P-438 Association of uterine cavity immune cells at the time of frozen embryo transfer with live birth
Authors: Rodriguez S , Ibrahim M , Padula L , Fisher E , Rivas K , Paidas M , Strbo N , Attia G 
Published: July 2024

Abstract

Study question
Is there an association between live birth and uterine cavity immune cell subsets collected at the time of frozen embryo transfer (FET)?

Summary answer
A positive correlation exists between live birth and number of uterine cavity immune cells, as well as T regulatory cells at the time of FET.

What is known already
Several strategies have been deployed to improve pregnancy rate during IVF, with little success. Although selecting the euploid embryo was thought to improve pregnancy rate, implantation rate of 50-60% has been reported. Endometrial receptivity plays a critical role in the success of implantation and could be the missing link in improving pregnancy rate. While certain cytokines within the endometrium are recognized for influencing implantation, information on uterine milieu at time of implantation is limited. Immune cell profile at the time of FET may reflect on endometrial receptivity and could be utilized as a marker for predicting implantation and successful pregnancy.

Study design, size, duration
Institutional Review Board approval was obtained for this study (#49174). This prospective observational cohort study was conducted at a single academic-based fertility clinic. A total of 24 subjects were recruited between May 2021 through December 2022. Eligible participants were those who had undergone at least one cycle of IVF with at least one frozen embryo available for FET. Live birth was defined as birth of one of more live-born infants.

Participants/materials, setting, methods
Under ultrasound guidance, trial transfer catheter was placed in lower uterine segment, then rinsed in IMDN medium with 10% FBS (lower). Embryo was placed in the upper uterus and the transfer catheter tip was rinsed in separate sample of above media (upper). Collected cells were centrifuged and stained for the following surface markers: CD45, CD3, CD19, CD4, CD8, gamma delta TCR, CD25, CD127, CD66b, CD14, CD16, CD56. Statistical analysis was performed with Pearson correlation test.

Main results and the role of chance
All participants underwent elective single embryo transfer under ultrasound guidance. Out of the 24 patients participating in the study, 13 patients had a live birth (54%), one patient had a biochemical pregnancy (4%), 10 patients did not achieve pregnancy (42%), and there were no miscarriages. When patients with a live birth were compared to those without a live birth, there was no significant difference in their age (P 0.87), body mass index (P 0.64) or ethnicity (P 0.94). There was no association between the relative expression and subtypes of different immune cells isolated from the lower uterine segment and pregnancy outcome. However, in cells isolated from upper uterus, there was a positive correlation between live birth and total immune cells (CD45+; P 0.04) and T regulatory cells (P 0.03) at the time of FET. There was no statistically significant association with live birth noted with the remainder of the analyzed cells, including neutrophils (CD66b), NK cells (CD56), T cells (CD3) and other subsets (GDT, CD8, CD4), B cells (CD19), monocytes (CD14), and Fc receptor positive cells (CD16).

Limitations, reasons for caution
Our study’s limitations include the number of participants and the range of flow cytometry parameters that were analyzed. With the use of multicolor flow cytometry, additional detailed data can be obtained on immune cell diversity and expression.

Wider implications of the findings
We posit that the uterine milieu during embryo transfer could influence implantation. Analyzing immune cell association with live birth can direct future studies toward identifying conditions conducive to implantation, paving the way for novel diagnostics and treatments to enhance implantation and subsequently live birth rate.

Trial registration number
not applicable