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Toxoplasma gondii infection during pregnancy can result in adverse pregnancy outcomes. Previously, we have reported that these outcomes are associated with the impaired function of decidual Treg cells; however, the detailed mechanisms involved were unclear. It has been reported that the suppressive capacity of Treg cells is dependent on PD-1 expression. The present study explored the role of decidual PD-1+ Treg cell function in adverse pregnancy outcomes due to T. gondii infection. Toxoplasma gondii–infected pregnant mice were sacrificed on gestational day 14 and their pregnancy outcomes were observed. The expression of PD-1 on decidual Treg cells and expressions of Foxp3, CTLA-4, TGF-β, and IL-10 on decidual PD-1+ and PD-1− Treg cells were determined using flow cytometry. The results showed that the expression of PD-1 on decidual Treg cells was clearly higher in the T. gondii–infected mice than in the normal mice. Meanwhile, the expressions of Foxp3, CTLA-4, TGF-β, and IL-10 on decidual PD-1+ Treg cells were higher in the infected mice than in the normal mice. The expressions were higher in decidual PD1+ Treg cells than in PD-1− Treg cells in the infected mice. However, these expressions on PD-1− Treg cells did not significantly differ between the infected and normal mice. Nonetheless, the absolute percentages of decidual PD-1+ Treg cells decreased significantly in the infected mice compared with those in the normal mice. These results suggest that T. gondii infection mainly influences the function of decidual PD-1+ Treg cells, which would result in an insufficiently immunotolerant microenvironment and consequently in adverse pregnancy outcomes. [ABSTRACT FROM AUTHOR]