Origin, antigenicity, and function of a secreted form of ORF2 in hepatitis E virus infection
Origin, antigenicity, and function of a secreted form of ORF2 in hepatitis E virus infection
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ORF2分泌形式在戊型肝炎病毒感染中的起源、抗原性和功能
DOI：
10.1073/pnas.1721345115
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发表时间：
2018-05-01
影响因子：
11.1
通讯作者：
Feng, Zongdi
中科院分区：
文献类型：
--
作者：
Yin, Xin;Ying, Dong;Feng, Zongdi
戊型肝炎病毒（HEV）是世界范围内急性肝炎的主要病因。最近的证据表明，hev感染的细胞释放一种分泌形式的ORF2蛋白（ORF2S），但其来源和功能尚不清楚。在这里，我们证明ORF2S和ORF2C（实际的衣壳蛋白）是不同的翻译产物，ORF2S对HEV生命周期不是必需的，而是抑制抗体介导的HEV中和。我们的研究结果对理解HEV复制周期和免疫逃避机制具有重要意义。本研究中鉴定的内部启动密码子在大多数HEV菌株中是高度保守的，这表明ORF2S的产生是HEV的进化保守功能。肠内传播的戊型肝炎病毒（HEV）采用一种独特的策略，通过掩盖其衣壳（由病毒ORF2基因编码），并以“准包膜”颗粒的形式在血液中循环，从而退出细胞。然而，最近的证据表明，大多数ORF2蛋白存在于患者血清和hev感染细胞培养的上清液中，以自由形式存在，与病毒颗粒无关。这种分泌形式的ORF2 （ORF2S）的起源和生物学功能尚不清楚。在这里，我们发现ORF2S的产生源于先前假设的衣壳蛋白的AUG起始密码子的翻译，而实际衣壳蛋白（ORF2C）的翻译是在先前未被识别的内部AUG密码子（第一个AUG下游的15个密码子）上启动的。在衣壳蛋白的N端添加15个氨基酸创建了一个信号序列，通过分泌途径驱动ORF2S分泌。与ORF2C不同，ORF2S是糖基化的，以二聚体的形式存在。尽管如此，ORF2S与衣壳表现出大量的抗原重叠，但预测结合假定的细胞受体的表位丢失了。与此一致的是，ORF2S不会阻止HEV细胞进入，而是抑制抗体介导的中和。这些结果揭示了HEV生物学中一个以前未被认识到的方面，并为该病毒的免疫逃避机制和发病机制提供了新的思路。
Significance Hepatitis E virus (HEV) is a main cause of acute hepatitis worldwide. Recent evidence suggests that HEV-infected cells release a secreted form of ORF2 protein (ORF2S) but its origin and function are unknown. Here we demonstrate that ORF2S and ORF2C (the actual capsid protein) are different translation products and that ORF2S is not essential for the HEV life cycle but inhibits antibody-mediated neutralization of HEV. Our results have important implications for understanding the HEV replication cycle and immune evasion mechanisms. The identified internal start codon in this study is highly conserved in most HEV strains, suggesting that the production of ORF2S is an evolutionary conserved function for HEV. The enterically transmitted hepatitis E virus (HEV) adopts a unique strategy to exit cells by cloaking its capsid (encoded by the viral ORF2 gene) and circulating in the blood as “quasi-enveloped” particles. However, recent evidence suggests that the majority of the ORF2 protein present in the patient serum and supernatants of HEV-infected cell culture exists in a free form and is not associated with virus particles. The origin and biological functions of this secreted form of ORF2 (ORF2S) are unknown. Here we show that production of ORF2S results from translation initiated at the previously presumed AUG start codon for the capsid protein, whereas translation of the actual capsid protein (ORF2C) is initiated at a previously unrecognized internal AUG codon (15 codons downstream of the first AUG). The addition of 15 amino acids to the N terminus of the capsid protein creates a signal sequence that drives ORF2S secretion via the secretory pathway. Unlike ORF2C, ORF2S is glycosylated and exists as a dimer. Nonetheless, ORF2S exhibits substantial antigenic overlap with the capsid, but the epitopes predicted to bind the putative cell receptor are lost. Consistent with this, ORF2S does not block HEV cell entry but inhibits antibody-mediated neutralization. These results reveal a previously unrecognized aspect in HEV biology and shed new light on the immune evasion mechanisms and pathogenesis of this virus.