IFITMs inhibit infection of diverse enveloped viruses, including the Influenza A virus, West Nile virus, Ebola virus, and others. Using single virus imaging, we have shown that IFITM3 concentrates at the sites of Influenza virus entry and allows lipid mixing but abrogates the release of viral content into the cytoplasm (Desai et al., 2014; Suddala et al., 2019). We have shown that IFITM3 imposes a negative curvature and increases rigidity of the cytoplasmic leaflet of a target membrane, thereby disfavoring the formation of fusion pores and trapping virions at a hemifusion intermediate. Viral hemifusion was detected by single-particle tracking in live cells and using correlative light-electron microscopy (CLEM) to delineate the influenza virus fusion intermediates arrested by IFITM3 (Figure 5; Buth et al., 2026). We have shown that cyclosporin A antagonizes the antiviral activity of IFITM3 by relocating it from the limiting membrane of endosomes to intraluminal vesicles (Buth et al., 2026). Despite progress in understanding the molecular basis for IFITM-mediated virus restriction in target cells, our recent work suggests a distinct mechanism of HIV-1 restriction by IFITMs incorporated into virions. IFITM mutants that lack antiviral activity, when expressed in target cells, potently reduce infectivity when incorporated into virions (Verma et al., 2026).
