Interferons (IFNs) act through cell-surface receptors to activate intracellular signaling cascades that culminate in transcription of interferon-stimulated genes (ISGs). Type I IFNs (including IFN-α and IFN-β) engage the IFNAR1/IFNAR2 heterodimer to activate JAK1/TYK2 and phosphorylate STAT1/STAT2, which, together with IRF9, form ISGF3 and bind ISREs to drive ISG expression. Type II IFN (IFN-γ) signals via IFNGR and generates STAT1 homodimers (GAF) that bind GAS elements. Type III IFNs (IFN-λ1–4) use IFNLR1/IL-10R2 but converge on the same JAK/STAT machinery. The study team reported that the number of inducible ISGs varies substantially by cell type and species, often estimated in the hundreds, and that PRR activation (for example, RLRs and TLRs) also contributes to ISG induction. This signaling architecture establishes a broad and cell-contextual antiviral state.
ISGs fall into multiple functional classes. Some act as direct antiviral effectors at the cellular level; others modulate signaling, antigen presentation, ubiquitination machinery, apoptosis or metabolic pathways. Genetic loss-of-function work has delineated major effector pathways, including the MX GTPases, the OAS–RNase L axis, PKR, the ISG15 ISGylation system, TRIM family members, and interferon-induced transmembrane proteins (IFITMs). These pathways target discrete steps of the viral lifecycle: blocking transcription, degrading viral RNA, arresting translation, sequestering viral proteins, or preventing virion release. The authors emphasized that expression breadth does not always predict potency; some modestly induced ISGs nevertheless provide strong restriction against particular viruses.
ISG15 was characterized as a ubiquitin-like modifier with a conserved C-terminal LRLRGG motif. The review notes that ISGylation is a reversible conjugation requiring E1, E2 and E3 enzymes, and that, unlike K48-linked ubiquitination, ISGylation does not universally mark proteins for degradation. Evidence cited includes mouse models: ISG15-deficient mice displayed increased susceptibility to influenza A and B, Sindbis virus and HSV-1, supporting an antiviral role in vivo. Viral antagonism of ISGylation was illustrated by viral proteases with de-ISGylation activity; mutation of an OTU domain protease in a porcine virus reduced in vitro viral growth.
The authors described multiple proposed mechanisms for ISG15 activity: modulation of multivesicular body trafficking and lysosomal degradation, interference with ubiquitination cascades, stabilization of signaling proteins (for example, USP18 in humans), and extracellular cytokine-like functions. Extracellular ISG15 has been reported to stimulate NK proliferation, induce IFN-γ from T cells, act as a neutrophil chemoattractant, and activate PBMCs through LFA-1 (CD11a) to increase IFN-γ and IL-10 secretion. The review also pointed out species-specific distinctions: the intracellular USP18–ISG15 interaction stabilizes USP18 in humans but not in mice. A limited set of ISG15-specific E3 ligases have been identified (for example, HERC5 in humans), and the authors state that the full complement of ISG15 ligases and the rules that govern ISGylation specificity remain incompletely defined.
The myxovirus resistance proteins (MX family) are dynamin-like GTPases induced by IFN. Humans have MX1 (encoding MXA) and MX2 (encoding MXB). MX proteins contain a GTPase domain, a central protein-interaction domain, and a C-terminal effector region. Functional studies reviewed indicate that MX proteins employ different antiviral strategies: trapping viral ribonucleoprotein complexes, interfering with viral polymerases, or blocking essential trafficking steps. MX1 variants localize to different compartments across species; for example, mouse MX1 is nuclear and strongly inhibitory for influenza-like viruses that replicate in the nucleus, while human MXA is cytoplasmic and can inhibit a broader set of viruses. The authors reported species and allele variability: avian MX proteins often lack antiviral activity, and polymorphisms (such as at specific residues in chicken MX) may determine activity. MX2 (MXB) is nuclear and, unlike MX1, has been shown to restrict retroviral processes such as integration for HIV-1 in human cells.
The review organized ten ISG families with mechanistic annotations: ISG15 (ISGylation), MX proteins (GTPase-mediated trapping), OAS/RNase L (2′–5′ oligoadenylate synthesis activating RNase L to degrade single-stranded RNA), IFIT family (cap-recognition and translation suppression), PKR (dsRNA-triggered translation arrest), viperin (production of ddhCTP), PML nuclear bodies (sequestration of viral proteins), APOBEC3s (cytidine deamination and hypermutation of viral DNA), IFITM proteins (inhibition of membrane fusion), and tetherin (retention of budding virions). For several effectors, the authors summarized documented viral antagonists: viral de-ISGylases that remove ISG15, viral proteins that avoid PKR activation or block OAS–RNase L signaling, and viral evasion of tetherin-mediated retention. The collective data illustrate a recurring virus–host arms race: strong evolutionary pressure on both effector proteins and viral antagonists.
Many mechanistic gaps remain. The review explicitly states that the precise fates of ISGylated substrates are unclear and likely substrate-specific rather than governed by a single rule. Only a limited number of ISG15 E3 ligases have been characterized, and the authors called attention to the need for additional identification and functional validation. Species differences—human versus mouse versus avian—limit direct extrapolation from model organisms to human biology; several ISG activities and regulatory interactions differ across species. The extracellular cytokine-like functions of ISG15 require further mechanistic clarification in vivo, and the review notes that some ISG effects observed in vitro, including paradoxical outcomes in ISG15-deficient cells, warrant deeper investigation. These limitations frame specific unanswered questions: which ISGylation events are essential in particular infections, how universal are proposed mechanisms across hosts, and what is the full complement of viral antagonists matched to individual ISGs?