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In 1999/2000 there was an outbreak of avian influenza A/H7N1 viruses among different avian species in Italy. The viruses which were still low pathogenic for domestic birds in the beginning of the outbreak became highly pathogenic in late 1999. In addition to a multibasic cleavage site, most of the later isolates also acquired glycans close to the receptor binding site of the HA either at position 123 or 149.
To demonstrate wether these glycosylation sites affect pathogenicity we generated recombinant viruses with no (G0), one (G1, G2) or both glycans (G1+2). We have previously shown that the G0 mutant grew to 100 times lower titers than G1+2 in cultured chicken fibroblasts and likewise influenced mean death time and tissue tropism of infected chicken embryos. G0 differed from G1, G2 and G1+2 by a significantly lower elution rate from chicken cells. Now we are able to demonstrate the same effect for turkey, goose and duck cells likewise influencing tissue tropism in infected embryos of these avian species, indicating that the evolution of glycans at position 149 and 123 of the HA generally contribute to pathogenicity in avian species.
All highly pathogenic isolates possessed mutations in a proposed nuclear export signal (NES) and a C-terminal deletion of 6 amino acids in the NS1 protein compared to the sequence of their low pathogenic ancestors.
In order to evaluate wether the mutations in the NS1 affect pathogenicity, we generated highly pathogenic recombinant viruses which contain the NES and/or the C-terminus of NS1 of a low pathogenic virus. These mutants showed reduced virus titers in chicken embryo cells and restricted spread of infection in chicken embryos compared to the highly pathogenic wildtype virus. Immunofluorescence analysis revealed that NS1 with the NES of the highly pathogenic viruses was predominantly found in the cytoplasm, whereas NS1 with the C-terminal truncation showed strong nucleolar accumulation. Enhanced nuclear export of NS1 was paralleled by a complete shut-down of Interferon-β production in infected cells, suggesting that this mechanism is responsible for the improved virus growth of the highly pathogenic variant.