Displacement of platelets from blood to spleen following intravenous injection of liposomes encapsulating dichloromethylene bisphosphonate.

Liposomes encapsulating dichloromethylene bisphosphonate (Cl2MBP-liposomes) have been shown to cause selective depletion of phagocytic macrophages. We have shown that intravenous injection of Cl2MBP-liposomes into mice induces an almost complete depletion of F4/80-positive cells (mature macrophages) in the liver and in the splenic red pulp, but not in the lung. Platelets in the mouse contain a large amount of 5-hydroxytryptamine (5HT; serotonin) and so, by measuring 5HT, it is possible to assess the translocation of platelets to tissues. The injection of Cl2MBP-liposomes was found to induce a prolonged and marked increase in 5HT that occurred selectively in the spleen. On the other hand, 5HT in the blood decreased by as much as 50%. These changes in 5HT corresponded well with each other in terms of both time course and dose-response relationship. To judge from measurements made at the peak of the response, the 5HT increase in the spleen corresponded to about 80% of the 5HT lost from the blood. Electron microscopic analysis revealed a great accumulation of platelets in the splenic cords. We have shown that aggregation and degranulation of platelets in the lung is involved in rapid anaphylactoid shock induced within 10 min of intravenous injection into mice of a lipopolysaccharide [Shibazaki, M., Nakamura, M., Endo, Y., 1996. Biphasic, organ-specific, and strain-specific accumulation of platelets induced in mice by a lipopolysaccharide from Escherichia coli and its possible involvement in shock. Infect. Immun. 64, 5290-5294; Endo, Y., Shibazaki, M., Nakamura, M., Takada, H., 1997. Contrasting effects of lipopolysaccharides (endotoxins) from oral black-pigmented bacteria and Enterobacteriaceae on platelets, a major source of serotonin, and on histamine-forming enzyme in mice. J. Infect. Dis. 175, 1404-1412]. In the present study, it was found that such shock was almost completely prevented in those mice in which platelets were displaced from the blood by Cl2MBP-liposomes. These results suggest that in the spleen the depletion of phagocytic macrophages may impair the function or structure of this organ. This may lead to the entry of platelets into the spleen in such large numbers as to reduce their level in the blood and result in their prolonged accumulation in the spleen. The Cl2MBP-liposome may be an excellent tool for the in vivo investigation of the role of platelets, as well as that of macrophages.