Pneumocystis carinii host origin defines the antibody specificity and protective response induced by immunization

To determine how the known host species-specific antigenic variation of Pneumocystis carinii would affect immune recognition, mice were immunized with either mouse- or ferret-derived P. carinii, with subsequent analysis of the immune response and the ability of the mice to resist infection after immunosuppression and challenge. Immunization with mouse-derived P. carinii produced a strong immune response to mouse but not ferret P. carinii. These mice were completely protected from P. carinii pneumonia when challenged by intratracheal inoculation with mouse P. carinii. In contrast, immunization with ferret P. carinii produced a limited antibody response to mouse P. carinii and had no protective effect. These results show that P. carinii from different host species are immunologically distinct, and any possible use of immunotherapy for P. carinii pneumonia in humans must take into consideration these biologically significant antigenic differences in P. carinii of animal origin.