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Figure 1. Expression of CD49d, CD184, and CD133 on CD34+ cells. A, Representative fluorescence-activated cell-sorting dot plots showing the expression of CD49d on CD34+ cells from the peripheral blood (PB) and bone marrow (BM) of patients with multiple sclerosis treated with natalizumab (NAT). For comparison, dot plots of granulocyte colony-stimulating factor (G-CSF)–mobilized CD34+ cells and BM-derived CD34+ cells of healthy control (HC) patients are shown. B, Percentage of CD34+ cells expressing CD49d, CD184, and CD133 as measured by fluorescence-activated cell sorting. Gray and black bars on the left side represent G-CSF–mobilized CD34+ cells from 5 HCs and PB-derived CD34+ cells from 17 patients with multiple sclerosis receiving NAT therapy. Gray and black bars on the right side represent BM-derived CD34+ cells from 7 HCs and from 9 patients with multiple sclerosis treated with NAT. Error bars indicate mean (SEM). FITC indicates fluorescein isothiocyanate; PE, phycoerythrin.

Figure 1. Expression of CD49d, CD184, and CD133 on CD34+ cells. A, Representative fluorescence-activated cell-sorting dot plots showing the expression of CD49d on CD34+ cells from the peripheral blood (PB) and bone marrow (BM) of patients with multiple sclerosis treated with natalizumab (NAT). For comparison, dot plots of granulocyte colony-stimulating factor (G-CSF)–mobilized CD34+ cells and BM-derived CD34+ cells of healthy control (HC) patients are shown. B, Percentage of CD34+ cells expressing CD49d, CD184, and CD133 as measured by fluorescence-activated cell sorting. Gray and black bars on the left side represent G-CSF–mobilized CD34+ cells from 5 HCs and PB-derived CD34+ cells from 17 patients with multiple sclerosis receiving NAT therapy. Gray and black bars on the right side represent BM-derived CD34+ cells from 7 HCs and from 9 patients with multiple sclerosis treated with NAT. Error bars indicate mean (SEM). FITC indicates fluorescein isothiocyanate; PE, phycoerythrin.

Figure 2. In vitro adhesion and migration assays. On the left side is shown the adhesion of CD34+ cells from peripheral blood (PB) (granulocyte colony-stimulating factor [G-CSF], n = 9; natalizumab [NAT], n = 8) and bone marrow (BM) (healthy control [HC] patients, n = 6; NAT, n = 9) to fibronectin. On the right side is shown the migration of CD34+ cells from PB (G-CSF, n = 15; NAT, n = 5) and BM (HC, n = 7; NAT, n = 8) along a stromal-derived factor 1 gradient. Gray bars represent G-CSF–mobilized CD34+ cells and BM-derived CD34+ cells of HCs. Black bars represent CD34+ cells of patients with multiple sclerosis treated with NAT. Error bars indicate mean (SEM).

Figure 2. In vitro adhesion and migration assays. On the left side is shown the adhesion of CD34+ cells from peripheral blood (PB) (granulocyte colony-stimulating factor [G-CSF], n = 9; natalizumab [NAT], n = 8) and bone marrow (BM) (healthy control [HC] patients, n = 6; NAT, n = 9) to fibronectin. On the right side is shown the migration of CD34+ cells from PB (G-CSF, n = 15; NAT, n = 5) and BM (HC, n = 7; NAT, n = 8) along a stromal-derived factor 1 gradient. Gray bars represent G-CSF–mobilized CD34+ cells and BM-derived CD34+ cells of HCs. Black bars represent CD34+ cells of patients with multiple sclerosis treated with NAT. Error bars indicate mean (SEM).

Figure 3. Bone marrow smears. Microscopic images of bone marrow smears from 3 healthy donors (A through C) and 3 patients with multiple sclerosis treated with natalizumab (D through F).

Figure 3. Bone marrow smears. Microscopic images of bone marrow smears from 3 healthy donors (A through C) and 3 patients with multiple sclerosis treated with natalizumab (D through F).

1.
Bonig H, Wundes A, Chang KH, Lucas S, Papayannopoulou T. Increased numbers of circulating hematopoietic stem/progenitor cells are chronically maintained in patients treated with the CD49d blocking antibody natalizumab.  Blood. 2008;111(7):3439-3441PubMedArticle
2.
Zohren F, Toutzaris D, Klärner V, Hartung HP, Kieseier B, Haas R. The monoclonal anti-VLA-4 antibody natalizumab mobilizes CD34+ hematopoietic progenitor cells in humans.  Blood. 2008;111(7):3893-3895PubMedArticle
3.
Warnke C, Smolianov V, Dehmel T,  et al.  CD34+ progenitor cells mobilized by natalizumab are not a relevant reservoir for JC virus.  Mult Scler. 2011;17(2):151-156PubMedArticle
4.
Jing D, Oelschlaegel U, Ordemann R,  et al.  CD49d blockade by natalizumab in patients with multiple sclerosis affects steady-state hematopoiesis and mobilizes progenitors with a distinct phenotype and function.  Bone Marrow Transplant. 2010;45(10):1489-1496PubMedArticle
5.
Clifford DB, De Luca A, Simpson DM, Arendt G, Giovannoni G, Nath A. Natalizumab-associated progressive multifocal leukoencephalopathy in patients with multiple sclerosis: lessons from 28 cases [published correction appears in Lancet Neurol. 2010;9(5):463].  Lancet Neurol. 2010;9(4):438-446PubMedArticle
6.
Ransohoff RM. Natalizumab and PML.  Nat Neurosci. 2005;8(10):1275PubMedArticleArticle
7.
Tan CS, Dezube BJ, Bhargava P,  et al.  Detection of JC virus DNA and proteins in the bone marrow of HIV-positive and HIV-negative patients: implications for viral latency and neurotropic transformation.  J Infect Dis. 2009;199(6):881-888PubMedArticle
8.
Steidl U, Haas R, Kronenwett R. Intercellular adhesion molecular 1 on monocytes mediates adhesion as well as trans-endothelial migration and can be downregulated using antisense oligonucleotides.  Ann Hematol. 2000;79(8):414-423PubMedArticle
9.
Kronenwett R, Steidl U, Kirsch M, Sczakiel G, Haas R. Oligodeoxyribonucleotide uptake in primary human hematopoietic cells is enhanced by cationic lipids and depends on the hematopoietic cell subset.  Blood. 1998;91(3):852-862PubMed
10.
Czibere A, Prall WC, Zerbini LF,  et al.  Exisulind induces apoptosis in advanced myelodysplastic syndrome (MDS) and acute myeloid leukaemia/MDS.  Br J Haematol. 2006;135(3):355-357PubMedArticle
11.
Steidl U, Bork S, Schaub S,  et al.  Primary human CD34+ hematopoietic stem and progenitor cells express functionally active receptors of neuromediators.  Blood. 2004;104(1):81-88PubMedArticle
12.
Adams GB, Chabner KT, Alley IR,  et al.  Stem cell engraftment at the endosteal niche is specified by the calcium-sensing receptor.  Nature. 2006;439(7076):599-603PubMedArticle
13.
Bruns I, Czibere A, Fischer JC,  et al.  The hematopoietic stem cell in chronic phase CML is characterized by a transcriptional profile resembling normal myeloid progenitor cells and reflecting loss of quiescence.  Leukemia. 2009;23(5):892-899PubMedArticle
14.
Sutherland HJ, Eaves CJ, Eaves AC, Dragowska W, Lansdorp PM. Characterization and partial purification of human marrow cells capable of initiating long-term hematopoiesis in vitro.  Blood. 1989;74(5):1563-1570PubMed
15.
Punzel M, Moore KA, Lemischka IR, Verfaillie CM. The type of stromal feeder used in limiting dilution assays influences frequency and maintenance assessment of human long-term culture initiating cells.  Leukemia. 1999;13(1):92-97PubMedArticle
16.
Nicoletti I, Migliorati G, Pagliacci MC, Grignani F, Riccardi C. A rapid and simple method for measuring thymocyte apoptosis by propidium iodide staining and flow cytometry.  J Immunol Methods. 1991;139(2):271-279PubMedArticle
17.
Vermes I, Haanen C, Steffens-Nakken H, Reutelingsperger C. A novel assay for apoptosis: flow cytometric detection of phosphatidylserine expression on early apoptotic cells using fluorescein labelled Annexin V.  J Immunol Methods. 1995;184(1):39-51PubMedArticle
Original Contributions
Nov 2011

Natalizumab and Impedance of the Homing of CD34+ Hematopoietic Progenitors

Author Affiliations

Author Affiliations: Departments of Hematology, Oncology, and Clinical Immunology (Drs Saure, Zohren, Schroeder, Bruns, Weigelt, Kobbe, and Haas and Mr Cadeddu) and Neurology (Drs Warnke, Hartung, and Kieseier), and Clinic for Pediatric Oncology, Hematology, and Clinical Immunology (Dr Fischer), University of Düsseldorf, and Institute for Virology, Heinrich-Heine-University (Dr Adams), Düsseldorf, Germany.

Arch Neurol. 2011;68(11):1428-1431. doi:10.1001/archneurol.2011.238
Abstract

Background Treatment with natalizumab, an antibody blocking the α4-integrin, is associated with increased numbers of circulating CD34+ cells in the peripheral blood of patients with multiple sclerosis.

Objective To determine whether natalizumab mobilizes CD34+ cells from or inhibits homing to the bone marrow (BM).

Design Fifty-two patients with relapsing-remitting multiple sclerosis treated with natalizumab were included. Flow cytometric analyses; polymerase chain reaction assays for JC (John Cunningham) virus DNA detection; and adhesion, migration, and apoptosis assays of immunomagnetically enriched peripheral blood and BM CD34+ cells were conducted. A comparison was made with CD34+ cells from granulocyte colony-stimulating factor–mobilized peripheral blood or steady-state BM of age- and sex-matched healthy donors.

Results We found adhesion and migration of peripheral blood–derived CD34+ cells to be reduced. In BM aspirates from natalizumab-treated patients, the cellularity, the proportion, and the adhesive capacity of CD34+ cells were normal. The JC virus was undetectable.

Conclusions Natalizumab mediates an increase in circulating CD34+ cells by interfering with homing to the BM. Thus, CD34+ cells appear unlikely to represent a source mobilizing JC virus out of the BM in patients treated with natalizumab.

Natalizumab (NAT), an approved treatment for relapsing-remitting multiple sclerosis (MS) (comarketed by Biogen Idec and Élan as Tysabri), is a recombinant humanized monoclonal IgG4 antibody directed against the α4-subunit of VLA-4. We and others13 have shown that management of patients with relapsing-remitting MS treated with NAT leads to a rapid and sustained increase in circulating CD34+ cells. A recent study4 in a small cohort of 8 MS patients was suggestive of mobilization of CD34+ cells out of the bone marrow (BM).

However, increased numbers of CD34+ cells in the peripheral blood (PB) could alternatively be the result of impaired homing of CD34+ cells to, rather than true mobilization of, the BM in patients treated with NAT. We addressed this by assessing the adhesive and migratory capacity of CD34+ cells in PB relevant for transendothelial egress in a cohort of MS patients treated with NAT. In addition, we studied the effect of NAT on coexpression of CD49d (α4-subunit of VLA-4) on and adhesive properties of BM CD34+ cells, the BM cellularity, the proportion of BM CD34+ cells, and JC (John Cunningham) virus DNA expression in BM aspirates of 9 MS patients and 8 healthy age- and sex-matched control patients. From our results, we conclude that CD34+ cells exposed to NAT in the PB are functionally impaired, whereas the CD34+ cells in the BM appear to be only marginally affected by the antibody. This argues against the use of NAT as an agent to mobilize hematopoietic progenitor cells. Furthermore, patients with relapsing-remitting MS appear not to be at risk of exhaustion of the CD34+ hematopoietic progenitor pool as a potential adverse effect of long-term treatment. In addition, the present findings add to recent data arguing against a BM release hypothesis in the pathogenesis of progressive multifocal leukoencephalopathy observed in MS patients treated with NAT.3,57

METHODS

Fifty-two patients with relapsing-remitting MS, receiving NAT once every month (median [range] number of infusions, 7 [2-33]), were included. Informed consent was obtained following the guidelines of the local ethics committee of Heinrich-Heine-University (Düsseldorf, Germany) in accordance with the Declaration of Helsinki. Before NAT infusion, 50 mL of EDTA-anticoagulated PB was collected.

Peripheral blood mononuclear cells were obtained by density centrifugation using density gradient media (lymphoprep; Technoclone GmbH, Vienna, Austria) as previously described.8 The CD34+ cells were separated by midiMACS magnetic separation system (Miltenyi Biotec GmbH, Bergisch Gladbach, Germany) as previously published.9 The CD34+ cells purified from leukapheresis products of 24 healthy donors, who had received a 4- to 5-day course of granulocyte colony-stimulating factor (G-CSF), served as control patients.

Bone marrow aspirates were obtained from 9 patients before NAT infusion (median interval after the last NAT administration, 28 days) and from 8 healthy controls (HCs). The CD34+ cells were positively selected after density gradient centrifugation as previously described.10 The proportion of CD34+ cells of BM samples were assessed after red blood cell lysis using fluorescence-activated cell sorting analysis. The BM cellularity (number of mononuclear cells per microliter) was measured by the computed mononuclear cells after density centrifugation, taking into account the volume of the BM aspirate.

Median (SEM) purity measured by flow cytometry was 97.1% (0.6%) for NAT samples and 97.8% (2.7%) for HC samples. Flow cytometric analysis, cell adhesion tests, and migration assays of selected CD34+ cells were performed as previously described.1113 Long-term culture-initiating assays were performed in accordance with described methods14,15 with modifications. Polymerase chain reaction assays for JC virus DNA of CD34+ separates were performed as previously published.3 Flow cytometric analysis of the cell cycle and cellular DNA content as well as of the phosphatidylserine expression were performed as described previously16,17 to detect apoptotic CD34+ cells. Statistical analysis was performed using SPSS statistical software, version 18 (IBM, Armonk, New York).

RESULTS

Circulating CD34+ cells of patients treated with NAT were more mature, showed impaired adhesive and migratory properties, and showed no increase in the rate of apoptosis. Circulating CD34+ cells of MS patients treated with NAT (NAT group) were compared with those of HCs mobilized by G-CSF (G-CSF group). The phenotype of circulating CD34+ cells was assessed by multicolor immunofluorescence analysis. As expected, the median (SEM) coexpression rate of CD49d was reduced in the NAT group (NAT [n = 17], 49.6% [7.4%]; G-CSF, 99.4% [2.2%]; P = .007) (Figure 1). Whereas median (SEM) CD184 (CXCR4) expression was similar between groups (NAT, 45.2% [5.4%] vs G-CSF, 40.0% [ 13.3%]), coexpression of CD133 was reduced in the NAT group (NAT, 37.5% [1.7%]; G-CSF, 54.3% [1.9%]; P < .001), suggestive of a more mature subpopulation (Figure 1). In line with this finding, the ability of NAT-exposed circulating CD34+ cells to initiate long-term cultures was poor (data not shown). The median (range) proportion of adhering CD34+ cells (NAT [n = 8], 15.6% [12.9%-21.6%]; G-CSF [n = 9], 25.5% [16.4%-35.3%]; P = .003) and the migratory capacity of circulating CD34+ cells (NAT, 1% [0.7%]; G-CSF, 32.3% [5.11%]; P = .003) (Figure 2) were reduced in the NAT group compared with those of HCs. The labeling of apoptotic cells showed no increase in the proportion of apoptotic CD34+ cells in the NAT group (NAT [n = 5], 4.5% [1.7%]; G-CSF [n = 4], 8.1% [1.4%]), confirmed by a small proportion (1.5% [1.7%]) of apoptotic cells in DNA content analysis of CD34+ cells of 5 patients treated with NAT.

The BM cellularity and the proportion of BM CD34+ cells are not affected by treatment with NAT. The CD34+ cells derived from the BM of 9 MS patients treated with NAT were compared with those of 7 HCs. The median (SEM) CXCR4-expression rate (NAT, 62.1% [5.1%]; HC, 54.8% [5.2%]) and CD133-expression rate (NAT, 73.3% [5.3]; HC, 67.8% [3.8%]) were similar between the 2 groups. Median coexpression of CD49d was lower in the NAT group (NAT, 98.3% [0.8%], mean fluorescent intensity, 66.1% [8.5%]; HC, 99.7% [0.2%], mean fluorescent intensity, 97.4% [34.3%]; P = .03) (Figure 1). There was no significant difference in the median (SEM) BM cellularity between 9 MS patients and 6 HCs (NAT, 16 000 [3000] mononuclear cells/μL; HC, 9000 [2100] mononuclear cells/μL; P = .15). The median (SEM) proportion of CD34+ cells (2.03% [0.42%]) as measured by fluorescence-activated cell sorting analysis and of blasts (1% [0.2%]) as obtained by cytomorphologic analysis were within the normal range (Figure 3).

The adhesive abilities of BM CD34+ cells of NAT-treated patients remain unaffected, whereas the migratory properties are impaired. We observed no significant difference in the median (SEM) adhesive abilities of CD34+ cells obtained from BM between the 2 groups (NAT [n = 9], 28.2% [0.7%]; HC [n = 6], 23.3% [1.5%]), whereas the median migratory capacity was significantly reduced in the NAT group (NAT [n = 8], 0.4% [0.1%]; HC [n = 7], 1.7% [ 0.6%]; P = .02) (Figure 2).

The JC virus DNA was not detectable in BM CD34+ cells of 9 MS patients treated with NAT. This was demonstrated by polymerase chain reaction assay.

COMMENT

Treatment of MS patients with NAT leads to a rapid and sustained increase in circulating CD34+ cells in the PB.13 Here, we present data showing that this increase is the result of impaired homing of CD34+ cells to, rather than true mobilization of, the BM. Therefore, we believe that the increased concentration of CD34+ cells in the PB is the result of a gradual accumulation of cells unable to return to homing sites, such as the BM. The following data support this view: (1) circulating CD34+ cells of MS patients treated with NAT revealed reduced adhesive and migratory properties; thus, transendothelial egress relevant for homing is most likely to be impaired; (2) adhesive properties of BM CD34+ cells are not impaired, whereas migratory capacity is reduced; thus, BM CD34+ cells are less likely to get mobilized out of the BM; (3) the BM cellularity and the proportion of BM CD34+ cells are not affected by treatment with NAT; thus, NAT treatment appears to have only a marginal influence on BM in our cohort.

These findings have implications for the clinical use of NAT in the treatment of MS, as well as for NAT as a potential agent to mobilize hematopoietic progenitor cells in hematology. First, coexpression analysis demonstrates that circulating CD34+ cells of MS patients treated with NAT are more mature and therefore have a poor ability to initiate long-term cultures. Thus, NAT is not a promising candidate to mobilize hematopoietic progenitors in hematology. Second, our findings argue against an antibody-induced exhaustion of the CD34+ progenitor pool as a potential adverse effect of long-term application of NAT in MS patients. Third, the BM has been hypothesized to be a relevant reservoir for the JC virus and the cases of progressive multifocal leukoencephalopathy observed in MS patients treated with NAT.3,6,7 We were unable to detect the JC virus in BM CD34+ cells within this study. We cannot exclude a sampling error in only 9 MS patients. However, the fact that NAT treatment impairs homing strongly argues against a hypothesis claiming NAT induced mobilized JC virus–infected BM cells to be involved in progressive multifocal leukoencephalopathy pathogenesis. This finding is in line with negative JC virus DNA findings in circulating CD34+ cells in a recently published study.3 Thus, other mechanisms of JC virus reactivation and central nervous system infection should be addressed to understand NAT-associated cases of progressive multifocal leukoencephalopathy.

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Article Information

Correspondence: Christian Saure, MD, Department of Hematology, Oncology, and Clinical Immunology, University of Düsseldorf, Moorenstr. 5, 40225 Düsseldorf, Germany (Christian.Saure@med.uni-duesseldorf.de).

Accepted for Publication: March 8, 2011.

Author Contributions:Study concept and design: Saure, Zohren, Schroeder, Kobbe, and Haas. Acquisition of data: Saure, Zohren, Schroeder, Bruns, Cadeddu, Weigelt, Fischer, Hartung, and Adams. Analysis and interpretation of data: Saure, Warnke, Zohren, Schroeder, Cadeddu, Fischer, Kobbe, Adams, Kieseier, and Haas. Drafting of the manuscript: Saure, Warnke, Schroeder, and Fischer. Critical revision of the manuscript for important intellectual content: Warnke, Zohren, Schroeder, Bruns, Cadeddu, Weigelt, Kobbe, Hartung, Adams, Kieseier, and Haas. Statistical analysis: Zohren. Obtained funding: Saure and Schroeder. Administrative, technical, and material support: Saure, Bruns, Cadeddu, Weigelt, Fischer, Kobbe, and Kieseier. Study supervision: Zohren, Weigelt, Kobbe, Hartung, and Haas.

Financial Disclosure: Drs Hartung and Kieseier have received honoraria for lecturing, travel expenses for attending meetings, and financial support for research from Bayer Health Care, Biogen Idec, Merck Serono, Novartis, sanofi-aventis, and TEVA Pharmaceutical Industries Ltd.

Funding/Support: This work was supported by a grant from the Forschungskommission der Medizinischen Fakultät, Düsseldorf, Germany (Dr Saure) and by grant 01GI1002 from the German Ministry for Education and Research (German Competence Network Multiple Sclerosis, Natalizumab-Pharmakovigilanzstudie) (Dr Kieseier).

References
1.
Bonig H, Wundes A, Chang KH, Lucas S, Papayannopoulou T. Increased numbers of circulating hematopoietic stem/progenitor cells are chronically maintained in patients treated with the CD49d blocking antibody natalizumab.  Blood. 2008;111(7):3439-3441PubMedArticle
2.
Zohren F, Toutzaris D, Klärner V, Hartung HP, Kieseier B, Haas R. The monoclonal anti-VLA-4 antibody natalizumab mobilizes CD34+ hematopoietic progenitor cells in humans.  Blood. 2008;111(7):3893-3895PubMedArticle
3.
Warnke C, Smolianov V, Dehmel T,  et al.  CD34+ progenitor cells mobilized by natalizumab are not a relevant reservoir for JC virus.  Mult Scler. 2011;17(2):151-156PubMedArticle
4.
Jing D, Oelschlaegel U, Ordemann R,  et al.  CD49d blockade by natalizumab in patients with multiple sclerosis affects steady-state hematopoiesis and mobilizes progenitors with a distinct phenotype and function.  Bone Marrow Transplant. 2010;45(10):1489-1496PubMedArticle
5.
Clifford DB, De Luca A, Simpson DM, Arendt G, Giovannoni G, Nath A. Natalizumab-associated progressive multifocal leukoencephalopathy in patients with multiple sclerosis: lessons from 28 cases [published correction appears in Lancet Neurol. 2010;9(5):463].  Lancet Neurol. 2010;9(4):438-446PubMedArticle
6.
Ransohoff RM. Natalizumab and PML.  Nat Neurosci. 2005;8(10):1275PubMedArticleArticle
7.
Tan CS, Dezube BJ, Bhargava P,  et al.  Detection of JC virus DNA and proteins in the bone marrow of HIV-positive and HIV-negative patients: implications for viral latency and neurotropic transformation.  J Infect Dis. 2009;199(6):881-888PubMedArticle
8.
Steidl U, Haas R, Kronenwett R. Intercellular adhesion molecular 1 on monocytes mediates adhesion as well as trans-endothelial migration and can be downregulated using antisense oligonucleotides.  Ann Hematol. 2000;79(8):414-423PubMedArticle
9.
Kronenwett R, Steidl U, Kirsch M, Sczakiel G, Haas R. Oligodeoxyribonucleotide uptake in primary human hematopoietic cells is enhanced by cationic lipids and depends on the hematopoietic cell subset.  Blood. 1998;91(3):852-862PubMed
10.
Czibere A, Prall WC, Zerbini LF,  et al.  Exisulind induces apoptosis in advanced myelodysplastic syndrome (MDS) and acute myeloid leukaemia/MDS.  Br J Haematol. 2006;135(3):355-357PubMedArticle
11.
Steidl U, Bork S, Schaub S,  et al.  Primary human CD34+ hematopoietic stem and progenitor cells express functionally active receptors of neuromediators.  Blood. 2004;104(1):81-88PubMedArticle
12.
Adams GB, Chabner KT, Alley IR,  et al.  Stem cell engraftment at the endosteal niche is specified by the calcium-sensing receptor.  Nature. 2006;439(7076):599-603PubMedArticle
13.
Bruns I, Czibere A, Fischer JC,  et al.  The hematopoietic stem cell in chronic phase CML is characterized by a transcriptional profile resembling normal myeloid progenitor cells and reflecting loss of quiescence.  Leukemia. 2009;23(5):892-899PubMedArticle
14.
Sutherland HJ, Eaves CJ, Eaves AC, Dragowska W, Lansdorp PM. Characterization and partial purification of human marrow cells capable of initiating long-term hematopoiesis in vitro.  Blood. 1989;74(5):1563-1570PubMed
15.
Punzel M, Moore KA, Lemischka IR, Verfaillie CM. The type of stromal feeder used in limiting dilution assays influences frequency and maintenance assessment of human long-term culture initiating cells.  Leukemia. 1999;13(1):92-97PubMedArticle
16.
Nicoletti I, Migliorati G, Pagliacci MC, Grignani F, Riccardi C. A rapid and simple method for measuring thymocyte apoptosis by propidium iodide staining and flow cytometry.  J Immunol Methods. 1991;139(2):271-279PubMedArticle
17.
Vermes I, Haanen C, Steffens-Nakken H, Reutelingsperger C. A novel assay for apoptosis: flow cytometric detection of phosphatidylserine expression on early apoptotic cells using fluorescein labelled Annexin V.  J Immunol Methods. 1995;184(1):39-51PubMedArticle
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