In vivo maintenance of T-lymphocyte unresponsiveness induced by thymic medullary epithelium requires antigen presentation by radioresistant cells

Regulation of thymic T regulatory cell differentiation by TECs in health and disease

The thymus produces self-limiting and self-tolerant T cells through the interaction between thymocytes and thymus epithelial cells (TECs), thereby generating central immune tolerance. The TECs are composed of cortical and medullary thymic epithelial cells, which regulate the positive and negative selection of T cells, respectively. During the process of negative selection, thymocytes with self-reactive ability are deleted or differentiated into regulatory T cells (Tregs). Tregs are a subset of suppressor T cells that are important for maintaining immune homeostasis. The differentiation and development of Tregs depend on the development of TECs and other underlying molecular mechanisms. Tregs regulated by thymic epithelial cells are closely related to human health and are significant in autoimmune diseases, thymoma and pregnancy. In this review, we summarize the current molecular and transcriptional regulatory mechanisms by which TECs affect the development and function of thymic Tregs. We also review the pathophysiological models of thymic epithelial cells regulating thymic Tregs in human diseases and specific physiological conditions.

Long-term prevention of chronic allograft rejection by regulatory T-cell immunotherapy involves host Foxp3-expressing T cells

Administration of donor-specific regulatory T cells prevents chronic rejection of BM and skin allografts in the mouse. Injected regulatory T cells induce the emergence of host regulatory T cells with similar specificity thus ensuring persistence of tolerance.

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

Shaping of the autoreactive regulatory T cell repertoire by thymic cortical positive selection

The main function of regulatory T lymphocytes is to keep autoimmune responses at bay. Accordingly, it has been firmly established that the repertoire of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) is enriched in autospecific cells. Differences in thymic-positive and/or -negative selection may account for selection of the qualitatively distinct regulatory and conventional T cell (Tconv) repertoires. It has previously been shown that precursors for Tregs are less sensitive to negative selection than Tconv precursors. Studies with TCR/ligand doubly transgenic mice suggested that an agonist ligand might induce positive selection of Treg (but not Tconv) cells. However, massive deletion of Tconv (but not Treg) cell precursors observed in these mice renders interpretation of such data problematic and a potential role for positive selection in generation of the autospecific Treg repertoire has remained therefore incompletely understood. To study this important unresolved issue and circumvent use of TCR/ligand-transgenic mice, we have developed transgenic mice expressing a single MHC class II/peptide ligand on positively selecting thymic cortical epithelial cells. We found that functional Treg (but not Tconv) cells specific for the single ligand were preferentially selected from the naturally diverse repertoire of immature precursors. Our data therefore demonstrate that thymic cortical positive selection of regulatory and Tconv precursors is governed by distinct rules and that it plays an important role in shaping the autoreactive Treg repertoire.

Anti-CD28 antibody-induced kidney allograft tolerance related to tryptophan degradation and TCR class II B7 regulatory cells

B7/CTLA-4 interactions negatively regulate T-cell responses and are necessary for transplant tolerance induction. Tolerance induction may therefore be facilitated by selectively inhibiting the B7/CD28 pathway without blocking that of B7/CTLA-4. In this study, we selectively inhibited CD28/B7 interactions using a monoclonal antibody modulating CD28 in a rat model of acute kidney graft rejection. A short-term treatment abrogated both acute and chronic rejection. Tolerant recipients presented few alloantibodies against donor MHC class II molecules, whereas untreated rejecting controls developed anti-MHC class I and II alloantibodies. PBMC from tolerant animals were unable to proliferate against donor cells but could proliferate against third-party cells. The depletion of B7+, non-T cells fully restored this reactivity whereas purified T cells were fully reactive. Also, NK cells depletion restored PBMC reactivity in 60% of tolerant recipients. Conversely, NK cells from tolerant recipients dose-dependently inhibited alloreactivity. PBMC anti-donor reactivity could be partially restored in vitro by blocking indoleamine-2,3-dioxygenase (IDO) and iNOS. In vivo, pharmacologic inhibition of these enzymes led to the rejection of the otherwise tolerated transplants. This study demonstrates that an initial selective blockade of CD28 generates B7+ non-T regulatory cells and a kidney transplant tolerance sustained by the activity of IDO and iNOS.

Induction of antigen-specific tolerance to bone marrow allografts with CD4+CD25+ T lymphocytes

Thymus-derived regulatory T lymphocytes of CD4(+)CD25(+) phenotype regulate a large variety of beneficial and deleterious immune responses and can inhibit lethal graft-versus-host disease in rodents. In vitro, CD4(+)CD25(+) T cells require specific major histocompatibility complex (MHC)/peptide ligands for their activation, but once activated they act in an antigen-nonspecific manner. In vivo, regulatory T cells are also activated in an antigen-specific fashion, but nothing is known about antigen specificity of their suppressor-effector function. Here we show that CD4(+)CD25(+) regulatory T lymphocytes isolated from naive mice and activated in vitro with allogeneic antigen-presenting cells (APCs) induced specific long-term tolerance to bone marrow grafts disparate for major and minor histocompatibility antigens; whereas "target" bone marrow was protected, third-party bone marrow was rejected. Importantly, in mice injected with a mix of target and third-party bone marrows, protection and rejection processes took place simultaneously. These results indicate that CD4(+)CD25(+) regulatory T cells can act in an antigen-specific manner in vivo. Our results suggest that CD4(+)CD25(+) regulatory T cells could in the future be used in clinical settings to induce specific immunosuppression.

T-helper cell tolerance to ubiquitous nuclear antigens

Systemic autoimmune diseases are characterized by the development of antinuclear autoantibodies. In order to understand the immunologic events leading to the development of such antibodies, knowledge of mechanisms of immune tolerance to nuclear antigens is required. By utilizing adoptive T-cell transfer strategies with transgenic mouse models expressing nuclear neo-self antigens, T-cell tolerance to the lupus-related nuclear antigens human La and nRNP A has been demonstrated. These findings also indicate the existence in normal animals of autoreactive B cells continuously presenting nuclear antigen, suggesting that nuclear antigens are not sequestered from the immune system. Investigations of CD4+ T-cell tolerance to non-nuclear antigens have revealed a number of mechanisms that protect the host from autoreactivity, including autoreactive T-cell deletion, regulatory T-cell development and anergy induction. Recent studies using T-cell receptor and neo-self nuclear antigen transgenic mice are revealing the importance of such mechanisms in maintaining tolerance to nuclear antigens. Mechanisms of tolerogenic antigen presentation, identification of tolerogenic antigen source(s) and the pathways leading to loss of tolerance to nuclear antigens in systemic autoimmune disease states are currently being sought.