MHC class II proteins mediate sialic acid independent entry of human and avian H2N2 influenza A viruses

Influenza A viruses (IAV) cause seasonal outbreaks that pose a substantial burden on human health. They are also a zoonotic threat as avian and swine IAV can be a source for pandemic influenza. Receptor specificity is a critical determinant of tropism, host range and transmissibility of IAV and thus, plays a crucial role in zoonotic IAV infections1-3. Avian, swine and human IAV bind sialic acid on host cell glycans as their common receptor but differ in sialic acid specificity4,5. In contrast, bat IAV of the H17 and H18 subtypes cannot use sialic acid and require MHC class II complexes for host cell entry6-8. It is unknown how this difference in receptor specificity evolved and if dual receptor specificity for sialic acid and MHC class II is possible. Here, we show that human H2N2 IAV and related avian H2N2 possess dual receptor specificity. In addition to their known sialic acid-dependent entry they can use MHC class II as alternative entry pathway, independent of sialic acid. Of note, MHC class II from humans, pigs, ducks, swans and chickens but not from bats can mediate H2 IAV entry and the ability to use this alternative entry pathway is conserved in current Eurasian avian H2 IAV. Our results demonstrate that IAV can possess dual receptor specificity for sialic acid and MHC class II and suggest a role for MHC class II-dependent entry in zoonotic IAV infections. We used primary human airway epithelial cells of bronchial origin and generated airway cultures at air-liqwid interface. These cultures were subjected to single cell RNA sequencing in order to identify the different cell types present along with their transcriptomic profile. In particular, we aimed to determine transcript levels for HLA-DR expression in the different cell types.