Normal human pregnancy is associated with an elevation in the immune suppressive CD25⁺ CD4⁺ regulatory T‐cell subset

CD4⁺ CD25⁺ T regulatory cells (T_Reg), suppress antigen‐specific immune responses and are important for allograft tolerance. During pregnancy the mother tolerates an allograft expressing paternal antigens (the fetus) requiring substantial changes in immune regulation over a programmed period of time. We analysed whether immune‐suppressive T_Reg cells were altered during pregnancy and therefore might play a part in this tolerant state. The presence of T_Reg cells was assessed in the blood of 25 non‐pregnant, 63 pregnant and seven postnatal healthy women by flow cytometry. We observed an increase in circulating T_Reg cells during early pregnancy, peaking during the second trimester and then a decline postpartum. Isolated CD25⁺ CD4⁺ cells expressed FoxP3 messenger RNA, a marker of T_Reg cells, and suppressed proliferative responses of autologous CD4⁺ CD25^– T cells to allogeneic dendritic cells. These data support the concept that normal pregnancy is associated with an elevation in the number of T_Reg cells which may be important in maintaining materno‐fetal tolerance.