Mice with disrupted ␤ 2-microglobulin (␤ 2 m ؊/؊), I-A (class II ؊/؊), or CD4 (CD4 ؊/؊) genes were examined for their capacity to resolve Chlamydia trachomatis genital tract infection. C57BL/6 and ␤ 2 m ؊/؊ mice resolved infection similarly and were culture negative by 4 to 5 weeks following infection. Conversely, major histocompatibility complex (MHC) class II ؊/؊ mice failed to resolve infection, and CD4 ؊/؊ mice showed a significant delay (2 weeks). Secondary challenge of C57BL/6, ␤ 2 m ؊/؊ , and CD4 ؊/؊ mice established that acquired protective immunity, which was characterized by an infection of shortened duration and reduced shedding of infectious organisms, developed. Serological analysis of C57BL/6 and ␤ 2 m ؊/؊ mice by enzyme-linked immunosorbent assays revealed no striking differences in the immunoglobulin subclass specificity of the anti-Chlamydia response, although some differences were observed in the magnitude of the immunoglobulin G2a (IgG2a) and IgG2b responses. Class II ؊/؊ mice produced lower-titered serum anti-Chlamydia antibodies of all isotypes. The serum antibody responses of CD4 ؊/؊ mice were similar to those of C57BL/6 mice, except that the anti-Chlamydia IgA response was delayed by approximately 3 weeks. Analysis of vaginal washes for Chlamydiareactive antibodies revealed the presence of IgG2a, IgG2b, and IgA in C57BL/6 and ␤ 2 m ؊/؊ mice and primarily of IgA in CD4 ؊/؊ mice. Vaginal washes from class II ؊/؊ mice were consistently antibody negative. Interestingly, the Chlamydia-specific IgA response in the vaginal washes of CD4 ؊/؊ mice was delayed, but its appearance coincided with decreased shedding of infectious organisms and resolution of infection. Our results demonstrate that MHC class II-restricted T-cell responses are necessary for the development of protective immunity to Chlamydia genital tract infection and that local (vaginal) anti-Chlamydia IgA antibody coincides with the resolution of infection. A substantive role for MHC class I-restricted T-cell responses in protective immunity to Chlamydia genital tract infection was not confirmed.

The importance of antibody-mediated immunity in primary and secondary Chlamydia trachomatis genital tract infections was examined by using a definitive model of B-cell deficiency, the MT/MT gene knockout mouse. Vaginally infected B-cell-deficient MT/MT mice developed a self-limiting primary infection that was indistinguishable from infection of control C57BL/6 mice. Sera and vaginal secretions from infected mice were analyzed for anti-Chlamydia antibodies. C57BL/6 mice produced high-titered serum anti-Chlamydia immunoglobulin G2a (IgG2a), IgG2b, and IgA antibodies, and vaginal washes contained predominately anti-Chlamydia IgA. Serum and vaginal washes from infected B-cell-deficient mice were negative for anti-Chlamydia antibody. T-cell proliferation and delayed-type hypersensitivity assays were used as measures of Chlamydia-specific cell-mediated immunity and were found to be comparable for C57BL/6 and B-cell-deficient mice. Seventy days following primary infection, mice were rechallenged to assess acquired immunity. B-cell-deficient mice which lack anti-Chlamydia antibodies were more susceptible to reinfection than immunocompetent C57BL/6 mice. However, acquired immune resistance was evident in both strains of mice and characterized by decreased shedding of chlamydiae and an infection of shorter duration. Thus, this study demonstrates that cell-mediated immune responses alone were capable of resolving chlamydial infection; however, in the absence of specific antibody, mice were more susceptible to reinfection. Therefore, these data suggest that both humoral and cell-mediated immune responses were important mediators of immune protection in this model, though cell-mediated immune responses appear to play a more dominant role.

Mucosal immunity to Chlamydia trachomatis in a mouse model of female genital tract infection is mediated predominantly by Th1-type cells, as shown by in vivo neutralization of cytokines involved in the Th1 vs Th2 pathways. Neutralization of IL-12 was associated with an apparent decrease in the infiltration of CD4+ T cells into infected tissues, systemic reductions in the production of IFN-gamma, and prolonged shedding of high levels of bacteria. Neutralization of IL-4 had no detectable effect on host immunity or on bacterial clearance. To dissociate the protective role of IL-12 from that of IL-12-induced IFN-gamma, resistance to C. trachomatis was compared in IL-12-depleted and IFN-gamma-deficient animals. IL-12-depleted mice displayed minimal bacterial clearance for 1 mo post-infection but eventually resolved genital tract infections completely. IFN-gamma-deficient mice, on the other hand, cleared 99.9% of genital Chlamydia within the first 3 wk but then developed systemic disease associated with dissemination of bacteria to multiple organs. Animals surviving this stage often maintained low level persistent infections within the urogenital tract. These results indicate that the bulk of chlamydial clearance from the genital mucosa is mediated by an IL-12-dependent, IFN-gamma-independent mechanism, while prevention of disseminated disease requires the action of IFN-gamma.