Different replication characteristics of historical pseudorabies virus strains in porcine respiratory nasal mucosa explants

Different alphaherpesviruses, including pseudorabies virus (PRV), are able to cross the basement membrane barrier in nasal respiratory epithelium. As a first step in investigating this invasion process, a detailed quantitative analysis system was set up to determine the kinetics of horizontal and vertical virus spread in nasal explants, using the virulent PRV strain 89V87. Plaque latitudes, total depths, depths measured from the basement membrane and volumes were determined at 0, 12, 24 and 36 h post inoculation (pi). PRV 89V87 was found to spread in a plaquewise manner and to cross the basement membrane between 12 and 24 hpi.During the 1960s–1970s, an increase in PRV virulence has been reported. To analyse potential differences in efficiency of infection and spread for different historical PRV strains, single infected cells and plaques of infected cells were quantified at 12 and 36 hpi in nasal mucosa explants for seven European PRV strains, isolated in the 1960s (Becker, NIA1), the 1970s (NS374, NIA3, 75V19) and later (89V87, 00V72). All viruses were used at second passage in cell culture, except for the Becker strain, which had an unknown passage history. Older strains, Becker, NIA1 and/or NS374, showed lower numbers of primary infectious centers, lower capacity to form plaques and/or lower capacity to cross the basement membrane. The observed differences in virus-mucosa interactions may aid in understanding the virulence increase of PRV. The quantitative assay established here will be of use in unravelling the mechanism of alphaherpesvirus-mediated invasion through the basement membrane.