An Evolutionarily Conserved Role for Autophagy in Antiviral Defense

Innate immunity is the first line of defense against viral infection and depends on the use of germline-encoded pattern recognition receptors (PRRs) that detect pathogen-associated molecular patterns (PAMPs). Virus recognition subsequently triggers an assortment of effector responses that are critical for clearing the invading pathogen. Among known antiviral pathways, autophagy, a biological process that mediates the degradation of cytoplasmic components, may be one of the most ancient. We previously demonstrated that autophagy is essential for resistance to the Rhabdovirus Vesicular stomatitis virus (VSV) in flies. However, the mechanism by which autophagy is triggered during VSV infection remained unclear. Through a targeted RNAi screen, I discovered that a previously uncharacterized Drosophila Toll receptor, Toll-7, is essential for restricting VSV replication in cells and adult flies. Moreover, Toll-7 was required to activate antiviral autophagy both in vitro and in vivo. Toll-7 interacted with VSV virions at the cell surface, demonstrating that Toll-7 is a bona fide PRR and the only known Toll receptor that physically associates with a pathogen. Taken together, these data have revealed Toll-7 as the link between VSV recognition and activation of antiviral autophagy. To investigate the range of Toll receptor-mediated antiviral immunity, I next challenged Toll receptor mutant flies with a panel of arthropod-borne viruses (arboviruses). These studies revealed a critical in vivo role for Toll-7 in restricting infection with Rift Valley fever virus (RVFV), a medically relevant pathogen in the bunyavirus family that causes disease in humans and livestock. Toll-7 mutant flies showed increased viral replication and mortality after RVFV infection due to impaired antiviral autophagy activation. Remarkably, autophagy was also required to restrict RVFV infection in mouse and human cells, demonstrating an evolutionarily conserved role for antiviral autophagy in flies and mammals. Autophagy activation using pharmacologic inducers of autophagy potently inhibited RVFV infection in flies and mammalian cells. Collectively, these data suggest that modulation of this conserved antiviral pathway may be a promising strategy for controlling RVFV infection, which lacks effective therapeutics and vaccines, in both mammals and the vector insect.