Protective immunity conferred by the C-terminal fragment of recombinant Pasteurella multocida toxin

Pasteurella multocida serogroup D, producing P. multocida toxin (PMT), is a causative pathogen of progressive atrophic rhinitis (PAR) in swine. To evaluate the protective immunity and vaccination efficacy of the truncated form of PMT, a C-terminal form of recombinant PMT (designated PMT2.3; amino acid residues 505 to 1285 of PMT) was expressed in an Escherichia coli expression system, and the humoral and cellular immune responses to PMT2.3 were investigated. PMT2.3 vaccination in mice led to high levels of the anti-PMT antibody with a high neutralizing antibody titer. PMT2.3 also induced a cellular immune response to PMT, as demonstrated by the lymphocyte proliferation assay. Furthermore, strong protection against a homologous challenge with P. multocida was also observed in mice vaccinated with PMT2.3. In PMT2.3 vaccination in swine, high levels of serum antibody titers were observed in offspring from sows vaccinated with PMT2.3. Offspring from sows vaccinated with PMT2.3 or toxoid showed a good growth performance as depicted by mean body weight at the time of sacrifice, as well as in average daily gain in the postweaning period. Low levels of pathological lesions in turbinate atrophy and pneumonia were also observed in these offspring. Therefore, we consider PMT2.3--in the truncated and nontoxic recombinant PMT form--to be an attractive candidate for a subunit vaccine against PAR induced by P. multocida infection.