Effects of active and passive immunization on Mycoplasma pulmonis-induced pneumonia in mice

Parenteral immunization of C3H mice with viable Mycoplasma pulmonis organisms protected them from pneumonia induced by intranasal inoculation of these organisms. Spleen cells obtained from immunized mice were ineffective in preventing syngeneic recipients from developint respiratory disease. In contrast, convalescent-phase serum enhanced the clearance of mycoplasmas from the respiratory tract of mice challenged with a small number of organisms. Further, although 'immune' serum had no detectable effect on the number of mycoplasmas in the respiratory tract of mice challenged with a large number of organisms, such animals did not develop pneumonia. Since the pneumonia appears to be the results of the host's immune response to the mycoplasma, it is suggested that the transferred 'immune' serum may act by suppressing the immune response so that mice develop less severe lung lesions. This suggestion is supported by the observation that the complement-fixing antibody response of passively immunized mice was suppressed.