The role of eutopic endometrial CD8+ T cells in adverse reproductive outcomes

Endometrial immune dysregulation is indicated in adverse reproductive outcomes, such as endometriosis-associated subfertility (EAS) and recurrent pregnancy loss (RPL). Endometriosis, defined as the growth of endometrium-like tissue outside of the uterus, affects 1 in 10 women, of which 35–50% suffer from EAS. RPL is defined as the loss of two or more subsequent clinically recognised pregnancies before week 24 and affects 1 in 50 pregnant women. This thesis investigates the role of CD8+ T (CD8 T) cells in endometriosis and RPL, the major T cell population constituting 25–35% of all endometrial leukocytes. I performed a systematic review of CD8 T cells in endometriosis, revealing conflicting data in endometriosis-associated tissues with no detailed phenotypic analysis of CD8 T cells in the endometrium and peripheral blood. Bulk mRNA sequencing of endometrial CD8 T cells from RPL patients (n = 13) and fertile controls (n = 11) demonstrated significantly altered CD8 T cell transcriptome between RPL patients with miscarriage versus live birth in the next pregnancy and between RPL groups and fertile controls. A 35-parameter panel was developed for full spectrum flow cytometry (Cytek®Aurora), and matched endometrial and peripheral blood samples of 10 RPL, 20 endometriosis and 8 control patients were analysed. I confirmed significant variations between the endometrium and peripheral blood. Endometrial CD8+ MAIT-like cells were increased in endometriosis versus non-endometriosis controls (p.adj < 0.05), and naïve CD8 T cells were increased in RPL patients with miscarriage versus live birth in their next pregnancy (p.adj = 0.021). Finally, samples were histologically examined across the menstrual cycle with singleplex and multiplex (ZellScannerONE®) immune imaging to study the spatial organisation of single immune cells, including intraepithelial lymphocytes and lymphoid aggregates (LAs) across the cycle in patients and controls. My findings on endometrial CD8 T cell transcriptomes offer valuable insights into modified immune responses during the window of implantation in RPL. Flow cytometry revealed systemic immune changes in endometriosis and localised CD8 T cell dysregulation in EAS and RPL. Qualitative differences in endometrial and myometrial immune populations across menstrual cycle phases in endometriosis indicate opportunities for future research to understand mechanisms at the foeto-maternal interface impacting embryo implantation and immune tolerance.