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Decidualization renders the endometrium transiently receptive to an implanting blastocyst although the underlying mechanisms remain incompletely understood. The aim of this study was to determine the role of chemokine CXCL16 and its receptor CXCR6 in the decidualization during pregnancy. Here, the expression of CXCL16 was investigated in endometrial tissues, decidua and placenta in this study. Compared with endometrial tissue, protein expression of CXCL16 was significantly higher in tissues from the fertile control samples, especially in villus. Meanwhile, the primary trophoblast cells and decidual stromal cells (DSCs) secreted more CXCL16 and expressed higher CXCR6 compared to endometrial stromal cells (ESCs) in vitro. Stimulation with the inducer of decidualization (8-bromoadenosine 3′,5′-cyclic with medroxyprogesterone acetate, 8-Br-cAMP plus MPA) significantly upregulated the expression of CXCL16 and CXCR6 in ESCs in vitro. After treatment with exogenous recombinant human CXCL16 (rhCXCL16) or trophoblast-secreted CXLC16, decidualised ESCs showed a significant decidual response, mainly characterised by increased prolactin (PRL) secretion. Simultaneously, PI3K/PDK1/AKT/Cyclin D1 pathway in decidualised ESCs were activated by rhCXCL16, and AKT inhibitor GS 690693 abolished the PRL secretion of ESCs that was triggered by rhCXCL16. Finally, the impaired CXCL16/CXCR6 expression could be observed at the maternal–foetal interface from patients who have experienced spontaneous abortion. This study suggests that the CXCL16/CXCR6 axis contributes to the progression of ESC decidualization by activating PI3K/PDK1/AKT/Cyclin D1 pathway. It unveils a new paradigm at the maternal–foetal interface in which CXCL16 is an initiator for the molecular crosstalk that enhances decidualization of ESCs.

The dysfunction of NK cells in women with endometriosis (EMS) contributes to the immune escape of menstrual endometrial fragments refluxed into the peritoneal cavity. The reciprocal communications between endometrial stromal cells (ESCs) and lymphocytes facilitate the development of EMS. However, the mechanism of these communications on cytotoxicity of natural killer (NK) cells in endometriotic milieus is still largely unknown. To imitate the local immune microenvironment, the co-culture systems of ESCs from patients with EMS and monocyte-derived macrophages or of ESCs, macrophages and NK cells were constructed. The cytokine levels in the co-culture unit were evaluated by ELISA. The expression of functional molecules in NK cells was detected by flow cytometry (FCM). The NK cell behaviors in vitro were analyzed by cell counting kit-8 and cytotoxic activation assays. After incubation with ESCs and macrophages, the expression of CD16, NKG2D, perforin and IFN-γ, viability and cytotoxicity of NK cells were significantly downregulated. The secretion of interleukin (IL)-1β, IL-10 and transforming growth factor (TGF)-β in the co-culture system of ESCs and macrophages was increased. Exposure with anti-IL-10 receptor β neutralizing antibody (αhIL-10Rβ) or αTGF-β could partly reverse these effects of ESCs and macrophages on NK cells in vitro. These results suggest that the interaction between macrophages and ESCs downregulates cytotoxicity of NK cells possibly by stimulating the secretion of IL-10 and TGF-β, and may further trigger the immune escape of ectopic fragments and promote the occurrence and the development of EMS.

It has been reported that the impaired cytotoxicity of natural killer (NK) cells and abnormal cytokines that are changed by the interaction between ectopic endometrial cells and immune cells is indispensable for the initiation and development of endometriosis (EMS). However, the mechanism of NK cells dysfunction in EMS remains largely unclear. Here, we found that NK cells in peritoneal fluid from women with EMS highly expressed indoleamine 2,3-dioxygenase (IDO). Furthermore, IDO⁺NK cells possessed lower NKp46 and NKG2D but higher IL-10 than that of IDO^-NK. Co-culture with endometrial stromal cells (nESCs) from healthy control or ectopic ESCs (eESCs) from women with EMS led to a significant increase in the IDO level in NK cells from peripheral blood, particularly eESCs, and an anti-TGF-β neutralizing antibody suppressed these effects in vitro. NK cells co-cultured with ESC more preferentially inhibited the viability of nESCs than eESCs did, and pretreating with 1-methyl-tryptophan (1-MT), an IDO inhibitor, reversed the inhibitory effect of NK cells on eESC viability. These data suggest that ESCs induce IDO⁺NK cells differentiation partly by TGF-β and that IDO further restricts the cytotoxicity of NK cells in response to eESCs, which provides a potential therapeutic strategy for EMS patients, particularly those with a high number of impaired cytotoxic IDO⁺NK cells.

Macrophages play an important role in the origin and development of endometriosis. Estrogen promoted the growth of decidual stromal cells (DSCs) by downregulating the level of interleukin (IL)-24. The aim of this study was to clarify the role and mechanism of IL-24 and its receptors in the regulation of biological functions of endometrial stromal cells (ESCs) during endometriosis. The level of IL-24 and its receptors in endometrium was measured by immunohistochemistry. In vitro analysis was used to measure the level of IL-24 and receptors and the biological behaviors of ESCs. Here, we found that the expression of IL-24 and its receptors (IL-20R1 and IL-20R2) in control endometrium was significantly higher than that in eutopic and ectopic endometrium of women with endometriosis. Recombinant human IL-24 (rhIL-24) significantly inhibited the viability of ESCs in a dosage-dependent manner. Conversely, blocking IL-24 with anti-IL-24 neutralizing antibody promoted ESCs viability. In addition, rhIL-24 could downregulate the invasiveness of ESCs in vitro. After co-culture, macrophages markedly reduced the expression of IL-24 and IL-20R1 in ESCs, but not IL-22R1. Moreover, macrophages significantly restricted the inhibitory effect of IL-24 on the viability, invasion, the proliferation relative gene Ki-67, proliferating cell nuclear antigen (PCNA) and cyclooxygenase2 (COX-2), and the stimulatory effect on the tumor metastasis suppressor gene CD82 in ESCs. These results indicate that the abnormally low level of IL-24 in ESCs possibly induced by macrophages may lead to the enhancement of ESCs’ proliferation and invasiveness and contribute to the development of endometriosis.