Decidual CD8(+)T cells exhibit both residency and tolerance signatures modulated by decidual stromal cells

Background During early pregnancy, tolerance of the semi-allogeneic fetus necessitates comprehensive modifications of the maternal immune system. How decidual CD8(+)T (CD8(+)dT) cells balance maternal tolerance of the fetus with defense from invading pathogens remains undefined. Methods We investigated the distribution patterns of CD8(+)T cells and their heterogeneity in paired peripheral blood and decidual tissue in the first trimester of pregnancy using flow cytometry and mRNA-Seq. Gene Set Enrichment Analysis was utilized to determine the transcriptional features of CD8(+)dT cells. Moreover, we examined activation of T cells when they were cocultured with trophoblasts, in addition to the effect of the fetal-maternal environment on peripheral CD8(+)T (CD8(+)pT) cells. Results We found that, compared with CD8(+)pT cells, CD8(+)dT cells consisted mainly of effector memory cells (T-EM) and terminally differentiated effector memory cells (T-EMRA). Both T-EM and T-EMRA subsets contained increased numbers of CD27(+)CD28(-) cells, which have been shown to possess only partial effector functions. In-depth analysis of the gene-expression profiles of CD8(+)dT cells revealed significant enrichment in T cell exhaustion-related genes and core tissue residency signature genes that have been found recently to be shared by tissue resident memory cells and tumor(-)infiltrating lymphocytes (TILs). In accordance with gene expression, protein levels of the exhaustion-related molecules PD-1 and CD39 and the tissue resident molecules CD103 and CXCR3 were increased significantly with almost no perforin secretion in CD8(+)dT cells compared with CD8(+)pT cells. However, the levels of granzyme B, IFN-gamma, and IL-4 in CD8(+)dT cells were increased significantly compared with those in CD8(+)pT cells. Both CD8(+)dT and CD8(+)pT cells were not activated after being cocultured with autologous trophoblast cells. Moreover, the production of granzyme B in CD103(+)CD8(+)dT cells decreased significantly compared with that in their CD103(-) counterparts. Coculture with decidual stromal cells and trophoblasts upregulated CD103 expression significantly in CD8(+)pT cells. Conclusions Our findings indicate that the selective silencing of effector functions of resident CD8(+)dT cells may favor maternal-fetal tolerance and that the decidual microenvironment plays an important role in promoting the residency of CD8(+)T cells and their tolerance-defense balance.