Objective    To investigate the binding of staphylococcal superantigen-like protein-5 (SSL5) to P-selectin glycoprotein ligand-1 (PSGL-1) on human umbilical cord blood-derived endothelial progenitor cells (EPCs) and the inhibitive effect of SSL5 on the adhesion of EPCs to P-selectin-coated surface.     Methods     SSL5 gene was amplified from the genome of Staphylococcus aureus NCTC 8325 and cloned into a vector for expressing recombinant SSL5 protein. Mononuclear cells were isolated from human umbilical cord blood by density gradient centrifugation and then cultured in vitro. The acetylated low-density lipoprotein (DiI-acLDL) uptake and Ulex europaeus agglutinin (FITC-UEA-1) interaction of the adherent cells were observed using a laser scanning confocal microscope. The expression of PSGL-1 on EPCs and the binding between SSL5 and PSGL-1 on EPCs were examined using a fluorescence-activated cell sorter. Calcein-AM-labeled EPCs were cultured with different concentrations of SSL5 (0

1

3

10 and 30 mg/L)

and the inhibitory effect of SSL5 on the adhesion of EPCs to P-selectin-coated surface was determined quantitatively.     Results    Almost all EPCs were positive in DiI-acLDL and FITC-UEA-1 staining. PSGL-1 was highly expressed on EPCs with a positive cell rate of 76.6%. The binding of SSL5 to PSGL-1 on EPCs was significantly and positively correlated with SSL5 concentration. SSL5 competitively inhibited the binding of anti-PSGL-1 monoclonal antibody (KPL-1) to EPCs. SSL5 significantly suppressed the adhesion of EPCs to P-selectin-coated surface

and 30 mg/L SSL5 presented similar inhibition effect to 10 mg/L KPL-1

showing significant difference as compared with the control group (P0.01).      Conclusion    SSL5 can bind to PSGL-1 on EPCs and inhibit the adhesion of EPCs to P-selectin-coated surface

which indicates that SSL5 may hinder the repair effect of EPCs on blood vessels by inhibiting the binding of EPCs to injured vascular endothelia or activated blood platelets.