Most humans lack virus neutralizing (VN) and haemagglutination inhibition (HI) antibodies to H5N1 avian
influenza viruses (AIVs), but cross-reactive neuraminidase inhibition (NI) antibodies and cell-mediated
immune (CMI) responses are common. These immune responses result largely from infections with seasonal
human H1N1 influenza viruses, but the protective effect of H1N1 infection-immunity against H5N1
infection has never been examined. To this purpose, we have used the pig model of influenza and a
low pathogenic (LP) H5N1 AIV. Pigs were inoculated intranasally with sw/Belgium/1/98 (H1N1) 4 weeks
before challenge with duck/Minnesota/1525/81 (H5N1). While the viruses failed to cross-react in HI and
VN tests, the H1N1 infection induced high levels of H5N1 cross-reactive NI antibodies. Cross-reactive CMI
was demonstrated by measurements of lymphoproliferation and IFN- secretion after in vitro restimulation
of peripheral blood mononuclear cells. All control pigs showed clinical signs and H5N1 virus isolation
from the respiratory tract post-challenge. The H1N1-immune pigs, in contrast, showed a complete clinical
protection and only 3 pigs out of 10 were H5N1 virus-positive. In a second and smaller experiment, H1N1
virus infection also conferred cross-protection against a LP H5N2 AIV, while cross-reactive immunity was
solely detected in tests for CMI. Our data further support the notion that immunity induced by seasonal
human H1N1 influenza virus infection may provide some protection against H5N1 or other H5 AIVs in
the absence of neutralizing H5 antibodies. Further studies should reveal whether cross-protection holds
against H5N1 viruses that are better adapted to replicate in mammals or with a more distantly related
N1.