Use of a high affinity DNA ligand in flow cytometry

To investigate the feasibility of using oligonucleotides in flow cytometry we describe a model system consisting of human neutrophil elastase (HNE) coated on 3.3 mu m beads and a high affinity DNA ligand for HNE isolated by in vitro selection (SELEX). In this system the fluoresceinated DNA ligand was equally effective as an anti-HNE antibody in detecting HNE on beads, The location on and the chemistry of attachment of fluorescein to the DNA ligand is critical for the sensitivity of detection. DNA constructs in which fluorescein was conjugated via an ethylene glycol tether to either the 5'-end or near the 3'-end gave much higher signals than did probes with fluorescein directly conjugated to either end. Second-step staining with strepavidin-conjugated phycoerythrin was accomplished using a biotinylated DNA ligand in the initial staining of HNE beads. These data suggest that instead of, or in addition to, antibodies high affinity oligonucleotide probes can be useful in diagnostic applications based on flow cytometry.