Overexpression of the human neonatal Fc-receptor α-chain in trophoblast-derived BeWo cells increases cellular retention of β2-microglobulin

Major histocompatibility complex (MHC) class I and MHC class I-type molecules such as the neonatal Fcγ-receptor, FcRn, are heterodimers consisting of a transmembrane α-chain non-covalently associated with β2-microglobulin (β2m). Human placental villous syncytiotrophoblast (STB) lacks MHC class I molecules, but express hFcRn that mediates materno-fetal transmission of immunoglobulin G (IgG). Trophoblast-derived BeWo cells that are used to study placental IgG transport likewise express β2m and low levels of hFcRn α-chain. The contribution of FcRn α-chain in retention and subcellular distribution of β2m in STB and BeWo cells is unclear. To investigate this issue, we increased expression of hFcRn α-chain in BeWo cells (BeWo/hFcRn) by cDNA transfection. Overexpressed hFcRn protein exhibited the characteristic pH-dependent IgG binding and association with β2m. In comparison to parental BeWo cells, β2m mRNA levels in BeWo/hFcRn cells were not significantly altered, but total cell-associated β2m protein was increased by 120%. Treatment of BeWo and BeWo/hFcRn cells with brefeldin A, an inhibitor of the secretory pathway, abrogated this effect, demonstrating that hFcRn α-chain expression retained otherwise secreted β2m. Flow cytometry revealed that β2m plasma membrane expression was unaffected by α-chain overexpression whereas by fluorescence microscopy a preferential staining of β2m in peripheral endosomes was observed.