Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 7, 2011
Publication Date: December 15, 2011
Citation: Mullins, M.A., Register, K.B., Bayles, D.O., Butler, J.E. 2011. Haemophilus parasuis exhibits IgA protease activity but lacks homologs of the IgA protease genes of Haemophilus influenzae. Veterinary Microbiology. 153(3-4):407-412. Interpretive Summary: Haemophilus parasuis causes Glasser's disease in swine and has a significant impact on swine health and production. It has been estimated to result in a loss of over $150 million per year to the U.S. pork industry on the basis of mortality alone; actual losses additionally include the costs of treatment and delayed time to market. While some strains are highly virulent, others infect pigs without causing clinical signs of disease. The mechanisms used by this bacterium to cause disease in pigs are not understood and, as a consequence, effective vaccines that target virulent strains are lacking. A closely related bacterial pathogen that infects humans uses a specialized protein to inactivate antibodies important for protecting the mucosal surfaces against infection by microbes. It is not known whether H. parasuis makes a similar protein. In this study we demonstrate that invasive strains of H. parasuis can inactivate swine mucosal antibody, suggesting this may be an important virulence factor in Glasser's disease. This is the first report to identify such activity in any animal pathogen. Our results provide novel information to scientists attempting to define the attributes of H. parasuis that contribute to invasive disease and may assist in the design of improved vaccines that specifically target disease-causing strains.
Technical Abstract: Haemophilus parasuis, the bacterium responsible for Glasser's disease, is a pathogen of significant concern in modern high-health swine production systems but there is little information regarding the identity or function of its virulence factors. Several important human mucosal pathogens, including the closely related bacterium Haemophilus influenzae, utilize IgA proteases to aid in defeating the host immune response and facilitate disease but it is unknown whether H. parasuis synthesizes any product with IgA protease activity. To better understand the mechanisms of Glasser's disease in swine, we attempted to detect, compare, and analyze the activity of IgA proteases in H. parasuis. Amplicons of the expected size were obtained by PCR from H. influenzae DNA using primer pairs specific for the iga gene, which encodes the human IgA1 protease. However, no amplicons were generated with any iga primer set from the DNA of five H. parasuis strains tested. DIG-labeled probes derived from iga of H. influenzae failed to detect a homolog in H. parasuis using either DNA dot blots or Southern blots. A second IgA1 protease gene, igaB, present in approximately one-third of H. influenzae strains, was also not detectable in H. parasuis. Neither iga nor igaB could be identified in the genomes of recently sequenced strains of H. parasuis. Western blotting, however, clearly demonstrated cleavage of swine IgA, but not human IgA1, following incubation with culture supernatants of some strains of H. parasuis. These results suggest that an IgA protease of H. parasuis attacks the swine IgA heavy chain at a site not found in human IgA1.