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.

Signals from platelet/endothelial cell adhesion molecule enhance the adhesive activity of the very late antigen-4 integrin of human CD34+ hemopoietic progenitor cells.

Adhesive interactions between human CD34+ hemopoietic progenitor cells and bone marrow stromal cells control the localization, proliferation, and differentiation of CD34+ cells. Changes in adhesive interactions may contribute to the mobilization of CD34+ cells to the blood induced by chemotherapy and cytokines. Thus, the identities and functional states of adhesion receptors are critical properties of CD34+ cells. Here, we confirm that the adhesion receptors very late antigen-4 (VLA-4), LFA-1, and platelet/endothelial cell adhesion molecule-1 (PECAM-1) are expressed on the CD34+ cell line KG1a and on CD34+ normal, steady state bone marrow cells. Therapeutically mobilized CD34+ cells express similar levels of PECAM-1 but reduced levels of VLA-4 and LFA-1 in comparison with steady state bone marrow cells. Integrin adhesive activity was measured from the binding of PKH 26- or phycoerythrin-labeled CD34+ cells to FITC-labeled Chinese hamster ovary (CHO) cells expressing vascular CAM-1 (VCAM-1) or intercellular CAM-1, which are ligands for VLA-4 and LFA-1, respectively. Incubation mixtures were analyzed by flow cytometry for the loss of free CD34+ cells and gain of CD34(+)-CHO cell aggregates. VLA-4 mediates the strong and specific adhesion of KG1a cells and bone marrow CD34+ cells to VCAM-1-transfected CHO cells. CD34+ cells mobilized with granulocyte colony stimulating factor (G-CSF) or cyclophosphamide also bind VCAM-1 via VLA-4. The VLA-4-mediated adhesion of all CD34+ cells to VCAM-1 is enhanced by Abs to the coexpressed adhesion receptor PECAM-1, implicating signals transmitted from PECAM-1 as determinants of VLA-4 integrin activity. VLA-4 function in CD34+ cells mobilized with G-CSF or cyclophosphamide is equivalent to steady state CD34+ cells. LFA-1 mediates minimal adhesion between CD34+ cells and intercellular CAM-1 transfected CHO cells and is refractory to PECAM-1 modulation. We infer that VLA-4, but not LFA-1, contributes to the constitutive adhesive phenotype of CD34+ cells. PECAM-1 is probably one of several receptors that control adhesive interactions between hemopoietic progenitors and target cells by regulating the activation states of specific integrins.

The mobilization of primitive hemopoietic progenitors into the peripheral blood.

There is considerable interest in the use of peripheral blood progenitor cells (PBPC) for hemopoietic rescue following high dose chemotherapy. Current regimens mobilize CD34+ with variable efficacy and there remains considerable empiricism in the design of these regimens. Some involve myelosuppression, some the administration of various cytokines alone or in combination, while a combination of chemotherapy and cytokines is employed in others. Certain protocols result in mobilization within one week while in others, maximal PBPC levels occur only after several weeks. Thus, procedures required for optimal mobilization of PBPC remain to be defined. An understanding of the mechanisms responsible for mobilization may lead to the development of improved mobilization strategies. Herein we review data that explore the mechanisms involved in the mobilization of PBPC in man. These data demonstrate that mobilization is associated with marked changes in the expression and function of cell adhesion molecules (CAMs) on hemopoietic progenitor cells (HPC), suggesting that the release of HPC into the blood involves a perturbation of the adhesive interactions between these cells and the marrow stroma that, in steady-state conditions, serve to restrict HPC to the bone marrow. Downregulation of c-kit is invariably associated with successful mobilization which, when combined with data from in vitro studies, implies a key role for stem cell factor (SCF) as an orchestrator of mobilization.