Although Tamoxifen injection promoted Ag presentation by only 4–8% of DCs in DIETER mice, it induced robust CD8+ T-cell tolerance that could not be broken by a subsequent LCMV infection. Importantly, the resulting CD8+ T-cell

tolerance was entirely Ag specific, as it did not affect T-cell responses against LCMV epitopes other than the ones expressed by the transgene. This suggested that a T-cell-intrinsic mechanism, such as inactivation or deletion of Ag-specific T cells, rather BTK inhibitor than a dominant mechanism is involved in the induction of peripheral tolerance by steady-state DCs in this model. Indeed, naïve T cells that were adoptively transferred into previously tolerized DIETER mice remained responsive [17]. Negative costimulation through inhibitory cell-surface receptors of the CD28 family Epigenetics Compound Library high throughput seems to be crucial for induction of T-cell tolerance by steady-state DCs. When coinhibitory signaling through programmed cell death 1 (PD1) or CTL protein 4 (CTLA4) was inhibited in DIETER mice, steady-state DCs failed to tolerize T cells, and CTLs were found to be massively primed when both receptors were blocked [17]. These findings demonstrated that PD1 and CTLA4 have nonredundant and complementary functions in T-cell tolerance induction by steady-state DCs. Interestingly, the costimulatory ligands CD80 and

CD86, which engage CTLA4, as well as the PD1 ligands PD-L1 and PD-L2, are expressed to higher levels on activated DCs than on steady-state DCs [18].

Thus, although ligation of PD1 and CTLA4 on T cells is crucial for tolerance induction by steady-state DCs, the expression level of their ligands on DCs does not govern the decision between tolerance and immunity. Another mechanism of induction of cell-intrinsic peripheral tolerance by steady-state DCs involves tryptophan metabolism. The rate-limiting enzyme of tryptophan catabolism indoleamine 2,3-dioxygenase (IDO) is expressed by steady-state DCs. DC-derived IDO promotes T-cell tolerance not only through mechanisms that depend on the catalytic function of IDO — such as local tryptophan depletion [19] and pheromone knyureine production [20] — but also through signaling events that involve IDO but are independent of its catalytic activity [21]. Together these different mechanisms of inducing T-cell intrinsic tolerance allow steady-state DCs to purge the naïve-T-cell repertoire in an Ag-specific manner of autoreactive T cells that have escaped negative selection in the thymus. In addition to promoting T-cell-intrinsic mechanisms of peripheral tolerance, steady-state DCs have been found to be essential for dominant peripheral tolerance, which mainly depends on the function of CD4+FOXP3+ regulatory T (Treg) cells.