Single-cell image analysis to assess ABC-transporter-mediated efflux in highly purified hematopoietic progenitors

BACKGROUND: Normal and malignant hematopoietic stem cells are characterized by their capacity to actively extrude fluorescent dyes. The contribution of different ATP-binding cassette (ABC) transporters to this phenomenon is largely unknown due to the small stem cell numbers limiting the use of standard methods to assess functional efflux. METHODS: We used epifluorescence microscopy (EFM) in combination with single-cell image analysis to study ABC-transporter-mediated efflux in highly purified, viable, CD34+CD38- cells sorted on an adhesive biolayer. P-glycoprotein and multidrug-resistant protein (MRP)-mediated efflux were quantitated using fluorescent substrates (rhodamine-123 and calcein acetoxymethyl ester [calcein-AM]) and specific inhibitors (verapamil and probenecid, respectively). RESULTS: The feasibility, sensitivity, and reproducibility of rhodamine-123 efflux quantitation using single-cell EFM was shown in cell lines and compared with standard flow cytometric assessment. P-glycoprotein-mediated transport was higher in CD34+CD38- cells than in more differentiated progenitors (mean efflux index = 2.24 +/- 0.35 and 1.14 +/- 0.11, respectively; P = 0.01). P-glycoprotein-mediated transport was the main determinant of the rhodamine "dull" phenotype of these cells. In addition, significant MRP-mediated efflux was demonstrated in CD34+CD38- and CD38+ cells (mean efflux index = 1.42 +/- 0.19 and 1.28 +/- 0.18, respectively). CONCLUSION: The described method is a valuable tool for assessing ABC-transporter-mediated efflux in highly purified single cells. Both P-glycoprotein and MRP-mediated efflux are present in human CD34+CD38- hematopoietic stem cells.