Effects of testosterone and estrogen on human CD4+ T-cell subsets during gender-affirming hormone therapy

Women have a higher burden of Type 2-driven diseases as well as Type 1-/Type 17-driven autoimmune diseases, suggesting insufficient counterbalancing by regulatory T cells (Tregs) as a possible shared mechanism. We investigated early immunological effects of gender-affirming hormone therapy (GAHT) in 19 transmen and 15 transwomen by analyzing CD4⁺ T-cell subsets before and three months after treatment initiation. Using 14-color flow cytometry and unsupervised FlowSOM clustering, we quantified 20 CD4⁺ T-cell metaclusters in peripheral blood. Feminizing GAHT in transwomen significantly reduced Treg frequencies, without affecting T-helper (Th)-1, Th2, or Th17 subsets. In contrast, masculinizing GAHT induced no major shifts in helper or regulatory subsets in transmen. These findings reveal early, hormone-specific immune remodeling during GAHT, characterized by reduced Tregs following feminizing therapy. Our results highlight Tregs as a key target of sex hormone-driven immune modulation and provide a cellular correlate to recent proteomic and transcriptomic studies.