Key Points Cell culture model of ISG15 deficiency replicates findings in ISG15−/− patient cells. Cause of resistance in ISG15−/− cells differs depending on duration of IFN treatment. ISG15−/− patients without serious viral disease do not prove ISGylation is unimportant. IFNs, produced during viral infections, induce the expression of hundreds of IFN-stimulated genes (ISGs). Some ISGs have specific antiviral activity, whereas others regulate the cellular response. Besides functioning as an antiviral effector, ISG15 is a negative regulator of IFN signaling, and inherited ISG15 deficiency leads to autoinflammatory IFNopathies, in which individuals exhibit elevated ISG expression in the absence of pathogenic infection. We have recapitulated these effects in cultured human A549-ISG15−/− cells and (using A549-UBA7−/− cells) confirmed that posttranslational modification by ISG15 (ISGylation) is not required for regulation of the type I IFN response. ISG15-deficient cells pretreated with IFN-α were resistant to paramyxovirus infection. We also showed that IFN-α treatment of ISG15-deficient cells led to significant inhibition of global protein synthesis, leading us to ask whether resistance was due to the direct antiviral activity of ISGs or whether cells were nonpermissive because of translation defects. We took advantage of the knowledge that IFN-induced protein with tetratricopeptide repeats 1 (IFIT1) is the principal antiviral ISG for parainfluenza virus 5. Knockdown of IFIT1 restored parainfluenza virus 5 infection in IFN-α–pretreated, ISG15-deficient cells, confirming that resistance was due to the direct antiviral activity of the IFN response. However, resistance could be induced if cells were pretreated with IFN-α for longer times, presumably because of inhibition of protein synthesis. These data show that the cause of virus resistance is 2-fold; ISG15 deficiency leads to the early overexpression of specific antiviral ISGs, but the later response is dominated by an unanticipated, ISG15-dependent loss of translational control.

中文翻译：

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干扰素刺激基因的直接抗病毒活性是导致 ISG15 缺陷细胞对副粘病毒耐药的原因

关键点 ISG15 缺陷的细胞培养模型复制了 ISG15-/- 患者细胞中的发现。ISG15-/- 细胞耐药的原因因 IFN 治疗的持续时间而异。没有严重病毒性疾病的 ISG15-/- 患者不能证明 ISG 化不重要。在病毒感染期间产生的 IFN 诱导数百个 IFN 刺激基因 (ISG) 的表达。一些 ISG 具有特定的抗病毒活性，而另一些则调节细胞反应。除了作为抗病毒效应器发挥作用外，ISG15 还是 IFN 信号传导的负调节剂，遗传性 ISG15 缺乏会导致自身炎症性 IFN 病变，其中个体在没有病原体感染的情况下表现出 ISG 表达升高。我们在培养的人 A549-ISG15-/- 细胞中概括了这些作用，并且（使用 A549-UBA7-/- 细胞）证实 ISG15 的翻译后修饰（ISGylation）不是调节 I 型 IFN 反应所必需的。用 IFN-α 预处理的 ISG15 缺陷细胞对副粘病毒感染具有抗性。我们还表明，对 ISG15 缺陷细胞进行 IFN-α 处理会显着抑制整体蛋白质合成，这导致我们询问耐药性是由于 ISG 的直接抗病毒活性还是由于翻译缺陷导致细胞不被允许。我们利用了 IFN 诱导的具有四肽重复序列 1 的蛋白质（IFIT1）是副流感病毒 5 的主要抗病毒 ISG 的知识。IFIT1 的敲低恢复了 IFN-α 预处理的 ISG15 缺陷细胞中的副流感病毒 5 感染，证实耐药性是由于干扰素反应的直接抗病毒活性。然而，如果细胞用 IFN-α 预处理更长的时间，可能会诱导抗性，这可能是因为抑制了蛋白质合成。这些数据表明，病毒抗药性的原因是2倍；ISG15 缺乏导致特定抗病毒 ISG 的早期过度表达，但后来的反应主要是意料之外的、依赖于 ISG15 的翻译控制丧失。