Immune system adaptation during gender-affirming testosterone treatment

Biological sex impacts human immune responses, modulating susceptibility and severity to immune-related diseases. Females generally mount more robust immune responses than males, resulting in lower infection severity and greater autoimmunity incidence. Here, we addressed the contribution of testosterone to human immune function by analyzing a cohort of subjects undergoing gender-affirming testosterone treatment. We performed systems-level immunomonitoring through mass cytometry, scRNA and scATAC-Sequencing, and proteome profiling of blood samples at baseline and following 3 and 12 months of treatment. Testosterone treatment was associated with a low-grade inflammatory profile, evidenced by upregulation of proinflammatory plasma proteome (e.g., EN-RAGE, OSM, TNF), and induction of an inflammatory transcriptional program associated with NFkB signaling, and TNF signaling. Following testosterone treatment, higher NFkB activity was revealed in CD4 T, CD8 T, and NK cells in scATACseq analyses. Further, testosterone increased monocytic inflammatory responses upon bacterial stimulation in vitro. Although testosterone was associated with this inflammatory profile, it also exerted negative effects on antiviral immunity. Firstly, the percentage of plasmacytoid dendritic cells (pDC) decreased over transition, with pDC also displaying phenotypic changes associated with lower IFN responses. Secondly, bulk transcriptomics analyses show an overall reduction of IFNa responses. Thirdly, testosterone treatment led to reduced IFNa production upon PBMCs stimulation with a viral agonist. Our results show that testosterone has broad effects on the human immune system, and significantly modulates important players in antiviral immunity and inflammatory response. Identifying pathways involved in immune sexual dimorphism will help define novel targets for effective prevention and treatment of immune-mediated diseases.