Tolerance was abrogated in TPH1 knockout mice, and this could be reconstituted with wild-type mast cells, but not by providing 5-hydroxytryptophan to bypass TPH1 and allow normal serotonin synthesis.[57] In a similar manner, arginase (ARG1) expression has often been associated with protective, type 2, macrophages within tissues,[58] and like IDO, has been implicated in regulating the immune response during pregnancy.[59, 60] Arginine is also the substrate for the inducible form of nitric oxide synthase (iNOS), which is normally associated with a Th1 effector

cell response, but under limiting concentrations of arginine in vitro, both arginase and iNOS can cause sufficient depletion Transmembrane Transporters modulator of this essential amino acid to cause mTOR inhibition and block T-cell proliferation.[51] Interleukin-4-induced 1 (IL4i1) was named for its induction in myeloid cells under Th2 conditions, and is also an enzyme that catabolizes find more amino acids, but with preference for those with a hydrophobic side chain such as phenylalanine.[61] Regulatory

T cells were able to induce many of these essential amino acid consuming enzymes in dendritic cells in vitro and within skin grafts in vivo,[51] whereas the enzymes that catabolize threonine (threonine dehydrogenase: TDH) and the branched chain amino acids (branched chain amino acid aminotransferase: BCAT1) were more closely associated with innate inflammation or wound healing,[51] suggesting that tissues have a built-in mechanism for protecting themselves Baf-A1 against immune attack under these circumstances. Intriguingly, long-term surviving, fully healed syngeneic skin grafts also had higher levels of these particular enzymes, as well as increased infiltration by FOXP3+ Treg cells, suggesting that self tolerance and allo-tolerance

within tissues may use similar mechanisms that depend on the availability of nutrients to T cells.[62] T-cell activation is primarily associated with glucose metabolism, even under aerobic conditions, as this not only provides a source of ATP for energy and effector cell activity, it generates the precursors for nucleotide synthesis and lipogenesis that are required for cell proliferation.[4] Under conditions of nutrient restriction and mTOR inhibition, however, it would be expected that T cells would switch to the more efficient pathways of ATP generation, such as oxidative phosphorylation and long-chain fatty acid oxidation, both of which require active mitochondria. Indeed, it has been shown that Treg cells have high levels of AMP kinase activity, which leads to mTOR inhibition, reduced levels of Glut1 and preferential lipid oxidation, effects that can be reversed in Glut1 over-expressing transgenic mice.[63] Evidence is now beginning to emerge that the metabolic pathways active in a T-cell are not only a response to activation and differentiation, but can actually be the trigger to determine their differentiation and cell fate.