PROJECT SUMMARY/ABSTRACT Chlamydia trachomatis is the obligate intracellular bacterium that causes the most prevalent STI worldwide. It is estimated that over 1.6 million of new cases of Chlamydia occurred in 2021 in the United States. Although most Chlamydia infections are asymptomatic, “silent” infections can lead to severe sequelae such as pelvic inflammatory disease, ectopic pregnancy and infertility. Current consensus is that prophylactic measures, such as a vaccine, are needed to flatten the ever-rising infection curve of Chlamydia. To date, there is no licensed human Chlamydia vaccine available, and only one subunit vaccine candidate completed a phase I clinical trial for safety. Understanding how protective immunity is developed in the female reproductive tract (FRT) against Chlamydia is essential for developing a vaccine. Prior research has established the essential role of CD4 T cells in protective adaptive immunity against Chlamydia. In contrast, the early innate defense mechanisms in the FRT are much less well understood in vivo. Our recent studies suggested that innate IFNg production during early Chlamydia infection is essential for preventing systemic bacterial dissemination. Moreover, increasing evidence suggest that innate immune cells can acquire immune memory characteristics through proper “training”. In this application, we propose to (i) explore the landscape of early immune responses to Chlamydia in the FRT and characterize the heterogenous Thy1+ cell population essential for protection; and (ii) characterize the myeloid cell infiltrates and determine whether trained immunity induced by BCG contribute to protective immunity and/or immunopathology during Chlamydia infection. We anticipate that identifying cellular populations and their effector functions responsible for protective immunity vs pathologic outcomes in the FRT will provide rational support for developing vaccines for STIs.