Pseudorabies virus expressing bovine herpesvirus 1 glycoprotein B exhibitsaltered neurotropism and increased neurovirulence

Herpesvirus glycoproteins play dominant roles in the initiation of infectio n of target cells in culture and thus may also influence viral tropism in v ivo, Whereas the relative contribution of several nonessential glycoprotein s to neurovirulence and neurotropism of Pseudorabies virus (PrV), an alphah erpesvirus which causes Aujeszky's disease in pigs, has recently been uncov ered in studies using viral deletion mutants, the importance of essential g lycoproteins is more difficult to assess, We isolated an infectious PrV mut ant, PrV-9112C2, which lacks the gene encoding the essential PrV glycoprote in B (gB) but stably carries in its genome and expresses the homologous gen e of bovine herpesvirus 1 (BHV-1) (A, Kopp and T. C. Mettenleiter, J. Virol , 66:2754-2762, 1992), Apart from exhibiting a slight delay in penetration kinetics, PrV-9112C2 was similar in its growth characteristics in cell cult ure to wild-type PrV, To analyze the effect of the exchange of these homolo gous glycoproteins in PrV's natural host, swine, 4-week-old piglets were in tranasally infected with 10(6) PFU of either wild-type PrV strain Kaplan (P rV-Ka), PrV-9112C2, or PrV-9112CZR, in which the PrV gB gene was reinserted instead of the BHV-1 gB gene. Animals infected with PrV-Ka and PrV-9112C2R showed a similar course of disease, i.e., high fever, marked respiratory s ymptoms but minimal neurological disorders, and excretion of high amounts o f virus. All animals survived the infection. In contrast, animals infected with PrV-9112C2 showed no respiratory symptoms acid developed only mild fev er. However, on day 5 after infection, all piglets developed severe central nervous system (CNS) symptoms leading to death within 48 to 72 h, Detailed histological analyses showed that PrV-9112C2R infected all regions of the nasal mucosa and subsequently spread to the CNS preferentially by the trige minal route. In contrast, PrV-9112C2 primarily infected the olfactory epith elium and spread via the olfactory route, In the CNS, more viral antigen an d significantly more pronounced histological changes resulting in more seve re encephalitis were found after PrV-9112C2 infection. Thus, our results de monstrate that replacement of PrV gB by the homologous BHV-1 glycoprotein r esulted in a dramatic increase in neurovirulence combined with an alteratio n in the route of neuroinvasion, indicating that the essential gB is involv ed in determining neurotropism and neurovirulence of PrV.