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Keywords:

  • epithelium;
  • lung disease;
  • mesenchymal stem cell;
  • mesenchymal stem cell transplantation;
  • mesenchymal stromal cell

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

Mesenchymal stem cells (MSC) are a population of tissue-resident adult progenitor cells that were originally identified in bone marrow, but have now been identified in many organs including the lung. Although their precise role in organ function remains incompletely defined, mounting evidence suggests that they are an important component of the parenchymal progenitor cell niche and orchestrate organ homeostasis and repair following injury. In this review, what is known about MSC biology will be outlined with particular emphasis on lung biology, and the therapeutic potential of MSC-based cell therapy will also be highlighted.


Abbreviations
BAL

bronchoalveolar lavage

bm-MSC

bone marrow mesenchymal stem cells

BOS

bronchiolitis obliterans syndrome

HLA

human leucocyte antigen

HSC

haematopoietic stem cell

lr-MSC

lung-resident mesenchymal stem cells

MSC

mesenchymal stem cells

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

Mesenchymal stem cells (MSC, sometimes also referred to as mesenchymal stromal cells) are a key component of the connective tissue framework which provides both structural and functional support to the parenchymal cells of multiple organs. It is now known that the maintenance and repair of parenchymal tissue is mediated by ‘adult’ stem cell populations which differentiate into their progeny, so creating a hierarchical structure. It is likely that the mesenchyme is ordered in a similar way, with one or more adult stem cell populations giving rise to differentiated progeny such as the fibroblast. The MSC is a key cell in the connective tissue hierarchy of many organs, including the lung, and given the intimate relationship between the epithelium and underlying mesenchyme in the lung, both during branching morphogenesis and in the adult,[1] it is likely that MSC play a crucial, but as yet largely unexplored, role in both the maintenance of epithelial integrity and the pathogenesis of lung disease.

MSC were first isolated in 1970 from suspensions of bone marrow and spleen.[2] They are characterized by their tendency to adhere to plastic (this remains the predominant means of isolating MSC from tissues); their ability to form colonies from single cells when plated ex vivo at clonogenic levels; their fibroblast-like appearance and their multipotent (fat, cartilage and bone) differentiation capacity. MSC have now been identified in many organs, including the lung in 2005.[3] Their exact biological function remains incompletely defined; however, one of their main functions in bone marrow is to create a niche for the haematopoietic stem cell (HSC) to support haematopoiesis.[4] It is likely that they have similar functions in other organs where they provide support to local progenitor cell (so called ‘adult’ or tissue stem cell) pools. For example, in the lung, MSC have recently been shown to increase the proliferative potential of a key progenitor cell—the bronchioalveolar stem cell,[5] and remarkably, to restore bioenergetics in lung epithelium through donation of mitochondria.[6, 7] Their function as the architect of organ repair and regeneration, as well as their immunosuppressive, antibacterial and anti-fibrotic properties and relative immune privilege has led to great interest in the therapeutic potential of ex vivo expanded MSC to treat many disease states. In this review, we will provide an overview of MSC biology before discussing their potential role in lung biology and pathobiology, and their promise as a novel therapeutic agent for patients with lung disease.

What Is an MSC?

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

The field of MSC research has been hampered by a lack of truly phenotype-specific and readily applicable MSC markers, with even the most up-to-date international standards falling short. According to the Mesenchymal and Tissue Stem Cell Committee of the International Society for Cellular Therapy Standards, an MSC has three key characteristics. Firstly, an MSC must be plastic adherent when cultured under standard conditions. Secondly, an MSC must express the cell surface proteins CD105, CD90 and CD73, but in addition, must also not express CD45, CD34, CD14/CD11b, CD79α/CD19 and human leucocyte antigen (HLA)-DR surface molecules. Finally, an MSC must also have the capacity to differentiate into the mesenchymal lineages—bone, cartilage and fat.[8] Of note, the listed cell surface markers are not specific for MSC (for instance the same markers are expressed by dermal fibroblasts[3, 9, 10]), further hampering the in vivo study of MSC function, which remains heavily reliant on the ability to demonstrate tri-lineage differentiation potential. In fact, bone marrow-derived MSC (bm-MSC) have turned out to be a heterogeneous group of progenitors and lineage-committed cells; the same is likely true of MSC populations in other organs such as the lung.[11] Developing a more precise understanding of the organ-specific hierarchical structure of mesenchymal populations is likely to be crucial to deciphering the role of MSC in (patho)biology.

Lung-Resident MSC

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

In 2005, Sabatini et al.[3] identified a plastic-adherent population of cells with fibroblast-like morphology, multi-lineage differentiation capacity and cell surface protein expression consistent with an MSC in human lung. Lung-resident MSC (lr-MSC) have since been isolated using plastic adherence from bronchoalveolar lavage (BAL)[12] as well as lung tissue digests.[13] Similar to other MSC populations, lr-MSC form structured colonies (colony-forming units) when grown in suitable media. They have been shown to retain the gender of the donor even many years after gender-mismatched lung transplantation, suggesting that they are either very long-lived or that they have an inherent capacity for self-renewal.[12]

Following on from these preliminary observations, much of our understanding of lr-MSC function has come from work focused on the role of MSC in the development of bronchiolitis obliterans syndrome (BOS) after lung transplantation. BOS is characterized histopathologically by fibrous obliteration of the bronchiolar region of the lung allograft, with one of the key drivers of the fibrotic response being recurrent airway injury and failure of epithelial repair. Lr-MSC increase in number during the early postoperative period and later, in lung transplant recipients who have organizing pneumonia and/or those who go on to develop BOS.[14] Furthermore, lr-MSC derived from the lung allograft are capable of fibrotic differentiation and notably, fibrogenesis can be induced by mediators known to be associated with BOS progression—interleukin-13 and transforming growth factor-β. In addition, lr-MSC from patients with BOS express increased levels of α-smooth muscle actin and collagen 1[15] when compared with lr-MSC from patients without BOS, consistent with a pro-fibrotic phenotype. Lr-MSC were also shown to highly express the forkhead/winged helix transcription factor forkhead box 1, a marker which was also shown to be expressed by the myofibroblasts within fibrotic lesions.[15] Together, these data are consistent with the idea that lr-MSC are somehow involved in BOS pathogenesis and that they may contribute precursor cells to the fibrotic lesions. What remains unknown is how big this contribution is, and whether differentiation of lr-MSC to a myofibroblastic phenotype is causal or is an epiphenomenon in a pro-fibrotic organ. Another key question is why, if lr-MSC are long-lived tissue-resident stem cells, do they appear in BAL at all?

Although the described BAL-derived MSC fulfil the currently accepted criteria for an MSC, it seems unlikely that a single homogeneous population of lr-MSC exists. Instead, it is probable that either multiple populations carry out specialized stem functions, or that, as is the case for bone marrow, a single true MSC progenitor population differentiates according to paracrine and autocrine soluble and contact-dependant cues to create a hierarchy of stem cells with increasingly specialized functions, but with retention of MSC cell surface marker expression and (to a greater or lesser extent) multi-lineage differentiation potential. It is likely that deconstructing the MSC hierarchy in the lung will provide much important information for understanding fibrotic lung diseases over the coming decade. A putative hierarchy of lung stem cell populations is provided in Figure 1.

figure

Figure 1. Putative pulmonary stromal cell hierarchy. Stem cell hierarchies have been well described for parenchymal cell populations in multiple organs, and similar hierarchies are likely to exist for stromal cell populations. In the lung, the mesenchymal stem cell may act as a slow-cycling stem cell which can give rise to both committed progenitor cells (termed here as mesenchymal stromal/support cells) and daughter stem cells (curved arrow), which enter a quiescent phase, leading to clonal persistence.[12] Mesenchymal stromal/support cells proliferate and differentiate into fibroblasts, myofibroblasts and possibly other as yet undefined cell types, such as the lipofibroblast.[16] While the mesenchymal stem cell is truly multipotent, tri-lineage differentiation potential (to bone, cartilage and fat) is lost with lineage commitment (as indicated by the hatched lines).

Download figure to PowerPoint

Another key piece in this puzzle will be determining how lr-MSC interact with lung epithelial progenitors and differentiated cell populations during both development and during the maintenance of epithelial integrity in the adult. During lung morphogenesis, the mesenchyme provides critical cues to the overlying epithelium so that the complicated branching structure of the bronchial tree and alveoli is eventually attained.[1] Given the proven role of MSC in creating the HSC niche in the adult, and the evidence for the trophic effect of lr-MSC on bronchioalveolar stem cell proliferation,[5] it is likely that at least some of this function persists in the adult, so that lr-MSC contribute to the organization of epithelial progenitor cell function during the response to epithelial injury.

As is the case for MSC derived from other organs, lr-MSC have been shown to be able to differentiate into non-mesodermal lineages, including epithelium.[17] When MSC derived from lung tissue are cultured in suitable media, they differentiate into cells expressing Clara cell secretory protein and aquaporin-5, markers of small airway and alveolar epithelial cells, respectively.[18] While these data demonstrate the multipotent diversity of lr-MSC, it is uncertain whether such transdifferentiation occurs in vivo. Certainly, given data from multiple other niches (see later), it seems unlikely that lr-MSC contribute directly to epithelial repair via transdifferentiation to any meaningful degree. Instead, current evidence points to the lr-MSC as the foremen that direct epithelial repair, rather than participating directly in this work themselves.

MSC Are Relatively Immunoprivileged

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

One feature of MSC biology which makes them an attractive cell therapy candidate is their ability to at least partially avoid immune cell recognition. Because MSC are poorly immunogenic, and can escape lysis by cytotoxic T cells and natural killer cells, they may be transplantable between HLA-mismatched individuals without the need for immunosuppression. This relative immunoprivilege has been confirmed in multiple studies using xenogeneic and major histocompatibility mismatched models. For instance, when human bm-MSC were transplanted into lambs, they were able to engraft and persist for up to 13 months.[19] While human MSC do not express HLA class II, they do express class I HLA, but expression is weak so that in co-culture experiments, human MSC fail to induce proliferation of allogeneic lymphocytes.[20] Lr-MSC are similarly weakly HLA class I positive and HLA class II negative.[12, 18, 21] Despite this body of work, recently, the degree of immunological privilege awarded MSC has been questioned. Studies in small animal cardiac models suggest that the immunogenicity of MSC increases with the differentiation state of the cells—so that multipotent cells remain immune-privileged, but major histocompatibility complex expression is upregulated in their terminally differentiated progeny (e.g. myocytes and endothelial cells), leading to destruction by complement- and/or cell-mediated lysis.[22, 23] However, even multipotent cells have been found to be more immunogenic than previously anticipated. In a primate model, multiple administrations of high-dose allogeneic MSC resulted in the production of alloantibodies in two of six animals.[24] MSC have also been reported to induce memory T-cell responses in a murine model[25] and furthermore, MSC express the activating natural killer cell-receptor ligands NKG2D and UL16,[26] limiting their ability to avoid lysis by natural killer cells.[27] The practical implication is that preclinical work in major histocompatibility complex-matched and/or immunosuppressed animals needs to be cautiously interpreted in the planning of human phase I studies which are likely to involve HLA mismatching, particularly if treatment with relatively well-differentiated cells and/or multiple treatments are given.

MSC Are Immunosuppressive In Vitro

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

MSC have been consistently found to be potently immunosuppressive and anti-inflammatory when studied in vitro, effects which are associated with inhibition of effector T-cell activation and/or expansion of regulatory T-cell numbers.[28, 29] While the T-cell suppressive effects of MSC have been recognized for many years,[30] the ability of MSC to favour CD4+ T-cell differentiation to the T regulatory cell rather than TH17 phenotype[31] is of particular interest—a property which has been consistently described in the literature and which provides a strong rationale for preclinical and clinical studies in transplantation.[32]

Both cell–cell contact and multiple soluble mediators have been implicated as mediators of the MSC effect on immune cells.[33] Krampera[34] and colleagues described in 2003 that MSC, but not conditioned media or MSC in a transwell, were capable of suppressing naïve and memory T-cell proliferation.[34] Contact-dependent mechanisms have also been found to be important in the induction of forkhead box P3 and CD25—the characteristic markers of the induced T regulatory cell phenotype.[35] However, multiple studies have also highlighted the importance of MSC-produced soluble factors in dampening the immune response. Of particular note, co-culture of bm-MSC and peripheral blood mononuclear cells results in prostaglandin E2 production by the MSC,[36] an effect abrogated by cyclo-oxygenase inhibition.[21, 31] Interestingly, prostaglandin E2 and cyclo-oxygenase-2 expression is increased in the presence of interferon-γ and TNF-α, suggesting that the level of influence MSC have over T lymphocyte populations is in turn regulated by the microenvironment in which they find themselves.[33, 37]

The immunosuppressive properties of MSC appear to be present regardless of the source of MSC, with those obtained from lung using BAL also able to potently inhibit T-cell proliferation in vitro.[21] Similar to bm-MSC, lr-MSC produce prostaglandin E2 which suppresses T-cell proliferation in the presence of mitogenic and allogeneic stimuli.[21] Prostaglandin E2 is also measurable in BAL, suggesting that lr-MSC may play an active immunosuppressive role in vivo. Lr-MSC isolated from digested parenchymal tissue also inhibit T-cell proliferation through the secretion of indoleamine-2,3-dioxygenase-1.[13]

In conclusion, MSC are remarkably immunoprivileged and are potently and broadly immunosuppressive in vitro. They are able to evade immune detection through low expression of HLA antigens and costimulatory molecules and influence a wide range of immune cells both directly and through the secretion of soluble mediators. It is these properties which have led to great interest in the potential for therapeutic efficacy in immune-mediated human disease.

MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

Initially, MSC were an appealing candidate cell for regenerative medicine because of their multipotency. Multiple in vitro studies have demonstrated that MSC can transdifferentiate not only into fat, cartilage and bone, but also cells of non-mesenchymal origin—for instance bronchial epithelium, renal epithelium, neuronal tissue and cardiomyocytes.[17] It was originally believed MSC could function as a reservoir population to replenish damaged and lost tissue after insult through engraftment and differentiation. This hypothesis has been tested across a variety of disease models characterized by abnormalities of both mesenchymal (e.g. osteogenesis imperfecta[38]) and non-mesenchymal tissues including brain,[39] heart[40] and kidney,[41] with frequent evidence of a treatment effect. However, initial enthusiasm was quickly tempered by data from a trial of MSC therapy for osteogenesis imperfecta which reported significant functional improvements with only very limited (<1%) levels of engraftment.[38] Since then, numerous studies have confirmed that functional improvements in the target organ are not due to the widespread engraftment and transdifferentiation of MSC, necessitating the development of alternate hypotheses for the efficacy of MSC treatment.

MSC as Orchestrators of Tissue Stem Cell Function

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

Complex organisms possess a remarkable capacity for extensive and sustained tissue renewal throughout a lifetime and this capacity for self-renewal is maintained by reservoirs of somatic tissue stem cells.[42] Organ homeostasis and repair following injury, and hence organism fitness, is therefore highly dependent on the appropriate function of tissue stem cells, the best studied being the HSC. It is now clear that one of the main functions of the bm-MSC is the maintenance of the HSC niche. Bm-MSC form a highly organized tripartite structure including the HSC and neural inputs which are tightly spatially associated within the bone marrow to create a niche to facilitate haematopoiesis.[4] The functional nature of this niche has been confirmed by experiments that have demonstrated that MSC express numerous HSC maintenance genes which are downregulated should HSC become mobilized. Selective MSC ablation results in HSC depletion and bone marrow homing of HSC infused into HSC-depleted mice is heavily MSC dependent.[4] It is likely that MSC in other organs will adopt similar roles when tissue stem cells are called into action to effect organ repair in the face of injury.

In the lung, McQualter and colleagues have demonstrated that the support of a stem cell common antigen-1+ mesenchymal cell is necessary and sufficient to support the proliferation and differentiation of a bronchiolar epithelial stem/progenitor cell into both airway and alveolar lineages. This support is provided, at least in part, through the secretion of the cytokines fibroblast growth factor-10 and hepatocyte growth factor.[43] Although it is currently unknown whether the murine stem cell common antigen-1+ mesenchymal cell is analogous to the human lr-MSC, their work is consistent with the idea that adult mesenchymal stromal cells are an essential element of the epithelial stem/progenitor cell niche in the adult lung; an idea supported by work from another group who have shown that parabronchial mesenchymal cells secrete fibroblast growth factor-10 to support epithelial regeneration from surviving epithelial progenitors following naphthalene-induced Clara cell ablation.[44] Furthermore, the functional improvements seen following the systemic administration of bm-MSC in a murine model of bronchopulmonary dysplasia have been shown to be mediated via expansion of bronchioalveolar stem cells.[5] Given this emerging body of work, it is likely that the orchestration of tissue stem cell function in the lung and other organs by MSC explains the apparent efficacy of MSC treatment in disease models in the absence of widespread engraftment and transdifferentiation.

The cues which MSC use to direct the function of tissue stem cells remain largely unknown, and at least outside the bone marrow, unexplored. However, intriguingly, MSC have recently been shown to rescue dysfunctional tissue stem cells by restoring bioenergetics through mitochondrial donation.[6, 7] Mitochondria were donated to eukaryotes approximately 1.5 billion years ago by an ancient prokaryote, facilitating aerobic respiration and the subsequent evolution of the eukaryotic cell, and eventually complex multicellular organisms. Mitochondrial dysfunction has recently been identified as a key trigger for cellular senescence and apoptosis, and may act as a break on tissue stem cell proliferation in organs with a generally slow turnover of stem cell pools, such as the lung.[45] The potential for MSC to restore mitochondrial function to tissue stem cells, and, in effect ‘recharge’ them to prevent stem cell ageing, is therefore of considerable interest. The lung has been a primary site of investigation of this phenomenon, with the first evidence for intercellular mitochondrial transfer being provided by Spees et al. in 2006[7] while studying A549 cells. A549 cells with dysfunctional mitochondria were rescued by small numbers of MSC which made contact with the target cells either through long, thin extensions or through broad areas of the cell surfaces with mitochondria streaming through the extended cytoplasmic projections into the target cell with resultant rescue of aerobic metabolism and growth kinetics.[7] These observations have recently been confirmed in a model of acute lung injury where the therapeutic effect of MSC appears to be at least in part mediated by the donation of functional mitochondria.[6]

Together, these data showcase the diversity and potential of MSC in regenerative medicine. MSC have been shown to be much more than a cellular population with immune-suppressive activity and multipotent capacity. Instead, MSC can promote tissue repair by coordinating the functionality of tissue stem cell pools and rescue aged, senescent or damaged tissue through mitochondrial transfer. Furthermore, this therapeutic potential has been confirmed in studies where ex vivo expanded MSC have been delivered intravenously or endobronchially in highly relevant models of severe lung disease, as outlined in Tables 1-6.

Table 1. Preclinical studies of MSC in the treatment of lung fibrosis
AuthorMSC sourceSpeciesInterventionModelOutcome
  1. BM, bone marrow; IV, intravenous; MSC, mesenchymal stem cells; Sca, stem cell common antigen.

Ortiz et al. 2003[46]BM—BALB/c mouseC57BL/6 mouse

5 × 105 cells.

Days 0 and 7.

IV via jugular vein.

Bleomycin[DOWNWARDS ARROW] Hydroxyproline—not significant with day 7 infusion
Cui et al. 2007[47]BM—Sprague–Dawley ratSprague–Dawley rat

5 × 105 cells.

Days 1 and 7.

IV via tail vein.

Bleomycin[DOWNWARDS ARROW] Hydroxyproline and lung fibrotic score—more pronounced with day 1 infusion
Zhao et al. 2008[48]BM—Sprague–Dawley ratSprague–Dawley rat

5 × 106 cells.

12 h.

IV via tail vein.

Bleomycin[DOWNWARDS ARROW] Hydroxyproline and pro-fibrotic cytokines
Moodley et al. 2009[49]Umbilical cord—humanSCID mouse

1 × 106 cells.

Day 1.

Bleomycin[DOWNWARDS ARROW] Hydroxyproline, collagen and pro-fibrotic cytokines
Bitencourt et al. 2011[50]Endogenous MSC (Sca-1+, CD44+, CD45)C57BL/6 mouseHyaluronidase treatment to promote MSC recruitment.Bleomycin[DOWNWARDS ARROW] Collagen content and fibrotic score
Table 2. Preclinical studies of MSC therapy in pulmonary arterial hypertension
AuthorMSC sourceSpeciesInterventionModelOutcomes
  1. BM, bone marrow; eNOS, endothelial nitric oxide synthase; IT, intratracheal; IV, intravenous; MSC, mesenchymal stem cells; S/C, subcutaneous.

Rochefort et al. 2005[51]BM—Wistar ratWistar rat

1 × 106 cells.

Twice weekly, 3 weeks.

IV.

Hypoxia (50 kPa hypobaric)

50–60% immediate pulmonary engraftment.

No difference versus controls.

Kanki-Horimoto et al. 2006[52]

BM—Sprague–Dawley rat.

Modified to overexpress eNOS.

Sprague–Dawley rat

1 × 106 wild-type cells or 5 × 105 eNOS cells.

IV via femoral vein.

60 mg/kg S/C monocrotaline 1 or 3 weeks prior to infusion

[DOWNWARDS ARROW] Right ventricular systolic pressure (both groups).

[UPWARDS ARROW] Survival—both groups, eNOS overexpressing cells had a more pronounced effect.

Baber et al. 2007[53]BM—Sprague-Dawley rat.Sprague–Dawley rat

3 × 106 cells.

Intratracheal.

60 mg/kg IV monocrotaline 2 weeks prior to IT injection

[DOWNWARDS ARROW] Pulmonary arterial pressure at 3 weeks.

Engraftment up to 3 weeks. Engrafting cells express von Willebrand factor.

Umar et al. 2009[54]

BM—Wistar rat.

Monocrotaline treated.

Wistar rat

1 × 106 cells.

IV.

60 mg/kg S/C monocrotaline 2 weeks prior to infusion

[DOWNWARDS ARROW] Pulmonary arterial pressure and right ventricular hypertrophy.

Normalized right ventricular ejection fraction.

Takemiya et al. 2010[55]

BM—Lewis rat.

Modified to overexpress prostacyclin.

Lewis rat

5 × 105 cells (wild-type or prostacyclin overexpressing)

IV

60 mg/kg S/C monocrotaline 2 weeks prior to infusion

[DOWNWARDS ARROW] Right ventricular pressure and right ventricle:left ventricle + septum ratio.

[UPWARDS ARROW] Survival for animals treated with prostacyclin overexpressing cells, but not wild-type cells.

[UPWARDS ARROW] Pulmonary engraftment in diseased animals and persisted for at least 2 weeks.

He et al. 2009[56] (Chinese)BM—Sprague–Dawley ratSprague–Dawley rat

5 × 106 cells.

IV via sublingual vein.

50 mg/kg S/C monocrotaline 22 days prior to infusion

[UPWARDS ARROW] Survival at 27 days postinfusion (90% vs 50%).

[DOWNWARDS ARROW] Pulmonary arterial pressure.

Extensive engraftment in neo-vascularized lung.

Jungebluth et al. 2011[57]BM—Lewis ratLewis rat

6 × 106 cells.

Intratracheal.

Ligation of left pulmonary artery at the time of MSC infusion[UPWARDS ARROW]Haemodynamics and gas exchange
Luan et al. 2011[58]BM—Sprague–Dawley ratSprague–Dawley rat

1 × 105 cells.

IV.

50 mg/kg monocrotaline 1 week prior to infusion[UPWARDS ARROW]Haemodynamics and survival
Liang et al. 2011[59]

BM—FVB/n mouse.

Modified to overexpress haeme oxygenase-1.

FVB/n mouse (wild-type or haeme oxygenase-1 overexpressing)IV via jugular vein (wild-type or haeme oxygenase-1 overexpressing)Normobaric hypoxia (8–10 %) for 5 weeks prior to infusion

Normalized right ventricular systolic pressure.

[DOWNWARDS ARROW] Right ventricular hypertrophy.

Efficacy appeared dependent on epithelial transdifferentiation.

[DOWNWARDS ARROW] Engraftment from 2 days postinfusion to very low levels by 1 week, but engraftment was higher in hypoxia.

Table 3. Preclinical studies of MSC therapy in bronchopulmonary dysplasia
AuthorMSC sourceSpeciesInterventionModelOutcomes
  1. AEC2, type II alveolar epithelial cell; BM, bone marrow; IV, intravenous; MSC, mesenchymal stem cells; TGF, transforming growth factor; TIMP, tissue inhibitor of metalloproteinases.

Aslam et al. 2009[60]BM—FVB mouseFVB mouse

5 × 104 cells or conditioned media (50 μL, 10×).

IV via superficial temporal vein at postnatal day 4.

Hyperoxic exposure (75%) for 14 days

[DOWNWARDS ARROW] Alveolar loss, inflammation ([DOWNWARDS ARROW] macrophages and neutrophils).

Prevented pulmonary arterial hypertension.

Conditioned media more effective—prevented alveolar loss and vessel remodelling.

Van Haaften et al. 2009[61]BM—Sprague–Dawley ratSprague–Dawley rat

1 × 105 cells.

Postnatal day 4 or 14.

Intratracheal.

Hyperoxic exposure (95%) for 14 days

MSC migrate towards injured lung tissue in vitro.

MSC acquire AEC2 phenotype.

[UPWARDS ARROW] Survival, exercise capacity.

Day 4 administration improves alveolar structures.

Partial protection of vasculature.

[DOWNWARDS ARROW] Pulmonary arterial hypertension.

Hansmann et al. 2012[62]

BM—FVB mouse.

Conditioned cell culture media.

FVB mouse

10 μg conditioned media.

IV via superficial temporal vein or jugular vein.

Hyperoxic exposure (75%) for 14 days

Reversal of alveolar injury, septal thickness and myofibroblast infiltration.

[UPWARDS ARROW] Lung function.

Reversal in pulmonary arterial hypertension, lung injury, abnormal peripheral pulmonary arterial remodelling.

Tropea et al. 2012[5]

BM—FVB mouse.

Cells or conditioned cell culture media.

FVB mouse

5 × 104 cells or conditioned media (50 μL, 10×).

Postnatal day 4.

IV via superficial temporal vein.

Hyperoxic exposure (75%) for 14 days

[UPWARDS ARROW] Lung architecture.

[UPWARDS ARROW] Number of bronchioalveolar stem cells.

Waszak et al. 2012[63]

BM—Sprague–Dawley rat.

Conditioned cell culture media.

Sprague–Dawley rat

1 μL/g conditioned media.

Postnatal days 1–20.

Intraperitoneal.

Hyperoxic exposure (90%) for 14 days.

Media obtained from P2 1.5 × 106 cells from normoxic or hyperoxic environment.

[DOWNWARDS ARROW] Pulmonary arterial hypertension.

Improved lung architecture.

Zhang et al. 2012[64]BM—Kunming mouseKunming mouse

1 × 105 cells.

7 days postnatal then weekly.

Intraperitoneal.

Hyperoxic exposure (60%) for 45 days

[UPWARDS ARROW] Survival.

MSC hone and engraft as AEC2 cells.

[DOWNWARDS ARROW] Fibrosis (TGFβ1, collagen 1α, TIMP-1).

Improved lung architecture.

Table 4. Preclinical studies of MSC therapy in acute lung injury
AuthorMSC sourceSpeciesInterventionModelOutcomesPutative mechanism
  1. AEC2, type II airway epithelial cell; BAL, bronchoalveolar lavage; BM, bone marrow; CFU, colony-forming units; E. coli, Escherichia coli; ICR, imprinting control region; IFN, interferon; IL, interleukin; IT, intratracheal; IV, intravenous; IgM, immunoglobulin M; KC, keratinocyte-derived cytokine; LPS, lipopolysaccharide; MIP, macrophage inflammatory protein; MSC, mesenchymal stem cells; NFKβ, nuclear factor kappa-light-chain-enhancer of activated B cells; PBS, phosphate-buffered saline; SiRNA, small interfering RNA.

Gupta et al. 2007[65]BM—C57BL/6 mouseC57BL/6 mouse

7.5 × 105 cells.

4 h post LPS treatment.

IT.

5 mg/kg E. coli-derived LPS.

IT.

[UPWARDS ARROW] Survival ([UPWARDS ARROW]42%).

[DOWNWARDS ARROW] Total protein in BAL, haemorrhaging, oedema.

Estimated engraftment <5%.

[DOWNWARDS ARROW] Pro-inflammatory cytokines (MIP-2, TNF-α).

[UPWARDS ARROW] Anti-inflammatory cytokines (IL-10, IL-13, IL-1ra).

Unknown

Suggested possible mediators: IL-10, IL-13 or IL-1ra

Mei et al. 2007[66]BM—C57BL/6J. Wild-type and angiopoietin-1 transfected.C57BL/6J mouse

2.5 × 105 cells.

30 min post LPS treatment.

Jugular venous cannula.

800 μg E. coli-derived LPS.

IT.

[DOWNWARDS ARROW] Neutrophilia, interalveolar septal thickening, interstitial oedema.

[DOWNWARDS ARROW] Pro-inflammatory cytokines (IFN-γ, TNF-α, IL-6, IL1β).

[DOWNWARDS ARROW] Apoptosis (caspases 3 and 7).

[DOWNWARDS ARROW] Pulmonary leakage with transfected MSC treatment (total protein, albumin and IgM).

<8% MSC retained at 72 h post-treatment.

Angiopoietin-1.

MSC modified to overexpress were more beneficial compared with wild-type cells.

Xu et al. 2007[67]BM—C57BL/6 mouseC57BL/6 mouse

5 × 105cells.

1 h post LPS treatment.

IV.

1 mg/kg E. coli-derived LPS.

Intraperitoneal.

Prevents vascular congestion.

[DOWNWARDS ARROW] Neutrophilia.

Prevents thickening of alveolar septae and oedema.

[DOWNWARDS ARROW] Pro-inflammatory cytokines (1L-1β, IFN-γ, IL-6, MIP-1α, TNF-α and KC).

Large number of MSC in lungs at 24 h but are not detectable at 14 days post-treatment.

No MSC were found to acquire AEC2, endothelial or fibroblast phenotypes.

In vitro co-culture with lung tissue digests promote proliferation and migration in MSC.

Unknown
Fang et al. 2010[68]BM—humanHuman ex vivo1 × 106 cellsTranswell co-culture. 2.5 × 105 epithelial cells in lower chamber wounded with 50 ng/mL of Cytomix.

[DOWNWARDS ARROW] Epithelial permeability.

Si-RNA interference of angiopoietin-1 ablates this effect.

Angiopoiten-1.

Functions by preventing actin stress fibre formation and claudin 18 disorganization via NFKβ suppression.

Krasnodembskaya et al. 2010[69]BM—humanIn vitro or C57BL/6 mouse

1 × 106 cells.

IT.

1 × 106 E. coli CFU. IT.

MSC and MSC-conditioned medium [DOWNWARDS ARROW] Gram negative and positive bacterial growth.

[DOWNWARDS ARROW] Neutrophil counts and MIP-2

MSC produce LL-37.

LL-37.

mRNA and protein expression by MSC increases after bacterial challenge.

In vivo neutralization of LL-37 reduces bacterial clearance.

Lee et al. 2009[70]BM—humanHuman ex vivo

5 × 106 cells or conditioned medium from 1 × 106 cells.

Intrapulmonary

6 mg E. coli LPS

[DOWNWARDS ARROW] Extravascular lung water.

[DOWNWARDS ARROW] Endothelial barrier permeability.

Restored alveolar fluid clearance.

Keratinocyte growth factor.

Partially functions by restoring amiloride-dependent sodium transport.

Kim et al. 2011[71]Umbilical cord—humanICR mouse

1 × 105 cells.

3 h post-treatment.

IT.

1 × 107 E. coli in 50 μL of PBS.

IT.

[UPWARDS ARROW] Survival.

[DOWNWARDS ARROW] Lung injury score, oedema and bacterial load (blood and BAL).

[DOWNWARDS ARROW] Myeloperoxidase activity.

[DOWNWARDS ARROW] Pro-inflammatory cytokines (IL-1α, IL-1β, IL-6, TNF-α, MIP).

Unknown.

Suggested possible mediators: keratinocyte growth factor and angiopoietin-1 suggested as possible mediators.

Liang et al. 2011[72]BM—Wistar ratWistar rat

1 × 106 cells.

2 h post LPS treatment.

IV via left tail vein.

8 mg/kg E. coli-derived LPS.

IV—right tail vein.

MSC honing correlated with degree of injury.

[DOWNWARDS ARROW] Pro-inflammatory cytokines (IL-β and TNF-α).

[DOWNWARDS ARROW] Total protein in BAL, myeloperoxidase activity, neutrophil count in BAL.

No change in pulmonary fibrosis score.

Non-significant trend suggests increased survival.

[UPWARDS ARROW] IL-10 in vitro alveolar macrophage and MSC co-culture.

Unknown

IL-10 shown to increase with MSC treatment, although not shown to be MSC produced.

Ionescu et al. 2012[73]BM—C57BL/6 mouseC57BL/6 mouse

0.25 × 106 cells or conditioned medium from 5 × 106 cells.

IT.

4 mg/kg E. coli-derived LPS.

IT.

[DOWNWARDS ARROW] Neutrophils, lung oedema and injury.

[UPWARDS ARROW] Arginase-1 and Ym1 activity.

Recombinant insulin-like growth factors could partially confer benefit.

Insulin-like growth factor. Suggested to promote M2 macrophage phenotype.
Tai et al. 2012[74]BM—Kunming mouseKunming mouse

5 × 106 cells.

IV via tail vein.

5μg E. coli-derived LPS.

IT.

[DOWNWARDS ARROW] Wet : dry ratio, BAL protein.

[DOWNWARDS ARROW] TNF-α, IL-6 and neutrophils in BAL.

[UPWARDS ARROW] IL-10 in BAL.

[DOWNWARDS ARROW] Myeloperoxidase activity.

Unknown

IL-10 increases with MSC treatment, although not shown to be MSC produced.

Table 5. Preclinical studies of MSC therapy in asthma
AuthorMSC sourceSpeciesInterventionModelOutcomes
  1. BAL, bronchoalveolar lavage; BM, bone marrow; IFN, interferon; Ig, immunoglobulin; IL, interleukin; iNOS, inducible nitric oxide synthase; IP, intraperitoneal; IT, intratracheal; IV, intravenous; KC, keratinocyte-derived cytokine; MIP, macrophage inflammatory protein; MSC, mesenchymal stem cells; PCNA, proliferating cell nuclear antigen; RW, ragweed; SMA, smooth muscle actin; STAT6, signal transducer and activator of transcription; TDI, toluene diisocyanate induced; TGF, transforming growth factor.

Bonfield et al. 2010[75]BM—humanBALB/c mouse

1 × 106 cells.

IV via tail vein.

Sensitized IP 10 μg ovalbumin. Challenged 1% wt/vol ovalbumin day 14 onwards, intranasally.

[DOWNWARDS ARROW] Eosinophils, macrophages and neutrophils.

[DOWNWARDS ARROW] Goblet cell hyperplasia, epithelial cell lining thickening and collagen deposition.

[DOWNWARDS ARROW]Serum IgE.

[DOWNWARDS ARROW] IFN-γ, IL-5 and IL-13, MIP-1α, KC and iNOS expression.

Nemeth et al. 2010[76]BM—C57BL/6J or BALB/c mouseC57BL/6J or BALB/c mouse

Cell number not stated.

IV.

Sensitized IP 50 μg ragweed. Challenged days 14 and 15 with 50 μg RW extract via IT and intranasal inoculation.

Improved histology.

[DOWNWARDS ARROW] BAL cell number.

[DOWNWARDS ARROW] IL-4, IL-13, IL-5.

[DOWNWARDS ARROW] IgG1 and IgE.

IL-4 and/or IL-13 increase TGF-β production through STAT6 pathway.

Goodwin et al. 2011[77]BM—C57BL/6 mouseC57BL/6 or BALB/c mouse

2 × 106 cells.

IV via tail vein.

Sensitized IP 20 μg ovalbumin on days 0 and 7. Challenged with aerosolized 1% ovalbumin for 30 min on day 14, 15 and 16.

Protects against airways hyperreactivity.

[DOWNWARDS ARROW] Eosinophil inflammation.

No change in CD4+ T-cell proliferation but altered antigen-specific CD4 T-cell differentiation.

[DOWNWARDS ARROW] Inflammation partially dependant on IFN-γ.

Firinci et al. 2011[78]BM—BALB/c mouseBALB/c mouse

1 × 106 cells.

IV via tail vein.

Sensitized IP 10 μg/100 μL ovalbumin on day 1 and 14. Challenged 3 days a week for 8 weeks from day 21. 2.5% ovalbumin aerosolized for 30 min.

MSC engrafted in injured animals.

Restored changes in basement membrane, subepithelial smooth muscle thickness.

[DOWNWARDS ARROW] Goblet and mast cell number.

Lower serum nitric oxide.

Lee et al 2011[79]BM—Sprague–Dawley ratBALB/c mouse

1 × 105 cells.

IV via tail vein.

Sensitized with 3% toluene diisocyanate intranasally once daily for 5 days. Challenged with 1% TDI for 1 h over 3 consecutive days.

[DOWNWARDS ARROW] Inflammatory index, eosinophils and neutrophils in BAL.

[DOWNWARDS ARROW] Collagen deposition, α-SMA, PCNA.

[DOWNWARDS ARROW] Airway remodelling.

Table 6. Preclinical studies of MSC in ischaemia reperfusion injury
AuthorOrganMSC sourceSpeciesInterventionModelOutcomes
  1. BM, bone marrow; IL, interleukin; IV, intravenous; MEK/ERK, mitogen-activated protein kinase - extracellular signal-regulated kinase; MSC, mesenchymal stem cells; STAT3, signal transducer and activator of transcription 3; TNF, tumour necrosis factor.

Manning et al. 2010[80]LungBM—Lewis ratLewis rat15 × 106 cells. IV. Cells engineered to carry IL-10 vector120 min ischaemia. Occlusion of left pulmonary artery, left pulmonary vein and left mainstem bronchus

[UPWARDS ARROW] Blood O2.

[DOWNWARDS ARROW] Lung injury score, lung permeability, wet–dry score and apoptosis.

Chen et al. 2012[81]LungBM–Sprague–Dawley ratSprague–Dawley rat

1 × 106 cells.

IV.

30 min ischaemia. Occlusion of left pulmonary artery, bronchus and pulmonary vein.

[DOWNWARDS ARROW] Lung injury, pro-inflammatory cytokines and apoptosis.

[UPWARDS ARROW] O2 partial pressure.

Effects enhanced when coupled with ischaemic post conditioning.

Pan et al. 2012[82]LiverBM—Sprague–Dawley ratSprague–Dawley rat

3 × 106.

IV.

60 min ischaemia. Occlusion of hepatic artery, portal vein and bile duct.

[UPWARDS ARROW] Histopathological changes.

[DOWNWARDS ARROW] Liver enzymes.

In vitro study suggests MEK/ERK pathway activation.

Poynter et al. 2011[83]HeartBM—C57BL/6C57BL/61 × 106 cells15 min equilibration and 25 min ischaemia

[UPWARDS ARROW] Improved functional recovery.

STAT3 knockout MSC had no effect.

[DOWNWARDS ARROW] TNF-α in wild-type MSC-treated animals.

STAT3 knockout MSC produced less insulin-like growth factor-1 but more hepatocyte growth factor.

Liu et al. 2012[84]KidneyBM—Sprague–Dawley rat.Sprague–Dawley rat

1 × 106 cells.

IV.

40 min ischaemia. Occlusion of left renal artery.

[UPWARDS ARROW] Regeneration.

[UPWARDS ARROW] Epidermal growth factor and haeme oxygenase-1.

[DOWNWARDS ARROW] In vitro oxidative stress and apoptosis.

The Lung—An Attractive Target for MSC Therapy

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

One of the difficulties with MSC therapy for other organs—the pulmonary first-pass effect—may be an inherent advantage when pulmonary biotherapy is considered. Following intravenous infusion, due to the filtering function of the pulmonary vasculature, only a small proportion of cells pass through into the systemic circulation.[85] This effect is particularly pertinent to MSC-based therapy due to the large physical size of MSC. The first-pass effect has impeded the development of regenerative therapy approaches such as MSC therapy for heart diseases[86] and has led to the development of sometimes hazardous strategies to deliver MSC directly to the affected organ. The ability to deliver cellular therapy to the lung via a simple intravenous approach is a major advantage and gives the potential for large-scale retention. Even more attractively, retained cells appear to target areas of injured lung.[87] Direct intratracheal, intrapulmonary[65] and intrapleural inoculation represent additional relatively non-invasive routes of administration which are peculiar to the lung.

MSC Therapy for Lung Disease—Preclinical Studies

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

Notwithstanding their inherent limitations, there is robust evidence for the efficacy of MSC therapy in preclinical models of acute lung injury, bronchopulmonary dysplasia, pulmonary fibrosis and pulmonary hypertension and some evidence of efficacy in asthma and post-transplant obliterative bronchiolitis. These data are summarized in Tables 1-6 and are particularly strong for acute lung injury. Acute lung injury studies (summarized in Table 4) have also contributed to most our understanding of the mechanism of action of MSC therapy. Most of these studies have utilized allogeneic MSC which have been isolated from bone marrow using plastic adherence and delivered intravenously or endobronchially. Of particular note are the studies which demonstrated that genetic engineering of MSC, for instance to overexpress haeme oxygenase[59] or prostacyclin,[55] conferred enhanced efficacy in pulmonary arterial hypertension.

The therapeutic potential for MSC in pulmonary fibrosis was first recognized when it was noted that lung fibrosis was diminished in a study designed to assess the effect of bleomycin on pulmonary MSC engraftment.[46] Since that study, multiple preclinical studies, summarized in Table 1, have demonstrated the therapeutic efficacy of MSC in the bleomycin model of lung fibrosis. Although there appears to be a consistent effect of MSC if delivered soon after the administration of bleomycin, the therapeutic effect diminishes considerably if treatment is delayed until 7 days after administration.[46, 47, 87] This effect highlights a well-known deficiency of the bleomycin model, and is particularly important to recognize because the timing of MSC delivery appears to determine the fate of the engrafting cell, with later delivery favouring the differentiation of MSC into cells which are pro-fibrotic.[87] The latter possibility remains an ongoing concern for investigators designing early-phase human trials and will be a key safety outcome.

Human Studies of MSC Therapy in Lung Disease

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

The multifaceted activity of MSC has translated into a large body of clinical trial activity outside the lung, most notably in the treatment of steroid refractory graft versus host disease following allogeneic bone marrow transplant, but also in other immune-mediated diseases like Crohn's disease, multiple sclerosis, lupus and in the renal transplant setting.[88] The tissue repair capability of MSC is being investigated in clinical trials for cardiac repair, bone disorders (osteogenesis imperfecta), bone fracture and following meniscectomy. However, there has only been one large clinical trial of MSC therapy for lung disease which is as yet unpublished. The safety and efficacy of intravenous ex vivo cultured adult human MSC for subjects with moderate to severe chronic obstructive pulmonary disease was tested in an industry-sponsored phase II, double-blind, placebo-controlled trial which has just been published.[89, 90] Human adult MSC were derived from normal healthy adult volunteer bone marrow donors. A total of 62 patients, with a diagnosis of moderate (n = 23) or severe (n = 39) chronic obstructive pulmonary disease were enrolled. All patients completed the planned course of four infusions without any evidence of infusional toxicity. Adverse event rates were comparable for patients receiving MSC and placebo, but the pulmonary function efficacy end-point was not met.[89, 91]

Our group has initiated two human phase I trials of MSC therapy for lung disease. In the first study (NCT01175655),[92] the primary objective is to establish the safety of infusions of MSC from related or unrelated HLA-identical or HLA-mismatched donors in the management of BOS after lung transplantation. The secondary objectives are to document changes in lung function, 6-min walk distance and survival following MSC infusion. Patients (n = 10) with single, bilateral or heart-lung allografts and deteriorating chronic allograft dysfunction manifesting as either BOS grades 2 & 3, or grade 1[93] with an additional risk factor for subsequent death, receive open-label treatment with 2 × 106 MSC/kg bodyweight intravenously twice weekly for 2 weeks. Thus far, six patients have been treated with no evidence of toxicity.[94] The second study (NCT01385644)[95] is an open-label, non-randomized, dose-escalation evaluation of the safety and feasibility of intravenous placental MSC treatment for subjects diagnosed with idiopathic pulmonary fibrosis. This study has recently closed to recruitment. MSC have been delivered intravenously to eight patients with moderate (forced vital capacity > 50% of predicted and diffusing capacity for carbon monoxide > 25% of predicted) idiopathic pulmonary fibrosis. Four received 1 × 106 cells/kg and the next four received 2 × 106 cells/kg. The primary end-point was again safety, with particular interest in the potential for acute infusional toxicity and in the medium term, increased fibrosis. There has been no evidence of acute infusional toxicity, but follow up is ongoing to assess the medium-term safety of MSC treatment for this indication. The only other study listed on http://www.clinicaltrial.gov as assessing MSC therapy for lung disease involves the intratracheal administration of umbilical cord blood-derived MSC to infants with bronchopulmonary dysplasia.[96] However, as confidence with respect to safety for the delivery of this therapeutic product to patients with even moderate to severe lung disease increases, and as the body of preclinical data steadily grows, no doubt the number of actively recruiting trials will increase. Notwithstanding this clinical trial activity, truly paradigm-changing therapies will depend on thoroughly defining the mode of action of MSC in the normal and diseased human lung. Once these mechanisms are defined, the pipeline of MSC-based treatments will be able to follow a more traditional, drug-like, developmental pathway with, for instance, the ability to reliably predict in vivo activity using in vitro potency assays.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References

While a number of lung diseases, most notably asthma, are now able to be relatively safely and effectively treated due to significant improvements in the available pharmacologics, substantial therapeutic gaps remain, particularly for degenerative and fibrotic lung diseases. It is pleasing to think that a deeper understanding of the role of tissue-resident stem cells in the maintenance of lung health, and hence the development of pharmacologic and/or cell therapies aimed at restoring that health, may 1 day fill these gaps. However, in order for this promise to be achieved safely, and in order to avoid a repeat of the problematic headlong introduction of gene therapies to large-scale clinical trials,[97] a deeper understanding of basic MSC biology will need to be acquired, alongside the careful conduct of early-phase human trials. Of particular interest in the lung will be the interrelationships between epithelial and MSC hierarchies, the make-up of the pulmonary stem cell niche and the role of the MSC in maintaining that niche.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. What Is an MSC?
  5. Lung-Resident MSC
  6. MSC Are Relatively Immunoprivileged
  7. MSC Are Immunosuppressive In Vitro
  8. MSC Display Multi-Lineage Differentiation Potential, but only Limited Organ Engraftment
  9. MSC as Orchestrators of Tissue Stem Cell Function
  10. The Lung—An Attractive Target for MSC Therapy
  11. MSC Therapy for Lung Disease—Preclinical Studies
  12. Human Studies of MSC Therapy in Lung Disease
  13. Conclusions
  14. References
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