Regulation of antigen expression on the placenta: A unique type of immunosuppression

The broadly shared pregnancy-associated (Pa) MHC class I antigen that elicits a nondestructive antibody response in the mother during normal pregnancy is located on the same cells (basophilic trophoblast) that carry the classic class I allele-specific antigens (RT1.A^a). These cells express the Pa antigen on the cell membrane as well as in their cytoplasm, whereas the classic class I allele-type-specific antigen (RT1.A^a) is expressed only in the cytoplasm of the basophilic trophoblast. The absence of antigenic expression on the cell membrane prevents the maternal recognition of these antigens. Therefore, selective regulation of antigenic expression on the trophoblastic cell membrane explains the escape of the placenta from the maternal immune destruction during normal pregnancy. A similar regulation mechanism may occur in neoplastic tissue, thus preventing their immune destruction by the host.