Immunopathology of Actinobacillus pleuropneumoniae infection in swine: Interaction of a secreted cytotoxin with the host immune system

This dissertation examined the role of cytotoxins secreted by Actinobacillus pleuropneumoniae serotypes 1 and 7 in the pathogenesis of disease in porcine and murine models. It examined the production of pathologic lesions in lymphoreticular organs such as the thymus and spleen, as well as in the lung, and the ability of cytotoxin-neutralizing convalescent antisera to prevent these lesions in the murine model. It further examined the ability of serum neutralizing antibodies and mucosal immune response to prevent acute disease in swine. A mucosal immune response was induced by a novel transthoracic intrapulmonary vaccination technique which enhanced specific immunoglobulin A antibodies at the respiratory mucosae and also in the blood serum. The induction of cytotoxin-neutralizing antibodies as well as mucosal immunoglobulins were found to reduce the total volume of lung tissue affected by pathologic lesions after experimental infection. Finally, the interaction of these cytotoxins with host leukocytes and susceptible cell lines was examined in an attempt to identify the cellular binding protein(s) for these cytotoxins. This was accomplished by: (1) reacting iodinated cell surface proteins of susceptible and nonsusceptible cell lines with the cytotoxins and immunoprecipitating toxin/binding-protein complexes with antibodies specific for the cytotoxins; and (2) comparing the cell specificity of these cytotoxins with their ability to specifically label a unique cellular protein on susceptible cells, but absent on non-susceptible cells. This was accomplished by the conjugation of a photoactive cross-linker to the cytotoxins. Cytotoxic culture supernatants from Actinobacillus pleuropneumoniae serotypes 1 and 7 specifically labeled a protein of approximately 100 kilodaltons present on susceptible cells, but failed to label a similar protein on nonsusceptible cells. This research should allow a more complete understanding of the pathogenesis of acute disease; factors which may contribute to the establishment of chronic disease; and also contribute to the understanding of the interaction of Actinobacillus pleuropneumoniae cytotoxins with the host immune system at both the organismal and cellular levels.