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Blood, Vol. 95 No. 12 (June 15), 2000:
pp. 4015-4016
CORRESPONDENCE
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To the Editor: |
Homogeneous expression of CXC chemokine receptor 4 (CXCR4)
on G-CSF-mobilized peripheral blood CD34+ cells
We read with interest the article by Chelucci et
al,1 reporting chemokine receptor 4 (CXCR4) mRNA and
protein expression on about 50% of steady-state circulating
CD34+ cells. This expression pattern was coincident with
that observed in resident normal CD34+ cells from bone
marrow, where CXCR4 was found on approximately 40% of
CD34+ progenitors2; moreover, bone marrow
CD34+CXCR4 and
CD34+CXCR4+ cells differ in their
colony-forming potential, with CD34+CXCR4+
cells originating from CD34+CXCR4 cells and
having the capacity to proliferate and differentiate into B and T
lymphocytes in vitro, but being incapable of forming myeloid progeny in
serum-free culture systems. From these data on steady-state
hematopoiesis, it can be asserted that
CD34+CXCR4 cells represent a more primitive
subset compared with CD34+CXCR4+ cells, which
have already initiated their commitment toward the lymphoid
lineage.2 Further investigations identified the expression of mRNA for CXCR4 in about 80% of samples of highly enriched
CD34+ cells collected by leukapheresis after
G-CSF-supported cytotoxic chemotherapy.3 In that study,
apparently in contrast with CXCR4-expression pattern on marrow resident
precursors, CXCR4 was found predominantly on a more immature subset of
circulating progenitors (CD34+/HLA-DR);
similarly, Ruiz et al recently confirmed the presence of primitive CD34+/CD90+/CXCR4+ cells among
steady-state circulating precursors.4 The above-mentioned discrepancies between CXCR4 expression patterns in marrow versus steady-state and G-CSF-exposed circulating precursors recall previous findings on peripheral CD34+ progenitors that homogeneously
express CD33 and CD38 antigens, unlike bone-marrow-resident
CD34+ cells, whose antigens, to the contrary, identify
distinct functional subsets.5
To settle the apparent inconsistency of available data on CXCR4
expression on human CD34+ precursors, we examined the
expression of CXCR4 protein on highly purified G-CSF-mobilized
peripheral blood CD34+ cells from 7 patients with
hematological malignancies. Moreover, CXCR4 expression was evaluated
during the differentiation of those CD34+ cells along the
erythroid (E), granulocytic/monocytic (GM), and megakaryocytic (Mk)
lineages in liquid culture; details on cell culture methodology have
been already published.6,7 Aliquots of unfractionated
peripheral blood progenitor cells (PBPC) or purified/cultured
CD34+ cells were incubated with a phycoerythrin
(PE)-conjugated anti-CXCR4 monoclonal antibody (mAb) (12G5 clone,
Pharmingen, San Diego, CA) and were costained with fluorescein
isothyocyanate (FITC)-conjugated mAb directed against CD34, CD33, CD61,
CD105, and glycophorin-A.8
CD34+ cells mobilized by chemotherapy and G-CSF
homogeneously expressed CD33, CD13, and HLA-DR antigens, thus
confirming previous observations (Figure, A).5 CXCR4 was
expressed on the majority of circulating G-CSF-mobilized
CD34+ cells (90%, range 86 to 93). CD34+ cells
were next fractionated into a CD105+ and a
CD105 population, as previously described6;
surprisingly, no differences in CXCR4 expression levels were detected
when comparing the 2 cell subsets (> 90% CXCR4+;
Figure, B). Coreceptor expression was also investigated during unilineage E, GM, and Mk differentiation pathways. CXCR4 expression was
rapidly down-regulated during erythroid differentiation, and a
negligible percentage of glycophorin-A+ cells were found to
coexpress CXCR4 after 7 days of culture (< 2%). CXCR4 levels
moderately decreased along the GM pathway; conversely, high levels of
coreceptor were found in Mk cultures, and increasing CD61 staining
intensities were associated with increasing CXCR4 levels (Figure,
C).
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CXCR4 expression.
(A) Expression of CXCR4 and CD33 antigens on freshly purified G-CSF
mobilized CD34+ progenitors from a representative
experiment. Markers were set according to isotypic controls. The
percentage of cell coexpressing both antigens is indicated. (B) CXCR4
expression on freshly purified CD34+CD105+ and
CD34+CD105 cell fractions from a
representative experiment. Markers were set according to isotypic
controls. (C) CXCR4 expression during unilineage E, GM, and Mk
differentiation. Markers were set according to isotypic controls. The
percentage of cells coexpressing CXCR4 and glycophorin-A, CD33, or CD61
differentiation antigens is indicated.
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Chemokine receptors belong to a family of 7 membrane-
spanning molecules coupled to G-proteins, and their
activation leads to calcium flux and intracellular and extracellular
signaling; the surface expression of different chemokine receptors on
leukocytes and the biological functions of their cognate ligands have
been thoroughly investigated.9 CXCR4 is
abundantly expressed on peripheral blood lymphocytes, monocytes,
thymocytes, pre-B cells, and dendritic cells; stromal cell-derived
factor 1 (SDF-1), the natural ligand for CXCR4, initially characterized
as a pre-B-cell stimulating factor, as well as a potent chemoattractant
for T lymphocytes, has been shown to potently mediate the chemotaxis of
CD34+ progenitors, suggesting a pivotal role in the
regulation of hematopoiesis.
Circulating CD34+ cells contain a progenitor pool
supporting prompt and durable trilineage hematopoietic reconstitution
after myeloablative chemotherapy.8 G-CSF has been recently
shown to upregulate CXCR4 expression on peripheral blood
CD34+CD38 cells from normal
donors10; interestingly, CXCR4 expression is crucial for
the engraftment of human stem cells and for the repopulation of
NOD/SCID mice.11 The present investigation suggests that
G-CSF might up-regulate CXCR4 expression on peripheral blood CD34+ cells, which differ profoundly from their
bone marrow counterpart, where CXCR4 is specifically associated
with lymphoid commitment2; in addition, the bone marrow
microenvironment might easily accomodate immigrating progenitor cells
that express high levels of CXCR4 following G-CSF mobilization or
stress conditions. The different functional significance of CXCR4 on
G-CSF-mobilized cells compared with bone marrow CD34+
cells is further suggested by the finding of similar CXCR4 levels on
CD34+CD105+ and
CD34+CD105 cell subsets, which represent
functionally distinct subpopulations of circulating
progenitors.6,7 Following the observation that CXCR4 might
be required for the migration of CD34+ progenitor cells
from the fetal liver to the bone marrow microenvironment during fetal
development,2 the dynamic alterations of CXCR4 expression
on human CD34+ hematopoietic progenitors during growth
factor-induced mobilization compared with steady-state hematopoiesis
could confer an enhanced homing capacity and deserve further investigation.
Sergio Rutella
Luca Pierelli
Giuseppina Bonanno
Giovanni Scambia
Giuseppe Leone
Carlo Rumi
Departments of Hematology and Gynecology
Catholic University
School of Medicine
Rome, Italy
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References |
1.
Chelucci C, Casella I, Federico M, et al.
Lineage-specific expression of human immunodeficiency virus (HIV) receptor/coreceptors in differentiating hematopoietic precursors: correlation with susceptibility to T- and M-tropic HIV and chemokine-mediated HIV resistance.
Blood.
1999;94:1590-1600[Abstract/Free Full Text].
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Ishii T, Nishihara M, Ma F, et al.
Expression of stromal cell-derived factor-1/pre-B cell growth-stimulating factor receptor, CXC chemokine receptor 4, on CD34+ human bone marrow cells is a phenotypic alteration for committed lymphoid progenitors.
J Immunol.
1999;163:3612-3620[Abstract/Free Full Text].
3.
Deichmann M, Kronenwett R, Haas R.
Expression of the human immunodeficiency virus type-1 coreceptors CXCR-4 (fusin, LESTR) and CXR-5 in CD34+ hematopoietic progenitor cells.
Blood.
1997;89:3522-3528[Abstract/Free Full Text].
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Ruiz ME, Cicala C, Arthos J, et al.
Peripheral blood-derived CD34+ progenitor cells: CXC chemokine receptor 4 and CC chemokine receptor 5 expression and infection by HIV.
J Immunol.
1998;161:4169-4176[Abstract/Free Full Text].
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Rumi C, Rutella S, Teofili L, et al.
RhG-CSF-mobilized CD34+ peripheral blood progenitors are myeloperoxidase-negative and non-cycling irrespective of CD33 or CD13 coexpression.
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Pierelli L, Scambia G, Bonanno G, et al.
CD34+/CD105+ cells are enriched in primitive circulating progenitors residing in G0 phase of the cell cycle and contain all bone marrow and cord blood CD34+/CD38low/ precursors.
Br J Haematol.
2000;108:610-620[Medline]
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7. Pierelli L, Marone M, Bonanno G, et al. Modulation of bcl-2 and p27 in
human primitive hematopoietic progenitors by autocrine TGF- is a
cell cycle independent effect and influences their hematopoietic
potential. Blood. In press.
8.
Rutella S, Rumi C, Laurenti L, et al.
Immune reconstitution after transplantation of autologous peripheral CD34+ cells: analysis of predictive factors and comparison with unselected progenitor transplants.
Br J Haematol.
2000;108:105-115[Medline]
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Wang J-F, Liu Z-Y, Groopman JE.
The  -chemokine receptor CXCR4 is expressed on the megakaryocytic lineage from progenitor to platelets and modulates migration and adhesion.
Blood.
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Konopleva M, Marini F, Leysath C, Champlin R, Andreeff M, Korbling M.
G-CSF induces CXCR4 expression on CD34+CD38 peripheral blood progenitor cells in vivo [abstract].
Blood.
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Peled A, Petit I, Kollet O, et al.
Dependence of human stem cell engraftment and repopulation of NOD/SCID mice on CXCR4.
Science.
1999;283:845-848[Abstract/Free Full Text].
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