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Restoration of virulence of escape mutants of H5 and H9 influenza viruses by their readaptation to mice

¹ The D. I. Ivanovsky Institute of Virology, 16 Gamaleya Street, 123098 Moscow, Russia
² Division of Virology, Department of Infectious Diseases, St Jude Children's Research Hospital, 332 North Lauderdale Street, Memphis, TN 38105-2794, USA

Correspondence
Nikolai V. Kaverin
kaverin{at}online.ru

Antigenic mapping of the haemagglutinin (HA) molecule of H5 and H9 influenza viruses by selecting escape mutants with monoclonal anti-HA antibodies and subjecting the selected viruses to immunological analysis and sequencing has previously been performed. The viruses used as wild-type strains were mouse-adapted variants of the original H5 and H9 isolates. Phenotypic characterization of the escape mutants revealed that the amino acid change in HA that conferred resistance to a monoclonal antibody was sometimes associated with additional effects, including decreased virulence for mice. In the present study, the low-virulence H5 and H9 escape mutants were readapted to mice. Analysis of the readapted variants revealed that the reacquisition of virulence was not necessarily achieved by reacquisition of the wild-type HA gene sequence, but was also associated either with the removal of a glycosylation site (the one acquired previously by the escape mutant) without the exact restoration of the initial wild-type amino acid sequence, or, for an H5 escape mutant that had no newly acquired glycosylation sites, with an additional amino acid change in a remote part of the HA molecule. The data suggest that such ‘compensating’ mutations, removing the damaging effects of antibody-selected amino acid changes, may be important in the course of influenza virus evolution.