中科院连发两篇文章 解析乙肝病毒感染新机制

　　T细胞免疫，特别是病毒特异性T细胞免疫在控制乙肝病毒感染和清除病毒中发挥核心作用，在慢性乙肝感染中高浓度的病毒抗原长期作用是诱导T细胞免疫耐受和特异性T细胞耗竭的直接原因，T细胞免疫功能受损直接导致病毒持续性感染和病毒难以清除。但长期以来乙肝慢性感染中T细胞抗病毒功能缺陷与免疫耐受的调控机制并不清楚，这阻碍了进一步设计有效的针对性免疫治疗方案和药物。

中科院微生物研究所孟颂东课题组研究发现在慢性乙肝患者T细胞中microRNA-146a水平在病毒抗原和炎症因子的作用下明显上调，microRNA-146a的上调通过靶向Stat1显著抑制T细胞的抗病毒功能，这为揭示病毒感染导致T细胞功能受损和免疫耐受提供了阐释。

有趣的是，课题组还发现乙肝病毒核心蛋白在60位氨基酸由亮氨酸突变为缬氨酸时会产生一个新的T细胞表位，同时导致病毒复制水平明显增加。通过临床资料分析发现该位点突变导致乙肝患者病毒水平和肝脏免疫损伤均急剧升高，提示T细胞免疫应答、病毒复制和肝脏病理损伤三者存在复杂相互作用。

以上研究为揭示乙肝慢性感染中T细胞免疫耐受与T细胞介导的“旁观”效应引发肝损伤提供了新的机制，为进一步了解乙肝感染T细胞免疫应答机制和新型药物设计提供了依据。成果分别发表在Journal of Immunology (191:193-301, 2013)和Journal of Virology (doi:10.1128/JVI.00577-13, 2013)。　

原文摘要：

MicroRNA-146a Feedback Suppresses T Cell Immune Function by Targeting Stat1 in Patients with Chronic Hepatitis B

More than 350 million people are chronically infected with hepatitis B virus, and dysfunctional T cell responses contribute to persistent viral infection and immunopathogenesis in chronic hepatitis B (CHB). However, the underlying mechanisms of T cell hyporesponsiveness remain largely undefined. Given the important role of microRNA-146a (miR-146a) in diverse aspects of lymphocyte function, we investigated the potential role and mechanism of miR-146a in regulating T cell immune responses in CHB. We found that miR-146a expression in T cells is significantly upregulated in CHB compared with healthy controls, and miR-146a levels were correlated with serum alanine aminotransaminase levels. Both inflammatory cytokines and viral factors led to miR-146a upregulation in T cells. Stat1 was identified as a miR-146a target that is involved in antiviral cytokine production and the cytotoxicity of CD4+ and CD8+ T cells. In vitro blockage of miR-146a in T cells in CHB greatly enhanced virus-specific T cell activity. Therefore, our work demonstrates that miR-146a upregulation in CHB causes impaired T cell function, which may contribute to immune defects and immunopathogenesis during chronic viral infection.

Hepatitis B Virus mRNA-Mediated miR-122 Inhibition Upregulates PTTG1-Binding Protein, Which Promotes Hepatocellular Carcinoma Tumor Growth and Cell Invasion

As the most abundant liver-specific microRNA, miR-122 is involved in diverse aspects of hepatic function and neoplastic transformation. Our previous study showed that miR-122 levels are significantly decreased in hepatitis B virus (HBV)-infected patients, which may facilitate viral replication and persistence (S. Wang, L. Qiu, X. Yan, W. Jin, Y. Wang, L. Chen, E. Wu, X. Ye, G. F. Gao, F. Wang, Y. Chen, Z. Duan, and S. Meng, Hepatology 55:730–741, 2012). Loss of miR-122 expression in patients with hepatitis B enhances hepatitis B virus replication through cyclin G1-modulated P53 activity.). In this study, we provide evidence that all HBV mRNAs harboring an miR-122 complementary site act as sponges to bind and sequester endogenous miR-122, indicating that the highly redundant HBV transcripts are involved in HBV-mediated miR-122 suppression. We next identified pituitary tumor-transforming gene 1 (PTTG1) binding factor (PBF) as a target of miR-122 and demonstrated that HBV replication causes an obvious increase in PBF levels. Furthermore, we observed that the miR-122 levels were decreased and PBF was upregulated in chronic hepatitis B (CHB) and hepatocellular carcinoma (HCC). Overexpression and knockdown studies both revealed that PBF enhances proliferation and invasion of HCC cells, and silencing PBF resulted in a dramatic reduction of HCC tumor growth in vivo. Mechanistic analysis demonstrated that PBF interacts with PTTG1 and facilitates PTTG1 nuclear translocation, subsequently increasing its transcriptional activities. Therefore, we identified a novel HBV mRNA-miR-122-PBF regulatory pathway that facilitates malignant hepatocyte growth and invasion in CHB which may contribute to CHB-induced HCC development and progression. Our work underscores the reciprocal interplay of host miRNA sequestration and depletion by viral mRNAs, which may contribute to chronic-infection-related cancer.