[48] This demonstrates that the tolerated, re-transplanted skin graft carried over within it perfectly functional effector T cells, but that FOXP3+ Treg cells were actively blocking their ability to reject and so maintained
the tolerant state within the graft. By studying the changes in gene expression of dendritic cells when they interact with Treg cells,[49, 50] it was found that in addition to the known down-regulation of co-stimulatory ligands and antigen presentation, there was up-regulation of a number of enzymes that either catabolize or use essential amino acids[51] (Fig. 3). In the context of a microenvironment with a restricted availability of nutrients, the local depletion of essential amino acids by these enzymes would buy GW-572016 be an effective mechanism to control the immune response via the mTOR nutrient sensing pathway. It has also been shown that the intracellular availability of leucine and consequently mTOR activation is controlled by T-cell-receptor-induced expression of the neutral amino acid transporter slc7a5 in Th1 and Th2 cells, where it is essential for their activation find more and differentiation, while Treg cells seem not to require this particular transporter.[52] The first example of such amino acid catabolism being able to
control the immune response was the expression of indoleamine 2,3 dioxygenase (IDO) in the placenta during pregnancy, which acts locally to deplete the essential amino acid tryptophan in order to block the maternal immune response to paternal
alloantigens.[53] This tryptophan-depleted microenvironment is sensed by general control non-repressed 2 (GCN2), ADP ribosylation factor which is one of the initiators of the integrated stress response, and leads to a block in the proliferation of CD8 effector T cells,[54] and is required for the survival of T cells, including CD4+ Treg cells, during periods of amino acid starvation.[51] GCN2, however, was not essential for T cells to sense the absence of essential amino acids in vitro,[51] neither is it required for the induction of tolerance to skin grafts in mice by co-receptor blockade (S. Cobbold, E. Adams and H. Waldmann, unpublished results). The induction of FOXP3 by stimulating naive CD4+ T cells in the presence of low doses of TGF-β in vitro was also unaffected by stimulating the GCN2 pathway with histidinol; whereas, inhibition of the mTOR pathway gave a synergistic increase in FOXP3 induction.[51] It has also now been shown that 1-methyltryptophan mediated blocking of IDO and tryptophan sensing can act via mTOR and PKCθ signalling.[55] Indoleamine 2,3 dioxygenase may have been recognized as the first example of immune regulation due to amino acid catabolism because, of all the essential amino acids, tryptophan is thought to be present at the lowest concentration.