Human Kupffer cell recognition and phagocytosis of apoptotic peripheral blood lymphocytes

Cells undergoing apoptosis are recognized and rapidly phagocytosed by macrophages before their degradation, thus preventing the inflammatory reaction and protecting tissues from the damaging effects of released potentially harmful intracellular contents. In spite of growing interest in the mechanisms leading to the engulfment of apoptotic cells, the molecular bases by which an apoptotic cell is recognized are not entirely understood. Among the several potential mechanisms by which a macrophage can identify a cell as apoptotic, the data reported in the present paper support the idea that Kupffer cells phagocytose apoptotic cells by means of lectin-like receptors. Human Kupffer cells, which possess galactose-specific binding sites, can recognize and phagocytose peripheral blood lymphocytes undergoing apoptosis after heat shock (43 degrees C) or cycloheximide treatment, but not normal living peripheral blood lymphocytes. The putative structure by which apoptotic peripheral blood lymphocytes are targeted as "edible" could be the molecular changes in the plasma membrane, In fact, our experiments indicate that the membranes of apoptotic peripheral blood lymphocytes express increased amounts of N-acetylgalactosamine, D-galactose, and mannose residues when compared with membranes of normal PBL. Phagocytosis was inhibited by adding to the culture medium sugar cocktail solution (glucose, N-acetylgalactosamine, methyl mannopyranoside, fucose, 80 mM final concentration) or to a lower extent by desialylated glycoproteins (lactosylated bovine serum albumin, asialofetuin, 2 mg/ml final concentration), but not by nondesialylated glycoproteins (fetuin, 2 mg/ml final concentration, bovine serum albumin, 20% final concentration). In addition, phagocytosis of apoptotic peripheral blood lymphocytes by human Kupffer cells was a very rapid process, being almost entirely completed within 15 min of incubation.