|Ph.D Student||Nahor Irit|
|Subject||Immunomodulatory Properties of Human Mesenchymal Stem Cells:|
Mechanisms of Action and Use in Cell Therapy
|Department||Department of Medicine||Supervisors||PROFESSOR EMERITUS Joseph Itskovitz|
|DR. Sonia Berrih|
|Full Thesis text|
Human Mesenchymal Stem Cells (hMSCs) are known for their immunoregulatory properties as they modulate the immune response both in vitro and in vivo although their mechanisms of action are not fully elucidated. The objective of this study was to investigate the mechanisms of action of hMSCs obtained from adult human adipose tissues in a co-culture model, combining hMSCs and allogenic PBMCs (peripheral blood mononuclear cells) in absence of exogenous stimulation. When PBMCs were co-cultured with hMSCs, we observed a striking decreased expression of CD8 receptor level on CD8 cells, together with a decreased CD28 and CD44 expression as well as impaired IFN-gamma and Granzyme B production. These effects were found to be specific to hMSCs and CD8 cells, since no such an effect was observed with other cell lines such as human fibroblast or HaCat cell line or in the CD4 subset accordingly. These effects required CD14 monocytes in direct contact with the CD8 cells, while the effects of hMSCs on the CD14 cells were essentially mediated by soluble factors. The CD14 monocytes exhibited a tolerogenic pattern when co-cultured with hMSCs, demonstrating a clear decrease in CD80 and CD86 co-stimulatory molecules and in HLA-ABC and HLA-DR histocompatibility molecules, along with an increase in the inhibitory receptors ILT-3 and ILT-4. MSCs effect over CD14 cells was found to be continuous, since pre-sensitized monocytes managed to reduce CD8 surface expression. In order to address whether hMSCs could present similar features in vivo, we used NOD SCID RAG2−/−γc−/− (NSG) immunodeficient mice strain, humanized with CD34 hematopoietic stem cells. Injection of hMSC in the reconstituted mice showed similar inhibitory effect, in particular decreased CD44 and CD28 on CD8 cells, as was evaluated in the spleen, BM, and blood in comparison to mice injected with PBS. Altogether, these results demonstrate that hMSCs shift the cytotoxic CD8 cells towards a suppressor phenotype, an effect mediated by antigen-presenting cells. We propose that hMSC have a physiological role in limiting the cytotoxic activity of CD8 cells in steady state as well in pathological situations.