Mesenchymal stromal cell crosstalk with the immune system

Abstract

Cell therapy is a promising treatment for several diseases. One of the most commonly used cell types are mesenchymal stromal cells (MSCs). MSCs are found in most connective tissues but for clinical use they are commonly harvested from bone marrow, adipose tissue and umbilical cord. MSCs have the ability to differentiate into connective tissues, such as adipocytes, chondrocytes and osteoblasts. Furthermore, they are known for their antiinflammatory, immunosuppressive and regenerative properties. They release a large number of immunomodulatory factors, such as indoleamine-pyrrole 2,3-dioxygenase and prostaglandin E2, as part of their mechanisms of action. MSCs are safe to transplant and allogenic cells can be used without adverse reactions. Clinically, MSCs have been used to treat numerous diseases, including graft-versus-host disease (GvHD), type 1 diabetes (T1D) and multiple sclerosis.

A goal of this thesis was to study the interaction of MSCs with the blood compartment in an attempt to recapitulate the fate of the cells after intravenous infusion. By exploring MSC interactions with active plasma, containing complement proteins and immune cells, such as monocytes, we aimed to gain new insights into the mechanisms of action involved in MSCmediated immunosuppression. We also studied whether MSC function was compromised in autoimmune diseases, using T1D as an example. Finally, to further decipher tissue microenvironment characteristics permissive for MSC responsiveness, we compared the immune profile in intestinal biopsies from patients receiving MSC transplantations to treat acute (a)GvHD.

In paper I, we investigated MSC interactions with components of peripheral blood. Using in vitro assays we demonstrated that proteins of the complement system (C3b/iC3b) bind to the cell surface of MSCs. We reported that MSCs survive complement binding and retain certain functional characteristics. Binding of complement proteins to the MSCs triggered enhanced phagocytosis by classical and intermediate subsets of monocytes. Monocyte-MSC interactions have been previously demonstrated to play a key role in MSC-mediated immunomodulation. These findings suggested that the therapeutic effects observed after intravenous delivery of MSCs may be mediated by interactions with the complement system. The subsequent skewing of monocytes, via phagocytosis, may also partly explain previous reports by our group regarding the fate of infused MSCs and their lack of tissue engraftment. In paper II, we compared the genotypic and phenotypic profiles of MSCs isolated from T1D and healthy donors. Evaluation of bone marrow mononuclear counts, colony forming unitfibroblasts and growth kinetics discerned no significant differences between the groups. Transcriptional comparisons, using microarray, indicated a number of differences between healthy and T1D donors, with respect to their expression of cytokines, immunomodulation and wound healing potential. However, these differences in gene expression did not reflect functional changes when tested in in vitro systems. We concluded that expanded MSCs from T1D donors were suitable for autologous therapy, thereby reducing risks associated with allogeneic treatments. In paper III, we highlighted the importance of the gut mucosa immune cell profile of aGvHD patients, in the responsiveness to MSC treatment. Gut mucosal biopsies, obtained for aGvHD diagnoses were profiled using immunohistochemistry with a panel of innate and adaptive immune cell markers. Distinct baseline immune cell milieus were seen between patients who later responded to MSC therapy compared to those who did not respond. The responder group presented with increased levels of CD8+ T cells and mast cells but decreased levels of CD4, CD56 and CD68. We concluded from this small pilot study that a pro-inflammatory profile within the gut at the point of MSC therapy may limit patient responsiveness. These findings need to be confirmed in larger patient cohorts but indicate that the patient’s immunological milieu should be considered when evaluating potential responsiveness to MSC therapy.

In conclusion, this thesis ties together three major considerations for the development and efficacy of intravenous MSC therapy. Using a combination of basic science and clinical investigations we have demonstrated the importance of MSC interactions with the innate immune compartment, including both the complement system and peripheral monocytes. We further, report that MSCs from patients with immunological disorders, such as T1D, retain their therapeutic potential, as assessed in vitro, confirming that autologous therapies are an option for these patient cohorts. Finally, we evaluate how the patient’s immune milieu may contribute to efficacy of MSC therapy and the need for further investigation of both the MSC biology in determining mode of therapeutic action, as well as, how the patient’s status itself may impede or promote MSC responsiveness. We conclude that MSCs represent a cell source with strong therapeutic potential in a broad number of diseases. With further investigation in the highlighted areas, we hope to improve efficacy and to fully understand how these cells contribute to healing and tissue regeneration.