Murine modeling of menstruation identifies immune correlates of protection during Chlamydia muridarum challenge

The menstrual cycle influences the risk of acquiring sexually transmitted infections (STIs), including those caused by the pathogen Chlamydia trachomatis (C. trachomatis). However, the underlying immune contributions are poorly defined. A mouse model simulating the repetitive immune-mediated process of menstruation could provide valuable insights into tissue-specific determinants of protection against chlamydial infection within the cervicovaginal and uterine mucosae of the female reproductive tract (FRT). Here, we used the pseudopregnancy approach for inducing menstruation in naïve C57Bl/6 mice and performed vaginal challenge with Chlamydia muridarum (C. muridarum) over the course of decidualization, endometrial tissue remodeling, and menstruation. This strategy identified that a time point over pseudopregnancy corresponding to the late luteal phase of the menstrual cycle correlated with reduced bacterial burden. By evaluating the early infection site following challenge at this time point, we found that a greater abundance of NK cell populations and proinflammatory signaling, including IFNγ, were strongly correlated with protection. FRT immune profiling in uninfected mice over pseudopregnancy or in pig-tailed macaques over the menstrual cycle identified periodic NK cell infiltration into the cervicovaginal tissues and luminal surface occurring over a similar time frame. Notably, these cell populations were transcriptionally distinct and enriched for programs associated with NK cell effector functions. Depletion of FRT NK cells during the late luteal phase time frame resulted in a loss of protection, enabling productive infection following C. muridarum challenge. This study shows that the pseudopregnancy murine menstruation model recapitulates dynamic changes occurring in mucosal immune states throughout the FRT as a result of endometrial remodeling and identifies NK cell localization at the FRT barrier site of pathogen exposure as essential for immune protection against primary C. muridarum infection.