MHC class II is a functional receptor for H3N2 Influenza A viruses and mediates host-specificity

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 conformations is a major host range determinant. However, some FLUAV subtypes (H17, H18, H19) were recently shown to 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 deacetylated, non-susceptible cells which ultimately led to cell death. Further, expression of SLA in deacetylated, 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, expanding the cell tropism and host range of the virus, with potential implications for the viral pathogenesis and adaptation to a new species.