Longitudinal study of an epitope-biased serum haemagglutination-inhibition antibody response in rabbits immunized with type A influenza virions.

This paper describes a longitudinal study of the antibody specificities generated to the haemagglutinin (HA), the major envelope glycoprotein of type A influenza virus particles, during primary and secondary antibody responses in rabbits. Two New Zealand White rabbits (no. 191, no. 192) and an English Half-Lop rabbit (no. 193) were immunized intravenously with beta-propiolactone-inactivated virus at 0, 28 and 56 days. Haemagglutination-inhibition (HI) antibody specificities were measured using neutralizing antibody double escape mutants selected with monoclonal antibodies (mabs) specific for an epitope in antigenic site A, site B and site D. In this regard the epitope reactivity to these mabs was represented as A+B-D-, A-B+D- and A-B-D+, where "+" and "-" represent the non-mutated and mutated epitopes respectively. The HI response was well developed at 7 days after primary immunization and at this time the response was equally divided between all three epitopes. Two rabbits (no. 191 and no. 193), showed a bias to the site B epitope from 14 days onwards, such that about half of the total HI activity was to this epitope and the other half was made up of HI antibody to the other two epitopes in approximately equal proportions. In the other New Zealand White rabbit (no. 192) a non-biased response extended throughout the primary response and for one week after the second immunization. Apart from this, a bias to a single epitope was clearly evident in all rabbits after the second immunization, and this constituted up to 70% of the total HI antibody response. The antibody response did not broaden and remained essentially unchanged even after a third immunizing injection. The observed bias to the site B epitope during the secondary response of HA-specific antibody is in accord with a previous cross-sectional study of nine other rabbits.