Antiviral Immune Responses to Invertebrate Iridescent Virus 6 in Drosophila

Abstract

The innate immune system is a critical first line of defense against invading pathogens. Innate immunity directly detects pathogens, sets up an appropriate adaptive response, and can directly kill pathogens. Drosophila may lack an adaptive immune response, but have a robust innate immune system with a variety of defense effector mechanisms. While the responses to bacteria, fungi, and RNA viruses have been well characterized, not much is known about the response to DNA viruses. My studies have set out to characterize the Drosophila immune response to a DNA virus, utilizing the large dsDNA virus, Invertebrate Iridescent Virus 6 (IIV-6). IIV-6 infection causes shortened lifespan, and in later stages of infection, flies present with abdominal swelling and iridescent blue color. Our objectives were to identify pathways flies use to protect themselves from IIV-6 infection, determine how this protection is mediated, and to identify any immune inhibitors that IIV-6 uses to suppress innate immune signaling. I have found that IIV-6 strongly up-regulates a class of stress proteins with unknown function, termed Turandots, after infection in vivo or in vitro. This induction is dependent upon viral replication, requires JAK-STAT activation, and activation of p38b MAPK. In addition, the unpaireds, which function as JAK-STAT ligands, are upregulated after IIV-6 infection in a p38b-dependent manner. Together, this data suggests that p38b activation leads to production of unpaired cytokines and activation of JAK-STAT signaling to induce Turandots. I have also found that IIV-6 infected cells secrete protective factors. This response is induced within 12 hours of IIV-6 infection, exosome-mediated, and provides robust protection to naive cells challenged with an mCherry-expressing strain of IIV-6. Additionally, IIV-6 inhibits two major immune responses in Drosophila, the IMD and Toll pathways. Stimulation of IIV-6 infected Drosophila S2* cells with either IMD or Toll stimulators results in very poor antimicrobial peptide responses. Yet, IMD and Relish are still cleaved upon stimulation in IIV-6 infected cells, indicating that the block is downstream. In support of this finding, IIV-6 infected flies respond very poorly to infection with the enterobacteria Erwinia carotovora carotovora compared to mock-injected flies.