Date Published: November 2, 2016
Publisher: Public Library of Science
Author(s): Jessica B. Graham, Jessica L. Swarts, Courtney Wilkins, Sunil Thomas, Richard Green, Aimee Sekine, Kathleen M. Voss, Renee C. Ireton, Michael Mooney, Gabrielle Choonoo, Darla R. Miller, Piper M. Treuting, Fernando Pardo Manuel de Villena, Martin T. Ferris, Shannon McWeeney, Michael Gale, Jennifer M. Lund, Richard J. Kuhn.
Infection with West Nile virus (WNV) leads to a range of disease outcomes, including chronic infection, though lack of a robust mouse model of chronic WNV infection has precluded identification of the immune events contributing to persistent infection. Using the Collaborative Cross, a population of recombinant inbred mouse strains with high levels of standing genetic variation, we have identified a mouse model of persistent WNV disease, with persistence of viral loads within the brain. Compared to lines exhibiting no disease or marked disease, the F1 cross CC(032×013)F1 displays a strong immunoregulatory signature upon infection that correlates with restraint of the WNV-directed cytolytic response. We hypothesize that this regulatory T cell response sufficiently restrains the immune response such that a chronic infection can be maintained in the CNS. Use of this new mouse model of chronic neuroinvasive virus will be critical in developing improved strategies to prevent prolonged disease in humans.
West Nile virus (WNV) is an emerging flavivirus, and a potential model pathogen for other viruses in its genus such as Zika, dengue, and yellow fever viruses. Since its introduction to North America in 1999, West Nile virus has spread throughout the USA to now cause significant morbidity and mortality in the Western hemisphere as well as worldwide. It is predicted that WNV and other flaviviruses will continue to cause a significant disease burden to the global population, with expansion possible particularly as mosquito habitats expand and bird migration patterns change with continued shifts in climate worldwide. WNV is neuroinvasive and can cause disease ranging from self-limiting febrile illness to disease of the central nervous system (CNS), including meningitis and encephalitis . Neuroinvasive infection and CNS disease can be particularly deadly and also leave survivors with long-term physical and cognitive disabilities . Additionally, a more chronic poliomyelitis-like syndrome can occur, in which patients experience neurologic weakness and/or tremor up to one year after their acute illness , and virus has been detected in patient urine years after the initial infection . However, there is limited knowledge available regarding the immune response to WNV in humans due to the high prevalence of subclinical infection that precludes identification of WNV-infected individuals and subsequent clinical and immune response evaluations. Thus, most knowledge of anti-WNV immunity comes from study of WNV infection using inbred mouse models of infection, generally using C57BL/6J (B6) mice.
We report here for the first time a mouse model of chronic WNV infection that can be used to elucidate the immunological mechanisms underlying chronic flaviviral infection and disease in humans. By intercrossing inbred CC strains, we create outbred mice with replicable genomes. Among these lines, we identified CC(032×013)F1 hybrid males as having sustained weight loss and virus detectable in the brain out to day 60 p.i., as well as neuropathology within various regions of the brain. Interestingly, this RIX was able to largely clear virus from the spleen by d7 p.i. (Fig 3B), yet sustained brain viral loads for at least 2 months p.i. (Fig 1B). Clearance of the peripheral infection correlated with a rapid innate immune response within the spleen, including IFIT1 and IFNβ expression that was elevated by day 2 p.i. at much higher levels than what was observed in the RIX lines without disease or in the RIX lines with the more traditional disease course (Fig 3A). In parallel with this robust innate immune response, however, the chronic RIX also has elevated Treg frequency and activation status, thereby suggesting that while the exuberant innate response allows for eventual peripheral clearance by day 7 p.i., it may also set up the host for a chronic infection at least in part due to enhanced immunoregulation. In the brain, the chronic CC(032×013)F1 also showed signs of a uniquely early innate immune response, though this was clearly not sufficient to quickly control virus replication in the CNS, perhaps because the innate immune expression of IFNß is negligible by d12, a time at which the response is just ramping up in the traditional disease RIX lines (Fig 3D). While IL-12 expression in CC(032×013)F1 is quickly elevated within the brain, this inflammatory and TH1-promoting response is not able to completely suppress virus replication, perhaps due to immunoregulation mediated by Tregs. Taken together, we find that chronic disease following WNV infection is defined by several components, including sustained weight loss, altered innate immunity, and increased regulatory T cell frequency; we also find that chronically infected mice have lower expression of genes associated with cytolysis.