PROBLEM: We have shown that most of the IgG present on term syncytiotrophoblast, membrane, microvesicles is bound to an 80 kDa protein antigen (R80K).
METHODS: Microvesicles were prepared from term human placenta, and the IgG eluted at pH3.
RESULTS: 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-lympho-cytes. 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.
CONCLUSION: 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.