Transport of transferrin across the blood-thymus barrier in young rats

The interaction of transferrin-peroxidase (Tf-HRP) with the capillary endothelium of the rat thymus was analyzed by diaminobenzidine (DAB) cytochemistry and electron microscopy. The thoracic aortas of young rats were cannulated and the upper bodies perfused with the Tf-HRP conjugate. In the thymus, plasmalemmal vesicles of the endothelium mediated the endocytosis and transport of Tf-HRP. Transcytosis of Tf-HRP appeared to occur by micropinocytosis, without morphological evidence of involvement by endothelial endosomes. DAB reaction product was commonly present in the subendothelial interstitial space and, in addition, was often localized in clathrin coated pits and vesicles in epithelial reticular cells that surround the thymic capillaries. In perfusions done at 4 degrees C, no binding of Tf-HRP to the lumenal membranes of capillaries was detected. The transport of Tf-HRP across the capillary endothelium in the thymus was not inhibited by competition with excess native transferrin; however, the uptake of Tf-HRP by epithelial reticular cells was completely inhibited by excess transferrin. Transferrin receptors were localized in the thymus by indirect immunocytochemistry using OX-26, a mouse anti-rat transferrin receptor monoclonal antibody. No transferrin receptors were detected on the capillary endothelium but diffuse reaction product was localized on the subjacent epithelial reticular cells. These results indicate that transport of Tf-HRP across the thymus capillary wall is independent of transferrin receptors at the level of the endothelial cells but that subsequent uptake of Tf-HRP by epithelial reticular cells is a transferrin receptor dependent process.