P-489 Uterine fluid-immune cell analysis at the time of the embryo transfer

Abstract Study question What are the subtypes of immune cells present in the upper and lower parts of the uterus at the time of frozen embryo transfer (FET)? Summary answer Our study is the first study confirming different immune cell subsets in the freshly collected uterine fluid at the time of the embryo transfer. What is known already Although endometrial receptivity is a key factor in influencing implantation in both naturally conceived and assisted reproductive technology (ART) cycles, very little is known about the endometrium milieu around the time of implantation. Previous studies have demonstrated the presence of several cytokines in the endometrium affect implantation. However, there is lacking data about the presence of immune cell subtypes within the endometrium and in the uterine cavity at the time of implantation. Study design, size, duration This study was approved by the Institutional Review Board (# 49174). The study was designed as a prospective observational cohort study between May 2021 and December 2022 at a single academic-based fertility center. All patients underwent at least one In Vitro Fertilization cycle (IVF) and have frozen embryos. Twenty-four participants were recruited for this study which was conducted during the frozen embryo transfer cycle (FET) regardless of the outcome of previous cycles. Participants/materials, setting, methods Trial transfer catheter was introduced under ultrasound guidance into lower uterine segment. Upon removal, the tip was rinsed in IMDN medium containing 10% FBS (lower). Embryo was then placed in upper uterus under ultrasound guidance. The tip of transfer catheter was rinsed in separate aliquot of the above media (upper). After centrifugation, pelleted cells were stained for the following surface markers: CD45, CD3, CD19, CD4, CD8, gamma delta TCR, CD25, CD127, CD66b, CD14, CD16, CD56. Main results and the role of chance Upon staining the pelleted cells, we were able to identify viable leukocytes from samples obtained from both, upper and lower uterus (0.1257 x 106 cells +/- SD 0.3167), (0.1230 x 106 cells +/- SD 0.1171), respectively. Among total viable cells, there was no significant difference in both the percent and number of CD45+ cells between the upper and lower uterus (9.880 +/- 6.983, 13.67 +/- 9.792, p = 0.1980) respectively. However, there was significantly higher expression of CD3 + (p = 0.006), CD19 + (p = 0.0319) and CD14 + (p = 0.0189) cells in samples collected from upper compare