Discovering Novel Intrinsic Antiviral Responses to Arboviruses: From Transcription to Intestinal Innate Immunity

Many (re) emerging pathogens are arthropod-borne, transmitted via an insect vector, and cause significant health and agricultural problems worldwide. Despite their significance, there are few vaccines and no targeted therapies that exist. This is at least in part due to our limited understanding of virus-host interactions and the mechanisms used by hosts to restrict infection. In particular, insect vectors play a critical role in the transmission and spread of these pathogens, but performing molecular and genetic studies has proven to be difficult. Drosophila is a model organism that shares a high degree of conservation with insect vectors and has a wealth of molecular and genetic tools for study. Hence, this thesis aims to provide a deeper understanding of the innate immune factors that restrict arthropod-borne viruses using this model organism. Using genetic approaches both in vitro and in vivo, two novel antiviral pathways are discovered and examined in this thesis. First, using RNA interference (RNAi) screening against disparate viruses in Drosophila, the transcriptional pausing pathway is found to be essential for antiviral insect immunity. This led to the characterization of a rapidly induced antiviral transcriptional program, half of which is genetically dependent on this regulatory mechanism and has pausing-associated chromatin features. These findings suggest transcriptional pausing primes virally induced genes by enhancing promoter accessibility to allow for rapid gene induction, thereby coordinating a robust and complex antiviral response. Subsequently, the ERK pathway is found to be part of this transcriptional response to viral infection. Not only is this nutrient responsive pathway induced by viral infection, but it also restricts disparate arboviral pathogens. Furthermore, ERK signaling is essential for antiviral defense in the insect intestinal epithelium. While wild type flies are refractory to oral infection by arboviruses, this innate restriction can be overcome chemically by oral administration of an ERK pathway inhibitor or genetically via the specific loss of ERK in the intestinal epithelial cells. In addition, vertebrate insulin that activates ERK signaling in the mosquito gut during a blood meal, can restrict viral infection in insect cells and protect against viral invasion of the gut epithelium. These studies collectively demonstrate that ERK signaling in the insect intestines potently restricts viral infection, suggesting that insects take advantage of signals in the meal to preemptively activate antiviral immunity