Assessment of immune cells in the uterine fluid at the time of the embryo transfer
Authors: Strbo Natasa , Rodriguez Suset , Padula Laura , Fisher Eva , Lyons Annabel , Rodriguez Carolina , Rivas Katelyn , Ibrahim Mohammed , Paidas Michael , Attia George 
Published: April 2024

AbstractProblemAlthough 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 that 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.Method of studyThis study was approved by the Institutional Review Board (# 225589). 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 (IVF) cycle and have frozen embryos. Twenty‐four participants were recruited for this study which was conducted during the frozen embryo transfer (FET) cycle regardless of the outcome of previous cycles. Two samples were acquired from each subject, denoted as lower and upper. A trial transfer catheter was introduced under ultrasound guidance into the lower uterine segment. Upon removal, the tip was rinsed in IMDM medium containing 10% FBS (lower uterus). A transfer catheter was then loaded with the embryo that was placed in the upper uterus under ultrasound guidance. The tip of the transfer catheter was rinsed in separate aliquot of the above media (upper uterus). 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 and acquired on Sony SP6800 Spectral Analyzer.ResultsUpon staining the pelleted cells, we were able to identify viable leukocytes from samples obtained from both, upper and lower uterus (0.125 × 106cells ± SD 0.32), (0.123 × 106cells ± SD 0.12), respectively. Among total viable cells, there was no significant difference in both percent and number of CD45+ cells between the upper and lower uterus (9.88% ± 6.98 SD, 13.67% ± 9.79 SD,p = .198) respectively. However, there was significantly higher expression of CD3+ (p = .006), CD19+ (p = .032) and CD14+ (p = .019) cells in samples collected from upper compared to lower uterus. Within all CD3+ cells, we found that gamma delta T cells (GDT) were the major population of T cells in both upper and lower uterus. In contrast, CD8+ T cells were significantly higher in the lower uterus when compared to the upper uterus (p = .009). There was no statistically significant difference in the expression of CD4+ T cells, T regulatory cells (CD4+CD25+CD127‐), NK cells (CD56+), neutrophils (CD66b+) and FcγRIII+ cells (CD16+) between upper and lower uterus.ConclusionsWe believe the immune milieu at the time of embryo transfer will affect implantation. Understanding the composition of immune cells will guide further research in identifying optimal immune milieus that favor implantation. Comprehensive analysis of endometrium is expected to lead to new diagnostic and therapeutic approaches to improve IVF outcomes.