T cell activation requires that the cell meet increased energetic and biosynthetic demands. We showed that exogenous nutrient availability regulated the differentiation of naïve CD4(+) T cells into distinct subsets. Activation of naïve CD4(+) T cells under conditions of glutamine deprivation resulted in their differentiation into Foxp3(+) (forkhead box P3-positive) regulatory T (Treg) cells, which had suppressor function in vivo. Moreover, glutamine-deprived CD4(+) T cells that were activated in the presence of cytokines that normally induce the generation of T helper 1 (TH1) cells instead differentiated into Foxp3(+) Treg cells. We found that α-ketoglutarate (αKG), the glutamine-derived metabolite that enters into the mitochondrial citric acid cycle, acted as a metabolic regulator of CD4(+) T cell differentiation. Activation of glutamine-deprived naïve CD4(+) T cells in the presence of a cell-permeable αKG analog increased the expression of the gene encoding the TH1 cell-associated transcription factor Tbet and resulted in their differentiation into TH1 cells, concomitant with stimulation of mammalian target of rapamycin complex 1 (mTORC1) signaling. Together, these data suggest that a decrease in the intracellular amount of αKG, caused by the limited availability of extracellular glutamine, shifts the balance between the generation of TH1 and Treg cells toward that of a Treg phenotype.

Results. In the CIA model of arthritis, MSCs did not confer any benefit. Both the clinical and the immunologic findings suggested that MSCs were associated with accentuation of the Th1 response. Using luciferaseexpressing MSCs, we were unable to detect labeled cells in the articular environment of the knee, suggesting that worsening of the symptoms was unlikely due to the homing of MSCs in the joints. Experiments in vitro showed that the addition of TNF␣ was sufficient to reverse the immunosuppressive effect of MSCs on T cell proliferation, and this observation was associated with an increase in interleukin-6 secretion.Conclusion. Our data suggest that environmental parameters, in particular those related to inflammation, may influence the immunosuppressive properties of MSCs.

Mitochondrial activity is central to tissue homeostasis. Mitochondria dysfunction constitutes a hallmark of many genetic diseases and plays a key role in tumor progression. The essential role of mitochondria, added to their recently documented capacity to transfer from cell to cell, obviously contributes to their current interest. However, determining the proper role of mitochondria in defined biological contexts was hampered by the lack of suitable experimental tools. We designed a protocol (MitoCeption) to directly and quantitatively transfer mitochondria, isolated from cell type A, to recipient cell type B. We validated and quantified the effective mitochondria transfer by imaging, fluorescence-activated cell sorting (FACS) and mitochondrial DNA analysis. We show that the transfer of minute amounts of mesenchymal stem/stromal cell (MSC) mitochondria to cancer cells, a process otherwise occurring naturally in coculture, results in cancer cell enhanced oxidative phosphorylation (OXPHOS) activity and favors cancer cell proliferation and invasion. The MitoCeption technique, which can be applied to different cell systems, will therefore be a method of choice to analyze the metabolic modifications induced by exogenous mitochondria in host cells.