Two Mechanisms for IgG Uptake in Cultured Human Trophoblast: Evidence for a Novel High Affinity Fc Receptor

ABSTRACT: The mechanism of IgG transport by the placental trophoblast was examined by studying IgG uptake by purified trophoblast maintained in culture. This model retains the ability to bind and endocytose human IgG from human serum. Comparison of the relative IgG uptake by the trophoblast among the four subclasses of both human and mouse IgG indicates that the trophoblast IgG receptor has different affinities from those described for the three known human Fcγ receptors, FcRγI, FcRγII, and FcRγIII. These results suggest the presence of a novel trophoblast Fcγ receptor. Although FcγRIII has been reported to be present on trophoblasts, immunocytochemical studies failed to detect binding to the cell surface of antibody-specific for FcγRIII, 3G8 MAb. In addition, blocking studies with MAb 3G8 did not interfere with IgG uptake. Scatchard analysis of human IgG uptake revealed a biphasic curve consistent with two distinct mechanisms for the transport of IgG by the trophoblast. The first is a higher affinity system (Ka = 1.7 × 107 M-1 1.7 × 104 binding sites/cell) which exhibits IgG subclass and species specificity, and the second is a low affinity system (Ka = 6.9 × 103 M-1, 7.5 × 107 binding sites/cell).