Leucocyte populations and cytokine regulation in human uteroplacental tissues

Human endometrial tissue and the decidualized endometrium in pregnancy cont ain relatively large numbers of leucocytes, the proportions of which vary d uring both the menstrual cycle and early pregnancy. CD3(+) T-cells, CD14(+) macrophages and a population of phenotypically unusual CD3(-)CD16(-)CD56(+) large granular lymphocytes (LGL) are present, whereas B-cells are virtua lly absent. Relative T-cell numbers decrease in first-trimester decidua; T- cells are therefore unlikely to have an important role in the immunological maintenance of pregnancy but could be more important in implantation, when their relative numbers are greater. Extensive numbers of class II MHC-posi tive tissue macrophages in both the endometrium and placenta will provide a n immediate antigen non-specific host defence to infection at this importan t site. Nevertheless, most attention has focused on a role for the predomin ant LGL endometrial cell population in the implantation and maintenance of pregnancy because, at the time of implantation, these LGLs comprise 70-80 % of all leucocytes in the endometrium. It is now well recognized that there is substantial and complex cytokine activity within human uteroplacental t issues; both leucocytic and non-leucocytic cells have been shown to be capa ble of producing a significant array of cytokines. However, to avoid excess ive pathological sequelae, such cytokine activity must be locally regulated . This has been highlighted by recent reports indicating that abnormal Th1- type cytokine responses could be a reason for immunological reproductive fa ilure in women. Key cytokines controlling differentiation into a Th1 (inter leukin 12) or Th2 (interleukin 4) type pattern both exist in unusual molecu lar forms at the human maternal-fetal tissue interface and hence might be f undamental regulatory elements controlling cytokine action locally by an an tagonistic action.