IMMUNIZATION STRATEGIES FOR THE CONTROL OF MANNHEIMIA HAEMOLYTICA PNEUMONIA IN BIGHORN SHEEP
Batra, Sai Arun
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Mannheimia haemolytica is an important pathogen of pneumonia in bighorn sheep (BHS, Ovis canadensis). Leukotoxin is the critical virulence factor of this bacterium. In an earlier study, an experimental vaccine containing leukotoxin and surface antigens of M. haemolytica protected BHS against challenge, but required multiple booster injections. Vaccination of wild BHS is difficult. But they can be vaccinated at the time of transplantation into new habitats. Administration of booster doses, however, is impossible. Therefore, a vaccine that does not require booster doses is the logical choice. We evaluated the potential of a modified live vaccine and a herpesvirus-vectored vaccine to protect BHS against M. haemolytica. M. haemolytica leukotoxin is encoded as an inactive protoxin by lktA. Acylation of the protoxin by lktC-encoded trans-acylase is believed to be necessary for its biological effects. Contrary to our expectation, an LktC mutant was lethal to BHS. Further analysis revealed that un-acylated leukotoxin is cytotoxic to BHS PMNs, albeit to a less degree than wild-type toxin. These results demonstrate that acylation is not necessary for cytotoxic activity of M. haemolytica Lkt, but enhances its potency. Bovine herpesvirus 1 (BHV-1), undergoes latency and reactivation in infected cattle. Hence BHV-1 engineered to express the protective immunogens of M. haemolytica is likely to express these immunogens every time it undergoes reactivation, simulating booster injections in infected animals. First, the ability of BHV-1 to infect, establish latency and reactivate in BHS was demonstrated. Then a chimeric protein comprising the leukotoxin-neutralizing epitopes and immuno-dominant epitopes of PlpE was constructed. Mice immunized with the chimeric protein expressed in mammalian cells developed leukotoxin-neutralizing and anti-surface antigen antibodies. BHV-1 was then engineered to express the chimeric protein by replacing the gene encoding glycoprotein gC with the gene encoding the chimeric protein. Intra-nasal administration of the recombinant virus induced variable levels of leukotoxin-neutralizing and anti-surface antigen antibodies in BHS. The antibodies induced, however, were inadequate to protect BHS from M. haemolytica challenge. These data indicate that BHV-1 is a suitable vector for immunization of BHS, but additional experimentation with the inserts are necessary to enhance the immune response to the M. haemolytica antigens.