Figure 1.

Interaction between uNK cells and HLA class I positive cells at the feto-maternal interface. Humans: Spiral arteries develop through the decidua and fill the intervillous space with blood that supplies the placenta. Placenta villi are lined with villous cytotrophoblast (VT) under a layer of syncitiotrophoblast (ST), both of which are HLA class I and HLA class II negative. The villi are attached to the decidua via fetal extravillous trophoblast (EV). Some EVT migrate and line the spiral arteries as endovascular trophoblast (ENDVT) and other EVT invade the decidua where they interact with maternal lymphocytes such as dNK cells. Both EVT and ENDT express non classical HLA class I -E/C/G but do not express classical HLA- class I -A/-B and are HLA class II negative. Mice: Invasive trophoblast giant cells (TGC) from fetal origin form the boundary of the implantation site. They invade the maternal decidua and remodel the spiral arteries (Spa) by displacing the endothelial cells (EC) and allow the maternal blood flow through the spongiotrophoblast (SPT) layer to the labyrinth (not shown here). The spongiotrophoblast (SPT) and the glycogen cells (GC) are the major endocrine compartment of the placenta. In B6 mice, TGC lack expression of non-classical MHC-I but express classical MHC-I H2-K which can interact with maternal lymphocytes in the decidua such as uNK cells.