Systemic and mucosal immune responses to H1N1 influenza virus infection in pigs

Influenza is a common respiratory disease in pigs, and since swine influenza viruses are zoonotic pathogens, they also pose human health risks. Pigs infected with influenza virus mount an effective immune response and are protected from subsequent challenge, whereas the currently available, inactivated-virus vaccine does not consistently confer complete protection to challenge. To develop and evaluate new vaccination strategies, it is imperative to fully understand the immune responses that are associated with protection following natural infection. Therefore, we have evaluated the phenotype and kinetics of immune responses to primary and re-challenge infection with H1N1 swine influenza virus in the pig. Through the use of isotype-specific antibody secreting cell ELISPOT assays, interferon-gamma ELISPOT assays and isotype-specific ELISAs on serum, nasal wash and bronchoalveolar lavage samples, we defined the humoral and cellular immune responses, both locally in the respiratory tract and systemically, to this viral infection. Virus-specific serum IgG, IgA, and HI titers all peaked 2-3 weeks after primary infection and did not substantially increase after re-challenge. The predominant virus-specific isotype in serum was IgG. Pigs responded with virus-specific IgG and IgA in both the upper (nasal washes) and lower (bronchoalveolar lavages) airways; IgA was the predominant isotype in both sites. Despite the fact that the pigs were completely protected from re-challenge, the antibody titers in the nasal washes increased. Results of the antibody-secreting cell ELISPOT assays demonstrated that the numbers of both IgG and IgA secreting cells in the nasal mucosa were dramatically higher than in any other tissue examined. In contrast, IFN-gamma secreting cells were predominantly localized to the spleen and tracheobronchial lymph nodes. These data will be helpful in the future development and evaluation of novel vaccines.