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Immune Recognition at the Maternal‐Fetal Interface: Overview

ABSTRACT: Trophoblast antigens at the maternal‐fetal interface that are capable of stimulating maternal immune responses have been studied. Candidates are blood group I and P, HA, FcΔ‐receptors, TLX, and phospholipids. Antigens I and P on trophoblast have been implicated in pregnancy loss but incompatible i,p mothers are rare. HLA‐G is expressed on cytotrophoblast; however, no evidence for HLA‐G allotypy or maternal responses to these molecules exists, although HLA‐G has been implicated in recruitment of suppressor T cells. Receptors for IgG (FcΔ‐RI, FcΔ‐RII and FcΔ‐III) are present on trophoblast but allotypy is limited to the NA1‐NA2 antigen system associated with FcΔ‐RIII on neutrophils. Maternal Fc‐γR blocking antibodies have been linked to pregnancy success. The TLX alloantigen system was described by using xenogeneic antisera. Idiotype‐antiidiotype regulated maternal responses to TLX are proposed as necessary for successful pregnancy. Several putative TLX monoclonal antibodies (Mab) recognize a regulator of complement activation called MCP (membrane cofactor protein, or CD46). Mab to MCP do not exhibit allotypy. Syncytial and cytotrophoblastic membranes are rich sources of MCP. Preliminary data suggest that a conformational site induced by C3b (iC3) binding to MCP may be responsible for TLX allotypy. Certain pregnancy loss patients produce antiphospholipid antibodies (aPA). Some investigators believe that aPA recognize a plasma protein cofactor, β2GPI and not phospholipid per se. We produced three Mab specific for β2GPI, one of which fails to recognize β2GPI bound to phospholipid. By using these Mab in immunocytochemical studies of normal term placentae, β2GPI was observed on syncytial and cytotrophoblast as well as on fetal stem vessel endothelia. These Mab indicate that the β2GPI present in placentae is phospholipid bound.