Extracellular ISG15 drives severe respiratory pathology during viral infection

Cytokine storm during respiratory viral infection is an indicator of disease severity and poor prognosis. Type 1 interferon (IFN-I) production and signaling have been reported to be causal in cytokine storm associated pathology in several respiratory viral infections, however, the mechanisms by which IFN-I promotes disease pathogenesis remain poorly understood. We report using Usp18-deficient, USP18 enzymatic inactive and Isg15-deficient mouse models that lack of deISGylation during persistent viral infection leads to severe immune pathology characterized by hematological disruptions, cytokine amplification, lung vascular leakage and death. Transcriptional studies of immune cells from mice bearing the enzymatic inactive version of USP18 (Usp18C61A) revealed a neutrophil influx in the lung of these animals during infection, associated with increased immature neutrophil content. These neutrophil differentiation changes were dependent on ISG15 as deletion of Isg15 from C61A animals reversed the accumulation of immature neutrophils. Importantly, neutrophil depletion reversed morbidity and mortality in Usp18C61A mice. In summary, we reveal Usp18 enzymatic function is crucial for regulating extracellular release of ISG15 which are accompanied by altered neutrophil differentiation, cytokine amplification and mortality following persistent viral infection.