A barrier function made up of the receptor preference of the virus and the paucity of receptors in human airways against avian influenza virus infection *
There are many kinds of influenza A virus distributed amongst bird species worldwide. Birds are thus regarded as an important supplier of new pandemic influenza viruses. Avian and human influenza viruses preferentially recognize α2,3 (SAα2,3) and α2,6 sialosaccharide (SAα2,6) as receptors. Since SAα2,3 can hardly be detected in the epithelium of the human upper respiratory tract, the receptor preference of avian influenza viruses to SAα2,3 and the paucity of SAα2,3 in the human airway have been thought to constitute an important barrier against avian influenza virus infection in humans. However, the highly virulent H5N1 avian influenza virus has sporadically infected humans and had severe, often fatal, results. Recently several research groups have reported the presence of SAα2,3 in the human airway. Therefore the above noted concept needs reexamining. In the present study, we examined the effectiveness of this barrier using human lung carcinoma A549 cells and avirulent avian influenza viruses. We found that a small amount of SAα2,3 was present in A549 cells, along with a large amount of SAα2,6. The avian viruses tested could enter A549 cells utilizing the small amount of SAα2,3 present as a receptor and synthesize viral antigens. However, viral entry efficiency differed among the viruses tested. These results indicated that the presence of a small amount of receptor on cell surfaces compromised the barrier function, and so some avian viruses have the potential to overcome the barrier to human infection. (Accepted on September 2, 2009)
