MHC class I antigen expression by molar trophoblast

It is an issue for debate why molar tissues are not rejected by an immunologically potent host, since all genes in complete moles and 2/3 genes in partial moles are considered to be paternally derived. Molar trophoblasts are in direct contact with host cells, and therefore HLA expression by these cells may hold the key to the elucidation of the immunological reaction between molar tissues and the host. It has been reported that villous trophoblasts are negative and extravillous trophoblasts are positive for HLA-A, B,C, but the expressed HLA-A,B,C molecule has been noted to lack their polymorphic determinants. We analyzed the reactivity of two monoclonal antibodies to a monomorphic determinant of HLA-A,B,C (W6/32 and Cappel anti-HLA-A,B,C) with molar trophoblasts, using three uteri containing complete moles and two containing partial moles. The reactivity was examined by an indirect immunoperoxidase method. The staining patterns were almost identical in complete moles and partial moles. Villous trophoblasts showed a negative reaction with both antibodies. On the other hand, extravillous trophoblasts exhibited intense staining for W6/32 and negative staining for Cappel anti-HLA-A,B,C, which may suggest that the expression of a constant region as well as a variant region of HLA-A,B,C molecule is incomplete.