| Pandemic influenza viruses represent one of the most serious health threats to humans worldwide. Pigs are thought to play an important role in the creation of pandemic viruses by acting as "mixing vessel" hosts for genetic reassortment. Although persistent circulation of human viruses in pigs may be a crucial prerequisite in the development of pandemic viruses in pigs, maintenance of human lineage viruses in the swine population appears to be relatively rare. The focus of this research was to examine factors necessary for human influenza viruses to efficiently infect and replicate in pigs.; In 1997, H3N2 influenza A viruses emerged among pigs in North America. The most commonly isolated viruses have a triple-reassortant genotype with genes of human- (HA/NA/PB1), swine- (NP/M/NS) and avian-lineage (PA/PB2) (e.g. A/Swine/Minnesota/593/99, Sw/MN). In contrast, a wholly human H3N2 virus (A/Swine/Ontario/00130/97, Sw/ONT) was isolated from a single piglet, but subsequently disappeared from the pig population. To test whether Sw/ONT's disappearance was due to an inability of this virus to infect and replicate efficiently in pigs, we infected pigs intranasally with Sw/ONT or SW/MN. We found that Sw/ONT was less pathogenic and that its infectivity was highly restricted in pigs. In order to further explore the virus infectivity differences, we developed an in vitro infection model system employing primary human (HRE) and porcine (PRE) respiratory epithelial cells. We then conducted single-step growth curve experiments. While Sw/MN infected PREs efficiently, the infectivity of Sw/ONT and other human influenza viruses was highly restricted. Using a plasmid-based reverse genetics system, we further demonstrated that Sw/ONT's restricted infectivity in PREs was primarily HA, and to a lesser extent, NA dependent. Lastly, we investigated whether this HA/NA-dependent restriction would translate to pigs. Intranasal infection of pigs with Sw/ONT expressing the SW/MN HA and NA resulted in a substantial increase in infectivity and pathogenicity compared to Sw/ONT. In contrast, there was a milder negative impact on infectivity and pathogenicity for the Sw/MN expressing the Sw/ONT envelope glycoproteins, suggesting that the HA/NA are important, though not the only, factors controlling infectivity of human-lineage influenza viruses in pigs. |
