Collagenase perfusion of the liver followed by pronase treatment of the cell suspension thus obtained gave a quantitative recovery of viable nonparenchymal liver cells (NPC). From these NPC, Kupffer (K) cells can be purified by attachment to tissue culture dishes. Tail vein injection of carbon 1-2 h before liver perfusion permitted stepwise calculation as well as visualization of carbon-containing K cells. When these K cells have been put into tissue culture medium with serum and incubated overnight, they exhibit typical macrophage characteristics. Phase-contrast and transmission electron microscopy showed typical macrophage morphology and scanning electron microscopy revealed well-spread cells with cytoplasmic projections and ruffled membranes. Endocytosis studies using radioactive colloidal gold and inert latex particles also indicated that these cells are highly active in pinocytosis and phagocytosis. Further characterization of K cells is the identification of Fc receptor on their membranes. Studies on lysosomal enzymes showed that purified K cells possess higher specific activities in beta-glucuronidase, acid DNase, and cathepsin D than in purified parenchymal cells.

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Ricin is a glycoprotein which is present in the seeds ofRicinus communis and is extremely toxic to animals and man (1, 2). After intravenous administration the LDs0 dose in mice is about 65 ng. The toxin inhibits the protein synthesis by inactivating the ribosomes. Ricin, which has a mol wt of 65,000 consists of two polypeptide chains joined together by an SS bond. The A chain or "effectomer" is an enzyme capable of inactivating specifically the 60S ribosomal subunit (2-8). The B chain or "haptomer" is responsible for the binding of the toxin to receptors on the cell surface, which is a prerequisite for toxic effect on intact cells. The binding is inhibited by lactose, and there is now good evidence that the B chain of the toxin interacts with membrane glycoproteins containing nonreducing terminal galactose residues (1, 9-13).The B chain of the toxin binds to animal cells in general including erythrocytes. Thus, to make the toxic effect selective against certain kinds of cells such as tumor cells, a number of points related to the introduction of the toxin into cells have to be clarified. In this study we have asked the following questions: (a) Can the toxin be introduced into cells through other cell membrane receptors than via the B-chain receptor? (b) Does internalization through endocytosis (most likely including exposure of the toxin to lysosomal enzymes) neutralize the toxic effect?In attempts to throw light on these questions we have studied the effect of [ricin.antiricin B] complexes on mouse macrophages and rat Kupffer cells. The results indicate that such complexes, subsequent to binding to Fc receptors of cells, are indeed internalized and lead to inhibition of protein synthesis and cell death. Materials and MethodsIsolation ofRicin. Ricin was extracted from the seeds ofRicinus communis (a batch of Castor beans obtained from Deutsche Rizinus Oelfabrik, Boley & Co, Krefeld-Urdingen, West Germany) and purified by chromatography using and Sepharose 4B columns (I,9,10). To prepare its constituent polypeptide chains, ricin was treated with 5% 2-mercaptoethanol in the presence of 0.5 M D-galactose, and the A and the B chain were separated on DE-52 and CM-52 columns as described elsewhere (1, 9, 10). Dilution of the toxin used in these experiments was always done in cell culture medium (containing 0.02% fetal calf serum).Preparation of Antitoxin. Antiserum against ricin was raised in rabbits. Ricin was first treated for 3 days with 5% formaldehyde at room temperature in 0.05 M sodium phosphate buffer

Liver and peritoneal macrophages under similar test conditions behaved in an identical manner with regard to accessory cell effects in the lymphocyte response to concanavalin A. When present in low concentrations (less than or equal to 3.3%) they stimulate lymphocytes, and when present in high concentrations (greater than or equal to 10%) they inhibit lymphocyte proliferation. These two effects are, however, mediated through totally different mechanisms. Stimulation was an early effect, required viable cells, was not affected by enzymatic treatment of macrophages, and was similar to the effect of 2-mercaptoethanol, allogeneic macrophages, and even non-macrophages. Inhibition occurred at a larger stage of lymphocyte transformation, was sensitive to collagenase and pronase treatment of macrophages, was more specifically due to macrophages, was reduced with allogeneic macrophages, and persisted after freeze-thawing of macrophages. Removal of Fc receptors or related segments of the surface of macrophages greatly reduced their inhibitory capacity, whereas removal of foreign surface receptors apparently had no consequence.