Selectins mediate adhesion events among leukocytes, platelets, and endothelial cells. Within the selectin family, P-selectin plays a predominant role in the recruitment of polymorphonuclear neutrophils into inflammatory sites and, thus, represents an important target for future therapies against various inflammatory diseases. Despite intense research efforts over the past 20 years, effective selectin inhibitors have not yet become available to clinicians. Since all 3 selectins bind sialylated and fucosylated oligosaccharides (for example, sialyl Lewis X), much of the work has focused on the generation of specific glycoconjugates that generally display relatively low binding affinities when presented as mono- or oligovalent ligands. In its natural environment, the chief P-selectin glycoprotein ligand (PSGL-1) indeed requires both the specific carbohydrate decorations and sulfated N-terminal tyrosine residues to bind P-selectin with high affinity (nanomolar range).
Molenaar and colleagues (page 3570) used phage-display libraries to isolate peptides containing the consensus sequence EWVDV, which can specifically inhibit human P-selectin but not mouse P-selectin or human E- or L-selectins. The binding affinity of an EWVDV-containing peptide for P-selectin was greatly enhanced (200-fold) when presented as a tetramer. Importantly, Molenaar et al also show that nanomolar concentrations of the tetrameric peptide can efficiently inhibit the adhesion of HL60 cells to P-selectin in static assays and increase rolling velocities of HL60 cells in a flow system. How do such small peptides do it? Because the EWVDV-containing peptide can inhibit the binding of a carbohydrate selectin ligand (sulfated Lewis A), the authors speculate that EWVDV may interact in close proximity of the carbohydrate recognition domain of P-selectin. The fact that EWVDV binding to P-selectin is only partially Ca++dependent suggests that the peptide may also interact outside the carbohydrate-binding pocket whose binding is strictly Ca++dependent. Since the consensus sequence contains 2 acidic residues (glutamic acid and aspartic acid), its overall negative charge may also promote interactions with the region of positive electrostatic potential previously shown to mediate high-affinity interactions with the sulfotyrosines of PSGL-1 (Somers et al, Cell. 2000;103:467-479). This feature might also explain the dramatically enhanced avidity of multimeric peptides. Beyond their physicochemical properties, the real litmus test for the peptides will be to determine their capacity to inhibit leukocyte adhesion on activated endothelium in vivo. This remarkable study, however, provides a critical first step in the generation of powerful selectin antagonists that have profound clinical relevance.
