Mesenchymal stem cells (MSCs) are present in most, if not all, tissues and are believed to contribute to tissue regeneration and the tissue immune microenvironment. Murine MSCs exert immunosuppressive effects through production of the nitric oxide synthase iNOS, while human MSCs utilize indoleamine 2,3-dioxygenase (IDO). Thus, studies of MSC-mediated immunomodulation in mice may not be informative in the setting of human disease, although this critical difference has been mainly ignored. To address this issue, we established a novel humanized system to model human MSCs, employing murine iNOS−/− MSCs that constitutively or inducibly express an ectopic human IDO gene . In this system, inducible IDO expression is driven by a mouse iNOS promoter that can be activated by inflammatory cytokine stimulation in a similar fashion as the human IDO promoter. These IDO-expressing humanized MSCs (MSC-IDO) were capable of suppressing T lymphocyte proliferation in vitro. In melanoma and lymphoma tumor models, MSC-IDO promoted tumor growth in vivo, an effect that was reversed by the IDO inhibitor 1-methyl-tryptophan. We found that MSC-IDO dramatically reduced both tumor-infiltrating CD8+ T cells and B cells. Our findings offer an important new line of evidence that interventional targeting of IDO activity could be used to restore tumor immunity in humans, by relieving IDO-mediated immune suppression of MSCs in the tumor microenvironment as well as in tumor cells themselves.

Mesenchymal stem cells (MSCs), derived from adult tissues, are multipotent progenitor cells, which hold great promise for regenerative medicine. Recent studies have shown that MSCs are immunosuppressive in vivo and in vitro in both animals and humans. However, the mechanisms that govern these immune modulatory functions of MSCs remain largely elusive. Some studies with bulk populations of MSCs indicated that soluble factors such as PGE2 and TGFb are important, while others support a role for cell-cell contact. In this study, we intended to clarify these issues by examining immunosuppressive effects of cloned MSCs. We derived MSC clones from mouse bone marrow and showed that the majority of these clones were able to differentiate into adipocytes and osteoblast-like cells. Importantly, cells from these clones exhibited strong inhibitory effects on TCR activation-induced T cell proliferation in vitro, and injection of a small number of these cells promoted the survival of allogeneic skin grafts in mice. Conditioned medium from MSC cultures showed some inhibitory effect on anti-CD3 induced lymphocyte proliferation independent of PGE2 and TGFb. In comparison, direct co-culture of MSCs with stimulated lymphocytes resulted in much stronger immunosuppressive effect. Interestingly, the suppression was bi-directional, as MSC proliferation was also reduced in the presence of lymphocytes. Taking together, our findings with cloned MSCs demonstrate that these cells exert their immunosuppressive effects through both soluble factor(s) and cell-cell contact, and that lymphocytes and MSCs are mutually inhibitory on their respective proliferation.

Background-Desensitization of the cyclic adenosine monophosphate signal protects cardiac myocytes against catecholamine stress, thus preventing the development of apoptosis. Molecular mechanisms of desensitization have been well studied at the level of adrenergic receptors but less so at the level of the effector enzyme, adenylyl cyclase (AC). Methods and Results-When the effects of long-term (1 to 2 weeks) isoproterenol infusion were compared between type 5 AC-null mice (AC5KO) and wild-type controls, we found that the subsequent responses of left ventricular ejection fraction to sudden intravenous isoproterenol challenge were reduced in AC5KO compared with wild-type mice (ie, physiological desensitization was more effective in AC5KO), consistent with enhanced downregulation of AC catalytic activity in AC5KO. One mechanism for the less effective desensitization in wild-type mice was paradoxical upregulation of type 5 AC protein expression. The number of apoptotic myocytes was similar at baseline but was significantly less in AC5KO after infusion. This was accompanied by a 4-fold greater increase in Bcl-2 and a 3-fold greater increase in phospho-Akt in AC5KO. The latter is most likely mediated by increased membrane localization of phosphoinositidedependent protein kinase 1, which is known to be inhibited by the cyclic adenosine monophosphate signal. Conclusions-The absence of type 5 AC results in more effective desensitization after long-term catecholamine stress and protects against the development of myocyte apoptosis and deterioration of cardiac function, potentially elucidating a novel approach to the therapy of heart failure.