Direct fluorochrome labeling of phage display library clones for studying binding specificities: applications in flow cytometry and fluorescence microscopy

Phage display technology is increasingly employed to identify high-affinity peptides and single-chain antibodies with binding specificities for a diversity of target types. The analysis of phage-binding sensitivity and specificity typically employs directly labeled secondary antiphage antibodies and potentially tertiary labels, such as fluorochromes and enzymes, when biotinylated antibodies are used. However, secondary or tertiary reagents may not be feasible or desirable for some target types and applications. Here, we present a simple approach for directly labeling phage clones with two common amine-reactive fluorochromes. We show that these fluorochromes label the pVIII major coat protein and that the binding selectivity of peptides displayed on the pIII protein of several well-characterized phage clones is maintained in flow cytometric analysis and immunofluorescence microscopy. Uniquely, such labeled phage, in part, represent self-propagating reagents because conjugation does not impair the ability to efficiently reproduce in bacteria, although relabeling with fluorochrome would be necessary. Our data suggest that primary labeled phage clones may be used similarly to primary antibody conjugates.