Survey of the Hemagglutinin (HA) Cleavage Site Sequence of H5 and H7 Avian Influenza Viruses: Amino Acid Sequence at the HA Cleavage Site as a Marker of Pathogenicity Potential

Abstract

The deduced amino acid sequence at the hemagglutinin (HA) cleavage site of 76 avian influenza (AI) viruses, subtypes H5 and H7, was determined by reverse transcription-polymerase chain reaction and cycle sequencing techniques to assess pathogenicity. Eighteen of the 76 viruses were isolated in 1993 and 1994 from various sources in the United States. In addition, 34 H5 (4 highly pathogenic [HP] and 30 non-highly pathogenic [non-HP]) and 24 H7 (3 HP and 21 non-HP) repository viruses, isolated between 1927 and 1992, were sequenced and the sequences compared to those in recent isolates. All repository HP H5 and H7 viruses studied had multiple basic amino acids adjacent to the HA cleavage site and most had basic amino acids in excess of the proposed minimum motif B-X-B-R (B = basic amino acids arginine or lysine, X = nonbasic amino acid, R = arginine) that has been associated with high pathogenicity. Of the non-HP viruses studied, 35 of 38 for H5 and 30 of 31 for H7 conformed to the motif B-X-X-R and B-X-R, respectively. Two non-HP H5 viruses had the motif X-X-X-R at the cleavage site and a third had the motif B-X-X-K (K = basic amino acid lysine). One non-HP H7 (A/Pekin robin/CA/30412-5/94) had four basic amino acids (K-R-R-R) adjacent to the cleavage site. Although the Pekin robin isolate did not produce disease in chickens under the conditions of the study it did have the amino acid sequence compatible with that in HP AI viruses and, therefore, is considered potentially HP. This is the first account of an H7 virus that is non-HP in chickens but meets the molecular criterion to be classified as HP.

Human influenza A H5N1 virus related to a highly pathogenic avian influenza virus

“…The presence of multiple basic aminoacids in the sequence motif at the cleavage site of the haemagglutinin molecule of both the human and avian isolates (figure 1) is known to be associated with high virulence of avian influenza viruses. [17][18][19] Experimental infection of chickens with HK97 showed that even after passaging in mammalian cells-once in the child and twice in MDCK cells-the virus remained highly pathogenic for chickens: all eight chickens inoculated intratracheally with MDCK-cell-grown HK97 died within 3 days of infection. Also, CkHK97 was shown to be highly pathogenic in experiments in which seven of eight birds inoculated intravenously with 0·2 mL of a one in ten dilution of infectious allantoic fluids died within 24 h of inoculation and the remaining bird within 48 h.…”

Isolation and characterization of H7N9 viruses from live poultry markets — Implication of the source of current H7N9 infection in humans

“…The HA gene of the three human H7N9 viruses and the avian and environmental H7N9 viruses in this study lacked the stretch of multi-basic amino acids at the HA cleavage site, which is the signature of highly pathogenic H5 and H7 influenza viruses [5,6]. However, human infections with this H7N9 virus still cause severe symptoms and a considerably high fatality rate.…”

The viral polymerase mediates adaptation of an avian influenza virus to a mammalian host

“…HPAIV possess polybasic cleavage sites in HA recognized by ubiquitous proteases of the subtilisin family (31)(32)(33). Such a cleavage site that is a marker for high pathogenicity is present in SC35 and SC35M.…”