Suppression of Mitogen-Induced Lymphoproliferation by Soluble IgG Fc Receptors in Retroplacental Serum in Normal Human Pregnancy

A competitive ELISA was used to quantify soluble IgG Fc receptors (FcR) in retroplacental serum (RPS) and peripheral serum (PS) from 10 women after uncomplicated full-term deliveries. The RPS contained significantly higher amounts of soluble FcR than did PS from the same individuals. RPS suppressed phytohaemagglutinin-stimulated lymphoproliferation as compared with PS, and a positive correlation ( r =0.66) was found between the degree of suppression and the difference in soluble FcR level between RPS and PS. Absorption of sera with Sepharose 4B coupled with heat-aggregated IgG strongly reduced the immunosuppressive activity, whereas absorption with Sepharose coupled with IgG F(ab')₂ fragments did not. When IgG-binding material was eluted from Sepharose beads and added to cell cultures, the immunosuppressive activity was restored. The data indicate that soluble FcR at physiological levels have immunosuppressive properties. FcR-mediated immunosuppression may be of importance for maintenance of local immunosuppression during pregnancy.