Protective Effect of Vaccines in Experimental Mycoplasma pneumoniae Disease

The mechanisms of immunity to Mycoplasma pneumoniae were investigated by evaluating different vaccination procedures in an experimental animal model. Hamsters were immunized by intranasal inoculation of broth cultures or by parenteral injections of saline-suspended organisms. All vaccinees received a standardized intranasal challenge which produced pneumonia in 94% of controls. Intranasal immunization with virulent organisms produced a 71% reduction in pneumonia. Subcutaneous and intraperitoneal inoculation of the same organisms yielded 56 and 61% reductions, respectively. Animals similarly immunized with an attenuated strain developed resistance to pneumonia only after intranasal infection. Serum antibody levels did not correlate with protection. Growth-inhibiting activity was demonstrated in bronchial washings of challenged animals, suggesting the development of local antibody in response to infection with M. pneumoniae. Crosschallenge studies were performed in animals vaccinated intranasally with virulent and avirulent variants of the same strain. The avirulent vaccine prevented pneumonia in animals challenged with homologous virulent organisms but not in those receiving an unrelated strain; the virulent vaccine provided protection to both homologous and heterologous challenge. These studies indicate that organism strain variation as well as vaccination technique are important determinants of the immune response to M. pneumoniae.