B7 family molecules: novel immunomodulators at the maternal-fetal interface.

The placenta utilizes both active and passive mechanisms to evade rejection by the maternal immune system. Recently, the mRNA for two newly cloned members of the B7 family of immunomodulatory cell-associated proteins have been identified in the human term placenta. In this article, we review the current knowledge of the B7 family member B7-H1, and discuss how it may participate in modulation of the maternal immune system at the maternal-fetal interface. B7-H1 has been found to possess immunostimulatory or immunoinhibitory properties, and immunohistological examination of first trimester and term placenta has revealed that this protein is abundant in the placenta. B7-H1 is highly expressed by both the syncytiotrophoblast and extravillous cytotrophoblast, both of which lie in direct contact with maternal blood and tissue. Further, treatment of the choriocarcinoma cell line, JEG-3, with recombinant human interferon (IFN)-gamma resulted in a dose-dependent increase in the abundance of the message for B7-H1, suggesting that IFN-gamma could regulate expression of B7-H1 by the trophoblast. These studies document that the positioning of B7-H1 at the maternal-fetal interface is such that it could participate in suppression of activated maternal leukocytes.