TORC2 is thought to control spatial aspects of cell growth, in particular CH5424802 cell polarity and responses to chemotactic signals via G-protein-coupled activation of RAS.[16] It has long been known that mTOR inhibition by rapamycin (which is used clinically in organ transplantation under the name Sirolimus) is potently immunosuppressive, partly because it blocks the ability of T cells to respond to interleukin-2 and consequently their ability to proliferate in response to antigen stimulation.[17] It is only more recently that is has become clear that the mTOR pathway also controls

the differentiation of different T helper cell subsets,[18] and in particular, the expression of forkhead box P3 (FOXP3), the ‘master’ transcription factor for regulatory T cells (Fig. 1). Downstream activation by mTOR of the T-cell receptor, CD28 co-stimulation EMD 1214063 concentration and cytokine-mediated PI3K signalling is generally required for the differentiation of effector T cells but is inhibitory for FOXP3 expression.[19, 20] Signalling downstream of the sphingomyelin phosphate receptor (S1PR), which is required for lymphocyte trafficking and exit from the lymph nodes, also acts to activate mTOR.[21] Interestingly, this pathway is also the target of a relatively new immunosuppressive drug known as Fingolimod/FTY720,[22]

which therefore might also have the potential to promote regulatory T (Treg) cell development.[23] Although the exact mechanism of FOXP3 inhibition by mTOR has not been clarified, there is some evidence for the involvement of a number of different pathways. These include poorly defined effects on FOXP3 translation via phosphorylation of ribosomal protein S6, and mTOR acting either indirectly via suppressor of cytokine signalling 3 (SOCS3)[24, 25] or directly on signal transducer and activator of transcription 3 (STAT3) downstream of interleukin-6 and the buy 5-FU satiety hormone leptin,[26] which then competes for the interleukin-2-driven STAT5 enhancement of foxp3 transcription.[27] In addition, two transcription factors promoting FOXP3 expression, FOXO3a[28, 29] and the transforming growth factor-β (TGF-β) signalling

component SMAD3, are negatively regulated by AKT downstream of TORC2.[30] Evidence from raptor (TORC1) deficient and rictor (TORC2) deficient mice has suggested that TORC1 tends to promote T helper type 1 (Th1) differentiation,[18] while TORC2 may bias the response to Th2 via AKT and PKCθ,[31] while inhibition of both complexes is required for optimal FOXP3+ Treg cell induction. Th17 cell development seems to be independent of TORC2, but is inhibited by rapamycin in favour of FOXP3+ Treg cells.[32] Hypoxia-induced factor (HIF) 1α, another downstream target of TORC1, has also been implicated as both a positive[33, 34] and a negative[35, 36] regulator of FOXP3 expression and it is also thought to bind directly to FOXP3 protein to target it for proteosomal degradation.