Identification of macrophages in sections of rabbit lung using acetoacetylated lipoproteins

Macrophages were labeled in sections of rabbit lung with acetoacetylated low density lipoprotein (LDL), a marker internalized by cultured macrophages but not by other connective tissue cells. Using a modified technique, thin slices of fresh rabbit lung were incubated in 3,3'-dioctadecylindocarbocyanine (DiI)-labeled, acetoacetylated LDL, fixed in paraformaldehyde, and sectioned. Alveolar macrophages incorporated the fluorescently labeled, modified LDL, but surrounding stroma and parenchyma did not stain. Our results indicate that DiI-labeled, acetoacetylated LDL may be used to identify mononuclear phagocytes in tissue sections.