We have examined whether active immunization with c13
protein, a hybrid
protein of the first 81
amino acids of the viral NS1 nonstructural
protein and the HA2 subunit of A/PR/8 (H1N1)
hemagglutinin, could protect BALB/c mice from challenge with A/PR/8 H1 subtype virus. Mice immunized with the c13
protein had a significant reduction of pulmonary virus titers with A/PR/8 (H1) virus, but failed to limit the replication of A/PC (H3) virus, which reflects the in vitro CTL activity of c13 immune spleen cells. We observed that the
epitope recognized by HA2 specific CTL, which are induced by a derivative of c13
protein, is highly conserved among H1 and H2 subtype virus strains. This led us to test whether active immunization with c13
protein would also limit pulmonary virus replication in mice infected with the A/TW virus, a virus of the H1 subtype, which was isolated in 1986, and with a virus of the H2 subtype, A/Japan/305/57. Immunized mice had significantly lower lung virus titers than did control mice, and did not possess any
neutralizing antibodies to the challenger viruses. These results indicate that active immunization with a fusion
protein containing the cross-reactive CTL
epitope protects mice from
influenza infection by inducing CTL against
influenza A H1 and H2 subtype virus strains, which markedly vary in their antibody binding sites on the HA1. The ability to induce active cross-reactive immunization with a fusion
protein which contains a highly conserved CTL
epitope offers a model for
vaccine approaches against viruses which undergo significant variations in their antibody binding sites.