Recovery of C. pneumoniae from the lung 14 and 28 days after intranasal inoculation of 5x105IFU 6-month-old (young) or 20-month-old (aged) C57BL/6 mice were immunized with 100 μg of Cpn CTL7 (vaccine) or VR1012 plasmid DNA (vector) and challenged intranasally with 5x105 IFU C. pneumoniae 12 days after completion of the immunization protocol. Mice were euthanized 14 days (A) or 28 days (B) after infection, the lungs were removed, and lysates prepared as described in Materials and Methods. Viable organisms were recovered and quantified by immunofluorescent staining using FITC-conjugated Chlamydia-specific antibody (Fitzgerald 61-C75). The groups of experimentally infected and uninfected age-matched control mice are listed on the X-axis and the number of inclusion forming units/ml of 10 % weight/volume homogenate (log10) is displayed on the Y-axis. Each dot represents the concentration (IFU/ml) of C. pneumoniae recovered from the organ homogenate of an individual mouse. The red bars indicate the geometric mean of all animals in each group (log10) and the red arrow indicates the limit of C. pneumoniae in the detection system. The “**” symbols indicate a statistically significant difference (p = 0.025) between the geometric mean of respiratory titers of aged, vaccinated mice and aged mice receiving vector alone.