What is the mechanism of Tim-3⁺ regulatory T (Treg)-cell accumulation in the decidua during early pregnancy and is its disruption associated with recurrent pregnancy loss (RPL)? IL-27 and Gal-9 secreted by trophoblasts activate the Tim-3 signaling pathway in CD4⁺ T cells and Treg cells and so promote accumulation of Tim-3⁺ Treg cells, the abnormal expression of IL-27 and Gal-9 is associated with impaired immunologic tolerance in RPL patients. Tim-3⁺ Treg cells are better suppressors of Teff cell proliferation, and display higher proliferative activity than Tim-3⁻ Treg cells. Tim-3⁺ Treg cells are tissue-specific promoters of T-cell dysfunction in many tumors. These cells express a unique factor that influences and shapes the tumor microenvironment. The animal study included 80 normal pregnant mice. In human study, decidua tissues in the first trimester for flow cytometry analysis were collected from 32 normal pregnant women and 23 RPL patients. Placenta tissues for immunohistochemistry analysis were collected from 15 normal pregnant women. Placenta tissues for western blot analysis were collected from 5 normal pregnant women, 5 RPL patients and 5 women who have experienced one miscarriage. Blood samples for in vitro experiments were collected from 30 normal pregnant women. This study was performed between January 2017 and March 2019. In this study, we investigated the kinetics of Tim-3⁺ CD4⁺ T-cell accumulation, and the proportions of Tim-3⁺ Treg cells throughout murine pregnancies using flow cytometry. We compared Tim-3 expression on decidual CD4⁺ T cells and Treg cells during normal pregnancies with expression on the same cell populations in women suffering from RPL. IL-27 and Gal-9 transcription and protein expression in the placenta were determined by RT-PCR and western blot, respectively. An in vitro co-culture model consisting of peripheral CD4⁺ T cells and primary trophoblasts from early pregnancy was used to mimic the maternal–fetal environment. The percentage of Tim-3⁺ Treg cells present in mouse uteri fluctuates as gestation proceeds but does not change in the spleen. Levels of Tim3⁺ Treg cells in uteri peaked at pregnancy Day 6.5 (E 6.5), then progressively diminished, and fell to non-pregnant levels by E18.5. In pregnant mice, Tim-3⁺ Treg cells constituted 40–70% of Treg cells in uteri but were present at much lower abundance in spleens. About 60% of decidual Treg cells were Tim-3 positive at E6.5. Of these decidual Tim3⁺ Treg cells, nearly 90% were PD-1 positive. However, only about 16% of Tim3⁻ Treg cells expressed PD-1. Blocking the Tim-3 signaling pathway decreased the proportion of Treg cells and led to embryo resorption. Moreover, much lower Tim-3 expression was observed on CD4⁺ T cells and Treg cells in women who had suffered from RPL at 6–9 gestational weeks compared with those who had normal pregnancies at matched gestations. In a normal pregnancy, Tim-3 expression on decidual CD4⁺ T cells is induced initially by IL-27. Then Gal-9-Tim-3 interaction promotes differentiation of decidual Tim-3⁺ CD4⁺ T cells into Treg cells. IL-27 and Gal-9 cooperatively induced Tim-3⁺ Treg cells in vitro. N/A We did not investigate the kinetics of human decidual Tim-3⁺ CD4⁺ T and Tim-3⁺ Treg cell populations throughout pregnancy due to limited availability of second and third trimester decidua. In addition, functional suppressive data on the decidual Tim-3⁺ Treg cells are lacking due to limited and low quantities of these cells in decidua. These findings might have therapeutic clinical implications in RPL. This study was supported by research grants from the National Natural Science Foundation of China (No. 81871186) and National Key Research & Developmental Program of China (2018YFC1003900, 2018YFC1003904). The authors declare no conflict of interest.

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