Anti–VLA4/VCAM-1—Induced Mobilization Requires Cooperative Signaling Through the kit/mkit Ligand Pathway

Marrow engraftment of hematopoietic stem and progenitor cells is independent of Galphai-coupled chemokine receptors

The mechanism of hematopoietic stem and progenitor cell (HSPC) homing to hematopoietic organs after transplantation is still poorly understood. There is evidence that HSPC homing is a multistep process involving integrins and other adhesion molecules as well as stimulation of cytokine and chemokine receptors, similar to the process of lymphocyte recirculation and leukocyte emigration. This study examined the effect of pertussis toxin (PT), an inhibitor of signaling by many Galphai protein-coupled chemokine receptors, on engraftment of HSPC. An in vitro incubation of total bone marrow cells in PT-supplemented media prior to transplantation into lethally irradiated syngeneic mice resulted in an increase in marrow repopulation and a parallel decrease of colony-forming unit-spleen (CFU-S) on day 13. PT treatment of Rh(low)Lineage(neg)Sca-1pos cells prior to transplant resulted in delayed spleen cell engraftment, but no observable difference in the bone marrow cellularity compared to animals transplanted with untreated cells. FACS analysis of hematopoietic organs revealed that myeloid cell recovery in the bone marrow was unaffected by PT treatment of HSPC. However, a reduced myeloid cell recovery in the spleen and an increased B lymphoid recovery in both the spleen and the bone marrow were observed in recipients of PT-treated grafts relative to untreated grafts. To test the hypothesis that PT inhibits proliferation rather than engraftment of HSPC in the spleen, the effect of PT on cytokine-stimulated proliferation of HSPC was tested. Although an inhibition of the growth of microcolonies in response to interleukin 6 as a single cytokine could be observed after PT treatment, colony growth of HSPC after steel factor or steel factor + interleukin 6 stimulation was unaffected by PT. This study demonstrates that bone marrow, but not splenic, recovery after HSPC transplantation is independent of PT-sensitive mechanisms. It is likely that PT inhibits spleen cell recovery by disrupting a Galphai-coupled homing receptor expressed by HSPC. These studies support the hypothesis that distinct mechanisms regulate splenic vs bone marrow engraftment of HSPC, and that B lymphocyte progenitors and HSPC can utilize a PT-resistant homing mechanism to localize in hematopoietic tissues after transplantation.

Mechanisms underlying hematopoietic stem cell mobilization induced by the CXC chemokine interleukin-8

The CXC chemokine interleukin-8 induces rapid mobilization of hematopoietic progenitor cells in mice and monkeys. Antibodies against the beta 2-integrin leukocyte function-associated antigen-1 completely blocked interleukin-8-induced mobilization. This was not due to a direct effect on the hematopoietic progenitor cells, as leukocyte function-associated antigen-1 was found not to be expressed on hematopoietic progenitor cells. Additional experiments showed that interleukin-8 induces the rapid release of the metalloproteinase gelatinase B, concurrent with the mobilization of hematopoietic progenitor cells. Mobilization could be completely prevented by anti-gelatinase B antibodies. Because neutrophils express leukocyte function-associated antigen-1 and high affinity interleukin-8 receptors, and release gelatinase B upon stimulation with interleukin-8, we hypothesized that neutrophils are key mediators in interleukin-8-induced stem cell mobilization. Further studies showed that mobilization by interleukin-8 was completely absent in mice rendered neutropenic with anti-granulocytic antibodies. Taken together, these data are consistent with an essential role for neutrophils in interleukin-8-induced stem cell mobilization.

Hematopoietic stem/progenitor cell mobilization. A continuing quest for etiologic mechanisms

The physiologic egress of mature hemopoietic cells and of hemopoietic stem/progenitor cells from bone marrow to the circulation are poorly understood processes. Likewise, the mechanism of their enforced emigration or mobilization through the use of several agents has not been unraveled. Although mobilization is suspected to be a multi-step process, involving sequential and/or overlapping changes in adhesion and migratory capacity, a model of molecular hierarchy, like the one governing the extravasation of mature leukocytes to tissues of inflammation, has not been worked out. Understanding the in vivo mechanism of mobilization has been a challenge. Signals emanating from both stromal cells and from hemopoietic cells are likely involved. However, dissecting out their roles, specificity, and interactions has been difficult. Nevertheless insightful information is rapidly emerging, especially with the current availability of many mouse models bearing targeted disruptions of cytoadhesion or signaling molecules.

Cytoskeleton and integrin-mediated adhesion signaling in human CD34+ hemopoietic progenitor cells

Significant progress has been made recently in the understanding of cell adhesion signaling. Many components of focal adhesion complexes have been identified in fibroblasts and endothelial cells, showing considerable overlap and complementarity between growth signaling mediated by growth factor receptors and adhesive signaling mediated by cell adhesion receptors such as integrins. These studies showed that the cytoskeleton is essential for the correct intracellular localization of large signaling complexes that regulate the cellular machinery. Although adhesive interactions are essential to maintain steady-state hemopoiesis, the study of the function and role of adhesive interactions in hemopoietic progenitor and stem cells is less advanced. As in fibroblasts, functional overlap between hemopoietic growth factor receptors and cell adhesion receptors has been demonstrated, with the cytoskeleton likely playing a critical role in integrating information provided by soluble factors and cell adhesion molecules constituting the hemopoietic microenvironment. The intention of this article is to give a critical review of the current knowledge about the cytoskeleton and integrin-mediated signaling in hemopoietic progenitor cells.

Adhesion receptor expression by hematopoietic cell lines and murine progenitors: modulation by cytokines and cell cycle status

Hematopoietic progenitor cells are incubated with cytokine combinations for in vitro expansion of stem cells and to enhance retrovirus-mediated gene transfer. Optimization of the engraftment of these treated cells would be critical to the success of stem cell transplantation or gene therapy. Previous studies demonstrated that a 48-hour incubation of donor BALB/c bone marrow with a mixture of four cytokines (IL-3, IL-6, IL-11, and SCF), resulted in expansion of primitive progenitor/stem cells but a loss of long-term engraftment in nonmyeloablated or myeloablated recipients. We have established the expression pattern for a number of adhesion receptors by normal hematopoietic progenitors and cell lines and the modulation in expression induced by cytokines or cell cycle progression to ascertain the molecular basis for such defective engraftment. Northern blot analysis demonstrated that the cytokine combination of IL-3, IL-6, IL-11, and SCF dramatically down-regulated alpha 4 integrin receptor expression in HL-60 cells. Synchronized FDC-P1 cells exhibited modulation of alpha 4 expression through cell cycle progression, both by quantitative RT-PCR and flow cytometry. Normal murine bone marrow lineage-depleted, Sca+ cells expressed a number of adhesion receptors, including alpha L, alpha 1, alpha 3, alpha 4, alpha 5, alpha 6, beta 1, L-selectin, CD44, and PECAM as assessed by flow cytometry, immunofluorescence, and RT-PCR. There was modulation of the expression of several of these receptors after incubation in the four cytokines for 24 and/or 48 hours: the proportion of cells expressing alpha L, alpha 5, alpha 6, and PECAM increased, whereas the proportion of cells expressing alpha 4 and beta 1 decreased, after cytokine incubation. There was a demonstrable concomitant decline in adhesion of these cells to fibronectin after the cytokine incubation, a finding that correlates with the decrease in expression of alpha 4. These changes in adhesion receptor expression and function with cytokines and during cell cycle transit may be critical to stem cell homing and engraftment after transplantation, as multiple receptors could be involved in the process of rolling, attachment to endothelium, endothelial transmigration, and migration within the marrow space.

Prolonged release and c-kit expression of haemopoietic precursor cells mobilized by stem cell factor and granulocyte colony stimulating factor

Mobilization of haemopoietic precursor cells into the circulation by the combination of cytokines, stem cell factor (SCF) and G-CSF in previously untreated patients with carcinoma of the breast resulted in increased yield of collected peripheral blood precursor cells (PBPC). This mobilization of PBPC by SCF with G-CSF lasted several days after ceasing the cytokines in comparison to the rapid fall of PBPC after ceasing G-CSF. Possible mechanisms for this increased and prolonged mobilization were investigated. Immunological phenotyping with CD38, Thy-1 and MDR-1 of the CD34-positive mobilized PBPC detected no difference in maturity compared to PBPC mobilized by G-CSF alone. However, the down-regulation of c-kit, which is associated with the mechanism of mobilization, was much greater in the PBPC mobilized by SCF and G-CSF. The potential clinical implication of increased and prolonged mobilization is increased yield, allowing transplantation of heavily pre-treated patients, transplantation with PBPC from a single apheresis, or PBSC support for multiple courses of high-dose therapy from one mobilization procedure.

Selective Transendothelial Migration of Hematopoietic Progenitor Cells: A Role in Homing of Progenitor Cells

To elucidate the mechanisms by which hematopoietic progenitor cells transmigrate via the bone marrow (BM) endothelial cells, we first established endothelial cell lines from BM and lung, and BM fibroblast cell lines; then we established an in vitro model of transendothelial migration of hematopoietic progenitor cells in the presence of chemoattractants secreted by BM fibroblast cells. The BM endothelial cells expressed vascular cell adhesion molecule-1 (VCAM-1), but the lung endothelial cells did not. The BM fibroblast cells secreted chemoattractants including stroma cell–derived factor (SDF)-1, which could attract hematopoietic progenitor cells to BM and activate the adhesion molecules expressed on hematopoietic progenitor cells after rolling along the endothelial cells. Anti–SDF-1 antibody inhibited the transendothelial migration of a hematopoietic progenitor cell line, FDCP-2. FDCP-2 that expressed very late activation antigen-4 (VLA-4) and normal progenitor cells transmigrated through BM endothelial cells but not lung endothelial cells, even if in the presence of chemoattractants produced by BM fibroblasts. Both anti–VLA-4 and anti–VCAM-1 antibodies inhibited the transendothelial migration of FDCP-2 cells and normal hematopoietic progenitor cells. These findings suggest that the transendothelial migration of hematopoietic progenitor cells is characteristic of BM endothelial cells, and that VLA-4/VCAM-1 and SDF-1 play important roles in the transendothelial migration and, consequently, homing of hematopoietic progenitor cells to BM.