Human decidual macrophages suppress IFN-γ production by T cells through costimulatory B7-H1:PD-1 signaling in early pregnancy

In human pregnancy, CD14+ decidual macrophages (DMs) are the dominant professional antigen-presenting cells in the decidua, comprising 20–30% of the local leukocyte population. Although the relevance of DMs to feto-maternal immune tolerance has been described, the molecular mechanisms underlying these functions have not been fully elucidated. B7-H1, a costimulatory ligand in the B7 family, negatively modulates T cell activity by binding to its corresponding receptor, PD-1. The present study aimed to investigate the functional significance of costimulatory interactions between DMs and T cells, with a particular focus on B7-H1:PD-1 signaling. An analysis of the expression profile of B7 ligands on human DMs revealed that B7-H1 was present on DMs isolated from early but not term pregnancies. B7-H1 was not expressed on the peripheral monocytes (PMs) of pregnant women. In response to IFN-γ, B7-H1 expression was induced on PMs and was enhanced on DMs, suggesting that this cytokine might be a key factor in the control of B7-H1 expression in the decidua. The majority of decidual T cells were noted to exhibit robust expression of PD-1, whereas the expression was limited to a small subpopulation of circulating T cells. Functional assays demonstrated that DMs are able to suppress T cell IFN-γ production via B7-H1:PD-1 interactions. This suppressive property was not observed for PMs, which lack B7-H1. B7-H1 on DMs may function as a key regulator of local IFN-γ production and thereby contribute to the development of appropriate maternal immune responses to the fetus in early pregnancy.