Macrocyclic peptide probes for immunomodulatory protein CD59: Potent modulators of bacterial toxin activity and antibody-dependent cytotoxicity.

CD59 is an immunomodulatory cell surface receptor associated with human disease. Despite its importance in complement regulation and bacterial pathogenesis, CD59 remains a challenging therapeutic target. Research to date has focused on antibody or protein-based strategies. Here we present a new approach to target CD59 using macrocyclic peptides with low nanomolar affinity for CD59. Through X-ray crystallographic studies and structure-activity relationship studies we identify key interactions which are essential for binding and activity. We find that the macrocyclic peptide CP-06 adopts a beta-hairpin structure and binds CD59 through an intermolecular beta-sheet, mimicking protein-protein interactions of biologically relevant CD59 interaction partners. We create dimeric and lipidated macrocyclic peptide conjugates as enhanced cell-active CD59 inhibitors and show that these probes can be used to modulate both complement-mediated killing of human cells and lytic activity of bacterial virulence factors. Together, our data provide a starting point for future development of macrocyclic peptides to target CD59 activity in diverse cellular contexts.