MHC class II functions as a host-specific entry receptor for representative human and swine H3N2 influenza A viruses

Influenza A viruses (FLUAV) utilize sialic acid to enter host cells via the envelope’s hemagglutinin (HA), and its affinity for host-specific sialic acid linkage configuration is a major host range determinant. However, some FLUAV subtypes (H17, H18, H19) use the major histocompatibility complex class II (MHCII) as an entry receptor instead of sialic acid (SA), challenging our knowledge about FLUAV tropism and interspecies transmission potential. Here, we show that H3N2 viruses can use MHCII as an alternative entry receptor in a host-specific manner, and adaptation of human viruses to pigs increases affinity for the MHCII swine leukocyte antigen (SLA). By using two prototypic human-seasonal (hVIC/11) and swine-adapted (sOH/04) H3N2 viruses we found that expression of the human (HLA) but not the swine MHCII conferred replication of hVIC/11 in desialylated, non-susceptible cells. Further, expression of SLA in non-susceptible cells conferred susceptibility to infection by sOH/04. Introduction of point mutations near the hVIC/11 HA receptor-binding site (RBS) allowed the use of both human and swine MHCII. Our findings revealed that MHCII can serve as a sialic acid-independent entry receptor to H3N2 FLUAV in a host-specific manner, with potential implications for the viral pathogenesis and adaptation to a new species. Human and swine H3N2 influenza A viruses were shown to use MHC class II as a sialic acid–independent, host-specific entry receptor, potentially expanding viral cell tropism. Furthermore, affinity for swine MHCII was higher in viruses adapted to pigs, suggesting a role in host range.