PROJECT SUMMARY Testosterone gender-affirming hormone therapy is associated with significant changes to the vaginal micro- environment, such that most transgender men develop vaginal atrophy and have a non-Lactobacillus dominated vaginal microbiota (i.e., vaginal dysbiosis). However, the association between vaginal atrophy, the unique microbiota of the testosterone-exposed vagina, and vaginal infections is unknown. No interventions for treating and preventing vaginal atrophy and vaginal dysbiosis have been evaluated for transgender men using testosterone. There is an urgent need to better characterize the unique vaginal microbiota of transgender men and assess the acceptability of potential interventions (such as intravaginal estrogen) to improve the sexual health of this population. Thus, the overall objective of this project is to investigate the social, biological, and epidemiological factors influencing vaginal dysbiosis in transgender men using testosterone. Our central hypothesis is that the severity of vaginal atrophy mediates the impact of testosterone on the vaginal microbiota and vaginal infections. In Aim 1, we will use a mixed-methods approach to identify and contextualize transgender men’s gynecological treatment preference and key barriers/facilitators to accessing gynecological care for vaginal infections, and to adapt existing scales for measuring vulvovaginal symptoms. In Aim 2, we will recruit a cross-sectional clinical sample of transgender men to assess if testosterone use and vaginal atrophy severity are associated with vaginal microbiota composition, taxonomic diversity, and/or vaginal infections. This aim will use 16S rRNA gene amplicon sequencing (V3-V4 hypervariable region) from self-collected vaginal swabs. These aims constitute the mentored research component of the candidate’s career development plan for this K01 submitted in response to NOT-MD-22-012. In parallel with this research, the candidate will pursue training in translational microbiome science, supported by an exemplary team of renowned investigators with expertise in the gynecological health of transgender men (Primary Mentor, Dr. Juno Obedin-Maliver), the vaginal microbiome (Co-Mentors Drs. David Relman, Christina Muzny, and Christopher Taylor), and the impact of hormones on genital tissues (Co-Mentor Dr. Bertha Chen). Stanford University offers a world class research infrastructure that fosters outstanding collaborative and innovative translational research. This research also leverages the expertise of Stanford’s LGBTQ+ Health Program and The PRIDE Study/ PRIDEnet, both of which are national leaders in LGBTQ+ healthcare and research, as well as Dr. Tordoff and her mentor’s significant experience conducting community-engaged research with transgender men. In summary, the strong mentoring environment and training plan are anticipated to fully prepare Dr. Tordoff to launch an independent research career. The proposed studies promise to address a critical ga...