HIV/AIDS thrives in the margins of society, where low education, unstable housing, and poverty heighten people's vulnerability to HIV. No population is more affected by these social injustices than transgender persons. A case in point is transgender women (TGW}-individuals who were assigned a male sex at birth but express their gender along a feminine spectrum. Sadly, TGW have some of the highest concentrated HIV epidemics in the world, with a pooled global prevalence of 19% and a 49-fold higher odds ratio of acquiring HIV than non-transgender adults. A key part of gender affirmation in TGW is feminizing hormone therapy (FHT), of which the main drug is the hormone estradiol. Although the physical female traits triggered by FHT are well established, less is known about the immunological effects of FHT in TGW. It is noteworthy that estradiol can modulate immune responses in many ways, including by upregulating CCR5 expression on CD4+ T-cells. Since activated CCR5+ CD4+ T-cells are highly permissive to HIV infection, a better understanding of how FHT impacts the male immune system may provide new insights into how to prevent HIV infection in TGW. In this regard, here we will model FHT in nonhuman primates to prospectively address two knowledge gaps about the impact of FHT on the male immune system. 1) Does FHT increase the availability of HIV-susceptible CD4+ T-cells in vivo? To answer this question, we will use flow cytometry to assess the frequency and phenotype of memory CD4+ Teens in blood and gut biopsies from male rhesus macaques receiving FHT (Group 1) or placebo (Group 2). By comparing the levels of activated CD4+ T-cells between animals in Groups 1 and 2, this analysis will reveal whether FHT modulates a crucial marker of HIV susceptibility in biological males. 2) Does FHT interfere with adeno-associated virus (AAV}-vectored immunoprophylaxis? Considering TGW's high risk of acquiring HIV, they stand to benefit greatly from AAV-mediated delivery of immunoglobulins as this approach can provide durable anti-HIV immunity after a one-time administration. Indeed, a single dose of an AAV vector encoding the potent and extremely broad HIV entry inhibitor eCD4-lg resulted in persistent expression of eCD4-lg and protection against stringent immunodeficiency virus challenges in rhesus macaques. However, given the immunoenhancing properties of estradiol, FHT may undermine AAV-driven eCD4-lg expression in TGW by amplifying host anti-drug antibodies (ADAs}-a major limiting factor for AAV delivery of biologics. To address this possibility, all animals in Groups 1 and 2 will be inoculated with AAV/eCD4-lg after 6 months of FHT or placebo therapy. We will then compare their serum levels of eCD4-lg and ADAs to determine the impact of FHT on AAV-mediated delivery of eCD4-lg in males. Ultimately, the experiments proposed here will begin to uncover how FHT can affect HIV susceptibility and the outcome of immune interventions in TGW.

This project will investigate whether feminizing hormone therapy (FHT) can contribute to the high incidence of HIV infection among transgender women. To that end, FHT will be modeled in male rhesus macaques to determine whether it increases the availability of HIV "target" cells in the gut mucosa and whether FHT interferes with a prophylactic immune intervention. Ultimately, this project will advance our understanding of the pharmacodynamics of FHT and how it affects immune responses in biological males.