IMMUNE-RESPONSES OF PIGLETS TO PASTEURELLA-MULTOCIDA TOXIN AND TOXOID

Experimental atrophic rhinitis (AR), serum antibody titres and in vitr o lymphoproliferation to Pasteurella multocida derived toxin (Pm-T) we re studied in piglets. Specific immune responses to Pm-T and Pm-T indu ced conchae atrophy were compared with AR immunity. This immunity was initiated by the Nobi-VAC(R) AR-T vaccine administered at various time s with respect to Pm-T challenge. Animals challenged with Pm-T develop ed conchae atrophy, but no antibodies nor cellular immune responses to Pm-T were detected. Vaccination 3 weeks before Pm-T challenge protect ed pigs against breakdown of nasal bony tissues. This protection was a ccompanied by an increase of serum antibodies and in vitro lymphoproli feration to Pm-T. Animals vaccinated 10 days before or after Pm-T chal lenge also had antibodies and cellular immune responses. However, thes e animals developed AR. In vitro, Pm-T appeared mitogenic for quiescen t (non-immune) peripheral lymphocytes and Concanavalin A stimulated ly mphocytes from some pigs. These in vitro lymphoproliferative responses could be partly abrogated by the addition of monomorphic anti-swine m ajor histocompatibility complex class II Do and DR specific monoclonal antibodies. We conclude that Pm-T is poorly immunogenic in vivo and d oes not initiate a protective Pm-T specific immune response. Pigs were protected from AR by vaccination, but protection was dependent on the timing of vaccine administration. We speculate that Pm-T modifies the immune response such that the response is not directed towards the to xin but to an unidentified component in the nose of piglets.