Avian Influenza Virus Infection of Immortalized Human Respiratory Epithelial Cells Depends upon a Delicate Balance between Hemagglutinin Acid Stability and Endosomal pH

• Published in: The Journal of biological chemistry 2015-04, Vol.290 (17), p.10627-10642
• Main Authors: Daidoji, Tomo, Watanabe, Yohei, Ibrahim, Madiha S., Yasugi, Mayo, Maruyama, Hisataka, Masuda, Taisuke, Arai, Fumihito, Ohba, Tomoyuki, Honda, Ayae, Ikuta, Kazuyoshi, Nakaya, Takaaki
• Format: Article
• Language: English
• Subjects: Animals; Avian Influenza Virus; Birds; Cell Line; Clone Cells; Dogs; Endosomal pH; Endosome; Endosomes - metabolism; Endosomes - virology; Epithelial Cell; Hemagglutinin; Hemagglutinin Glycoproteins, Influenza Virus - chemistry; Hemagglutinin Glycoproteins, Influenza Virus - metabolism; Humans; Hydrogen-Ion Concentration; Influenza A Virus, H5N1 Subtype - pathogenicity; Influenza A Virus, H5N1 Subtype - physiology; Influenza in Birds - metabolism; Influenza in Birds - transmission; Influenza in Birds - virology; Influenza, Human - metabolism; Influenza, Human - transmission; Influenza, Human - virology; Membrane Fusion; Microbiology; Protein Stability; Receptors, Cell Surface - metabolism; Respiratory Mucosa - metabolism; Respiratory Mucosa - pathology; Respiratory Mucosa - virology; Tropism; Viral Protein; Virulence - physiology; Virus Entry; Virus Internalization
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M114.611327

The highly pathogenic avian influenza (AI) virus, H5N1, is a serious threat to public health worldwide. Both the currently circulating H5N1 and previously circulating AI viruses recognize avian-type receptors; however, only the H5N1 is highly infectious and virulent in humans. The mechanism(s) underlying this difference in infectivity remains unclear. The aim of this study was to clarify the mechanisms responsible for the difference in infectivity between the current and previously circulating strains. Primary human small airway epithelial cells (SAECs) were transformed with the SV40 large T-antigen to establish a series of clones (SAEC-Ts). These clones were then used to test the infectivity of AI strains. Human SAEC-Ts could be broadly categorized into two different types based on their susceptibility (high or low) to the viruses. SAEC-T clones were poorly susceptible to previously circulating AI but were completely susceptible to the currently circulating H5N1. The hemagglutinin (HA) of the current H5N1 virus showed greater membrane fusion activity at higher pH levels than that of previous AI viruses, resulting in broader cell tropism. Moreover, the endosomal pH was lower in high susceptibility SAEC-T clones than that in low susceptibility SAEC-T clones. Taken together, the results of this study suggest that the infectivity of AI viruses, including H5N1, depends upon a delicate balance between the acid sensitivity of the viral HA and the pH within the endosomes of the target cell. Thus, one of the mechanisms underlying H5N1 pathogenesis in humans relies on its ability to fuse efficiently with the endosomes in human airway epithelial cells. Background: The mechanism of H5N1 pathogenesis in humans remains unclear. Results: SAEC-T clones were poorly susceptible to previously circulating avian influenza viruses but were completely susceptible to H5N1. Conclusion: Infectivity depends on a delicate balance between acid stability of viral hemagglutinin and endosomal pH in infected cells. Significance: These findings could explain why H5N1 is directly transmitted to humans from birds, resulting in serious illness.