Physiological and immunological changes in the brain associated with lethal eastern equine encephalitis virus in macaques

Aerosol exposure to eastern equine encephalitis virus (EEEV) can trigger a lethal viral encephalitis in cynomolgus macaques which resembles severe human disease. Biomarkers indicative of central nervous system (CNS) infection by the virus and lethal outcome of disease would be useful in evaluating potential medical countermeasures, especially for therapeutic compounds. To meet requirements of the Animal Rule, a better understanding of the pathophysiology of EEEV-mediated disease in cynomolgus macaques is needed. In this study, macaques given a lethal dose of clone-derived EEEV strain V105 developed a fever between 2-3 days post infection (dpi) and succumbed to the disease by 6 dpi. At the peak of the febrile phase, there was a significant increase in the delta electroencephalography (EEG) power band associated with deep sleep as well as a sharp rise in intracranial pressure (ICP). Viremia peaked early after infection and was largely absent by the onset of fever. Granulocytosis and elevated plasma levels of IP-10 were found early after infection. At necropsy, there was a one hundred- to one thousand-fold increase in expression of traumatic brain injury genes (LIF, MMP-9) as well as inflammatory cytokines and chemokines (IFN-gamma, IP-10, MCP-1, IL-8, IL-6) in the brain tissues. Phenotypic analysis of leukocytes entering the brain identified cells as primarily lymphoid (T, B, NK cells) with lower levels of infiltrating macrophages and activated microglia. Massive amounts of infectious virus were found in the brains of lethally-infected macaques. While no infectious virus was found in surviving macaques, quantitative PCR did find evidence of viral genomes in the brains of several survivors. These data are consistent with an overwhelming viral infection in the CNS coupled with a tremendous inflammatory response to the infection that may contribute to the disease outcome. Physiological monitoring of EEG and ICP represent novel methods for assessing efficacy of vaccines or therapeutics in the cynomolgus macaque model of EEEV encephalitis. Author summary Eastern equine encephalitis virus (EEEV) is a mosquito-transmitted virus that can cause highly lethal encephalitis in a subset of infected humans. Due to being highly virulent and infectious by aerosol, a need exists to develop vaccines or therapeutics to prevent lethal disease, and FDA approval of such an intervention would likely require use of the FDA's Animal Rule. To further characterize the cynomolgus macaque model of EEEV after aerosol exposure, we conducted a detailed analysis of the pathological progression of encephalitis in these animals. Physiological condition of the animals was assessed after infection using electroencephalography (EEG) and intracranial pressure (ICP) along with immunological characterization of the inflammatory cells in the brain. We found overwhelming virus infection of the CNS in fatal infections coupled with a strong CNS inflammatory response. Novel findings by EEG and ICP will guide comprehensive studies evaluating the efficacy of medical countermeasures.

Automated summary

Exposure to aerosolized Eastern equine encephalitis virus (EEEV) in cynomolgus macaques can lead to lethal viral encephalitis resembling severe human disease. Pathophysiological understanding of EEEV-mediated disease in macaques is crucial, with fever, EEG changes, intracranial pressure rise, and inflammatory response indicating severe CNS infection and potential for disease outcome. Developing vaccines or therapeutics for EEEV in the highly virulent and infectious aerosol context may require FDA Animal Rule approval, and physiological monitoring techniques like EEG and ICP show promise in assessing efficacy of medical countermeasures in this model.