T cell tolerance by clonal elimination in the thymus

Epitope recognition in HLA-DR3 transgenic mice immunized to TSH-R protein or peptides

Development of Graves' disease is related to HLA-DR3. The extracellular domain (ECD) of human TSH receptor (hTSH-R) is a crucial antigen in Graves' disease. hTSH-R peptide 37 (amino acids 78-94) is an important immunogenic peptide in DR3 transgenic mice immunized to hTSH-R. This study examined the epitope recognition in DR3 transgenic mice immunized to hTSH-R protein and evaluated the ability of a mutant hTSH-R peptide to attenuate the immunogenicity of hTSH-R peptide 37. DR3 transgenic mice were immunized to recombinant hTSH-R-ECD protein or peptides. A mutant hTSH-R 37 peptide (ISRIYVSIDATLSQLES: 37 m), in which DR3 binding motif position 5 was mutated V>A, and position 8 Q>S, was synthesized. 37 m should bind to HLA-DR3 but not bind T cell receptors. DR3 transgenic mice were immunized to hTSH-R 37 and 37 m. Mice immunized to hTSH-R-ECD protein developed strong anti-hTSH-R antibody, and antisera reacted strongly with hTSH-R peptides 1-5 (20-94), 21 (258-277), 41 (283-297), 36 (376-389), and 31 (399-418). Strikingly, antisera raised to hTSH-R peptide 37 bound to hTSH-R peptides 1-7 (20-112), 10 (132-50), 33 (137-150), 41, 23 (286-305), 24 (301-320), 36, and 31 as well as to hTSH-R-ECD protein. Both antibody titers to hTSH-R 37 and reaction of splenocytes to hTSH-R 37 were significantly reduced in mice immunized to hTSH-R 37 plus 37 m, compared with mice immunized to hTSH-R 37 alone. The ability of immunization to a single peptide to induce antibodies that bind hTSH-R-ECD protein, and multiple unrelated peptides, is a unique observation. Immunogenic reaction to hTSH-R peptide 37 was partially suppressed by 37 m, and this may contribute to immunotherapy of autoimmune thyroid disease.

Elimination of self-reactive T cells in the thymus: a timeline for negative selection

Two-photon microscopy and flow cytometry reveal the timing of thymocyte death and the surprisingly close coupling between cell death and phagocytosis during negative selection in thymic slices.

The elimination of autoreactive T cells occurs via thymocyte apoptosis and removal by thymic phagocytes, but the sequence of events in vivo, and the relationship between thymocyte death and phagocytic clearance, are unknown. Here we address these questions by following a synchronized cohort of thymocytes undergoing negative selection within a three-dimensional thymic tissue environment, from the initial encounter with a negative selecting ligand to thymocyte death and clearance. Encounter with cognate peptide–MHC complexes results in rapid calcium flux and migratory arrest in auto-reactive thymocytes over a broad range of peptide concentrations, followed by a lag period in which gene expression changes occurred, but there was little sign of thymocyte death. Caspase 3 activation and thymocyte loss were first detectable at 2 and 3 hours, respectively, and entry of individual thymocytes into the death program occurred asynchronously over the next 10 hours. Two-photon time-lapse imaging revealed that thymocyte death and phagocytosis occurred simultaneously, often with thymocytes engulfed prior to changes in chromatin and membrane permeability. Our data provide a timeline for negative selection and reveal close coupling between cell death and clearance in the thymus.

As an important safeguard against autoimmunity, T cells bearing autoreactive T cell antigen receptors are eliminated during their development in the thymus, a process known as negative selection. Although much is known about the molecular events involved in negative selection, surprisingly little is known about the dynamic aspects of the process. Here we examine a synchronized population of developing T cells (thymocytes) undergoing negative selection within three-dimensional living thymic tissue. We show that the initial encounter with negative selecting ligands results in migratory arrest, but in spite of this synchronous early response, individual thymocytes then undergo delayed and asynchronous entry into the death program between 2 and 12 hours thereafter. Using time-lapse two-photon imaging, we reveal that thymocyte death and the clearance of the dead cells invariably occur together, with many thymocytes already engulfed by a macrophage before the cell death-related changes in chromatin and membrane permeability are evident. These data provide a timeline of the major events during negative selection, and suggest close coupling between thymocyte death and clearance by macrophages.

Identification of CD3+CD4-CD8- T cells as potential regulatory cells in an experimental murine model of graft-versus-host skin disease (GVHD)

We have developed K14-mOVA transgenic (Tg) mice that express membrane-associated ovalbumin (mOVA) under the control of a K14 promoter as well as double Tg mice by crossing them with OT-I mice that have a T cell receptor (TCR) recognizing OVA peptide. When injected with CD8⁺ OT-I cells, K14-mOVATg mice develop graft-vs-host disease (GvHD), whereas double Tg mice are protected. This suggests that, in double Tg mice, regulatory mechanisms may prevent infused OT-I cells from inducing GvHD. We demonstrated that, after adoptive transfer, TCRαβ⁺CD3⁺CD4^-CD8^-NK1.1^- double negative (DN) T cells are increased in the peripheral lymphoid organs and skin of double Tg mice and exhibit a Vα2⁺Vβ5⁺TCR that is the same TCR specificity as OT-I cells. These DN T cells isolated from tolerant double Tg mice proliferated in response to OVA peptide and produced IFN-γ in the presence of IL-2. These cells could also suppress the proliferation of OT-I cells and were able to specifically kill activated OT-I cells through Fas/Fas ligand interaction. These findings suggest that DN T cells that accumulate in double Tg mice have regulatory functions and may play a role in the maintenance of peripheral tolerance in vivo.

Medullary thymic epithelial cells, the indispensable player in central tolerance

Crosstalk between thymocytes and thymic epithelial cells is critical for T cell development and the establishment of central tolerance. Medullary thymic epithelial cells (mTECs) play important roles in the late stage of T cell development, especially negative selection and Treg generation. The function of mTECs is highly dependent on their characteristic features such as ectopic expression of peripheral tissue restricted antigens (TRAs) and their master regulator-autoimmune regulator (Aire), expression of various chemokines and cytokines. In this review, we summarize the current understanding of cellular and molecular mechanisms of mTEC development and its functions in T cell development and the establishment of central tolerance. The open questions in this field are also discussed. Understanding the function and underlying mechanisms of mTECs will contribute to the better control of autoimmune diseases and the improvement of immune reconstitution during aging or after infection, chemotherapy or radiotherapy.

Modulating T regulatory cells in cancer: how close are we?

Regulatory T cells (Tregs) are a specialized subset of CD4 T cells that have an indispensable role in maintaining immune homeostasis and tolerance. Although studies in mice and humans have clearly highlighted that the absence of these cells results in severe autoimmunity and inflammation, increased Treg numbers and/or function is not always beneficial. This is best exemplified in certain cancers where increased Tregs promote cancer progression by interfering with immune surveillance. Conversely, in other types of cancers that have an inflammatory component, Tregs can inhibit cancer progression by dampening inflammation. In this review article, we provide a historical perspective of the discovery of Tregs, followed by a summary of the existing literature on the role of Tregs in malignancy.

Mixed chimerism through donor bone marrow transplantation: a tolerogenic cell therapy for application in organ transplantation

PURPOSE OF REVIEW

Organ transplantation is the state-of-the-art treatment for end-stage organ failure; however, long-term graft survival is still unsatisfactory. Despite improved immunosuppressive drug therapy, patients are faced with substantial side effects and the risk of chronic rejection with subsequent graft loss. The transplantation of donor bone marrow for the induction of mixed chimerism has been recognized to induce donor-specific tolerance a long time ago, but safety concerns regarding toxicities of current bone marrow transplantation (BMT) protocols impede widespread application.

RECENT FINDINGS

Recent studies in nonhuman primates and kidney transplant patients have demonstrated successful induction of allograft tolerance even though--in contrast to murine models--only transient chimerism was achieved. Progress toward the development of nontoxic murine BMT protocols revealed that Treg therapy is a potent therapeutic adjunct eliminating the need for cytotoxic recipient conditioning. Furthermore, new insight into the mechanisms underlying tolerization of CD4 and CD8 T cells in mixed chimeras has been gained and has identified possible difficulties impeding clinical translation.

SUMMARY

This review will address the recent advances in murine models as well as findings from the first clinical trials for the induction of tolerance through mixed chimerism. Both the potential for more widespread clinical application and the remaining hurdles and challenges of this tolerance approach will be discussed.

Structural basis for self-recognition by autoimmune T-cell receptors

T-cell receptors (TCRs) recognize peptides presented by major histocompatibility complex molecules (pMHC) to discriminate between foreign and self-antigens. Whereas T-cell recognition of foreign peptides is essential for protection against microbial pathogens, recognition of self-peptides by T cells that have escaped negative selection in the thymus can lead to autoimmune disease. Structural studies of autoimmune TCR-pMHC complexes have provided insights into the mechanisms underlying self-recognition and escape from thymic deletion. Two broad categories of self-reactive TCRs can be clearly distinguished: (i) TCRs with altered binding topologies to self-pMHC and (ii) TCRs that bind self-pMHC in the canonical diagonal orientation, but where there are structural defects or suboptimal anchors in the self-ligand. For both categories, however, the overall stability of the autoimmune TCR-pMHC complex is markedly reduced compared to anti-microbial complexes, allowing the autoreactive T cells to evade negative selection, yet retain the ability to be activated by self-antigens in target organs. Additionally, the structures provide insights into TCR cross-reactivity, which can contribute to autoimmunity by increasing the likelihood of self-pMHC recognition. Efforts are now underway to understand the impact of structural alterations in autoimmune TCR-pMHC complexes on higher order assemblies involved in TCR signaling, as well as on immunological synapse formation.

Helios marks strongly autoreactive CD4+ T cells in two major waves of thymic deletion distinguished by induction of PD-1 or NF-κB

Acquisition of self-tolerance in the thymus requires T cells to discriminate strong versus weak T cell receptor binding by self-peptide-MHC complexes. We find this discrimination is reported by expression of the transcription factor Helios, which is induced during negative selection but decreases during positive selection. Helios and the proapoptotic protein Bim were coinduced in 55% of nascent CCR7(-) CD4(+) CD69(+) thymocytes. These were short-lived cells that up-regulated PD-1 and down-regulated CD4 and CD8 during Bim-dependent apoptosis. Helios and Bim were also coinduced at the subsequent CCR7(+) CD4(+) CD69(+) CD8(-) stage, and this second wave of Bim-dependent negative selection involved 20% of nascent cells. Unlike CCR7(-) counterparts, Helios(+) CCR7(+) CD4(+) cells mount a concurrent Card11- and c-Rel-dependent activation response that opposes Bim-mediated apoptosis. This "hollow" activation response consists of many NF-κB target genes but lacks key growth mediators like IL-2 and Myc, and the thymocytes were not induced to proliferate. These findings identify Helios as the first marker known to diverge during positive and negative selection of thymocytes and reveal the extent, stage, and molecular nature of two distinct waves of clonal deletion in the normal thymus.

Recognition of self and altered self by T cells in autoimmunity and allergy

T cell recognition of foreign peptide antigen and tolerance to self peptides is key to the proper function of the immune system. Usually, in the thymus T cells that recognize self MHC + self peptides are deleted and those with the potential to recognize self MHC + foreign peptides are selected to mature. However there are exceptions to these rules. Autoimmunity and allergy are two of the most common immune diseases that can be related to recognition of self. Many genes work together to lead to autoimmunity. Of those, particular MHC alleles are the most strongly associated, reflecting the key importance of MHC presentation of self peptides in autoimmunity. T cells specific for combinations of self MHC and self peptides may escape thymus deletion, and thus be able to drive autoimmunity, for several reasons: the relevant self peptide may be presented at low abundance in the thymus but at high level in particular peripheral tissues; the relevant self peptide may bind to MHC in an unusual register, not present in the thymus but apparent elsewhere; finally the relevant self peptide may be post translationally modified in a tissue specific fashion. In some types of allergy, the peptide + MHC combination may also be fully derived from self. However the combination in question may be modified by the presence of other ligands, such as small drug molecules or metal ions. Thus these types of allergies may act like the post translationally modified peptides involved some types of autoimmunity.

Tumor dormancy and cancer stem cells: two sides of the same coin?

Increasing evidence suggests that tumor dormancy represents an important mechanism underlying the observed failure of existing therapeutic modalities to fully eradicate cancers. In addition to its more established role in maintaining minimal residual disease after treatment, dormancy might also critically contribute to early stages of tumor development and the formation of clinically undetectable micrometastatic foci. There are striking parallels between the concept of tumor dormancy and the cancer stem cell (CSC) theory of tumor propagation. For instance, the CSC hypothesis similarly predicts that a subset of self-renewing cancer cells-that is CSCs-is responsible for tumor initiation, bears the preferential ability to survive tumor therapy, and persists long term to ultimately cause delayed cancer recurrence and metastatic progression. Additionally, many of the biological mechanisms involved in controlling the dormant state of a tumor can also govern CSC behavior, including cell cycle modifications, alteration of angiogenic processes, and modulation of antitumor immune responses. In fact, quiescence and immune escape are emerging hallmark features of at least some CSCs, indicating significant overlap between dormant cancer populations and CSCs. Herein, we crucially dissect whether CSCs occupy specific roles in orchestrating the switch between dormancy and exuberant tumor growth. We elucidate how recently uncovered CSC biological features could enable these cells to evade immunologic clearance and regulate cancer expansion, relapse, and progression. We propose that the study of CSC immunobiological pathways holds the promise to critically advance our understanding of the processes mediating tumor dormancy. Ultimately, such research endeavors could unravel novel therapeutic avenues that efficiently target both proliferating and dormant CSCs to minimize the risk of tumor recurrence in cancer patients.

Immunotherapy in autoimmune type 1 diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease affecting millions of people worldwide. The disease is characterized by the loss of self-tolerance to the insulin-producing β-cells in the pancreas, the destruction of β-cells, and finally the development of chronic hyperglycemia at diagnosis of T1D. Its incidence and prevalence are rising dramatically, highlighting the need for immunotherapeutic strategies able to prevent or treat the disease in a safe and specific manner. Immunotherapeutic strategies are being developed, and aim to restore immunological self-tolerance, thereby limiting unwanted immunity and β-cell destruction. Foxp3+ regulatory T (Treg) cells exert essential functions to maintain and restore immunological self-tolerance. The identification of the transcription factor Foxp3 as the specification factor for the Treg cell lineage facilitated our understanding in the biology of Treg generation and function. This review highlights the current understanding of immunotherapeutic approaches as preventative and curative measures for autoimmune T1D. It includes an overview on early immunointervention studies, which made use of general immunosuppressive agents such as cyclosporin A, followed by a discussion on newly emerging clinical trials. Besides non-antigen-specific therapies, particular attention is given to antigen-specific generation of Foxp3+ Treg cells and their potential use to limit autoimmunity such as T1D.

Treg vaccination with a strong-agonistic insulin mimetope

Foxp3(+) regulatory T (Treg) cells serve as a vital mechanism of negative regulation to maintain immunological self-tolerance thereby suppressing immune-mediated inflammation. The identification of the transcription factor Foxp3 as the specification factor for the Treg cell lineage facilitated our understanding in the biology of Treg generation and function. In the past, we carefully studied the extrathymic conversion of naive CD4(+) T cells into Foxp3(+) expressing Treg cells and found that this process is most efficient upon subimmunogenic supply of strong-agonistic T cell receptor (TCR) ligands avoiding activation of antigen-presenting and T cells. In contrast, weak-agonistic antigens fail to efficiently induce stable Foxp3(+) Treg cells irrespective of the applied dose. Here, we discuss the specific requirements for the establishment of Treg vaccination protocols to interfere with autoimmunity such as Type 1 diabetes.

Apoptosis and inflammation

During the last few decades it has been recognized that cell death is not the consequence of accidental injury, but is the expression of a cell suicide programme. Kerr et al. (1972) introduced the term apoptosis. This form of cell death is under the influence of hormones, growth factors and cytokines, which depending upon the receptors present on the target cells, may activate a genetically controlled cell elimination process. During apoptosis the cell membrane remains intact and the cell breaks into apoptotic bodies, which are phagocytosed. Apoptosis, in contrast to necrosis, is not harmful to the host and does not induce any inflammatory reaction. The principal event that leads to inflammatory disease is cell damage, induced by chemical/physical injury, anoxia or starvation. Cell damage means leakage of cell contents into the adjacent tissues, resulting in the capillary transmigration of granulocytes to the injured tissue. The accumulation of neutrophils and release of enzymes and oxygen radicals enhances the inflammatory reaction. Until now there has been little research into the factors controlling the accumulation and the tissue load of granulocytes and their histotoxic products in inflammatory processes. Neutrophil apoptosis may represent an important event in the control of intlamtnation. It has been assumed that granulocytes disintegrate to apoptotic bodies before their fragments are removed by local macrophages. Removal of neutrophils from the inflammatory site without release of granule contents is of paramount importance for cessation of inflammation. In conclusion, apoptotic cell death plays an important role in inflammatory processes and in the resolution of inflammatory reactions. The facts known at present should stimulate further research into the role of neutrophil, eosinophil and macrophage apoptosis in inflammatory diseases.

Reduced TCR signaling potential impairs negative selection but does not result in autoimmune disease

TCR signal attenuation selectively favors Foxp3 expression and T reg cell lineage commitment.

Negative selection and regulatory T (T reg) cell development are two thymus-dependent processes necessary for the enforcement of self-tolerance, and both require high-affinity interactions between the T cell receptor (TCR) and self-ligands. However, it remains unclear if they are similarly impacted by alterations in TCR signaling potential. We generated a knock-in allele (6F) of the TCR ζ chain gene encoding a mutant protein lacking signaling capability whose expression is controlled by endogenous ζ regulatory sequences. Although negative selection was defective in 6F/6F mice, leading to the survival of autoreactive T cells, 6F/6F mice did not develop autoimmune disease. We found that 6F/6F mice generated increased numbers of thymus-derived T reg cells. We show that attenuation of TCR signaling potential selectively impacts downstream signaling responses and that this differential effect favors Foxp3 expression and T reg cell lineage commitment. These results identify a potential compensatory pathway for the enforcement of immune tolerance in response to defective negative selection caused by reduced TCR signaling capability.

Do MHCII-presented neoantigens drive type 1 diabetes and other autoimmune diseases?

The strong association between particular MHCII alleles and type 1 diabetes is not fully understood. Two ideas that have been considered for many years are that autoimmunity is driven by (1) low-affinity CD4(+) T cells that escape thymic negative selection and respond to certain autoantigen peptides that are particularly well presented by particular MHCII molecules, or (2) CD4(+) T cells responding to neoantigens that are absent in the thymus, but uniquely created in the target tissue in the periphery and presented by particular MHCII alleles. Here we discuss the recent structural data in favor of the second idea. We review studies suggesting that peptide antigens recognized by autoimmune T cells are uniquely proteolytically processed and/or posttranslationally modified in the target tissue, thus allowing these T cells to escape deletion in the thymus during T-cell development. We postulate that an encounter with these tissue-specific neoantigenic peptides presented by the particular susceptible MHCII alleles in the peripheral tissues when accompanied by the appropriate inflammatory milieu activates these T-cell escapees leading to the onset of autoimmune disease.

Persistent infection of thymic epithelial cells with coxsackievirus B4 results in decreased expression of type 2 insulin-like growth factor

It has been hypothesized that a disturbance of central self-tolerance to islet β cells may play a role in the enteroviral pathogenesis of type 1 diabetes. Whether enteroviruses can induce an impaired expression of β-cell self-antigens in thymic epithelial cells has been investigated in a murine thymic epithelial (MTE) cell line. This cell line was permissive to the diabetogenic group B4 coxsackievirus (CV-B4) strain CV-B4 E2 and spontaneously expressed type 2 insulin-like growth factor (Igf2), the dominant self-antigen of the insulin family. In this model, a persistent replication of CV-B4 E2 was obtained, as attested to by the prolonged detection of intracellular positive- and negative-strand viral RNA by reverse transcription-PCR (RT-PCR) and capsid protein VP1 by immunofluorescent staining and by the release of infectious particles in culture supernatants. The chronic stage of the infection was characterized by a low proportion of VP1-positive cells (1 to 2%), whereas many cells harbored enteroviral RNA, as displayed by RT-PCR without extraction applied directly to a few cells. Igf2 mRNA and IGF-2 protein were dramatically decreased in CV-B4 E2-infected MTE cell cultures compared with mock-infected cultures, whereas housekeeping and interleukin-6 (Il6) gene expression was maintained and Igf1 mRNA was decreased, but to a lower extent. Inoculation of CV-B3, CV-B4 JVB, or echovirus 1 resulted in a low level of IGF-2 in culture supernatants as well, whereas herpes simplex virus 1 stimulated the production of the protein. Thus, a persistent infection of a thymic epithelial cell line with enteroviruses like CV-B4 E2 can result in a disturbed production of IGF-2, a protein involved in central self-tolerance toward islet β cells.

Tolerance to MHC class II disparate allografts through genetic modification of bone marrow

Induction of molecular chimerism through genetic modification of bone marrow is a powerful tool for the induction of tolerance. Here we demonstrate for the first time that expression of an allogeneic MHC class II gene in autologous bone marrow cells, resulting in a state of molecular chimerism, induces tolerance to MHC class II mismatched skin grafts, a stringent test of transplant tolerance. Reconstitution of recipients with syngeneic bone marrow transduced with retrovirus encoding H-2I-A^b (I-A^b) resulted the long-term expression of the retroviral gene product on the surface of MHC class II-expressing bone marrow derived cell types. Mechanistically, tolerance was maintained by the presence of regulatory T cells, which prevented proliferation and cytokine production by alloreactive host T cells. Thus, the introduction of MHC class II genes into bone marrow derived cells through genetic engineering results in tolerance. These results have the potential to extend the clinical applicability of molecular chimerism for tolerance induction.

Identification of a human Th1-like IFNγ-secreting Treg subtype deriving from effector T cells

Characteristics and function of effector T-cells with regulatory properties (induced Treg, "iTreg") in humans are ill defined. Here we report that a proportion of activated, initially CD4(+)CD25(-)CD127(+) effector T-cells from human peripheral blood can convert into T-cells with regulatory activity while concomitantly secreting IFNγ. Upon short-term culture in vitro these cells expressed a panel of common Treg markers, including FOXP3, CD25, GITR, HLA-DR and CTLA-4 in parallel with the Th1-specific transcription factor T-bet. Despite their own IFNγ secretion they effectively suppressed IFNγ secretion in effector T cells in parallel with inhibition of their proliferation. Highly purified IFNγ(+)iTreg shared many functional properties with nTreg: Their suppressive activity was antigen-independent, contact-mediated and cytokine-independent. Of note, in contrast to nTreg an inhibitor of TGF-β1 signalling promoted the proliferation of IFNγ(+)iTreg, without abrogating their suppressive function. In addition in vivo in tonsils of patients with chronic tonsillitis an IFNγ-secreting subpopulation of the CD4(+)CD25(-)CD127(+)CD45RA(-) memory T helper cell population was detected, which exhibited regulatory properties as well. Our results support the existence of Th1-like adaptive Tregs in humans that express a robust regulatory phenotype, comparable to nTreg and at the same time share characteristics of Th1 cells. According to our in vitro data IFNγ(+)iTreg can emerge from activated effector T cells and downregulate Th1-mediated immune responses, supporting the hypothesis of effector T cell plasticity as a means for proper initiation and self regulation of inflammatory processes. This report characterizes a new subpopulation of human adaptive regulatory T-cells that derive from effector Th-cells and concomitantly express Th1-specific T-bet and IFNγ with Foxp3.

Allospecific CD4(+) effector memory T cells do not induce graft-versus-host disease in mice

We studied whether allospecific CD4(+) effector memory T cells (T(EM)) could induce graft-versus-host disease (GVHD) using a novel GVHD model induced solely by CD4(+) T cell receptor transgenic TEa cells. Allospecific T(EM) generated in a lymphopenic host bore a typical memory phenotype. Moreover, these cells were able to elicit a faster and more effective proliferative response on challenge with alloantigen in vitro and to mediate "second-set" skin graft rejection in vivo. However, these allospecific T(EM) were unable to induce GVHD. Allospecific T(EM) recipients became tolerant to alloantigen as a result of clonal deletion. Even though allospecific T(EM) were able to respond to alloantigen initially, the expansion of these cells and inflammatory cytokine production during GVHD were dramatically decreased. The inability of allospecific T(EM) to sustain the alloresponse may be a result of enhanced activation-induced cell death. These observations provide insight into how allospecific CD4(+) T(EM) respond to alloantigen during GVHD and underscore the fundamental differences in alloresponses mediated by allospecific T(EM) in graft rejection and GVHD settings.

T-cell tolerance: central and peripheral

Somatic recombination of TCR genes in immature thymocytes results in some cells with useful TCR specificities, but also many with useless or potentially self-reactive specificities. Thus thymic selection mechanisms operate to shape the T-cell repertoire. Thymocytes that have a TCR with low affinity for self-peptide-MHC complexes are positively selected to further differentiate and function in adaptive immunity, whereas useless ones die by neglect. Clonal deletion and clonal diversion (Treg differentiation) are the major processes in the thymus that eliminate or control self-reactive T cells. Although these processes are thought to be efficient, they fail to control self-reactivity in all circumstances. Thus, peripheral tolerance processes exist wherein self-reactive T cells become functionally unresponsive (anergy) or are deleted after encountering self-antigens outside of the thymus. Recent advances in mechanistic studies of central and peripheral T-cell tolerance are promoting the development of therapeutic strategies to treat autoimmune disease and cancer and improve transplantation outcome.

Prolonged acceptance of skin grafts induced by B cells places regulatory T cells on the histopathology scene

The participation of regulatory T (Treg) cells in B cell-induced T cell tolerance has been claimed in different models. In skin grafts, naive B cells were shown to induce graft tolerance. However, neither the contribution of Treg cells to B cell-induced skin tolerance nor their contribution to the histopathological diagnosis of graft acceptance has been addressed. Here, using male C57BL/6 naive B cells to tolerize female animals, we show that skin graft tolerance is dependent on CD25⁺ Treg cell activity and independent of B cell-derived IL-10. In fact, B cells from IL-10-deficient mice were able to induce skin graft tolerance while Treg depletion of the host inhibited 100% graft survival. We questioned how Treg cell-mediated tolerance would impact on histopathology. B cell-tolerized skin grafts showed pathological scores as high as a rejected skin from naive, non-tolerized mice due to loss of skin appendages, reduced keratinization and mononuclear cell infiltrate. However, in tolerized mice, 40% of graft infiltrating CD4⁺ cells were FoxP3⁺ Treg cells with a high Treg:Teff (effector T cell) ratio (6:1) as compared to nontolerized mice where Tregs comprise less than 8% of total infiltrating CD4 cells with a Treg:Teff ratio below 1:1. These results render Treg cells an obligatory target for histopathological studies on tissue rejection that may help to diagnose and predict the outcome of a transplanted organ.

Clonal deletion and the fate of autoreactive thymocytes that survive negative selection

Clonal deletion of autoreactive thymocytes is important for self-tolerance, but the intra-thymic signals that induce clonal deletion have not been clearly identified. We now report that clonal deletion during negative selection requires CD28 costimulation of autoreactive thymocytes at the CD4⁺CD8^lo intermediate stage of differentiation. Autoreactive thymocytes were prevented from undergoing clonal deletion by either absent CD28 costimulation or transgenic over-expression of the anti-apoptotic factors Bcl-2 or Mcl-1, with surviving thymocytes differentiating into anergic T cell receptor αβ⁺ double negative thymocytes that preferentially migrated to the intestine where they re-expressed CD8α and were sequestered as CD8αα intraepithelial lymphocytes (IELs). This study identifies CD28 costimulation as the intrathymic signal required for clonal deletion and identifies CD8αα IELs as the developmental fate of autoreactive thymocytes that survive negative selection.

Historical overview of immunological tolerance

A fundamental property of the immune system is its ability to mediate self-defense with a minimal amount of collateral damage to the host. The system uses several different mechanisms to achieve this goal, which is collectively referred to as the "process of immunological tolerance." This article provides an introductory historical overview to these various mechanisms, which are discussed in greater detail throughout this collection, and then briefly describes what happens when this process fails, a state referred to as "autoimmunity."

Pathogenic CD8 T cells in multiple sclerosis and its experimental models

A growing body of evidence suggests that autoreactive CD8 T cells contribute to the disease process in multiple sclerosis (MS). Lymphocytes in MS plaques are biased toward the CD8 lineage, and MS patients harbor CD8 T cells specific for multiple central nervous system (CNS) antigens. Currently, there are relatively few experimental model systems available to study these pathogenic CD8 T cells in vivo. However, the few studies that have been done characterizing the mechanisms used by CD8 T cells to induce CNS autoimmunity indicate that several of the paradigms of how CD4 T cells mediate CNS autoimmunity do not hold true for CD8 T cells or for patients with MS. Thus, myelin-specific CD4 T cells are likely to be one of several important mechanisms that drive CNS disease in MS patients. The focus of this review is to highlight the current models of pathogenic CNS-reactive CD8 T cells and the molecular mechanisms these lymphocytes use when causing CNS inflammation and damage. Understanding how CNS-reactive CD8 T cells escape tolerance induction and induce CNS autoimmunity is critical to our ability to propose and test new therapies for MS.

Intravenous injection of endogenous microbial components abrogates DSS-induced colitis

BACKGROUND

The etiology of inflammatory bowel diseases (IBD) is largely unknown, but appears to be perpetuated by uncontrolled responses to antigenic components of the endogenous flora. Tolerance to antigenic stimulation can be achieved by exposure to a given antigen in high amounts (high dose tolerance). Colitis induced by feeding of Dextran Sodium Sulfate (DSS) is an often-used animal model mimicking clinical and histological features of human IBD.

AIMS

We investigated whether treatment with high doses of endogenous bacterial components can affect the response to these antigenic components and thus impact the course of the inflammatory response induced by DSS.

METHODS

129/SvEv mice were injected intravenously in the tail vein with lysates prepared from fecal material of conventionally-raised mice. Control mice received a solution of bacterial antigen-free lysates prepared from fecal material of germ-free mice. Seven days later, colitis was induced in these mice by introducing DSS (3.5%) in the drinking water for 5 days. Onset and course of the inflammatory response was monitored by assessment of weight loss. Mice were sacrificed at day 7 post colitis induction and tested for histopathologic injury, intestinal cytokine release, and systemic response to bacterial antigens.

RESULTS

Intravenous injection with fecal lysates reduced intestinal and antigen-stimulated systemic pro-inflammatory cytokine release and prevented DSS-induced weight loss and intestinal injury.

CONCLUSION

Pretreatment with high amount of endogenous bacterial components has a profound tolerogenic effect on the systemic and mucosal immune responses resulting in reduced intestinal inflammation and abrogates colitis-induced weight loss.

Respiratory syncytial virus vaccine development

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract viral disease in infants and young children. Presently, there are no explicit recommendations for RSV treatment apart from supportive care. The virus is therefore responsible for an estimated 160,000 deaths per year worldwide. Despite half a century of dedicated research, there remains no licensed vaccine product. Herein are described past and current efforts to harness innate and adaptive immune potentials to combat RSV. A plethora of candidate vaccine products and strategies are reviewed. The development of a successful RSV vaccine may ultimately stem from attention to historical lessons, in concert with an integral partnering of immunology and virology research fields.

A role for differential variable gene pairing in creating T cell receptors specific for unique major histocompatibility ligands

A limited set of T cell receptor (TCR) variable (V) gene segments are used to create a repertoire of TCRs that recognize all major histocompatibility complex (MHC) ligands within a species. How individual αβTCRs are constructed to specifically recognize a limited set of MHC ligands is unclear. Here we have identified a role for the differential pairing of particular V gene segments in creating TCRs that recognized MHC class II ligands exclusively, or cross-reacted with classical and nonclassical MHC class I ligands. Biophysical and structural experiments indicated that TCR specificity for MHC ligands is not driven by germline-encoded pairwise interactions.Rather, identical TCRβ chains can have altered peptide-MHC (pMHC) binding modes when paired with different TCRα chains. The ability of TCR chain pairing to modify how V region residues interact with pMHC helps to explain how the same V genes are used to create TCRs specific for unique MHC ligands.

Dickkopf-3, an immune modulator in peripheral CD8 T-cell tolerance

In healthy individuals, T cells react against incoming pathogens, but remain tolerant to self-antigens, thereby preventing autoimmune reactions. CD4 regulatory T cells are major contributors in induction and maintenance of peripheral tolerance, but a regulatory role has been also reported for several subsets of CD8 T cells. To determine the molecular basis of peripheral CD8 T-cell tolerance, we exploited a double transgenic mouse model in which CD8 T cells are neonatally tolerized following interaction with a parenchymal self-antigen. These tolerant CD8 T cells have regulatory capacity and can suppress T cells in an antigen-specific manner during adulthood. Dickkopf-3 (DKK3) was found to be expressed in the tolerant CD8 T cells and to be essential for the observed CD8 T-cell tolerance. In vitro, genetic deletion of DKK3 or blocking with antibodies restored CD8 T-cell proliferation and IL-2 production in response to the tolerizing self-antigen. Moreover, exogenous DKK3 reduced CD8 T-cell reactivity. In vivo, abrogation of DKK3 function reversed tolerance, leading to eradication of tumors expressing the target antigen and to rejection of autologous skin grafts. Thus, our findings define DKK3 as a immune modulator with a crucial role for CD8 T-cell tolerance.

Structural basis for the killing of human beta cells by CD8(+) T cells in type 1 diabetes

The structural characteristics of the engagement of major histocompatibility complex (MHC) class II-restricted self antigens by autoreactive T cell antigen receptors (TCRs) is established, but how autoimmune TCRs interact with complexes of self peptide and MHC class I has been unclear. Here we examined how CD8(+) T cells kill human islet beta cells in type 1 diabetes via recognition of a human leukocyte antigen HLA-A*0201-restricted glucose-sensitive preproinsulin peptide by the autoreactive TCR 1E6. Rigid 'lock-and-key' binding underpinned the 1E6-HLA-A*0201-peptide interaction, whereby 1E6 docked similarly to most MHC class I-restricted TCRs. However, this interaction was extraordinarily weak because of limited contacts with MHC class I. TCR binding was highly peptide centric, dominated by two residues of the complementarity-determining region 3 (CDR3) loops that acted as an 'aromatic-cap' over the complex of peptide and MHC class I (pMHCI). Thus, highly focused peptide-centric interactions associated with suboptimal TCR-pMHCI binding affinities might lead to thymic escape and potential CD8(+) T cell-mediated autoreactivity.

Cellular therapies supplement: the role of granulocyte macrophage colony-stimulating factor and dendritic cells in regulatory T-cell homeostasis and expansion

Regulatory T cells are a subset of T cells with inhibitory function that are critical for protection against autoimmunity and immunopathology. A failure to maintain adequate regulatory T-cell numbers in the periphery results in autoimmune manifestations, highlighting the importance of the continuous maintenance of peripheral regulatory T cells. The cellular and molecular requirements for regulatory T-cell homeostasis and expansion are not fully understood but involve a complex interplay among dendritic cells, conventional T cells, and regulatory T cells. In addition, soluble factors such as the cytokine granulocyte macrophage colony-stimulating factor may play a role in enhancing these interactions. In this review, we discuss our National Blood Foundation-funded studies relating to the role of granulocyte macrophage colony-stimulating factor and dendritic cells in controlling regulatory T-cell homeostasis and expansion.

Immunological tolerance during fetal development: from mouse to man

The development of the adaptive immune system has been studied in the mouse primarily because it is easier to access fetal tissues and because there exists a rich array of probes for analysis of various components of the immune system. While much has been learned from this exercise, it is also clear that different species show substantial temporal variation in the development of the immune system during early life. In mice, for instance, mature α/β T cells first appear in the periphery during the final stages of fetal gestation and only increase in number after birth (Friedberg and Weissman, 1974); in humans, on the other hand, the first mature α/β T cells are seen in peripheral tissues at 10-12 gestational weeks (g.w.) and are circulating in significant numbers by the end of the second trimester (Ceppellini et al., 1971; Haynes et al., 1988; Hayward and Ezer, 1974; Kay et al., 1970). Although the functional implications of these differences remain unclear, it is likely that there are significant biological consequences associated with the relatively early development of the peripheral adaptive immune system in humans, for example, with respect to the development of peripheral tolerance as well as to the response to antigens that might cross the placenta from the mother (e.g., cells bearing noninherited maternal alloantigens, infectious agents, food antigens, and the like). Here, we will review studies of immune system ontogeny in the mouse and in humans, and then focus on the possible functional roles of fetal T cell populations during development and later in life in humans.

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.

Extra-thymically induced regulatory T cells: do they have potential in disease prevention?

Fopx3(+) Treg safeguard against autoimmune diseases and immune pathology. The extrathymic conversion of naïve T cells into Foxp3(+) regulatory T cells can be achieved in vivo by the delivery of strong-agonist ligands under subimmunogenic conditions. Tolerogenic vaccination with strong-agonist mimetopes of self-antigen to promote self-antigen specific tolerance may represent the most specific and safest means of preventing autoimmunity. This review discusses the requirements for induction of dominant tolerance exerted by Foxp3(+) Tregs in autoimmunity with special emphasis on their impact to interfere with T1D. The future goals are the understanding of self-non-self discrimination at the cellular and molecular level, which should then enable investigators to develop clinical vaccination protocols that specifically interfere with unwanted immune responses.

The multiple roles of the CD8 coreceptor in T cell biology: opportunities for the selective modulation of self-reactive cytotoxic T cells

Short peptide fragments generated by intracellular protein cleavage are presented on the surface of most nucleated cells bound to highly polymorphic MHCI molecules. These pMHCI complexes constitute an interface that allows the immune system to identify and eradicate anomalous cells, such as those that harbor infectious agents, through the activation of CTLs. Molecular recognition of pMHCI complexes is mediated primarily by clonally distributed TCRs expressed on the surface of CTLs. The coreceptor CD8 contributes to this antigen-recognition process by binding to a largely invariant region of the MHCI molecule and by promoting intracellular signaling, the effects of which serve to enhance TCR stimuli triggered by cognate ligands. Recent investigations have shed light on the role of CD8 in the activation of MHCI-restricted, antigen-experienced T cells and in the processes of T cell selection and lineage commitment in the thymus. Here, we review these data and discuss their implications for the development of potential therapeutic strategies that selectively target pathogenic CTL responses erroneously directed against self-derived antigens.

Affinity maturation of human CD4 by yeast surface display and crystal structure of a CD4-HLA-DR1 complex

Helper T-cell activation generally requires the coreceptor CD4, which binds MHC class II molecules. A remarkable feature of the CD4-MHC class II interaction is its exceptionally low affinity, which ranges from K(D) = ∼200 μM to >2 mM. Investigating the biological role of the much lower affinity of this interaction than those of other cell-cell recognition molecules will require CD4 mutants with enhanced binding to MHC class II for testing in models of T-cell development. To this end, we used in vitro-directed evolution to increase the affinity of human CD4 for HLA-DR1. A mutant CD4 library was displayed on the surface of yeast and selected using HLA-DR1 tetramers or monomers, resulting in isolation of a CD4 clone containing 11 mutations. Reversion mutagenesis showed that most of the affinity increase derived from just two substitutions, Gln40Tyr and Thr45Trp. A CD4 variant bearing these mutations bound HLA-DR1 with K(D) = 8.8 μM, compared with >400 μM for wild-type CD4. To understand the basis for improved affinity, we determined the structure of this CD4 variant in complex with HLA-DR1 to 2.4 Å resolution. The structure provides an atomic-level description of the CD4-binding site on MHC class II and reveals how CD4 recognizes highly polymorphic HLA-DR, -DP, and -DQ molecules by targeting invariant residues in their α2 and β2 domains. In addition, the CD4 mutants reported here constitute unique tools for probing the influence of CD4 affinity on T-cell activation and development.

Mutagenesis of beryllium-specific TCRs suggests an unusual binding topology for antigen recognition

Unconventional Ags, such as metals, stimulate T cells in a very specific manner. To delineate the binding landscape for metal-specific T cell recognition, alanine screens were performed on a set of Be-specific TCRs derived from the lung of a chronic beryllium disease patient. These TCRs are HLA-DP2-restricted and express nearly identical TCR Vβ5.1 chains coupled with different TCR α-chains. Site-specific mutagenesis of all amino acids comprising the CDRs of the TCRA and TCRB genes showed a dominant role for Vβ5.1 residues in Be recognition, with little contribution from the TCR α-chain. Solvent-exposed residues along the α-helices of the HLA-DP2 α- and β-chains were also mutated to alanine. Two β-chain residues, located near the proposed Be binding site of HLA-DP2, played a dominant role in T cell recognition with no contribution from the HLA-DP2 α-chain. These findings suggest that Be-specific T cells recognize Ag using an unconventional binding topology, with the majority of interactions contributed by TCR Vβ5.1 residues and the HLA-DP2 β1-chain. Thus, unusual docking topologies are not exclusively used by autoreactive T cells, but also for the recognition of unconventional metal Ags, such as Be.

Characterization of antigen-binding and MHC class II-bearing T cells with suppressive activity in response to tolerogenic stimulus

Antigen specific non-responsiveness is generally developed through clonal deletion, anergy, and suppression. The term "suppression" is being considered as a functional immune deficit that can be adoptively transferred by regulatory/suppressor T cells. Following tolerance induction protocols the aim of the present study was to characterize the T cells involved in antigen-specific suppression. After defining the immunogenic and tolerogenic protocols in vitro and in vivo, it was shown that CD90(+) cells from tolerogenic mice were able to reduce specific antibody production when adaptively transferred to immunized mice. These cells were shown to highly express CD25 and Foxp3, co-localizing with CD4 and MHC class II antigens (MHCII), while a small percentage of these cells (8-14%) was binding free antigen. Further isolation of CD90(+)MHCII(+) and CD90(+)HSA(+) from mice having received the tolerogenic treatment and adaptive transfer to immunized mice showed that CD90(+)MHCII(+) cells were able to suppress antigen-specific response only when transferred along with the second antigenic challenge, while CD90(+)HSA(+) cells were suppressive only when given along with the first antigenic challenge. The suppressive effect of these two sub-populations could also be obtained in in vitro spleen cell proliferation assays in response to the HSA antigenic stimulus. Both in vitro and in vivo tolerogenic treatments were shown to correlate with soluble MHCII production in culture supernatants or serum respectively. Increase of MHCII in the serum could only be detected upon adaptive transfer of CD90(+)HSA(+) cells, whereas transfer of CD90(+)MHCII(+) cells resulted in increased levels of IL-10 and IFN-γ in the serum. These results defined at least two different levels of suppression, one during the onset which was antigen-specific and MHC restricted, and another non-specific at the end of an immune response.

Immunophysiology and pathology of inflammation in the testis and epididymis

The ability of spermatogenic cells to evade the host immune system and the ability of systemic inflammation to inhibit male reproductive function represent two of the most intriguing conundrums of male reproduction. Clearly, an understanding of the underlying immunology of the male reproductive tract is crucial to resolving these superficially incompatible observations. One important consideration must be the very different immunological environments of the testis, where sperm develop, and the epididymis, where sperm mature and are stored. Compared with the elaborate blood-testis barrier, the tight junctions of the epididymis are much less effective. Unlike the seminiferous epithelium, immune cells are commonly observed within the epithelium, and can even be found within the lumen, of the epididymis. Crucially, there is little evidence for extended allograft survival (immune privilege) in the epididymis, as it exists in the testis, and the epididymis is much more susceptible to loss of immune tolerance. Moreover, the incidence of epididymitis is considerably greater than that of orchitis in humans, and susceptibility to sperm antibody formation after damage to the epididymis or vas deferens increases with increasing distance of the damage from the testis. Although we still know relatively little about testicular immunity, we know less about the interactions between the epididymis and the immune system. Given that the epididymis appears to be more susceptible to inflammation and immune reactions than the testis, and thereby represents the weaker link in protecting developing sperm from the immune system, it is probably time this imbalance in knowledge was addressed.

The golden anniversary of the thymus

The immunological function of the thymus was first documented 50 years ago by using neonatally thymectomized mice, while studying its role in virus-induced leukaemia. Since then, an enormous wealth of reports has helped to define the importance of this primary lymphoid organ. In this article, I summarize the key advances that have led to our current knowledge of the functions of the thymus and its T cells in immunity.

T cell anergy as a strategy to reduce the risk of autoimmunity

Some self-reactive immature T cells escape negative selection in the thymus and may cause autoimmune diseases later. In the periphery, if T cells are stimulated insufficiently by peptide-major histocompatibility complex, they become inactive and their production of cytokines changes, a phenomenon called "T cell anergy". In this paper, we explore the hypothesis that T cell anergy may function to reduce the risk of autoimmunity. The underlying logic is as follows: Since those self-reactive T cells that receive strong stimuli from self-antigens are eliminated in the thymus, T cells that receive strong stimuli in the periphery are likely to be non-self-reactive. As a consequence, when a T cell receives a weak stimulus, the likelihood that the cell is self-reactive is higher than in the case that it receives a strong stimulus. Therefore, inactivation of the T cell may reduce the danger of autoimmunity. We consider the formalism in which each T cell chooses its response depending on the strength of stimuli in order to reduce the risk of autoimmune diseases while maintaining its ability to attack non-self-antigens effectively. The optimal T cell responses to a weak and a strong stimulus are obtained both when the cells respond in a deterministic manner and when they respond in a probabilistic manner. We conclude that T cell anergy is the optimal response when a T cell meets with antigen-presenting cells many times in its lifetime, and when the product of the autoimmunity risk and the number of self-reactive T cells has an intermediate value.

Promyelocytic leukemia zinc finger turns on the effector T cell program without requirement for agonist TCR signaling

Thymocytes expressing the NKT cell semi-invariant αβ TCR are thought to undergo agonist interactions with CD1d ligands prior to expressing promyelocytic leukemia zinc finger (PLZF), a broad complex, tramtrack, bric-a-brac, poxvirus, and zinc finger transcription factor that directs acquisition of the effector program of these innate-like T cells. Whether PLZF can mediate this effector conversion independently of agonist signaling has not been investigated. We demonstrated that transgenic (Tg) expression of PLZF under the CD4 promoter induced the innate effector program in two different MHC class II-restricted TCR-Tg Rag1(-/-) models examined. In CD4 thymocytes expressing a fixed Tg TCR β-chain, the associated TCRα sequences in wild-type and PLZF-Tg mice overlapped extensively, further demonstrating that PLZF could induce the effector program in most CD4 T cells that would normally be selected as naive cells. In contrast, PLZF altered the negative selection of thymocytes expressing TCR β-chains reactive against several retroviral superantigens. Thus, PLZF is remarkable in that it is a transcription factor capable of inducing an effector program in the absence of T cell agonist interactions or cell division. Its expression may also enhance the survival of agonist-signaled thymocytes.

Sertoli cell-specific deletion of the androgen receptor compromises testicular immune privilege in mice

In the mammalian testis, meiotic and postmeiotic germ cell antigens are granted immune privilege. Both local immune suppression and specialized intercellular junctions between somatic Sertoli cells have been proposed to contribute to a highly restricted and effective blood-testis barrier (BTB) that helps maintain tolerance to germ cell antigens. Several studies have suggested that androgens play a role in immune suppression, although direct evidence for this is lacking. We previously reported that Sertoli cell-specific ablation of the androgen receptor (Ar) decreases expression of Cldn3, an androgen-regulated gene and component of Sertoli cell tight junctions, and increases the permeability of the BTB to biotin, a small-molecular-weight tracer. The physiological consequences of Sertoli cell-specific Ar (S-Ar) ablation on immune privilege are unknown. Here we show that in the testes of S-Ar mutant mice, the ultrastructure of Sertoli cell tight junctions is defective and testicular IgG levels are elevated. The interstitium of S-Ar mutant testes becomes populated with macrophages, neutrophils, plasma cells, and eosinophils, and serum samples of mutant mice contain antibodies against germ cell antigens. Together, these results suggest that Sertoli cell-specific deletion of the androgen receptor results in loss of testicular immune privilege. Suppressed levels of androgen signaling may be a contributing factor in idiopathic male infertility.

Transplant tolerance: bench to bedside--26th annual Samuel Jason Mixter Lecture

Ever since the first demonstration that transplant tolerance can be induced in neonatal mice by injection of donor bone marrow cells shortly after birth,1 the establishment of tolerance for organ transplants has been a major goal in the field of transplantation immunology. Indeed, an understanding of tolerance has both theoretical and practical implications for transplantation: from a theoretical point of view, an understanding of tolerance is basic to the requirement for self-/nonself-discrimination in the immune system; from a practical point of view, the induction of tolerance could relieve patients from the need for long-term immunosuppressant medications, thereby avoiding the adverse effects and complications that are a major limitation to the success of clinical transplant at present.

Continuous activation of the CD122/STAT-5 signaling pathway during selection of antigen-specific regulatory T cells in the murine thymus

Signaling events affecting thymic selection of un-manipulated polyclonal natural CD25(+)foxp3(+) regulatory T cells (nTreg) have not been established ex vivo. Here, we report a higher frequency of phosphorylated STAT-5 (pSTAT-5) in nTreg cells in the adult murine thymus and to a lesser extent in the periphery, compared to other CD4(+)CD8(-) subsets. In the neonatal thymus, the numbers of pSTAT-5(+) cells in CD25(+)foxp3(-) and nTreg cells increased in parallel, suggesting that pSTAT-5(+)CD25(+)foxp3(-) cells might represent the precursors of foxp3(+) regulatory T cells. This "specific" pSTAT-5 expression detected in nTreg cells ex vivo was likely due to a very recent signal given by IL-2/IL-15 cytokines in vivo since (i) it disappeared rapidly if cells were left unstimulated in vitro and (ii) was also observed if total thymocytes were stimulated in vitro with saturating amounts of IL-2 and/or IL-15 but not IL-7. Interestingly, STAT-5 activation upon IL-2 stimulation correlated better with foxp3 and CD122 than with CD25 expression. Finally, we show that expression of an endogenous superantigen strongly affected the early Treg cell repertoire but not the proportion of pSTAT-5(+) cells within this repertoire. Our results reveal that continuous activation of the CD122/STAT-5 signaling pathway characterize regulatory lineage differentiation in the murine thymus.

Regulatory T cells in colorectal cancer: from biology to prognostic relevance

Regulatory T cells (Tregs) were initially described as "suppressive" lymphocytes in the 1980s. However, it took almost 20 years until the concept of Treg-mediated immune control in its present form was finally established. Tregs are obligatory for self-tolerance and defects within their population lead to severe autoimmune disorders. On the other hand Tregs may promote tolerance for tumor antigens and even hamper efforts to overcome it. Intratumoral and systemic accumulation of Tregs has been observed in various types of cancer and is often linked to worse disease course and outcome. Increase of circulating Tregs, as well as their presence in mesenteric lymph nodes and tumor tissue of patients with colorectal cancer de facto suggests a strong involvement of Tregs in the antitumor control. This review will focus on the Treg biology in view of colorectal cancer, means of Treg accumulation and the controversies regarding their prognostic significance. In addition, a concise overview will be given on how Tregs and their function can be targeted in cancer patients in order to bolster an inherent immune response and/or increase the efficacy of immunotherapeutic approaches.