Trends in Immunology
ReviewISG15: In Sickness and in Health
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.
References (79)
- et al.
Interferon-stimulated genes and their antiviral effector functions
Curr. Opin. Virol.
(2011) Durbin, Induction and function of type I and III interferon in response to viral infection
Curr. Opin. Virol.
(2011)Molecular characterization of the interferon-induced 15-kDa protein. Molecular cloning and nucleotide and amino acid sequence
J. Biol. Chem.
(1986)Interferon-induced proteins. Purification and characterization of a 15,000-dalton protein from human and bovine cells induced by interferon
J. Biol. Chem.
(1984)A 15-kDa interferon-induced protein is derived by COOH-terminal processing of a 17-kDa precursor
J. Biol. Chem.
(1988)Interferon induces a 15-kilodalton protein exhibiting marked homology to ubiquitin
J. Biol. Chem.
(1987)- et al.
The interferon-inducible 15-kDa ubiquitin homolog conjugates to intracellular proteins
J. Biol. Chem.
(1992) Precursor processing of pro-ISG15/UCRP, an interferon-beta-induced ubiquitin-like protein
J. Biol. Chem.
(1999)Crystal structure of the interferon-induced ubiquitin-like protein ISG15
J. Biol. Chem.
(2005)Human homologue of ariadne promotes the ubiquitylation of translation initiation factor 4E homologous protein, 4EHP
FEBS Lett.
(2003)