Glycoprotein D-negative pseudorabies virus can spread transneuronally via direct neuron-to-neuron transmission in its natural host, the pig, but not after additional inactivation of gE or gI.
Envelope glycoprotein D (gD) is essential for entry of pseudorabies virus (PRV) into cells but is not required for the subsequent steps in virus replication. Phenotypically complemented gD mutants can infect cells and can spread, both in vitro and in mice, by direct cell-to-cell transmission. Progeny virions released by infected cells are noninfectious because they lack gD. The aim of this study was to determine the role of gD in the neuropathogenicity of PRV in its natural host, the pig. We investigated whether gD-negative PRV can spread transneuronally via synaptically linked neurons of the olfactory and trigeminal routes. High doses of a phenotypically complemented gD mutant and gD mutants that are unable to express either gI or gI plus gE were inoculated intranasally in 3- to 5-week-old pigs. Compared with the wild-type virus, the virulence of the gD mutant was reduced. However, pigs inoculated with the gD mutant still developed fever and respiratory signs. Additional inactivation of either gI or gI plus gE further decreased virulence for pigs. Immunohistochemical examination of infected pigs showed that a PRV gD mutant could replicate and spread transneuronally into the central nervous system (CNS). Compared with the wild-type virus, the gD mutant had infected fewer neurons of the CNS on day 2. Nevertheless, on day 3, the gD-negative PRV had infected more neurons and viral antigens were present in second- and third-order neurons in the olfactory bulb, brain stem, and medulla oblongata. In contrast, gD mutants which are unable to express either gI or gI plus gE infected a limited number of first-order neurons in the olfactory epithelium and in the trigeminal ganglion and did not spread transneuronally or infect the CNS. Thus, transsynaptic spread of PRV in pigs can occur independently of gD. Possible mechanisms of transsynaptic transport of PRV are discussed.