Impact of testosterone-based gender affirming hormone therapy on toll-like receptor transcript levels and peripheral blood leukocyte counts in transmasculine individuals

Hüseyin Cihan^1,2,3,4Özge Güngör⁵Gökcen Ünal Kocabas⁶Esma Pehlivan Köroglu⁶Kübra Gülpinar⁶Erhan Pariltay⁵Fatma Ömür Ardeniz⁷Guy T’Sjoen⁸Jonatan Leffler⁹Bettina Winzeler^2,3Banu Yürekli^6*

• ¹Universitat Zurich Medizinische Fakultat, Zürich, Switzerland
• ²Zurich University Hospital, Department of Endocrinology, Zurich, Switzerland
• ³Basel University Hospital, Innovation Focus Gender Variance, Basel, Switzerland
• ⁴Ege University Faculty of Medicine, Izmir, Türkiye
• ⁵Ege University Hospital, Department of Medical Genetics, Izmir, Türkiye
• ⁶Ege University Hospital, Department of Endocrinology, Izmir, Türkiye
• ⁷Ege University Hospital, Department of Allergy and Immunology, Izmir, Türkiye
• ⁸Ghent University Hospital, Department of Endocrinology, Izmir, Türkiye
• ⁹University of Western Australia, Telethon Kids Institute, Perth, Australia

Background: Sex differences in immune responses are partly attributed to sex hormones, with testosterone generally associated with dampened immune responses. However, the specific impact of testosterone on innate immune system in humans remains unclear. We examined changes in toll-like receptor (TLR) transcript levels and peripheral blood leukocyte parameters during masculinizing hormone therapy in transmasculine individuals. Methods: We conducted a 6-month prospective observational study in 20 transmasculine individuals initiating testosterone-based gender affirming hormone therapy. Peripheral blood mononuclear cells were collected at baseline and after 6 months of therapy. Gene transcript levels of TLR1-8, TLR 10, MD2 and CD14 were quantified by RT-qPCR. Serum hormones and hematologic parameters were measured by standard assays. Results: After 6 months of testosterone therapy, serum testosterone levels rose to values comparable to the range in cisgender men. Lymphocyte and monocyte counts increased (p=0.02 and p=0.006, respectively). TLR8 and TLR 10 mRNA levels were increased relative to baseline (p=0.003 and p=0.001, respectively). TLR2 and MD2 transcript levels showed nominal declines (unadjusted p<0.05) but after correction did not remain significant. No other TLRs showed significant changes. Exploratory correlation analyses shows that the magnitude of estradiol decline and monocyte expansion was associated with changes in TLR1, TLR2 and TLR8 transcript levels. Conclusions: In this cohort, testosterone-based gender affirming hormone therapy is associated with upregulation of TLR8 and TLR10 transcripts, suggesting that sex hormone shifts (increased testosterone and decreased estradiol) may modulate certain innate immune receptors at the transcript level. These findings provide insights into how gender-affirming hormone therapy can influence innate immune system in transmasculine individuals.