Expression of placental alkaline phosphatase does not correlate with IgG binding, internalization and transcytosis

The human homologue of FcRn, an IgG Fc receptor expressed in rat villous syncytiotrophoblasts, might be involved in IgG transfer from the maternal to the fetal circulation. However, because the receptor does not bind IgG at the physiological pH of the maternal blood (pH 7.4), FcRn is probably not involved in the initial uptake of IgG. A role in IgG internalization has been suggested for placental alkaline phosphatase (PLAP), which is highly expressed on the apical surface of syncytiotrophoblasts. To determine whether PLAP does indeed have a role in IgG uptake, we analysed the ability of PLAP to bind, internalize and transcytose IgG in BeWo choriocarcinoma cells endogenously expressing the protein, or in Madin-Darby canine kidney (MDCK) cells transfected with the PLAP cDNA. Although PLAP expression in MDCK cells resulted in increased IgG binding to intact cells, binding was not correlated with the level of PLAP expressed in the different cell lines. Furthermore our findings do not support a role for PLAP in IgG endocytosis or transcytosis.