Nondepleting anti-CD4 has an immediate action on diabetogenic effector cells, halting their destruction of pancreatic beta cells

Coreceptor therapy has distinct short- and long-term tolerogenic effects intrinsic to autoreactive effector T cells

Immunotherapies are needed in the clinic that effectively suppress beta cell autoimmunity and reestablish long-term self-tolerance in type 1 diabetes. We previously demonstrated that nondepleting αCD4 and αCD8α antibodies establish rapid and indefinite remission in recent-onset diabetic NOD mice. Diabetes reversal by coreceptor therapy (CoRT) is induced by suppression of pathogenic effector T cells (Teff) and the selective egress of T cells from the pancreatic lymph nodes and islets that remain free of infiltration long-term. Here, we defined CoRT-induced events regulating early Teff function and pancreatic residency, and long-term tolerance. TCR-driven gene expression controlling autoreactive Teff expansion and proinflammatory activity was suppressed by CoRT, and islet T cell egress was sphingosine-1 phosphate-dependent. In both murine and human T cells, CoRT upregulated the Foxo1 transcriptional axis, which in turn was required for suppression and efficient pancreatic egress of Teff. Interestingly, long-term tolerance induced in late-preclinical NOD mice was marked by reseeding of the pancreas by a reduced CD8+ Teff pool exhibiting an exhausted phenotype. Notably, PD-1 blockade, which rescues exhausted Teff, resulted in diabetes onset in protected animals. These findings demonstrate that CoRT has distinct intrinsic effects on Teff that impact events early in induction and later in maintenance of self-tolerance.

Evolving Antibody Therapies for the Treatment of Type 1 Diabetes

Type 1 diabetes (T1D) is widely considered to be a T cell driven autoimmune disease resulting in reduced insulin production due to dysfunction/destruction of pancreatic β cells. Currently, there continues to be a need for immunotherapies that selectively reestablish persistent β cell-specific self-tolerance for the prevention and remission of T1D in the clinic. The utilization of monoclonal antibodies (mAb) is one strategy to target specific immune cell populations inducing autoimmune-driven pathology. Several mAb have proven to be clinically safe and exhibit varying degrees of efficacy in modulating autoimmunity, including T1D. Traditionally, mAb therapies have been used to deplete a targeted cell population regardless of antigenic specificity. However, this treatment strategy can prove detrimental resulting in the loss of acquired protective immunity. Nondepleting mAb have also been applied to modulate the function of immune effector cells. Recent studies have begun to define novel mechanisms associated with mAb-based immunotherapy that alter the function of targeted effector cell pools. These results suggest short course mAb therapies may have persistent effects for regaining and maintaining self-tolerance. Furthermore, the flexibility to manipulate mAb properties permits the development of novel strategies to target multiple antigens and/or deliver therapeutic drugs by a single mAb molecule. Here, we discuss current and potential future therapeutic mAb treatment strategies for T1D, and T cell-mediated autoimmunity.

Therapies to Suppress β Cell Autoimmunity in Type 1 Diabetes

Type 1 diabetes (T1D) is an autoimmune disease that is generally considered to be T cell-driven. Accordingly, most strategies of immunotherapy for T1D prevention and treatment in the clinic have targeted the T cell compartment. To date, however, immunotherapy has had only limited clinical success. Although certain immunotherapies have promoted a protective effect, efficacy is often short-term and acquired immunity may be impacted. This has led to the consideration of combining different approaches with the goal of achieving a synergistic therapeutic response. In this review, we will discuss the status of various T1D therapeutic strategies tested in the clinic, as well as possible combinatorial approaches to restore β cell tolerance.

Route of Antigen Presentation Can Determine the Selection of Foxp3-Dependent or Foxp3-Independent Dominant Immune Tolerance

It has been shown that dominant tolerance, namely in transplantation, requires Foxp3⁺ regulatory T cells. Although most tolerance-inducing regimens rely on regulatory T cells, we found that induction of tolerance to proteins in aluminum hydroxide can be achieved in Foxp3-deficient mice using nondepleting anti-CD4 Abs. This type of tolerance is Ag specific, and tolerant mice retain immune competence to respond to unrelated Ags. We demonstrated with chicken OVA-specific TCR-transgenic mice that the same tolerizing protocol (CD4 blockade) and the same target Ag (OVA) achieves Foxp3-dependent transplantation tolerance to OVA-expressing skin grafts, but Foxp3-independent tolerance when the Ag is provided as OVA-aluminum hydroxide. In the latter case, we found that tolerance induction triggered recessive mechanisms leading to elimination of effector cells and, simultaneously, a dominant mechanism associated with the emergence of an anergic and regulatory CTLA-4⁺IL-2^lowFoxp3^- T cell population, where the tolerance state is IL-10 dependent. Such Foxp3-independent mechanisms can improve the efficacy of tolerance-inducing protocols.

Anti-coreceptor therapy drives selective T cell egress by suppressing inflammation-dependent chemotactic cues

There continues to be a need for immunotherapies to treat type 1 diabetes in the clinic. We previously reported that nondepleting anti-CD4 and -CD8 Ab treatment effectively reverses diabetes in new-onset NOD mice. A key feature of the induction of remission is the egress of the majority of islet-resident T cells. How this occurs is undefined. Herein, the effects of coreceptor therapy on islet T cell retention were investigated. Bivalent Ab binding to CD4 and CD8 blocked TCR signaling and T cell cytokine production, while indirectly downregulating islet chemokine expression. These processes were required for T cell retention, as ectopic IFN-γ or CXCL10 inhibited Ab-mediated T cell purging. Importantly, treatment of humanized mice with nondepleting anti-human CD4 and CD8 Ab similarly reduced tissue-infiltrating human CD4⁺ and CD8⁺ T cells. These findings demonstrate that Ab binding of CD4 and CD8 interrupts a feed-forward circuit by suppressing T cell-produced cytokines needed for expression of chemotactic cues, leading to rapid T cell egress from the islets. Coreceptor therapy therefore offers a robust approach to suppress T cell-mediated pathology by purging T cells in an inflammation-dependent manner.

The fate of CD4+ T cells under tolerance-inducing stimulation: a modeling perspective

Non-depleting anti-CD4 monoclonal antibodies (MAbs) induce long-term dominant tolerance mediated by regulatory T cells in several animal models of transplantation, allergy and autoimmunity. However, despite many studies on tolerance induction following CD4 blockade, the consequences of this intervention on T-cell kinetics are still unknown. Mathematical models have been useful to understand lymphocyte dynamics, estimating rates of proliferation and cell death following an intervention. Using the same strategy, we found that CD4(+) T cells activated in vitro in the presence of non-depleting anti-CD4 MAbs are prevented from undergoing optimal proliferation and show a higher frequency of apoptosis. Although the changes are small, during the course of a proliferative response, they lead to very distinct final levels of cell numbers. The importance of these mechanisms, predicted by the mathematical model, was validated by showing that lck-driven Bcl-x(L) transgenic mice, bearing T cells resistant to apoptosis, fail to become tolerant to skin grafts following CD4-blockade. Our data show that, in addition to induction of regulatory T cells, CD4 blockade has a marked effect in the effector T-cell pool by the combined action of hindering proliferation while favoring apoptosis. It is, therefore, the combination of all those mechanisms that leads to stable tolerance.

Long-term remission of diabetes in NOD mice is induced by nondepleting anti-CD4 and anti-CD8 antibodies

Residual β-cells found at the time of clinical onset of type 1 diabetes are sufficient to control hyperglycemia if rescued from ongoing autoimmune destruction. The challenge, however, is to develop an immunotherapy that not only selectively suppresses the diabetogenic response and efficiently reverses diabetes, but also establishes long-term β-cell-specific tolerance to maintain remission. In the current study, we show that a short course of nondepleting antibodies (Abs) specific for the CD4 and CD8 coreceptors rapidly reversed clinical disease in recent-onset diabetic NOD mice. Once established, remission was maintained indefinitely and immunity to foreign antigens unimpaired. Induction of remission involved selective T-cell purging of the pancreas and draining pancreatic lymph nodes and upregulation of transforming growth factor (TGF)-β1 by pancreas-resident antigen-presenting cells. Neutralization of TGF-β blocked the induction of remission. In contrast, maintenance of remission was associated with tissue-specific immunoregulatory T cells. These findings demonstrate that the use of nondepleting Ab specific for CD4 and CD8 is a robust approach to establish long-term β-cell-specific T-cell tolerance at the onset of clinical diabetes.

Overcoming self-destruction in the pancreas

Type 1 diabetes (T1D) is an autoimmune disease where insulin producing pancreatic beta cells are progressively destroyed. In the absence of a cure, exogenous insulin is given to maintain glucose homeostasis. Tolerogenic strategies to halt destruction and facilitate recovery of beta cells are being explored. This disease is under polygenic control; the identification of specific candidate pathways to target for drug discovery or corrective therapy would enhance therapeutic options. Whilst islet transplantation combined with immune suppression has shown some efficacy, the availability of beta cells restricts its general use. Differentiation protocols directing cultured stem cells into pancreatic tissue are under development. The availability of embryonic stem cells from a mouse model of T1D should facilitate dissection of the causes and progression of this disease.

Non-depleting Anti-CD4 Antibody not only Prevents Onset but Resolves Sialadenitis in NOD Mice

The non-obese diabetic (NOD) mouse spontaneously develops lymphocytic infiltrates in the salivary glands (sialadenitis) and provides an useful rodent model of human Sjogren's syndrome (SS). Non-depleting anti-CD4 antibodies have been shown to ameliorate Type 1 diabetes in NOD mice and also vasculitis in MRL/lpr mice. This study shows that a short course of treatment with the non-depleting anti-CD4 monoclonal antibody, YTS 177, completely prevents salivary infiltration and reverses ongoing pathology in the salivary gland.

Patients with chronic pancreatitis have islet progenitor cells in their ducts, but reversal of overt diabetes in NOD mice by anti-CD3 shows no evidence for islet regeneration

Monoclonal antibodies to T-cell coreceptors have been shown to tolerise autoreactive T-cells and prevent or even reverse autoimmune pathology. In type 1 diabetes, there is a loss of insulin-secreting beta-cells, and a cure for type 1 diabetes would require not only tolerance induction but also recovery of the functional beta-cell mass. Although we have previously shown that diabetic mice have increased numbers of ductal progenitors in the pancreas, there is no evidence of any increase of insulin-secreting cells in the ducts. In contrast, in the adult human pancreas of patients with chronic pancreatitis, we can demonstrate, in the ducts, increased numbers of insulin-containing cells, as well as cells containing other endocrine and exocrine markers. There are also significantly increased numbers of cells expressing the homeodomain protein, pancreatic duodenal homeobox-1. Anti-CD3 has been shown to reverse overt diabetes in NOD mice; thus, we have used this model to ask whether monoclonal antibody-mediated inhibition of ongoing beta-cell destruction enables islet regeneration to occur. We find no evidence that such monoclonal antibody therapy results in either regeneration of insulin-secreting beta-cells or of increased proliferation of islet beta-cells.

Fluorine-19 MRI for visualization and quantification of cell migration in a diabetes model

This article describes an in vivo imaging method for visualizing and quantifying a specific cell population. Cells are labeled ex vivo with a perfluoropolyether nanoparticle tracer agent and then detected in vivo using (19)F MRI following cell transfer. (19)F MRI selectively visualizes only the labeled cells with no background, and a conventional (1)H image taken in the same imaging session provides anatomical context. Using the nonobese diabetic mouse, an established model of type 1 diabetes, (19)F MRI data were acquired showing the early homing behavior of diabetogenic T cells to the pancreas. A computational algorithm provided T cell counts in the pancreas. Approximately 2% of the transferred cells homed to the pancreas after 48 hr. The technique allows for both unambiguous detection of labeled cells and quantification directly from the in vivo images. The in vivo quantification and cell trafficking patterns were verified using (19)F spectroscopy and fluorescence microscopy in excised pancreata. The labeling procedure did not affect T-cell migration in vivo. This imaging platform is applicable to many cell types and disease models and can potentially be used for monitoring the trafficking of cellular therapeutics.

Induction of dominant tolerance using monoclonal antibodies

Monoclonal antibodies (MAb) have been shown to be effective in inducing immune tolerance in transplantation and autoimmunity. Several different MAb have tolerogenic properties and their effect has been studied in a range of experimental animal models and, in some cases, in clinical trials. The tolerant state seems to be maintained by CD4+ regulatory T cells (Treg), induced in the periphery, capable of suppressing other T cells specific for the same antigens or antigens presented by the same antigen presenting cells. Furthermore, following the initial induction of Treg cells under MAb treatment, Treg cells themselves can maintain the tolerant state in a dominant way in the absence of the therapeutic MAb or other immunosuppressive agents, and are able to recruit other T cells into the regulatory pool--a process named infectious tolerance.

Pharmacokinetics/pharmacodynamics of nondepleting anti-CD4 monoclonal antibody (TRX1) in healthy human volunteers

PURPOSE

TRX1 is a nondepleting anti-CD4 monoclonal IgG1 antibody being developed to induce tolerance by blocking CD4-mediated functions. The purpose of this study is to describe the pharmacokinetics (PK) and pharmacodynamics (PD) of TRX1 and to develop a receptor-mediated PK/PD model that characterizes the relationships between serum TRX1 concentration and total and free CD4 expression in healthy male volunteers.

METHODS

Nine subjects from three dosing cohorts in double-blinded, placebo-controlled phase I clinical study was included in the analysis. Serum TRX1 levels were determined using enzyme-linked immunosorbent assay. Blood total and free CD4 receptor levels were determined by using flow cytometric analyses. The receptor-mediated PK/PD model was developed to describe the dynamic interaction of TRX1 binding with CD4 receptors.

RESULTS AND CONCLUSIONS

TRX1 displayed nonlinear pharmacokinetic behavior and the CD4 receptors on T cells were saturated and down-modulated following treatment with TRX1. Results from in vitro studies using purified human T cells suggested that CD4-mediated internalization may constitute one pathway by which CD4 is down-modulated and TRX1 is cleared in vivo. The developed receptor-mediated PK/PD model adequately described the data. This PK/PD model was used to simulate PK/PD time profiles after different dosing regimens to help guide the dose selection in future clinical studies.

MAdCAM-1 is needed for diabetes development mediated by the T cell clone, BDC-2.5

The NOD-derived islet-reactive CD4(+) T cell clone, BDC-2.5, is able to transfer diabetes to neonatal non-obese diabetic (NOD) mice but is unable to transfer disease to either adult NOD or NOD scid recipients. Transfer of diabetes to adult recipients by BDC-2.5 is only accomplished by cotransfer of CD8(+) T cells from a diabetic donor. To understand why this CD4(+) T cell clone is able to mediate diabetes in neonatal but not the adult recipients we examined the ability of the clone to traffic in the different recipients. Our studies showed that MAdCAM-1 has a very different expression pattern in the neonatal and adult pancreas. Blockade of this addressin prevents the clone from transferring diabetes to neonatal mice, suggesting that the differential pancreatic expression of MAdCAM-1 in neonatal and adult pancreas provides an explanation of the differences in diabetes development.

Pathogenic T-cell clones in autoimmune diabetes: more lessons from the NOD mouse

T-cell clones that can efficiently transfer diabetes to prediabetic nonobese diabetic (NOD) mice provide a powerful approach to dissecting the autoimmune disease process and for investigating immunoregulation. Diabetogenic T-cell clones carried in culture allow for detailed analysis of T-cell effector function and in vivo activity, and thus the contribution of a single clonotype to pathogenesis can be studied. As T cells comprising most or all of the repertoire in T-cell receptor transgenic (TCR-Tg) mice, diabetogenic T-cell clones have led to new variations on the NOD mouse model of autoimmune disease. T-cell clones are being used to screen peptide libraries and proteomic arrays to identify the autoantigens that drive these clones in vivo and to extend our knowledge of the processes that give rise to these antigens. With the identification of peptide agonists and natural ligands, the development of MHC-peptide multimers has been possible. These reagents can track T cells in vivo and thus provide new approaches for disease diagnosis and therapy as well as a versatile set of tools for basic research on how T cells contribute to autoimmune disease.

Induction of immunological tolerance/hyporesponsiveness in baboons with a nondepleting CD4 antibody

Tolerance induction with anti-CD4 Abs is well established in rodent transplant and autoimmune disease models, but has yet to be demonstrated in non-human primates or in clinical studies. In retrospect, failure of anti-CD4 Abs to induce tolerance in primates may be technical, a consequence of insufficient dosing and Ab properties influencing immunogenicity and cell depletion. To circumvent these possible limitations, we constructed a novel anti-CD4 mAb, TRX1, humanized to reduce immunogenicity and Fc-modified to prevent cell depletion. Using equine immune globulin (equine Ig) as a model Ag, we examined the tolerance-inducing capacity of TRX1 in baboons. During the induction phase, TRX1 inhibited the humoral response to equine Ig in a dose-dependent manner, with complete suppression of response at the highest dose tested (40 mg/kg). Upon challenge, anti-equine Ig responses were generated in baboons treated with 1 and 10 mg/kg doses of TRX1 and in control animals. In higher dosing cohorts (20 and 40 mg/kg), however, the immune response to equine Ig was modulated in seven of nine animals, including complete unresponsiveness to Ag challenges in two animals. Five of nine were hyporesponsive to equine Ig, generating titers 50- to 250-fold lower than control groups. Repeated challenge resulted in titers falling to baseline or near baseline, with two of five hyporesponsive animals becoming unresponsive to Ag. All animals responded to neoantigen immunization, indicating that the modified response to equine Ig was Ag specific. These studies demonstrate that anti-CD4 Ab-mediated, Ag-specific tolerance can be achieved in baboons without long term immune suppression.

Autoimmune diabetes: ongoing development of immunological intervention strategies targeted directly against autoreactive T cells

It is well known that autoimmunity associated with the onset of insulin-dependent diabetes mellitus (IDDM) involves the generation of autoreactive T and B cells. The findings that diabetics mount humoral and cellular immune responses against islet cell antigens (ICAs) have led to the testing of ICAs and their analogs as candidates for therapeutic agents for better treatment of IDDM at its prediabetic and diabetic stages. Apart from this type of approach, various immunological intervention strategies aimed at direct targeting of the autoreactive T cells have also been investigated. The present review covers the ongoing aspects of these developments focusing on the preclinical findings made in NOD (nonobese diabetic) mice which have been commonly used as a disease model for human autoimmune diabetes. Other types of approaches involving the mobilization of regulatory T cells to indirectly control or modulate the pathological activity of autoreactive T cells will not be discussed within this scope.

DEPLETING ANTI-CD4 MONOCLONAL ANTIBODY CURES NEW-ONSET DIABETES, PREVENTS RECURRENT AUTOIMMUNE DIABETES, AND DELAYS ALLOGRAFT REJECTION IN NONOBESE DIABETIC MICE1

BACKGROUND

The prevention of recurrent autoimmunity is a prerequisite for successful islet transplantation in patients with type I diabetes. Therapies effective in preserving pancreatic beta-cell mass in patients with newly diagnosed diabetes are good candidates for achieving this goal. Anti-CD3 monoclonal antibody (mAb) and antilymphocyte antisera are the only therapies to date that have cured early diabetic disease in the nonobese diabetic (NOD) mouse. We investigated whether other immunosuppressive therapies, including short-term depleting anti-CD4 mAb or costimulation blockade, would affect the disease progression in recently diabetic NOD mice. We also evaluated the effect of the anti-CD4 mAb on syngeneic and allogeneic graft survival in diabetic NOD recipients.

METHODS AND RESULTS

We demonstrate that a short course of anti-CD4 mAb early after hyperglycemia onset cured diabetes. Normal islets and islets with CD4+ and CD8+ T-cell peri-insulitic infiltrate were found in the pancreata of cured NOD mice. A similar regimen prevented the recurrence of autoimmune diabetes in NOD/severe combined immunodeficient disease (SCID) islet isografts and delayed the rejection of allogeneic C57BL/6 islet allografts in diabetic female NOD mice. The co-transfer of diabetogenic splenocytes with splenocytes from anti-CD4 mAb-treated and cured NOD mice into 7-week-old, irradiated, NOD male mice was not able to protect from diabetes occurrence. This indicates that an anti-CD4-mediated cure of diabetes is independent of the induction of immunoregulatory T cells. Anti-CD154 mAb and cytotoxic T-lymphocyte antigen 4 immunoglobulin were ineffective in early-onset diabetes.

CONCLUSION

Our results provide the first evidence that newly established autoimmune islet destruction in NOD mice responds to a short course of anti-CD4 mAb. In contrast, costimulation blockade is ineffective in this clinically relevant model.

Chronic autoimmune disease caused by somatic mutation to T-lymphocyte regulatory receptors

Chronic autoimmune diseases are a common cause of death and disability in the developed world. Despite this, their aetiology is unknown and researchers work without an accepted hypothesis as to how these diseases arise. Chronic autoimmunity could result from spontaneous somatic mutation to an autoreactive T helper lymphocyte, causing impairment of the receptor mechanism by which it communicates with regulatory T-cells. This would result in a dysregulated autoreactive T-cell clone. Current experimental evidence suggests this is at least possible, if not probable, and would explain many of the epidemiological and clinical features of chronic autoimmune diseases.

Nondepleting anti-CD4 and soluble interleukin-1 receptor prevent autoimmune destruction of syngeneic islet grafts in diabetic NOD mice

BACKGROUND

Successful islet transplantation in type 1 diabetes requires tolerance induction of both allo- and autoreactive T-cell responses. Monoclonal antibodies targeting the CD4 coreceptor on T-helper cells have been shown to be effective in this regard. In type 1 diabetes, there is some evidence to suggest that cytokines such as interleukin (IL)-1 may be involved in beta-cell destruction. The high glucose levels associated with type 1 diabetes are also known to be toxic to beta cells.

METHOD

The tempo of T-cell and macrophage infiltration into syngeneic islets transplanted into diabetic nonobese diabetic (NOD) mice was examined by immunohistochemistry. We investigated the ability of a nondepleting anti-CD4 monoclonal antibody (YTS177) to induce tolerance to syngeneic islet grafts in female spontaneous diabetic NOD mice and in an adoptive transfer model of diabetes in NOD mice. The spontaneous model was used to test the effect on graft function of perioperative insulin therapy in mice treated with YTS177. The ability of soluble interleukin (sIL)-1 receptor (R) type II (sIL-1RII) to inhibit IL-1 effects in syngeneic islet transplants was also assessed.

RESULTS

Cellular infiltration of CD3 cells and macrophages into the islet graft coincided with loss of graft function in untreated mice. Self-tolerance to beta cells was restored with YTS177, allowing long-term graft survival in a proportion of animals. The use of perioperative insulin therapy increased the number of successful grafts in spontaneously diabetic NOD mice treated with YTS177. The combination of YTS177 with sIL-1RII significantly improved the rates of graft survival compared with graft survival in YTS177-treated spontaneously diabetic NOD mice.

CONCLUSIONS

Nondepleting anti-CD4 antibodies restore self tolerance to syngeneic islet transplants in diabetic NOD mice. Insulin therapy improves graft survival in mice treated with YTS177. Preventing the action of IL-1 greatly improves graft survival induced with YTS177.

Remission and pancreas isograft survival in recent onset diabetic NOD mice after treatment with low-dose anti-CD3 monoclonal antibodies

Diabetes in NOD mice is an autoimmune disease similar to Type I diabetes in humans. Prior to hypoglycemia, changes in the islet infiltrate led to autoreactive T cell activation and destruction of the insulin-producing beta cells. If T cell activation can be inhibited before beta cell destruction is complete, islet cell rescue and regeneration can occur. Female NOD mice > 100 days old with blood glucose levels > 20 mM/l for less than 7 days were selected as 'recent onset' mice. Untreated, all of these animals would die of diabetes in < 40 days. Mice treated with anti-CD4 (GK1.5) achieved 14.3% permanent remission, while those treated with anti-CD8 (53.6.7) showed 33.3% permanent remission. Mice treated with anti-CD3 (145-2C1) also achieved 33.3% permanent remission, but 14% of these died of first dose syndrome. In mice treated with a low dose of anti-CD3 (10 microg KT3), which did not induce first dose syndrome, 50% remained in remission for > 100 days. This dose of mAb reduced insulitis but did not deplete splenic CD3 cells. When mice in remission were challenged with a vascularized pancreas isograft at 50 days, 9/22 remained normal and 13/22 had recurrent disease in both transplanted and native pancreas. Of the long-surviving isografts 7/9 were in KT3 treated recipients. Histology showed activated T cell infiltration in the native and transplanted pancreases of mice with transient remission. Benign insulitis with macrophages, B cells, CD4 > CD8 T cells and low levels of IL-2R, IL-2, IFN-gamma and IL-4 was seen in islets from the native pancreas and in long surviving pancreas isografts in mice that remained in remission. Thus, using low dose KT3, it was possible to halt the development of diabetes in 50% of animals treated soon after diagnosis, despite significant islet cell destruction at this stage. Of the KT3 treated mice in permanent remission, 70% had re-established tolerance to autoantigen and did not destroy vascularized pancreas isografts.

Combination therapy with sirolimus and interleukin-2 prevents spontaneous and recurrent autoimmune diabetes in NOD mice

Sirolimus is an immunosuppressant that inhibits interleukin (IL)-2 signaling of T-cell proliferation but not IL-2-induced T-cell apoptosis. Therefore, we hypothesized that administration of IL-2, together with sirolimus, might shift T-cell proliferation to apoptosis and prevent autoimmune destruction of islet beta-cells. We found that sirolimus and IL-2 therapy of female NOD mice, beginning at age 10 weeks, was synergistic in preventing diabetes development, and disease prevention continued for 13 weeks after stopping sirolimus and IL-2 therapy. Similarly, sirolimus and IL-2 were synergistic in protecting syngeneic islet grafts from recurrent autoimmune destruction after transplantation in diabetic NOD mice, and diabetes did not recur after stopping sirolimus and IL-2 combination therapy. Immunocytochemical examination of islet grafts revealed significantly decreased numbers of leukocytes together with increased apoptosis of these cells in mice treated with sirolimus and IL-2, whereas beta-cells were more numerous, and significantly fewer were apoptotic. In addition, Th1-type cells (gamma-interferon-positive and IL-2(+)) were decreased the most, and Th2-type cells (IL-4(+) and IL-10(+)) and Th3-type cells (transforming growth factor-beta1(+)) were increased the most in islet grafts of sirolimus and IL-2-treated mice. We conclude that 1) combination therapy with sirolimus and IL-2 is synergistic in protecting islet beta-cells from autoimmune destruction; 2) diabetes prevention continues after withdrawal of therapy; and 3) the mechanism of protection involves a shift from Th1- to Th2- and Th3-type cytokine-producing cells, possibly due to deletion of autoreactive Th1 cells.

Experimental chronic Chagas' disease myocarditis is an autoimmune disease preventable by induction of immunological tolerance to myocardial antigens

The protozoan Trypanosoma cruzi causes chronic Chagas' disease myocarditis (CCDM) in infected mammals. The pathogenesis of CCDM, however, is still unclear. Indirect evidence for either parasite- or heart-specific immune responses playing a pathogenic role is available. In this work, the participation of autoimmunity in the development of CCDM is demonstrated in mice in which immunological tolerance to heart antigens was induced or strengthened prior to their infection by T. cruzi. Tolerance was induced by heart antigen administration in the presence of complete Freund's adjuvant and anti-CD4 antibodies. Tolerized mice developed less intense CCDM than control non-tolerized animals that had received only anti-CD4 and adjuvant. This result confirms the important notion that tolerance to self, and in particular to heart antigens, may be reinforced/induced in normal animals, and raises the possibility that analogous interventions may prevent the development of CCDM in millions of T. cruzi -infected human beings.

Immunotherapy of insulin-dependent diabetes mellitus

Immunotherapy of diabetes is now focusing on induction of tolerance to beta cell antigens using either soluble antigens or monoclonal anti-T-cell antibodies. These approaches have reached the clinical arena. At the experimental level, strategies are being developed that use or target cytokines (with gene therapy) or stimulate regulatory T cells.

Tolerogenic strategies to halt or prevent type 1 diabetes

A variety of therapeutic strategies have been developed to tolerize autoreactive T cells and prevent autoimmune pathology. In terms of type 1 diabetes, prevention strategies can inhibit the priming and expansion of autoreactive T cells; however, a cure for diabetes would require tolerance to be established in the presence of primed effector cells together with replacement of the destroyed beta cell mass. Replacement of beta cells could be accomplished by transplantation of islets or stem cells or through islet regeneration. We will focus here on tolerogenic strategies that have been used to prevent onset of type 1 diabetes and discuss the potential for a cure.

Therapeutic aspects of tolerance

The immune system is naturally unresponsive to 'self' antigens. Improved knowledge of mechanisms underlying self tolerance is giving rise to a new generation of immunosuppressive agents, that can exploit these mechanisms and so reduce the nature and level of medication that needs to be given long-term to control diseases where the immune system does harm.