Why doesn’t H5N1 pass from person to person as easily as it passes from bird to bird? After all, H5N1 can replicate very efficiently in someone’s lungs.
Japanese researchers now think they have an answer to this vexing question. The bird virus, they have found, preferentially binds to cells in different regions of the human airway from those favoured by human influenza viruses.
Flu viruses infecting humans and birds are known to home in on slightly different versions of the same molecule, found on the surface of cells that line the respiratory tract. Yoshihiro Kawaoka and colleagues report in today’s Nature the effect this has on patients. Whereas the version of the molecule preferentially bound by human viruses is more prevalent on cells higher up in the airway, the molecule that is preferentially targeted by avian viruses tends to be found on cells deep within the lungs, in the air sacs, or alveoli, of the lung.
This may explain why human-to-human transmission of H5N1 remains uncommon, explain the authors. The virus may preferentially enter cells deep down inside the lungs, meaning that an infected person is less likely to spread the virus by coughing or sneezing. The researchers add, however, that should the virus ever acquire the ability to infect cells higher up in the airway then it may make the leap to a human to human infectious disease.
Parallel findings are also published today in Science, by Thijs Kuiken and colleagues. They have identified alveoli type II pneumocytes, scavenging cells within the lumen of the alveoli as the cells to which H5N1 predominantly attaches. These findings are in contrast to the received wisdom that avian influenza viruses have little or no affinity for cells of the human respiratory tract.