“Mesenchymal stromal cells: There is life in Death” - Seminar Summary

This post is a summary of a seminar by Prof. Francesco Dazzi that I have attended.

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Mesenchymal stem cells (MSC) have been implicated in a multitude of effects, but the claims cannot be justified purely by stem cell activity. Prof. Francesco Dazzi and his team have been studying the immunomodulatory effects of MSC in an attempt to identify their mechanisms better.

The main problem that arises when working with MSC is that they are not only heterogeneous and poorly characterised, there is also high heterogeneity regarding tissue source, as bone marrow, peripheral blood, cord blood, and other sources can be used for the derivation of MSC. The media used for the cultivation of MSC and different product specifications also contribute to the high variability in available cells and can potentially impact their immunosuppressive potency (Trento et al., 2018). However, a new MSC manufacturing protocol has been tested in a study, and it was shown that the MSC generated with it do not only have a more consistent quality but are also promising as a therapy for steroid-refractory acute graft versus host disease (GvHD) (Bader et al., 2018).

So far, studies have determined that immunomodulation by MSC is fast-acting and highly effective. Due to a low expression of human-leucocyte antigens and co-stimulatory molecules on the surface of MSC, donor-recipient matching is not required as it would be in most other types of transplants, allowing for more straightforward therapeutic use and less concern about tissue rejection (Avivar-Valderas et al., 2019).

The immunomodulatory effect of MSC has been demonstrated to rely on the exposure to T-cell-derived IFN-γ, which T-cells produce as an inflammatory response at the site of interest. This specificity helps avoid a systemic effect of MSC (Krampera et al., 2006).

The immunomodulation by MSC is characterised by the production of anti-inflammatory cytokines, the recruitment of monocytes, the polarisation of macrophages into the M2 type, the inhibition and differentiation of dendritic cells, and the generation and recruitment of MDSCs (Cheung and Dazzi, 2018). All these effects dampen the immune response, which explains the multitude of effects MSC can exhibit when used in a therapeutic setting.

So far, there are 1021 studies recorded as clinical trials with MSC, of which 418 are currently active. The areas of interest are, as mentioned before, GvHD, solid organ transplants (Popp et al., 2008), autoimmune diseases (Panés et al., 2016), chronic degenerative disorders, and genetic diseases that stand in connection with inflammation.

In GvHD, allogeneic haemopoietic stem cell transplants have been shown to elicit a good response without killing the patient, and even a small response is already sufficient to impact survival. The results observed one week after treatment has started allow for a prediction of the overall outcome.
Not just the qualities of the MSC but also those of the recipients themselves and the surrounding circumstances play an important role, as shown by some other studies. The timing of the MSC administration is crucial in GvHD; a prophylactic infusion has a considerably lower effect than treatment when the disease arises (Introna et al., 2014).

Transfused MSC do not usually engraft, and the clinical responses are unpredictable as the pharmacological dynamics are not known, making it difficult to predict outcomes and improving treatment. To better understand the mechanics of MSC treatment, the fate of MSC after they have been infused was studied in a GvHD model. Observed had been rapid caspase activity within the infused MSC an hour after administration, leading to apoptosis of the cells. The apoptosis, however, did not impair the immunosuppression. In fact, apoptosis induced by cytotoxic cells in the recipient turned out to be necessary for the desired immunomodulatory effect. The efficiency with which a patient’s immune system kills the MSC directly affects the patient’s response and predicts overall survival. The observations resulted in the conclusion that cytotoxicity of the recipient’s immune cells is the only important factor associated with the response (Cheung et al., 2019).

The cytotoxic cells in question are natural killer cells and cytotoxic T-cells belonging to the host. They are responsible for inducing apoptosis in the administered MSC, but their interaction does not interfere with antigen-specific killing, which avoids an unwanted, generalised immunosuppression (Krampera et al., 2006).

With the response of the recipient cytotoxic cells being crucial to successful MSC-based therapies, the question posed itself if there was a way to circumvent this need somehow. In one experiment, apoptotic MSC (ApoMSC) have been administered directly to GvHD patients. These ApoMSC still, as hoped, exhibited immunosuppressive effects. Their presence was followed by the induction of indoleamine 2,3-dioxygenase (IDO) in phagocytotic macrophages (Cheung and Dazzi, 2018).

Figure 1 shows the effects of MSC in both a living and apoptotic state. Again, it becomes apparent that the MSC do not need to be alive to elicit downstream effects, their presence in an apoptotic state attracts macrophages which then start producing IDO, IL-10, and TGF-β, all anti-inflammatory cytokines (Galipeau and Sensébé, 2018).

Please check out the figure in the original paper, due to copyright

_{Figure 1 MSC Fitness, Function, and Fate Theorem, Galipeau and Sensébé, 2018}

Further research showed that in vivo, the education of monocytes by ApoMSC is regulated by COX2/PGE2 activity. Under COX2 suppression, production of IDO is equally suppressed, and ApoMSC do not affect the immune response.
While the determination of IDO activity would be helpful to directly monitor the effects of MSC, the method to accomplish this is still in need of considerable improvement. Instead, the soluble factor PGE2 can be used as a better tool, as a patient who responds to the MSC with immunosuppression and tissue regeneration also shows higher PGE2 levels (Cheung et al., 2019).

The results of the study of ApoMSC are the first that demonstrate evidence for apoptotic cells having the ability to be immunomodulatory in a clinical setting, removing the recipient’s cytotoxic cells from the equation. In theory, this could lead to more consistent effects of MSC therapies and a better prediction of outcomes even before the cells have been administered. Further research into this direction is needed and might advance therapies for a multitude of diseases.

Bibliography

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Cheung, T.S., Galleu, A., Bonin, M. von, Bornhäuser, M., Dazzi, F., 2019. Apoptotic mesenchymal stromal cells induce prostaglandin E2 in monocytes: implications for the monitoring of mesenchymal stromal cell activity. Haematologica 104, e438–e441. https://doi.org/10.3324/haematol.2018.214767

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Popp, F.C., Eggenhofer, E., Renner, P., Slowik, P., Lang, S.A., Kaspar, H., Geissler, E.K., Piso, P., Schlitt, H.J., Dahlke, M.H., 2008. Mesenchymal stem cells can induce long-term acceptance of solid organ allografts in synergy with low-dose mycophenolate. Transpl. Immunol., Minimization of Immunosuppression in Organ Transplantation 20, 55–60. https://doi.org/10.1016/j.trim.2008.08.004

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