In the case of avian influenza viruses of the H7 subtype,

which tend to present preferential tropism for ocular tissues in humans [22], mechanical and innate defences associated with the human eye likely require invasive insults, such as physical abrasion, to allow avian influenza virus infection of the ocular epithelia. Therefore, the relative limited accessibility of receptors used by avian influenza viruses in human hosts may contribute to the relative rarity of their transmission to humans. Sialic acids with α2,6 linkage to galactose are more abundantly distributed in the upper regions of the respiratory tract [60], [68] and [73] and are the cellular receptors used by human influenza VX-770 nmr viruses, adapted to and circulating in the human population [54]. They are expressed abundantly on respiratory epithelial cells of the upper respiratory tract, trachea and bronchi [64], [78] and [79] and likely are more accessible to influenza virus particles than sialic acids with α2,3 linkage to galactose. Preferred affinity for these cellular receptors thus may favour successful cross-species transmission of zoonotic influenza viruses from animal reservoirs to humans. Sialic acids

with α2,6 linkage to galactose are not expressed on respiratory or intestinal epithelial cells of ducks [80], but are expressed on respiratory and intestinal epithelial cells of terrestrial birds, such as chicken and quail [80]. Accordingly, avian influenza LBH589 manufacturer viruses using these cellular receptors do circulate in these species. It is the case for some strains of LPAIV H9N2 and of LPAIV and HPAIV of the H7 subtype, which have caused human infection [81], [82], [83] and [84]. Recently, LPAIV of the H6 subtype were shown to infect mammalian hosts without prior adaptation and Linifanib (ABT-869) may have dual

affinity for sialic acids with α2,3 and with α2,6 linkage to galactose [85]. Likewise, respiratory epithelial cells of swine were shown to harbour both types of sialic acids [60] and swine influenza viruses circulating endemically in pig populations typically bind to sialic acids with α2,3 and with α2,6 linkage to galactose [86] and [87]. This may explain the more frequent occurrence of cross-species transmission of swine influenza viruses to humans compared to that of avian influenza viruses. The receptor binding site of influenza virus HA protein is a shallow depression at the top of the protein to which sialic acids bind. Key amino-acids within or close to the receptor binding site and conferring α2,3 or α2,6 receptor binding affinity have been identified in the HA protein of influenza viruses of the H1, H2, H3, H4, H5 and H9 subtypes (Table 2). Portals of entry other than the respiratory epithelium were suggested for HPAIV H5N1, yet the sites of initial virus attachment and infection following non-respiratory routes of entry remain unclear.