Identification of a previously unknown antigen-specific regulatory T cell and its mechanism of suppression

Regulatory T cells in the control of transplantation tolerance and autoimmunity

A role for immunoregulatory T cells in the maintenance of self-tolerance and in transplantation tolerance has long been suggested, but the identification of such cells has not been achieved until recently. With the characterisation of spontaneously occurring CD4+CD25+ and NK1.1+ T subpopulations of T cells as regulatory cells in rodents and in humans, together with several in vitro generated regulatory T-cell populations, it seems possible that 'customised' regulatory cells possessing antidonor specificity may become therapeutic tools in clinical transplantation tolerance.

Regulatory T cells in transplantation tolerance

The identification and characterization of regulatory T (T(Reg)) cells that can control immune responsiveness to alloantigens have opened up exciting opportunities for new therapies in transplantation. After exposure to alloantigens in vivo, alloantigen-specific immunoregulatory activity is enriched in a population of CD4+ T cells that express high levels of CD25. In vivo, common mechanisms seem to underpin the activity of CD4+CD25+ T(Reg) cells in both naive and manipulated hosts. However, the origin, allorecognition properties and molecular basis for the suppressive activity of CD4+CD25+ T(Reg) cells, as well as their relationship to other populations of regulatory cells that exist after transplantation, remain a matter of debate..

A unique population of extrathymically derived alpha beta TCR+CD4-CD8- T cells with regulatory functions dominates the mouse female genital tract

A better understanding of the regulatory role of genital tract T cells is much needed. In this study, we have analyzed the phenotype, distribution, and function of T lymphocytes in the female genital tract of naive, pregnant, or Chlamydia trachomatis-infected C57BL/6 mice. Unexpectedly, we found that the dominant lymphocyte population (70-90%) in the genital tract was that of CD3(+)alphabetaTCR(int)CD4(-)CD8(-) T cells. Moreover, these cells were CD90(low) but negative for the classical T cell markers CD2 and CD5. The CD3(+)B220(low) cells were NK1.1 negative and found in nude mice as well as in mice deficient for MHC class II, beta(2)-microglobulin, and CD1, indicating extrathymic origin. They dominated the KJ126(+)Vbeta8.2(+) population in the genital tract of DO11.10 OVA TCR-transgenic mice, further supporting the idea that the CD3(+)B220(low) cells are truly T cells. The function of these T cells appeared not to be associated with immune protection, because only CD4(+) and CD8(+) T cells increased in the genital tract following chlamydial infection. Notwithstanding this, the infected, as well as the uninfected and the pregnant, uterus was dominated by a high level of the CD3(+)CD4(-)CD8(-)B220(low) cells. Following in vitro Ag or polyclonal stimulation of the CD3(+)CD4(-)CD8(-)B220(low) cells, poor proliferative responses were observed. However, these cells strongly impaired splenic T cell proliferation in a cell density-dependent manner. A large fraction of the cells expressed CD25 and produced IFN-gamma upon anti-CD3 plus anti-CD28 stimulation, arguing for a strong regulatory role of this novel T cell population in the mouse female genital tract.

Role of double-negative regulatory T cells in long-term cardiac xenograft survival

A novel subset of CD3(+)CD4(-)CD8(-) (double negative; DN) regulatory T cells has recently been shown to induce donor-specific skin allograft acceptance following donor lymphocyte infusion (DLI). In this study, we investigated the effect of DLI on rat to mouse cardiac xenotransplant survival and the ability of DN T cells to regulate xenoreactive T cells. B6 mice were given either DLI from Lewis rats, a short course of depleting anti-CD4 mAb, both DLI and anti-CD4 treatment together, or left untreated. DLI alone did not prolong graft survival when compared with untreated controls. Although anti-CD4-depleting mAb alone significantly prolonged graft survival, grafts were eventually rejected by all recipients. However, the combination of DLI and anti-CD4 treatment induced permanent cardiac xenograft survival. We demonstrate that recipients given both DLI and anti-CD4 treatment had a significant increase in the total number of DN T cells in their spleens when compared with all other treatment groups. Furthermore, DN T cells harvested from the spleens of DLI plus anti-CD4-treated mice could dose-dependently inhibit the proliferation of syngeneic antidonor T cells. Suppression mediated by these DN T cells was specific for antidonor T cells as T cells stimulated by third-party Ags were not suppressed. These results demonstrate for the first time that a combination of pretransplant DLI and anti-CD4-depleting mAb can induce permanent survival of rat to mouse cardiac xenografts and that DN T regulatory cells play an important role in preventing long-term concordant xenograft rejection through the specific suppression of antidonor T cells.

Development of CD4+ T cell lines that suppress an antigen-specific immune response in vivo

It has been suggested for many years that the regulation of the immune system for the maintenance of peripheral tolerance may involve regulatory/suppressor T cells. In the past few years, several investigators have demonstrated that these cells can be generated in vitro. It has also been shown that they can inhibit the progression of various autoimmune disease models when infused into susceptible mice. We have generated two murine T cell lines in the presence of KLH-specific T cell clones from BALB/c or DBA2 mice. The lines are characterized by a low proliferative response to mitogens, the capacity to secrete high amounts of IL-10 and TGF-beta, and small amounts of IFN-gamma. Interestingly, these cells are unable to produce IL-2, IL-4 or IL-5. The study of the surface phenotype of both lines revealed CD4+, CD25high, CD44low and CTLA-4- cells. When injected intravenously in (CBy.D2) F1 mice, these cells were able to inhibit 50-100% of the TNP-specific antibody production, when the hapten was coupled to KLH. In the present study we offer another evidence for the existence of regulatory T cells in the T lymphocyte repertoire, suggesting that they can also regulate immune responses to foreign antigens. Furthermore, we demonstrate an alternative pathway to generate these cells different from approaches used thus far.

T-regulatory cells: are we re-discovering T suppressors?

Regulatory T cells are shown to originate form the thymus and their role is to maintain self-tolerance to intra-thymic as well as extra-thymic self-antigens. Their mode of action, using in vivo and in vitro systems, has led to different conclusions as to the need of cell-cell interactions or regulation upon suppressive cytokines. The more we study regulatory T cells the more we find similarities to the old notion of the suppressor T cell network. The limited knowledge in molecular technology in the early 70s and 80s discouraged investigators to further scrutinize the issue and the terms T suppressors and contra-suppressors that were coined back then have been forgotten over the years. It is now time to remember the work of these investigators and attempt to explain their findings using the current knowledge and technology.

Donor-lymphocyte infusion induces transplantation tolerance by activating systemic and graft-infiltrating double-negative regulatory T cells

Donor-lymphocyte infusion (DLI) before transplantation can lead to specific tolerance to allografts in mice, nonhuman primates, and humans. We and others have demonstrated a role for regulatory T cells in DLI-induced, donor-specific transplantation tolerance, but it is not known how regulatory T cells are activated and where they execute their function. In this study, we observed, in both transgenic and normal mice, that DLI before transplantation is required for activation of alphabeta-T-cell-receptor-positive, CD3(+)CD4(-)CD8(-) double-negative (DN) regulatory T cells in the periphery of recipient mice. More interestingly, DLI induced DN regulatory T cells to migrate preferentially to donor-specific allogeneic skin grafts and to form a majority of graft-infiltrating T cells in accepted skin allografts. Furthermore, both recipient-derived peripheral and graft-infiltrating DN T cells were able to suppress and kill antidonor CD8(+) T cells in an antigen-specific manner. These data indicate that DLI may induce donor-specific transplantation tolerance by activating recipient DN regulatory T cells in the periphery and by promoting migration of regulatory T cells to donor-specific allogeneic skin grafts. Our results also show that DN regulatory T cells can eliminate antidonor T cells both systemically and locally, a finding suggesting that graft-infiltrating T cells can be beneficial to graft survival.

The role of CD8+ T cells during allograft rejection

Organ transplantation can be considered as replacement therapy for patients with end-stage organ failure. The percent of one-year allograft survival has increased due, among other factors, to a better understanding of the rejection process and new immunosuppressive drugs. Immunosuppressive therapy used in transplantation prevents activation and proliferation of alloreactive T lymphocytes, although not fully preventing chronic rejection. Recognition by recipient T cells of alloantigens expressed by donor tissues initiates immune destruction of allogeneic transplants. However, there is controversy concerning the relative contribution of CD4+ and CD8+ T cells to allograft rejection. Some animal models indicate that there is an absolute requirement for CD4+ T cells in allogeneic rejection, whereas in others CD4-depleted mice reject certain types of allografts. Moreover, there is evidence that CD8+ T cells are more resistant to immunotherapy and tolerance induction protocols. An intense focal infiltration of mainly CD8+CTLA4+ T lymphocytes during kidney rejection has been described in patients. This suggests that CD8+ T cells could escape from immunosuppression and participate in the rejection process. Our group is primarily interested in the immune mechanisms involved in allograft rejection. Thus, we believe that a better understanding of the role of CD8+ T cells in allograft rejection could indicate new targets for immunotherapy in transplantation. Therefore, the objective of the present review was to focus on the role of the CD8+ T cell population in the rejection of allogeneic tissue.

Allograft acceptance despite differential strain-specific induction of TGF-beta/IL-10-mediated immunoregulation

We examined the immune approaches that C57BI/6 and BALB/c mice take when treated to accept cardiac allografts. C57BI/6 mice accept DBA/2 cardiac allografts when treated with gallium nitrate (GN) or anti-CD40L mAb (MR1). These allograft acceptor mice fail to mount donor-reactive delayed type hypersensitivity (DTH) responses, and develop a donor-induced immunoregulatory mechanism that inhibits DTH responses. In contrast, BALB/c mice accept C57BI/6 cardiac allografts when treated with MR1 but not with GN. These allograft acceptor mice display modest donor-reactive DTH responses, and do not develop donor-induced immune regulation of DTH responses. Real-time PCR analysis of rejecting graft tissues demonstrated no strain-related skewing in the production of cytokines mRNAs. In related studies, C57BI/6 recipients of cytokine and alloantigen educated syngeneic peritoneal exudate cells (PECs) failed to mount DTH responses to the alloantigens unless neutralizing antibodies to transforming growth factor-beta (TGF-p were present at the DTH site demonstrating regulation of cell-mediated alloimmune responses. In contrast, BALB/c recipients of cytokine-and alloantigen-educated PECs expressed strong DTH responses to alloantigens demonstrating a lack of regulated alloimmunity. In conclusion, C57BI/6 mice respond to immunosuppression by accepting cardiac allografts and generating TGF-beta-related regulation of donor-reactive T cell responses, unlike BALB/c mice that do not generate these regulatory responses yet still can accept cardiac allografts.

The nature and mechanisms of DN regulatory T-cell mediated suppression

Regulatory T cells have been reported to enhance survival of transplanted allografts. We have recently identified and cloned a novel CD3(+)CD4(-)CD8(-) (double negative, DN) regulatory T cell from mice that were given a single class I mismatched donor lymphocyte infusion and permanently accepted donor-specific skin allografts. When infused into naïve syngeneic mice, these DN T cells prolonged the survival of class I mismatched donor skin allografts. Here we further characterize the nature and mechanism of DN T-cell mediated suppression. This present study reveals that DN T cells are able to specifically eliminate activated syngeneic CD8(+) T cells that share the same T cell receptor (TCR) specificity as DN T cells in vitro. Similarly, we found that, along with an increase of recipient DN T cells in the peripheral blood, anti-donor CD8(+) T cells were also eliminated in vivo following a donor lymphocyte infusion. We further demonstrate that DN T regulatory cells do not mediate suppression by competition for growth factors or antigen presenting cells (APC) nor by modulation of APC, but require cell contact with the activated target CD8(+) T cells. This contact can be mediated either by the TCR on CD8(+) T cells that recognize constitutively expressed or acquired MHC molecules on DN T cells, or by the TCR on DN T cells that recognize constitutively expressed MHC molecules on CD8(+) T cells. Together, these data extend our previous findings, and expand the conditions in which DN T cells can potentially be used to specifically suppress allogeneic immune responses.

Regulatory T cells in transplantation

There has recently been an explosion of renewed interest in regulatory T cells, particularly those within the CD4(+)CD25(+) population. It is becoming increasingly apparent that these cells exist not only as naturally occurring cells that may contribute to the maintenance of self-tolerance, but they also have the potential to prevent rejection of allografts in experimental models. Such cells have now been identified in humans as well as in rodents.

Insights into the immunopathogenesis of multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS). Significant progress has been made in our understanding of the etiology of MS. MS is widely believed to be an autoimmune disease that results from aberrant immune responses to CNS antigens. T cells are considered to be crucial in orchestrating an immunopathological cascade that results in damage to the myelin sheath. This review summarizes the currently available data supporting the idea that myelin reactive T cells are actively involved in the immunopathogenesis of MS. Some of the therapeutic strategies for MS are discussed with a focus on immunotherapies that aim to specifically target the myelin reactive T cells.

The immune regulatory function of lymphoproliferative double negative T cells in vitro and in vivo

Lymphoproliferative (lpr) mice, which lack functional Fas receptor expression and develop autoimmune lymphoproliferative disease, have an accumulation of T cell receptor-alphabeta(+)CD4(-)CD8(-) (double negative T cells [DNTC]) in the periphery. The function of the accumulating DNTC is not clear. In this study we demonstrate that B6/lpr DNTC can dose dependently kill syngeneic CD8(+) and CD4(+) T cells from wild-type B6 mice through Fas/Fas ligand interactions in vitro. We also demonstrate that B6/lpr DNTC that are activated and expand in vivo are able to specifically down-regulate allogeneic immune responses mediated by syngeneic Fas(+)CD4(+) and CD8(+) T cells in vivo. B6/lpr DNTC that have been preactivated in vivo by infusion of either class I- (bm1) or class II- (bm12) mismatched allogeneic lymphocytes are able to specifically enhance the survival of bm1 or bm12, but not third-party skin allografts when adoptively transferred into naive B6(+/+) mice. These findings clearly demonstrate that B6/lpr DNTC have a potent immune regulatory function in vitro and in vivo. They also provide new insights into the mechanisms involved in the development of autoimmune disease in lpr mice.

Linked unresponsiveness: early cytokine gene expression profiles in cardiac allografts following pretreatment of recipients with bone marrow cells expressing donor MHC alloantigen

Linked unresponsiveness operates to induce specific unresponsiveness to fully mismatched vascularized allografts in recipients pretreated with anti-CD4 antibody and syngeneic bone marrow cells expressing a single donor MHC class I alloantigen. The aim of the study was to evaluate early post transplant cytokine expression in allografts where linked unresponsiveness was required for long term graft survival. CBA (H2(k)) mice were pretreated with CBK (H2(k)+K(b)) bone marrow cells under the cover of anti-CD4 antibody 28 days before transplantation of a CBK or a C57BL/10 (H2(b)) cardiac allograft. In both cases graft survival was prolonged (MST=100 days). Intragraft expression for interferon (IFN)-gamma, interleukin (IL)-2, IL-4, IL-10, IL-12(p40), IL-18, iNOS, transforming growth factor (TGF)-beta(1) and C-beta was determined on day 1.5, 3, 7 and 14 after transplantation. Whereas rejecting allografts displayed a sharp peak in IFN-gamma, IL-2, IL-4 and IL-10 expression, non-rejecting allografts were characterized by an initial TGF-beta(1) and IFN-gamma production. An increasing IL-4 expression towards day 14 was a unique feature of linked unresponsiveness. All non-rejecting allografts were characterized by an increasing IL-12(p40) production towards day 14. In summary, the early cytokine expression pattern in allografts after bone marrow induced operational tolerance is influenced by the quantity of donor alloantigens expressed on the graft as well as on the bone marrow inoculum.

A composite picture of TcR alpha/beta(+) CD4(-)CD8(-) T Cells (alpha/beta-DNTCs) in humans with autoimmune lymphoproliferative syndrome

The discovery of an unusual T-cell subset characterized by the expression of the alpha/beta T-cell receptor without expression of either CD4 or CD8 [alpha/beta-double-negative T cells (alpha/beta-DNTCs)] provided critical insights in the evaluation of a "new" lymphoproliferative disorder known as autoimmune lymphoproliferative syndrome (ALPS). ALPS is a disorder of defective Fas-mediated lymphocyte apoptosis, manifested by accumulation of alpha/beta-DNTCs and other lymphocyte subsets, leading to lymphadenopathy and splenomegaly, autoimmunity, and an increased risk of lymphoma. The expanded population of alpha/beta-DNTCs from ALPS patients has a remarkable uniform phenotype that is for the most part similar to alpha/beta-DNTCs from mice with defective Fas (lpr) or Fas ligand (gld). This is in contrast to the minor alpha/beta-DNTC compartment in healthy individuals that contains multiple, immunophenotypically distinct subpopulations. Current data indicate that alpha/beta-DNTCs from ALPS patients are derived from cytotoxic CD8(+) T cells, chronically activated in vivo but anergic in vitro. Their anergic state may be related to persistent modifications of O-linked carbohydrates on cell surface molecules, such as CD43 and CD45, as well as to the increased presence of interleukin-10. Although largely consistent with a model of (linear) CD8(+) cytotoxic T-cell differentiation, the expression patterns of certain surface molecules, such as CD27 and CD28, are not consistent with this model. This may be the result of the perturbed homeostasis of lymphocytes in ALPS, thereby revealing pathways of differentiation and immunophenotypes, including phenotypes pertaining to cell surface glycosylation that are hidden from view in healthy individuals.

Acquisition of functional MHC class II/peptide complexes by T cells during thymic development and CNS-directed pathogenesis

This study provides evidence that both rat and mouse thymic and splenic T cells express significant levels of MHC class II glycoproteins (MHCII) in vivo. Derivation of rat and mouse chimeras revealed that a major source of MHCII on thymic T cells was acquired from radioresistant host APC. Expression of MHC on thymic T cells appeared physiologically relevant because presentation of rat myelin basic protein (RMBP) by nonadherent, radiosensitive thymic T cells was associated with the adoptive transfer of tolerance. Mature MBP-specific effector T cells isolated from the CNS in both rat and mouse models of EAE also expressed significant levels of MHCII. Adoptive transfer of activated B10.PL MBP/I-A(u)-restricted TCR transgenic T cells into F1(C57BL/6 x B10.PL) mice revealed acquisition of allogeneic I-A(b) on encephalitogenic CNS-derived T cells. Overall, this study indicates that immature and mature T cells in rats and mice acquire functional MHCII in vivo during thymic development and pathogenic inflammation.

Inability to induce tolerance through direct antigen presentation

Both the direct and indirect antigen presentation pathways are important mechanisms for T cell-mediated allograft rejection. Studies using knockout mice and monoclonal antibodies have demonstrated that CD4+ T cells are both necessary and sufficient for the rejection of allogeneic tissues, including skin, heart, and islet. Furthermore, combined blockade of the CD28/B7 and CD154/CD40 costimulatory pathways induces tolerance in multiple CD4+ T-cell dependent allograft models. In this study, we addressed the T-cell requirement for costimulation in direct antigen presentation. We demonstrated that class II-specific alloreactive T-cell receptor transgenic T cells were sufficient to mediate allograft rejection independent of costimulatory blockade. Analysis of the costimulatory capacity of different antigen presenting cell (APC) populations demonstrated that APCs resident within the donor skin, Langerhans cells, are potent stimulators not requiring CD28- or CD154-dependent costimulation for direct major histocompatibility complex (MHC) antigen presentation. These results complement previous work examining the role of costimulation on CD8+ T cells, supporting a model in which the effectiveness of costimulatory blockade in the setting of transplantation may be selective for the indirect pathway of MHC alloantigen presentation.

T-cell signalling and autoimmunity: molecular mechanisms of disease

The genetic manipulation of mice has led to insights into the molecular mechanisms of autoimmune disease. Recent studies have begun to identify ways in which signalling cascades can be disrupted that preclude the development of autoimmunity. This review outlines a new model for the induction of T-cell-mediated autoimmune diseases. I highlight recent data that illustrate the ways in which the altered survival of T cells and defects in the inhibitory signalling pathways of T cells can contribute to autoimmunity.

Role for thymic and splenic regulatory CD4+ T cells induced by donor dendritic cells in allograft tolerance by LF15-0195 treatment

A 20-day treatment with LF15-0195, a deoxyspergualine analogue, induced allograft tolerance in a fully MHC-mismatched heart allograft model in the rat. Long-term allografts displayed minimal cell infiltration with no signs of chronic rejection. CD4+ spleen T cells from tolerant LF15-0195-treated recipients were able to suppress in vitro proliferation of allogeneic CD4+ T cells and to transfer tolerance to second syngeneic recipients, demonstrating dominant suppression by regulatory cells. A significant increase in the percentage of CD4+CD25+ T cells was observed in the thymus and spleen from tolerant LF15-0195-treated recipient. In vitro direct stimulation with donor APCs demonstrated that CD4+ regulatory T cells proliferated weakly and expressed low levels of IFN-gamma, IL-10, and IL-2. CD4+CD25+ cell depletion increased IL-2 production by CD4+CD25- thymic cells, but not splenic cells. Moreover, tolerance was transferable with splenic and thymic CD4+CD25+ cells, but also in 50% of cases with splenic CD4+CD25- cells, demonstrating that CD25 can be a marker for regulatory cells in the thymus, but not in the periphery. In addition, we presented evidences that donor APCs were required to induce tolerance and to expand regulatory CD4+ T cells. This study demonstrates that LF15-0195 treatment induces donor APCs to expand powerful regulatory CD4+CD25+/- T cells present in both the central and peripheral compartments.

Longitudinal study of antimyelin T-cell reactivity in relapsing-remitting multiple sclerosis: association with clinical and MRI activity

In multiple sclerosis (MS), T-cells are considered to be critical in coordinating an immunopathological cascade that results in myelin damage. We investigated whether clinical disease activity or brain inflammatory activity as measured by magnetic resonance imaging (MRI) was associated with changes in autoreactive T-cell reactivities in MS patients. To this end, a longitudinal study was performed in which T-cell-related immune parameters and clinical parameters (including MRI) were monitored in seven relapsing-remitting (RR) MS patients and two healthy controls with bimonthly intervals over a period of 18 months. The serial evaluation of antimyelin (MBP, PLP, MOG) T-cell responses revealed highly dynamic shifts and fluctuations from one pattern to another in a patient-dependent manner. In some of the patients, changes in T-cell-related immune variables were found to concur with MRI activity and generally preceded clinical relapses. These alterations include: increased number of myelin-reactive IFN-gamma secreting T-cells, detection of clonally expanded myelin-reactive T-cells, elevated proinflammatory and decreased antiinflammatory cytokine production, upregulation of ICAM-1 membrane expression and highly increased serum levels of soluble VCAM-1. However, not all exacerbations and MRI changes were associated with changes in antimyelin reactivity. Some of the observed immune alterations were also detected in the healthy controls, indicating that additional regulatory mechanisms-which may be defective in MS-play a role in the downregulation of potentially pathological T-cell responses. In conclusion, this study provides further support for an important role of myelin-reactive T-cells in the pathogenesis of MS. In addition, the observed dynamic changes in the antimyelin T-cell reactivity pattern may be a major obstacle for the development of antigen-specific immunotherapies.

The puzzling world of murine T regulatory cells

T regulatory cells are essential for downregulation of undesired immune responses and prevention of autoimmune diseases, organ rejection, and graft versus host disease. This review describes the considerable progress made in the recent years in the characterization of the many subsets that constitute the puzzled world of murine T regulatory cells.

Intercellular transfer of antigen-presenting cell determinants onto T cells: molecular mechanisms and biological significance

Upon physiological stimulation, receptors with tyrosine kinase activity (RTK) are rapidly internalized together with their soluble ligands. T cell activation is the consequence of recognition by the T cell receptor (TCR) of specific peptide-major histocompatibility protein complexes (peptide-MHC) present at the membrane of antigen-presenting cells (APC). The TCR belongs to the RTK family and is known to be endocytosed upon ligand recognition. It differs from most other RTK in that its ligand, the peptide-MHC complex, is membrane bound and the TCR-ligand interaction is quite weak. Recent experiments have shown that the TCR ligand becomes internalized by T cells upon stimulation. Here we review current knowledge on the molecular mechanisms by which the membrane-bound MHC molecules can be transferred onto T cells, and propose hypotheses on the role this phenomenon could play in physio-pathological situations involving T cells.

Interactions between double positive thymocytes and high affinity ligands presented by cortical epithelial cells generate double negative thymocytes with T cell regulatory activity

Previous studies on thymocyte differentiation by using reaggregate cultures (RC) of double positive T cell receptor (TCR) transgenic thymocytes and the thymic epithelial cell line ANV indicated that low concentrations of high affinity ligands for the TCR were efficient inducers of thymocyte maturation to CD4 single positive (SP) functional cells. In this study, it is demonstrated that, when high concentrations of high affinity ligands are used in this RC system, double positive (DP) cells down-modulate expression of both coreceptors and that, as a result, large numbers of double negative (DN) cells are generated. These DN cells proliferated modestly in response to stimulation by antigen, and this response was considerably augmented by the addition of IL-2 to the cultures. Notably, these antigen-stimulated DN cells produced large amounts of IL-10. When the DN cells generated in RC were cocultured with naive TCR transgenic T cells in the presence of antigen, they suppressed the proliferative response of the naive T cells. Thus, high affinity ligands, when presented to DP thymocytes by cortical thymic epithelial cells in reaggregate cultures, rather than causing deletion of the immature thymocytes, induce their differentiation into immunoregulatory DN cells, suggesting a distinct mechanism by which self tolerance may be maintained.

Collagen II-pulsed antigen-presenting cells genetically modified to secrete IL-4 down-regulate collagen-induced arthritis

We explored the possibility that pulsed antigen-presenting cells (APC) provide a model vector system for site-specific delivery of immunosuppressive proteins during collagen-induced arthritis (CIA), an animal model for rheumatoid arthritis. Thus, mice were treated with either B cells or macrophages engineered to secrete IL-4 and loaded (or not) with type II collagen (CII). Systemic injection of an IL-4-producing B cell hybridoma resulted in a reduction of arthritis severity which was further improved when APC were incubated with CII before their transfer. Unmanipulated B cells loaded with CII also exerted a potent suppressive effect. Likely, clinical amelioration was observed in mice given at priming syngeneic bone marrow-derived macrophages producing IL-4 and pulsed with CII in comparison to the other groups. When the same dose of cells was transferred at disease onset, a moderate beneficial effect was observed. Whatever the APC inoculated, the beneficial effect did not rely upon an IL-4-driven shift towards Th2 phenotype. Systemic administration of fluorescent dye labeled macrophages to arthritic mice has shown that some of these cells rapidly migrate to joints. Moreover, IL-4 transfected macrophages retained their potent capacity to present CII peptides to T cells. These findings validate the use of CII peptide-loaded engineered APC as therapeutic vector cells in CIA and allow consideration of this strategy for the administration of various anti-inflammatory proteins.

TCR/self-antigen interactions drive double-negative T cell peripheral expansion and differentiation into suppressor cells

Mature CD4-CD8- alphabeta+ T cells (DNTC) in the periphery of TCR transgenic mice are resistant to clonal deletion in cognate Ag-expressing (Ag+) mice. Previously, we have characterized DNTC populations bearing the alloreactive 2C TCR in Ag-free (Ag-) and Ag+ mice. Despite appearing functionally anergic when challenged with cognate Ag in vitro, Ag-experienced DNTC exhibit markers of activation/memory, a lowered threshold of activation, ex vivo cytolytic activity, and the ability to rapidly secrete IFN-gamma. Remarkably, these memory-like DNTC also possess potent immunoregulatory properties, competing effectively for bystander-produced IL-2 and suppressing autoreactive CD8+ T cell proliferation via a Fas/FasL-dependent cytolytic mechanism. The fact that DNTC recovered from Ag+ mice possess markers and attributes characteristic of naive CD8+ T cells that have undergone homeostasis-induced proliferation suggested that they may be derived from a similar peripheral expansion process. Naive DNTC adoptively transferred into Ag-bearing hosts rapidly acquire markers and functional attributes of DNTC that have continually developed in the presence of Ag. Thus, the peripheral selection and maintenance of such autoreactive cells may serve to negatively regulate potential autoimmune T cell responses.

Intercellular exchange of class II MHC complexes: ultrastructural localization and functional presentation of adsorbed I-A/peptide complexes

Activated rat T cells, like human T cells, synthesize class II MHC glycoproteins (MHCII) and absorb MHCII from neighboring T cells. This study focused on interactions of myelin basic protein (MBP)-specific T cells that either synthesized MHCII or absorbed MHCII during activation to assess cellular structures associated with presentation of functional MHCII/peptide complexes. Synthesis of MHCII by CD4(+)TCR(+) T cells involved I-A(+) multivesicular MHC class II-like compartments (MIIC), release of MHCII(+) vesicles, and expression of MHCII on a dendritic arborization. T-cell-mediated adsorption of MHCII was a saturable process that required close cell proximity, actin polymerization, and a permissive temperature. Adsorbed MHCII existed on vesicles that were intimately associated with the responder cell membrane. T cells bearing adsorbed vesicular MHCII presented antigen and were specifically lysed by CD4(+) T cell responders, but when labeled with anti-MHCII antibody were not susceptible to complement-mediated lysis. In summary, this study reveals vesicular compartments associated with synthesis and intercellular exchange of functional MHCII/peptide complexes.

Human CD4(+)CD25(+) cells: a naturally occurring population of regulatory T cells

Despite thymic deletion of cells with specificity for self-antigens, autoreactive T cells are readily detectable in the normal T-cell repertoire. In recent years, a population of CD4(+) T cells that constitutively express the interleukin-2 receptor-alpha chain, CD25, has been shown to play a pivotal role in the maintenance of self-tolerance in rodent models. This study investigated whether such a regulatory population exists in humans. A population of CD4(+)CD25(+) T cells, taken from the peripheral blood of healthy individuals and phenotypically distinct from recently activated CD4(+) T cells, was characterized. These cells were hyporesponsive to conventional T-cell stimuli and capable of suppressing the responses of CD4(+)CD25(-) T cells in vitro. Addition of exogenous interleukin-2 abrogated the hyporesponsiveness and suppressive effects of CD4(+)CD25(+) cells. Suppression required cell-to-cell contact but did not appear to be via the inhibition of antigen-presenting cells. In addition, there were marked changes in the expression of Notch pathway molecules and their downstream signaling products at the transcriptional level, specifically in CD4(+)CD25(+) cells, suggesting that this family of molecules plays a role in the regulatory function of CD4(+)CD25(+) cells. Cells with similar phenotype and function were detected in umbilical venous blood from healthy newborn infants. These results suggest that CD4(+)CD25(+) cells represent a population of regulatory T cells that arise during fetal life. Comparison with rodent CD4(+)CD25(+) cells suggests that this population may play a key role in the prevention of autoimmune diseases in humans.

Persistence of dominant T cell clones in accepted solid organ transplants

Donor/recipient MHC class II matching is beneficial to the survival of allogeneic kidneys in humans and swine. In the latter, tolerance to class I-disparate grafts can be induced by a short course of immunosuppression, a peripheral mechanism that implicates regulatory T cells. Absence of treatment will lead to prompt rejection. Rejected grafts are infiltrated by dominant alloaggressive T cells, whereas there is still speculation on the specificity and function of T cells invading accepted tissues. To characterize the TCR repertoire of graft-infiltrating T cells (GITC) in accepted kidneys, we have used the RT-PCR-based spectratyping technique to assess the length polymorphism of the porcine TCRbeta chain complementary-determining region 3 (CDR3). Results show that T cells infiltrating accepted kidneys (n = 5) express a restricted polymorphism of the CDR3 length, whereas PBL from the same animal have the polymorphic distribution of CDR3 lengths found in naive animals; that the skewed Vbeta repertoire in accepted grafts involved distinct Vbeta subfamilies in otherwise MHC-identical recipient animals; that GITC clonal dominance is not caused by immunosuppression because a second kidney, accepted without drug treatment, exhibits the same TCR Vbeta CDR3 profiles than those detected in the first graft; and that intragraft clonal dominance intensifies with time, indicating progressive preeminence of nonaggressive GITC clones. Collectively, these data represent the first example, in a preclinical model, of the emergence of nonaggressive intragraft clones, which may be involved in the induction/maintenance of local tolerance to allogeneic tissues.

Alloantigen-induced specific immunological unresponsiveness

When the immune system encounters alloantigen it can respond in any one of a number of different ways. The choice that is made will take into account factors such as where, when and how the contact with the alloantigen takes place, as well as the environmental conditions that prevail at the time the alloantigen is encountered. Alloantigen administration before transplantation either alone or in combination with therapeutic agents that modulate the functional activity of the responding leucocytes can be a powerful way of inducing specific unresponsiveness to alloantigens in vivo. The molecular mechanisms that influence the way the outcome of the immune response to alloantigen develops, either activation or unresponsiveness to the triggering antigen, hold the key to our ability to manipulate the immune system effectively by exposing it to donor antigen for therapeutic purposes. This review will focus on alloantigen-induced immunological unresponsiveness and how insights into the mechanisms of unresponsiveness have driven the development of novel tolerance-induction strategies that show promise for translation into the clinic in the future.

Xenotransplantation and tolerance

The application of xenotransplantation faces daunting immunological hurdles, some of which might be overcome with the induction of tolerance. Porcine organs transplanted into primates are subject to several types of rejection responses. Hyperacute rejection mediated by naturally occurring xenoreactive antibodies and complement can be overcome without tolerance. Acute vascular rejection and cellular rejection, however, may present important opportunities for immunological tolerance, and humoral rejection might be approached by various mechanisms including (i) clonal deletion, (ii) anergy, (iii) immune deviation, (iv) induction of immunoregulatory or suppressor cells, or (v) veto cells. B-cell tolerance, useful for preventing humoral rejection, might be approached through clonal anergy. It remains to be determined, however, whether tolerance induction is required for xenotransplantation and by which means the various mechanisms of tolerance can be applied in the setting of xenotransplantation. Regardless, the study of tolerance will surely expand understanding of the physiology and pathophysiology of the immune system.

An overview of regulatory T cells

The induction of tolerance is essential for the maintenance of immune homeostasis and for the prevention of autoimmune diseases. To induce tolerance the immune system uses several mechanisms, including the deletion of autoreactive T cells, the induction of anergy and active suppression of autoimmune responses. The mechanisms of thymic deletion and anergy of autoreactive T cells are well characterized, whereas active suppression by T regulatory cells, which has recently emerged as an essential component of the immune response to induce peripheral tolerance, is less well understood. Results from seminal studies by a number of laboratories have renewed interest in (CD4(+)) T cells with regulatory properties and some of the researchers who have been involved in this effort have contributed to this Forum on regulatory T cells. This general overview on regulatory T cells comments on recent results in the field of regulatory T cells and presents our current knowledge on Tr1 T cells.

Transplantation tolerance induced by mixed chimerism

Although short- and long-term results after organ transplantation have improved considerably in recent years, morbidity and mortality rates in graft recipients remain high. The induction of lifelong donor-specific tolerance would dramatically improve outcome after organ transplantation. Although many tolerance protocols have been successful in rodent studies, most of these approaches have failed when attempted in large animals or humans. Robust tolerance, in contrast, has been demonstrated with mixed chimerism regimens not only in rodents but also in large animal models, including non-human primates. Furthermore, mixed chimerism protocols have been developed that would be feasible in cadaveric, and thus in thoracic, transplantation. The induction of mixed hematopoietic chimerism is therefore an attractive experimental approach for development of clinical tolerance protocols. One of the obstacles to widespread clinical application of this concept is the remaining toxicity of the host conditioning. Recent advances, however, have led to substantially milder protocols that could become clinically acceptable in the foreseeable future. This article provides a short overview of the basic mechanisms by which immunologic tolerance may be induced, describes the concept of mixed chimerism as a promising approach for clinical tolerance induction, and reviews recent progress in developing clinically feasible mixed chimerism protocols.

Procoagulants in fetus rejection: the role of the OX-2 (CD200) tolerance signal

The spontaneous loss of normal karyotype embryos may be initiated or prevented by the maternal immune system. In mice, loss between the time of implantation (day 4.5) and formation of a vascularized placenta (day 9.5) when the embryo is too large to survive by diffusion alone, is analogous to occult pregnancy failure in humans. They are called occult because usually the woman does not know she is pregnant. From studies in mice, these early losses have a different mechanism than abortion of a vascularized placenta (analogous to clinically evident human spontaneous miscarriage). The latter depend on the activation of the novel prothrombinase fgl2 on the fetal trophoblast and in maternal decidua by the T helper-1 (Th1) type cytokines TNF- alpha+gamma -interferon that arise from NK cells and NK gammadelta T cells; conversion of prothrombin to thrombin which in turn generates IL8 that activates polymorphonuclear leukocytes leads to embryonic death. These inflammatory processes are counteracted by Th2/3-type cytokines that arise in part from V gamma 1 delta 6 T cells reacting to, as yet, unidentified trophoblast antigens in the presence of the 'tolerance signaling molecule' OX-2. By contrast, peri-implantation losses (between implantation and formation of a vascularized placenta, analogous to occult losses in humans) appear to be dependent upon perforin(+)cells, complement activation, and products of alphabeta T and NK alphabeta T cells, but not on TNF- alpha or procoagulant activation. Similarities and differences between findings in the mouse and human, and the potential evolutionary significance of mechanisms affecting reproductive success are reviewed.

MHC class II-regulated central nervous system autoaggression and T cell responses in peripheral lymphoid tissues are dissociated in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis

We dissected the requirements for disease induction of myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis in MHC (RT1 in rat) congenic rats with overlapping MOG peptides. Immunodominance with regard to peptide-specific T cell responses was purely MHC class II dependent, varied between different MHC haplotypes, and was linked to encephalitogenicity only in RT1.B(a)/D(a) rats. Peptides derived from the MOG sequence 91-114 were able to induce overt clinical signs of disease accompanied by demyelinated CNS lesions in the RT1.B(a)/D(a) and RT1(n) haplotypes. Notably, there was no detectable T cell response against this encephalitogenic MOG sequence in the RT1(n) haplotype in peripheral lymphoid tissue. However, CNS-infiltrating lymphoid cells displayed high IFN-gamma, TNF-alpha, and IL-4 mRNA expression suggesting a localization of peptide-specific reactivated T cells in this compartment. Despite the presence of MOG-specific T and B cell responses, no disease could be induced in resistant RT1(l) and RT1(u) haplotypes. Comparison of the number of different MOG peptides binding to MHC class II molecules from the different RT1 haplotypes suggested that susceptibility to MOG-experimental autoimmune encephalomyelitis correlated with promiscuous peptide binding to RT1.B and RT1.D molecules. This may suggest possibilities for a broader repertoire of peptide-specific T cells to participate in disease induction. We demonstrate a powerful MHC class II regulation of autoaggression in which MHC class II peptide binding and peripheral T cell immunodominance fail to predict autoantigenic peptides relevant for an autoaggressive response. Instead, target organ responses may be decisive and should be further explored.

Advances in transplant immunobiology

The present review briefly addresses the most recent knowledge acquired in the field of transplant immunology. A particular emphasis is placed on articles published during the past 12-18 months that have focused on allorecognition, dendritic cells and tolerance.

Anti-CD3 epsilon F(ab')2 prevents graft-versus-host disease by selectively depleting donor T cells activated by recipient alloantigens

Transplantation tolerance is facilitated by activation-induced apoptosis of peripheral T cells triggered by specific AG: Abs specific for the nonpolymorphic CD3 component of the TCR complex bind to APCs through Fc-FcR interactions, mimic MHC-peptide, and activate polyclonal T cells. In contrast, F(ab')(2) of anti-CD3epsilon Abs do not activate naive T cells but induce apoptosis of Ag-activated, cycling T cells. Here, we report that treatment with anti-CD3epsilon F(ab')(2) can selectively induce apoptosis of donor T cells that recognize a recipient alloantigen, thereby preventing graft-vs-host disease initiated by a TCR-transgenic T cell population. The selective elimination of Ag-activated T cells by non-FcR-binding anti-CD3epsilon Abs could serve as an ideal strategy to prevent graft-vs-host disease and allograft rejection or to treat autoimmune disorders.

Regulatory functions of self-restricted MHC class II allopeptide-specific Th2 clones in vivo

We studied T-cell clones generated from grafts of rejecting and tolerant animals and investigated the regulatory function of Th2 clones in vitro and in vivo. To prevent allograft rejection, we treated LEW strain recipient rats of WF strain kidney grafts with CTLA4Ig to block CD28-B7 costimulation. We then isolated epitope-specific T-cell clones from the engrafted tissue, using a donor-derived immunodominant class II MHC allopeptide presented by recipient antigen-presenting cells. Acutely rejected tissue from untreated animals yielded self-restricted, allopeptide-specific T-cell clones that produced IFN-gamma, whereas clones from tolerant animals produced IL-4 and IL-10. Adoptive transfer into naive recipients of Th1 clones, but not Th2 clones, induced alloantigen-specific delayed-type hypersensitivity (DTH) responses. In addition, Th2 clones suppressed DTH responses mediated by Th1 clones in vivo and blocked Th1 cell proliferation and IFN-gamma production in vitro. A pilot human study showed that HLA-DR allopeptide-specific T-cell clones generated from patients with chronic rejection secrete Th1 cytokines, whereas those from patients with stable graft function produce Th2 cytokines in response to donor-specific HLA-DR allopeptides. We suggest that self-restricted alloantigen-specific Th2 clones may regulate the alloimmune responses and promote long-term allograft survival and tolerance.

The role of different subsets of T regulatory cells in controlling autoimmunity

T regulatory cells-in addition to clonal deletion and anergy-are essential for the downregulation of T cell responses to both foreign and self antigens, and for the prevention of autoimmunity. Recent progress has been made in characterising the different subsets of T regulatory cells, the factors that drive their differentiation, and their mode of action. The resolution of these mechanisms will make it possible to use T regulatory cells therapeutically in human autoimmune diseases.

Review article: molecular signals and genetic reprogramming in peripheral T-cell differentiation

Rearrangement of gene segments occurs in T lymphocytes during thymic development as the T-cell receptor (TCR) is first expressed, allowing T cells to become central regulators of antigen specificity in the acquired immune system. However, further development of T cells occurs after population of peripheral lymphoid tissues, which can result in T-cell expansion and differentiation into effectors of various immune function, or progression to memory T cells, anergic cells or death by apoptosis. This review focuses on more recent developments concerning the choices that peripheral T cells make between first encountering antigen through TCR recognition and death. These decisions are associated with a process of genetic reprogramming that alters the behaviour of cells so that immune responses are appropriately regulated.