Effect of Pasteurella multocida toxin on in vivo immune responses in piglets.

Atrophic rhinitis is characterized by a lack of an immune response towards Pasteurella multocida toxin (Pm-T). The effects of intranasal administration of Pm-T on cellular and T-cell dependent antibody responses of piglets against various other antigens were studied in a 3 by 2 factorial arrangement of treatments: three levels of challenge with Pm-T (either once; on three consecutive days; or no Pm-T challenge), and with or without simultaneous immunization with a "cocktail' containing Keyhole limpet haemocyanin (KLH), ovalbumin (OA) and tetanus toxoid (TT). Challenge with Pm-T affected but did not abolish the in vivo humoral immune response against OA, KLH, or TT. Pigs treated once with Pm-T showed lower total antibody responses, caused by lower IgG responses to the antigens. We found no clear consistent effects of Pm-T on cellular immune responses to the various T-cell-dependent antigens in vitro. The lack of immune responsiveness to Pm-T in Pm-T challenged pigs is discussed. The absence of detectable immune responses does not depend on a general suppression of the cellular immune system.