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 synthesi s to the mucosal lumen is mediated by the polymeric Ig receptor (pIgR) . During transcytosis, a disulfide bond forms between pIgR and pIgA, r esulting in secretion of a covalently linked complex. To dissect furth er the intracellular processing and functions of pIgR, we have express ed 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 ex tracellular domain of pIgR in culture supernatants. The function of tr ansfected pIgR was confirmed by measuring vectorial transcytosis of I- 125-labeled pIgA and its disulfide bonding to pIgR. Species specificit y 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 p IgR, with rat pIgA being transported to the greatest extent in each ca se. However, disulfide bonding was observed only with human pIgR, and was found to occur mainly inside the cell. Our results suggest that co nformational differences between human and rabbit pIgR may account for differences in disulfide bonding to pIgA, and show that efficient tra nscytosis of pIgA is correlated better with noncovalent than covalent binding to pIgR.