HLA Class I Expression on the Materno‐Fetal Interface

PROBLEM: The conditions that permit the genetically dissimilar (haplo‐non‐identical) human fetus to evade rejection by its mothers immune system have been the subject of intense interest for several years. As the placental cells, which are in contact with maternal blood or tissue, are devoid of HLA class II antigens, the interest has focused on the expression of HLA class I molecules. METHOD OF STUDY: Recent findings on the constitutive, transcriptional, and translational expression of HLA class I molecules on anatomically and morphologically different sub‐populations of trophoblast cells will form the basis of this article. RESULTS: The expression of HLA class I molecules in the trophoblast cells, forming the materno‐fetal junctional zone is inhomogeneous. It differs depending on the differentiation and location of trophoblast cells within the placenta and furthermore on the stage of gestation. On the transcriptional level HLA‐A, ‐B, ‐C, ‐E, and ‐G could be detected on individual trophoblast populations, whereas only HLA‐C and HLA‐G seem to be translated to protein. CONCLUSIONS: The expression of HLA class I antigens by trophoblast cells is not simply suppressed. Instead, less polymorphic HLA‐G and HLA‐C antigens are carefully selected from the class I multigene family. This gives rise to the assumption that these two HLA class I molecules play an important role in the maintenance of pregnancy.