Pathophysiology of Severe Yellow Fever

About This Grant

PROJECT SUMMARY/ABSTRACT Yellow fever (YF) is a serious mosquito-borne disease (~80,000 deaths annually) endemic to Africa and South America that is poised for emergence/re-emergence on a global scale. YF is caused by the yellow fever virus (YFV), which replicates primarily in the liver where it causes significant damage and organ dysfunction. Although treatment of YF is limited to supportive care, a highly effective live-attenuated YF vaccine (known as “17D”) was created in the 1930s. 17D differs from its parental strain, “Asibi,” by only 22 amino acids; however, the mutations responsible for the attenuation of 17D remain unknown. A major barrier to understanding 17D attenuation and YF pathogenesis is the lack of an immunocompetent small animal model that faithfully recapitulates important aspects of human YF disease. In collaboration with physicians and pathologists in Brazil, we have shown that infection of hamsters with hamster-adapted YFV-Asibi recapitulates human YF extremely accurately. Using this model, we have begun to tackle the most pressing questions in the YFV field. We developed a highly flexible reverse genetics system for YFV that enabled us to create large panels of hamster-adapted (HA)-17D/Asibi chimeric viruses. Using disease in the hamster as our primary readout, we identified two mutations in17D’s NS2B gene that, when introduced into HA-Asibi, completely abolish YF disease but have a relatively small impact on viral replication. It is well established that YFV-Asibi and YFV-17D evoke significantly different antiviral type I interferon (IFN) response, with YFV-17D inducing significantly more IFN than YFV-Asibi. Therefore, in Aim 1, we will first study the relationship between these two NS2B mutations, IFN induction, viral replication, and attenuation. We will determine the molecular basis for differential IFN responses between YFV-Asibi and YFV-17D. And finally, we will identify and characterize the innate immune factors that are antagonized by virulent, but not attenuated, YFV. Using the hamster model and specimens collected from YF patients in Brazil, we recently discovered that gastrointestinal (GI) damage plays a central role in the development of YF intoxication by allowing gut bacteria to opportunistically spread within the virus-ravaged host. Thus, intoxication is a septic-shock-like syndrome. There is now an urgent need to understand the drivers of GI damage in YF. Our preliminary studies have shown that YFV does not infect cells in the GI tract. Therefore, in Aim 2, we will seek to understand how YFV infection causes severe GI damage. First, we will determine the anatomical distribution of GI damage in the hamster model. Next, we will determine the anatomic distribution of vascular damage in YF. And finally, we will determine if the viral toxin NS1 is responsible for causing YFV-mediated GI damage. This project integrates viral genetics, antiviral immunity, and recent insights into YF intoxication into a new unified mechanism of YFV pathogenesis. This research has real-world implications for improving vaccine safety/efficacy and developing interventions aimed at reducing the morbidity and mortality of YF.