Antigen expression by cells near the maternal-fetal interface.

The purpose of the present study was to evaluate the potential for immunologic interaction between the mother and fetus by documenting 1) fetal and maternal cell histocompatibility antigen (HLA) expression and 2) populations of immunologically relevant cells near the maternal-fetal interface through all stages of normal pregnancy. Mesenchymal cells in extraembryonic tissues demonstrated a gradual and progressive development of both class I and class II HLA, with class I expression preceding class II. Coordinated development of expression of two subclasses of class II HLA-D, HLA-DR and HLA-DQ, by fetal mesenchymal cells was noted. In adjacent tissue, maternal decidual cells were strongly class I HLA positive; but in contrast to fetal cells, expression of HLA-D subclasses was discoordinate. HLA-DR was present throughout gestation but HLA-DQ expression was detectable only in second and third trimester tissues. Immunologically relevant cells were present in both fetal and maternal tissues. The major leukocyte population and the major class II-bearing cell type at the maternal-fetal interface was of monocyte/macrophage lineage. T and B lymphocytes were present only in very low densities (1-3% of all cells), whereas at all stages of gestation, macrophages were present in high density in both the fetal mesenchyme (14-25%) and in maternal decidua (27-32%). Documentation of class I and class II HLA expression and the cell types available to participate in immunologic events at the maternal-fetal interface may assist in understanding the immunologic basis of the maternal-fetal relationship during successful pregnancy.