Steady-state egress of hematopoietic progenitor cells can be rapidly amplified by mobilizing agents such as AMD3100, the mechanism, however, is poorly understood. We report that AMD3100 increased the homeostatic release of the chemokine stromal cell derived factor-1 (SDF-1) to the circulation in mice and non-human primates. Neutralizing antibodies against CXCR4 or SDF-1 inhibited both steady state and AMD3100-induced SDF-1 release and reduced egress of murine progenitor cells over mature leukocytes. Intra-bone injection of biotinylated SDF-1 also enhanced release of this chemokine and murine progenitor cell mobilization. AMD3100 directly induced SDF-1 release from CXCR4⁺ human bone marrow osteoblasts and endothelial cells and activated uPA in a CXCR4/JNK-dependent manner. Additionally, ROS inhibition reduced AMD3100-induced SDF-1 release, activation of circulating uPA and mobilization of progenitor cells. Norepinephrine treatment, mimicking acute stress, rapidly increased SDF-1 release and progenitor cell mobilization, whereas β2-adrenergic antagonist inhibited both steady state and AMD3100-induced SDF-1 release and progenitor cell mobilization in mice. In conclusion, this study reveals that SDF-1 release from bone marrow stromal cells to the circulation emerges as a pivotal mechanism essential for steady-state egress and rapid mobilization of hematopoietic progenitor cells, but not mature leukocytes.

The mechanisms of hematopoietic progenitor cell egress and clinical mobilization are not fully understood. Herein, we report that in vivo desensitization of Sphingosine-1-phosphate (S1P) receptors by FTY720 as well as disruption of S1P gradient toward the blood, reduced steady state egress of immature progenitors and primitive Sca-1⁺/c-Kit⁺/Lin⁻ (SKL) cells via inhibition of SDF-1 release. Administration of AMD3100 or G-CSF to mice with deficiencies in either S1P production or its receptor S1P₁, or pretreated with FTY720, also resulted in reduced stem and progenitor cell mobilization. Mice injected with AMD3100 or G-CSF demonstrated transient increased S1P levels in the blood mediated via mTOR signaling, as well as an elevated rate of immature c-Kit⁺/Lin⁻ cells expressing surface S1P₁ in the bone marrow (BM). Importantly, we found that S1P induced SDF-1 secretion from BM stromal cells including Nestin⁺ mesenchymal stem cells via reactive oxygen species (ROS) signaling. Moreover, elevated ROS production by hematopoietic progenitor cells is also regulated by S1P. Our findings reveal that the S1P/S1P₁ axis regulates progenitor cell egress and mobilization via activation of ROS signaling on both hematopoietic progenitors and BM stromal cells, and SDF-1 release. The dynamic cross-talk between S1P and SDF-1 integrates BM stromal cells and hematopoeitic progenitor cell motility.

We develop an optical whole-body imaging technique for monitoring normal and leukemic hematopoietic cell homing in vivo. A recently developed near-infrared (NIR) lipophilic carbocyanine dye $1,1′$-diocta-decyl-$3,3,3′,3′$-tetramethylindotricarbocyanine iodide (DiR) is used to safely and directly label the membranes of human leukemic Pre-B ALL G2 cell lines as well as primary murine lymphocytes and erythrocytes. DiR has absorption and fluorescence maxima at 750 and $782nm$, respectively, which corresponds to low light absorption and autofluorescence in living tissues. This allows us to obtain a significant signal with very low background level. A charge-coupled device (CCD)-based imager is used for noninvasive whole-body imaging of DiR-labeled cell homing in intact animals. This powerful technique can potentially visualize any cell type without use of specific antibodies conjugated with NIR fluorescent tag or loading cells with transporter-delivered NIR fluorophores. Thus, in vivo imaging based on NIR lipophilic carbocyanine dyes in combination with advanced optical techniques may serve as a powerful alternative or complementation to other small animal imaging methods.

The CD45 phosphatase is uniquely expressed by all leukocytes, but its role in regulating hematopoietic progenitors is poorly understood. We show that enhanced CD45 expression on bone marrow (BM) leukocytes correlates with increased cell motility in response to stress signals. Moreover, immature CD45 knockout (KO) cells showed defective motility, including reduced homing (both steady state and in response to stromal-derived factor 1) and reduced granulocyte colony-stimulating factor mobilization. These defects were associated with increased cell adhesion mediated by reduced matrix metalloproteinase 9 secretion and imbalanced Src kinase activity. Poor mobilization of CD45KO progenitors by the receptor activator of nuclear factor κB ligand, and impaired modulation of the endosteal components osteopontin and stem cell factor, suggested defective osteoclast function. Indeed, CD45KO osteoclasts exhibited impaired bone remodeling and abnormal morphology, which we attributed to defective cell fusion and Src function. This led to irregular distribution of metaphyseal bone trabecules, a region enriched with stem cell niches. Consequently, CD45KO mice had less primitive cells in the BM and increased numbers of these cells in the spleen, yet with reduced homing and repopulation potential. Uncoupling environmental and intrinsic defects in chimeric mice, we demonstrated that CD45 regulates progenitor movement and retention by influencing both the hematopoietic and nonhematopoietic compartments.