Mammalian mesenchymal stem cells (MSCs) have been shown to be strongly immunosuppressive in both animal disease models and human clinical trials. We have reported that the key molecule mediating immunosuppression by MSCs is species dependent: indoleamine 2,3-dioxygenase (IDO) in human and inducible nitric oxide synthase (iNOS) in mouse. In the present study, we isolated MSCs from several mammalian species, each of a different genus, and investigated the involvement of IDO and iNOS during MSC-mediated immunosuppression. The characterization of MSCs from different species was by adherence to tissue culture plastic, morphology, specific marker expression, and differentiation potential. On the basis of the inducibility of IDO and iNOS by inflammatory cytokines in MSCs, the tested mammalian species fall into two distinct groups: IDO utilizers and iNOS utilizers. MSCs from monkey, pig, and human employ IDO to suppress immune responses, whereas MSCs from mouse, rat, rabbit, and hamster utilize iNOS. Interestingly, based on the limited number of species tested, the iNOS-utilizing species all belong to the phylogenetic clade, Glires. Although the evolutionary significance of this divergence is not known, we believe that this study provides critical guidance for choosing appropriate animal models for preclinical studies of MSCs.

IL-17 is one of the most potent and most actively investigated proinflammatory cytokines. In this study, we examined the effect of IL-17 on mesenchymal stem cells (MSCs) under the influence of inflammatory cytokines. Ironically, IL-17 dramatically enhanced the immunosuppressive effect of MSCs induced by IFNγ and TNFα, revealing a novel role of IL-17 in immunosuppression. Interestingly, we found that this action of IL-17 was dependent on the promoted expression of a key immune suppressive molecule, inducible nitric oxide synthase (iNOS), in MSCs. In a concanavalin A (ConA)-induced hepatitis mouse model, we found that IL-17 also enhanced the in vivo immunosuppressive effect of MSCs in an iNOS-dependent manner. Moreover, this promoting effect of IL-17 was found to be exerted through enhancing mRNA stability by modulating the protein level of ARE/poly(U)-binding/degradation factor 1 (AUF1), a well-known factor that promotes mRNA decay. In auf1(-/-) MSCs, IFNγ and TNFα could induce maximal immunosuppressive effect, both in vitro and in vivo, without the need for IL-17. Thus, our studies demonstrated that in the presence of MSCs, IL-17 promotes immunosuppression.

Multiple sclerosis (MS) is a chronic and debilitating autoimmune disease, characterized by chronic inflammatory demyelination in the nervous tissue and subsequent neurological dysfunction. Spermidine, a natural polyamine, has been shown to affect inflammation in some experimental models. We show here that spermidine could alleviate experimental autoimmune encephalomyelitis (EAE), a model for MS, through regulating the infiltration of CD4+ T cells and macrophages in central nervous system. Unexpectedly, we found that spermidine treatment of MOG-specific T cells did not affect their pathogenic potency upon adaptive transfer; however, spermidine diminished the ability of macrophages in activating MOG-specific T cells ex vivo. Depletion of macrophages in diseased mice completely abolished the therapeutic effect of spermidine, indicating a critical role of spermidine-activated macrophages. Mechanistically, spermidine was found to specifically suppress the expression of interleukin-1beta (IL-1β), IL-12 and CD80 while enhance the expression of arginase 1 in macrophages. Interestingly, macrophages from spermidine-treated mice could also reverse EAE progression, while pretreatment of those macrophages with the arginase 1 inhibitor abrogated the therapeutic effect. Therefore, our studies revealed a critical role of macrophages in spermidine-mediated treatment on EAE and provided novel information for better management of MS.