文摘
Nucleotide and amino acid sequence homologies of the entire RNA segments of six H5N1 human influenza viruses were divided into two or three variant groups. This result was compatible with that of phylogenetic analysis, which showed that trees of the eight genes were composed of two or three minor branch clusters. The PB2, PA, NP and M proteins of human H5N1 viruses were further characterized by the presence of previously reported amino acid sequences peculiar to those of human strains. The virulence of A/Hong Kong/483/97 (HK483) in mice exhibited a sharp contrast to that of A/Hong Kong/156/97 (HK156). Virulence of the former even remained after constant repeated passage in eggs, while the latter rapidly decreased in virulence for mice after only a few passages in eggs. It became apparent that Madin–Darby canine kidney (MDCK) cell-grown virus (HK156-CK) and its clones derived from mouse brain produced a fatal infection in mice through intranasal (i.n.) or intracerebral (i.c.) routes. Conversely, the pathogenicity shown by the egg-derived parental virus (HK156-E3) and its clones were considerably lower in mice after i.n. or i.c. infection. A series of experimental infections revealed that the marked differences in neurovirulence among viruses could be attributed to whether or not viruses were transmitted from the lung to the brain. In amino acid sequence comparison of the entire proteins, it was suggested that a total of six amino acid differences in the PB1(residues 456 and 712), PA (residue 631), HA (residue 211), NP (residue 127) and NS1 (residue101) proteins related closely to changes in pathogenicity or neurovirulence of the HK156 virus in mice. As a result, it was evident that less pathogenic and neurovirulent strains appeared through adaptation and selection of variants during passage in eggs.
