Comparative pathogenesis of recombinant rabies vaccine strain SAD-L16 and SAD-D29 with replacement of Arg333 in the glycoprotein after peripheral inoculation of neonatal mice: less neurovirulent strain is a stronger inducer of neuronal apoptosis.

Less neurovirulent strains of rabies virus have been recognized to be stronger inducers of neuronal apoptosis in vitro than more neurovirulent strains, but few studies have clarified whether this also applies in vivo. A comparative study was performed in two-day-old ICR mice inoculated in a hindlimb thigh muscle with recombinant rabies virus vaccine strain SAD-L16 (L16) or SAD-D29 (D29), which contains an attenuating substitution of Arg333 in the rabies virus glycoprotein. Histopathological and immunohistochemical analyses of brains were performed at early daily time points and in moribund animals. Both viruses caused progressive limb weakness; mortality with L16 was 100% at day 7 post-inoculation (p.i.) and 75% at 17 days p.i. for D29 and Kaplan-Meyer survival curves were significantly different. L16 spread to the brain more quickly than D29, and both viruses produced multifocal lesions in the brainstem and cerebellum associated with inflammatory changes and neuronal apoptosis. There was more disseminated involvement of the brain and many more infected neurons in L16 infection, particularly in the neostriatum, hippocampus, and cerebral cortex. Both viruses induced neuronal apoptosis, which was most marked in the brainstem tegmentum and internal granular layer of the cerebellum. In light of the lower burden of infection and smaller number of neurons infected with D29, this less virulent virus was a stronger inducer of neuronal apoptosis than the more virulent L16. These findings support previous in vitro studies indicating that there is an inverse relationship between pathogenicity and apoptosis. Induction of apoptosis, which is an innate mechanism in which the host restricts viral spread, may contribute to severe clinical neurological disease when there is viral invasion into the central nervous system.