An 80-kDa syncytiotrophoblast alloantigen bound to maternal alloantibody in term placenta

We have shown that most of the IgG present on term syncytiotrophoblast, membrane, microvesicles is bound to an 80 kDa protein antigen (R80K).

Microvesicles were prepared from term human placenta, and the IgG eluted at pH3.

When IgG antibody was eluted at pH3 and reacted with acid-treated vesicles of other placentae, the alloantibody always bound to the preparation from which it was obtained, but only to about 10% of acid-treated preparations from other placentae. A similar polymorphic protein found in association with IgG antibody was found in term horse placentae. Cross-reactivity of the antibodies between species was not found. Using binding of labelled antibody, complement dependent cytotoxicity and FACS two-color analysis, the human polymorphic antigen was present on peripheral blood monocytes and B-lymphocytes. The R80k antigen on intact microvesicles was resistant to trypsin, but after acid elution of IgG, trypsin released a soluble 50 kDa fragment which reacted with the acid-eluted IgG antibody.

The presence of antibodies to R80K in all term placentae studied, including first pregnancies, suggests that development of this alloantibody may be a normal requirement for successful pregnancy.