Interferon-Induced ISG15 Conjugation Inhibits Influenza A Virus Gene Expression and Replication in Human Cells

The ubiquitin-like ISG15 protein, as well as its conjugating enzymes, is induced by type I interferons (IFNs). Experiments using ISG15 knockout (ISG15(-/-)) mice established that ISG15 and/or its conjugation inhibits the replication of influenza A virus. However, in contrast to the virus inhibition results for mice, the rates of virus replication in ISG15(+/+) and ISG15(-/-) mouse embryo fibroblasts in tissue culture were similar. Here we focus on human tissue culture cells and on the effect of ISG15 and/or its conjugation on influenza A virus gene expression and replication in such cells. We demonstrate that IFN-induced antiviral activity against influenza A virus in human cells is significantly alleviated by inhibiting ISG15 conjugation using small interfering RNAs directed against ISG15-conjugating enzymes. IFN-induced antiviral activity against influenza A virus protein synthesis was reduced 5- to 20-fold by suppressing ISG15 conjugation. The amounts of the viral proteins that were restored by these siRNA treatments were approximately 40 to 50% of the amounts produced in cells that were not pretreated with IFN. Further, we show that ISG15 conjugation inhibits influenza A virus replication 10- to 20-fold at early times after infection in human cells. These results show that ISG15 conjugation plays a substantial role in the antiviral state induced by IFN in human cells. In contrast, we show that in mouse embryo fibroblasts ISG15 conjugation not only does not affect influenza A virus replication but also does not contribute to the IFN-induced antiviral activity against influenza A virus gene expression.