Infectious tolerance describes the process of CD4 ؉ regulatory T cells (Tregs) converting naïve T cells to become additional Tregs. We show that antigen-specific Tregs induce, within skin grafts and dendritic cells, the expression of enzymes that consume at least 5 different essential amino acids (EAAs). T cells fail to proliferate in response to antigen when any 1, or more, of these EAAs are limiting, which is associated with a reduced mammalian target of rapamycin (mTOR) signaling. Inhibition of the mTOR pathway by limiting EAAs, or by specific inhibitors, induces the Treg-specific transcription factor forkhead box P3, which depends on both T cell receptor activation and synergy with TGF-␤.amino acid catabolism ͉ foxp3 ͉ mTOR inhibitor ͉ regulatory T cells ͉ rapamycin

Pharmacological modulation is known to temper the immune capacity of DC, enhancing the notion that modulated Ag-bearing DC might be used therapeutically to induce tolerance. We have investigated phenotypic features shared by such DC, and queried their potential to tolerize in different settings. Immature, IL-10, TGF-b and 1a,25-dihydroxyvitamin D 3 -modulated BMDC all induced tolerance to male skin in female TCR transgenic A1.RAG mice, and the modulated DC also tolerized after exposure to the TLR4-ligand LPS. Transcript profiling revealed that this was achieved despite retaining much of the normal LPS-maturation response. No shared tolerance-associated transcripts could be identified. Equivalent BMDC could not tolerize in Marilyn TCR-transgenic mice. Simultaneous presentation of both A1.RAG and Marilyn peptide-Ag (Dby-H2E k and Dby-H2A b ) on immature (C57BL/6JxCBA/Ca) F1 BMDC also only achieved tolerance in A1.RAG mice. Both strains registered Ag, but Foxp3 1 Treg were only induced in A1.RAG mice. In contrast, Marilyn T cells showed greater proliferation and an inflammatory bias, in response to Ag presented by immature F1 BMDC in vitro. In summary, while pharmacological agents can skew DC to reinforce their immature tolerogenic phenotype, the outcome of presentation is ultimately an integrated response including T-cell-intrinsic components that can over-ride for immune activation.Key words: DC . TCR transgenic mice . Transplantation tolerance Supporting Information available online IntroductionDespite enthusiasm to exploit tolerogenic DC clinically [1][2][3], what constitutes a tolerogenic DC in vivo, and how it achieves tolerance, is still poorly understood. Immunological outcome was thought to be related directly to the maturation status of DC, with immature DC presenting for tolerance [4][5][6]; however, this simple view rapidly failed to account for accumulating experimental observations [7,8]. The functional diversity of DC has more recently been explained by the integration of multiple factors within a particular anatomical location causing the differentiation of appropriately tuned ''effector'' DC [9,10]. Although key co-inhibitory receptors, such as ILT3/4, PIR-B, PD-L1 and ICOSL, and immunoregulatory molecules, such as IDO and histone deacetylase HDAC11, have been variously implicated in driving tolerance [11][12][13][14][15][16], it is still not clear to what degree such molecules are universal players.A concern using immature DC in vivo is that they might become activated, particularly within the context of inflammation. If DC are to be used therapeutically, then the tolerant phenotype must be robust and stable. Numerous agents have been used to manipulate à These authors contributed equally to this study.Ãà These authors contributed equally to this study as senior authors. 1728DC in vitro to generate maturation-resistant cells for adoptive cell therapy [2,9]; however, attempts to induce tolerance to allografts in conventional models have generally failed unless combined with other immunosuppressive...