Immunohistologic evidence that villitis in human normal term placentas is an immunologic lesion.

Villitis of unestablished origin is a lesion in placentas from normal and high-risk pregnancies. We have studied villitis areas in 25 normal term placentas for immune cells, coagulation components, and endothelial markers. Villitis areas were filled with activated (HLA-DR, HLA-DP, and HLA-DQ reactive) macrophages. B lymphocytes were not identified, and T lymphocytes were of the helper (CD4) phenotype. Antibodies to coagulation components revealed perivascular and trophoblastic basement membrane deposits of factor IX, increased numbers of platelets, and fetal stem vessels that did not react with endothelial markers. These findings suggest helper T lymphocytes activate macrophages that mediate coagulation activation and alter endothelium. This combination of immunologic events results in tissue changes that are histologically diagnosed as villitis. It is not known what triggers these immunologic events, but the finding of villitis in normal placentas suggests the causative factor(s) is present in all pregnancies.