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Respiratory and systemic humoral and cellular immune responses of pigs to a heterosubtypic influenza A virus infection.

The level of heterosubtypic immunity (Het-I) and the immune mechanisms stimulated by a heterosubtypic influenza virus infection were investigated in pigs. Pigs are natural hosts for influenza virus and, like humans, they host both subtypes H1N1 and H3N2. Marked Het-I was observed when pigs were infected with H1N1 and subsequently challenged with H3N2. After challenge with H3N2, pigs infected earlier with H1N1 did not develop fever and showed reduced virus excretion compared with non-immune control pigs. In addition, virus transmission to unchallenged group-mates could be shown by virus isolation in the non-immune control group but not in the group infected previously with H1N1. Pigs infected previously with homologous H3N2 virus were protected completely. After challenge with H3N2, pigs infected previously with H1N1 showed a considerable increase in serum IgG titre to the conserved extracellular domain of M2 but not to the conserved nucleoprotein. These results suggest that antibodies against external conserved epitopes can have an important role in broad-spectrum immunity. After primary infection with both H1N1 and H3N2, a long-lived increase was observed in the percentage of CD8(+) T cells in the lungs and in the lymphoproliferation response in the blood. Upon challenge with H3N2, pigs infected previously with H1N1 again showed an increase in the percentage of CD8(+) T cells in the lungs, whereas pigs infected previously with H3N2 did not, suggesting that CD8(+) T cells also have a role in Het-I. To confer broad-spectrum immunity, future vaccines should induce antibodies and CD8(+) T cells against conserved antigens.