INFLUENCE OF EXPERIMENTALLY-INDUCED ENDOGENOUS PRODUCTION OF CORTISOLON THE IMMUNE CAPACITY IN SWINE

Field studies have suggested that 'stressors', such as transportation and mixing, might interfere with the immune competence of pigs. Theref ore, an experimental model was established to study the influence of e levated concentrations of circulating cortisol on the immune capacity in swine. Three experimental groups, with six pigs in each, were immun ized twice, 4 weeks apart, with Mycoplasma hyopneumoniae antigen. Endo genous production of cortisol was induced by intramuscular injection o f adrenocorticotropic hormone (ACTH) twice daily. One group received A CTH during the week before and after the second immunization, one grou p during the week after the second immunization only, while one group served as untreated controls. The treatment with ACTH induced high, bu t physiological, concentrations of cortisol in plasma. Simultaneously, the number of lymphocytes per milliliter blood decreased while the ne utrophil number increased. The elevated concentrations of cortisol als o coincided with reduced proliferation and interleukin-2 production by blood lymphocytes stimulated with the mitogens concanavalin A and phy tohemagglutinin in vitro, while the responses to pokeweed mitogen were less affected. The suppression of mitogen responses was more pronounc ed in cultures of whole blood than in cultures of purified peripheral blood mononuclear cells (PBMC). Antibody production, induced by M. hyo pneumoniae in cultures of purified PBMC was also inhibited by ACTH tre atment. Both the rate of increase and the magnitude of the antibody pr oduction induced by the primary immunization were reduced. In contrast , no effects of ACTH treatment were recorded for the response to the s econd immunization or on the serum levels of antibodies to M. hyopneum oniae. The ability of blood leukocytes to produce interferon-alpha (IF N-alpha) at exposure in vitro to fixed pseudorabies virus adsorbed to porcine kidney cells increased in all animals shortly after the second immunization with M. hyopneumoniae. The influence of cortisol on the IFN-alpha-producing capacity was dependent on whether the test was car ried out in whole blood cultures or in cultures with purified PBMC. Th is finding further emphasizes that the relevance of in vitro assays fo r measuring in vivo phenomena must be carefully scrutinized.