Replication Fitness of Human Influenza B Viruses in Swine Primary Respiratory Epithelial Cells

Influenza B virus (IBV) is a significant pathogen and contributes to seasonal influenza epidemics worldwide yearly. Humans are thought to be a reservoir and amplification host for IBV. However, natural infections of IBVs in other species such as pigs probably from a spillover event from humans can sporadically occur. Here, we examined and compared the replication fitness of two human IBV lineages in swine primary respiratory epithelial cells (nasal turbinate, trachea, and lung). IBV strains used in this study included two Victoria lineage viruses (B/Brisbane/60/2008 (BR08) and B/HongKong/286/2017 (HK17)) and two Yamagata lineage viruses (B/Florida/04/06 (FL06) and B/Utah/09/2014 (UT14)). Results of our experiments showed that IBVs replicated efficiently in swine primary respiratory epithelial cells regardless of the virus lineage with higher titers observed at 33°C than at 37°C. HK17 (Victoria lineage) grew to the highest titers in nasal turbinate and lung cells, while the other Victoria lineage strain BR08 showed a modest replication fitness. In sum, the swine primary respiratory epithelial cells representing the upper, middle, and lower respiratory tract of swine support IBV replication, which can be further explored as the in vitro model system to study IBV adaptation to swine, the mixing vessel in generating zoonotic influenza viruses.