ISG-15, beyond its functions in the cell: a mini review

Abstract

Interferon-stimulated gene 15 (ISG15) is an interferon-stimulated gene and a ubiquitin-like protein, traditionally known for its role in ISGylation. In addition to its intracellular functions, recent studies have revealed a novel role for extracellular ISG15, particularly in the context of viral infections. Beyond type I interferons, various stimuli, including viral and bacterial infections, have been found to trigger its secretion. Notably, the integrin receptor LFA-1 has been identified as a receptor for extracellular ISG15. Despite these advancements, the precise mechanisms by which extracellular ISG15 functions-such as the pathways regulating its secretion and receptor interactions-remain unclear. Viral proteins and de-ISGylating enzymes are known to influence ISG15 secretion levels, thereby impacting its immunomodulatory potential. This mini-review summarizes the existing studies aimed at understanding the mechanisms behind the secretion and functions of extracellular ISG15, with a particular focus on its immunomodulatory effects during infection. We also explore the contrasting roles of extracellular ISG15 in mice and humans, highlighting the need for more species-specific research. Further investigation into the role of extracellular ISG15 may uncover novel therapeutic strategies for infectious diseases, cancer, and inflammatory conditions.

Keywords: Extracellular; ISG-15; Innate immunity; Interferon; LFA-1.

Fig. 1

Summary diagram of the stimuli leading to ISG15 secretion, potential mechanisms of ISG15 secretion, and the known functions of extracellular ISG15

Fig. 2

(A) Human ISG15 (PDB 1Z2M) with residues important for function highlighted. The molecule comprises an N-terminal domain (Cyan), a hinge region (pink) and a C-terminal domain (green). Two cysteine residues Cys78 and Ser146 (red) were implicated in multimerization. Note that in this published crystal structure Cys78 was intentionally mutated to a serine. Leu72, Ser83 and Leu85 (orange) are implicated in secretion and a bundle of polar residues Tyr96, Arg99, Thr101, Gln102 and Thr103 (blue) are implicated in signaling, putatively interacting with LFA-1. (B) Two ubiquitin molecules (extracted from PDB 2K6D, chain B, coloured yellow) are aligned to the two domains of ISG15, showing good structural homology. (C) Structural alignment of murine ISG15 (ribbon in light purple) onto human ISG15. While the N-termini are aligned in good agreement, the C-termini adopt different relative orientations to the N-termini due to flexibility of the hinge region

Fig. 3

Structure of part of the LFA-1 ectodomain (PDB 5E6R). Enlarged from the inlet shows the aligned structure of the ICAM-3/LFA-1 αI complex (PDB 1T0P). The two molecules are linked up by a magnesium (II) ion. However ISG15 is expected to bind to a different, previously uncharacterized site on this domain and the binding is metal-independent