Allo‐Antigenicity of Trophoblast Cells*

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Abstract
Trophoblast cells of the placenta represent the frontier of the conceptus at the feto‐maternal interface, exposed to the maternal immune system. The presence or the absence of major histocompatibility (MHC) antigens on trophoblast cells, thus, is a key determinant of immune interactions between the mother and the allogeneic fetus during pregnancy. This article critically reviews the studies of the alloantigenic status of murine and human trophoblast cells, including the published data in the mouse and unpublished data in the human in the authors' own laboratory. It is shown that, in both these species, a significant proportion of trophoblast cells express Class 1 but not Class 2 MHC antigens in a manner directly accessible to the maternal immune system. However, the lack of immunogenicity of semiallogeneic trophoblast cells can not solely be explained on the basis of the absence of Class 2 molecules, despite the presence of Class 1 molecules. It is proposed either that the Class 1 MHC antigens on the trophoblast cells are presented in a nonimmunogenic manner to the maternal T cells or else that certain cells at the feto‐maternal interface (eg, fetally derived trophoblast cells, maternally derived lymphoid cells, or decidual cells) elaborate potent immunosuppressor molecules capable of abrogating T cell alloreactivity.