Transcellular transport of polymeric IgA in the rat hepatocyte: Biochemical and morphological characterization of the transport pathway

Polymeric IgA (plgA) is transported by liver parenchymal cells (hepatocytes) from blood to bile via a receptor-mediated process. We have studied the intracellular pathway taken by a TEPC15 mouse myeioma pIgA. When from 1 µg to 1 mg ¹²⁵I-pIgA was injected into the saphenous vein of a rat, 36% was transported as intact protein into the bile over a 3-h period. The concentration of transported ¹²⁵I-pIgA was maximal in bile 30-60 min after injection, and ~80% of the total ¹²⁵I-pIgA ultimately transported had been secreted into bile by 90 min. A horseradish peroxidase-pIgA conjugate (¹²⁵I-pIgA-HRP) was transported to a similar extent and with kinetics similar to that of unconjugated ¹²⁵I-pIgA and was therefore used to visualize the transport pathway. Peroxidase cytochemistry of livers fixed in situ 2.5 to 10 min after ¹²⁵I-pIgA-HRP injection demonstrated a progressive redistribution of labeled structures from the sinusoidal area to intermediate and bile canalicular regions of the hepatocyte cytoplasm. Although conjugate-containing structures began accumulating in the bile canalicular region at these early times, no conjugate was present in bile until 20 min. From 7.5 to 45 min after injection ~30% of the labeled structures were in regions that contained Golgi complexes and lysosomes; however, we found no evidence that either organelle contained ¹²⁵I-pIgA-HRP. At least 85% of all positive structures in the hepatocyte were vesicles of 110-160-nm median diameters, with the remaining structures accounted for by tubules and multivesicular bodies. Vesicles in the bile canalicular region tended to be larger than those in the sinusoidal region. Serial sectioning showed that the ¹²⁵I-pIgA-HRP-containing structures were relatively simple (predominantly vesicular) and that extensive interconnections did not exist between structures in the sinusoidal and bile canalicular regions.