Genetic relationships, serological cross-reaction and cross-protection between H1N2 and other influenza A virus subtypes endemic in European pigs.

This study examines the genetic relationships between the recently emerged H1N2 swine influenza virus and viruses of H1N1 and H3N2 subtypes, and the extent of protection against H1N2 challenge in pigs immune after infection or vaccination with the other subtypes. There was low amino acid homology (70.4-71.9%) in the haemagglutinin (HA) gene between H1N1 viruses used for primary infection or vaccination and the H1N2 challenge strain, with 94-99 amino acid changes between these viruses involving all five antigenic sites. The NA genes of H3N2 viruses used for primary infection or vaccination showed higher amino acid homology with H1N2 (88.3-92.6%), while nucleoprotein (95.5-96.3% nucleotide identity) and matrix (96.8-98.4%) genes were most conserved between the three subtypes. Pigs immune as a result of intranasal inoculation with either H1N1 or H3N2 showed partial clinical protection against H1N2 challenge, and nasal virus excretion was 2 days shorter than in naive pigs. Moreover, dually infected (H1N1 + H3N2)-immune pigs showed complete clinical protection and H1N2 virus replication in the lungs and nasal secretions was either undetectable or markedly reduced. In contrast, a double vaccination with a commercial H1N1 and H3N2-based vaccine did not protect against H1N2 challenge. Haemagglutination inhibition (HI) or virus neutralisation (VN) tests of swine sera revealed little if any antigenic cross-reactivity between subtypes. These data suggest that serum HI or VN antibodies are not essential in heterosubtypic protection, but that mucosal or cellular immunity are probably involved. It is still unknown whether this type of cross-subtype protection will also occur in infection-immune pigs in the field.