MHC class I-related neonatal Fc receptor for IgG is functionally expressedin monocytes, intestinal macrophages, and dendritic cells

The neonatal Fc receptor (FcRn) for IgG, an MHC class I-related molecule, f unctions to transport Ige across polarized epithelial cells and protect IgG from degradation, However, Little is known about whether FcRn is functiona lly expressed in immune cells, We show here that FcRn mRNA was identifiable in human monocytes, macrophages, and dendritic cells, FcRn heavy chain was detectable as a 45-kDa protein in monocytic U937 and THP-1 cells end in pu rified human intestinal macrophages, peripheral blood monocytes, and dendri tic tells by Western blot analysis, FcRn colocalized in vivo with macrosial in (CD68) and Ncl-Macro, two macrophage markers, in the lamina propria of h uman small intestine, The heavy chain of FcRn was associated with the beta (2)-microglobulin (beta (2)m) light chain in U937 and THP-1 cells. FcRn bou nd human IgG at pH 6.0, but not at pH 7.5. This binding could be inhibited by human Ige Fc, but not Fab, FcRn could be detected on the cell surface of activated, but not resting, THP-1 cells, Furthermore, FcRn was uniformly p resent intracellularly in all blood monocytes and intestinal macrophages, F cRn was detectable on the cell surface of a significant fraction of monocyt es at lower levels and on a smalt subset of tissue macrophages that express ed high levels of FcRn on the cell surface. These data show that FcRn is fu nctionally expressed end its cellular distribution is regulated in monocyte s, macrophages, and dendritic cells, suggesting that it may confer novel Ig G binding functions upon these cell types relative to typical Fc gamma Rs: Fc gamma RI, Fc gamma RII and Fc gamma RIII.