The roles of glycoprotein E, glycoprotein I, and US9 in neuronal spread of herpes simplex virus type 1

In animal models of infection, glycoprotein E (gE) is required for efficient HSV-1 spread from the inoculation site to the cell bodies of innervating neurons (retrograde direction). Retrograde spread in vivo is a multi-step process in that HSV-1 first spreads between epithelial cells at the inoculation site, then infects neurites and finally travels by retrograde axonal transport to the neuron cell body. To better understand the role of gE in retrograde spread, we used a compartmentalized neuron culture system in which neurons were infected in the presence or absence of epithelial cells. We found that gE-deleted HSV-1 (NS-gEnull) retained retrograde axonal transport activity when added directly to neurites, in contrast to the retrograde spread defect of this virus in animals. However, overlaying neurites with epithelial cells revealed a 100-fold retrograde spread defect for NS-gEnull. Therefore, gE-mediated spread between epithelial cells and neurites likely explains the retrograde spread defect of gE-deleted HSV-1 in vivo. Three viral proteins, gE, gI, and Us9, have been implicated in alphaherpesvirus anterograde spread in several animal models and neuron culture systems. We sought to better define the roles of gE, gI, and Us9 in HSV-1 anterograde spread using a compartmentalized neuron culture system. We found that no anterograde spread occurred in the absence of gE or gI. However, we did detect anterograde spread in the absence of Us9 using two independent Us9-deleted viruses. We confirmed the Us9 finding in different murine models of neuronal spread. We examined viral transport into the optic nerve and spread to the brain after retinal infection; the production of zosteriform disease after flank inoculation; and viral spread to the spinal cord after flank inoculation. In all models, anterograde spread occurred in the absence of Us9, although in some cases at reduced levels. This finding contrasts with gE- and gI-deleted viruses, which display no anterograde spread in any animal model. Thus, gE and gI are essential for HSV-1 anterograde spread, while Us9 is dispensable