Expression library immunization confers partial protection against Chlamydia muridarum genital infection

Protective sequences of Chlamydia muridarum were identified as potential vaccine candidates by screening a genomic DNA expression library and assessing the immune responses of mice immunized with individual library clones following vaginal challenge with live Chlamydia. Groups of female BALB/c mice were immunized intra-abdominally by gene gun delivery of DNA three times at three-weekly intervals with individual library clones expressing chlamydial protein fragments and humoral and cell-mediated immune responses were evaluated. Chlamydia-specific cytokines including tumour necrosis factor-α (TNF-α) interleukin-10 (IL-10), interleukin-4 (IL-4), interleukin-12 (IL-12) and interferon-γ (IFN-γ) were detected in mice immunized either with selected DNA clones in spleen cells (0.2–135.2 pg/mL) or lymph nodes (0.15–84.9 pg/mL). The most protective antigen identified was TC0512, a putative outer membrane protein (OMP). Immunization of mice with this clone elicited T-helper type-1 (Th-1) and T-helper type-2 (Th-2) cytokines as well as and IgG1 and IgG2a in sera of these animals. Ten days after the last immunization, animals were challenged intra-vaginally with 5 × 104 inclusion-forming units (IFUs) of C. muridarum. At 9 days following challenge TC0512 showed a 73% reduction in the number of recoverable Chlamydia compared with vector only immunized controls. Six additional clones were identified that also conferred varying degrees of protection against live chlamydial challenge. Significant protection against the initial stages of infection was shown by two DNA clones (encoding hypothetical proteins) and five clones showed enhanced clearance of chlamydial infection following DNA immunization and live chlamydial challenge. These results demonstrate that the C. muridarum genome can be screened for individual vaccine candidates by genetic immunization and that the screen produces novel and partially protective vaccine candidates.