How ISG15 combats viral infection

Abstract

Interferon (IFN)-stimulated gene product 15 (ISG15) is a ubiquitin-like protein critical for the control of microbial infections. ISG15 appears to serve a wide variety of functions, which regulate multiple cellular responses contributing to the development of an antiviral state. ISG15 is a versatile molecule directly modulating both host and virus protein function which regulate many signaling pathways, including its own synthesis. Here we review the various roles ISG15 plays in the antiviral immune response, and examine the mechanisms by which viruses attempt to mitigate or exploit ISG15 activity.

Keywords: Coronavirus; ISG15; Innate immune response; Nairovirus; Viral pathogenesis; deISGylase.

(Figure 1)

(1) Upon infection viral RNA or DNA is released into the cytoplasm where it is detected by RIG-I or other RLRs. After binding to the nucleic acid the RLR will undergo a conformational change, exposing the CARDs. (2) The exposed CARDs can be ubiquitinated or ISGylated. Ubiquitinated CARDs activate MAVS, which leads to downstream activation of IRF3. Activated IRF3 increases transcription at the ISREs for IFNαand IFNβ. ISGyaltion of RIG-I inhibits this pathway by marking RIG-I for proteasomal degradation. (3) Type 1 IFNs bind extracellularly to IFNAR, activating JAK1. (4) Activated JAK1 phosphorylates STAT1 and STAT2, which bind to IRF9 to form ISGF3. ISGF3 binds to the ISREs of many ISGs including ISG15, Ube1L, UbcH8, and HERC5. (5) proISG15 is processed by an unknown protease into functional free ISG15. (6)USP18 can inhibit activation of the JAK/STAT pathway by binding competitively with IFNAR2 preventing JAK1 activation. (7) ISG15 stabilizes the interaction between IFNAR2 and USP18 by inhibiting SKP2 mediated Ub conjugation. This prevents proteasomal degradation of USP18, effectively down regulating production of ISG15. ISGylation of RIG-I CARDs downregulates MAVS activation and IFN production.

(Figure 2)

Extracellular ISG15 directly affects pathogenesis or activates various immune cell types. (1) It inhibits the virus replication at the earliest stages, potentially by preventing entry into the cell. (2) It is an effective chemoattractant for neutrophils. It binds to the LFA-1 receptor on dendritic cells, NK cells, and T cells. This receptor is present in macrophages but has yet to be confirmed as the mechanism by which ISG15 activates macrophages. (3) In dendritic cells it initiates maturation and IFN-γ production. (4) In macrophages it causes polarization to the M1 phenotype, resulting in production of reactive oxygen species and nitric oxide. It also increased autophagy and mitophagy of infected cells and organelles. (5) In T cells and NK cells upon co-stimulation with IL-12 it stimulates production of IFN-γ. (6) In NK cells it also stimulates production of IL-10, which inhibits IFN-γ production in T cells. (7) Exosomal trafficking is one of the proposed methods by which ISG15 may be secreted from a cell but that mechanism is still unclear.

(Figure 3)

Viral countermeasures to ISG15 production and activity. (1) The Vif protein from HIV-1 degrades STAT1 and IE1 from HCMV sequesters STAT2, both of which are critical to upregulating ISG15 synthesis. (2) Vaccinia virus E3 protein sequesters free ISG15, preventing it from conjugating to targets or acting as a signaling protein. (3) pUL50 of HCMV binds to and causes proteasomal degradation of Ube1L, an essential E1 activator protein that facilitates ISGylation. (4) KSHV and HCMV both produce proteins that interfere with the HERC5 E3 ligase. Both reduce ISG15 conjugation to target proteins. (5) Viral DUBs and deISGylases such as OTUs and PLPs cleave the conjugation between ISG15 and target proteins, returning ISGylated proteins to their normal state. (6) NS1B from IBV sequesters ISGylated proteins, preventing them from being incorporated into oligomers.