Characterization of the surface proteins of influenza A (H5N1) viruses isolated from humans in 1997-1998

Influenza A (H5N1) viruses infected humans in Hong Kong between May and Dec ember, 1997. Sixteen viruses, including 6 from fatal cases, were isolated d uring this outbreak. Molecular analysis of the surface proteins genes encod ing the hemagglutinin (HA) and neuraminidase (NA) of these H5N1 isolates, o f a subtype not previously known to infect humans, are presented. The 16 hu man H5 HA sequences contain multiple basic amino acids adjacent to the clea vage site, a motif associated with highly pathogenic avian influenza A viru ses. The phylogenetic relationship among both avian and human H5 hemaggluti nins indicates that the human isolates are related directly to isolates tha t circulated among chickens in the live poultry markets in Hong Kong prior to and during the outbreak in humans. HA sequences from the human isolates and a recent chicken isolate represent a separate clade, within which there are two subgroups that are distinguishable antigenically and by the presen ce of a potential glycosylation site. Likewise the N1 neuraminidases of the human H5 isolates represent a clade that is evolutionarily distinct from p reviously characterized N1 neuraminidases. The recent human H5N1 virus NA g enes are avian-like, indicating direct introduction from an avian source ra ther than evolution of a human N1 NA. All of the 16 human NA genes encode a shortened stalk due to a 19-amino acid deletion, also found in the recent avian H5N1 isolates from Hong Kong. Two unique amino acids were identified in the N1 NAs of the recent human isolates; however, it is not known if the se residues influence host range. Neither the HA nor the NA genes of the hu man H5N1 virus isolates show evidence of adaptive changes during the outbre ak. Although analyses of the surface protein genes of the H5N1 viruses from this outbreak did not provide immediate answers regarding the molecular ba sis for virulence, the analyses provided clues to potentially important are as of the genes worth further investigation. (C) 1999 Academic Press.