LIVER SINUSOIDAL CELLS Identification of a Subpopulation for Erythroeyte Catabolism

Sequestration and degradation of red blood cells (RBC) are believed to occur in part in the liver, but the magnitude and cellular localization of this process remain uncertain. This problem was studied in rats by investigating isolated parenehymal and sinusoidal cell populations of the liver. After digesting the perfused liver with pronase, hepatic sinusoidal cells were isolated free of RBC and debris. Of the isolated cells, 90 % were phagocytic, as judged by their uptake of colloidal ~gSAu or of aggregated albumin-lalI administered in vivo After administration of spherocytic (heat-treated) RBC, however, only about one quarter of the isolated cells were found to contain phagocytized RBC. This apparently distinct popula~ tion of RBC-phagocydzing cells is designated as '°erythrophagocytic (EP)" ceils. The EP cell population was further characterized functionally by its specific phagocytosis of coltoidaI carbon and of 99mtechnetium-sulfur colloid and histochemically by its peroxidase activity. The role of the EP population in the catabolism of RBC-hemoglobin was studied in isolated hepatic sinusoidal ceils by assay of microsomal heme oxygenase (MHO), which is the inducible enzyme system that converts heine to bilirubin. The M H O activity of individual sinusoidal isolates was related directly to their content of EP ceils Assay of the M H O activity of the whole spleen and of the total EP cell population of the liver suggested that these two tissues may be of comparable importance in their ability to degrade RBC-hemoglobin. Senescent red blood cells (RBC) disappear from the circulation of the rat at a rate of about 0.67 % per day (2). Sequestration is thought to take place primarily in the spleen, liver, and bone marrow (3-5) by virtue of the relatively large number of reticuloendothelial cells in these tissues. The removal of RBC is believed to occur by a process of phagocytosis followed by intracellular digestion of the red cell matrix and hemoglobin (6). The protein moiety of hemoglobin and the RBC matrix appear to be degraded primarily by acid hydrolases (7), while fission of the protoporphyrin ring of the hemoglobin-heme is catalyzed by microsomal heme oxygenase (MHO). This microsomal enzyme system was recently shown to cleave ferriprotoporphyrin IX at its a-methene bridge and thereby convert the heme to bilirubin IX-o~ (8, 9), The enzyme is present in relatively high specific activity in spleen, liver, and bone marrow (10) and is inducible by substrate (10-12). Within the liver, it is believed that RBC are sequestered and degraded by sinusoidal cells with phagocytic properties, frequently referred to as Kupffer cells (5). Recent light and electron microscope studies (13, 14) suggested the possibility that only a fraction of the total sinusoidal cells may be concerned with RBC phagocytosis. If this concept is correct, the RBC-phagocytizing cell ought to be T~IE Jolra~aI~ oF CELL BIoLoaY • VoLtr~ 54, 197~ • pages 107-119 107 on O cber 9, 2017 jcb.rress.org D ow nladed fom identifiable by its high M H O activity and by morphological evidence of RBC sequestration. In the present experiments, it was possible to separate hepatic parenchymal and sinusoidal cells into pure and viable fractions and to identify a populat ion of sinusoidal ceils that is specifically concerned with RBC phagocytosis. These socalled erythrophagocytic (EP) ceils differ from other sinusoidal cells in their phagocytic and histochemical properties. Moreover, indirect evidence indicates that EP cells contain most, if not all, of the M H O activity present in isolates of hepatic sinusoidal cells. Par t of this work has been presented in abstract form (1). M A T E R I A L S A N D M E T H O D S