Characterization of immunity to Pasteurella multocida
The immunity produced by Pasteurella multocida vaccination is not well-understood. A series of in vivo and in vitro tests was used to characterize the immunity induced by vaccination with an inactivated P. multocida bacterin;In vivo tests. Passive protection experiments demonstrated that antiserum obtained from vaccinated turkeys provided temporary protection against challenge infection in nonvaccinated turkeys. Newly hatched chickens and turkeys were treated with cyclophosphamide, or by surgical bursectomy in a manner designed to suppress antibody response but leave cell-mediated responses intact. The birds were vaccinated and challenge infected with virulent P. multocida. Antibody response was found to be of major importance in protection;Chickens were treated with niridazole, an immunosuppressant that does not affect antibody response but does suppress cell-mediated response. The chickens were vaccinated and challenge infected with virulent P. multocida. Cell-mediated immunity did not appear to be of major importance in resistance to challenge infection;Birds were tested for the presence of delayed type hypersensitivity. Vaccinated chickens did not develop skin test reactions to P. multocida extracts. It was not possible to demonstrate conclusively that delayed type hypersensitivity was induced by vaccination;In vitro tests. The lymphocyte transformation assay, and the leukocyte migration inhibition test were used to test for the presence of cell-mediated immunity in vaccinated chickens. Leukocytes were isolated from peripheral blood by dextran sedimentation, or from spleens by ficoll-paque density gradient centrifugation. Lymphocyte transformation was measured by ('3)H-methyl-thymidine incorporation. Lymphocytes from P. multocida vaccinated chickens underwent blast transformation when cultured in the presence of sonicate soluble fraction or capsular material from P. multocida. However, stimulation indices rarely exceeded four indicating that the cellular response was minimal. The leukocyte migration inhibition test did not indicate the presence of cell-mediated immunity to P. multocida;The results indicated that there is a correlation between the presence of agglutinating antibody and resistance to challenge infection. Cell-mediated response does not appear to play a major role in immunity to challenge infection. It was concluded that the development of P. multocida vaccines should be oriented toward producing a sustained antibody response rather than a cellular response.