Pigs aerogenously immunized with genetically inactivated (ghosts) or irradiated Actinobacillus pleuropneumoniae are protected against a homologous aerosol challenge despite differing in pulmonary cellular and antibody responses.

Aerosol immunization is a safe way to induce complete protection against pleuropneumonia in pigs caused by the lung pathogenic bacterium Actinobacillus pleuropneumoniae. In order to determine the local immune responses of vaccinees in concomitant with protection, lung lining fluid before and 3 weeks after immunization from pigs immunized three times with aerosols of either genetically inactivated ghosts which represent whole cell envelope preparations, or irradiated bacteria were examined following an homologous aerosol challenge. Specific antibody isotypes in the bronchoalveolar lavage were assayed by whole cell ELISAs. Total and relative numbers of cells including lymphocyte subsets were determined. In both vaccinated groups a net influx of plasma cells and lymphocytes, as well as a significant increase of specific IgG occurred. Concurrently, the CD4+/CD8+ ratio was found to increase after aerosol immunization. The lymphocyte subsets of IgG+ and IgA+ cells were found significantly higher in the group immunized with irradiated bacteria when compared to pigs immunized with bacterial ghosts. The latter group showed a significant increase of IgA, IgM, and a net influx of lymphoid blasts and granulocytes in the bronchoalveolar lining fluid. Although differences between the local immune responses of both immunized groups occurred, a significant increase of specific IgG and a net influx of plasma cells and lymphocytes were found to be associated with complete protection against a homologous aerosol challenge infection.