The novel immunosuppressant FTY720 activates sphingosine 1-phosphate receptors (S1PRs) that affect responsiveness of lymphocytes to chemokines such as stromal cell-derived factor 1 (SDF-1), resulting in increased lymphocyte homing to secondary lymphoid organs. Since SDF-1 and its receptor CXCR4 are also involved in bone marrow (BM) homing of hematopoietic stem and progenitor cells (HPCs), we analyzed expression of S1PRs and the influence of FTY720 on SDF-1/ CXCR4-mediated effects in human HPCs. By reverse transcriptase-polymerase chain reaction (RT-PCR), S1PRs were expressed in mobilized CD34 ؉ HPCs, particularly in primitive CD34 ؉ /CD38 ؊ cells. Incubation of HPCs with FTY720 resulted in prolonged SDF-1-induced calcium mobilization and actin polymerization, and substantially increased SDF-1-dependent in vitro transendothelial migration, without affecting VLA-4, VLA-5, and CXCR4 expression. In nonobese diabeticsevere combined immunodeficient (NOD/ SCID) mice, the number of CD34 ؉ /CD38 ؊ cells that homed to the BM after 18 hours was significantly raised by pretreatment of animals and cells with FTY720, tending to result in improved engraftment. In addition, in vitro growth of HPCs (week-5 cobblestone area-forming cells [CAFCs]) was 2.4-fold increased. We conclude that activation of S1PRs by FTY720 increases CXCR4 function in HPCs both in vitro and in vivo, supporting homing and proliferation of HPCs. In the hematopoietic microenvironment, S1PRs are involved in mi IntroductionHoming of hematopoietic stem and progenitor cells (HPCs) to the bone marrow is an active and rapid process that takes place less than one day after transplantation, as demonstrated in the nonobese diabetic-severe combined immunodeficient (NOD/SCID) mouse model 1 as well as in recent in vivo tracking experiments using a bioluminescence technique. 2 In addition to adhesion molecules, proteolytic enzymes, and cytokines, especially chemokines such as stromal cell-derived factor 1 (SDF-1, CXCL12) are involved in HPC trafficking. CXCR4, the receptor for SDF-1, is expressed on HPCs and plays a central role in both progenitor cell homing and mobilization. [3][4][5][6][7][8][9] Blocking of CXCR4 with monoclonal antibodies has been shown to reduce homing of transplanted human progenitors to the bone marrow of NOD/SCID mice. 3 In the adult human bone marrow, SDF-1 was found to be expressed particularly by endothelial cells and along the endosteum region, suggesting SDF-1 mediates both early steps of HPC homing and final lodgement of stem cells in the niches of the bone marrow microenvironment. 10 Stimulation of CXCR4 by SDF-1 also activates adhesion molecules (eg, VLA-4) important for the homing process and may modulate proliferation of progenitors and stem cells. [11][12][13] In addition, there is indirect evidence that the SDF-1/ CXCR4 interaction is important for stem cell homing after transplantation in humans, since the migratory capacity of human progenitor cells in response to SDF-1 is correlated with hematopoietic recovery after stem cell tra...

Sphingosine 1-phosphate (S1P) is an ubiquitously present extracellular lipid mediator that is released by several cell types, particularly by activated platelets. The effects of S1P are mediated by a specific family of G protein-coupled sphingosine 1-phosphate receptors (S1P1-S1P5). We demonstrate that S1P acts on hematopoietic progenitor cells as a chemotactic factor, attracting peripheral blood CD34(+) cells in vitro. Furthermore, constant activation of S1P receptors augments CXCR4-mediated signal transduction induced by stromal cell-derived factor 1 (SDF-1). These effects are most likely mediated by the S1P1 receptor consistently expressed in both primitive and committed CD34(+) hematopoietic progenitor cells (HPCs). In vivo, sustained activation of S1P1 by a receptor agonist during the homing process resulted in increased engraftment. Given the fact that activated platelets represent a major source of extracellular S1P, SDF-1-mediated stem cell homing may occur at sites of tissue injury in addition to the bone marrow. This could explain the previously observed contribution of primary hematopoietic stem cells to tissue repair in myocardial infarction and other diseases.

No abstract