The molecular mechanisms by which mesenchymal stem cells (MSCs) suppress T-cell proliferation are poorly understood, and whether a soluble factor plays a major role remains controversial. Here we demonstrate that the T-cell-receptor complex is not a target for the suppression, suggesting that downstream signals mediate the suppression. We found that Stat5 phosphorylation in T cells is suppressed in the presence of MSCs and that nitric oxide (NO) is involved in the suppression of Stat5 phosphorylation and T-cell proliferation. The induction of inducible NO synthase (NOS) was readily detected in MSCs but not T cells, and a specific inhibitor of NOS reversed the suppression of Stat5 phosphorylation and T-cell proliferation. This production of NO in the presence of MSCs was mediated by CD4 or CD8 T cells but not by CD19 B cells. Furthermore, inhibitors of prostaglandin synthase or NOS restored the proliferation of T cells, whereas an inhibitor of indoleamine 2,3-dioxygenase and a transforming growth factor-␤-neutralizing antibody had no effect. Finally, MSCs from inducible NOS ؊/؊ mice had a reduced ability to suppress T-cell proliferation. Taken IntroductionBecause mesenchymal stem cells (MSCs) differentiate into osteocytes, chondrocytes, myotubes, and adipocytes, 1-3 they are expected to become a source of cells for regenerative therapy. Also, MSCs support hematopoietic stem cell engraftment 4-9 and modulate immunologic responses by unknown mechanisms. [9][10][11][12][13][14] Here, we investigated the molecular mechanisms by which MSCs suppress T-cell proliferation.Transforming growth factor-␤ (TGF-␤), hepatocyte growth factor, indoleamine 2,3-dioxygenase (IDO), and prostaglandin E2 (PGE 2 ) have been reported to mediate T-cell suppression by MSCs. [13][14][15] Specifically, neutralizing antibodies against TGF-␤ or hepatocyte growth factor, 13 an inhibitor of IDO,14 or an inhibitor of prostaglandin production reverse the inhibition of T-cell proliferation by MSCs. 15 In addition, some reports have shown that a soluble factor is the major mediator of suppression, [13][14][15][16][17] whereas some reports have demonstrated that T-cell-MSC contact is required for this suppression. [12][13][14]16,17 In the current study, we sought to resolve these conflicting results by using a mouse bone marrow-derived MSC system.One candidate soluble factor for T-cell suppression is nitric oxide (NO) because it is known to inhibit T-cell proliferation. [18][19][20][21][22][23][24][25] NO is produced by NO synthases (NOSs), of which there are 3 subtypes: inducible NOS (iNOS), endothelial NOS, and neuronal NOS. Like MSCs, it has been known that macrophages suppress T-cell proliferation. This suppression was reported to be mediated by NO inhibition of Stat5 phosphorylation. 18,19 Also, MSCs were reported to produce NO when they differentiate into chondrocytes. 26 We therefore investigated whether MSCs can produce NO and whether NO is involved in their ability to suppress T-cell proliferation. Materials and methods MaterialsN-nitro-L-a...

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Immunological effects of IL-21 on T-, B-, and NK-cells have been reported, but the role for IL-21 in graft-versus-host disease (GVHD) remains obscure. Here we demonstrate that morbidity and mortality of GVHD was significantly reduced after bone marrow transplantation with splenocytes from IL-21R−/− mice compared to those from wild type mice. To further confirm our observation, we generated a decoy receptor for IL-21. GVHD was again less severe in mice receiving bone marrow cells transduced with the IL-21 decoy receptor than control mice These results suggest that IL-21 critically regulates GVHD and that blockade of the IL-21 signal may represent a novel strategy for the prophylaxis for GVHD.

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Mesenchymal stem cells (MSCs) have been identified in animals, especially in bone marrow. As stem cells, they have the ability to differentiate into multiple cell types. This potential raises exciting therapeutic possibilities. A recent report described the successful use of MSCs for the treatment of graft-versus-host disease; however, the scientific community has yet to define the molecular mechanisms of immunomodulation by MSCs. This review summarizes what is known and discusses the conflicting data with regard to the mechanisms of immunomodulation by MSCs.