Abstract

Interactions between SDF-1 and CXCR4 determine in part the retention, migration and mobilization of hematopoietic stem cells and endothelial progenitor cells during steady state hematopoiesis and following a variety of stimuli. Chronic idiopathic myelofibrosis (CIMF) is characterized by constitutive mobilization of CD34+ hematopoietic stem/progenitor cells as well as endothelial progenitor cells (

Barosi et al,
Blood
98
:
3249
,
2001
and J Clin Oncol, in press, 2005). This abnormal cell trafficking has been attributed to dysfunction of the bone marrow (BM) microenvironment and/or the presence of a proteolytic environment which disrupts adhesive interactions which favor retention of CD34+ cells within the BM (
Xu et al,
Blood
105
:
4508
,
2005
). We examined the role of SDF-1/CXCR4 in CD34+ cell trafficking in patients with CIMF. The median level of plasma SDF-1 in CIMF patients (N=27) was 2008 pg/ml (955–2981 pg/ml) which was significantly higher than that found in the plasma of normal individuals (N=13, 1411 pg/ml, 945–1803, p<0.001). Plasma SDF-1 levels in patients with polycythemia vera were intermediate in value (N=10, 1777 pg/ml, 1400–2496 pg/ml). When BM biopsy specimens from normal individuals were immunostained with anit-human SDF-1 monoclonal antibody, SDF-1 was found to be distributed in BM blood vessels of varying sizes, including small arterioles, venules, capillaries and sinusoids. In contrast, the BM biopsies from CIMF patients demonstrated a significant increase in SDF-1 deposition. Increased SDF-1 deposition was found in blood vessels, in areas adjacent to bone trabeculae and within the BM matrix surrounding hematopoietic cells. This abnormal distribution of BM SDF-1 was no longer observed following curative therapy with allogeneic stem cell transplantation. IM PB CD34+ cells were characterized by decreased expression of CXCR4 as well as by decreased in vitro migration in response to SDF-1 (p<0.001). The expression of CXCR4 by IM PB CD34+ cells was, however, significantly increased by incubation with SCF and IL-6 (7.2% to 18.0%). The expression of the cell-bound MMP-2 and MMP-9 by IM CD34+ cells was significantly increased by pre-incubation with SDF-1 in vitro (p<0.01). SDF-1 expression has been previously shown to be regulated by hypoxia inducible factor-1 (HIF-1α). Utilizing Western Blotting, we were able to demonstrate that HIF-1α protein levels were significantly increased in the CD34+ cells of 5/14 patients with CIMF. HIF-1α levels can be regulated by external stimuli such as hypoxia, cytokines or by the constitutive expression of pp60−src (c-src). The elevated HIF-1α expression of the CD34+ cells in a subpopulation of IM were associated with increased expression of c-src. The abnormal distribution of SDF-1 in CIMF, therefore, has multiple biological consequences including: down modulating the CXCR4 expression of CD34+ cells, up regulating CD34+ cell-bound MMP-2 and MMP-9, and promoting the growth and survival of BM stromal cells. These multiple effects of SDF-1 in CIMF likely alter the trafficking of CD34+ cells as well as promote BM fibrosis and osteosclerosis.

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