Neutrophil-derived MMP-9 mediates synergistic mobilization of hematopoietic stem and progenitor cells by the combination of G-CSF and the chemokines GROβ/CXCL2 and GROβT /CXCL2Δ4

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Abstract
Mobilized peripheral blood stem cells (PBSCs) are widely used for transplantation, but mechanisms mediating their release from marrow are poorly understood. We previously demonstrated that the chemokines GROβ/CXCL2 and GROβT/CXCL2Δ4 rapidly mobilize PBSC equivalent to granulocyte colony-stimulating factor (G-CSF) and are synergistic with G-CSF. We now show that mobilization by GROβ/GROβT and G-CSF, alone or in combination, requires polymorphonuclear neutrophil (PMN)–derived proteases. Mobilization induced by GROβ/GROβT is associated with elevated levels of plasma and marrow matrix metalloproteinase 9 (MMP-9) and mobilization and MMP-9 are absent in neutrophil-depleted mice. G-CSF mobilization correlates with elevated neutrophil elastase (NE), cathepsin G (CG), and MMP-9 levels within marrow and is partially blocked by either anti–MMP-9 or the NE inhibitor MeOSuc-Ala-Ala-Pro-Val-CMK. Mobilization and protease accumulation are absent in neutrophil-depleted mice. Synergistic PBSC mobilization observed when G-CSF and GROβ/GROβT are combined correlates with a synergistic rise in the level of plasma MMP-9, reduction in marrow NE, CG, and MMP-9 levels, and a coincident increase in peripheral blood PMNs but decrease in marrow PMNs compared to G-CSF. Synergistic mobilization is completely blocked by anti–MMP-9 but not MeOSuc-Ala-Ala-Pro-Val-CMK and absent in MMP-9–deficient or PMN-depleted mice. Our results indicate that PMNs are a common target for G-CSF and GROβ/GROβT-mediated PBSC mobilization and, importantly, that synergistic mobilization by G-CSF plus GROβ/GROβT is mediated by PMN-derived plasma MMP-9.

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