Highly pathogenic avian influenza viruses (HPAIVs) of hemagglutinin H5 and H7 subtypes emerge after introduction of low pathogenic avian influenza viruses (LPAIVs) from wild birds into poultry flocks followed by subsequent circulation and evolution. The acquisition of multiple basic amino acids at the endoproteolytical cleavage site of the hemagglutinin (HA) is a molecular indicator for high pathogenicity, at least for infections of gallinaceous poultry. Apart from the well-studied significance of the multi-basic HA cleavage site there is only limited knowledge on other alterations in the HA and neuraminidase (NA) molecules associated with changes in tropism during emergence of HPAIVs from LPAIVs. We hypothesized that changes in tropism may require alterations of sialyloligosaccharide specificities of HA and NA. To test this hypothesis we compared a number of LPAIVs and HPAIVs for their HA-mediated binding and NA-mediated desialylation of a set of synthetic receptor analogs, namely, α2-3 sialylated oligosaccharides. NA substrate specificity correlated with structural groups of NAs and did not correlate with pathogenic potential of the virus. In contrast, all HPAIVs differed from LPAIVs by a higher HA receptor-binding affinity towards trisaccharides 3´SLN and SiaLe(c) and by the ability to discriminate between non-fucosylated and fucosylated sialyloligosaccharides 3´SLN and SiaLe(x). These results suggest that alteration of the receptor-binding specificity accompanies emergence of the HPAIV from their low pathogenic precursors.

IMPORTANCE: Here, we have found for the first time correlations of receptor-binding properties of the HA with a highly pathogenic phenotype of poultry viruses. Our study suggests that enhanced receptor-binding affinity of HPAIVs for a typical "poultry-like" receptor, 3´SLN, is provided by substitutions in the receptor binding site of HA which appeared in HA of LPAIVs in a course of transmission of LPAIVs from wild waterfowl into poultry flocks with subsequent adaptation in poultry. The identification of LPAIVs with receptor characteristics of HPAIVs argue that sialic acid-binding specificity of the HA may be used as a novel phenotypic marker of HPAIVs.