Scavenger endothelial cells of vertebrates: A nonperipheral leukocyte system for high-capacity elimination of waste macromolecules
Studies over the last two decades have shown that mammalian nonmacrophagic liver endothelial cells clear the blood from numerous physiological and foreign waste macromolecules, such as polysaccharides and proteins released during extracellular matrix turnover, intracellular macromolecules, modified serum proteins, and bacterial and fungal proteins [Smedsrød, B., Pertoft, H., Gustafson, S. & Laurent, T. C. (1990) Biochem. J. 266, 313-327]. These macromolecules are released daily in gram-amounts in a normal human body and are effectively taken up and degraded by the liver endothelial cells. Recent studies show that bony fishes harbor a similar system of specialized nonmacrophagic scavenger endothelial cells in either kidney [Smedsrød, B., Gjøen, T., Sveinbjørnsson, B. & Berg, T. (1993) J. Fish Biol. 42, 279-291] or heart [Sørensen, K. K., Melkko, J. & Smedsrød, B. (1998) J. Exp. Biol. 201, 1707-1718], but not in liver. Using specific and extremely effective endocytosis, these fish scavenger endothelial cells function as their mammalian counterpart to eliminate soluble waste macromolecules from the circulation. We show here that species from all seven vertebrate classes carry a population of nonmacrophagic scavenger endothelial cells that efficiently eliminate an array of circulating waste macromolecules. Thus representing an important part of the vertebrate innate immune system, these scavenger endothelial cells display the following distribution in the different vertebrate classes: Gills in Agnatha and Chondrichtyes; heart or kidney in Osteichtyes; and liver in Amphibia, Reptilia, Aves, and Mammalia.