IMMUNOHISTOCHEMICAL AND IN SITU HYBRIDIZATION ANALYSIS OF BALB/C AND C3H/HEN MICE AFTER TC-83 VACCINATION AND CHALLENGE WITH VIRULENT VENEZUELAN EQUINE ENCEPHALITIS VIRUS

Abstract

Venezuelan equine encephalitis (VEE) virus is an alphavirus responsible for causing febrile illness in humans. This virus is normally transmitted to humans by mosquitos but is also infectious by the aerosol route. A live-attenuated vaccine, TC-83 is presently used to vaccinate military and laboratory personnel who would be at risk for exposure. This vaccine has generally been effective in protecting animals from subsequent challenge with virulent VEE. Vaccination with TC-83 protects BALB/c mice from overt illness and death after subcutaneous (s.c.) or aerosol challenge with virulent Trinidad donkey virus (TrD), a strain of VEE virus. Vaccinated C3H/HeN mice are protected from death after s.c. challenge but only 30% survive aerosol challenge. This study was designed to address the question of whether vaccination protects the brain and olfactory regions from viral infection after s.c. or aerosol challenge. Six week old female C3H/HeN and BALB/c mice were vaccinated with TC-83 or inoculated with Earle's minimal essential medium (EMEM) s.c. and challenged six weeks later with TrD either s.c. or by aerosol. Tissues were taken at days 1, 2, 3, 5, 7, and 9 after challenge and were processed for immunohistochemistry (IHC) or in situ hybridization (ISH) analysis. C31-I/HeN mice vaccinated and challenged with TrD by aerosol developed viral infection of the olfactory mucosa, olfactory tract and brain. Viral antigen was limited to the cerebral cortex of the brain, and whole body tissues were free of detectable viral antigen. Vaccinated BALB/c mice challenged by aerosol and C3H/HeN mice challenged s.c. were free of detectable viral antigen in the olfactory mucosa, brain and whole body tissues. These findings in this study suggest that 70% of the vaccinated C3H/HeN mice challenged by aerosol die from viral infection of the brain.