CHARACTERIZATION OF THE P-SELECTIN LIGAND ON HUMAN HEMATOPOIETIC PROGENITORS

The adhesive characteristics of hematopoietic stem and progenitor cell s may partly regulate their proliferation and differentiation and may be critical in the homing of transplanted stem cells. Using quantitati ve adhesion assays, we have examined the characteristics of activated platelet adhesion to CD34(+) cells in human blood and to the KG1a fell line. Approximately 85-95% of CD34(+) cells from both sources bound t hrombin-activated platelets; like mature neutrophils, activated platel et binding was maximal within 2 minutes of coincubation. Activated pla telet adhesion to CD34(+) cells was completely inhibited by chelation of calcium or by preincubation with the G1 blocking monoclonal antibod y (MoAb) to platelet P-selectin. Using MoAbs to P-selectin glycoprotei n ligand-1 (PSGL-1), we demonstrated that PSGL-1 was present on the su rface of CD34(+) cells; preincubation of CD34(+), cells with the PL1 b locking MoAb to PSGL-1 completely inhibited activated platelet adhesio n to CD34(+) cells. Furthermore, treatment of CD34(+) cells with O-sia loglycoprotein endopeptidase, which destroyed the PL1 epitope of PSGL- 1, also abolished activated platelet-CD34(+) cell binding. By contrast , MoAb directed against control epitopes of PSGL-1 or endopeptidase-se nsitive epitopes of the CD34 molecule had no effect on activated plate let adhesion to CD34(+) cells. Unlike mature neutrophils that, when ac tivated, decrease P-selectin-dependent platelet adhesion because of re distribution of PSGL-1, phorbol ester treatment of CD34(+) cells had n o effect on their ability to bind activated platelets or PSGL-1 MoAbs. This study identifies PSGL-1 on CD34(+) cells as the ligand for plate let P-selectin and suggests functional differences between mature and precursor hematopoietic cells in the regulation of surface PSGL-1 expr ession.