Type I interferonopathies are a group of hereditary diseases characterized by excessive activation of the type I interferon (IFN-I) system. These cytokines provide antiviral protection and regulate inflammation. Interferonopathies are monogenic diseases because they arise from mutations in individual genes.
One mechanism underlying interferonopathies is increased IFN-I production, which occurs in mutations affecting genes involved in nucleic acid metabolism – such as TREX1, SAMHD1, and ADAR1 – as well as genes encoding sensor proteins and signalling pathway components, including RIG-I, IFIH1, STING, and COPA. Such mutations lead to the accumulation of molecules that activate or enhance immune signalling pathways. As a result, cells produce excessive amounts of type I interferons.
Another mechanism involves not increased interferon production but an enhanced cellular response to interferon. After IFN-I binds to the IFNAR receptor, signaling proteins JAK1, TYK2, STAT1, STAT2, and IRF9 become activated. This results in increased activity of interferon-stimulated genes (ISGs). Mutations that enhance the function of these signaling proteins – for example, in JAK1 or STAT1 – lead to excessive signaling.
A similar effect occurs with loss-of-function mutations in proteins that normally restrain this pathway, such as USP18 and ISG15. Mutations have also been identified in a specific region of STAT2 that disrupts its ability to maintain USP18 function.
Despite shared features of type I interferonopathies – persistently elevated IFN-I levels in blood, increased ISG expression, and basal ganglia calcifications – clinical manifestations vary depending on the specific genetic mutation.
Many interferonopathies associated with excessive IFN-I production – such as Aicardi–Goutières syndrome – are characterized by chilblain-like lesions. These appear as purplish papules and nodules on fingers and toes, typically developing in areas exposed to lower temperatures. Lung involvement is generally not described in this syndrome.
By contrast, diseases characterized by persistently activated IFN-I signaling or increased ISG activity – such as gain-of-function mutations in STING or ISG15 deficiency – may be associated with inflammatory lung diseases.
Cutaneous manifestations of ISG15 deficiency also differ from those of chilblains: lesions are more commonly found on the back, trunk, and upper thighs. These differences reflect distinct regulatory mechanisms of the interferon signaling pathway. During chronic IFN-I signaling, part of the signal may be limited by the accumulation of negative regulators. However, in ISG15-deficient cells, ISGs persistently activate, leading to diverse pathological manifestations.
The ISG15 protein is a negative regulator of IFNAR signaling. It binds to the protein USP18 and protects it from proteasomal degradation. USP18, in turn, displaces JAK1 from the IFNAR2 receptor complex, thereby terminating interferon signaling.
ISG15 also performs additional functions. Free secreted ISG15 can enhance IFN-γ production in CD8 T cells and NK cells via the LFA-1 receptor. In addition, ISG15 can be conjugated to proteins via ISGylation. This process involves the enzymes UBE1L, UBCH8, and HERC5. Although the molecular mechanism of ISGylation has been well described, its functional consequences remain incompletely understood.
Individuals with ISG15 deficiency exhibit characteristic clinical features, including basal ganglia calcifications, spontaneous skin lesions (without vasculitis but with perivascular inflammation), and interstitial lung disease.
In vitro experiments indicate that ISG15 is important for skin structure: its absence disrupts collagen synthesis and the TGFβ response. In addition, ISG15 deficiency may manifest as susceptibility to mycobacterial infections, likely due to the loss of its extracellular activity.
Although the genetic causes of ISG15 deficiency and the molecular mechanisms of enhanced IFN-I signaling have been described, the processes leading to inflammatory tissue damage remain poorly understood. American researchers investigated the pathophysiology of ISG15 deficiency in the context of clinically significant inflammatory tissue damage and described a new pathogenic ISG15 mutation.
A Novel ISG15 Mutation And Severe Pulmonary Fibrosis
Researchers described the case of a 16-year-old patient from Qatar, born to consanguineous parents, who developed severe restrictive lung disease. Functional tests revealed markedly reduced lung capacity – down to 27% – along with hypoxemia and secondary pulmonary hypertension. Infectious causes were excluded.
The patient was found to carry a homozygous ISG15 mutation (c.463insC). The same mutation was identified in her older brother, who developed severe pulmonary fibrosis. The disease resulted in right ventricular dysfunction and pulmonary hypertension, and at age 28, he underwent bilateral lung transplantation. Examination of the explanted lungs confirmed end-stage disease with extensive interstitial fibrosis and inflammatory features.
The patient showed increased expression of ISGs (IFI27, MX1, SIGLEC1, IFIT1) in blood. ISG15 mRNA was present, but functional assays demonstrated the absence of ISG15 protein and ISGylation.
To prevent progression of fibrosis, the patient was treated with oral steroids and the JAK1/JAK2 inhibitor ruxolitinib.
Comparison Of Skin Samples From An ISG15-Deficient Patient And Healthy Individuals
Because lung biopsies from the Qatari patient and her brother were unavailable, researchers analyzed skin biopsies from another patient with ISG15 deficiency – a woman born in the United States in 2015 – and compared them with samples from healthy individuals.
Due to the near-complete loss of ISG15 production, the patient developed ulcerative skin lesions. One lesion contained abundant Aspergillus hyphae penetrating the subcutaneous tissue. Prior use of topical corticosteroids worsened the ulcers. The affected tissue was surgically removed, and the lesion healed with severe scarring two years after its discovery.
Comparison Of Skin Biopsies. The general structure of skin layers remained preserved. However, affected areas showed signs of abnormal cell death, including nonspecific tissue damage and sclerosis. Additional analysis revealed increased levels of apoptosis markers – CASP3, CASP8, BAX, BAK1, and CYCS – particularly in the dermis.
Researchers also observed markedly increased expression of interferon-stimulated genes – IFIT1, USP18, MX1, IFI27, and IFI44L – in 60–90% of skin biopsies from patients with ISG15 deficiency, but not in the control group.
Elevated apoptosis markers were most pronounced in inflamed tissue regions, suggesting that enhanced IFN-I signalling may contribute to cell death in ISG15-deficient cells. In vitro experiments confirmed that ISG15-deficient cells exhibit increased sensitivity to the cytotoxic effects of IFN-I. These cells showed increased apoptosis and impaired migration during wound repair. No significant contribution of necroptosis or autophagy was observed.
Inflammation And Wound Healing
Inflammation following acute injury is a normal component of wound healing. Tissue damage activates resident macrophages, triggering the release of chemokines that recruit neutrophils and monocytes to the injury site. Neutrophils arrive first and facilitate the recruitment of monocytes, followed later by infiltration of T cells.
During the acute phase, macrophages adopt a proinflammatory M1 phenotype, enabling the removal of pathogens and damaged tissue. After clearance is complete, they transition to an anti-inflammatory M2 phenotype that promotes resolution of inflammation and tissue repair.
In chronic wounds, this transition is impaired: M1 macrophages remain active, sustain inflammation, and prevent healing, ultimately leading to scar formation.
Role Of Immune Cells In ISG15 Deficiency
Analysis of immune cells revealed increased presence of proinflammatory M1 macrophages and enhanced production of inflammatory cytokines, indicating chronic macrophage activation at sites of tissue injury, prolonging wound healing and promoting fibrosis.
Notably, increased TGFβ production was observed in macrophages and in the epidermis of patients with ISG15 deficiency.
In vitro experiments showed that under the influence of IFN-α and TGFβ, lung epithelial cells lacking ISG15 rapidly converted into myofibroblasts. Prolonged exposure of fibroblasts to IFN-α and TGFβ also induced myofibroblast activation in ISG15-deficient cells, whereas such changes did not occur in wild-type cells.
Model Of Inflammatory Tissue Damage In ISG15 Deficiency
Based on these findings, the researchers proposed the following model. ISG15 deficiency amplifies the effects of IFN-I, disrupting cell migration and proliferation and impairing normal wound healing, leading to chronic tissue injury and abnormal macrophage activation.
Activated macrophages begin secreting TGFβ, which triggers epithelial–mesenchymal transition (EMT) in epidermal cells. The resulting myofibroblasts produce additional TGFβ, spreading EMT to neighboring tissue areas and further promoting fibrosis.
Conclusion
ISG15 deficiency increases susceptibility to apoptosis and fibrosis, resulting in skin and lung damage and functional impairment. Severe pulmonary fibrosis poses a serious threat to life and, in some cases, requires transplantation.
In addition, chronic wounds increase the risk of infection, while skin fibrosis results in the loss of hair follicles, sweat glands, and other structures necessary for maintaining normal tissue homeostasis.
