Age-dependent Powassan Virus Lethality and Neuropathogenesis in Mice

Powassan viruses (POWV) are emergent tick-borne flaviviruses that cause severe neurologic disease in humans. Subcutaneous inoculation of C57BL/6 mice with POWV (strain LI9) resulted in overt brain damage resembling spongiform encephalitis. Noting higher POWV lethality in older mice, we assessed neurovirulence as a function of age. We found that POWV LI9 inoculation was lethal in 80% of 50 wk old mice, 10-15 dpi, and that lethality was sequentially reduced in 40, 30, 20, 10 wk old mice to <10%. Lethality was conferred by 2-20 POWV FFUs, and POWV neuropathology was evident as early as 5 dpi, with lethal disease 10-15 dpi correlated with sustained POWV RNA levels in brains of aged mice. Histology of POWV infected 50 wk old murine brains revealed severe spongiform neuronal necrosis, microgliosis, and inflammation with increased brainstem and cerebellar damage. These findings delineate an age-dependent murine model of lethal POWV infection that mirrors human POWV disease and permits analysis of age-dependent neurovirulence determinants. Significance Our findings establish a novel age-dependent lethal animal model to study encephalitic POWV disease in vivo. These initial findings demonstrate that following peripheral inoculation, non-neuroadapted POWV LI9 is neuroinvasive and enters the brains of young and aged mice. However, POWV LI9 lethality is strictly age-dependent and correlated with increased viral load in the brains of aged mice. POWV rapidly directs neuronal loss and spongiform lesions, microglial activation and causes prolonged inflammation that fails to clear POWV from the brains of aged mice. Our results provide a lethal murine model of POWV neurovirulence that mirrors the prevalence of severe human POWV encephalitis in the elderly. This lethal murine POWV model provides mechanisms for defining POWV protective responses of the young, revealing determinants of age-dependent POWV lethality and evaluating potential POWV therapeutics. SUMMARY Powassan virus is an emerging tick-borne flavivirus linked to severe neurologic disease in aged individuals. Here we describe an age-dependent mouse model of POWV pathogenesis. SUBJECTS Powassan virus, flavivirus, neurovirulence, neuroinvasion, neurotropic, spongiform encephalopathy, microgliosis, neuroinflammation