The bat influenza A virus subtype H18N11 induces nanoscale MHCII clustering upon host cell attachment.

Prior to the discovery of bat influenza A virus (IAV) subtypes H17N10 and H18N11, all IAVs were thought to bind sialic acid residues via hemagglutinin (HA) to mediate attachment and subsequent viral entry. However, H17 and H18 engage a proteinaceous receptor: the major histocompatibility complex class II (MHCII). The mechanistic details of this hitherto unknown protein-mediated entry are not understood. Given that conventional IAVs rely on multivalent binding to sialylated glycans, we hypothesized that bat HA similarly interacts with multiple MHCII molecules. Using photoactivated localization microscopy (PALM) on fixed and live cells, we demonstrate that bat IAV particles attach to pre-existing MHCII clusters and induce a further increase in cluster size upon binding. To measure the impact of viral attachment on the dynamics of MHCII, we employ an "inverse attachment" approach, immobilizing viral particles on coverslips before seeding live MHCII-expressing cells on top. Single-molecule tracking reveals that the mobility of MHCII is indeed slowed down in viral proximity leading to a local enrichment of MHCII molecules beneath the viral particle. These findings suggest that viral attachment induces MHCII clustering, a process similar to the MHCII dynamics observed during the formation of an immunological synapse.