GLYCOPROTEIN D-NEGATIVE PSEUDORABIES VIRUS CAN SPREAD TRANSNEURONALLYVIA 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 vi rus (PRV) into cells hut is not required for the subsequent steps in v irus replication. Phenotypically complemented gD mutants can infect ce lls and can spread, both in vitro and in mice, by direct cell-to-cell transmission. Progeny virions released by infected cells are noninfect ious 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 p ig. We investigated whether gD-negative PRV can spread transneuronally via synaptically linked neurons of the olfactory and trigeminal route s. High doses of a phenotypically complemented gD mutant and gD mutant s that are unable to express either gI or gI plus gE were inoculated i ntranasally in 3- to 5-week-old pigs. Compared with the wild-type viru s, the virulence of the gD mutant was reduced. However, pigs inoculate d with the gD mutant still developed fever and respiratory signs. Addi tional inactivation of either gI or gI plus gE further decreased virul ence 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, t he gD mutant had infected fewer neurons of the CNS on day 2. Neverthel ess, on day 3, the gD-negative PRV had infected more neurons and viral antigens were present in second- and third-order neurons in the olfac tory 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 C NS. Thus, transsynaptic spread of PRV in pigs can occur independently of gD. Possible mechanisms of transsynaptic transport of PRV are discu ssed.