Trends in Immunology
Volume 38, Issue 2, February 2017, Pages 79-93
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Review
ISG15: In Sickness and in Health

https://doi.org/10.1016/j.it.2016.11.001Get rights and content

Trends

ISG15 is a ubiquitin-like molecule induced by type I IFN.

ISG15 can both be conjugated to other proteins and exist as a free protein. Soluble ISG15 can induce IFN-γ.

Murine ISG15 appears to be largely antiviral, as often expected for ISGs, and knockout mice are more susceptible to several, although not all, viruses.

However, humans deficient for ISG15 do not have an increased susceptibility to viral infections.

ISG15 stabilizes ubiquitin-specific peptidase 18 (USP18) in humans and, thus, acts as a negative regulator of IFN signaling. Therefore, in humans in vivo, ISG15 appears to act primarily to dampen IFN-induced inflammation, explaining the lack of susceptibility observed in patients.

ISG15 is a type I interferon (IFN)-inducible gene encoding a protein with pleiotropic functions, acting both as a soluble molecule and as a protein modifier. Surprisingly, and despite the antiviral functions of ISG15 described in mice, humans born with inactivating mutations of ISG15 do not present with any overt viral phenotype, but are highly susceptible to environmental mycobacteria and have autoinflammatory disease presentations. In vitro, ISG15 deficiency also leads to persistently high levels of type I IFN-stimulated gene expression and to increased resistance to all viruses tested to date. This suggests that ISG15 deficiency increases antiviral responses in humans, in stark contrast to expectations based on mouse experiments. We discuss here the roles of each of the forms of ISG15 in health and disease, as well as the differences between species.

Section snippets

History of Discoveries Relating to ISG15

Type I IFNs are cytokines with well-defined antiviral activities. They signal through the IFN-α/β receptor (IFNAR), and this signaling ultimately results in the formation of a phosphorylated IFN-stimulated gene factor 3 (ISGF3) complex comprising phosphorylated signal transducer and activator of transcription 1 (pSTAT1)/pSTAT2/IFN-regulatory transcription factor 9 (IRF9) in the nucleus. This complex initiates the transcription of approximately 500 IFN-stimulated genes (ISGs) 1, 2, 3. ISG15 was

ISG15 Conjugation

ISG15 conjugation (ISGylation) is widely documented and there has been considerable speculation about the function of ISG15 in this form. Similar to ubiquitin, free ISG15 can be conjugated through a series of enzymatic reactions involving E1/E2/E3 ligases to a lysine residue on a target protein 6, 9, 12, 13. In the case of ISG15, conjugation is rendered possible by the C-terminal LRLRGG motif, which is exposed after the removal of eight amino acids at the C terminus of the protein [14].

Free ISG15

ISG15 was initially thought to act only intracellularly. However, in 1991, ISG15 was detected in medium from human lymphocytes and monocytes that had been treated with IFN-β. More than 50% of total ISG15 was found to be present in the medium 24 h after interferon stimulation, consistent with an extracellular role for ISG15 [10]. Similarly, free ISG15 was detected in serum samples from humans treated with recombinant type I IFN [27]. Recombinant ISG15 was also shown to induce IFN-γ (type II IFN)

Human ISG15 Deficiency

Six individuals with ISG15 deficiency have recently been identified 29, 30. Given its IFN-inducible nature, it was presumed that IFN acted as an antiviral effector. However, these ISG15-deficient humans presented with susceptibility to environmental mycobacteria, basal ganglia calcifications, and, contrary to expectations, no particular susceptibility to viral infections. In fact, ISG15-deficient patients have an enhanced type I IFN signature in the blood, increasing their resistance to viral

Murine in vivo Studies

Almost all of what was known about ISG15 function in vivo before the identification of ISG15-deficient patients resulted from experimental studies in mice. The mouse model is one of the most widely used animal models for inferring in vivo biological phenotype. For ISG15, in vivo functional data were generated over more than a decade in mice. Some of the findings suggested that ISG15 had no role in antiviral defenses, whereas other suggested a robust antiviral role (Table 2). Combined, at the

Murine in vitro Studies

Variable results have also been reported concerning the effect of ISG15 on viruses in mouse cells in vitro. Data for ISG15 in vitro in murine systems strongly suggest that this protein has a virus- or condition-specific immunological role. IAV, HSV-1, γHV68, and SB have been shown to grow equally well in ISG15-deficient mouse embryonic fibroblasts (MEFs) and WT MEFs [45] (Table 4). However, many examples of antiviral effects of ISG15 in vitro have also been reported. Zhang et al. described how

Human in vitro Experiments

Building on the experiments carried out on mice in vivo and in vitro, many studies were carried out on human cell lines in vitro to determine the role of ISG15, before the identification of ISG15-deficient humans. These studies typically involved the transient overexpression or siRNA knockdown of ISG15, to determine the role of this protein in antiviral responses.

Shortly after the identification of ISG15 as a probable antiviral protein, Okumura et al. explored the role of this protein in HIV-1

Concluding Remarks

The immune functions of ISG15 are clearly more complex than initially thought. Humans with ISG15 deficiency are susceptible to environmental mycobacterial infections due to a lack of free secreted ISG15, which is required for the effective induction of type II IFNs. Mice lacking ISG15 are also more susceptible to mycobacteria than are their WT littermates [29]. However, in vivo phenotypic resemblances between the two species appear to stop there. Humans lacking ISG15 also display a profound

Acknowledgments

We would like to thank Justin Taft, Jennie Altman, Sofija Buta, Marta Martin-Fernandez, Phillip Cohen, Sandra Pellegrini, and Conor Gruber for their critical reading and advice.

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