**Molecular Mechanisms Underlying Bone Marrow Homing of Hematopoietic Stem Cells**

Aysegul Ocal Sahin and Miranda Buitenhuis

*Department of Hematology and Erasmus MC Stem Cell Institute for Regenerative Medicine, Erasmus MC, Rotterdam, The Netherlands* 

#### **1. Introduction**

184 Advances in Hematopoietic Stem Cell Research

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Huijgens & A. M. Drager (2004). "Bone marrow stromal proteoglycans regulate megakaryocytic differentiation of human progenitor cells." *Exp Cell Res* 299(2): 383The formation of blood cells, also called hematopoiesis, is a complex process that occurs in the bone marrow and depends on correct regulation of hematopoietic cell fate decisions. Aberrant regulation of hematopoiesis can result in the development of severe malignant and non-malignant hematological disorders, including leukemia. Hematopoietic stem cell transplantation is the most powerful treatment modality for a large number of those malignancies. Successful hematopoietic recovery after transplantation depends on homing of hematopoietic stem cells to the bone marrow and subsequent lodging of those cells in the bone marrow microenvironment.

Homing is a rapid, coordinated process in which circulating hematopoietic stem and progenitor cells actively enter the bone marrow within a few hours after transplantation (Figure 1). Rolling and firm adhesion of those cells to endothelial cells in small marrow sinusoids is followed by trans-endothelial migration across the endothelium/extracellular matrix barrier. Finally, in irradiated recipients, hematopoietic stem cells anchor to their specialized niches within the bone marrow compartment near osteoblasts and initiate longterm repopulation (Lo Celso et al., 2009). In absence of available niches in, for example, nonirradiated recipients, HSCs tend to be more randomly distributed throughout the bone marrow (Lo Celso et al., 2009). Since the first bone marrow transplantation decades ago, research has focused on understanding the mechanisms underlying homing of hematopoietic stem cells to the bone marrow. This chapter will focus on recent studies that have extended our understanding of the molecular mechanisms underlying adhesion, migration and bone marrow homing of hematopoietic stem cells.

### **2. Selectins and bone marrow homing**

A first step in the process of bone marrow homing is initial tethering and rolling of hematopoietic stem and progenitor cells along the endothelial wall of blood vessels. It has been demonstrated that selectins play an important role in bone marrow homing of hematopoietic stem and progenitor cells by regulating these processes. Intravital microscopy in bone marrow sinoids and venules of mice deficient for individual selectins revealed that rolling of hematopoietic progenitor cells involves both P and E-selectin, but not L-selectin (Mazo et al.,

Molecular Mechanisms Underlying Bone Marrow Homing of Hematopoietic Stem Cells 187

Fig. 1. Homing of hematopoietic Stem Cells to the bone marrow. 1) Initial tethering and 2) rolling are the first steps in bone marrow homing. These processes are mediated by both E-

hematopoietic stem cells to the endothelial wall. 4) Firmly attached hematopoietic stem cells can subsequently transmigrate through the endothelial layer and 5) basal lamina, consisting of fibronectin, collagen and laminin. Integrins involved in these steps are CD49d/CD29, CD49e/CD29 and CD49f/CD29. 6) Finally, hematopoietic stem cells migrate towards the

Integrins are, in addition to selectins, also implicated in playing an important role in regulation of bone marrow homing. Several in vitro studies with blocking antibodies have, for example, shown that both CD49d/CD29 (41 or VLA-4) and CD11a/CD18 (L2 or LFA-1) play an important role in adhesion of hematopoietic stem and progenitor cells to endothelial cells and subsequent transendothelial migration (Imai et al., 1999; Peled et al., 2000; Voermans et al., 2000). In addition, spontaneous migration of CD34+ hematopoietic progenitors underneath a bone marrow derived stromal cell layer, was found to be significantly inhibited by a peptide that blocks CD49d/CD29 integrin binding (Burger et al., 2003). However, adhesion of CD34+ cells to fibronectin was found to be primarily dependent on CD49e/CD29 (51 or VLA-5) and not CD49d/CD29 (Peled et al., 2000). In addition, chemotaxis of peripheral blood CD34+ progenitor cells on recombinant fibronectin appears to be mediated, at least in part, by CD49e/CD29 (Carstanjen et al., 2005). The importance of

and P-selectin. 3) SDF-1 mediated integrin activation induces firm adhesion of the

SDF-1 gradient to the osteoblasts.

**3. Integrins and bone marrow homing** 

1998). Similarly, coating of a surface with immobilized P- or E-Selectin was sufficient to induce rolling of human CD34+ hematopoietic progenitor cells under flow conditions (Xia et al., 2004). A next step in bone marrow homing is transendothelial migration. This process requires firm adhesion of hematopoietic stem and progenitor cells to endothelial cells. Although CD34+ hematopoietic progenitor cells are capable of binding to fluid-phase P- and E-selectin (Xia et al., 2004), in vitro adhesion to bone marrow derived endothelial cells under static conditions has been shown not to depend on E-selectin (Naiyer et al., 1999). Transwell experiments performed to study the importance of E-selectin in migration of human hematopoietic progenitor cells through a confluent layer of bone marrow derived endothelial cells, precultured with IL-1B to induce E-selectin expression, yielded contradictory results. While Naiyer et al. have demonstrated with blocking antibodies that E-selectin is important for transendothelial migration (Naiyer et al., 1999), no significant inhibition in transendothelial migration could be observed by Voermans et al. who performed similar experiments (Voermans et al., 2000). Transplantation of lethally irradiated recipient mice deficient for both P-and E-selectin with wild type bone marrow cells resulted in reduced recruitment of hematopoietic progenitors to the bone marrow and enhanced levels of circulating hematopoietic progenitors, indicating that selectins indeed play an important role in bone marrow homing (Frenette et al., 1998).

Ligands for E-selectin include the PSGL-1 glycoform CLA, CD43 and the CD44 glycoform HCELL (Dimitroff et al., 2001; Merzaban et al., 2011). These ligands are all expressed on mouse Lin-Sca-1+c-Kit+ hematopoietic stem and progenitor cells and human CD34+ hematopoietic progenitor cells (Merzaban et al., 2011). Immune precipitation experiments revealed that although E-selectin can bind to CLA and CD43 in both mouse and human cells, the interaction between E-selectin and CD44 only occurs in human cells (Merzaban et al., 2011). These studies indicate that the molecular mechanism underlying bone marrow homing may be different for mouse and human hematopoietic stem cells. This hypothesis was confirmed by the observation that human CD34+ hematopoietic progenitor cells exhibit a stronger E-selectin binding capacity compared to mouse Lin-Sca-1+c-kit+ cells (Merzaban et al., 2011). In contrast to PSGL-1 which is also expressed in mature hematopoietic cells, CD44 appears to be predominantly expressed on primitive human CD34+ hematopoietic progenitor cells (Dimitroff et al., 2001). Rolling experiments performed under physiological flow conditions revealed that CD44 mediates E-selectin-dependent rolling interactions over a wider shear range in comparison to PSGL-1 and promotes rolling interactions on human bone marrow endothelial cells (Dimitroff et al., 2001). Silencing of CD44 expression in human cells with shRNAs was sufficient to decrease E-selectin binding under physiologic shear conditions, while enforced CD44 expression in Lin-Sca-1+c-kit+ cells conversely increased E-selectin adherence, resulting in improved bone marrow homing in vivo (Merzaban et al., 2011). In addition, treatment of mice with blocking antibodies against CD44 resulted in an increase in committed progenitors in the peripheral blood, suggesting that CD44 is important for lodging of hematopoietic progenitors in the bone marrow (Vermeulen et al., 1998). It has also been demonstrated that the selectin ligands must be alpha1-3 fucosylated to form glycan determinants such as sialyl Lewis x (sLe(x)). Inadequate alpha1-3 fucosylation of umbilical cord blood derived CD34+CD38-/low cells resulted in reduced interaction with both E-selectin and P-selectin, while increasing the level of cellsurface sLe(x) determinants augmented binding to fluid-phase P- and E-selectin, improved cell rolling on P- and E-selectin under flow and enhanced engraftment of human hematopoietic cells in bone marrows of irradiated NOD/SCID mice (Xia et al., 2004).

1998). Similarly, coating of a surface with immobilized P- or E-Selectin was sufficient to induce rolling of human CD34+ hematopoietic progenitor cells under flow conditions (Xia et al., 2004). A next step in bone marrow homing is transendothelial migration. This process requires firm adhesion of hematopoietic stem and progenitor cells to endothelial cells. Although CD34+ hematopoietic progenitor cells are capable of binding to fluid-phase P- and E-selectin (Xia et al., 2004), in vitro adhesion to bone marrow derived endothelial cells under static conditions has been shown not to depend on E-selectin (Naiyer et al., 1999). Transwell experiments performed to study the importance of E-selectin in migration of human hematopoietic progenitor cells through a confluent layer of bone marrow derived endothelial cells, precultured with IL-1B to induce E-selectin expression, yielded contradictory results. While Naiyer et al. have demonstrated with blocking antibodies that E-selectin is important for transendothelial migration (Naiyer et al., 1999), no significant inhibition in transendothelial migration could be observed by Voermans et al. who performed similar experiments (Voermans et al., 2000). Transplantation of lethally irradiated recipient mice deficient for both P-and E-selectin with wild type bone marrow cells resulted in reduced recruitment of hematopoietic progenitors to the bone marrow and enhanced levels of circulating hematopoietic progenitors, indicating that

selectins indeed play an important role in bone marrow homing (Frenette et al., 1998).

hematopoietic cells in bone marrows of irradiated NOD/SCID mice (Xia et al., 2004).

Ligands for E-selectin include the PSGL-1 glycoform CLA, CD43 and the CD44 glycoform HCELL (Dimitroff et al., 2001; Merzaban et al., 2011). These ligands are all expressed on mouse Lin-Sca-1+c-Kit+ hematopoietic stem and progenitor cells and human CD34+ hematopoietic progenitor cells (Merzaban et al., 2011). Immune precipitation experiments revealed that although E-selectin can bind to CLA and CD43 in both mouse and human cells, the interaction between E-selectin and CD44 only occurs in human cells (Merzaban et al., 2011). These studies indicate that the molecular mechanism underlying bone marrow homing may be different for mouse and human hematopoietic stem cells. This hypothesis was confirmed by the observation that human CD34+ hematopoietic progenitor cells exhibit a stronger E-selectin binding capacity compared to mouse Lin-Sca-1+c-kit+ cells (Merzaban et al., 2011). In contrast to PSGL-1 which is also expressed in mature hematopoietic cells, CD44 appears to be predominantly expressed on primitive human CD34+ hematopoietic progenitor cells (Dimitroff et al., 2001). Rolling experiments performed under physiological flow conditions revealed that CD44 mediates E-selectin-dependent rolling interactions over a wider shear range in comparison to PSGL-1 and promotes rolling interactions on human bone marrow endothelial cells (Dimitroff et al., 2001). Silencing of CD44 expression in human cells with shRNAs was sufficient to decrease E-selectin binding under physiologic shear conditions, while enforced CD44 expression in Lin-Sca-1+c-kit+ cells conversely increased E-selectin adherence, resulting in improved bone marrow homing in vivo (Merzaban et al., 2011). In addition, treatment of mice with blocking antibodies against CD44 resulted in an increase in committed progenitors in the peripheral blood, suggesting that CD44 is important for lodging of hematopoietic progenitors in the bone marrow (Vermeulen et al., 1998). It has also been demonstrated that the selectin ligands must be alpha1-3 fucosylated to form glycan determinants such as sialyl Lewis x (sLe(x)). Inadequate alpha1-3 fucosylation of umbilical cord blood derived CD34+CD38-/low cells resulted in reduced interaction with both E-selectin and P-selectin, while increasing the level of cellsurface sLe(x) determinants augmented binding to fluid-phase P- and E-selectin, improved cell rolling on P- and E-selectin under flow and enhanced engraftment of human

Fig. 1. Homing of hematopoietic Stem Cells to the bone marrow. 1) Initial tethering and 2) rolling are the first steps in bone marrow homing. These processes are mediated by both Eand P-selectin. 3) SDF-1 mediated integrin activation induces firm adhesion of the hematopoietic stem cells to the endothelial wall. 4) Firmly attached hematopoietic stem cells can subsequently transmigrate through the endothelial layer and 5) basal lamina, consisting of fibronectin, collagen and laminin. Integrins involved in these steps are CD49d/CD29, CD49e/CD29 and CD49f/CD29. 6) Finally, hematopoietic stem cells migrate towards the SDF-1 gradient to the osteoblasts.

### **3. Integrins and bone marrow homing**

Integrins are, in addition to selectins, also implicated in playing an important role in regulation of bone marrow homing. Several in vitro studies with blocking antibodies have, for example, shown that both CD49d/CD29 (41 or VLA-4) and CD11a/CD18 (L2 or LFA-1) play an important role in adhesion of hematopoietic stem and progenitor cells to endothelial cells and subsequent transendothelial migration (Imai et al., 1999; Peled et al., 2000; Voermans et al., 2000). In addition, spontaneous migration of CD34+ hematopoietic progenitors underneath a bone marrow derived stromal cell layer, was found to be significantly inhibited by a peptide that blocks CD49d/CD29 integrin binding (Burger et al., 2003). However, adhesion of CD34+ cells to fibronectin was found to be primarily dependent on CD49e/CD29 (51 or VLA-5) and not CD49d/CD29 (Peled et al., 2000). In addition, chemotaxis of peripheral blood CD34+ progenitor cells on recombinant fibronectin appears to be mediated, at least in part, by CD49e/CD29 (Carstanjen et al., 2005). The importance of

Molecular Mechanisms Underlying Bone Marrow Homing of Hematopoietic Stem Cells 189

 homing is CD49f (6). In contrast to CD49d that appears to primarily be involved in bone marrow homing of short-term repopulating hematopoietic stem cells, CD49f is thought to be important for homing of both short-term and long-term stem cells (Qian et al., 2006). In contrast, similar experiments with fetal liver cells revealed that, in contrast to CD49d which appeared to be important for homing of both hematopoietic stem and progenitor cells, CD49f is only important for homing of hematopoietic progenitors but not stem cells (Qian et al., 2007). These studies indicate that CD49d and Cd49f play differential roles during homing of cord blood and fetal liver derived hematopoietic stem and progenitor cells (Qian et al., 2007). In contrast, bone marrow homing was not affected in a more recent study in which also mouse bone marrow derived hematopoietic stem and progenitor cells pretreated with blocking antibodies directed against CD49f were transplanted in recipient mice (Bonig et al., 2009). In addition, blocking CD49f in human and primate bone marrow derived hematopoietic stem and progenitor cells, but not mobilized peripheral blood or cord blood derived cells that express little or no CD49f, resulted in enhanced bone marrow homing in a xenogeneic transplant model and significantly improved engraftment levels (Bonig et al., 2009). Finally, intravenous injection of anti-CD49f antibodies, in contrast to antibodies against CD49d integrin, did not mobilize progenitors or enhance cytokine-induced mobilization by G-CSF, suggesting that CD49f is not essential for lodging of hematopoietic stem and progenitor cells in the bone marrow (Qian et al., 2006). Additional research is

required to investigate whether or not CD49f regulates bone marrow homing.

chemoattractants for hematopoietic stem and progenitor cells (Kim et al., 2011).

**4. Chemoattractants involved in migration of hematopoietic stem cells** 

Chemoattractants play an important role in directing migration of hematopoietic stem and progenitor cells to the bone marrow. Several studies have demonstrated that Stromal cell Derived Factor 1 (SDF-1), also known as CXC chemokine ligand 12 (CXCL12) (Tashiro et al., 1993) acts as a chemoattractant for hematopoietic stem and progenitor cells and is important for their transendothelial migration (Aiuti et al., 1997; Naiyer et al., 1999; Mohle et al., 1998; Kim & Broxmeyer, 1998; Glass et al., 2011). Further investigation, utilizing a large panel of CC and CXC chemokines, suggested that the only chemokine capable of inducing migration of murine hematopoietic stem and progenitor cells appears to be SDF-1 (Liesveld et al., 2001; Wright et al., 2002). Although the chemokine receptors CCR3 and CCR9 were also expressed at mRNA level, their ligands could not induce migration (Wright et al., 2002). Similarly, examination of a panel of chemokines and cytokines in transendothelial migration assays revealed that SDF-1 is also important for migration of human hematopoietic progenitors through a confluent layer of endothelial cells (Liesveld et al., 2001). However, to a lesser extent, also other chemokines and cytokines, including CCL2 (MCP-1), CCL5 (RANTES), CXCL10 (IP-10), IL-8 and SCF could also induce transendothelial migration (Liesveld et al., 2001). In addition, LTD4, a ligand for CysLT(1), a G protein-coupled receptor recognizing inflammatory mediator of the cysteinyl leukotriene family, which is highly expressed in hematopoietic progenitors, has been demonstrated to up-regulate CD49d/CD29 and CD49e/CD29 dependent adhesion of hematopoietic progenitors (Boehmler et al., 2009) and to induce chemotaxis and in vitro transendothelial migration (Bautz et al., 2001). Recently, a role for the proteolysis-resistant bioactive lipids sphingosine-1-phosphate and ceramide-1-phosphate in regulation of bone marrow homing has been suggested. Conditioning of mice for transplantation resulted in enhanced levels of these lipids in the bone marrow. In addition, both lipids appear to be

both CD49d/CD29 and CD49e/CD29 in directional migration through the basal lamina, which is composed of the extracellular matrix proteins laminin, collagen, and fibronectin, has been examined utilizing a three dimensional extra cellular matrix-like gel. In contrast to the dominant role of CD49e/CD29 in facilitating static adhesion to fibronectin, SDF-1– induced directional migration of CD34+ cells was found to be dependent on both CD49d/CD29 and CD49e/CD29 integrins (Peled et al., 2000). These studies suggest that both CD49d/CD29 and CD49e/CD29 play an important role in migration of hematopoietic stem and progenitor cells in general. However, transwell migration experiments with endothelial cells from different origin showed that CD49d/CD29 is only involved in migration of hematopoietic progenitors through a confluent layer of bone marrow derived, but not human umbilical vein derived, endothelial cells (Peled et al., 2000). In addition, inhibition of CD49e/CD29 alone was not sufficient to inhibit migration through both types of endothelial cells. However, an additive effect was observed when antibodies for CD11a/CD18, CD49d/CD29 and CD49e/CD29 were mixed together (Peled et al., 2000). These results suggest that the mechanisms underlying hematopoietic stem cell migration through endothelial walls of blood vessels depends on the origin of the endothelial cells and the VCAM-1 expression level.

As described above, deletion of both P- and E-selectin in recipient mice significantly reduced bone marrow homing after transplantation of wild type HPCs. Treatment of these mice with a blocking antibody against VCAM-1, thereby prohibiting interaction with CD49d/CD29, was sufficient to further reduce bone marrow homing after transplantation (Frenette et al., 1998), suggesting that both selectins and integrins are important for optimal bone marrow homing. In addition, the capacity of cells either deficient for CD49d (Scott et al., 2003) or pretreated with CD49d antibodies (Vermeulen et al., 1998; Papayannopoulou et al., 2001; Qian et al., 2006; Carstanjen et al., 2005) to migrate to bone marrow has been shown to be impaired resulting in delayed short-term engraftment (Scott et al., 2003). Furthermore, treatment of mice with blocking antibodies against CD49d resulted in an increase in the number of committed progenitors in the peripheral blood, suggesting that CD49d is also important for lodging of hemaopoietic progenitors in the bone marrow (Vermeulen et al., 1998). Since antibodies directed against mouse CD49d can bind to both CD49d/CD29 and CD49d/ITGB7 (47), and CD49d/ITGB7 is also expressed on mouse Lin-Sca-1+c-Kit+ cells, it was hypothesized that in addition to CD49d/CD29, CD49d/ITGB7 could also be involved in bone marrow homing. Indeed, inhibition of CD49d/ITGB7or its substrate MadCam-1 significantly reduced, but not completely abrogated, bone marrow homing after transplantation (Katayama et al., 2004). In contrast, other integrins, including CD11a, appear not be involved in bone marrow homing (Vermeulen et al., 1998). Transplantation studies with hematopoietic stem cells deficient for CD18 indicated that also CD18 is not essential for bone marrow homing. However, since inhibition of CD49d/CD29 in CD18 deficient hematopoietic stem cells resulted in more dramatic reduction in bone marrow homing in comparison to inhibition of CD49d/CD29 in wild type mice, it was suggested that CD18 can contribute to bone marrow homing when the function of CD49d/CD29 is compromised (Papayannopoulou et al., 2001). In addition to CD49d, CD49e/CD29 has also been implicated in playing a role in regulation of bone marrow homing. Treatment of hematopoietic progenitors with an antibody directed against CD49e/CD29 was sufficient to partially reduce homing of those cells to the bone marrow but not to the spleen (Wierenga et al., 2006; Carstanjen et al., 2005). Another integrin implicated in regulation of bone marrow

both CD49d/CD29 and CD49e/CD29 in directional migration through the basal lamina, which is composed of the extracellular matrix proteins laminin, collagen, and fibronectin, has been examined utilizing a three dimensional extra cellular matrix-like gel. In contrast to the dominant role of CD49e/CD29 in facilitating static adhesion to fibronectin, SDF-1– induced directional migration of CD34+ cells was found to be dependent on both CD49d/CD29 and CD49e/CD29 integrins (Peled et al., 2000). These studies suggest that both CD49d/CD29 and CD49e/CD29 play an important role in migration of hematopoietic stem and progenitor cells in general. However, transwell migration experiments with endothelial cells from different origin showed that CD49d/CD29 is only involved in migration of hematopoietic progenitors through a confluent layer of bone marrow derived, but not human umbilical vein derived, endothelial cells (Peled et al., 2000). In addition, inhibition of CD49e/CD29 alone was not sufficient to inhibit migration through both types of endothelial cells. However, an additive effect was observed when antibodies for CD11a/CD18, CD49d/CD29 and CD49e/CD29 were mixed together (Peled et al., 2000). These results suggest that the mechanisms underlying hematopoietic stem cell migration through endothelial walls of blood vessels depends on the origin of the endothelial cells and

As described above, deletion of both P- and E-selectin in recipient mice significantly reduced bone marrow homing after transplantation of wild type HPCs. Treatment of these mice with a blocking antibody against VCAM-1, thereby prohibiting interaction with CD49d/CD29, was sufficient to further reduce bone marrow homing after transplantation (Frenette et al., 1998), suggesting that both selectins and integrins are important for optimal bone marrow homing. In addition, the capacity of cells either deficient for CD49d (Scott et al., 2003) or pretreated with CD49d antibodies (Vermeulen et al., 1998; Papayannopoulou et al., 2001; Qian et al., 2006; Carstanjen et al., 2005) to migrate to bone marrow has been shown to be impaired resulting in delayed short-term engraftment (Scott et al., 2003). Furthermore, treatment of mice with blocking antibodies against CD49d resulted in an increase in the number of committed progenitors in the peripheral blood, suggesting that CD49d is also important for lodging of hemaopoietic progenitors in the bone marrow (Vermeulen et al., 1998). Since antibodies directed against mouse CD49d can bind to both CD49d/CD29 and CD49d/ITGB7 (47), and CD49d/ITGB7 is also expressed on mouse Lin-Sca-1+c-Kit+ cells, it was hypothesized that in addition to CD49d/CD29, CD49d/ITGB7 could also be involved in bone marrow homing. Indeed, inhibition of CD49d/ITGB7or its substrate MadCam-1 significantly reduced, but not completely abrogated, bone marrow homing after transplantation (Katayama et al., 2004). In contrast, other integrins, including CD11a, appear not be involved in bone marrow homing (Vermeulen et al., 1998). Transplantation studies with hematopoietic stem cells deficient for CD18 indicated that also CD18 is not essential for bone marrow homing. However, since inhibition of CD49d/CD29 in CD18 deficient hematopoietic stem cells resulted in more dramatic reduction in bone marrow homing in comparison to inhibition of CD49d/CD29 in wild type mice, it was suggested that CD18 can contribute to bone marrow homing when the function of CD49d/CD29 is compromised (Papayannopoulou et al., 2001). In addition to CD49d, CD49e/CD29 has also been implicated in playing a role in regulation of bone marrow homing. Treatment of hematopoietic progenitors with an antibody directed against CD49e/CD29 was sufficient to partially reduce homing of those cells to the bone marrow but not to the spleen (Wierenga et al., 2006; Carstanjen et al., 2005). Another integrin implicated in regulation of bone marrow

the VCAM-1 expression level.

 homing is CD49f (6). In contrast to CD49d that appears to primarily be involved in bone marrow homing of short-term repopulating hematopoietic stem cells, CD49f is thought to be important for homing of both short-term and long-term stem cells (Qian et al., 2006). In contrast, similar experiments with fetal liver cells revealed that, in contrast to CD49d which

appeared to be important for homing of both hematopoietic stem and progenitor cells, CD49f is only important for homing of hematopoietic progenitors but not stem cells (Qian et al., 2007). These studies indicate that CD49d and Cd49f play differential roles during homing of cord blood and fetal liver derived hematopoietic stem and progenitor cells (Qian et al., 2007). In contrast, bone marrow homing was not affected in a more recent study in which also mouse bone marrow derived hematopoietic stem and progenitor cells pretreated with blocking antibodies directed against CD49f were transplanted in recipient mice (Bonig et al., 2009). In addition, blocking CD49f in human and primate bone marrow derived hematopoietic stem and progenitor cells, but not mobilized peripheral blood or cord blood derived cells that express little or no CD49f, resulted in enhanced bone marrow homing in a xenogeneic transplant model and significantly improved engraftment levels (Bonig et al., 2009). Finally, intravenous injection of anti-CD49f antibodies, in contrast to antibodies against CD49d integrin, did not mobilize progenitors or enhance cytokine-induced mobilization by G-CSF, suggesting that CD49f is not essential for lodging of hematopoietic stem and progenitor cells in the bone marrow (Qian et al., 2006). Additional research is required to investigate whether or not CD49f regulates bone marrow homing.
