AMD3100 Signals Via the Nervous System, Inducing Release to the Circulation of Bone Marrow SDF-1, Which Is Crucial for Progenitor Cell Mobilization.

AMD3100, a bicyclam antagonist of the chemokine receptor CXCR4 in vitro, has been shown to induce rapid mobilization of human and murine maturing leukocytes and immature hematopoietic stem and progenitor cells in vivo. In addition, AMD3100 combined with G-CSF, synergistically augments mobilization of human progenitor cells (Broxmeyer & Srour et al, JEM, 2005). However, the mechanism of AMD3100-induced mobilization is currently poorly understood. We report that AMD3100-induced mobilization in mice was accompanied with rapid increase in functional SDF-1 concentrations in the circulation and their parallel decrease in the bone marrow within 1 hour. Biotinylated SDF-1 (bSDF-1) directly injected into the femur was detected in the peripheral blood, adjacent bones and spleen as early as 10 minutes post administration. Interestingly, AMD3100 induced significant elevations in bone marrow-derived bSDF-1 concentrations in the peripheral blood. Similarly, G-CSF induced mobilization was initiated (24 hours post a single injection of G-CSF), by SDF-1 release to the circulation. Administration of neutralizing antibodies against CXCR4 to either untreated or AMD3100 treated mice markedly reduced SDF-1 levels in the peripheral blood, coinciding with increased retention levels of this ligand in the bone marrow. In vitro, AMD3100 directly induced SDF-1 release from the human osteoblast cell line MG-63 in a bell shaped dose response. Inhibition of CXCR4-dependent release of SDF-1 during homeostasis or upon treatment with AMD3100, correlated with selective reduction in recruitment of hematopoietic progenitor cells but not mature leukocytes to the circulation. Importantly, injection of neutralizing antibodies against SDF-1 (but not matched control antibodies) resulted in decreased steady state egress and AMD3100-induced mobilization of hematopoietic progenitor cells. Rapid recruitment (within 1 hour) of hematopoietic progenitor cells and maturing leukocytes out of the bone marrow as well as SDF-1 release were dependent on signals from the nervous system. Administration of the b2 adrenergic agonist (clenbuterol) inhibited endogenous SDF-1 and exogenous bSDF-1 release to the circulation and reduced progenitor cell egress, both during steady state and AMD3100-induced mobilization, while administration of the b2 adrenergic antagonist (propranolol) resulted in opposite effects. Based on our results we propose a model in which egress and mobilization of immature progenitor cells differs from that of maturing leukocytes and is more dependent on SDF-1/CXCR4 interactions. In addition to hematopoietic progenitor cells, also bone marrow stromal cells induce homeostatic secretion of SDF-1, which is increased during mobilization and stress induced recruitment. Secretion of this ligand is also CXCR4-dependent, revealing orchestrated mutual and reciprocal SDF1/CXCR4 interactions and a cross-talk with the nervous system, which regulates progenitor cell egress and recruitment.