Endometrial immune cell profile at the time of frozen embryo transfer as prognostic indicator of live birth

Introduction Endometrial receptivity is essential for implantation in both natural and ART cycles, yet the cellular and molecular environment of the endometrium during this window remains poorly characterized. While cytokines affecting implantation have been studied, data on immune cell subtypes in the endometrium are limited. The objective of this study was to determine the association between endometrial immune cell profile at the time of transfer and live birth in patients undergoing frozen embryo transfer (FET) using the index cycle. Methods This exploratory prospective observational cohort study (IRB#20190139) included 48 patients undergoing a hormone replacement FET cycle between May 2022 and May 2024. After ultrasound-guided FET, the catheter tip was rinsed in IMDM + 10% FBS, centrifuged, and stained for CD45, CD3, CD19, CD4, CD8, γδ TCR, CD25, CD127, CD66b, CD14, CD16, and CD56. The primary outcome was live birth. Secondary outcomes included miscarriage, biochemical pregnancy, and ectopic pregnancy. Results Elective single embryo transfer was performed for all the patients. There were 24 live births (50%), four miscarriages (8.3%), and three biochemical pregnancies (6.3%). There was no significant difference in demographics of patients that had a live birth compared to those who did not achieve implantation. There was an increased percentage of γδ T cells in the group of live birth compared to non-pregnant group (p=0.019). In contrast, an increased percentage of neutrophils (CD66b + ) was noted in patients that did not achieve implantation (p<0.003). Importantly, we found receiver operating characteristic (ROC) curve area under the curve (AUC) of 0.72 with 95% confidence interval (CI) 0.5504 to 0.8989 for γδ T cells and AUC is 0.75 (95% CI 0.5681 to 0.9319) for CD66b + cells, confirming the overall ability of these two tests to discriminate between patients who will achieve a live birth vs. ones who will have failed implantation. Discussion Our findings suggest that the uterine immune environment during FET may be associated with implantation outcomes. Characterization of endometrial immune cell profiles could provide insights into biological factors linked to implantation and live birth, although their clinical utility remains to be determined. To our knowledge, this study is among the first to describe associations between immune cell profiles assessed during the index FET cycle and subsequent IVF outcomes, supporting a potential role for endometrial immune composition in pregnancy success.