In vivo stimulation of polymeric Ig receptor transcytosis by circulating polymeric IgA in rat liver.

Binding of human polymeric IgA ligand to its epithelial cell polymeric Ig receptor, plgR, has been shown to stimulate plgR apical transcytosis in an in vitro system, based on polarized confluent MDCK cells expressing rabbit plgR. The present study aimed at testing whether such a stimulation also occurs in vivo. Transcytosis of plgR was monitored by rat liver output of total secretory component (SC) into bile, measured by radial immunodiffusion as the sum of free SC and plgA-bound SC. Whereas in the perfused rat liver system addition of plgA to the perfusate showed no effect, i.v. injection of human and rat plgA, but not of monomeric IgA nor PBS, in living rats significantly increased total bile SC output for more than 1 h. Furthermore, depletion of the normal plgA level circulating in the liver before injecting more plgA was not required to show the stimulation. Our data thus strongly suggest that stimulation of liver plgR transcytosis by plgA ligand binding is physiologically relevant, helping to quickly adjust plgA transport into bile to increase circulating plgA levels, without need for increased SC/plgR synthesis. [ABSTRACT FROM PUBLISHER]