Identification of interferon-stimulated gene 15 as an antiviral molecule during Sindbis virus infection in vivo

The innate immune response, and in particular the alpha/beta interferon (IFN-α/β) system, plays a critical role in the control of viral infections. Interferons α and β exert their antiviral effects through the induction of hundreds of interferon-induced (or -stimulated) genes (ISGs). While several of these ISGs have characterized antiviral functions, their actions alone do not explain all of the effects mediated by IFN-α/β. To identify additional IFN-induced antiviral molecules, we utilized a recombinant chimeric Sindbis virus to express selected ISGs in IFN-α/β receptor (IFN-α/βR)^-/- mice and looked for attenuation of Sindbis virus infection. Using this approach, we identified a ubiquitin homolog, interferon-stimulated gene 15 (ISG15), as having antiviral activity. ISG15 expression protected against Sindbis virus-induced lethality and decreased Sindbis virus replication in multiple organs without inhibiting the spread of virus throughout the host. We establish that, much like ubiquitin, ISG15 requires its C-terminal LRLRGG motif to form intracellular conjugates. Finally, we demonstrate that ISG15's LRLRGG motif is also required for its antiviral activity. We conclude that ISG15 can be directly antiviral.