Evaluating the Zoonotic Potential of Non-Human Influenza D Virus

Influenza viruses are respiratory pathogens known to infect a wide variety of vertebrate species and cause annual epidemics in addition to sporadic pandemics. These viruses belong to the Orthomyxoviridae family and are divided into three genera – influenzas A, B, and C. In 2011, a novel virus sharing approximately 50% amino acid homology with human influenza C virus (ICV) was isolated from pigs showing symptoms of respiratory infection in Oklahoma. Further studies revealed that this newly isolated virus was unable to reassort with human ICV to produce viable progeny and exhibited no cross-recognition with human ICV polyclonal antibodies. These findings suggested the virus isolate was genetically and antigenically divergent from human ICV, thus a new genus within the Orthomyxoviridae family was established, termed influenza D virus (IDV). In 2017 and 2018, the Animal Influenza Ecology and Epidemiology Research Program at The Ohio State University collected nasal swabs and nasal wipes from swine being sold at jackpot shows and identified several IDV strains. To investigate the potential for IDV transmission to humans, we infected primary human nasal epithelial cells (hNECs) with three swine isolates of IDV (D/swine/Kentucky/17TOSU1262/2017, D/swine/Ohio/18TOSU0287/2018, and D/swine/Kentucky/17SW1262/2017) and determined infectious virus production and epithelial cell tropism at temperatures consistent with the upper (32˚C) and lower (37˚C) human respiratory tract. The IDV strains replicated to high infectious virus titer in MDCK cells and hNEC cultures, indicating they were capable of infecting cells of the human respiratory tract. A comparison of chemokine and cytokine induction in hNEC cultures following IDV infection demonstrated decreased expression in 50% of factors tested when compared to ICV infected cultures. High titer replication in conjunction with repression of the innate immune response suggests IDV is capable of human infection. These studies provide a better understanding of the ability of IDV to infect and cause disease in humans and suggest increased surveillance for this genus of influenza virus is necessary