Age-dependent Powassan Virus Lethality is Directed by Glial Cell Activation and Divergent Neuroinflammatory Cytokine Responses in a Murine Model

Powassan virus (POWV) is an emergent tick-borne flavivirus that causes fatal encephalitis in the elderly and long-term neurologic sequelae in survivors. How age contributes to severe POWV encephalitis remains an enigma and there are currently no animal models that reflect age-dependent POWV neuropathology. Inoculating C57BL/6 mice with a POWV strain (LI9) currently circulating in Ixodes ticks, resulted in age-dependent POWV lethality with overt spongiform brain damage 10-15 dpi. Infection of 50 week old mice resulted in 82% lethality 10-15 dpi that was sequentially reduced by age to 7.1% in 10 week old mice. LI9 encephalitis resulted in early neuronal depletion, with severe CNS damage, persistent inflammatory gliosis and long-term spongiform pathology in survivors (30 dpi). In all mice POWV LI9 was neuroinvasive and reached maximum POWV loads in the CNS 10 dpi. Coincident with murine lethality, in 50 week old mice maximum POWV CNS levels persisted 15 dpi, while instead decreasing by 2-4 logs in 10-30 week old mice. Although glial cells were highly activated in all POWV infected mice, differences in age-dependent CNS cytokine responses were striking 15 dpi. In 50 week old mice POWV induced Th1-type cytokines (IFNg, IL-2, IL-12, IL-4, TNFa, IL-6), suggesting a pro-inflammatory M1 microglial activation cascade. In contrast, POWV induced Th2-type cytokines (IL-10, TGFb, IL-4) in 10 week old mice consistent with a neuroprotective M2 microglial phenotype. These findings reflect differences in neurodegenerative versus neuroprotective glial cell responses that correlate with divergent CNS viral clearance and age-dependent POWV LI9 lethality. Discrete age-dependent CNS cytokine responses suggest neuroinflammatory targets as potential POWV therapeutics. These studies establish a highly lethal POWV murine model and reveal a hyperinflammatory mechanism of age-dependent POWV lethality that mirrors human POWV severity and long-term CNS sequelae in the elderly.