**7. Discussion**

This review aimed to compile the evidence supporting specific mechanisms of action that may contribute to tendon regeneration in MSC-based cellular therapies. The analysis of the recent literature demonstrated an imbalance between the numbers of studies investigating tenogenic differentiation in vitro and ECM regeneration in vivo and the numbers of studies elucidating other potential mechanisms. This is conceivable as most studies investigating MSC in the context of tendon disease did not specifically aim at clarifying the mechanisms of action. Particularly, the in vivo studies mostly addressed MSC efficacy, at which ECM characteristics are reasonable outcome parameters. Still, despite the overlap with tissue engineering, the overrepresentation of tenogenic differentiation studies may reflect a delay in the field of tendon research. Tendon pathophysiology itself is still not well-understood, making it challenging to transfer the rapidly changing perception of MSC into experimental settings relevant to tendon disease in a timely manner. Yet, it can be anticipated that the general understanding of MSC mechanisms will be successively incorporated into tendon research in the following years.

Taking into account the existing data, the best-evidenced beneficial effect of MSC in tendon regeneration is the improved ECM regeneration. MSCs may also protect and rescue resident tendon cells, but only few data support this hypothesis so far. Both, ECM regeneration and tendon cell protection, are likely to be mediated by a range of mechanisms acting in concert. These may be active over long periods of time, as the engraftment of MSC within tendon lesions was repeatedly demonstrated.

The possible mechanisms mediating ECM regeneration include ECM synthesis and targeted remodeling by the engrafted MSC, inhibition of MMP over-activation, modulation of immune cells with suppression of macrophagemediated matrix degradation, and modulation of growth factor signaling. Last but not least, the rescue of resident tendon cells could prevent ongoing ECM degeneration, and their trophic support and stimulation by MSC-derived growth factors could re-initiate ECM synthesis and a healthy state of ECM remodeling driven by the tenocytes. A varying extent of evidence supports these different mechanisms, with the collectively most convincing data available for ECM synthesis, immunomodulation, and VEGF-mediated angiogenesis. **Figure 3** illustrates the possible interplay between the different mechanisms and their potential synergies.

The figure summarizes the currently known mechanisms of MSC that may contribute to tendon regeneration. Mechanisms for which there is conclusive evidence from in vivo studies are designated in bold typeface.

However, there may also be antagonisms between different mechanisms, although the evidence is not yet entirely conclusive. Perhaps, tenogenic differentiation and immunomodulation may not occur at the same time. Tenogenic differentiation was shown to interfere with the immunomodulatory potential of MSC [93], and inflammatory environment compromised tenogenic MSC properties [84]. Yet, some in vivo studies revealed anti-inflammatory effects of combined MSC and

**83**

moment.

**Figure 3.**

modulatory mechanisms.

*Mechanisms of action of MSC in tendon healing.*

*Mechanisms of Action of Multipotent Mesenchymal Stromal Cells in Tendon Disease*

tenogenic growth factor administration [123, 125], although it remained unclear if the MSCs had undergone tenogenic differentiation. It is possible that the context, i.e., the stage of tendon disease, may favor one mechanism over the other. For example, immune cells such as macrophages are not predominating during subclinical stages [6], and the macrophage polarization pattern is distinct in acute vs. chronic disease [8], which will certainly impact on the activation of MSC immuno-

A range of limitations impedes a coherent interpretation of the existing data.

These include the different treatment approaches chosen and models used, which make it difficult to elucidate specific reasons for contradictory findings. Inter-donor variability is a further issue that may obscure clarity of findings in studies using human or large animal MSCs [100, 143]. Furthermore, although tenogenic differentiation has extensively been studied, there is neither a consensus on differentiation protocols nor have specific markers for tenogenic differentiation been used consistently. Next, the limited understanding of tendon (patho)physiology makes it difficult to judge whether certain effects observed are beneficial or rather detrimental, e.g., with respect to MMP or TGF-β activity. Last but not least, the illustrated imbalance between evidence levels for particular mechanisms makes it difficult to draw a comprehensive picture at the

*DOI: http://dx.doi.org/10.5772/intechopen.83745*

*Mechanisms of Action of Multipotent Mesenchymal Stromal Cells in Tendon Disease DOI: http://dx.doi.org/10.5772/intechopen.83745*

### **Figure 3.**

*Tendons*

in response to TGF-β.

**7. Discussion**

ing years.

demonstrated.

potential synergies.

from in vivo studies are designated in bold typeface.

therapies is still limited. Further research is crucial to improve our ability to exploit these effects and, last not least, to prevent potential negative effects associated with some growth factors, such as hypervascularization in response to VEGF or fibrosis

This review aimed to compile the evidence supporting specific mechanisms of action that may contribute to tendon regeneration in MSC-based cellular therapies. The analysis of the recent literature demonstrated an imbalance between the numbers of studies investigating tenogenic differentiation in vitro and ECM regeneration in vivo and the numbers of studies elucidating other potential mechanisms. This is conceivable as most studies investigating MSC in the context of tendon disease did not specifically aim at clarifying the mechanisms of action. Particularly, the in vivo studies mostly addressed MSC efficacy, at which ECM characteristics are reasonable outcome parameters. Still, despite the overlap with tissue engineering, the overrepresentation of tenogenic differentiation studies may reflect a delay in the field of tendon research. Tendon pathophysiology itself is still not well-understood, making it challenging to transfer the rapidly changing perception of MSC into experimental settings relevant to tendon disease in a timely manner. Yet, it can be anticipated that the general understanding of MSC mechanisms will be successively incorporated into tendon research in the follow-

Taking into account the existing data, the best-evidenced beneficial effect of MSC in tendon regeneration is the improved ECM regeneration. MSCs may also protect and rescue resident tendon cells, but only few data support this hypothesis so far. Both, ECM regeneration and tendon cell protection, are likely to be mediated by a range of mechanisms acting in concert. These may be active over long periods of time, as the engraftment of MSC within tendon lesions was repeatedly

The possible mechanisms mediating ECM regeneration include ECM synthesis and targeted remodeling by the engrafted MSC, inhibition of MMP over-activation, modulation of immune cells with suppression of macrophagemediated matrix degradation, and modulation of growth factor signaling. Last but not least, the rescue of resident tendon cells could prevent ongoing ECM degeneration, and their trophic support and stimulation by MSC-derived growth factors could re-initiate ECM synthesis and a healthy state of ECM remodeling driven by the tenocytes. A varying extent of evidence supports these different mechanisms, with the collectively most convincing data available for ECM synthesis, immunomodulation, and VEGF-mediated angiogenesis. **Figure 3** illustrates the possible interplay between the different mechanisms and their

The figure summarizes the currently known mechanisms of MSC that may contribute to tendon regeneration. Mechanisms for which there is conclusive evidence

However, there may also be antagonisms between different mechanisms, although the evidence is not yet entirely conclusive. Perhaps, tenogenic differentiation and immunomodulation may not occur at the same time. Tenogenic differentiation was shown to interfere with the immunomodulatory potential of MSC [93], and inflammatory environment compromised tenogenic MSC properties [84]. Yet, some in vivo studies revealed anti-inflammatory effects of combined MSC and

**82**

*Mechanisms of action of MSC in tendon healing.*

tenogenic growth factor administration [123, 125], although it remained unclear if the MSCs had undergone tenogenic differentiation. It is possible that the context, i.e., the stage of tendon disease, may favor one mechanism over the other. For example, immune cells such as macrophages are not predominating during subclinical stages [6], and the macrophage polarization pattern is distinct in acute vs. chronic disease [8], which will certainly impact on the activation of MSC immunomodulatory mechanisms.

A range of limitations impedes a coherent interpretation of the existing data. These include the different treatment approaches chosen and models used, which make it difficult to elucidate specific reasons for contradictory findings. Inter-donor variability is a further issue that may obscure clarity of findings in studies using human or large animal MSCs [100, 143]. Furthermore, although tenogenic differentiation has extensively been studied, there is neither a consensus on differentiation protocols nor have specific markers for tenogenic differentiation been used consistently. Next, the limited understanding of tendon (patho)physiology makes it difficult to judge whether certain effects observed are beneficial or rather detrimental, e.g., with respect to MMP or TGF-β activity. Last but not least, the illustrated imbalance between evidence levels for particular mechanisms makes it difficult to draw a comprehensive picture at the moment.
