Project Summary/Abstract: Trans-females who have sex with men (TFSM) and men are a population disproportionately affected by the HV/AIDS epidemic. Although often grouped together with cis-men who have sex with men (MSM) due to the practice of receptive anal intercourse, TFSM have a unique immune phenotype due to life-long exposure to female sex hormones such as estradiol (E2). As E2 can modulate multiple systemic and mucosal immune parameters, it is critical to determine whether long-term synthetic estrogen exposure in those who are genetically male, adversely impact HIV susceptibility. Currently, there is a gap in the knowledge as to the extent to which long-term synthetic E2 therapy affects systemic immune responsiveness, and rectal mucosal immune microenvironment including the microbiome, in the context of HIV infection. The goal of this proposal is to identify dysregulation in systemic and mucosal immune pathways that are relevant to the risks of acquiring HIV. We will evaluate HIV uninfected TFSM who are starting gender- affirming hormone therapy (GAHT) and follow them up after 3 months (cohort 1), as well as those who have been on GAHT for at least 1 year (cohort 2). This study design allows us to compare immune condition and function in TFSM without E2 therapy, with short-term E2 therapy and with long-term E2 therapy. We have unique access to this population through Dr. Goldstein, the Director of Clinical Research at Whitman Walker Institute, Washington DC, who is actively involved in transgender health research and a co-investigator on this proposal. We propose to recruit 30-40 participants per group and collect blood for analysis of systemic immune responses and rectal swabs for analysis of mucosal immune microenvironment and microbiome. There is a growing interest in the National Institutes of Health (NIH) toward funding transgender research. However, data is severely lacking in our understanding of immuno-biological mechanisms that may affect HIV acquisition in TFSM. Further, this proposal is responsive to the recently issued NOSI, NOT-MD-19-001 (Research on the Health of Sexual and Gender Minority (SGM) Populations). Assessment of data obtained from this study will spur further research that may alert the scientific and medical community of unexplored risks and lead to well-informed recommendations and interventions.