The overall goal of this project is to develop a vaccine against Chlamydia. Chlamydia trachomatis (Ct) is an obligate intracellular gram-negative organism and the most common cause of bacterial sexually transmitted infections (STIs) worldwide. Ct infections in women cause long-term sequelae i.e. pelvic inflammatory disease (PID), ectopic pregnancy and infertility, and are often associated with other STIs (HIV, HPV, gonorrhea and syphilis). Antibiotics do not prevent recurring infections. Development of a vaccine against Ct is a largely unmet need. Previous whole organisms-based vaccines only conferred limited protection from Ct ocular infections, with short-lived, serovar/subgroup-specific immunity and hypersensitivity reactions in some vaccinated individuals. The Chlamydia major outer membrane protein, MOMP, is a prime antigen candidate for a subunit vaccine. MOMP, a trimeric porin with 8 surface exposed loops, contains regions of sequence variability (variable domains, VDs) within loops 2, 3, 5 and 6 and regions of constant amino acid sequences (constant domains, CDs) flanking the VDs. The VDs and CDs contain B-cell and T-cell epitopes that elicit neutralizing antibodies and CD4+ T cell responses responsible for protective cellular immunity. The MOMP VDs are unique for each serovar and, if combined in a single vaccine, could induce broad coverage against all isolates. Development of a MOMP-based vaccine, however, presents barriers for production and use in humans. To fill the current gaps, in our previous R03 AI123885, we engineered novel recombinant chimeric antigens inserting the whole loops 2, 3, 5 and 6 of the C. muridarum (Cm) MOMP into a carrier protein structurally similar to MOMP, the PorB porin fromthe human commensal Neisseria lactamica. The recombinant PorB/VD antigens are suitable for recombinant production in a folded conformation, are immunogenic, induce cross-reactive antibodies to recombinant MOMP, native MOMP and whole Cm and are protective in a mouse model of Cm respiratory infection. Our approach addresses the shortcomings of MOMP production without hindering induction of functional, protective host immune responses to Chlamydia. The hypothesis is that equivalent Ct MOMP-based constructs, appropriately adjuvanted, will elicit protective immune responses against Ct infection and disease, which can be tested in a mouse model of transcervical Ct challenge. The Aims are 1) To design, express and purify Ct MOMP serovar F-based PorB/VD1- 3, PorB/VD1-4 and PorB/VD1-2-4 and examine structure properties of the recombinant constructs; 2) To define humoral and cellular immune responses to the PorB/VD constructs in mice using adjuvants that are suitable for use in humans without reactogenic responses and 3) To evaluate protection against Ct serovar F in the mouse model of Ct transcervical challenge. The results of our studies will fill a critical unmet need for improving public health worldwide and provide the groundwork for a broadly pr...