Plasma testosterone concentration is correlated with circulating immune cell abundance in transgender young people on genderaffirming hormone treatment

Alice Amy White^1,2Thomas Pearce^1,3Isabelle Coenen¹Xander Bickendorf^1,4Julia K Moore^4,5Penelope Strauss⁶Liz A Saunders^4,7Georgia Chaplyn⁴Aris Siafarikas^1,2,4Ashleigh Lin⁷Martyn French^2,8Christian Tjiam^10,9Deborah Strickland¹Jonatan Leffler^11*

• ¹Translational Immunology Team, The Kids Research Institute Australia, Perth, Australia
• ²Medical School, University of Western Australia, Perth, Western Australia, Australia
• ³School of Biomedical Sciences, University of Western Australia, Perth, Western Australia, Australia
• ⁴Gender Diversity Service, Child and Adolescent Health Service, Nedlands, Western Australia, Australia
• ⁵School of Psychiatry, University of Western Australia, Perth, Western Australia, Australia
• ⁶Youth Mental Health Team, The Kids Research Institute Australia, Perth, Australia
• ⁷School of Global and Population Health, University of Western Australia, Perth, Western Australia, Australia
• ⁸Pathwest Laboratory Medicine, Nedlands, Western Australia, Australia
• ⁹Vaccine Trials Group, The Kids Research Institute Australia, Perth, Australia
• ¹⁰Department of Clinical Immunology, Pathwest Laboratory Medicine, Nedlands, Western Australia, Australia
• ¹¹Translational Immunology, The Kids Research Institute Australia, Perth, Australia

Sex hormones, such as oestrogen and testosterone, display significant immune modulatory properties. This is highly relevant for transgender (trans) people who undergo gender-affirming hormone (GAH) treatment. However, only a limited number of studies have evaluated the immunological impact of GAH treatments, and almost none have assessed the impact in trans young people. Biological samples collected from trans young people (n = 47) and controls not taking GAH (n = 53), were recruited to the Gender and IMmunity study (GIM) (n = 100). Participants had taken testosterone or oestrogen for at least 6 months. Immune profiles were evaluated using an 18-colour flow cytometry panel. In addition, the commercially available 37-parameter MaxPar panel was used for analysis of a subset of samples (n = 36) by mass cytometry (CyTOF). Immune cell abundance was compared across experimental groups, and correlated with plasma concentrations of oestrogen and testosterone using multiple regression models. From multiple comparisons analyses grouped by birth-registered sex, several differences were detected in the trans groups compared to control groups, in particular relating to abundance of B and T cell subsets. These differences appeared to be mainly associated with levels of plasma testosterone. The most notable differences were in trans males, who had lower numbers of CD11c + B cells and higher numbers of CD4 + regulatory T cells (Tregs) compared to control females. Using CyTOF, further analysis of B and T cells subsets revealed the frequency of naïve B cells was higher in trans males compared to control females. This also correlated with testosterone concentration in this group. Differences in the abundance of other T cell subsets were detected in both trans males and trans females, however only a decrease in CD161 + T effector memory cells in trans males, compared to control females, was associated with lower testosterone levels. This cross-sectional observational study of young trans individuals suggests that testosterone treatment may have immune modulatory effects, which should be investigated further, including functional studies. While oestrogen treatment was associated with differences in some immune cells in trans females compared with controls, these were generally not associated with plasma oestrogen