SELECTION OF NEUTRALIZING ANTIBODY ESCAPE MUTANTS WITH TYPE-A INFLUENZA-VIRUS HA-SPECIFIC POLYCLONAL ANTISERA - POSSIBLE SIGNIFICANCE FOR ANTIGENIC DRIFT

Ten antisera were produced in rabbits by two or three intravenous inje ctions of inactivated whole influenza type A virions. All contained ha emagglutination-inhibition (HI) antibody directed predominantly to an epitope in antigenic site B and, in addition, various amounts of antib odies to an epitope in site A and in site D. The ability of untreated antisera to select neutralization escape mutants was investigated by i ncubating virus possessing the homologous haemagglutinin with antiseru m adjusted to contain anti-B epitope HI titres of 100, 1000 and 10000 HIU/ml. Virus-antiserum mixtures were inoculated into embryonated hen' s eggs, and progeny virus examined without further selection. Forty pe rcent of the antisera at a titre of 1000 HIU/ml selected neutralizing antibody escape mutants as defined by their lack of reactivity to Mab HC10 (site B), and unchanged reactivity to other Mabs to site A and si te D epitopes. All escape mutant-selecting antisera had a ratio of ant i-site B (HC10)-epitope antibody:other antibodies of greater than or e qual to 2.0:1. The antiserum with the highest ratio (7.4:1) selected e scape mutants in all eggs tested in four different experiments. No ant iserum used at a titre of 10000 HIU/ml allowed multiplication of any v irus. All antisera used at a titre of 100 HIU/ml permitted virus growt h, but this was wild-type (wt) virus. We conclude that a predominant e pitope-specific antibody response, a titre of greater than or equal to 1000 HIU/ml, and a low absolute titre of other antibodies (less than or equal to 500 HIU/ml) are three requirements for the selection of es cape mutants. None of the antisera in this study could have selected e scape mutants without an appropriate dilution factor, so the occurrenc e of an escape mutant-selecting antiserum in nature is likely to be a rare event.