Regulatory T cells, inflammation, and endoplasmic reticulum stress in women with defective endometrial receptivity.

An adequate immune response is considered a key factor in the control of endometrial receptivity and fertility in women. Considerable evidence has associated adverse immune responses with infertility problems, and proinflammation molecules have been reported to be involved in compromised endometrial receptivity and fetus implantation 1 and 2. However, the precise mechanisms through which the immune response controls fetal implantation and pregnancy are still matter of debate, and the data in the literature are controversial. The population of leukocytes in human endometrium has been extensively studied and consists mainly in T cells, natural killer (NK) cells, and macrophages. CD3+ T cells represent the second most abundant subset of leukocytes in the human endometrium; they have been extensively studied in women with defective endometrial receptivity, but with controversial results. Few studies have shown no difference in the number of CD3+ T cells in the peripheral blood and endometrium of women with recurrent miscarriage (RM) compared with healthy women 3 and 4. In contrast, one study has demonstrated a significant reduction of CD3+ T cells in the peripheral blood in women with RM (5). In addition, several reports have shown an increase of the absolute number of NK cells expressing the marker CD56 in the peripheral blood of women with RM 6 and 5. In agreement, the report by Clifford et. al. (7) showed an increased number of CD56+ cells in the endometrium of women with RM. By contrast, Quenby et al. (8) found a lower number of CD56+ NK cells in women with RM who successively had a live birth compared with those who miscarried. Elevated NK-cell cytotoxicity has been proposed as negative prognostic factor in women with RM and in women who have repeated in vitro fertilization failure (RIF) 6 and 9. Several immunologic mechanisms have been reported to be essential in implantation and pregnancy outcomes. In this context, it has been established that T-helper (TH) cells play a central role in modulating immune response during pregnancy. Numerous studies attempted to support the theory that pregnancy is a TH2 anti-inflammatory state 10, 11 and 12. Although some studies have confirmed this hypothesis, others have not achieved comparable results 13 and 14. Because the paradigm TH1/TH2 is likely not sufficient to describe the many immunologic changes in adaptive immunity during pregnancy, the local and systemic spectrum of inflammatory molecules during implantation is thought to play a critical role in pregnancy success. High levels of proinflammatory cytokines such as macrophage inflammatory protein -1β (MIP-1β), tumor necrosis factor α (TNF-α), osteoprotegerin (OPG), and interleukin 15 (IL-15) have been positively correlated with the pregnancy outcome of the patients with RIF (15). Recent evidence also indicates that regulatory T cells (Treg), a T-cell subset essential for the maintenance of immune tolerance, provide a permissive immune microenvironment for the semiallogeneic fetus (16). Expression of forkhead-box-P3 (FoxP3) transcription factor is necessary for Treg cell function, and it represents the major lineage marker of this subset (17). However, the number and precise role of Treg cells in endometrial tissue have been poorly explored in the context of human unexplained infertility. Recent findings also support the evidence that endoplasmic reticulum (ER) stress and its homeostatic response, the unfolded protein response (UPR), play a crucial role in both innate and adaptive immune response (18). Excessive amounts of cytokines can trigger calcium release from the ER and reactive oxygen species (ROS) production, leading to ER stress and inflammation and thereby altering cell physiology (18). Furthermore, ER stress-associated UPR signals are involved in the maintenance of lymphocyte homeostasis and viability (19). Heat shock 70-kDa protein 5 (HSPA5), also known as binding protein or glucose-regulated protein 78, represents a key factor during ER stress as it is both a target and a regulator of the UPR (20). Increased levels of HSPA5 are thus a well-recognized marker of ER stress and UPR activation. Besides HSPA5, several other molecular markers, including X box binding protein 1 (XBP-1) mRNA splicing, are widely used to monitor ER stress and the UPR activation (18). Our study tested the hypothesis that an altered immune response during the window of implantation might be associated to ER stress in women with different but overlapping receptivity problems, such as RM and RIF.