In addition, CD69 might act specifically on the Treg cell subset, directly suppressing the activity of effector T cells [56]. After MSC/CD4+CD25– co-cultures, we observed that SSc cells were able

to induce normally functioning Tregs from the T lymphocytes of HC and SSc patients. As Pritelivir CD69 expression by Tregs has been associated with the production of TGF-β [55], we analysed the surface expression of this molecule in induced Tregs. Interestingly, although the CD69 surface expression was decreased in circulating SSc Tregs, an increased expression of this molecule was observed in induced cells without differences between patients and controls. Consistent with this evidence, Rapamycin induced SSc Tregs showed a normal ability to inhibit immunoproliferation of CD4+ T cells. We observed an increase of TGF-β production in the supernatants of SSc–MSC co-cultures, and this

production was associated with an increase of TGF-β gene expression in the SSc–MSCs. During SSc, IL-6 and TGF-β are involved not only in immunoregulatory mechanisms but also in the pathogenesis of the fibrotic process, which is the main feature of the disease. Further experiments are ongoing in our laboratory in order to evaluate the role of these cytokines, produced by MSCs, on collagen production as well as on modulation of the myofibroblast phenotype. These cAMP findings might suggest that, during SSc, an adaptive cytokine profile with an increase in both TGF-β and IL-6 expression avoids senescence interfering with MSC activity, thus maintaining their role in inducing fully functional Tregs. In this work we did not investigate the immunosuppressive role of senescent SSc–MSCs on dendritic cell functions, already shown in other conditions. It is well known that these cells produce higher levels of IL-10 and

might contribute to the specific cytokine milieu in the disease [57]. Furthermore, recent reports showed that dendritic cells might express TGF-β and support fibrogenesis [58]. In this setting, the possible modulation of dendritic cells might offer a new future target for MSC therapeutic application. The in-vitro immunosuppressive activity of MSCs is mediated by direct interaction with lymphocytes at a MSC : PBMC ratio of 1:1 [59]. This raises a question: are these MSC : PBMC ratios achieved normally in vivo, when MSC are utilized clinically in the clinical setting? Indeed, according to the immunosuppression observed in vivo [60], relatively high numbers of MSC should be injected to obtain this effect. This may be of great relevance in planning the dose of MSC to administer. However, some difficulties in obtaining a sufficient number of MSCs for clinical purposes have been described previously [61].