Opsonic effect of globulins on phagocytosis of liposomes by bone marrow cells.

Abstract

In recent years there has been a great surge in the development of particulate drug carriers for chemotherapy and diagnostic medicine. However, to realise the full potential of various drug carriers such as liposomes, it is necessary to understand how these particles are cleared from the circulation when injected intravenously, and what determines their recognition by the reticuloendothelial cells in the liver, spleen and bone marrow. In the circulation the drug carriers become coated with plasma components and this process is thought to determine their recognition by the reticuloendothelial cells. Liposomes coated in vifro with opsonins such as immunoglobulin, components of the complement system, fibronectin. C-reactive protein etc., when injected intravenously are cleared rapidly from the circulation (1). Recently we have demonstrated the presence of opsonins in serum which exhibit tissue specificity and described their properties which differentiate between 'liver-specific' and 'spleen-specific' opsonins responsible for the enhancement o f phagocytosis of liposomes by Kupffer cells and splenic macrophages respectively (2,3). However, so far there has been no clear indication to suggest the identity of the tissue-specific opsonin involved in phagocytosis of the injected liposomes. Bone marrow is one of the major organs of the reticuloendothelial system and plays a small but significant role in the clearance of foreign particulates including liposomes. In vifro serum is found to enhance the uptake of cholesterol-containing liposomes in the bone marrow cells. This opsonic activity of serum is destroyed when the serum is heated at 55°C for 30 min. This suggests that heat-labile complement may be involved in the phagocytosis of liposomes. However, when dialysed or EGTA-chelated serum is used, a complete loss of its opsonic activity occurs. Addition of the serum dialysate or divalent cations to the dialysed serum does not reinstall its lost opsonic activity (4). Thus these results shed doubt on the possibility of involvement of the complement and hence we searched for another heat-labile serum factor which may be responsible for enhancing phagocytosis of liposomes in these cells. Heat-labile pand y-globulin are known to enhance phagocytosis of certain strains of bacteria (1) and p-globulin is also known to enhance uptake of anionic liposomes into the perfused liver (5). Hence we examined the effect of various serum globulins on the uptake of neutral high cholesterol liposomes in the freshly prepared bone marrow Cells. The results in Table 1 show that y-globulin and immunoglobulin in the absence or presence of serum have little effect on the uptake of liposomes in the bone marrow cells, whereas p-globulin in the absence of serum stimulates the uptake of liposomes by 5-fold as compared to the control and it retains its opsonin activity in the presence of serum. Thus these results suggest that p-gloublin may be the heat-labile serum factor that enhances phagocytosis of liposomes in the bone marrow. Table 1