Comparative studies of transcytosis and assembly of secretory IgA in Madin-Darby canine kidney cells expressing human polymeric Ig receptor.

Epithelial transport of polymeric IgA (pIgA) from its site of synthesis to the mucosal lumen is mediated by the polymeric Ig receptor (pIgR). During transcytosis, a disulfide bond forms between pIgR and pIgA, resulting in secretion of a covalently linked complex. To dissect further the intracellular processing and functions of pIgR, we have expressed the entire coding sequence of human pIgR cDNA in Madin-Darby canine kidney (MDCK) cells. Cloned transfected cells express human pIgR, as detected by immunofluorescence and by quantification of the cleaved extracellular domain of pIgR in culture supernatants. The function of transfected pIgR was confirmed by measuring vectorial transcytosis of 125I-labeled pIgA and its disulfide bonding to pIgR. Species specificity of transcytosis was determined by comparing transport of human, rat, and mouse pIgA in MDCK cells expressing either human or rabbit pIgR. pIgA from all three species was transported by both human and rabbit pIgR, with rat pIgA being transported to the greatest extent in each case. However, disulfide bonding was observed only with human pIgR, and was found to occur mainly inside the cell. Our results suggest that conformational differences between human and rabbit pIgR may account for differences in disulfide bonding to pIgA, and show that efficient transcytosis of pIgA is correlated better with noncovalent than covalent binding to pIgR.