-γ expression at week 5 post-inoculation correlated with an upregulation of NK-cell markers, indicating the importance of a non-cytolytic mechanism in the initial viral control. Later during the marked decline in replication, the upregulated expression of adaptive immune cell markers indicated an additional cytolytic mechanism of viral clearance, along with a dampening of liver injury. The safety and efficacy of small agonist molecules targeting pattern recognition receptors (PRRs) have been evaluated in animal models and are currently tested in patients with CHB. However, other PRRs capable of sensing HBV are less investigated. The presence and activation of viral nucleic acid sensing PRRs and their downstream signaling pathways was determined in liver and blood of woodchucks during AHB resolution. In the liver, receptor expression did not change early post-inoculation, suggesting a stealth-like behavior of WHV. During resolution, however, most receptors were strongly upregulated in liver but not in blood. Previously unexplored viral DNA sensing receptors showed comparable or even higher expression than well-described receptors. Furthermore, the intrahepatic expression of these receptors was downregulated during CHB, when compared to their peak expression during AHB resolution. Treatment of primary hepatocytes from woodchucks with CHB with HSV-60 or poly(dA:dT) upregulated the IFI16 or ZBP1/DAI and AIM2 receptor signaling pathways, respectively, and produced effector cytokines which significantly reduced WHV replication and secretion. Combination treatment with both agonists enhanced the antiviral effect, but not indefinitely once saturating levels of type I IFNs were achieved. As observed in AHB, future immunotherapies for CHB should consider activating more than one PRR, preferentially located within infected hepatocytes, to induce antiviral efficacious levels of cytokines acting directly at the site viral replication.
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