Expression of TAP1 by human trophoblast.

Successful placentation in the human is dependent on the trophoblast evading recognition and destruction by the maternal immune system. However, invasive cytotrophoblast express HLA-G which may be able to present peptide to T cells. Transporter proteins are essential for peptide presentation and major histocompatibility complex (MHC) class I assembly. We have determined their expression by trophoblast in relation to HLA-G, using immunohistochemistry. Anti-transporter protein antibody (TAP1) labeling closely paralleled that of MHC class I, but the intensity of its expression was much greater on the HLA-G+ extravillous cytotrophoblast than any other fetal or maternal tissue in the first trimester and at term. This suggests that the extravillous cytotrophoblast are very actively assembling MHC class I antigens with peptides. However, expression of MHC class I by the cytotrophoblast was not correspondingly elevated. This pattern could result from HLA-G being shed from the surface of the trophoblast, a process which may play a central role in protecting the fetus from maternal immune attack.