IMMUNITY AND VACCINE DEVELOPMENT IN PASTEURELLA-MULTOCIDA INFECTIONS

The role of LPS in immunity was studied using monoclonal antibodies (M Abs) and active immunisation experiments. A panel of six MAbs produced against Pasteurella multocida serotype B:2 reacted with the LPS of se rotypes B:2 and B:5, but not with other serotypes. The MAbs could opso nise P. multocida for phagocytosis by mouse macrophages, but were not bactericidal in the presence of complement. They conferred only partia l passive protection in mice. Similar results showing only partial pro tection were obtained when purified LPS was used to actively immunise mice prior to challenge, suggesting that LPS plays a partial role in i mmunity to infection. The aroA gene from P. multocida serotypes A:1 an d A:3 was cloned and inactivated by insertion of a kanamycin resistanc e gene. The mutated gene was re-introduced onto the chromosome by alle lic exchange. The resultant aroA mutants were highly attenuated in a m ouse model system, with a 6-log decrease in ID50. Virulence could be r estored by complementation with a functional aroA gene. Mice immunised with two doses of the live mutants were protected against lethal chal lenge with the homologous parental strain, but not against the heterol ogous strain. P. multocida A:1 and A:3 expressed unique proteins when grown in iron-restricted medium. Moreover, the outer membrane (OM) fra ctions of these cells contained novel proteins of 75 kDa, 85 kDa and 9 4 kDa molecular mass. Mice were immunised with OM fractions prepared f rom serotype A:3 grown in iron-restricted (OM Fe-) or iron-replete (OM Fe+) media. When low challenge doses were used, both immunogens prote cted mice against serotype A:3, but only the OM Fe- fraction protected mice against heterologous challenge with serotype A:1. When higher ch allenge doses were used, only partial protection was observed.