Our findings provide evidences supporting involvement of HLA-G in the immunosuppressive properties of MSCs. These results emphasize the potential application of MSCs as a relevant therapeutic candidate in transplantation.

Mesenchymal stem cells (MSC) inhibit the response of allogeneic T lymphocytes in culture. Because the mechanisms of this effect may differ according to the existence of cell contact, we investigated the differences in gene expression of inhibitory molecules during MSC-T lymphocyte coculture when cell contact does and does not occur. Human MSC and T lymphocytes were cultured together in standard and transwell cultures. MSC gene expression was analyzed by semiquantitative real-time RT-PCR. MSC elicited a high dose-dependent inhibition of T lymphocytes in cultures with cell contact, but inhibition occurred even without cell contact. In both cases, we observed significant upregulation of IDO, LIF, and HLA-G, along with downregulation of HGF and SDF1. In cultures with cell contact, IL-10 and TGF-β transcripts were expressed in a significantly higher level than in cultures without this contact. Furthermore, in the latter, the increased inhibition of T-cell proliferation was positively correlated with IDO gene expression and negatively correlated with SDF1 gene expression. MSC appear to induce T-cell tolerance by two distinct mechanisms. The first of these, which does not require cell contact, induces expression of the tolerogenic genes IDO, LIF, and HLA-G. The second mechanism, which is contact dependent, modulates IL-10 and TGF-β gene expression. These two mechanisms probably play separate roles in MSC-induced tolerance in allogeneic hematopoietic stem cell transplantation.

Gene targeting studies have shown that T cell receptor (TCR)-β gene expression and recombination are inhibited after deletion of an enhancer (Eβ) located at the 3′ end of the ∼500-kb TCR-β locus. Using knockout mouse models, we have measured, at different regions throughout the TCR-β locus, the effects of Eβ deletion on molecular parameters believed to reflect epigenetic changes associated with the control of gene activation, including restriction endonuclease access to chromosomal DNA, germline transcription, DNA methylation, and histone H3 acetylation. Our results demonstrate that, in early developing thymocytes, Eβ contributes to major chromatin remodeling directed to an ∼25-kb upstream domain comprised of the Dβ-Jβ locus regions. Accordingly, treatment of Eβ-deleted thymocytes with the histone deacetylase inhibitor trichostatin A relieved the block in TCR-β gene expression and promoted recombination within the Dβ-Jβ loci. Unexpectedly, however, epigenetic processes at distal Vβ genes on the 5′ side of the locus and at the 3′ proximal Vβ14 gene appear to be less dependent on Eβ, suggesting that Eβ activity is confined to a discrete region of the TCR-β locus. These findings have implications with respect to the developmental control of TCR-β gene recombination, and the process of allelic exclusion at this locus.

The therapeutic potential of bone marrow-derived human mesenchymal stem cells (hMSC) has recently been brought into the spotlights of many fields of research. One possible application of the approach is the repair of tissue injuries related to side effects of radiotherapy. The first challenge in cell therapy is to assess the quality of the cell and the ability to retain their differentiation potential during the expansion process. Efficient delivery to the sites of intended action is also necessary. We addressed both challenges using hMSC cultured and then infused to non-obese diabetes/severe combined immunodeficiency (NOD/SCID) mice submitted to total body irradiation. Furthermore, we tested the impact of additional abdominal irradiation superimposed to total body irradiation (TBI), as a model of local therapeutic irradiation. Our results showed that the hMSC used for transplant have been expanded without significant loss in their differentiation capacities. After transplantation into adult unconditioned mice, hMSC not only migrate in bone marrow but also into other tissues. Total body irradiation increased hMSC implantation in bone marrow and muscle and further led to engraftment in brain, heart and liver. Local irradiation in addition to TBI, increased homing of injected cells to the injured tissues and to other tissues outside the local irradiation field. Morphological recovery of irradiated tissues after MSC transplantation and/or differentiation of MSC into specific organ cell types needs to be investigated. This study suggests that using the potential of hMSC to home to various organs in response to tissue injuries might be a strategy to repair the radiation-induced damages.