Acquired immunity to Chlamydia pneumoniae is dependent on gamma interferon in two mouse strains that initially differ in this respect after primary challenge.

The role of gamma interferon (IFN-gamma) in a Chlamydia pneumoniae mouse model was studied by in vivo neutralization in two inbred mouse strains. During primary C. pneumoniae infection, neutralization of IFN-gamma increased both the numbers of bacteria and the pneumonia score in the lungs of C57BL/6 mice but not BALB/c mice. During reinfection, the bacterial counts in the lungs were increased by IFN-gamma neutralization in both mouse strains. Thus, the effect of IFN-gamma neutralization was dependent on the genetic background in primary infection. However, IFN-gamma appeared to be equally important in both mouse strains during reinfection.

Immunity to Chlamydia pneumoniae induced by vaccination with DNA vectors expressing a cytoplasmic protein (Hsp60) or outer membrane proteins (MOMP and Omp2).

Immune responses induced by intramuscular DNA immunization with Chlamydia pneumoniae genes coding for the major outer membrane protein (MOMP), cysteine-rich outer membrane protein 2 (Omp2) or the heat shock protein 60 (Hsp60) were studied. BALB/c mice were vaccinated intramuscularly three times at 3-week intervals and challenged intranasally 2 weeks after the last injection. Immunization with pmomp or phsp60 showed 1.2-1.5 log reduction in the mean lung bacterial counts after the challenge. Specific antibodies were detected only in sera of the mice immunized with pomp2 and phsp60. Although immunization with pomp2 resulted in a strong serum antibody response against Omp2 protein, it failed to protect the mice. Immunization with any of the three vaccines did not reduce the severity of histologically assessed pneumonia, but resulted in significantly higher lymphoid reaction in the lung indicating immunological memory.