Mutation of a conserved alpha-herpesvirus transport effector is an effective vaccine strategy

Neuroinvasive members of the alpha-herpesvirinae include human (i.e. herpes simplex virus type I; HSV-1) and veterinary (i.e. pseudorabies virus; PRV) pathogens that routinely invade the peripheral nervous system of an immunocompetent host. Although infections are not typically associated with significant symptoms, debilitating diseases including shingles, encephalitis, and blindness may occur. Spread into the nervous system is also accompanied by the establishment of life-long latent infections in sensory ganglia. Very little is known regarding the mechanisms that underlie this remarkable neuroinvasive trait. Alphaherpesviruses enter axon nerve endings by fusing with the plasma membrane, and subsequent microtubule-based axon transport delivers the viral genome to neuronal nuclei resident in sensory ganglia. The viral pUL37 protein remains associated with the virus particle as it travels in axons, and as such is a candidate transport effector protein. We recently identified three conserved surface-exposed regions in the UL37 crystal structure. Our data demonstrate that mutation of one of these regions, designated R2, prevents sustained retrograde transport of both PRV and HSV-1 following entry into neurons in culture and in vivo. R2 mutants are avirulent and display bidirectional motion in axons, which results in zero net displacement towards the neural soma. Despite this dramatic neural defect, the R2 mutants propagate with wild-type kinetics in epithelial cells. This unique combination of properties prompted us to evaluate the R2 mutant as a live-attenuated vaccine, and demonstrate that they effectively protect the nervous system from wild-type infections and encephalitic disease.