The intercellular spread of alphaherpesviruses

Abstract

There are three prominent alphaherpesviruses that infect humans: Herpes Simplex virus-1, Herpes Simplex virus-2, and Varicella Zoster virus. Each virus is harbored within 15-98% of the population. Prototypical infections involve only a handful of intercellular spread events within a host. Most spread events involve neurons. Only one virion is thought to be successfully transmitted from a neuron to another cell -- but this has yet to be verified during infectious spread within a host. In this dissertation, we used Pseudorabies virus to trace the number of virions spreading infection from infected neurons to uninfected cells. Pseudorabies virus is a prominent model alphaherpesvirus that infects mice. To quantify the number of virions transmitted between cells, we intravitreally injected different, genetically engineered Pseudorabies virus strains and quantified spread to the murine central nervous system. We calculated the average number of expressed viral genomes per infected neuron by utilizing the Poisson distributions of neurons expressing one, two, or three different viral strains. We found that when a neuronal axon transmitted infection to cells, a cell became infected with one virion on average. In contrast, when neuronal soma or dendrites transmitted the viral strains to surrounding cells, each cell expressed three viral genomes on average. Most importantly, we discovered that the absence of a specific alphaherpesviral protein, US9, diminished the capacity of the virus to infect a cell with a plurality of viral particles. This dissertation advanced the field of herpesviral research in two ways: by quantifying the number of alphaherpesviral particles transmitted between infected neurons in a host and identifying a viral protein instrumental in determining the number virions transmitted from neurons to other cells. The average number of viral particles infecting cells within a host determines the viral genetic dosage and impacts viral gene expression, viral replicative rates, and viral diversification.