Hepatitis E virus (HEV), a single-stranded positive-sense RNA virus, generally causes self-limiting acute viral hepatitis, although chronic HEV infection has recently become a significant clinical problem in immunocompromised individuals, especially in solid-organ transplant recipients. Innate immunity, via the type I interferon (IFN) response, plays an important role during the initial stages of a viral infection. IFN-stimulated gene 15 (ISG15), an IFN-induced ubiquitin-like protein, is known to have an immunomodulatory role and can have a direct antiviral effect on a wide spectrum of virus families. In the present study, we investigated the antiviral effect as well as the potential immunomodulatory role of ISG15 during HEV replication. The results revealed that HEV induced high levels of ISG15 production both in vitro (Huh7-S10-3 liver cells) and in vivo (liver tissues from HEV-infected pigs); however, ISG15 is not required for virus replication. We also demonstrated that ISG15 silencing potentiates enhanced type I IFN-mediated signaling, resulting in an increase in the type I IFN-mediated antiviral effect during HEV replication. This observed enhanced type I IFN signaling correlated with an increase in IFN-stimulated gene expression levels during HEV replication. Furthermore, we showed that PKR and OAS1 played important roles in the ISG15-mediated type I IFN sensitivity of HEV. Taken together, the results from this study suggest that ISG15 plays an important immunomodulatory role and regulates HEV sensitivity to exogenous type I IFN.

IMPORTANCE Hepatitis E virus (HEV) infection typically causes self-limiting acute viral hepatitis. However, chronic HEV infection has recently become a significant clinical problem in immunocompromised patients. Pegylated interferon (IFN) has been used to treat chronic HEV infection in solid-organ transplant patients with some success. However, the mechanism behind the type I IFN-mediated antiviral effect against HEV remains unclear. This report demonstrates that ISG15 induced by HEV replication in Huh7-S10-3 human liver cells plays an immunomodulatory role by negatively regulating type I IFN signaling and, thus, HEV sensitivity to type I IFN. Our results also show that PKR and OAS1 play important roles in the ISG15-mediated type I IFN sensitivity of HEV.

戊型肝炎病毒（HEV）是一种单链正义RNA病毒，通常会引起自限性急性病毒性肝炎，尽管慢性HEV感染最近已成为免疫功能低下的个体（尤其是在实体器官移植受者中）的重要临床问题。通过I型干扰素（IFN）应答的先天免疫在病毒感染的初始阶段起着重要的作用。IFN刺激的基因15（ISG15）是一种IFN诱导的泛素样蛋白，已知具有免疫调节作用，并且可以对多种病毒家族具有直接的抗病毒作用。在本研究中，我们调查了ISV15在HEV复制过程中的抗病毒作用以及潜在的免疫调节作用。结果表明，戊型肝炎病毒在体外均可诱导高水平的ISG15产生（Huh7-S10-3肝细胞）和体内（来自受HEV感染的猪的肝脏组织）; 但是，病毒复制不需要ISG15。我们还证明，ISG15沉默增强了增强的I型IFN介导的信号传导，导致HEV复制过程中I型IFN介导的抗病毒作用增加。该观察到的增强的I型IFN信号传导与HEV复制期间IFN刺激的基因表达水平的增加相关。此外，我们表明PKR和OAS1在ISG15介导的HEV I型IFN敏感性中起着重要作用。两者合计，这项研究的结果表明ISG15发挥重要的免疫调节作用，并调节HEV对外源I型IFN的敏感性。

重要事项戊型肝炎病毒（HEV）感染通常会引起自限性急性病毒性肝炎。但是，慢性HEV感染最近已成为免疫受损患者的重要临床问题。聚乙二醇化干扰素（IFN）已用于治疗实体器官移植患者的慢性HEV感染，并取得了一些成功。然而，I型IFN介导的抗HEV抗病毒作用的机制尚不清楚。该报告证明，由HEV在Huh7-S10-3人肝细胞中复制引起的ISG15通过负调控I型IFN信号传导，从而增强HEV对I型IFN的敏感性，发挥免疫调节作用。我们的结果还表明，PKR和OAS1在ISG15介导的HEV I型IFN敏感性中起重要作用。