Viral infections in the respiratory tract are common, and, in recent years, severe acute respiratory syndrome coronavirus 2 outbreaks have highlighted the effect of viral infections on antiviral innate immune and inflammatory reactions. Specific treatments for numerous viral respiratory infections have not yet been established and they are mainly treated symptomatically. Therefore, understanding the details of the innate immune system underlying the airway epithelium is crucial for the development of new therapies. The present study aimed to investigate the function and expression of interferon (IFN)‑stimulated gene (ISG)60 in non‑cancerous bronchial epithelial BEAS‑2B cells exposed to a Toll‑like receptor 3 agonist. BEAS‑2B cells were treated with a synthetic TLR3 ligand, polyinosinic‑polycytidylic acid (poly IC). The mRNA and protein expression levels of ISG60 were analyzed using reverse transcription‑quantitative PCR and western blotting, respectively. The levels of C‑X‑C motif chemokine ligand 10 (CXCL10) were examined using an enzyme‑linked immunosorbent assay, and the effects of knockdown of IFN‑β, ISG60 and ISG56 were examined using specific small interfering RNAs. Notably, ISG60 expression was increased in proportion to poly IC concentration, and recombinant human IFN‑β also induced ISG60 expression. By contrast, knockdown of IFN‑β and ISG56 decreased ISG60 expression, and ISG60 knockdown reduced CXCL10 and ISG56 expression. These findings suggested that ISG60 is partly implicated in CXCL10 expression and that ISG60 may serve a role in the innate immune response of bronchial epithelial cells. The present study highlights ISG60 as a potential target for new therapeutic strategies against viral infections in the airway.

Brain capillary endothelial cells are the component of blood brain barrier, and the first line of defense against viruses invading into brain. We demonstrate that treatment of hCMEC/D3 cells, a human brain capillary endothelial cell line, with a Toll-like receptor 3 (TLR3) agonist polyinosinic-polycytidylic acid (poly IC) induces the expression of interferon (IFN)-stimulated gene 60 (ISG60), and this reaction was mediated by IFN-β. Knockdown of ISG60 increased the poly IC-induced expression of IFN-β and an IFN-β-inducible chemokine CXCL10. This indicates that ISG60 constitutes a negative feedback loop in the downstream of TLR3/IFN-β. ISG60 in brain capillary endothelial cells may contribute to prevent excess immune reactions associated with viral infections.

Treatment of cells with interferons (IFNs) induces the phosphorylation of signal transducer and activator of transcription 1 (STAT1), leading to the expression of hundreds of IFN-stimulated genes (ISGs). ISGs exert various antiviral and pro-inflammatory reactions. We have previously reported that ISG56 and ISG54 are induced by polyinosinic-polycytidylic acid (poly IC), an authentic agonist for Toll-like receptor 3 (TLR3), in U373MG human astrocytoma cells. ISG56 and ISG54 are also named as IFN-induced proteins with tetratricopeptide repeats (IFIT) 1 and IFIT2, respectively. In the present study, we demonstrated that poly IC induces the expression of ISG60, also named as IFIT3, in U373MG cells. RNA interference experiments showed that the induction of ISG60 by poly IC was mediated by TLR3, IFN-β, ISG56 and ISG54, whereas ISG60 is involved in poly IC-induced expression of ISG56, ISG54 and a chemokine CXCL10. The level of phosphorylated STAT1 was enhanced by poly IC, and it was inhibited by knockdown of ISG56, ISG54 or ISG60. These results suggest that there is a positive feedback loop between phosphorylated STAT1 and these ISGs.