Harnessing the power of recombinant rabies viruses to make safer vaccines and study viral clearance from the brain
The central goal of this thesis work was to advance the understanding of rabies virus (RABV) immunogenicity and pathogenicity in order to develop novel treatments for RABV and other infectious diseases. This was accomplished in two different studies, both using recombinant RABV in murine models of infection. The first study evaluated the effect that viral replication has on immunogenicity; the second evaluated RABV pathogenesis by elucidating neuronal cell fate after viral infection.
Previously, attenuated RABV was shown to be an effective viral vaccine vector, capable of eliciting immune responses to other pathogens. To address safety concerns associated with the use of live viral vectors, we developed a replication-deficient RABV. This was accomplished by the genomic deletion of the RABV glycoprotein, rendering the virus incapable of spread beyond the initially infected cell. Furthermore, this virus is apathogenic following intranasal inoculation. HIV-1 Gag was incorporated into the RABV genome in order to measure antigen-specific immune responses in vivo. When directly compared to a replication-competent RABV expressing HIV-1 Gag, we measured equivalent Gag-specific CD8+ T-cell responses, but lower overall antibody responses. These data suggest RABV immunogenicity is not entirely dependent on replication and antigen dose, and highlights the potential for replication-deficient vaccine vectors. G-deleted RABV vectors hold great promise for development of safer human vaccines, though efficacy in non-human primate models are needed.
The second part of my thesis investigated the fate of the neuron after RABV infection. RABV pathogenesis—or how it causes disease—may be due to neuronal death or dysfunction, though this has not been determined. In order to study neuronal integrity after RABV infection in vivo , we used a Cre reporter mouse model. These mice constitutively and ubiquitously express membrane-targeted tandem dimer Tomato (tdTomato); upon exposure to Cre recombinase, the tdTomato gene is deleted, and membrane-targeted EGFP is induced. Cre reporter mice were infected with a sub-lethal dose of RABV-expressing Cre recombinase, and infected cells were permanently labeled with EGFP. We were able to monitor the long-term survival of RABV-infected cells after clearance of viral antigen by monitoring for EGFP-labeled cells. Our results show that experimental RABV does not induce cell loss as a result of direct or indirect cytopathic effects of the virus, nor a cytotoxic immune response. To determine if these surviving cells were functionally different from their uninfected neighbors, we isolated them by fluorescence activated cells sorting (FACS). Microarray analysis of previously-infected neurons showed transcriptional differences compared to uninfected cells, particularly in the areas of cell-to-cell signaling and nervous system development/function. Furthermore, gene expression patterns were predictive of defects in neurite growth and disorganization of the cytoskeleton, cytoplasm, and microtubule dynamics. These results may guide the treatment of rabies and other CNS infections by restoring impaired neuronal function.