Intracellular ISG15 is an interferon (IFN)-α/β-inducible ubiquitin-like modifier which can covalently bind other proteins in a process called ISGylation; it is an effector of IFN-α/β-dependent antiviral immunity in mice1–4. We previously published a study describing humans with inherited ISG15 deficiency but without unusually severe viral diseases5. We showed that these patients were prone to mycobacterial disease and that human ISG15 was non-redundant as an extracellular IFN-γ-inducing molecule. We show here that ISG15-deficient patients also display unanticipated cellular, immunological and clinical signs of enhanced IFN-α/β immunity, reminiscent of the Mendelian autoinflammatory interferonopathies Aicardi–Goutières syndrome and spondyloenchondrodysplasia6–9.We further show that an absence of intracellular ISG15 in the patients’ cells prevents the accumulation of USP1810,11, a potent negative regulator of IFN-α/β signalling, resulting in the enhancement and amplification of IFN-α/β responses. Human ISG15, therefore, is not only redundant for antiviral immunity, but is a key negative regulator of IFN-α/β immunity. In humans, intracellular ISG15 is IFN-α/β-inducible not to serve as a substrate for ISGylation-dependent antiviral immunity, but to ensure USP18-dependent regulation of IFN-α/β and prevention of IFN-α/β-dependent autoinflammation.

ISG15 is an interferon (IFN)-α/β-inducible, ubiquitin-like intracellular protein. Its conjugation to various proteins (ISGylation) contributes to antiviral immunity in mice. We describe human patients with inherited ISG15 deficiency and mycobacterial, but not viral diseases. The lack of intracellular ISG15 production and protein ISGylation was not associated with cellular susceptibility to any viruses tested, consistent with the lack of viral diseases in these patients. By contrast, the lack of mycobacterium-induced ISG15 secretion by leukocytes — granulocytes in particular — reduced the production of IFN-γ by lymphocytes, including natural killer cells, probably accounting for the enhanced susceptibility to mycobacterial disease. This experiment of Nature shows that human ISGylation is largely redundant for antiviral immunity, but that ISG15 plays an essential role as an IFN-γ-inducing secreted molecule for optimal antimycobacterial immunity.

ISG15 is an interferon (IFN)-α/β-induced ubiquitin-like protein. It exists as a free molecule, intracellularly and extracellularly, and conjugated to target proteins. Studies in mice have demonstrated a role for Isg15 in antiviral immunity. By contrast, human ISG15 was shown to have critical immune functions, but not in antiviral immunity. Namely, free extracellular ISG15 is crucial in IFN-γ-dependent antimycobacterial immunity, while free intracellular ISG15 is crucial for USP18-mediated downregulation of IFN-α/β signalling. Here we describe ISG15-deficient patients who display no enhanced susceptibility to viruses in vivo, in stark contrast to Isg15-deficient mice. Furthermore, fibroblasts derived from ISG15-deficient patients display enhanced antiviral protection, and expression of ISG15 attenuates viral resistance to WT control levels. The species-specific gain-of-function in antiviral immunity observed in ISG15 deficiency is explained by the requirement of ISG15 to sustain USP18 levels in humans, a mechanism not operating in mice.