A study of the mechanisms of immunity to Pasteurella haemolytica infection

Immunity to Pasteurella haemolytica was studied using models of P. haemolytica infection in both lambs and mice. Immunisation of specific pathogen-free lambs with a . vaccine containing a sodium salicylate extract of P. haemolytica type A1 protected against challenge with P. haemolytica type A1, superimposed upon infection with parainfluenza type 3 virus. Lymphocyte transformation responses of peripheral blood leucocytes from vaccinated lambs increased markedly after challenge. The passive transfer of hyperimmune serum, produced by repeated vaccination of lambs with the same vaccine, did not protect recipients from challenge. Serum from vaccinated lambs had a bacteriostatic effect upon P. haemolytica in vitro, although this serum failed to facilitate phagocytosis of P. haemolytica by alveolar macrophages in vitro. Two models of P. haemolytica infection in mice were investigated. Intranasal infection of mice with P. haemollytica caused lesions in the lung 3 days later. Bacteria were cleared from the'lung by 48 hours after infection, and because similar lesions were produced by the introduction of heat-killed P. haemolytica, this model was not investigated further. P. haemolytica introduced to mice intraperitoneally in gastric mucin multiplied in the liver and spleen. The effect of vaccination with various extracts of P. haemolytica upon growth in the liver was investigated. Vaccines containing sodium salicylate extracts of P. haemolytica types Al, A6, A8, A11 and T10 protected against challenge with homologous strains, and little cross-protection between the various strains was observed. The model of intraperitoneal challenge of mice was used to investigate the commercial Pasteurella vaccine, Carovax. When given 12 or 24 hours before challenge, the vaccine protected against challenge with P. haemolytica type A1, and also Escherichia coli, indicating that protection was non-specific. When the vaccine was given 1 hour, 48 hours, 7 days or 14 days before challenge with. P. haemolytica type A1, no such protection was observed. An enzyme-linked immunosorbent assay was developed for detection of antibodies to P. haemolytica in the sera of vaccinated mice. There was considerable cross-reaction between sera from mice immunised with sodium salicylate extracts of the different types of P. haemolytica