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

Suset Rodriguez^1*Laura Padula²Eva Fisher²Bria Slater²Sujad Younis²Carmel Awadallah¹Aryana Mohtasham Gharagozloo¹Roberta Carlotta Rubino¹Mohammed Ibrahim¹Michael Paidas¹Natasa Strbo²George Roshdy Attia¹

• ¹Department of Obstetrics, Gynecology and Reproductive Sciences, Division of Reproductive Endocrinology, and Infertility, University of Miami Miller School of Medicine, Miami, United States
• ²Department of Microbiology and Immunology, University of Miami Miller School of Medicine, Miami, United States

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 incompletely characterized. While cytokines influencing implantation have been studied, data on specific immune cell subtypes in the endometrium are limited. This exploratory prospective observational cohort study (IRB#20190139) aimed to determine the association between endometrial immune cell profiles at the time of transfer and live birth in patients undergoing frozen embryo transfer (FET) using the index cycle. A total of 48 patients undergoing hormone replacement FET between May 2022 and May 2024 were included. 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, with secondary outcomes including miscarriage, biochemical pregnancy, and ectopic pregnancy. Elective single embryo transfer was performed in all patients. Among participants, 24 achieved live birth (50%), four had miscarriages (8.3%), and three had biochemical pregnancies (6.3%). Demographics did not differ significantly between patients with live birth and those who did not achieve implantation. The percentage of γδ T cells was higher in patients with live birth compared to non-pregnant patients (p=0.019), whereas neutrophils (CD66b+) were increased in patients who failed implantation (p<0.003). Receiver operating characteristic (ROC) analysis yielded an area under the curve (AUC) of 0.72 (95% CI 0.5504–0.8989) for γδ T cells and 0.75 (95% CI 0.5681–0.9319) for CD66b+ cells, supporting the ability of these measures to discriminate between patients who achieved live birth versus those who did not. These 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 factors linked to implantation and live birth. To our knowledge, this study is among the first to describe associations between immune cell profiles assessed during the index FET cycle and IVF outcomes, supporting a potential role for endometrial immune composition in pregnancy success