PROJECT SUMMARY Chlamydia trachomatis (Ct) is the most common bacterial sexually transmitted infection worldwide and causes serious medical complications in women, including pelvic inflammatory disease, ectopic pregnancy, and infertility. Therefore, a vaccine is urgently needed. One obstacle to successful vaccine development is a lack of a comprehensive understanding of the protective immune response to Ct infection, including the role of epitope-specific antibodies. The overall objective of this fellowship proposal is to finely map the antibody response in women with an acute history of Ct infection, determine immunodominant epitopes of Ct antigens, investigate differential antibody responses between women with a positive Ct infection outcome (defined as no Ct reinfection at 3-months follow-up or spontaneous resolution of Ct infection without antibiotic treatment) and women who experience Ct reinfection, and determine the functionality and protective capacity of these antibody responses. The central hypothesis to be tested is that antibody responses will differ between patients who had a positive Ct infection outcome than those who experienced a reinfection of Ct and that these epitope- specific antibodies to adhesion factors of Ct will play an important role in protection (binding to Ct-infected cells, neutralization, antibody-mediated neutrophil killing, and protection from a Ct challenge in murine models). The long-term goal is to inform vaccine design through better understanding of protective immune responses, culminating in an effective Ct vaccine. The following specific aims will be used to test the hypothesis: Specific Aim 1: Define the natural serum antibody response to urogenital Ct infection in women. Utilizing Deep Sequence-Coupled Biopanning, a novel technology that uses a small amount of human sera to finely map the antibody response at an epitope-level, I will elucidate the natural antibody response to Ct infection. This will empirically determine immunodominant epitopes and B cell epitopes to 24 Ct antigens. I will also define differential antibody responses between women who experienced a positive Ct infection outcome compared to those who experienced a Ct reinfection within 3-months of antibiotic treatment. Specific Aim 2: Investigate Ct adhesion factor epitope-specific antibody functions. Antibodies to immunodominant epitopes and differential antibody response between patient cohorts (SA1) will be tested for functionality to determine protective capacity against Ct infection. Using in vitro techniques, I will determine if these epitope-specific antibodies can bind to Ct, have neutralizing capacity, and can function in antibody- mediated neutrophil Ct killing. I will also determine if these antibodies can protect against a Ct infection in a murine model by monitoring Ct infection, bacterial shedding, Ct clearance, and upper genital tract pathology. Together, this proposal will investigate the specificity, functionality, and role of epitope-specific antibodies in protection against Ct infection and has the potential to significantly advance the Ct field by understanding protective immune responses and informing future vaccine design.

PROJECT NARRATIVE Chlamydia trachomatis (Ct) is the most common bacterial sexually transmitted infection worldwide and infection can result in long-term medical consequences for women, including pelvic inflammatory disease, ectopic pregnancy, and infertility. The prevalence of Ct infection has not sufficiently decreased, despite effective antibiotic treatment, suggesting that a vaccine is urgently needed. This proposal will investigate the specificity, functionality, and role of epitope-specific antibodies in protection against Ct infection to inform future vaccine design and development.