West Nile virus (WNV) is an emerging neuroinvasive flavivirus that now causes significant morbidity and mortality worldwide. The innate and adaptive immune responses to WNV infection have been well studied in C57BL/6J inbred mice, but this model lacks the variations in susceptibility, immunity, and outcome to WNV infection that are observed in humans, thus limiting its usefulness to understand the mechanisms of WNV infection and immunity dynamics. To build a model of WNV infection that captures human infection outcomes, we have used the Collaborative Cross (CC) mouse model. We show that this model, which recapitulates the genetic diversity of the human population, demonstrates diversity in susceptibility and outcomes of WNV infection observed in humans. Using multiple F1 crosses of CC mice, we identified a wide range of susceptibilities to infection, as demonstrated through differences in survival, clinical disease score, viral titer, and innate and adaptive immune responses in both peripheral tissues and the central nervous system. Additionally, we examined the Oas1b alleles in the CC mice and confirmed the previous finding that Oas1b plays a role in susceptibility to WNV; however, even within a given Oas1b allele status, we identified a wide range of strain-specific WNV-associated phenotypes. These results confirmed that the CC model is effective for identifying a repertoire of host genes involved in WNV resistance and susceptibility. The CC effectively models a wide range of WNV clinical, virologic, and immune phenotypes, thus overcoming the limitations of the traditional C57BL/6J model, allowing genetic and mechanistic studies of WNV infection and immunity in differently susceptible populations.

Regulatory T cells (Tregs) are well known for their role in dampening the immune responses to self-antigens and thereby limiting autoimmunity. However, they must also permit immune responses to occur against foreign infectious agents. Using a mouse model of West Nile virus (WNV) infection, we examined the role of Tregs in the generation of effector and memory T cell responses in the secondary lymphoid organs (SLO) as well as the infected tissues. We found that Treg numbers and activation increase in both the SLO and CNS after infection. Using the Foxp3DTR knock-in mice, we found that Treg-deficient mice had increased antigen-driven production of IFN-γ from both CD4+ and CD8+ T cells in both the spleen and CNS during the effector phase. In mice lacking Tregs, there were greater numbers of short-lived effector CD8+ T cells in the spleen during the peak of the immune response, but the memory CD8+ T cell response was impaired. Specifically, we demonstrate that Treg-dependent production of TGF-β results in increased expression of CD103 on CD8+ T cells, thereby allowing for a large pool of resident memory T cells to be maintained in the brain after infection.

Summary The Collaborative Cross (CC) is a panel of reproducible recombinant inbred mouse strains with high levels of standing genetic variation, thereby affording unprecedented opportunity to perform experiments in a small animal model containing controlled genetic diversity while allowing for genetic replicates. Here, we advance the utility of this unique mouse resource for immunology research, as it allows for both examination and genetic dissection of mechanisms behind adaptive immune states in mice with distinct and defined genetic makeups. This approach is founded on quantitative trait locus mapping: identifying genetically variant genome regions associated with phenotypic variance in traits-of-interest. Furthermore, the CC can be utilized for mouse model development; distinct strains have unique immunophenotypes and immune properties, making them suitable for research on particular diseases and infections. Here, we describe variation in cellular immune phenotypes across F1 crosses of CC strains, and reveal quantitative trait loci responsible for several immune phenotypes.

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(032x013)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.

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