CD4+ T Cells and Antibody Are Required for Optimal Major Outer Membrane Protein Vaccine-Induced Immunity to Chlamydia muridarum Genital Infection

Despite effective antimicrobial chemotherapy, control of Chlamydia trachomatis urogenital infection will likely require a vaccine. We have assessed the protective effect of an outer membrane protein-based vaccine by using a murine model of chlamydial genital infection. Female mice were first vaccinated with Chlamydia muridarum major outer membrane protein (MOMP) plus the adjuvants CpG-1826 and Montanide ISA 720; then they were challenged with C. muridarum. Vaccinated mice shed 2 log₁₀ to 3 log₁₀ fewer inclusion-forming units (IFU) than ovalbumin-vaccinated or naïve animals, resolved infection sooner, and had a lower incidence of hydrosalpinx. To determine the relative contribution of T cells to vaccine-induced protection, mice were vaccinated, depleted of CD4⁺ or CD8⁺ T cells, and then challenged vaginally with C. muridarum. Depletion of CD4⁺ T cells, but not depletion of CD8⁺ T cells, diminished vaccine-induced protection, with CD4-depleted mice shedding 2 log₁₀ to 4 log₁₀ more IFU than CD8-depleted or nondepleted mice. The contribution of antibodies to vaccine-induced protection was demonstrated by the absence of protective immunity in vaccinated B-cell-deficient mice and by a 2 log₁₀ to 3 log₁₀ decrease in bacterial shedding by mice passively administered an anti-MOMP serum. Thus, optimal protective immunity in this model of vaccine-induced protection depends on contributions from both CD4⁺ T cells and antibody.New cases of sexually transmitted diseases number more than 340 million worldwide annually and pose a formidable health risk to infected individuals (67-69). It is estimated that Chlamydia trachomatis, the causative agent of chlamydia, is responsible for more than 92 million of these cases. In the United States, where C. trachomatis infections are the infections most commonly reported to the Centers for Disease Control and Prevention, there are more than 4 million new cases each year (14, 67). As a bacterial agent of infection, C. trachomatis can be eradicated efficiently with appropriate antibiotic treatment, but more than 50% of infected individuals are asymptomatic and therefore lack the impetus to seek treatment (14). When left untreated, infection in women can lead to pelvic inflammatory disease, ectopic pregnancy, and tubal factor infertility and can cause severe and sometimes irreparable damage to the reproductive organs (14, 67). To combat the high rate of infection and disease, the development of an efficacious vaccine is critical.Trachoma vaccine trials using whole organisms in the 1950s and 1960s had mixed results, with some studies inducing only partial, serovar-specific, short-lived immunity (4). In one study, a subset of vaccine trial participants experienced an increased incidence of disease and exacerbated pathology relative to that of their unvaccinated counterparts upon reexposure to chlamydiae, which led many researchers to abandon the use of whole organisms in immunizations (7, 8). Since then, no other human vaccine trials targeting ocular or urogenital C. trachomatis infections have been published. Instead, researchers have focused their efforts on animal models of ocular and genital infection.To this end, the murine model of chlamydial genital infection, which closely mimics acute genital infection in women, has been employed extensively for the study of the immunological parameters of infection and for vaccine development. Mice infected with C. muridarum naturally resolve infection in approximately 4 weeks and develop long-lived adaptive immunity that markedly protects against reinfection (3, 37). Infection elicits Chlamydia-specific CD4⁺ T cells, CD8⁺ T cells, and antibody, but only CD4⁺ T cells are necessary for resolution of the primary infection (42). In contrast, immunity to reinfection is dominated by both protective CD4⁺ T cells and antibody (38). Clearance and immunity are highly dependent on a Th1-type response, specifically gamma interferon (IFN-γ)-secreting CD4 cells (9). On the other hand, Th2 responses are associated with scarring and immunopathology (62). For example, antibody responses dominated by IgG1 are not protective and may be associated with an increase in pathology, whereas anti-chlamydial antibodies of the IgG2a and IgG2b isotypes are associated with a protective response (51).Using knowledge of the protective response gleaned from the murine model of infection-induced immunity, investigators have made modest strides toward the development of an efficacious vaccine. Studies utilizing whole elementary body (EB) immunization have induced significant protection, though most of these studies have limited real-world application. One notable example that induced almost sterilizing immunity involved the passive transfer of dendritic cells pulsed ex vivo with nonviable chlamydiae (65). Subunit antigen vaccines represent the bulk of vaccine studies, and vaccines based on combinations of a number of chlamydial antigens, adjuvants, and delivery systems have had various degrees of success in preventing infection (8, 22, 62). Chlamydial antigens, including secreted proteins, such as chlamydial protease-like activity factor (CPAF) (16, 33, 43-46), and membrane associated proteins, such as PorB (26, 30) and IncA (33), have also been used in subunit vaccines; however, the vast majority of studies have focused on the major outer membrane protein (MOMP), an immunodominant antigen in both human and animal studies (22, 62). Novel delivery systems, including Vibrio cholerae ghosts and cationic liposomes, have been introduced into chlamydial vaccine research, and while initial studies have shown incomplete protection, these systems may have the potential to elicit protective responses against chlamydial genital infection when used in conjunction with appropriate antigens (2, 19, 20, 23).Despite substantial effort, no vaccine licensed for human use is currently available. Recently, a MOMP-based vaccine utilizing the adjuvants CpG-1826 and Montanide ISA 720, which together drive a strong Th1-type response, has been shown to confer considerable protection when mice are challenged directly in the upper genital tract with C. muridarum (51). In our current study, we sought to determine if this vaccine protected against vaginal challenge (the natural route of infection) and to evaluate the contributions of T cells and antibody to the vaccine-induced protective response. We found that the MOMP vaccine conferred significant protection against vaginal challenge and protected against infection-induced pathology (hydrosalpinx). Furthermore, optimal protection was dependent on both CD4⁺ T cells and antibody.