| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Letters to the Editor |
Department of Human Genetics, Baylor College of Medicine, Houston, Texas
Blood and Marrow Transplantation, The University of Texas M. D. Anderson Cancer Center, Houston, Texas
To the Editor:
Tavor et al. (1) investigated the role of CXCR4 in the migration and development of human acute myelogenous leukemia (AML) stem cells in NOD/severe combined immunodeficient (SCID) mice. They reported that human AML cells home to the murine bone marrow (BM) and spleen in an SDF-1/CXCR4-dependent manner.
Recently, we investigated the correlation between the level of expression of CXCR4 on AML CD34+ cells and their ability to engraft the BM of NOD/SCID mice (2) . We found the levels of CXCR4 expression on AML CD34+ cells to be decreased compared with normal progenitors. However, based on analysis of 11 AML cases with different CXCR4 levels, we found that engraftment of AML in NOD/SCID mice was independent of CXCR4. We demonstrated that not all AMLs engraft the murine BM, as described also by others (3, 4, 5) . Cells with virtually absent CXCR4 expression were able to engraft, and cells from two patients with high expression of CXCR4 did not. Anti-CXCR4 antibody failed to block the engraftment of two AMLs into NOD/SCID mice.
Engraftment of AML in all cases in the study by Tavor et al. (1) could be partially explained by the use of NOD/SCIDB2mnull mice (4) or by the small number of patient samples tested for homing and for engraftment (n = 6).
Tavor et al. (1) demonstrated high expression of internal CXCR4 in all AML cases, even in the absence of external CXCR4. However, the correlation between cell external or internal CXCR4 and the ability to engraft is missing. It is also interesting to note that the number of homing cells was higher in patient 8 than in patient 11, despite the lower CXCR4 expression. In addition, in 7 of 21 cases studied, CXCR4 expression was not determined. It is not reported which AML samples were used for engraftment studies, what their CXCR4 expression was, and whether the remaining 15 AML samples engrafted.
With regard to homing (see Fig. 5 in ref. 1 ), the frequency of homing cells is reported in the range of 0.01%, i.e., 100 cells/106. No background was reported for this assay, but Fig. 6 (see ref. 1 ) indicates a background of 0.1% to 0.2%. A frequency of 0.01% would therefore be well below the level of sensitivity of this assay. Furthermore, in Fig. 6C of ref. 1 , the number of mice injected per sample is not indicated, nor is the actual percentage of leukemic cells found in the mouse BM. In the homing experiment, only one mouse per sample was analyzed.
In conclusion, we feel that the report by Tavor et al. (1) does not establish a correlation between CXCR4 and homing or engraftment of AML.
REFERENCES
Department of Immunology, Weizmann Institute of Science, Rehovot, Israel
In Response:
The goal of our study (1) was to reveal the roles of CXCR4 signaling in bone marrow (BM) homing and repopulation of human acute myelogenous leukemia (AML) cells in transplanted immunodeficient mice. Homing experiments were performed with NOD/severe combined immunodeficient (SCID) B2mnull mice, which have lower innate immunity, leading to higher permissiveness for human cell homing. NOD/SCID mice were used for all of the engraftment experiments. The low background levels for homing were not indicated (see Fig. 5 of ref. 1 ) because neutralizing anti-CXCR4 treatment (10 µg of 12G5 monoclonal antibody coinjected with the human cells without washing of excess antibody to prevent expression and function of new receptors) significantly reduced homing. The background levels are either below the level of detection or in the range of 0% to 0.0001% in the murine BM and 0% to 0.0005% in the spleen as described previously with normal human progenitors (2) . Setting background levels for homing assays is more stringent than engraftment analysis to exclude any false positive event that may affect the initial low frequency of homing cells. Engraftment, however, yields a much higher percentage of human cells within the murine BM, and therefore this analysis allows a higher and reasonable background of about 0.1% to 0.2%, due to nonspecific staining of murine cells that can be eliminated by less stringent settings of the gates. In Fig. 6C, three untreated mice and two mice treated with anti-CXCR4 for each of the four experiments were used to reveal the role of CXCR4 in AML repopulation. Moreover, results were represented as a percentage of the control due to the high variability in engraftment levels from different donors.
Monaco et al. (3) did not find a significant correlation between CXCR4 expression and engraftment levels by AML cells; however, a positive trend was observed. Rombouts et al. (4) revealed a correlation between CXCR4 expression by AML CD34+ progenitors and engraftment in NOD/SCID mice and that levels of CXCR4 are also a negative predictor of overall survival and relapse-free survival of patients. In our study, pretreatment of primary AML cells with neutralizing CXCR4 antibodies significantly blocked homing to the BM and spleen of transplanted mice in all six tested samples and prevented repopulation in cells from six other patients.
Monaco et al. (3) pretreated AML cells from two patients before injection and did not observe inhibition of engraftment. Our studies did not include CXCR4 blocking before transplantation because blocking of homing will always lead to blocking of repopulation, as demonstrated with normal human progenitors. However, the different effect of anti-CXCR4 treatment is most probably due to washings of excess CXCR4 of the high dose used by Monaco et al. (3). Membrane expression of CXCR4 is dynamically regulated. We previously showed that CXCR4high sorted cells washed from excess Ab after sorting, display newly expressed receptors on their membrane, enabling partial recovery of receptor function and homing/repopulation potential (5). Newly expressed receptors will be neutralized by unwashed excess Abs, while washed cells can reexpress functional receptors during the long homing and repopulation processes but not during the short in vitro migration assay as shown by Monaco et al. (3) . In addition, Plett et al. (6) showed that low concentrations of anti-CXCR4 can also activate and enhance engraftment of normal human progenitors, suggesting that anti-CXCR4 antibodies may act as agonist in certain conditions because the antibody binds the same site as the ligand SDF-1.
REFERENCES
This article has been cited by other articles:
|
|
 |
|
  H. Bonig, G. V. Priestley, and T. Papayannopoulou Hierarchy of molecular-pathway usage in bone marrow homing and its shift by cytokines Blood, January 1, 2006; 107(1): 79 - 86. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Cancer Research | Clinical Cancer Research |
| Cancer Epidemiology Biomarkers & Prevention | Molecular Cancer Therapeutics |
| Molecular Cancer Research | Cancer Prevention Research |
| Cancer Prevention Journals Portal | Cancer Reviews Online |
| Annual Meeting Education Book | Meeting Abstracts Online |
