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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2007;27:283.)
© 2007 American Heart Association, Inc.

Vascular Biology

M-CSF Accelerates Neointimal Formation in the Early Phase After Vascular Injury in Mice

The Critical Role of the SDF-1–CXCR4 System

Yuji Shiba; Masafumi Takahashi; Toru Yoshioka; Noriyuki Yajima; Hajime Morimoto; Atsushi Izawa; Hirohiko Ise; Kiyohiko Hatake; Kazuo Motoyoshi; Uichi Ikeda

From the Division of Cardiovascular Sciences, Department of Organ Regeneration (Y.S., M.T., T.Y., N.Y., H.M., A.I., H.I., U.I.), Shinshu University Graduate School of Medicine, Matsumoto; the Cancer Chemotherapy Center (K.H.), Japanese Foundation for Cancer Research, Tokyo; and the Third Department of Internal Medicine (K.M.), National Defense Medical College, Saitama, Japan.

Correspondence to Masafumi Takahashi, MD, PhD, Division of Cardiovascular Sciences, Department of Organ Regeneration, Shinshu University Graduate School of Medicine, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan. E-mail masafumi{at}sch.md.shinshu-u.ac.jp

	Abstract

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Objective— Since the macrophage colony-stimulating factor (M-CSF) has been shown to stimulate differentiation and proliferation of monocyte/macrophage lineage and to be involved in the process of neointimal formation after vascular injury, we tested the effects of M-CSF on the recruitment of bone marrow-derived progenitor cells in neointimal formation after vascular injury in mice.

Methods and Results— Wire-mediated vascular injury was produced in the femoral artery of C57BL/6 mice. Recombinant human M-CSF [500 µg/(kg·day)] or saline (control) was administered for 10 consecutive days, starting 4 days before the injury. Treatment with M-CSF accelerated neointimal formation in the early phase after injury, and this neointimal lesion mainly consisted of bone marrow-derived cells. M-CSF treatment had no effect on the mobilization of endothelial progenitor cells (EPCs: CD34⁺/Flk-1⁺) and reendothelialization after injury. The stromal cell-derived factor-1 (SDF-1) was markedly expressed in the neointima and media after injury, whereas CXCR4⁺ cells were observed in the neointima. Further, a novel CXCR4 antagonist, AMD3100, significantly attenuated the M-CSF-induced neointimal formation.

Conclusions— These findings suggest that M-CSF accelerated neointimal formation after vascular injury via the SDF-1–CXCR4 system, and the inhibition of this system has therapeutic potential for the treatment of cardiovascular diseases.

We tested the effects of M-CSF on the recruitment of bone marrow-derived progenitor cells in neointimal formation after vascular injury in mice. The findings obtained from this study demonstrated that M-CSF accelerated neointimal formation in the early phase after vascular injury via the SDF-1–CXCR4 system.

Key Words: angioplasty • cytokines • inflammation • restenosis • vascular biology

	Introduction

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The vascular endothelium forms a biological interface between circulating blood components and various tissues in the body. This monolayer of endothelial cells locally monitors systemically generated stimuli, and alters the functional state of the vessels. This adaptive mechanism contributes to normal homeostasis; however, nonadaptive changes in the endothelial structure and function, provoked by pathophysiological stimuli, may induce "endothelial dysfunction," which plays an important role in the initiation and progression of cardiovascular diseases. In particular, the loss of endothelial cells because of vascular injury leads to the migration and proliferation of vascular smooth muscle cells (SMCs), resulting in neointimal formation. Further, the vascular injury initiates an inflammatory healing response that involves the expression of growth factors and cytokines and promotes neointimal formation. The resultant neointimal formation is the pathological basis of atherosclerosis and restenosis following percutaneous coronary intervention (PCI) such as angioplasty and stenting.

See page 263

The recruitment, activation, and proliferation of monocytes/macrophages in the vessel wall make important contribution to the process of atherosclerosis and restenosis. The presence of activated monocytes/macrophages at the site of the vascular injury leads to the release of vasoactive molecules, cytokines, and growth factors, which can induce the migration and proliferation of SMCs. However, recent evidence indicates that a part of the population of endothelial progenitor cells (EPCs) are derived from the monocyte/macrophage lineage cells, and these participate in the neovascularization of ischemic tissues.^1–3 In addition, monocyte/macrophage lineage-derived EPCs secrete large amounts of angiogenic factors, such as vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF)³; this suggests that monocytes/macrophages can promote neovascularization. Further, the bone marrow-derived EPCs may contribute to the process of reendothelialization, termed "vascular repair," and prevent neointimal formation after vascular injury.⁴ However, it remains unclear whether the monocyte/macrophage lineage cells play a substantial role in neointimal formation after vascular injury.

The macrophage colony-stimulating factor (M-CSF) is a multifunctional proinflammatory cytokine that regulates the differentiation, proliferation, and survival of monocytic progenitor cells,⁵ and plays a role in the differentiation of monocytes to macrophages in the arterial wall. Recent investigations suggest that M-CSF plays an important role in human atherosclerotic lesions^6,7 and in experimental animal models of atherosclerosis^8,9; M-CSF activates monocytes/macrophages and promotes the proliferation of these cells and SMCs. Further, an increased expression of M-CSF after vascular injury has been demonstrated.¹⁰ We previously reported that in patients with coronary artery diseases, M-CSF levels in the coronary sinus blood increased after PCI, and this increase was related to the development of restenosis.¹¹

In the present study, we postulate that M-CSF might play a role in the mobilization of bone marrow-derived progenitor cells, reendothelialization, and neointimal formation after vascular injury. We demonstrated that exogenous M-CSF treatment accelerated neointimal formation in the early phase after vascular injury, and this formation was mediated through a system comprising a key stem cell homing factor, stromal cell-derived factor (SDF-1: CXCL12), and the SDF-1 receptor CXCR4. The findings obtained from this study may provide new insights into the role of M-CSF and the SDF-1–CXCR4 system in the pathogenesis of neointimal formation after vascular injury.

	Materials and Methods

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All materials and methods are detailed in the online supplement available at http://atvb.ahajournals.org.

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

 
Effect of M-CSF on Neointimal Formation After Vascular Injury
We first assessed whether M-CSF was upregulated in the site of vascular injury, and found the expression of M-CSF at the injured artery (Figure 1A). Previously, we demonstrated that neointimal formation is initiated at 7 days and completed at 21 days after wire-mediated vascular injury in mice.¹² Hence, we evaluated the effect of M-CSF treatment on neointimal formation 7, 14, and 21 days after vascular injury. As expected, histological analysis showed that neointimal formation was initiated on the 7^th day; it increased until the 14^th day, and was completed by the 21^st day after the injury in control mice (Figure 1B). In contrast, marked neointimal formation was observed at 7 days after the injury in M-CSF-treated mice. Quantitative analysis revealed that the I/M ratio in the M-CSF-treated mice significantly increased at 7 days after the injury as compared with that in the control mice (Figure 1C, P<0.05); however, there was no difference between the I/M ratio of the control and M-CSF-treated mice at 21 days after the injury (Figure 1D).

View larger version (68K): [in this window] [in a new window]   Figure 1. Effect of M-CSF on neointimal formation after vascular injury. A, The injured and uninjured (intact) femoral arteries were excised at 7 days after injury. Immunohistochemical analysis for M-CSF was performed. B through D, M-CSF [500 µg/(kg·day), n=15] or saline (control, n=16) was administered for 10 consecutive days, starting 4 days before the vascular injury. The femoral arteries were excised at 7, 14, and 21 days after the injury. The sample sections were stained with HE, and neointimal formation was evaluated. B, Representative photographs of HE staining. C and D, Bar graphs show the I/M ratio quantified by NIH Image. Data are mean±SEM (each n=5 to 8).

Becaues the histology of neointimal formation at 7 days in M-CSF-treated mice appears to be different from that at 21 days, we performed an immunohistochemical analysis for macrophages (F4/80), endothelial cells (CD31), and SMCs (-SMA). As shown in Figure 2A, endothelial cells on the surface of the lesion and macrophages and SMCs in the lesion were observed. This finding was consistent with the histological features of the lesion at 21 days after injury (data not shown). Next, we assessed whether M-CSF treatment affected reendothelialization at 4 and 7 days after injury and found that M-CSF treatment had no effect (Figure 2B). These results indicate that M-CSF treatment accelerated neointimal formation but not reendothelialization after vascular injury. We further observed the expression of M-CSF receptor, c-fms, in the neointimal lesion of the injured arteries (Figure 2C).

View larger version (64K): [in this window] [in a new window]   Figure 2. Immunohistochemical staining for endothelial cells, macrophages, and smooth muscle cells. M-CSF [500 µg/(kg·day)] or saline (control) was administered for 10 consecutive days, starting 4 days before the vascular injury. The femoral arteries were excised at 7 days after the injury. Immunohistochemical staining for endothelial cells (CD31), macrophages (F4/80), and SMCs (-SMA) was performed. A, Representative photographs of CD31, F4/80, and -SMA expression. B, Bar graphs show the reendothelialization ratio determined by CD31 expression at 4 and 7 days after injury quantified by NIH Image. Data are mean±SEM (n=3 to 4). C, Immunohistochemical staining for c-fms was performed. Irrelevant IgG was used as a negative control.

Contribution of Bone Marrow-Derived Cells
To determine the contribution of bone marrow-derived cells to accelerated neointimal formation after vascular injury, we used bone marrow-transplanted mice whose bone marrow was replaced with that of ROSA26 mice. In control mice, almost no ß-galactosidase-positive cells were detected, whereas a large number of ß-galactosidase-positive cells were detected in M-CSF-treated mice (Figure 3A). These findings suggest that the accelerated neointimal lesion induced by M-CSF mainly consisted of bone marrow-derived cells.

View larger version (30K): [in this window] [in a new window]   Figure 3. Contribution of bone marrow-derived cells in the early phase. A, Bone marrow-transplanted mice (ROSA26C57BL/6) were developed, and wire-mediated vascular injury was produced 8 weeks after bone marrow transplantation. M-CSF [500 µg/(kg·day), n=10] or saline (control, n=10) was administered for 10 consecutive days, starting 4 days before vascular injury. The femoral arteries were excised 7 days after the injury, and X-gal staining was performed. B through D, The percentage of Mac-1+/Gr-1– (B), CD34+/Flk-1+ (C), CD34–/CD14+ (D), and CXCR4+ (E) cells was assessed by using flow cytometry after saline (control) or M-CSF [500 µg/(kg·day)] was administered for 4 consecutive days. Data are mean±SEM (each, n=5). E, Double staining for Mac-1+ and CXCR4+ cells was performed.

To explore the types of bone marrow-derived cells that were recruited in the neointimal lesion, we assessed the number of Mac-1⁺/Gr-1^– (monocytes/macrophages), CD34⁺/Flk-1⁺ (EPCs),¹³ CD34^–/CD14⁺, and CXCR4⁺ cells in the peripheral circulation in the control and M-CSF-treated mice. Flow cytometry analysis revealed that M-CSF treatment significantly increased the number of Mac-1⁺/Gr-1^– cells (P<0.05), but not CD34⁺/Flk-1⁺ and CD34^–/CD14⁺ cells (Figure 3B though 3D). Interestingly, the number of CXCR4⁺ cells was also significantly increased by M-CSF treatment (Figure 3E, P<0.05). Further, double staining for Mac-1 and CXCR4 showed that M-CSF-increased peripheral CXCR4⁺ cells contained Mac-1⁺ cells (Figure 3F).

Further, at 21 days after injury, we evaluated the contribution of the bone marrow-derived cells to neointimal formation by using bone marrow-transplanted mice whose bone marrow had been replaced with that of GFP mice. Because it was difficult to discriminate GFP-expressing cells from other types of cells in the presence of autofluorescence of the injured artery,¹² we identified the bone marrow-derived cells by immunohistochemical analysis using the anti-GFP antibody. Consistent with the report by Tanaka et al,¹⁴ a considerable number of GFP⁺ cells were detected in the neointima and media after the injury (supplemental Figure I). Many GFP⁺ cells in the neointima of the injured artery were positive for the staining against macrophages and SMCs. However, a small number of CD31-positive endothelial cells on the luminal surface of the artery were GFP-positive.

Expression of SDF-1 and CXCR4
Because SDF-1 is a ligand for CXCR4, we performed immunohistochemical analysis to detect SDF-1 in the injured arteries. As shown in Figure 4A, no SDF-1 expression was observed in uninjured arteries, whereas striking SDF-1 expression was observed in the injured arteries of the control and M-CSF-treated mice. Quantitative analysis showed that there was no significant difference in SDF-1 expression levels between control and M-CSF-treated mice (Figure 4B). Further, double immunofluorescence staining showed that SDF-1 was mainly expressed in the neointima and media, whereas CXCR4 was mainly expressed in the neointima (Figure 4C).

View larger version (65K): [in this window] [in a new window]   Figure 4. Expression of SDF-1 and CXCR4. M-CSF [500 µg/(kg·day), n=7] or saline (control, n=7) was administered for 10 consecutive days, starting 4 days before vascular injury. The injured and uninjured (intact) femoral arteries were excised at 7 days after injury. A, Immunohistochemical analysis for SDF-1 was performed. B, Bar graph shows the SDF-1 expression quantified by NIH image. Data are mean±SEM (each, n=3 to 4). C, Double immunofluorescence staining for SDF-1 (green) and CXCR4 (red) was performed.

Effect of CXCR4 Antagonist on Neointimal Formation
To explore the role of the SDF-1–CXCR4 system, we used a CXCR4 antagonist, AMD3100. AMD3100 [300 µg/(kg·hour)] was subcutaneously administered for 7 days after the vascular injury using a micro-osmotic pump. Consistent with previous reports,¹⁵ the administration of AMD3100 significantly increased the number of circulating white blood cells (WBCs), particularly, neutrophils and lymphocytes, as compared with M-CSF treatment alone (Figure 5A through 5C, P<0.05). Additionally, M-CSF treatment markedly increased the I/M ratio at 7 days after the injury (Figure 5D and 5E, P<0.01). AMD3100 treatment significantly reduced the increase in the I/M ratio that was caused by M-CSF treatment (P<0.05). Immunohistochemical analysis revealed that the number of CXCR4⁺ cells obviously decreased in the neointima of the AMD3100-treated mice (Figure 5D), but there was no significant difference of the reendothelialization after injury (Figure 5F).

View larger version (45K): [in this window] [in a new window]   Figure 5. Effect of AMD3100 on neointimal formation. M-CSF [500 µg/(kg·day), n=7] or saline (control, n=7) was administered for 10 consecutive days, starting 4 days before vascular injury. AMD3100 [300 µg/(kg·hour): AMD alone and M-CSF+AMD, each n=7] was administered using a micro-osmotic pump for 7 days after the injury. Blood samples were collected and the femoral arteries were excised at 7 days after the injury. The sample sections were stained with HE and neointimal formation was evaluated. Immunohistochemical analysis for CXCR4 was also performed. A through C, Bar graph shows the number of peripheral WBCs (A), neutrophils (B), and lymphocytes (C). Data are mean±SEM (n=7). D, Representative photographs of HE and CXCR4 staining. E and F, The bar graphs show the I/M ratio (E) and reendothelialization (F), which were quantified by NIH Image. Data are mean±SEM (n=7).

Effect of M-CSF on CXCR4 Expression In Vitro
To investigate the mechanism by which M-CSF increases the number of CXCR4⁺ cells in peripheral circulation, peripheral and bone marrow MNCs were incubated for 24 hours in the presence or absence of M-CSF, and then analyzed for the expression of Mac-1 and CXCR4. M-CSF treatment significantly increased Mac-1⁺ cells in peripheral MNCs (Figure 6A, P<0.05). However, M-CSF showed no effect on the CXCR4⁺ cells in the peripheral or bone marrow MNCs, although G-CSF decreased CXCR4⁺ cells in bone marrow MNCs (Figure 6B though 6D).

View larger version (31K): [in this window] [in a new window]   Figure 6. Effect of M-CSF on CXCR4 expression in vitro. Peripheral or bone marrow MNCs were incubated for 24 hours in the presence or absence of M-CSF (100 ng/mL) or G-CSF (100 ng/mL), and then the expression levels of Mac-1 and CXCR4 were analyzed by flow cytometry. Bar graphs show the fold change in the number of Mac-1+ cells in peripheral MNCs (A), CXCR4+ cells in peripheral MNCs (B), and CXCR4+ cells in bone marrow MNCs (D). Data are mean±SEM (each, n=8 to 10). C, Representative histogram of CXCR4 expression in bone marrow MNCs.

Involvement of Inflammatory Cytokines
Because the inhibition of CXCR4 signaling partially attenuated the accelerated neointimal formation by M-CSF, we investigated whether inflammatory cytokines, such as MCP-1, interleukin (IL)-12p70, IL-10, IL-6, and tumor necrosis factor (TNF)-, are involved in this process. M-CSF treatment significantly increased the serum of MCP-1 levels (P<0.05), but not that of other inflammatory cytokines (supplemental Figure II).

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
The major findings of this study are: (1) M-CSF treatment accelerated neointimal formation in the early phase of vascular injury; this neointimal lesion mainly consisted of bone marrow-derived cells. (2) M-CSF treatment had no effect on EPC mobilization after the injury and reendothelialization of the injured artery. (3) M-CSF treatment increased the number of peripheral CXCR4⁺ cells; this increase was possibly attributable to the mobilization of CXCR4⁺ cells from the bone marrow. (4) SDF-1 expression markedly increased in the neointima and media after the vascular injury; a number of CXCR4⁺ cells were observed in the neointima. (5) A CXCR4 antagonist, AMD3100 significantly attenuated neointimal formation in the early phase after vascular injury in M-CSF-treated mice. These findings suggest that M-CSF treatment accelerates neointima formation in the early phase after vascular injury via the SDF-1–CXCR4 system.

Increasing evidence indicates the importance of vascular progenitor cells derived from the bone marrow in vascular development, homeostasis, and remodeling. In particular, the bone marrow-derived EPCs could promote early reendothelialization of the denuded vessels after injury and potentiate their vascular repair¹⁶; this suggests the therapeutic potential of EPC transplantation for the treatment of cardiovascular diseases. Because colony-stimulating factors could mobilize bone marrow stem cells into the peripheral circulation, granulocyte CSF (G-CSF) and granulocyte-macrophage CSF (GM-CSF) have been recently noted as clinical application of stem cell therapy.^17,18 However, the effect of M-CSF on vascular repair has not been investigated. Here, we showed that exogenous M-CSF treatment significantly accelerated neointimal formation in the early phase after vascular injury.

In the present study, we used a wire-mediated vascular injury model because this model allows us to reproduce complete endothelial cell denudation and neointimal formation after injury.^12,19 This model induces the robust neointimal formation at 21 days after injury even in the control mice; this suggests that neointimal formation in the control mice could catch up with that in M-CSF-treated mice, and the lesion size in the late phase was similar between the control and M-CSF-treated mice. Xu et al²⁰ recently showed the importance of M-CSF-c-fms system in the vascular remodeling in a murine cuff-replacement model. We also detected c-fms-positive cells were accumulated in the M-CSF-induced neointimal lesion, indicating the role of M-CSF-c-fms system in the process of neointimal formation after injury. Interestingly, Tanaka et al¹⁴ demonstrated that the contribution of bone marrow cells to neointimal formation markedly differs between wire-mediated vascular injury and cuff-replacement models, and suggest that the wire-mediated vascular injury is suitable to investigate the role of bone marrow cells in the vascular remodeling after injury.

We demonstrated that M-CSF had no effect on the mobilization of EPCs, and reendothelialization after vascular injury. Although Harraz et al² reported that CD34^– angioblasts were a subset of CD14⁺ monocytic cells and that these monocytes have the potential to transdifferentiate into endothelial cells, we could not detect increase of peripheral CD34^–/CD14⁺ cells by M-CSF treatment. Recently, Minamino et al²¹ showed that M-CSF increased Sca-1⁺/Lin^–, Flk-1⁺/CD45^–, and Sca-1⁺/c-kit⁺/CD45^– cells as EPCs in the peripheral circulation. Further studies are needed to clarify the involvement of monocytic cell-derived EPCs in the accelerated neointimal formation by the treatment with M-CSF.

We clearly showed that M-CSF increased the number of CXCR4⁺ cells in peripheral circulation, whereas the vascular injury induced SDF-1 expression in the injured artery. In this regard, a recent study identified G-CSF downregulation of CXCR4 expression as a mechanism for mobilization of bone marrow myeloid cells.²² We showed that G-CSF clearly reduced CXCR4 expression in bone marrow-derived MNCs whereas M-CSF had no effect; this suggests that G-CSF and M-CSF may mobilize CXCR4⁺ cells by distinct mechanisms. The mobilized CXCR4⁺ cells were recruited; they interacted with SDF-1 and contribute to accelerated neointimal formation, indicating that the SDF-1–CXCR4 system plays an important role in neointimal formation after vascular injury. Although Weber and colleagues^23,24 recently reported that the SDF-1–CXCR4 system contributed to the recruitment of bone marrow-derived SMC progenitor cells and neointimal formation after vascular injury in apoE^–/– mice, we could not detect the mobilization of CXCR4⁺ cells after vascular injury in wild-type mice (data not shown). Therefore, it is possible that SDF-1–CXCR4 system may play a role in vascular repair under specific conditions such as hypercholesterolemia and M-CSF treatment. More recently, Zhang et al²⁵ also demonstrated that the SDF-1–CXCR4 system contributed to neointimal formation after carotid artery ligation in endothelial nitric oxide synthase (eNOS) deficient mice. Thus, these investigations strongly support the findings of our study. We showed that although CXCR4⁺ cells were recruited, M-CSF treatment had no effect on reendothelialization after vascular injury. Additionally, this finding was supported by Weber et al²⁴ who reported that the neutralization of SDF-1 did not alter the reendothelialization after vascular injury in apoE^–/– mice. Conversely, Walter et al²⁶ reported that the bone marrow MNCs or EPCs of heterozygous CXCR4^+/– mice displayed reduced CXCR4 expression and attenuated neovascularization capacity, suggesting that the CXCR4⁺ cells function as EPCs in ischemic tissue. Taken together, we postulate that the CXCR4⁺ cells have the potential to function as both EPCs and SMC progenitor cells according to the circumstances, and the SDF-1–CXCR4 system may contribute to the pathogenesis of cardiovascular diseases.

The present study showed that M-CSF treatment increased the level of MCP-1 in serum. Because MCP-1 is a major chemokine that induces the recruitment and activation of monocytes,^27,28 the accumulation of monocytes/macrophages at the injured artery might be mediated, at least in part, via MCP-1 induction.

Treatment with AMD3100 attenuated the M-CSF-induced neointimal formation after vascular injury; this suggests a therapeutic potential of this compound in treating the development of atherosclerosis and restenosis after PCI. Interestingly, recent investigations demonstrated that AMD3100 treatment rapidly mobilizes CD34⁺ hematopoietic stem cells from the bone marrow into peripheral circulation and synergistically enhances the mobilization of CD34⁺ cells in combination with G-CSF.^29–31 More recently, Capoccia et al³² reported that G-CSF combined with AMD3100 promoted angiogenesis in a murine model of hindlimb ischemia. In our study, however, the reendothelialization was not affected by AMD3100. This discrepancy might be attributable to continuous or transient inhibition of CXCR4 signaling. In the study by Capoccia et al,³² AMD3100 was given by a single injection, while AMD3100 was given by a continuous infusion using an osmotic pump in our study. Therefore, we postulate that continuous CXCR4 inhibition abrogates the chemotactic activity for SDF-1 and homing to the site of vascular injury. Supporting this, a recent study demonstrated that continuous inhibition of CXCR4 signaling impaired functional capacity of EPCs and inhibited angiogenesis.²⁶ Further investigations are required to use clinical application of this compound.

In summary, we demonstrated that M-CSF mobilized CXCR4⁺ cells from the bone marrow into peripheral circulation, and vascular injury induced SDF-1 expression in the injured artery. The bone marrow-derived CXCR4⁺ cells were recruited to the site of the injured artery where they interacted with SDF-1 resulting in the early development of neointimal formation. Further, we showed that the CXCR4 antagonist, AMD3100 significantly inhibited M-CSF-induced neointimal formation. These findings suggest that M-CSF accelerated neointimal formation after vascular injury, at least in part, via the SDF-1–CXCR4 system, and that the inhibition of the SDF-1–CXCR4 pathway might have therapeutic potential in the treatment of vascular injury.

	Acknowledgments

 
We thank Junko Nakayama, Tomoko Hamaji, and Kazuko Misawa for excellent technical assistance, Muneo Yamada (Morinaga Milk Industry Co Ltd) for providing M-CSF, and Masaru Okabe (Osaka University) for providing GFP mice.

Sources of Funding

This study was supported by research grants from the Ministry of Education, Culture, Sports, Science, and Technology (to M.T. and U.I.), the Ministry of Health, Labor, and Welfare (to M.T. and U.I.), and Mitsubishi Pharma Research Foundation (to M.T.).

Disclosures

None.

	Footnotes

 
Original received July 24, 2006; final version accepted September 28, 2006.

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