The maintenance of transplantation tolerance induced in adult mice after short-term treatment with nonlytic monoclonal antibodies to CD4 and CD8 was investigated. CD4+ T cells from tolerant mice disabled naïve lymphocytes so that they too could not reject the graft. The naïve lymphocytes that had been so disabled also became tolerant and, in turn, developed the capacity to specifically disable other naïve lymphocytes. This process of "infectious" tolerance explains why no further immunosuppression was needed to maintain long-term transplantation tolerance.

A major aim in immunology has been to understand how the immune system evokes characteristic responses to infection, foreign tissue grafts and tumours. The current view of immunoregulation is based mainly on studies of lymphocyte subsets, either in vitro or by adoptive transfer to irradiated recipients. Many reagents are available for defining T-cell subsets, but only recently have there been helper T-cell-specific antibodies against the mouse equivalent of the Leu3/T4 (man) and W3/25 (rat) antigens. It is clear that monoclonal antibodies will eventually replace antilymphocyte globulin for immunosuppression in organ grafting, but although there has been some clinical success, most monoclonal reagents cause only transient reductions in their target cells in vivo. This uncertainty in the potency of monoclonal antibodies has led some workers to consider them as targeting agents for such highly cytotoxic drugs as ricin A (ref. 21). We show here that unmodified monoclonal antibodies can be extremely effective at depleting cells in vivo and can be used for the selective manipulation of different aspects of the immune response.

Recently, agonist antibodies to glucocorticoid-induced tumor necrosis factor receptor (GITR) (tumor necrosis factor receptor superfamily 18) have been shown to neutralize the suppressive activity of CD4 ؉ CD25 ؉ regulatory T cells. It was anticipated that this would be the role of the physiological ligand. We have identified and expressed the gene for mouse GITR ligand and have confirmed that its interaction with GITR reverses suppression by CD4 ؉ CD25 ؉ T cells. It also, however, provides a costimulatory signal for the antigen-driven proliferation of naïve T cells and polarized T helper 1 and T helper 2 clones. RT-PCR and mAb staining revealed mouse GITR ligand expression in dendritic cells, macrophages, and B cells. Expression was controlled by the transcription factor NF-1 and potentially by alternative splicing of mRNA destabilization sequences.

Induction of transplantation tolerance with certain therapeutic nondepleting monoclonal antibodies can lead to a robust state of peripheral “dominant” tolerance. Regulatory CD4+ T cells, which mediate this form of “dominant” tolerance, can be isolated from spleens of tolerant animals. To determine whether there were any extra-lymphoid sites that might harbor regulatory T cells we sought their presence in tolerated skin allografts and in normal skin. When tolerated skin grafts are retransplanted onto T cell–depleted hosts, graft-infiltrating T cells exit the graft and recolonize the new host. These colonizing T cells can be shown to contain members with regulatory function, as they can prevent nontolerant lymphocytes from rejecting fresh skin allografts, without hindrance of rejection of third party skin. Our results suggest that T cell suppression of graft rejection is an active process that operates beyond secondary lymphoid tissue, and involves the persistent presence of regulatory T cells at the site of the tolerated transplant.

Transplantation tolerance can be induced in mice by grafting under the cover of nondepleting CD4 plus CD8 or CD154 mAbs. This tolerance is donor Ag specific and depends on a population of CD4+ regulatory T cells that, as yet, remain poorly defined in terms of their specificity, origin, and phenotype. Blocking of the Ag-specific response in vitro with an anti-CD4 mAb allowed T cells from monospecific female TCR-transgenic mice against the male Ag Dby, presented by H-2Ek, to express high levels of foxP3 mRNA. foxP3 induction was dependent on TGF-β. The nondepleting anti-CD4 mAb was also able to induce tolerance in vivo in such monospecific TCR-transgenic mice, and this too was dependent on TGF-β. As in conventional mice, acquired tolerance was dominant, such that naive monospecific T cells were not able to override tolerance. Splenic T cells from tolerant mice proliferated normally in response to Ag, and secreted IFN-γ and some IL-4, similar to control mice undergoing primary or secondary graft rejection. High levels of foxP3 mRNA, and glucocorticoid-induced TNFR superfamily member 18 (GITR)+ CD25+ T cells were found within the tolerated skin grafts of long-term tolerant recipients. These data suggest that regulatory T cells maintaining transplantation tolerance after CD4 Ab blockade can be induced de novo through a TGF-β-dependent mechanism, and come to accumulate in tolerated grafts.

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

Diabetogenic T-cells can be detected in pre-diabetic nonobese diabetic (NOD) mice after transfer in NOD-SCID recipients. Here we demonstrate that 6-week-old pre-diabetic NOD mice, >2 months before disease onset, already harbor pathogenic T-cells in equal numbers to overtly diabetic animals. The delay in diabetes appearance is explained by the presence of regulatory CD4 ؉ CD25؉ T-cells that control diabetogenic effectors and that are, in our hands, transforming growth factor (TGF)-␤-dependent. Our present results suggest, however, that diabetes onset is only partly explained by a decline in this regulatory T-cell activity. Another major factor appears to be the progressive resistance of diabetogenic cells to TGF-␤-dependent mediated inhibition. We propose that progression to overt disease correlates with the pathogenic T-cell's escape from TGF-␤-dependent T-cell-mediated regulation.

CD4+CD25+ T cells have been proposed as the principal regulators of both self-tolerance and transplantation tolerance. Although CD4+CD25+ T cells do have a suppressive role in transplantation tolerance, so do CD4+CD25− T cells, although 10-fold less potent. Abs to CTLA-4, CD25, IL-10, and IL-4 were unable to abrogate suppression mediated by tolerant spleen cells so excluding any of these molecules as critical agents of suppression. CD4+CD25+ T cells from naive mice can also prevent rejection despite the lack of any previous experience of donor alloantigens. However, this requires many more naive than tolerized cells to provide the same degree of suppression. This suggests that a capacity to regulate transplant rejection pre-exists in naive mice, and may be amplified in “tolerized” mice. Serial analysis of gene expression confirmed that cells sorted into CD4+CD25+ and CD4+CD25− populations were distinct in that they responded to TCR ligation with very different programs of gene expression. Further characterization of the differentially expressed genes may lead to the development of diagnostic tests to monitor the tolerant state.