Immune modulation by mesenchymal stem cells

Mesenchymal stem cells (MSCs) were discovered as adherent cells in the bone marrow stroma that could form bone and stimulate hematopoiesis. Later, MSCs were shown to be hypoimmunogenic and to suppress proliferation of activated T cells. Cytotoxic T lymphocytes (CTLs) constitute important effector cells of the immune system, but can also cause severe tissue destruction. The first article in this thesis shows that MSCs suppressed activation of CTLs against allogeneic peripheral blood mononuclear cells (PBMCs). MSCs did not affect the lysis performed by already activated CTLs. PBMCs but not MSCs were lysed by CTLs although both target cells were derived from the same individual. Additionally, MSCs potentiated NK-cell mediated lysis of K562, and were resistant to lysis by NK cells, despite a KIR-ligand mismatch. Cytokines are important mediators in immune signaling. MSCs increased the levels of interleukin-2 (IL-2), IL-2 Receptor, and IL-10 in mixed lymphocyte cultures (MLCs), while the levels decreased in mitogen-stimulated cultures, as presented in paper 11. Inhibition of prostaglandin E2 synthesis partially restored proliferation in mitogen-stimulated cultures inhibited by MSCs, but not in MLCs. These results indicate possible different mechanisms of inhibition by MSCs after mitogenic and allogeneic stimulation of PBMCs. MSCs also suppressed phorbol myristate acetate activation of PBMCs, indicating that MSCs exert the suppressive function downstream of the receptor level. MSCs stimulated the production of immunoglobulin G (IgG) by splenic mononuclear cells (MNCs) and enriched B cells. This stimulation by MSCs was mediated by soluble factors when MNCs were used as responder cells. In contrast, when enriched B cells were co-cultured with MSCs, cell-cell contact was required for increased IgG production. MSCs did not induce proliferation of splenic MNCs. When MSCs were added to stimulated MNCs, they both stimulated and inhibited IgG production induced by lipopolysaccharide, cytomegalovirus or varicella zoster virus, depending on the degree of stimulation. The resistance to lysis of MSCs reported in paper 1, was explored further using alloreactive and peptide-specific CTL clones in paper IV. MSCs were resistant to lysis compared to other cells with similar expression of HLA class I. MSCs as target cells generated only weak tyrosine phosphorylation in CTLs. Furthermore, CD25 upregulation and CD3 and CD8 downregulation were minimal. We also showed that MSCs faile