Different hemagglutinin cleavage site variants of H7N7 in an influenza outbreak in chickens in Leipzig, Germany.

The hemagglutinin (HA) genes from four avian H7N7 influenza A isolates, from a single outbreak, were shown to possess different cleavage sites that contain varying numbers of basic amino acid residues (KKKKR, KRKKR, KKRKKR, KKKKKKR). All four variants are highly pathogenic in chickens and share an immediate common ancestral HA with A/tern/Potsdam/342-6/79 (H7N7) and A/swan/Potsdam/63-6/81 (H7N7). These viruses are nonpathogenic and contain no extra basic amino acids at the cleavage site of their HA. During evolution a common precursor virus acquired different sequences at the cleavage site of the HA and became highly pathogenic in chickens. In vitro assays revealed that the HA from A/chicken/Leipzig/79 with KKKKR at the cleavage site was only partially cleaved (41%), compared to 93-100% cleavage of the other HAs. Since all four viruses were highly pathogenic in chickens, these findings confirm that the degree of pathogenicity in vivo is not exclusively determined by the degree of HA cleavability.