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Molecular changes in virulent mutants arising from avirulent avian influenza viruses during replication in 14-day-old
1Department of Virology and Molecular Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38101.
Abstract:
The emergence of virulent avian influenza viruses in poultry is unpredictable. To gain insight into the mechanism for this event, we sought to identify the molecular changes in virulent mutants that occur during replication in 14-day-old embryonated chicken eggs. After three passages in 14-day-old eggs, avirulent H5 viruses with the K/R-K-K/T-R sequence at the hemagglutinin (HA) cleavage site became virulent in chickens, concomitantly acquiring high HA cleavability, whereas those with the R-E-T-R sequence did not. None of the test viruses converted to a virulent phenotype when passaged in 10-day-old eggs. Nucleotide sequence analysis indicated that the virulent mutants either lost a glycosylation site near the HA cleavage site or acquired an additional arginine at the latter. Avirulent viruses that became virulent after passage in older eggs included an H5N2 avian strain with the R-K-T-R sequence that was isolated in 1993, indicating that viruses with this sequence motif, which are currently circulating in bird populations, should be considered potentially virulent. Failure to generate virulent mutants from viruses with R-E-T-R at the HA cleavage site underscores the pathogenic heterogeneity among avian influenza viruses.
Insights
Avian influenza viruses can become virulent through specific mutations at the hemagglutinin (HA) cleavage site, particularly in older chicken eggs. Viruses with certain HA sequences circulating today may pose a potential threat to poultry.
Area of Science:
- Virology
- Molecular Biology
- Poultry Science
Background:
- Avian influenza virus (AIV) virulence emergence in poultry is unpredictable.
- Understanding the molecular mechanisms driving AIV virulence is crucial for disease control.
Purpose of the Study:
- To identify molecular changes in AIVs during replication that lead to increased virulence.
- To investigate the role of the hemagglutinin (HA) cleavage site sequence in AIV virulence.
Main Methods:
- Passaging of avirulent H5 AIVs in 14-day-old embryonated chicken eggs.
- Assessment of viral virulence and hemagglutinin (HA) cleavability in chickens.
- Nucleotide sequence analysis of AIVs to identify mutations.
Main Results:
- Avirulent H5 AIVs with specific K/R-K-K/T-R HA cleavage site sequences acquired virulence after passage in 14-day-old eggs.
- Virulent mutants exhibited high HA cleavability, loss of a glycosylation site, or additional arginine at the HA cleavage site.
- Avirulent viruses with R-E-T-R HA cleavage site sequences did not become virulent under the same conditions.
- Passage in younger (10-day-old) eggs did not induce virulence.
Conclusions:
- The HA cleavage site sequence is a critical determinant of AIV virulence.
- Specific HA motifs, like R-K-T-R, found in currently circulating strains, indicate potential for virulence.
- Pathogenic heterogeneity exists among AIVs, influenced by HA sequence and host factors.