Modulation of the humoral immune response of swine and mice mediated by toxigenic Pasteurella multocida.

Progressive atrophic rhinitis is an upper respiratory tract disease of pigs caused by toxigenic strains of the bacterium Pasteurella multocida. In this study the effect of P. multocida on the humoral immune response of pigs and mice was investigated. Pigs were given live intranasal challenge with either a toxigenic strain or a non-toxigenic strain of P. multocida, or were given daily intranasal instillation of a cell-free lysate of the toxigenic strain. Mice were given a live intranasal challenge of either a toxigenic or a non-toxigenic strain of P. multocida. All of the animals were immunised with ovalbumin and serum concentrations of anti-ovalbumin antibodies were quantified and compared between different treatment groups and control animals. Intranasal challenge with toxigenic P. multocida caused a significant reduction in the levels of anti-ovalbumin IgG in both species. A similar effect was seen in pigs given a cell-free extract of toxigenic P. multocida. Whilst the mechanism of this suppression is unclear, we surmise that immunomodulation of the host is an important virulence factor for toxigenic P. multocida, and could be an important function of the toxin. This immunomodulatory effect may enhance colonisation of P. multocida aiding horizontal transmission and may predispose to concurrent infection with other potential pathogens.