Microscopic localization of PEG-liposomes in a rat model of focal infection

In the present study the microscopic localization of polyethylene glycol PE G) liposomes in infected tissues was studied with both light microscopy (LM ) and transmission electron microscopy (TEM) in rats with focal intramuscul ar Staphylococcus aureus infection. PEG-liposomes containing colloidal gold were prepared and injected intravenously in rats with focal S. aureus infe ction and tissues were dissected at 24 h post injection. Sections were cut and liposomes were visualized for microscopic evaluation using silver enhan cement. Uptake of PEG-liposomes was visualized by both scintigraphy and LM in the abscess, liver and spleen. In the infected area, the liposomes were mainly found in the vicinity of blood vessels. TEM showed that the liposome s were localized in the macrophages and to a lesser extent in endothelial c ells in the infectious tissue. In the liver, the liposomes appeared mainly localized in Kupffer cells. In the spleen, uptake was only seen in cells of the red pulp and in cells around the central arteries. Our microscopic obs ervations indicate that uptake and retention of PEG-liposomes in the infect ious focus is a result of enhanced extravasation due to increased vascular permeability and subsequent phagocytosis of PEG-liposomes by macrophages in the infected tissue. (C) 2001 Elsevier Science BY. All rights reserved.