Natural killer cell-mediated recognition of human trophoblast.

Large numbers of natural killer cells are present within the maternal decidua close to the extravillous trophoblast cells. Most of these natural killer cells express high levels of inhibitory NK receptors (KIR) specific for HLA class I molecules. Since trophoblast cells only express HLA-C and HLA-G, the KIR expressed by decidual NK cells can only recognize these HLA class I molecules in order to avoid NK-mediated rejection of fetal tissues. We show that HLA-C recognition by decidual NK cells can be mediated by p58, LIR-1 and (indirectly) by CD94/NKG2A receptors. On the other hand, HLA-G recognition is not only mediated by LIR-1 and (indirectly) by CD94/NKG2A but also by a newly identified receptor termed p49. The p49 receptor, unlike the other KIR, appears to be selectively expressed by decidual NK cells.