Abstract. A cDNA for the rabbit low Mr polymeric immunoglobulin (poly-Ig) receptor was expressed in an immortalized rabbit mammary cell line. The intracellular routing of the receptor and its cell surface expression was analyzed in stably transfected cells grown on permeable supports. Initially the cells formed a monolayer with no transmural electrical resistance. All monolayer cells expressed the poly-Ig receptor and cytokeratin 7 filaments characteristic of luminal mammary cells but absent in myoepithelial cells. Within 7 d in culture, the cells underwent cytodifferentiation and formed a bilayer with a transepithelial electrical resistance of,,,,500 fl x cm 2. Upper layer cells formed tight junctions with adjacent cells and gap junctions with basal cells. Expression of the poly-Ig receptor and cytokeratin 7 was restricted to the cells from the upper layer. The kinetics of receptor biosynthesis and processing was similar to that reported for rabbit mammary gland and rat liver. The receptor was T HE (poly-Ig) t polymeric immunoglobulin receptor, which mediates transcytosis of polymeric-IgA antibodies across glandular or mucosal epithelia, plays a crucial role in mucosal immunity. The receptor allows the antibodies produced in the interstitial space by local plasma cells to reach the environment where they interact with and eventually cross-link or neutralize pathogens (12). It also stabilizes the antibodies. After cleavage of the receptor (54), the extracellular domain, called secretory component (SC), remains tightly bound to the antibodies rendering them resistant to proteolytic degradation (29). The rabbit poly-Ig receptor has been cloned and sequenced (35), and its biosynthesis has been extensively studied in rat liver (57, 58, 63, 64), cultivated rat hepatocytes (38), rabbit mammary gland (53, 55), and in a human tumor cell line (32). The receptor is synthesized as a transmembrane glycoprotein (26, 34) which is terminally glyeosylated in the Golgi complex in,~30 rain (53, 63, 64). During its intracellula
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