Can "bone marrow transplants" [or Hematopoietic stem cell transplantation] be used to replace stem cells in older people whose stem cell counts decline? [MSC homing]

[Alex K Chen]

https://ashpublications.org/blood/article/106/6/1901/21603/How-do-stem-cells-find-their-way-home

https://stemcellsjournals.onlinelibrary.wiley.com/doi/full/10.1002/stem.3242

"stem cell engraftment" is the term

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Homing is the first—and a fairly rapid—process (measured in hours and no longer than 1-2 days) in which circulating hematopoietic cells actively cross the blood/BM endothelium barrier and lodge at least transiently in the BM compartment by activation of adhesion interactions prior to their proliferation

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The chemokine SDF-1 is expressed by immature human osteoblasts in the endosteum region.²⁰  This chemokine and its receptor CXCR4, constitutively expressed by human and murine BM endothelium,^20,27  are essential for stem cell seeding of the murine BM during fetal development and for definitive repopulation in adult mice that received transplants (reviewed in Cottler-Fox et al⁵ ).

 

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5.1 Methods to enhance homing

Because stem cell transplantation relies on homing of adequate stem cell numbers to reconstitute marrow function, in cases of inadequate autologous stem cell mobilization or in the case of single cord blood transplants with low stem cell availability, methods to enhance stem cell homing are needed (Table 1). Direct injection of stem cells into marrow has been attempted to bypass the low efficiency of engraftment after intravenous injection, but these approaches have not yet had broad application in clinical transplantation.^{99, 100}

TABLE 1. Methods to enhance hematopoietic stem cell homing

Method	Mediator(s)	Reference
Stromal cell coculture to increase HSPC CXCR4 expression	CXCL12/endothelial cells	81-85
Modulation of CXCR4/CXCL12 to increase HSPC CXCR4 expression	PGE2/HDAC inhibitors	86-89
Fucosylation of selectin ligands	Fucosyltransferases	90, 91
Erythropoietin receptor modulation on HSPCs	Hyperbaric oxygen	92-95
CD26 cleavage of CXCL12	Diprotin A/sitagliptin	96-98
Modulation of ROS levels	S1P	50

• Abbreviations: EPO-R, erythropoietin receptor; HDAC, histone deacetylase; HSPC, hematopoietic stem and progenitor cell; PGE2, prostaglandin E2; ROS, reactive oxygen species; S1P; sphingosine-1-phosphate.

 

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  6 MSC HOMING

  MSCs are defined by adherence to plastic and their capacity to differentiate into various connective tissue lineages. They have angiogenic, proliferative, and immune suppressive capacity as well.⁹⁵ MSCs are thought to have potential for improving clinical outcomes for inflammatory and degenerative diseases. They can be administered intravenously or in situ, or they may be mobilized and recruited to injury sites. Each of these modes of delivery requires MSC homing. Less is known about MSC homing than about leukocyte and HSPC homing. Means to track the fate of MSCs in vivo are needed as is an understanding of the chemoattractants which guide them to sites of injury.⁹⁵ It is in fact controversial whether MSCs localize to tissue due to passive entrapment in small vessels or if there are active mechanisms to guide them to specific tissues.⁹⁵ Nonetheless, there is much evidence that MSC homing uses many of the same steps as HSPC homing. Homing of MSCs is thought to be very inefficient, and like HSPC homing involves tethering by selectins, activation by cytokines, arrest by integrins, diapedesis using matrix remodelers, and extravascular migration toward chemokine gradients.⁹⁷ MSCs express CD44 which initiates rolling. Which selectin MSCs use is not well understood as they do not express PSGL-1. Galectin-1 or CD24 may serve as ligands for P-selectin on MSCs.^{98, 111} The activation step is mediated by chemokines such as CXCL12 or monocyte chemoattractant protein-1 (MCP-1), and this serves to increase the affinity of the integrins which then cause cell arrest. Integrin arrest is probably mediated mostly by CD49d (α4β1) which binds to VCAM-1 (CD106) on endothelial cells.¹¹² Overexpression of CD49d on MSCs can increase homing to marrow, and MSCs themselves can express VCAM and ICAM.¹¹³ In order to traverse the endothelial basement membrane, MSCs secrete matrix metalloproteinases (MMPs). Which MMPs are involved is not entirely understood, but MMP-1 has been found to have a role in tissue invasion by MSCs.¹¹⁴ Matrix components such as tropoelastin have also been found to increase homing and proliferation of MSCs in development and in wound repair.¹¹⁵ Last, as in HSPC homing, the MSCs must migrate to the site of injury guided by chemotactic signals such as PDGFα,¹¹⁶ insulin growth factor-1,¹¹⁷ CXCL12, and other chemokines. Hepatocyte growth factor and epidermal growth factor have also been shown to be involved in MSC chemotaxis and homing.¹¹⁸ CXCR4 is expressed by MSCs, and some studies have shown that MSCs also express CXCR7, the other receptor for CXCL12.¹¹⁹

   

   

Edited December 1, 2023 by InquilineKea

[Alex K Chen]

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The first step in homing of HSPCs to marrow is interaction with the sinusoidal endothelium. Endothelial cells direct HSPCs to the marrow and slow cell movement under the shear stress of blood flow so that the HSPC can tether to and roll along the endothelium.³¹ The shear stress is lower in sinusoidal vessels which aids the homing process. Homing of HSPCs requires selectin receptors with galactosyl and mannosyl specificities.³² Loss of function studies have shown that homing to the marrow niche is impaired with both P- and E-selectin deficiencies.^33,34

After selectin-mediated braking, HSCs migrate on adhesion ligands presented by the vascular endothelium. These include ligands for CD49d-f, CD11b, and CD11c such as vascular cell adhesion molecule-1 (CD106; VCAM-1), fibronectin, laminin, mucosal vascular addressin cell adhesion molecule 1 (MAdCAM-1), and intercellular adhesion molecule-1 (CD54; ICAM-1). The stem cells have to migrate against the direction of blood flow, and through antibody blocking studies, this has been found to be dependent on LFA-1 (lymphocyte function-associated antigen-1; CD11a) on the HSPCs.³⁵ HSPCs also express several other adhesion molecules such as L-selectin (CD62L) which binds GlyCAM-1 (glycosylation dependent cell adhesion molecule), MAdCAM-1, P-selectin glycoprotein ligand-1 (CD162; PSGL-1), CD34, sialyl Lewis^x (sLe^x), and PCLP1 (podocalyxin-like protein).³⁶ The P (CD62P)- and E-(CD62E) selectins are expressed on the surface of the endothelial cells. P-selectin binds to CD162, sLe^x, and CD24, and E-selectin binds to CD15, SLe^X, CD162, and other ligands. Interactions with selectins allow the leukocytes to roll on the endothelial surface after which adherence and tethering occur. The initial binding of stem cell integrins to endothelial ligands is weak, but CXCL12 and c-kit ligand signaling interactions trigger conformational changes in these integrins which foster further rolling and adhesion interactions as well as cytoskeletal changes. The VLA-4–VCAM-1 (CD49d/CD106) interaction has prime importance in homing,³⁷ and α5β1,α4β7, α1β2 and α6 integrins may also contribute independently to homing.³⁸

Once rolling and adhesion of the HSPC occurs, the expression of VCAM-1, ICAM-1, and E- and P-selectin on marrow sinusoidal endothelium is upregulated to facilitate the tethering process.³⁹ Unlike the case in other vascular beds, E-selectin is constitutively expressed by marrow endothelial cells. After tethering and adhesion, the cell must then penetrate through the endothelial cell cytoplasm or through cell-cell junctions. Both murine and human HSPCs are able to form podosomes which allow direct transcellular migration, but in conditions where vascular endothelial cadherin function is lost, increased permeability and paracellular migration can occur.⁴⁰ They then migrate between adventitial cells to the final place of lodgment. This relies on intrinsic amoeboid migratory ability of stem cells and invasion by means of matrix degradation which occurs as a result of secretion of matrix metalloproteinases MMP-2 and MMP-9 by the HSPCs.⁴¹ Some HSCs lodge near arterioles which are thought to maintain a low reactive oxygen species environment (ROS^lo) whereas stem cells near the sinusoidal endothelial cells have higher levels of ROS. High ROS can contribute to differentiation and mobilization whereas ROS^lo states lead to quiescence and self-renewal.⁴² CD44 and hyaluronic acid cooperate with CXCL12 in the transendothelial migration and in the final lodgment within the marrow.⁴³ Marrow laminins can also influence progenitor cell cycling and homing to the marrow. Laminin α4, α3, and α5 isoforms are all present, and laminin 421 (α4β3ϒ1) is a major component⁴⁴ and is located near venous sinuses and in close association with HSPCs in murine marrow. Figure 1 illustrates the proposed steps in homing for HSPCs.

 

 

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A shortcut to the BM: intraosseous stem cell transplantation

In an attempt to improve the efficiencies of BM homing, seeding, and repopulation, transplanted cells were injected directly into the BM cavity of immune-deficient NOD/SCID and normal mice (intraosseous [IO], intra–bone marrow [IBM], or intrafemur [IF] transplantation). Studies with enriched human progenitors demonstrate superior seeding efficiencies when the endothelial and ECM barriers are bypassed. IBM injections yielded a 15-fold higher frequency of SRCs in sorted CB CD34⁺/CD38^- cells in comparison with conventional intravenous injection. Increased engraftment and improved abilities of BM cells obtained from primary recipients to repopulate recipients of secondary transplants were also documented. Simultaneous blocking of the integrins VLA-4/5, which have been implicated in the homing and repopulation processes, profoundly inhibit engraftment of human progenitor cells injected intraosseously. Interestingly, CXCR4 neutralization similarly abolishes engraftment by human CD34⁺-enriched cells administered in both protocols, demonstrating an essential role for this receptor not only in SRC homing to the BM but also in BM seeding and colonization.⁴⁰  Thus, this method enables researchers to dissect different steps of BM repopulation. Dao et al¹²⁰  revealed that CD34 expression by human progenitors is dynamic, where positive cells can lose and regain CD34 expression in vivo. CB-derived Lin^-CD34^- primitive cells display rare and low repopulation potential when transplanted via the tail vein,^50,66  as opposed to high SRC activity when transplanted intraosseously,⁴¹  despite low expression of CXCR4 and reduced migration to SDF-1.^41,63,79  These progenitors acquire in vivo expression of the CD34⁺ marker, demonstrating slower differentiation and reconstitution kinetics but higher absolute numbers of CD45⁺ and immature human CD34⁺ cells in comparison with human cells initially expressing CD34.^41,65  Applying intrafemoral injection of CB CD34⁺CD38^-/lowCD36^- purified cells, Dick's group could identify a new short-term SRC subset that can migrate to other bones, colonizing them with myeloid and erythroid cells.⁴³  Taken together, this method enables the revealing of repopulating human stem cell subsets that can hardly be detected when injected intravenously.¹²¹  Of interest, when total CB mononuclear cells (MNCs) were transplanted, increased engraftment, including a dramatic increase in immature human CD34⁺ cells, were documented transiently only 1 month after intraosseous transplantation compared with intravenous transplantation, but not 2 and 3 months after transplantation.⁴² 

 

 

Edited December 1, 2023 by InquilineKea

[Alex K Chen]

A shortcut to the BM: intraosseous stem cell transplantation

In an attempt to improve the efficiencies of BM homing, seeding, and repopulation, transplanted cells were injected directly into the BM cavity of immune-deficient NOD/SCID and normal mice (intraosseous [IO], intra–bone marrow [IBM], or intrafemur [IF] transplantation). Studies with enriched human progenitors demonstrate superior seeding efficiencies when the endothelial and ECM barriers are bypassed. IBM injections yielded a 15-fold higher frequency of SRCs in sorted CB CD34⁺/CD38^- cells in comparison with conventional intravenous injection. Increased engraftment and improved abilities of BM cells obtained from primary recipients to repopulate recipients of secondary transplants were also documented. Simultaneous blocking of the integrins VLA-4/5, which have been implicated in the homing and repopulation processes, profoundly inhibit engraftment of human progenitor cells injected intraosseously. Interestingly, CXCR4 neutralization similarly abolishes engraftment by human CD34⁺-enriched cells administered in both protocols, demonstrating an essential role for this receptor not only in SRC homing to the BM but also in BM seeding and colonization.⁴⁰  Thus, this method enables researchers to dissect different steps of BM repopulation. Dao et al¹²⁰  revealed that CD34 expression by human progenitors is dynamic, where positive cells can lose and regain CD34 expression in vivo. CB-derived Lin^-CD34^- primitive cells display rare and low repopulation potential when transplanted via the tail vein,^50,66  as opposed to high SRC activity when transplanted intraosseously,⁴¹  despite low expression of CXCR4 and reduced migration to SDF-1.^41,63,79  These progenitors acquire in vivo expression of the CD34⁺ marker, demonstrating slower differentiation and reconstitution kinetics but higher absolute numbers of CD45⁺ and immature human CD34⁺ cells in comparison with human cells initially expressing CD34.^41,65  Applying intrafemoral injection of CB CD34⁺CD38^-/lowCD36^- purified cells, Dick's group could identify a new short-term SRC subset that can migrate to other bones, colonizing them with myeloid and erythroid cells.⁴³  Taken together, this method enables the revealing of repopulating human stem cell subsets that can hardly be detected when injected intravenously.¹²¹  Of interest, when total CB mononuclear cells (MNCs) were transplanted, increased engraftment, including a dramatic increase in immature human CD34⁺ cells, were documented transiently only 1 month after intraosseous transplantation compared with intravenous transplantation, but not 2 and 3 months after transplantation.⁴² 

[Alex K Chen]

 

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6.1 Methods to enhance MSC homing

To improve efficacy of MSC therapies, improved homing to tissues is required. Since MSC homing efficiency is poor (<10%) in various imaging studies such as whole body positron emission tomography (PET) imaging with radiolabeled MSCs, and intravenously injected MSCs may be trapped in the lung, methods are needed to enhance MSC homing.¹³³ These techniques, which can generally increase homing by two- to threefold, have been previously reviewed in detail⁹⁷ and are summarized briefly here.

Genetic modifications have used permanent overexpression of homing factors via viral transduction such as overexpression of CXCR4.¹³⁴ Overexpression of VLA-4 has also been attempted in rat models.¹³⁵ Direct administration of MSCs into the target tissue has been used to increase homing efficiency, but this does not always yield superior outcomes when compared with intravenously administered MSCs, and this may be tissue and inflammatory state-dependent.¹³⁶ Cell surface modifications have included modifications of CD44 through exofucosylation, allowing MSCs to utilize E- and L-selectin for homing.¹³⁷ In vitro priming has also focused on CD44 and CXCR4. For example, culture in hypoxic conditions induced HIF-1α, which then increased the expression of CXCR4 and CX3CR1.¹³⁸ Treatment with TNF-α may increase chemokine receptor expression including CCR3, CCR4, and CCR5.¹³⁹ Interleukin-1β can induce CXCR3-mediated chemotaxis to enhance MSC transendothelial migration.¹⁴⁰ This is mediated through p38 MAPK signaling, and CXCL9 ligand secretion was also enhanced, thus promoting transendothelial migration.¹⁴⁰ In murine models, trapping of intravenously administered MSCs in the pulmonary vasculature can be reduced by intravenous pretreatment with sodium nitroprusside. It has not yet been shown if this increases homing to injured organs.¹⁴¹ Many of these homing modification modalities have not yet entered human clinical trials examining MSC therapy efficacy.

Others have attempted to modulate MSC homing by altering the target tissue through overexpression of chemokines or through chemokine-coated scaffold implantation.¹⁴² Pulsed ultrasound applied to the tissue of interest may also increase MSC homing.¹⁴³ Targeted release of CXCL12 by ROS-sensitive nanoparticles can increase marrow stromal cell chemotaxis and homing to tissues with vascular injuries related to electrical burns.¹⁴⁴ Magnetic guidance of MSCs to injured tissues has been effective in rodent models.¹⁴⁵ To enhance MSC therapy effectiveness, their homing, retention, differentiation capacity, and immune modulatory ability due to indoleamine 2,3,-dioxygenase secretion will need to be better understood. When MSCs recruited by inflammatory stimuli home to tissues, they may have an immune suppressive function which modulates pro-inflammatory cells such as T cells.¹⁴⁶ This could affect subsequent cell recruitment and retention. If homing and stimulation of native cells to sites of tissue injury can be promoted, this would form an attractive alternative to costly and time consuming in vitro cell expansion and infusion protocols.¹⁴⁷ Many obstacles to adequately track MSC homing both from exogenous infusion and from native tissues exist, and these have been previously reviewed.¹⁴⁸ Table 2 summarizes interventions designed to modify MSC homing.

 

 

Edited December 1, 2023 by InquilineKea

[Alex K Chen]

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6529790/

 

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6.1 Methods to enhance MSC homing

To improve efficacy of MSC therapies, improved homing to tissues is required. Since MSC homing efficiency is poor (<10%) in various imaging studies such as whole body positron emission tomography (PET) imaging with radiolabeled MSCs, and intravenously injected MSCs may be trapped in the lung, methods are needed to enhance MSC homing.¹³³ These techniques, which can generally increase homing by two- to threefold, have been previously reviewed in detail⁹⁷ and are summarized briefly here.

 

 

Edited December 1, 2023 by InquilineKea