Characterization of regulatory T cells in decidua of miscarriage cases with abnormal or normal fetal chromosomal content

Decreased regulatory T (Treg) cells have been reported in cases of recurrent pregnancy loss. To understand the role of Treg cells in human pregnancy, we have studied the frequency, localization and characterization of Treg cells in the decidua. The frequency of Foxp3⁺ cells among CD3⁺CD8^- cells at the decidua basalis in cases of miscarriage with a normal embryo karyotype (n=10) was significantly lower than in normally progressing pregnancies (n=10). However, those frequencies in miscarriage with an abnormal embryo karyotype were similar to normally progressing pregnancies. Next, we used flow cytometry to study Treg cell expression of the proliferation marker Ki67 and functional Treg marker CCR5. The frequency of Foxp3⁺CD4⁺ T cells in miscarriage with a normal embryo (n=10) was significantly lower than those in normally progressing pregnancies (n=15) and in miscarriage with an abnormal embryo (n=14). In miscarriage with a normal embryo, the population of Ki67^-Foxp3⁺CD4⁺ T cells was significantly smaller than in normal pregnancy. However, the frequencies of Ki67⁺Foxp3⁺CD4⁺ cells and CCR5⁺Foxp3⁺CD4⁺ cells were not different between the three groups. These data suggest that increased Ki67^- Treg cells in the decidua basalis may play an important role in the induction of immune tolerance, and that immune-medicated pregnancy loss may be caused by decreased Ki67^- Treg cells in the implantation site.