The CCL17-CCR4 axis between endometrial stromal cells and macrophages contributes to the high levels of IL-6 in ectopic milieu

Endometriosis is a chronic inflammatory disease characterized by the elevation of proinflammatory cytokines, such as IL-6, in the peritoneal fluid. However, the precise mechanism of the highly elevated IL-6 levels in ectopic milieu remains unclear. The aim of this study was to investigate whether the cross talk between endometrial stromal cells (ESCs) and macrophages contributes to the elevated IL-6 production. Samples of endometrium and ectopic tissues were obtained from patients with or without endometriosis. The peripheral blood samples were collected from healthy volunteers. Enzyme-linked immunosorbent assay (ELISA) was for IL-6 levels in peritoneal fluid and cell culture supernatant. In-Cell Western assay was used for protein expression of CCL17 and phosphorylation levels of ERK, JNK, and P38. Immunohistochemistry was performed on normal, eutopic endometrium and ectopic tissues to analyze CCL17 expression. Flow cytometry was applied to detect the expression of CCR4, IL-6, and the phosphorylation levels of NF-B. Patients with endometriosis have higher levels of IL-6 in peritoneal fluid compared to the control. The co-culture of ESCs and macrophages produce more IL-6 than cultured alone, respectively. The eutopic endometrium had significantly higher expression of CCL17 compared to normal endometrium, and the ectopic tissues had the highest expression. IL-6 induced CCL17 secretion in ESCs via activating JNK signaling pathway, CCL17 upregulated the expression of its receptor CCR4 on macrophages. Furthermore, CCL17-CCR4 axis subsequently led to excessive IL-6 production in macrophages by activating NF-B. These findings suggest that the cross talk between ESCs and macrophages promotes the expression of CCL17 in ESCs and CCR4 on macrophages, which contributes to the high levels of IL-6 in ectopic milieu.