Approximately 8  % to 12  % of couples in the reproductive age group suffer from infertility worldwide [1]. Despite the rapid development of in vitro fertilization-embryo transfer (IVF-ET) and its widespread use in patients with infertility, its success rate is only approximately 40  % [2]. IVF-ET failures can result from one or multiple factors, and implantation failure (IF) is one of the most common causes of IVF-ET [3]. The incidence of IF in healthy women is approximately 8  % to 33  % [4]. Patients with IF show a negative urine or blood test result for human chorionic gonadotropin (HCG) or have elevated HCG but without a gestational sac confirmed by ultrasound [5]. Many infertile patients still require alternative therapies, including acupuncture, to enhance the success rate and pregnancy outcomes of IVF-ET [6].

Acupuncture is one of the most widely applied complementary therapies for IVF-ET, which regulates the neuroendocrine-immune axis by stimulating the skin [7]. A recent meta-analysis indicated that acupuncture could significantly improve the clinical pregnancy rate (CPR) and live birth rate (LBR) in IVF-ET [8]. Studies have shown that acupuncture can improve endometrial receptivity (ER) and CPR in patients undergoing IVF-ET by regulating related molecules [9] and remodeling microvessels [10]. It can also alter the immune microenvironment of the endometrium to improve blastocyst implantation [11], but the underlying mechanism is still not fully understood.

As we know it, the immune tolerance of the maternal–fetal interface is essential for successful embryo implantation. It involves a complex molecular network regulated by human leukocyte antigen (HLA)-G, various immune cells, and cytokines; HLA-G may be a key starting point for this molecular network [12]. The nonclassical HLA class I HLA-G is expressed by extravillous trophoblasts (EVTs) and contributes to trophoblast invasion, uterine vascular remodeling, and maintenance of a local immunosuppressive state [13]. Normal expression of HLA-G in EVTs is necessary for a successful pregnancy, and abnormal expression can lead to spontaneous abortion, preeclampsia, and other adverse pregnancy outcomes [14]. Membrane-bound and soluble HLA-G bind to inhibitory receptors expressed on various immune effector cells, especially natural killer (NK) and T cells, leading to their attenuated functions and protection of the fetus from maternal immune rejection [15]. Additionally, HLA-G regulates the balance of Th1/Th2 cytokines. Cytokines produced by Th1 cells, such as interferon (INF)-γ and interleukin (IL)−2, suppress the growth of trophoblasts and promote allograft rejection and inflammatory response in maternal uterine blood vessels thus adversely affecting implantation [16]. Cytokines produced by Th2 cells, such as IL-4, IL-6, and IL-10, inhibit inflammatory responses and promote immune tolerance [17]. The immune response during normal pregnancy is a special Th2 phenomenon mainly mediated by Th2 cytokines [18]. HLA-G shifts the Th1/Th2 balance towards Th2 immunity, which is conducive to pregnancy [19].

To the best of our knowledge, whether acupuncture can improve maternal–fetal immune tolerance and embryo implantation by regulating HLA-G expression has not been explored. Progesterone (P4) can increase the expression of HLA-G [20], whereas acupuncture can promote embryo implantation by upregulating the expression of P4 and the P4 receptor (PGR) [21]. Therefore, we assume that acupuncture can promote maternal-fetal immune tolerance and embryo implantation by upregulating the expression of P4 and HLA-G. In this study, we used pregnant mare serum gonadotropin (PMSG) combined with HCG to stimulate ovulation in rats, established an ovarian hyperstimulation model, and observed the effects of acupuncture on embryo implantation and maternal–fetal immune tolerance. The expression of estrogen (E2), P4 and their receptors, RT-BM1 (the counterpart of HLA-G in rats), activating receptors (Ly49s3 and Ly49s5), and inhibitory receptors (Ly49i3, Ly49i4, and Ly49i5) on immunocytes was detected in the serum and maternal–fetal interface of rats using ELISA, Western blotting, and real-time PCR. We aimed to provide theoretical support for the combination of acupuncture and modern assisted reproductive technology (ART) to improve pregnancy rates and lay a foundation for the application of acupuncture in the reproductive field.