RECEPTOR-MEDIATED ENDOCYTOSIS AND PHAGOCYTOSIS BY RAINBOW-TROUT HEAD KIDNEY SINUSOIDAL CELLS

Endocytosis by rainbow trout head kidney sinusoidal cells was studied following intravenous administration of various ligands. Immunohistochemistry was used to demonstrate that endothelial cells took up the soluble ligands ovalbumin and dinitrophenylated human serum albumin (DNP33HSA). Ovalbumin and DNP33HSA are endocytosed via the mannose- and the scavenger receptor, respectively. Sinusoidal macrophages took up DNP33HSA but only occasionally ovalbumin. The particulate ligand, colloidal human serum albumin (cHSA) (0·2-1μm) was taken up by both endothelial cells and sinusoidal macrophages. Endothelial cells were estimated to account for at least 15% of uptake of intravenously injected 125 I-tyramine cellobiose-labelled cHSA (125I-TC-cHSA) by head kidney cells. Uptake of 125I-TC-cHSA could be demonstrated in cultured head kidney macrophages. Intracellular transport of in vivo internalized 125I-TC-cHSA to lysosomal compartments occurred at a slower rate compared to ligands endocytosed via the mannose- and scavenger-receptors. Electron microscopic studies of head kidney endothelium that had taken up colloidal carbon, revealed that sinusoidal endothelial cells possess the ultra-structural apparatus characteristic for cells active in endocytosis of blood-borne ligands; namely well defined coated pits along the luminal plasma membrane, numerous intracellular vesicles and large, lysosome-like dense bodies. Thus, the results of this study suggest that both head kidney endothelial cells and sinusoidal macrophages are important ‗scavenger‘ cells in rainbow trout. © 1994 Academic Press. All rights reserved.