Redox regulation of age-associated defects in generation and maintenance of T cell self-tolerance and immunity to foreign antigens

Thymic atrophy reduces naive T cell production and contributes to increased susceptibility to viral infection with age. Expression of tissue-restricted antigen (TRA) genes also declines with age and has been thought to increase autoimmune disease susceptibility. We find that diminished expression of a model TRA gene in aged thymic stromal cells correlates with impaired clonal deletion of cognate T cells recognizing an autoantigen involved in atherosclerosis. Clonal deletion in the polyclonal thymocyte population is also perturbed. Distinct age-associated defects in the generation of antigen-specific T cells include a conspicuous decline in generation of T cells recognizing an immunodominant influenza epitope. Increased catalase activity delays thymic atrophy, and here, we show that it mitigates declining production of influenza-specific T cells and their frequency in lung after infection, but does not reverse declines in TRA expression or efficient negative selection. These results reveal important considerations for strategies to restore thymic function.

Significant association between tumor mutational burden and immune-related adverse events during immune checkpoint inhibition therapies

More than 2000 immuno-oncology agents are being tested or are in use as a result of the cancer immunotherapy revolution. Manipulation of co-inhibitory receptors has achieved tumor eradication in a minority of patients, but widespread immune-related adverse events (irAEs) compromised tolerance to healthy self-tissues in the majority. We have proposed that a major mechanism of irAEs is similar to a graft-versus-malignancy effect of graft-versus-host disease. To verify our hypothesis, we retrieved post-marketing data of adverse events from the U.S. Food and Drug Administration Adverse Event Reporting System. A significant positive correlation was revealed in 7677 patients between the reporting odds ratio of irAEs during immune checkpoint inhibitor therapy and the corresponding tumor mutational burden across 19 cancer types. These results can be interpreted to mean that the ICI drugs unleashed T cells against “altered-self,” self, and tumors resulting in better overall survival.

Anti-rods/rings autoantibody generation in hepatitis C patients during interferon-α/ribavirin therapy

Chronic inflammation associated with hepatitis C virus (HCV) infection can lead to disabling liver diseases with progression to liver cirrhosis and hepatocellular carcinoma. Despite the recent availability of more effective and less toxic therapeutic options, in most parts of the world the standard treatment consists of a weekly injection of pegylated interferon α (IFN-α) together with a daily dose of ribavirin. HCV patients frequently present circulating non-organ-specific autoantibodies demonstrating a variety of staining patterns in the indirect immunofluorescence assay for antinuclear antibodies (ANA). Between 20% to 40% of HCV patients treated with IFN-α and ribavirin develop autoantibodies showing a peculiar ANA pattern characterized as rods and rings (RR) structures. The aim of this article is to review the recent reports regarding RR structures and anti-rods/rings (anti-RR) autoantibody production by HCV patients after IFN-α/ribavirin treatment. Anti-RR autoantibodies first appear around the sixth month of treatment and reach a plateau around the twelfth month. After treatment completion, anti-RR titers decrease/disappear in half the patients and remain steady in the other half. Some studies have observed a higher frequency of anti-RR antibodies in relapsers, i.e., patients in which circulating virus reappears after initially successful therapy. The main target of anti-RR autoantibodies in HCV patients is inosine-5’-monophosphate dehydrogenase 2 (IMPDH2), the rate-limiting enzyme involved in the guanosine triphosphate biosynthesis pathway. Ribavirin is a direct IMPDH2 inhibitor and is able to induce the formation of RR structures in vitro and in vivo. In conclusion, these observations led to the hypothesis that anti-RR autoantibody production is a human model of immunologic tolerance breakdown that allows us to explore the humoral autoimmune response from the beginning of the putative triggering event: exposure to ribavirin and interferon.

[Regulatory T-cells: modern approaches to optimization of their numbers]

Regulatory T-cells (Tregs) are important components of the complex adaptive system of the body responsive to environmental challenges. Tregs ensure peripheral tolerance and play an important role in control of inflammatory reactions. Several subsets of Tregs have been described. Naturally occurring CD4+CD25+ Tregs are recognized as a major subset of immune cells responsible for peripheral immune self-tolerance. Another subtype of Tregs is inducible. Such Tregs are generated in the periphery and realize their suppressive potential largely in the form of anti-inflammatory activity. The latter plays an important role in cooperation of three principal anti-inflammatory mechanisms that developed in the course of evolution: macrophages possessed of suppressive activity, Tregs, and stress hormones. Normally, all the three mechanisms of inflammation control are in equilibrium. However, the balance may be disturbed with ageing due to repeated episodes of stress and HPA axis activation. As a result, secretion of stress hormones coupled to antigen overload leads to Treg accumulation. In the course of time activation of the HPA axis is replaced by its inhibition manifested both as a decrease of the baseline cortisol level and a reduction of stress-induced cortisol response. Cortisol present in blood at low concentrations is no longer capable of controlling inflammation and Tregs become a principal mechanism of anti-inflammatory machinery. Superfluous Treg accumulation results in the development of functional somatic syndromes, such as chronic fatigue syndrome, and (in some patients) in the growth of tumours resulting from the suppression of anticancer immunity. On the other hand, the lack of adequate antigen loading in the childhood may delay Treg maturation. Allergy and asthma manifestations may be a consequence of such Treg insufficiency. Thus, both excess and deficiency of Tregs may be at the bottom of morbid conditions. The advances in modern pharmacology open up opportunities for developing new methods to control the Treg level.

Immunochemical termination of self-tolerance

The ability to selectively induce a strong immune response against self-proteins, or increase the immunogenicity of specific epitopes in foreign antigens, would have a significant impact on the production of vaccines for cancer, protein-misfolding diseases, and infectious diseases. Here, we show that site-specific incorporation of an immunogenic unnatural amino acid into a protein of interest produces high-titer antibodies that cross-react with WT protein. Specifically, mutation of a single tyrosine residue (Tyr(86)) of murine tumor necrosis factor-alpha (mTNF-alpha) to p-nitrophenylalanine (pNO(2)Phe) induced a high-titer antibody response in mice, whereas no significant antibody response was observed for a Tyr(86) --> Phe mutant. The antibodies generated against the pNO(2)Phe are highly cross-reactive with native mTNF-alpha and protect mice against lipopolysaccharide (LPS)-induced death. This approach may provide a general method for inducing an antibody response to specific epitopes of self- and foreign antigens that lead to a neutralizing immune response.

The influence of cyclosporin A on lymphocyte attenuator expression

B and T lymphocyte attenuator (BTLA), a recently identified immune inhibitory receptor, has been demonstrated to have the ability to maintain self-tolerance and transplant-tolerance in mice. However, little is known about the effects of immunosuppressive drugs on the expression of BTLA. In the present study, we observed that the immunosuppressive drug cyclosporin A (CsA) could significantly reduce BTLA but not CD25 and CD69 expression on CD4+ T cells during activation in vitro, while rapamycin (RPM) had little effect on it. Exogenous interleukin-2 (IL-2) failed to reverse the inhibitory effect that CsA had on BTLA expression. Furthermore, phorbol 12-myristate 13-acetate (PMA) or ionomycin alone could efficiently induce BTLA protein expression on CD4+ and CD8+ T cells, while CsA significantly suppressed BTLA expression in this system. The present data indicate that the regulation of BTLA expression on CD4+ T cells does not depend on IL-2 and T cell activation but depends on calcineurin-dependent and calcineurin-independent pathways. The observation that CsA significantly inhibits BTLA expression on CD4+ T cells during activation, suggests that CsA might block the immune tolerance induced by BTLA and potentially increase the susceptibility to autoimmune diseases and graft rejection.

Human CD4+CD25+ regulatory T cells: proteome analysis identifies galectin-10 as a novel marker essential for their anergy and suppressive function

CD4(+)CD25(+)Foxp3(+) regulatory T cells (CD25(+) Treg cells) direct the maintenance of immunological self-tolerance by active suppression of autoaggressive T-cell populations. However, the molecules mediating the anergic state and regulatory function of CD25(+) Treg cells are still elusive. Using differential proteomics, we identified galectin-10, a member of the lectin family, as constitutively expressed in human CD25(+) Treg cells, while they are nearly absent in resting and activated CD4(+) T cells. These data were confirmed on the mRNA and protein levels. Single-cell staining and flow cytometry showed a strictly intracellular expression of galectin-10 in CD25(+) Treg cells. Specific inhibition of galectin-10 restored the proliferative capacity of CD25(+) Treg cells and abrogated their suppressive function. Notably, first identified here as expressed in human T lymphocytes, galectin-10 is essential for the functional properties of CD25(+) Treg cells.

Subversion of B lymphocyte tolerance by hydralazine, a potential mechanism for drug-induced lupus

Accumulating evidence indicates that epigenetic alterations contribute to exacerbated activation or deregulation of the mechanisms that maintain tolerance to self-antigens in patients with lupus, a systemic autoimmune disease that can be triggered by medications taken to treat a variety of conditions. Here, we tested the effect of hydralazine, an antihypertensive drug that triggers lupus, on receptor editing, a chief mechanism of B lymphocyte tolerance to self-antigens. Using mice expressing transgenic human Igs, we found that hydralazine impairs up-regulation of RAG-2 gene expression and reduces secondary Ig gene rearrangements. Receptor editing was also partially abolished in a dose-dependent manner by a specific inhibitor of MEK1/2. Adoptive transfer of bone marrow B cells pretreated with hydralazine or with a MEK inhibitor to naïve syngeneic mice resulted in autoantibody production. We conclude that, by disrupting receptor editing, hydralazine subverts B lymphocyte tolerance to self and contributes to generation of pathogenic autoreactivity. We also postulate that inhibition of the Erk signaling pathway contributes to the pathogenesis of hydralazine-induced lupus and idiopathic human lupus.

Immunization with an apoptotic cell-binding protein recapitulates the nephritis and sequential autoantibody emergence of systemic lupus erythematosus

The initial events predisposing to loss of tolerance in patients with systemic lupus erythematosus (SLE) are largely unknown, as are the events that precipitate the transition from preclinical to overt disease. We hypothesized that induction of murine SLE would require tipping the balance between tolerance and immunity in two ways: 1) an immunogen that could take advantage of apoptotic cells as a scaffold for epitope spread, and 2) an immune activator that would generate a strong and persistent T cell response to the inciting immunogen. We show that immunization of C57BL/6 and BALB/c mice with human beta(2)-glycoprotein I, an apoptotic cell-binding protein, in the presence of LPS induces a long-lived, potent response to beta(2)-glycoprotein I that results in epitope spread to multiple SLE autoantigens. SLE-specific autoantibodies emerged in a sequential manner that recapitulated the order seen in human SLE. Moreover, immunized mice developed overt glomerulonephritis closely resembling human lupus nephritis.

Cyclosporine Preserves the Anergic State of Human T Cells Induced by Costimulation Blockade In Vitro

BACKGROUND

Costimulation blockade based tolerance-inducing therapies might be disrupted by adjunct conventional immunosuppressive drug use. In the current study, we evaluated the compatibility of various immunosuppressive agents on costimulation blockade-based immunosuppression and T-cell anergy induction of human alloreactive T-cells in vitro. T-cell anergy is crucial in transplantation tolerance.

METHODS

T cell anergy was induced in human mixed lymphocyte cultures in vitro, by monoclonal antibodies directed against the costimulatory ligands CD40 and CD86. The effect of coadministration of conventional immunosuppressive drugs (CsA, rapamycin or FK506) on the inhibitory potential of costimulation blockade and the induction and maintenance of T cell anergy was analyzed.

RESULTS

We found that monoclonal antibodies against CD40 and CD86 and the simultaneous use of conventional immunosuppressive drugs resulted in strong immunosuppression of proliferation and cytokine production. Rapamycin, in contrast to FK506 and CsA, facilitated T-cell apoptosis. However, drug cotreatment prevented costimulation blockade induced T-cell anergy. Induction of human T-cell anergy in vitro required approximately 5 days of culture. Coadministration of drugs at day 5 after the start of mAb treatment, when anergy was established, did not increase the immunosuppressive effect of mAb treatment. But interestingly, in the majority of experiments, in contrast to rapamycin and FK506, CsA did not affect the anergic state when given after T-cell anergy induction. Moreover, the cell death facilitating potential of rapamycin vanished when used later after T-cell activation.

CONCLUSIONS

Timing and choice of conventional drug are crucial in the success of costimulation blockade-based tolerance induction therapies.

Bell's palsy associated with IFN-alpha and ribavirin therapy for hepatitis C virus infection

First-line therapy for hepatitis C virus (HCV) infection comprises interferon-alpha (IFN-alpha) and ribavirin for 6 or 12 months. Mild complications of therapy are common, but more serious complications are rare. Three patients with chronic HCV infection, acquired through injecting drug use, developed idiopathic facial paralysis (Bell's palsy) during therapy, with spontaneous resolution after withdrawal of treatment. Large-scale cohort studies reveal that IFNs are associated rarely with neurologic complications, and only one previous report has linked IFN-alpha therapy and Bell's palsy. We postulate that IFN-alpha therapy led to a breakdown of peripheral tolerance to myelin sheath antigens, leading to neuropathy, just as IFN-alpha therapy can cause autoimmune thyroiditis through breakdown of tolerance to native thyroid antigens.

Immunological study of IFNbeta-1a-treated and untreated multiple sclerosis patients: clarifying IFNbeta mechanisms and establishing specific dendritic cell immunotherapy

OBJECTIVES

A comparative immunological evaluation of multiple sclerosis (MS) patients receiving IFNbeta treatment and patients who are not receiving treatment may help clarify IFNbeta neurological mechanisms and lead the way to an effective dendritic cell (DC) immunotherapy. This type of study helps clarify the pathological function of T cells and DCs within the TH1/TH2/TH3 network as well as the specific interactions between TH1/TH2/TH3 cytokines implicated in MS pathological mechanisms and determine the best way of reestablishing the TH1/TH2/TH3 network equilibrium.

METHODS

We studied network interactions between TH1/TH2/TH3 cytokine levels in serum and supernatants of whole blood and CD14+ monocyte-derived DCs in the remission phase of the disease and in correlation to the Expanded Disability Status Scale (EDSS).

RESULTS

We found that TH1 dysregulation results in a disruption of the maturation and activation of dendritic and T cells, and a lack of T-regulating cells for the induction of self-tolerance; IFNbeta mechanisms restore regulation by reestablishing the network balance but fail to resolve the disease completely due to in vivo IL12p70 network interactions leading to the deletion of self-aggressive cells.

CONCLUSIONS

Our results indicate that a specific DC immunotherapy could cure rather than treat MS. The best point to reestablish the normal physiological cycle is at the immature DC stage which can be done in vitro with treated peripheral blood CD14+ cells and used in vivo to stimulate the expansion of specific regulatory T cells.

In vivo cyclophosphamide and IL-2 treatment impedes self-antigen-induced effector CD4 cell tolerization: implications for adoptive immunotherapy

The development of T cell tolerance directed toward tumor-associated Ags can limit the repertoire of functional tumor-reactive T cells, thus impairing the ability of vaccines to elicit effective antitumor immunity. Adoptive immunotherapy strategies using ex vivo expanded tumor-reactive effector T cells can bypass this problem; however, the susceptibility of effector T cells to undergoing tolerization suggests that tolerance might also negatively impact adoptive immunotherapy. Nonetheless, adoptive immunotherapy strategies can be effective, particularly those utilizing the drug cyclophosphamide (CY) and/or exogenous IL-2. In the current study, we used a TCR-transgenic mouse adoptive transfer system to assess whether CY plus IL-2 treatment rescues effector CD4 cell function in the face of tolerizing Ag (i.e., cognate parenchymal self-Ag). CY plus IL-2 treatment not only enhances proliferation and accumulation of effector CD4 cells, but also preserves the ability of these cells to express the effector cytokine IFN-gamma (and to a lesser extent TNF-alpha) in proportion to the level of parenchymal self-Ag expression. When administered individually, CY but not IL-2 can markedly impede tolerization, although their combination is the most effective. Although effector CD4 cells in CY plus IL-2-treated self-Ag-expressing mice eventually succumb to tolerization, this delay results in an increased level of in situ IFN-gamma expression in cognate Ag-expressing parenchymal tissues as well as death via a mechanism that requires direct parenchymal Ag presentation. These results suggest that one potential mechanism by which CY and IL-2 augment adoptive immunotherapy strategies to treat cancer is by impeding the tolerization of tumor-reactive effector T cells.

LF 15-0195 inhibits the development of rat central nervous system autoimmunity by inducing long-lasting tolerance in autoreactive CD4 T cells

Experimental autoimmune encephalomyelitis (EAE) is a T cell-dependent autoimmune disease induced in susceptible animals by a single immunization with myelin basic protein (MBP). LF 15-0195 is a novel immunosuppressor that has been shown to have a potent immunosuppressive effect in several pathological manifestations. The purpose of this study was to investigate the effect of this drug on the induction and progression of established rat EAE and to dissect the mechanisms involved. We show that LF 15-0195 administration at the time of MBP immunization reduces the incidence and severity of EAE in Lewis rats. This drug also inhibits ongoing and passively induced EAE, indicating that LF 15-0195 affects already differentiated pathogenic lymphocytes. Compared with lymph node cells from untreated rats, lymphocytes from MBP-immunized rats treated with LF 15-0195 proliferated equally well in response to MBP in vitro, while their ability to produce effector cytokines and to transfer EAE into syngeneic recipients was significantly reduced. This phenomenon is stable and long-lasting. Indeed, neither IL-12 nor repeated stimulation with naive APC and MBP in vitro rendered MBP-specific CD4 T cells from protected rats encephalitogenic. In conclusion, LF 15-0195 treatment suppresses EAE by interfering with both the differentiation and effector functions of autoantigen-specific CD4 T cells.

CpG DNA induces self and cross-hyporesponsiveness of RAW264.7 cells in response to CpG DNA and lipopolysaccharide: alterations in IL-1 receptor-associated kinase expression

Exposure of macrophages to LPS induces a state of hyporesponsiveness to subsequent challenge with LPS. It has not been known whether previous exposure to CpG DNA induces a similar suppressive response to subsequent stimulation with CpG DNA. In the present study, we demonstrate that pretreatment with CpG DNA induces suppression of cytokine release in a murine macrophage-like cell RAW264.7 in response to subsequent challenge by CpG DNA. Additionally, CpG DNA-mediated activation of mitogen-activated protein kinases, including c-Jun NH(2)-terminal kinase, extracellular signal-regulated kinase, and p38, and activation of transcription factors AP-1, CREB, NF-kappaB, and STAT1 are greatly suppressed in the cells pre-exposed to CpG DNA. Pretreatment with CpG DNA also partially inhibited LPS-mediated production of cytokines and activation of mitogen-activated protein kinases and transcription factors. Neither LPS nor CpG DNA treatment inhibited Toll-like receptor 4, MD2, Toll-like receptor 9, myeloid differentiation factor 88, Toll/IL-1R domain-containing adaptor protein, Tollip, and TNF-alpha receptor-associated factor 6 expression. Interestingly, CpG DNA or LPS stimulation led to the inhibition of IL-1R-associated kinase expression. These results indicate that CpG DNA-induced refractory of RAW264.7 cells may be, at least in part, due to suppressed IL-1R-associated kinase expression.

Tryptophan catabolism in nonobese diabetic mice

Autoimmune diseases including insulin-dependent diabetes mellitus (IDDM) are characterized by the loss of tolerance to self determinants, activation of autoreactive lymphocytes, and subsequent damage to target organs. Recent evidence suggests that the development of autoimmune diabetes in the nonobese diabetic mouse (NOD), an animal model of IDDM, is under the control of dendritic cells. The potent antigen-presenting capacity of dendritic cells can be strongly influenced by the cell maturation state and by the cytokine milieu, and in fact these cells may acquire disparate functional abilities, from immunity to tolerance. We have previously demonstrated that, in the DBA/2 mouse, IFN-gamma potentiates the tolerogenic potential of a subset of splenic dendritic cells via activation of the enzyme indoleamine 2,3-dioxygenase (IDO) and production of tryptophan catabolites capable of inducing apoptosis in T cells. In the present study, we wanted to examine whether dendritic cells from NOD mice could be subjected to regulation by proinflammatory cytokines in the same fashion as in conventional mice. We found that IFN-gamma does not potentiate the tolerogenic effects of dendritic cells from NOD mice at four weeks of age. This finding correlates with a low expression of IDO activity, thus suggesting that poor expression of IDO by dendritic cells may play a role in the development of diabetes.

T cell apoptosis by kynurenines

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-catabolizing enzyme that, expressed by different cell types, has regulatory effects on T cells resulting from tryptophan depletion in specific local tissue microenvironments. The discovery that inhibition of IDO activity reduces the survival of MHC-mismatched fetuses in mice and that the risk of fetal allograft rejection correlates with the degree of parental tissue incompatibility has led to the hypothesis that IDO activity protects fetal allografts from maternal T cell-mediated immunity. Different mechanisms, however, might contribute to IDO-dependent immune regulation. We have found that tryptophan metabolites in the kynurenine pathway, such as 3-hydroxyanthranilic and quinolinic acids, will induce the selective apoptosis in vitro of murine thymocytes and Th1 but not Th2 cells. T cell apoptosis was observed at relatively low concentrations of kynurenines, did not require Fas/Fas ligand interactions and was associated with the activation of casapase-8 and the release of cytochrome c from mitochondria. In vivo, the two kynurenines caused depletion of specific thymocyte subsets in a fashion qualitatively similar to dexamethasone. These data may represent the first experimental evidence for the involvement of tryptophan catabolism in the regulation of T cell apoptosis and maintenance of peripheral T cell tolerance.

Cyclosporin-A differentially affects apoptosis during in vivo rat thymocyte maturation

Maturation arrest and interference with selection are two well-documented effects of cyclosporin-A (CsA) on the thymus. We recently hypothesized that these effects are related and owing to the reduced T-cell receptor (TCR)-CD3 complex-mediated signal transduction in thymocytes upon CsA treatment. In this hypothesis, the maturation arrest is the result of the additional depletion of thymocytes that normally survive by positive selection, whereas the impaired self-tolerance induction is caused by an increased survival of thymocytes that normally undergo negative selection. In this view, it is anticipated that CsA differentially affects thymocyte apoptosis during in vivo thymocyte maturation. Indeed, we report in this study a strong increase in apoptotic cells in the thymic cortex on in situ analysis. Simultaneously, the number of apoptotic cells had decreased at the cortico-medullary zone which is held to be the site for negative selection. Rapamycin (Rapa) also interferes with thymocyte maturation by inhibiting cytokine-driven proliferation. Hence, Rapa preferentially affects the early maturational stages of thymocyte development and is considered not to alter thymocyte selection and subsequent apoptotic events. Indeed, the number of apoptotic events appears not to be altered. However, possibly owing to the decrease in cortical macrophages, the apoptotic cells revealed an atypical enumeration around blood vessels. Taken together, our results favour the hypothesis that the dominant effect of CsA on the thymus is the reduction of the TCR-CD3 complex-mediated signal transduction in thymocytes upon interaction with stromal cells. Furthermore, the preferential localization of apoptotic cells next to blood vessels upon Rapa administration may indicate that endothelial cells are a back-up system for the removal of apoptotic cells.

Autoimmune disorders

Autoimmune disorders affect people of all ages, ethnicity, social class and gender. The authors briefly review the normal immune response and consider how this might break down, resulting in autoimmune disorders. Susceptibility of individuals to such disorders is discussed and examples of specific conditions are outlined.

Initiation of autoimmunity by a reactive metabolite of a lupus-inducing drug in the thymus

Drug-induced lupus is a side effect of deliberate ingestion of various medications, but its etiology, underlying mechanisms, and pathogenesis are puzzling. In vivo metabolic transformation of lupus-inducing drugs to reactive products explains how a heterogeneous set of drugs can mediate the same disease syndrome. Evidence has accumulated that drugs are transformed by extracellular oxidation from reactive oxygen species and myeloperoxidase produced when neutrophils are activated, maximizing the in situ accumulation of reactive drug metabolites within lymphoid compartments. The metabolite of procainamide, procainamide hydroxylamine, displays diverse biologic properties, but no apparent autoimmune effect has been observed. However, when procainamide hydroxylamine was introduced into the thymus of young adult normal mice, a delayed but robust autoimmune response developed. Disruption of central T-cell tolerance by intrathymic procainamide hydroxylamine resulted in the production of chromatin-reactive T cells that apparently drove the autoantibody response in the periphery. Drug-induced autoantibodies in this mouse model were remarkably similar to those in patients with procainamide-induced lupus. Therefore, this system has considerable promise to provide insight into the initiating events in drug-induced lupus and may provide a paradigm for how other xenobiotics might induce systemic autoimmunity.

T-cell recognition of lipid peroxidation products breaks tolerance to self proteins

Peroxidation of polyunsaturated fatty acids in lipoproteins and cell membrane phospholipids occurs in many situations in the body, both under normal and pathological conditions. Low-density lipoprotein is particularly prone to oxidation and is believed to be a pathogenetic component in atherogenesis. Both antibody responses and T-cell responses to oxidatively modified lipoproteins have been demonstrated in humans as well as in animal models. However, little is known about how these responses arise or how T cells recognize these antigens. In the present study, mice were immunized with homologous albumin covalently modified with a series of defined aldehydes which are known to be generated during lipid peroxidation. T-cell hybridomas from immunized animals demonstrated major histocompatibility complex-restricted and protein sequence-dependent responses to modified albumin, but not to native albumin. In addition to the response to modified epitopes, some aldehyde modifications resulted in strong antibody responses also to the non-modified protein. This T-cell-dependent break of tolerance constitutes a novel pathway for induction of autoimmunity by lipid peroxidation. The findings have implications in many situations where lipid peroxidation products are generated, including atherosclerosis and inflammatory and infectious diseases.

Environmentally induced autoimmune diseases: potential mechanisms

Environmental and other xenobiotic agents can cause autoimmunity. Examples include drug-induced lupus, toxic oil syndrome, and contaminated l-tryptophan ingestion. Numerous mechanisms, based on (italic)in vitro(/italic) evidence and animal models, have been proposed to explain how xenobiotics induce or accelerate autoimmunity. The majority of these can be divided into three general categories. The first is those inhibiting the processes involved in establishing tolerance by deletion. Inhibiting deletion can result in the release of newly generated autoreactive cells into the periphery. The second mechanism is the modification of gene expression in the cells participating in the immune response, permitting lymphocytes to respond to signals normally insufficient to initiate a response or allowing the antigen-presenting cells to abnormally stimulate a response. Abnormal gene expression can thus disrupt tolerance maintained by suppression or anergy, permitting activation of autoreactive cells. The third is the modification of self-molecules such that they are recognized by the immune system as foreign. Examples illustrating these concepts are presented, and related mechanisms that have the potential to similarly affect the immune system are noted. Some mechanisms appear to be common to a variety of agents, and different mechanisms appear to produce similar diseases. However, evidence that any of these mechanisms are actually responsible for xenobiotic-induced human autoimmune disease is still largely lacking, and the potential for numerous and as yet unidentified mechanisms also exists.

Self-reactive forbidden clones are confined to pathways of intermediate T-cell receptor cell differentiation even under immunosuppressive conditions

It is believed that self-reactive forbidden T-cell clones are generated by 'failure' of the pathway of T-cell differentiation in the thymus, if it is disturbed. We examined how such forbidden clones are generated under immunosuppressive conditions. Mice were treated with an injection of deoxyspergualin, FK506, or cycloporin A. From day 3, the number of cells yielded by various organs decreased. Because of the resistance of intermediate (int) T-cell receptor (TCR) cells (i.e. TCRint cells), they became more prominent in proportion than TCRhigh cells. TCRhigh cells are conventional T cells generated through the mainstream in the thymus, whereas TCRint cells are primordial T cells generated by the extrathymic pathway or an alternative intrathymic pathway. Similar to untreated mice, forbidden V beta 3+ and V beta 11+ clones in C3H/He (Mls-1b2a) mice were confined to TCRint cells after treatment; there was no leakage of forbidden clones into TCRhigh cells in the thymus and periphery. In parallel with the increase in the proportion of TCRint cells, the proportion of forbidden clones also increased under immunosuppressive states, especially in the liver. Liver mononuclear cells isolated from treated mice still had the potential to mediate autologous killing. The present results suggest that the generation of self-reactive clones is highly restricted to the pathways of TCRint cell differentiation even under immunosuppressive conditions.

The regulation of autoimmunity through CD4+ T cells

Our experiments imply that it is possible to use monoclonal antibody therapy to reestablish self tolerance to self antigens. This can be achieved by using a short course of an nd anti-CD4 antibody thus avoiding the problem of long term immunosuppression. The mechanism by which such a state of self tolerance is achieved remains to be clarified but possible mechanisms include deletion or anergy of autoreactive T cells or some form of suppression mediated through local cytokine production. As this antibody induced state of tolerance can be reversed in the NOD mouse by cyclophospamide deletion cannot be the method by which autoreactivity is prevented. The mixing experiments which have been described in the thyroiditis experiments strongly suggest that anery is not the mechanism. It therefore remains most likely that tolerance induced following administration of nd anti-CD4 is an active process maintained through the production of an inhibitory cytokine. This ability to reprogram the immune system using monoclonal antibodies makes it not beyond the realms of possibility that individuals suffering from IDDM may become tolerant of their beta cell antigens and thus be able to regenerate their own beta cell mass. If this could indeed occur it might mean that a lifetime of insulin injections and the development of the life threatening complications that may accompany a disease like IDDM may be avoided.