Merocyanine 540 as a flow cytometric probe of membrane lipid organization in leukocytes

Merocyanine 540 (MC540) is a fluorescent probe that binds preferentially to membranes with loosely packed lipids. When combined with flow cytometry, it provides a novel methodology for rapidly and quantitatively assessing lipid organization in individual leukocytes. Analysis of cells stained simultaneously with MC540 and 1-[4-trimethylammoniumphenyl]-6-phenyl-1,3,5-hexatriene, to normalize for surface area, revealed that all leukocytes in peripheral blood bind equivalent amounts of dye per unit surface area. This result indicates that the lipids of the plasma membranes of all types of circulating cells are organized similarly. Upon activation with appropriate stimuli, lymphocytes, monocytes, and neutrophils all bound increased amounts of dye per unit surface area, indicating a change in lipid organization to a less-ordered state. Cells stained with MC540 were sorted on the basis of their fluorescence intensity yielding populations homogeneous with respect to lipid organization. Thus not only can MC540 and flow cytometry be combined for analyzing the organization of lipids in individual leukocytes, but sorted populations can be obtained for further biochemical, structural, and functional analyses.