Determining the replication kinetics and cellular tropism of the ruminant-associated Influenza D virus on primary human airway epithelial cells

Influenza viruses are notorious pathogens that frequently cross the species barrier with often severe consequences for both animal and human health. In 2011, a novel member of the Orthomyxoviridae family, Influenza D virus (IDV), was identified in the respiratory tract of pigs with influenza-like symptoms and subsequently also in cattle, a species that previously never was associated with influenza virus infection. Epidemiological surveys among livestock demonstrated that IDV is worldwide distributed among ruminants, but the most striking observation is the detection of IDV-directed antibodies among humans with occupational exposure to livestock. As a first step toward identifying the zoonotic potential of the newly emerging IDV we determined the replication kinetics and cell tropism at the primary site of replication using an in vitro respiratory epithelium model of humans. The inoculation of IDV on human airway epithelial cell (hAEC) cultures revealed efficient replication kinetics and apical progeny virus release of IDV at different body temperatures. Intriguingly, the replication characteristics of IDV revealed many similarities to the human-associated Influenza C virus, including the predominant cell tropism for ciliated cells. Moreover, analysis of the host response during IDV infection revealed only a pronounced upregulation of Type III interferon (IFN) transcripts. Nevertheless, viral progeny virus is replication competent and can be efficiently sub-passaged in hAEC cultures from different donors. Highlighting, that there is no intrinsic impairment of IDV replication within the human respiratory epithelium and might explain why IDV-directed antibodies can be detected among humans with occupational exposure to livestock.\n\nImportanceInfluenza viruses are notorious pathogens that frequently cross the species barrier with often severe consequences for both animal and human health. In 2011, a novel member of the Orthomyxoviridae family, Influenza D virus (IDV), was identified among pigs with influenza-like symptoms and subsequently also in cattle. IDV infections in humans have not yet been described, although IDV-directed antibodies have been found among people with occupational exposure to livestock. This observation suggests a possible spillover from livestock to humans. Using an in vitro human respiratory epithelium model we demonstrate there is no inherent restriction for IDV to replicate within the human respiratory epithelium and this might explain why IDV-directed antibodies are detected among humans with occupational exposure to livestock.