Magnetofluorescent liposomes for increased sensitivity of immunofluorescence.
BACKGROUND: Immunofluorescence and immunomagnetism are important technologies for analysis and sorting of cells according to specific marker molecules. Due to the limited sensitivity at least several thousands of antigens per cell are required for optical detection. Molecules expressed at low copy numbers cannot be analysed, although they may be of considerable functional importance. OBJECTIVES: Development of a magnetic and fluorescent staining reagent for analysis and sorting of cells according to antigens expressed at low number. To this end, uniformly sized, antibody-conjugated liposomes loaded with large amounts of dye molecules and small magnetic particles were generated. STUDY DESIGN: A method for the preparation of homogeneously sized large unilamellar liposomes which contain carboxyfluorescein, magnetic particles and surface-bound antibodies had to be developed. These liposomes were then tested for their ability to enhance immunofluorescence compared to conventional staining in a model system and by staining of CD25 on resting B and T cells. RESULTS AND CONCLUSION: Large unilamellar liposomes, homogeneous in size and loaded with fluorescein and magnetic beads can be prepared by combining membrane extrusion and magnetic filtration. Hapten-specific antibodies conjugated to their surface make them a universal tool for immunofluorescence. With such liposomes, intensity of fluorescent staining can be increased 100-1000-fold without increased background fluorescence, compared to conventional fluorochrome-conjugated antibodies. Due to the simultaneous magnetic labelling, stained cells can easily be isolated by MACS. The magnetofluorescent liposomes proved to be useful for improvement of sensitivity of detection and physical separation in general and to visualize and sort cells according to antigens expressed at low levels. The high affinity IL2 receptor CD25 is expressed in low copy number on a significant fraction of resting B and T lymphocytes in human peripheral blood, as can be shown exclusively by the magnetofluorescent liposomes.