B7 Family Molecules Are Favorably Positioned at the Human

The human placenta utilizes both active and passive mecha-nisms to evade rejection by the maternal immune system. We investigated the pattern of expression of the B7 family of im-munomodulatory molecules B7-H1 (PD-L1), B7-2 (CD86), and B7-1 (CD80) at the term maternal-fetal interface. Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses showed that B7-H1 mRNA is abundant in term placenta and that cytotrophoblasts are sources of this message. Immu-nohistochemistry demonstrated that B7-H1 is constitutively ex-pressed by the syncytiotrophoblast and by extravillous cytotro-phoblasts, both of which are juxtaposed to maternal blood and tissue. By contrast, placental stromal cells, including macro-phages, lacked the protein. Expression of B7-H1 protein was low in first-trimester placenta compared to second- and third-tri