Embryo transfer is the rate-limiting process of in vitro fertilization (IVF). An effective crosstalk between a good-quality embryo and a well-prepared endometrium is vital for successful pregnancy. When euploid blastocysts were selected with preimplantation genetic testing and transferred, the ongoing pregnancy rate is only 50 % (Casper, 2020). In addition, a well-balanced equilibrium of peripheral and uterine immune micro-environment is fundamental for successful embryo implantation. It is hypothesized that patients with implantation failure may be unable to recruit the necessary lymphocytes (Turocy and Williams, 2021), such as patients with recurrent implantation failure (RIF). RIF is defined as the failure to achieve a clinical pregnancy after 2–3 IVF cycles with 1–4 good-quality embryos (Cimadomo et al., 2021).

The peripheral and uterine immune cells during implantation window included uterine natural killer cells (uNK), macrophages, T cells (Vallve-Juanico et al., 2019). These cells produce cytokines, interleukins, and growth factors, which may be a positive effect on the endometrial thickness and endometrial receptivity (Yu et al., 2016). As the most abundant leukocytes in a normal endometrium, uterine natural killer (uNK) cells have been demonstrated to be main factors of pregnancy with their effects on angiogenesis, vascular remodeling, trophoblast invasion, and cytokine production (Polese et al., 2014). The second abundant leukocytes in the endometrium are uterine macrophages, which play a critical role in appropriate embryo recognition and tolerance for a successful implantation (Jena et al., 2019). Although these immune cells infiltrate into the endometrium during implantation window, whether these immune cells in peripheral blood can be used as a predictor of prognosis in IVF has not been clarified.

According to the phenotype and function, macrophages are mainly divided into M1 and M2 macrophages. Proinflammatory (M1) and anti-inflammatory (M2) activities are alternatively activated depending on the tissue microenvironment and playing different roles based on their functions and cytokine productions (Jena et al., 2019). Disruptions of macrophages might decline the endometrial receptivity that results in unfavorable pregnancy disorders, such as repeated implantation failure (RIF).

M1 macrophages are believed to be dominant phenotype that promote embryo attachment to the decidua by inflammatory cytokines and mediators in the peri-implantation period (Zhang et al., 2017). However, recent studies prove that M2 is the predominant phenotype of decidual macrophages in early pregnancy (Zhang et al., 2019, Valero-Pacheco et al., 2022). Most of M2 cells express scavenger receptors CD163 and CD206, and might be of clinical importance as it could regulate intraplaque vessel sprouting and function in angiogenesis (Krop et al., 2023). A few studies demonstrated that M2-like macrophages accumulated in the endometrium at the implantation period of mice (Care et al., 2013, Ono et al., 2020). For human, the association of uterine M2 macrophages and pregnancy outcomes were contradictory according to different studies (Ono et al., 2020, Fan et al., 2021, Diao et al., 2020).

The above studies focused on the effect of decidual macrophages on embryo implantation. However, it is invasive and forbidden to obtain the endometrium tissues during the same timing of embryo transfer in implantation window. Therefore, we investigated if macrophages and other immune cell subgroups in the peripheral blood are associated with pregnancy outcomes during frozen embryo transfer (FET). The present study aimed to study the correlation between circulating M2 monocytes and pregnancy outcome in FET patients.