Interactions between Mycoplasma hyopneumoniae and the porcine immune system
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Mycoplasmal pneumonia of swine (MPS), caused by Mycoplasma hyopneumoniae, has been identified as a major source of loss of revenue for swine producers all over the world. The nature of microscopic lesions of MPS strongly indicates that the immune system is involved in the development of these lesions. Better knowledge about the interactions between M. hyopneumoniae and the porcine immune system should lead to a better understanding of the pathogenesis of MPS;The nonspecific mitogenic effect of M. hyopneumoniae membranes for swine lymphocytes and the possible involvement of this effect in development of lesions in vivo was evaluated by intratracheal inoculation of such membranes into naturally-born, respiratory disease free and Cesarean-derived, colostrum-deprived (CDCD) pigs. Evaluation of peripheral blood lymphocytes (PBL) activity in vitro, using the lymphocyte stimulation test, revealed no difference in the response of control and membrane-inoculated pigs nonspecific mitogens and mycoplasmal antigens. However, bronchial lymph node lymphocytes (BLNL) from membrane-inoculated pigs had a stronger response to M. hyopneumoniae antigens than BLNL from control pigs indicating the reaction of local lymphocytes was antigen-specific. The nonspecific mitogenicity of the membranes was confirmed by using CDCD pigs as experimental animals. Although gross lesions of pneumonia were not detected, microscopic lesions of peribronchiolar and perivascular lymphoid hyperplasia resembling those observed in MPS were induced in conventionally-reared and CDCD pigs following administration of membranes;In studying the various immune responses of pigs following experimental infection with M. hyopneumoniae it was found that the antigen-specific response of BLNL to stimulation by M. hyopneumoniae antigen was stronger and developed more rapidly than the response of PBL. Development of local humoral immunity, indicated by presence of specific antibodies in lung washings (LW) and of immunoglobulin-bearing cells in pneumonic lesions was detected by ELISA and indirect fluorescent antibody test respectively. Although IgA antibodies were predominant in most LW from infected pigs, increasing intensity in the ELISA reactions by IgG antibodies as well as relatively large numbers of IgG-bearing cells were noted in the recovery stage of the infection.