CTLA-4 downregulates epitope spreading and mediates remission in relapsing experimental autoimmune encephalomyelitis

Advances in immune checkpoint-based immunotherapies for multiple sclerosis: rationale and practice

Beyond the encouraging results and broad clinical applicability of immune checkpoint (ICP) inhibitors in cancer therapy, ICP-based immunotherapies in the context of autoimmune disease, particularly multiple sclerosis (MS), have garnered considerable attention and hold great potential for developing effective therapeutic strategies. Given the well-established immunoregulatory role of ICPs in maintaining a balance between stimulatory and inhibitory signaling pathways to promote immune tolerance to self-antigens, a dysregulated expression pattern of ICPs has been observed in a significant proportion of patients with MS and its animal model called experimental autoimmune encephalomyelitis (EAE), which is associated with autoreactivity towards myelin and neurodegeneration. Consequently, there is a rationale for developing immunotherapeutic strategies to induce inhibitory ICPs while suppressing stimulatory ICPs, including engineering immune cells to overexpress ligands for inhibitory ICP receptors, such as program death-1 (PD-1), or designing fusion proteins, namely abatacept, to bind and inhibit the co-stimulatory pathways involved in overactivated T-cell mediated autoimmunity, and other strategies that will be discussed in-depth in the current review. Video Abstract.

Systemic sclerosis in a patient with muscle dystrophy

Systemic sclerosis is an autoimmune disease that can result in lung fibrosis, and is strongly associated with the presence of serum anti-topoisomerase-I autoantibodies. A young man with genetic muscular dystrophy caused by titin-cap/telethonin (TCAP) gene mutation, developed a severe restrictive lung disease due to a fibrosing interstitial pneumonia secondary to systemic sclerosis with positive anti-topoisomerase-I antibodies. Using amino acid sequence alignment and protein structure modelling, we found that mutant telethonin exposes an amino acid sequence with significant homology to an immunodominant site of topoisomerase-I. Abnormal telethonin results in a loss of integrity of the sarcomere structure, which might result in rhabdomyolysis and abnormal protein exposure to the immune system. Our preliminary analysis suggests a possible role for mutant sarcomere protein telethonin as an immunogenic target recognised by anti-topoisomerase-I antibodies, which could explain the development of systemic sclerosis in this particular patient.

The Role of Cytotoxic T-Lymphocyte Antigen 4 in the Pathogenesis of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune neurodegenerative disorder of the central nervous system that presents heterogeneous clinical manifestations and course. It has been shown that different immune checkpoints, including Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4), can be involved in the pathogenesis of MS. CTLA-4 is a critical regulator of T-cell homeostasis and self-tolerance and represents a key inhibitor of autoimmunity. In this scopingreview, we resume the current preclinical and clinical studies investigating the role of CTLA-4 in MS with different approaches. While some of these studies assessed the expression levels of CTLA-4 on T cells by comparing MS patients with healthy controls, others focused on the evaluation of the effects of common MS therapies on CTLA-4 modulation or on the study of the CTLA-4 blockade or deficiency in experimental autoimmune encephalomyelitis models. Moreover, other studies in this field aimed to discover if the CTLA-4 gene might be involved in the predisposition to MS, whereas others evaluated the effects of treatment with CTLA4-Ig in MS. Although these results are of great interest, they are often conflicting. Therefore, further studies are needed to reveal the exact mechanisms underlying the action of a crucial immune checkpoint such as CTLA-4 in MS to identify novel immunotherapeutic strategies for MS patients.

Pulsed Radiation Therapy to Improve Systemic Control of Metastatic Cancer

Radiation therapy (RT) is emerging as an interventional modality in the cancer-immunity cycle, augmenting the activation of an adaptive immune response against tumors. RT, particularly in combination with immunotherapy, can enhance immune memory effects and shape the tumor-directed T-cell populations. However, a single cycle of RT delivered to a limited number of polymetastatic lesions is rarely sufficient to achieve systemic control. We hypothesize that several rounds of RT, akin to several rounds of immunotherapeutic drugs, is likely to provide greater clinical benefit to patients with metastatic disease. We propose that the repeated exposure to tumor antigens released by “pulsed-RT” (i.e., treating 2-4 tumor lesions with 3 irradiation cycles given one month apart) may amplify the adaptive immune response by expanding the tumor-specific T-cell receptor repertoire, the production of high-affinity tumor antibodies, and the generation of memory lymphocytes and thereby improve immune control of systemic disease.

A Functionally Distinct CXCR3+/IFN-γ+/IL-10+ Subset Defines Disease-Suppressive Myelin-Specific CD8 T Cells

Multiple sclerosis (MS) is an immune-mediated demyelinating disease of the CNS. We have previously demonstrated that CNS-specific CD8 T cells possess a disease-suppressive function in MS and variations of its animal model, experimental autoimmune encephalomyelitis (EAE), including the highly clinically relevant relapsing-remitting EAE disease course. Regulatory CD8 T cell subsets have been identified in EAE and other autoimmune diseases, but studies vary in defining phenotypic properties of these cells. In relapsing-remitting EAE, PLP_178-191 CD8 T cells suppress disease, whereas PLP_139-151 CD8 T cells lack this function. In this study, we used this model to delineate the unique phenotypic properties of CNS-specific regulatory PLP_178-191 CD8 T cells versus nonregulatory PLP_139-151 or OVA_323-339 CD8 T cells. Using multiparametric flow cytometric analyses of phenotypic marker expression, we identified a CXCR3⁺ subpopulation among activated regulatory CD8 T cells, relative to nonregulatory counterparts. This subset exhibited increased degranulation and IFN-γ and IL-10 coproduction. A similar subset was also identified in C57BL/6 mice within autoregulatory PLP_178-191 CD8 T cells but not within nonregulatory OVA_323-339 CD8 T cells. This disease-suppressing CD8 T cell subpopulation provides better insights into functional regulatory mechanisms, and targeted enhancement of this subset could represent a novel immunotherapeutic approach for MS.

Central Nervous System Demyelination Associated With Immune Checkpoint Inhibitors: Review of the Literature

Immune checkpoint inhibitors (ICI) are a novel class of antineoplastic treatment that enhances immunity against tumors. They are associated with immune adverse events, and several neurological syndromes have been described, including multiple sclerosis and atypical demyelination. We performed a systematic literature review of case reports with neurological immune adverse events that presented with central nervous system demyelination, up to December 2019. We found 23 cases: seven with myelitis, four isolated optic neuritis, one neuromyelitis optica spectrum disorder, five multiple sclerosis, and six with atypical demyelination. Ipilimumab was the most frequently used ICI (11/23). The median time to develop symptoms from the onset of ICI was 6.5 weeks [range 1.0–43.0], and from last ICI dose was 14 days [range 0–161]. Anatomopathological examination was performed in four cases, with the finding of a T-cell mediated immune response. Outcomes were generally favorable after immunosuppression: 18 patients had improvement or a full recovery, three patients did not respond to treatment, three patients died, and in one, treatment was not reported. We describe the patients' clinical presentation, treatment administered, and outcomes. We further speculate on possible pathophysiological mechanisms and discuss potential treatments that may be worth investigating.

Generation of highly activated, antigen-specific tumor-infiltrating CD8+ T cells induced by a novel T cell-targeted immunotherapy

The induction of tumor-targeted, cytotoxic T lymphocytes has been recognized as a key component to successful immunotherapy. DPX-based treatment was previously shown to effectively recruit activated CD8⁺ T cells to the tumor. Herein, we analyze the unique phenotype of the CD8⁺ T cells recruited into the tumor in response to DPX-based therapy, and how combination with checkpoint inhibitors impacts T cell response. C3-tumor-bearing mice were treated with cyclophosphamide (CPA) for seven continuous days every other week, followed by DPX treatment along with anti-CTLA-4 and/or anti-PD-1. Efficacy, immunogenicity, and CD8⁺ T cells tumor infiltration were assessed. The expression of various markers, including checkpoint markers, peptide specificity, and proliferation and activation markers, was determined by flow cytometry. tSNE analysis of the flow data revealed a resident phenotype of CD8⁺ T cells (PD-1⁺TIM-3⁺CTLA-4⁺) within untreated tumors, whereas DPX/CPA treatment induced recruitment of a novel population of CD8⁺ T cells (PD-1⁺TIM-3⁺CTLA-4⁻) within tumors. Combination of anti-CTLA-4 (ipilimumab) with DPX/CPA versus DPX/CPA alone significantly increased survival and inhibition of tumor growth, without changing overall systemic immunogenicity. Addition of checkpoint inhibitors did not significantly change the phenotype of the newly recruited cells induced by DPX/CPA. Yet, anti-CTLA-4 treatment in combination with DPX/CPA enhanced a non-antigen specific response within the tumor. Finally, the tumor-recruited CD8⁺ T cells induced by DPX/CPA were highly activated, antigen-specific, and proliferative, while resident phenotype CD8⁺ T cells, seemingly initially exhausted, were reactivated with combination treatment. This study supports the potential of combining DPX/CPA with ipilimumab to further enhance survival clinically.

The Role of Antigen Spreading in the Efficacy of Immunotherapies

The introduction and the unexpected efficacy of checkpoint inhibitors (CPI) and more recently of chimeric antigen receptor T cells (CAR T-cells) in the treatment of malignant diseases boosted the efforts in the development and clinical application of immunotherapeutic approaches. However, the definition of predictive factors associated with clinical responses as well as the identification of underlying mechanisms that promote the therapeutic efficacy remain to be determined. Starting from the first immunotherapeutic trials, it became evident that vaccine-induced tumor-specific T cells or the adoptive transfer of ex vivo-expanded T lymphocytes can recognize and eliminate malignant cells leading to long-lasting remissions in some patients. In addition, a phenomenon called epitope spreading, which was observed in responding patients, seemed to increase the efficiency possibly representing an important predictive factor. This review will focus on experimental and clinical evidence for the induction of epitope spreading and its role in the maintenance of an efficient antitumor immune response in cancer immunotherapy.

Therapeutic intervention in relapsing autoimmune demyelinating disease through induction of myelin-specific regulatory CD8 T cell responses

Multiple Sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system (CNS). We have shown that CNS-specific CD8 T cells (CNS-CD8) possess a disease suppressive function in MS and its animal model, experimental autoimmune encephalomyelitis (EAE). Previous studies have focused on the role of these cells predominantly in chronic models of disease, but the majority of MS patients present with a relapsing-remitting disease course. In this study, we evaluated the therapeutic role of CD8 T cells in the context of relapsing-remitting disease (RR-EAE), using SJL mice. We found that PLP_178-191- and MBP_84-104-CD8 ameliorated disease severity in an antigen-specific manner. In contrast, PLP_139-151-CD8 did not suppress disease. PLP_178-191-CD8 were able to reduce the number of relapses even when transferred during ongoing disease. We further ascertained that the suppressive subset of CD8 T cells was contained within the CD25 ⁺ CD8 T cell compartment post-in vitro activation with PLP_178-191. Using Listeria monocytogenes (LM) encoding CNS antigens to preferentially prime suppressive CD8 T cells in vivo, we show that LM infection induced disease suppressive CD8 T cells that protected and treated PLP_178-191 disease. Importantly, a combination of PLP_178-191-CD8 transfer boosted by LM-PLP_175-194 infection effectively treated ongoing disease induced by a non-cognate peptide (PLP_139-151), indicating that this approach could be effective even in the context of epitope spreading. These data support a potential immunotherapeutic strategy using CD8 transfer and/or LM vaccination to boost disease regulatory CD8 T cells.

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Adoptive transfer of CD8 T cells ameliorates RR-EAE in an antigen specific manner .

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Listeria monocytogenes (LM) can be used to prime disease-ameliorating CD8 T cells in RR-EAE .

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Activated CD25 ⁺ CD8 T cells preferentially harbor the disease-suppressive activity .

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Relapses in RR-EAE can be curbed using a dual strategy of CD8 T cell transfer and LM boosting .

[Physiopathological mechanisms of immune-related adverse events induced by anti-CTLA-4, anti-PD-1 and anti-PD-L1 antibodies in cancer treatment]

Recently, the emergence of anti-CTLA-4 and anti-PD-1/PD-L1 antibodies called immune check-point inhibitors (ICPI) has modified the landscape of anti-cancer treatments. These therapeutics are associated with immune related adverse events that affect many organs, most commonly skin, digestive tract, endocrine glands and lungs. This review summarizes the main physiopathological hypotheses on the mechanisms of these toxicities. In most cases, the T lymphocytes hyperactivation induced by ICPI generates a specific response directed against tumor antigens, leading to anti-tumor activity in tumor tissues but also side effects in normal tisues called "on-target". The CD8+ cytotoxic T lymphocytes-mediated cell lysis induces the release of neoantigens, tumor antigens and auto-antigens from normal tissues, respectively. This phenomenon called "epitope spreading" leads to diversification of the T cell repertoire and thus to reduced immune tolerance, which is exacerbated by inhibition of regulator T lymphocytes. Furthermore, the predominant activation of Th1 and Th17T lymphocytes mediated by ICPI induced an increased production of pro-inflammatory cytokines such as interferon-γ (IFNγ) and interleukine-17 (IL-17). These two mechanisms are responsible for the so called "off-target" toxicities. The roles of cross-reactivity with the intestinal microbiota, hypersensitivity and the specific effect of PD-L2 remain to be determined. Better knowledge of these mechanisms will improve patient care and help predict patients at risk of developing severe toxicities to ICPIs.

Immune receptors CD40 and CD86 in oral keratinocytes and implications for oral lichen planus

Lichen planus (LP) is a chronic T-cell-mediated mucocutaneous inflammatory disease that targets stratified epithelia, including those lining the oral cavity. The intraoral variant of LP (OLP) is associated with interferon (IFN)-γ production by infiltrating T lymphocytes; however, the role of epithelial cells in the etiopathogenesis OLP is not completely understood. There is however a growing body of evidence regarding the involvement of epithelial-derived cytokines, immune receptors, and costimulatory molecules in the pathobiological processes that promote and sustain OLP. In the present study, we used a reverse transcriptase-polymerase chain reaction assay to assess whether CD40-a receptor found mainly on antigen presenting cells-and the costimulatory molecule CD86 were expressed in oral keratinocytes (three strains of primary normal oral keratinocytes and the H357 cell line) in the presence or absence of IFN-γ. To further characterize the involvement of CD40 in OLP, expression and distribution of receptor and ligand (CD40/CD154) in tissues from OLP were evaluated by immunohistochemistry. The present results are the first to show that both CD40 and CD86 are constitutively expressed at low levels in oral keratinocytes and that their expression was enhanced by IFN-γ stimulation. The intensity of CD40 staining in OLP tissues was strong. Taken together, the results strongly suggest that CD40 and CD86 play a role in the pathophysiology of oral inflammatory diseases such as OLP.

Monoclonal Antibodies in Preclinical EAE Models of Multiple Sclerosis: A Systematic Review

Monoclonal antibodies (mAb) are promising therapeutics in multiple sclerosis and multiple new candidates have been developed, hence increasing the need for some agreement for preclinical mAb studies. We systematically analyzed publications of experimental autoimmune encephalomyelitis (EAE) studies showing effects of monoclonal antibodies. A PubMed search retrieved 570 records, out of which 122 studies with 253 experiments were eligible based on experimental design, number of animals and presentation of time courses of EAE scores. Analysis of EAE models, treatment schedules, single and total doses, routes of administration, and onset of treatment from pre-immunization up to 35 days after immunization revealed high heterogeneity. Total doses ranged from 0.1 to 360 mg/kg for observation times of up to 35 days after immunization. About half of experiments (142/253) used total doses of 10-70 mg/kg. Employing this range, we tested anti-Itga4 as a reference mAb at varying schedules and got no, mild or substantial EAE-score reductions, depending on the mouse strain and onset of the treatment. The result agrees with the range of outcomes achieved in 10 reported anti-Itga4 experiments. Studies comparing low and high doses of various mAbs or early vs. late onset of treatment did not reveal dose-effect or timing-effect associations, with a tendency towards better outcomes with preventive treatments starting within the first week after immunization. The systematic comparison allows for extraction of some "common" design characteristics, which may be helpful to further assess the efficacy of mAbs and role of specific targets in preclinical models of multiple sclerosis.

Co-stimulatory and Co-inhibitory Pathways in Autoimmunity

The immune system is guided by a series of checks and balances, a major component of which is a large array of co-stimulatory and co-inhibitory pathways that modulate the host response. Although co-stimulation is essential for boosting and shaping the initial response following signaling through the antigen receptor, inhibitory pathways are also critical for modulating the immune response. Excessive co-stimulation and/or insufficient co-inhibition can lead to a breakdown of self-tolerance and thus to autoimmunity. In this review, we will focus on the role of co-stimulatory and co-inhibitory pathways in two systemic (systemic lupus erythematosus and rheumatoid arthritis) and two organ-specific (multiple sclerosis and type 1 diabetes) emblematic autoimmune diseases. We will also discuss how mechanistic analysis of these pathways has led to the identification of potential therapeutic targets and initiation of clinical trials for autoimmune diseases, as well as outline some of the challenges that lie ahead.

Neuroimmunotherapies Targeting T Cells: From Pathophysiology to Therapeutic Applications

Therapeutic options for multiple sclerosis (MS) have significantly increased over the last few years. T lymphocytes are considered to play a central role in initiating and perpetuating the pathological immune response. Currently approved therapies for MS target T lymphocytes, either in an unspecific manner or directly by interference with specific T-cell pathways. While the concept of "T-cell-specific therapy" implies specificity and selectivity, currently approved approaches come from a general shaping of the immune system towards anti-inflammatory immune responses by non-T-cell-selective immune suppression or immune modulation (e.g., interferons-immune modulation approach) to a depletion of immune cell populations involving T cells (e.g., anti-CD52, alemtuzumab-immune selective depletion approach), or a selective inhibition of distinct molecular pathways in order to sequester leucocytes (e.g., natalizumab-leukocyte sequestration approach). This review will highlight the rationale and results of different T-cell-directed therapeutic approaches coming from basic animal experiments to clinical trials. We will first discuss the pathophysiological rationale for targeting T lymphocytes in MS leading to currently approved treatments acting on T lymphocytes. Furthermore, we will disuss previous promising concepts that have failed to show efficacy in clinical trials or were halted as a result of unexpected adverse events. Learning from the discrepancies between expectations and failures in practical outcomes helps to optimize future research approaches and clinical study designs. As our current view of MS pathogenesis and patient needs is rapidly evolving, novel therapeutic approaches targeting T lymphocytes will also be discussed, including specific molecular interventions such as cytokine-directed treatments or strategies enhancing immunoregulatory mechanisms. Based on clinical experience and novel pathophysiological approaches, T-cell-based strategies will remain a pillarstone of MS therapy.

Epitope Spreading of Autoantibody Response to PLA2R Associates with Poor Prognosis in Membranous Nephropathy

The phospholipase A2 receptor (PLA2R1) is the major autoantigen in idiopathic membranous nephropathy. However, the value of anti-PLA2R1 antibody titers in predicting patient outcomes is unknown. Here, we screened serum samples from 50 patients positive for PLA2R1 for immunoreactivity against a series of PLA2R1 deletion mutants covering the extracellular domains. We identified reactive epitopes in the cysteine-rich (CysR), C-type lectin domain 1 (CTLD1), and C-type lectin domain 7 (CTLD7) domains and confirmed the reactivity with soluble forms of each domain. We then used ELISAs to stratify 69 patients positive for PLA2R1 by serum reactivity to one or more of these domains: CysR (n=23), CysRC1 (n=14), and CysRC1C7 (n=32). Median ELISA titers measured using the full-length PLA2R1 antigens were not statistically different between subgroups. Patients with anti-CysR-restricted activity were younger (P=0.008), had less nephrotic range proteinuria (P=0.02), and exhibited a higher rate of spontaneous remission (P=0.03) and lower rates of renal failure progression (P=0.002) and ESRD (P=0.01) during follow-up. Overall, 31 of 69 patients had poor renal prognosis (urinary protein/creatinine ratio >4 g/g or eGFR<45 ml/min per 1.73 m(2) at end of follow-up). High anti-PLA2R1 activity and epitope spreading beyond the CysR epitope were independent risk factors of poor renal prognosis in multivariable Cox regression analysis. Epitope spreading during follow-up associated with disease worsening (n=3), whereas reverse spreading from a CysRC1C7 profile back to a CysR profile associated with favorable outcome (n=1). We conclude that analysis of the PLA2R1 epitope profile and spreading is a powerful tool for monitoring disease severity and stratifying patients by renal prognosis.

Eluding anaphylaxis allows peptide-specific prevention of the relapsing stage of experimental autoimmune encephalomyelitis

We have used a peptide derived from Acanthamoeba castellanii (ACA) to treat the relapsing phase of EAE that develops in SJL mice following immunization with the PLP 139-151 peptide. The native sequence of the ACA 81-95 peptide that shares key residues with the PLP 139-151 peptide is weakly encephalitogenic in SJL mice but is not recognized by antiserum from SJL mice immunized with PLP 139-151. A single amino acid change to the ACA 81-95 peptide sequence significantly enhanced its encephalitogenicity. When administered to SJL mice as a nonlinear peptide octamer, the modified ACA peptide prevented relapsing episodes of EAE in SJL mice previously immunized with the PLP 139-151 encephalitogenic peptide.

Exploiting apoptosis for therapeutic tolerance induction

Immune tolerance remains the most promising yet elusive strategy for treating immune-mediated diseases. An experimental strategy showing promise in phase 1 clinical studies is the delivery of Ag cross-linked to apoptotic leukocytes using ethylene carbodiimide. This approach originated from demonstration of the profound tolerance-inducing ability of i.v. administered Ag-coupled splenocytes (Ag-SP) in mice, which has been demonstrated to treat T cell-mediated disorders including autoimmunity, allergy, and transplant rejection. Recent studies have defined the intricate interplay between the innate and adaptive immune systems in Ag-SP tolerance induction. Innate mechanisms include scavenger receptor-mediated uptake of Ag-SP by host APCs, Ag representation, and the required upregulation of PD-L1 expression and IL-10 production by splenic marginal zone macrophages leading to Ag-specific T cell regulation via the combined effects of cell-intrinsic anergy and regulatory T cell induction. In this paper, we discuss the history, advantages, current mechanistic understanding, and clinical potential of tolerance induction using apoptotic Ag-coupled apoptotic leukocytes.

SOCS3 deletion in T lymphocytes suppresses development of chronic ocular inflammation via upregulation of CTLA-4 and expansion of regulatory T cells

Suppressors of cytokine signaling (SOCS) proteins are negative-feedback regulators of the JAK/STAT pathway, and SOCS3 contributes to host immunity by regulating the intensity and duration of cytokine signals and inflammatory responses. Mice with Socs3 deletion in myeloid cells exhibit enhanced STAT3 signaling, expansion of Th1 and Th17 cells, and develop severe experimental autoimmune encephalomyelitis. Interestingly, development of the unique IL-17/IFN-γ double-producing (Th17/IFN-γ and Tc17/IFN-γ) subsets that exhibit strong cytotoxic activities and are associated with pathogenesis of several autoimmune diseases has recently been shown to depend on epigenetic suppression of SOCS3 expression, further suggesting involvement of SOCS3 in autoimmunity and tumor immunity. In this study, we generated mice with Socs3 deletion in the CD4 T cell compartment (CD4-SOCS3 knockout [KO]) to determine in vivo effects of the loss of Socs3 in the T cell-mediated autoimmune disease, experimental autoimmune uveitis (EAU). In contrast to the exacerbation of experimental autoimmune encephalomyelitis in myeloid-specific SOCS3-deleted mice, CD4-SOCS3KO mice were protected from acute and chronic uveitis. Protection from EAU correlated with enhanced expression of CTLA-4 and expansion of IL-10-producing regulatory T cells with augmented suppressive activities. We further show that SOCS3 interacts with CTLA-4 and negatively regulates CTLA-4 levels in T cells, providing a mechanistic explanation for the expansion of regulatory T cells in CD4-SOCS3 during EAU. Contrary to in vitro epigenetic studies, Th17/IFN-γ and Tc17/IFN-γ populations were markedly reduced in CD4-SOCS3KO, suggesting that SOCS3 promotes expansion of the Th17/IFN-γ subset associated with development of severe uveitis. Thus, SOCS3 is a potential therapeutic target in uveitis and other autoinflammatory diseases.

Immunoregulatory molecules are master regulators of inflammation during the immune response

The balance between pro- and anti-inflammatory signalling is critical to maintain the immune homeostasis under physiological conditions as well as for the control of inflammation in different pathological settings. Recent progress in the signalling pathways that control this balance has led to the development of novel therapeutic agents for diseases characterized by alterations in the activation/suppression of the immune response. Different molecules have a key role in the regulation of the immune system, including the receptors PD-1 (Programmed cell Death 1), CTLA-4 (Cytotoxic T-Lymphocyte Antigen 4) and galectins; or the intracellular enzyme IDO (indoleamine 2,3-dioxygenase). In addition, other molecules as CD69, AhR (Aryl hydrocarbon Receptor), and GADD45 (Growth Arrest and DNA Damage-inducible 45) family members, have emerged as potential targets for the regulation of the activation/suppression balance of immune cells. This review offers a perspective on well-characterized as well as emergent negative immune regulatory molecules in the context of autoimmune inflammatory diseases.

Immune checkpoints in central nervous system autoimmunity

A number of autoimmune diseases, including multiple sclerosis, are mediated by self-reactive T cells that have escaped the deletional mechanisms of central tolerance. Usually, these T cells are kept at bay through peripheral tolerance mechanisms, including regulation through coinhibitory receptors and suppression by regulatory T cells. However, if these mechanisms fail, self-reactive T cells are activated and autoimmune responses ensue. This review outlines how the coinhibitory receptors CTLA-4 (cytotoxic T-lymphocyte antigen-4), PD-1 (programed death-1), Tim-3 (T-cell immunoglobulin- and mucin domain-containing molecule 3), and TIGIT (T-cell immunoreceptor with immunoglobulin and ITIM domains) act at different checkpoints to inhibit autoreactive T cells and suppress the development of central nervous system autoimmunity. Loss of each of these receptors predisposes to autoimmunity, indicating a non-redundant role in maintaining peripheral tolerance. At the same time, their functional patterns seem to overlap to a large degree. Therefore, we propose that only the concerted action of a combination of inhibitory receptors is able to maintain peripheral tolerance and prevent autoimmunity.

Female-biased incidence of experimental autoimmune encephalomyelitis reflects sexually dimorphic expression of surface CTLA-4 (CD152) on T lymphocytes

BACKGROUND

Autoimmune reactions occur naturally and in most cases are controlled by regulatory mechanisms. However, unwanted autoimmune responses still appear in 5% to 7% of the population, in strikingly greater frequencies in women compared with men. The chronic inflammation characteristic of autoimmune diseases is mainly initiated and maintained by autoreactive CD4(+) T lymphocytes. Costimu-lation is required for an optimal response of T lymphocytes: CD28 is a T-cell activator, whereas CTLA-4 (cytotoxic T-lymphocyte antigen 4, also known as CD152) downregulates T-cell activity. Together these costimulatory molecules provide a balance in T-cell immune response.

OBJECTIVE

The aim of this study was to elucidate the role of the inhibitory receptor CTLA-4 in the quality of sex-specific immune responses.

METHODS

At the German Rheumatism Research Center (DRFZ), Berlin, Germany, between 2006 and 2010, we tested mouse strains commonly used for the development of experimental autoimmune encephalomyelitis, the animal model of multiple sclerosis (MS). The SJL mouse strain not only mimics MS pathogenesis, but also exhibits the female predominance that occurs in patients with MS.

RESULTS

Cells derived from SJL females revealed increased proliferation and a doubled frequency of T-helper (Th)1- and Th2-like cytokines, compared with their male counterparts. Moreover, activated Th cells from male mice express significantly higher frequencies (61%) of CTLA-4 expressed at the cell surface in comparison with those of females (46%). Accordingly, close to 50% reduction of CTLA-4 expression occurred in cells of both sexes after the addition of estrogen. We observed that interferon (IFN)-γ(high) production in females occurred in a higher frequency in CD4(+) T cells cultured under neutral conditions (24.6% in females, 15.9% in males). Moreover, we observed that the IFN-yhigh producers were mainly present in females (4.5% vs 0.4% in males).

CONCLUSION

Our results suggest that induction of CTLA-4 expression could serve as a target for an immunomodulatory strategy to downregulate immune responses in sexually dimorphic autoimmune diseases.

Therapeutic approaches to multiple sclerosis: an update on failed, interrupted, or inconclusive trials of immunomodulatory treatment strategies

Multiple sclerosis (MS) continues to be a therapeutic challenge, and much effort is being made to develop new and more effective immune therapies. Particularly in the past decade, neuroimmunologic research has delivered new and highly effective therapeutic options, as seen in the growing number of immunotherapeutic agents and biologics in development. However, numerous promising clinical trials have failed to show efficacy or have had to be halted prematurely because of unexpected adverse events. Some others have shown results that are of unknown significance with regard to a reliable assessment of true efficacy versus safety. For example, studies of the highly innovative monoclonal antibodies that selectively target immunologic effector molecules have not only revealed the impressive efficacy of such treatments, they have also raised serious concerns about the safety profiles of these antibodies. These results add a new dimension to the estimation of risk-benefit ratios regarding acute or long-term adverse effects. Therapeutic approaches that have previously failed in MS have indicated that there are discrepancies between theoretical expectations and practical outcomes of different compounds. Learning from these defeats helps to optimize future study designs and to reduce the risks to patients. This review summarizes trials on MS treatments since 2001 that failed or were interrupted, attempts to analyze the underlying reasons for failure, and discusses the implications for our current view of MS pathogenesis, clinical practice, and design of future studies. In order to maintain clarity, this review focuses on anti-inflammatory therapies and does not include studies on already approved and effective disease-modifying therapies, albeit used in distinct administration routes or under different paradigms. Neuroprotective and alternative treatment strategies are presented elsewhere.

PD-1 ligands expressed on myeloid-derived APC in the CNS regulate T-cell responses in EAE

Disease progression in experimental autoimmune encephalomyelitis (EAE) is regulated by programmed death receptor 1 (PD-1) and its ligands, B7-H1 (programmed death ligand 1 (PD-L1)) and B7-DC (PD-L2). B7-H1 and B7-DC have negative regulatory effects upon binding PD-1 on activated T cells and B7-H1 deficiency increases severity of both diabetes and EAE. However, the role of PD-L expression on different APC in the CNS in regulating local T-cell function during relapsing EAE has not been examined. Our data show that the majority of CNS CD4+ T cells isolated during acute EAE are PD-1+, and T cells specific for relapse-associated epitopes express PD-1 upon antigen stimulation in the CNS. B7-H1 and B7-DC are differentially expressed on discrete APC populations in the inflamed CNS. B7-H1 and PD-1 have mainly inhibitory functions on CNS T cells. B7-H1 negatively regulates the stimulation of activated PD-1+ T(H) cells, in co-cultures with microglia and different CNS-infiltrating APC presenting endogenously processed peptides. The preponderance of IFN-gamma+ versus IL-17+ T cells in the CNS of B7-H1(-/-) mice suggests that B7-H1 more selectively suppresses T(H)-1 than T(H)-17 responses in vivo. In contrast, blockade of B7-DC has less pronounced regulatory effects. Overall, the results demonstrate that B7-H1 expressed by CNS myeloid APC negatively regulates T-cell activation during acute relapsing EAE.

Neural precursor cells inhibit multiple inflammatory signals

Intravenous neural precursor cell (NPCs) injection attenuates experimental autoimmune encephalomyelitis by reducing autoreactive T cell encephalitogenicity in lymph nodes in vivo. Here we examined NPC-lymphocyte interactions in vitro. NPCs inhibited the induction of T cell activation marker IL-2-Receptor alpha, ICOS, PD-1 and CTLA-4 and inhibited T cell proliferation. NPCs inhibited T cell activation and proliferation in response to Concavalin-A and to anti-CD3/anti-CD28, which are T cell receptor (TCR)-mediated stimuli, but not in response to phorbol myristate acetate/ionomycin, a TCR-independent stimulus. The suppressive effect was not mediated via downregulation of CD3epsilon or induction of apoptosis. We next examined NPCs effects on inflammatory-cytokine signaling. NPCs impaired IL-2-mediated phosphorylation of JAK3 in lymphocytes, and inhibited IL-6 mediated proliferation of B9 murine hybridoma cells. In conclusion, NPCs ameliorate TCR-mediated T cell activation and inhibit inflammatory cytokines' signaling in immune cells. These findings may underlie the broad anti-inflammatory effects of NPCs in vivo.

Diminished myelin-specific T cell activation associated with increase in CTLA4 and Fas molecules in multiple sclerosis patients treated with IFN-beta

Multiple sclerosis (MS) is a chronic inflammatory disease of the white matter of the central nervous system (CNS) characterized by focal areas of demyelination. Interferon-beta (IFN-beta) provides an effective treatment that lessens the frequency and severity of exacerbations in relapsing-remitting multiple sclerosis (RRMS), but the mechanisms by which IFN-beta is efficient remain uncertain. The data presented here demonstrate that IFN-beta impairs the proliferative response to myelin basic protein (MBP) and myelin, as well as increasing the expression of the CTLA4 intracellular molecule. Moreover, this treatment increases the expression of surface Fas molecules and of the soluble form of these molecules. Our hypothesis is that the increase in Fas and CTLA4 molecules in MS patients may lead to lymphocyte apoptosis, which suggests possible mechanisms underlying the therapeutic response to IFN-beta.

Modulation of dendritic cell development by immunoglobulin G in control subjects and multiple sclerosis patients

Intravenous immunoglobulin (IVIg) preparations are reportedly effective in inhibiting the relapse of multiple sclerosis (MS), but few reports have investigated the effect of IVIg on dendritic cells (DCs), which are thought to be involved in such relapses. In the system that uses monokines to differentiate DCs from peripheral blood monocytes (Mo-DCs), we investigated the effect of immunoglobulin G (IgG) on these antigen-presenting cells. Using monocytes derived from healthy volunteers, IgG partially inhibited the expression of CD1a, a marker of immature DCs (imDCs), and CD40 and CD80, which are markers associated with T cell activation. In contrast, IgG enhanced the expression of CD83, a marker of mature DCs (mDCs). Furthermore, IgG markedly inhibited the expression of CD49d [very late activation antigen (VLA)-4 alpha4-integrin], the adhesion molecule required for mDCs to cross the blood-brain barrier. We obtained similar results on all the aforementioned cell surface molecules investigated in both healthy controls and MS patients. In addition, IgG treatment of cells from both healthy controls and MS patients inhibited the production of interleukin (IL)-12, a cytokine associated with mDC differentiation, but did not inhibit the production of IL-10. These results suggested the possibility that IgG treatment, apart from its known ability to regulate inflammation, may help to prevent relapses of MS by controlling DC maturation, consequently inhibiting invasion of immune cells into the central nervous system and affecting the cytokine profile.

Experimental autoimmune encephalomyelitis can be prevented and cured by infection with Trypanosoma cruzi

Trypanosoma cruzi is an intracellular parasite that induces a strong Th1-type response and immunosuppression during the acute phase of infection. To study how the infection with T. cruzi would modulate the development of an autoimmune disease, we immunized C57BL/6 mice and IL-10 or iNOS knock-out mice of the same background with the encephalitogenic MOG 35-55 peptide and infected them with T. cruzi. Our results demonstrate that infection with T. cruzi completely prevents EAE development and furthermore induces complete and lasting remission in mice that were infected with this parasite after they had developed clinical EAE. Nitric oxide and IL-10 participate in triggering the mechanisms associated with EAE suppression by the infection. Decreased lymphoproliferation and increased frequencies of Annexin-positive cells and of T cells bearing CD95, CD95L or CTLA-4 were observed in the spleen from immunized/infected mice, as well as lower IL-2 and increased TGF-beta production in comparison with only immunized mice. Our results indicate that several effector and regulatory mechanisms of the immune response that arise during the acute phase of T. cruzi infection lastingly affect the expansion and/or effector functions of encephalitogenic cells, preventing the onset or inducing complete remission of EAE.

Mapping of the immunodominant T cell epitopes of the protein topoisomerase I

OBJECTIVES

To identify the immunodominant T cell epitopes of the topoisomerase I protein in patients with systemic sclerosis (SSc) and control subjects, using computational analysis software (TEPITOPE) and T cell proliferation assays.

METHODS

Six oligopeptides, predicted by TEPITOPE software as potential topoisomerase protein epitopes, were used to perform T cell proliferation assays in 21 patients with SSc and 15 healthy controls.

RESULTS

A positive response to at least one of the peptides was seen in 10/21 patients and 7/15 healthy controls. Among responders, the proliferative response was limited to a single peptide in 6/7 healthy controls, whereas 5/10 patients responded to more than one peptide. In responding patients a significant correlation was found between disease duration and number of peptides inducing a response (p = 0.007).

CONCLUSIONS

Several T cell epitopes of the topoisomerase I protein have been identified and evidence has been found to suggest epitope spreading in patients with SSc.

Elimination of an immunodominant CD4+ T cell epitope in human IFN-beta does not result in an in vivo response directed at the subdominant epitope

The BALB/cByJ mouse strain displays an immunodominant T cell response directed at the same CD4(+) T cell epitope peptide region in human IFN-beta, as detected in a human population-based assay. BALB/cByJ mice also recognize a second region of the protein with a lesser magnitude proliferative response. Critical residue testing of the immunodominant peptide showed that both BALB/cByJ mice and the human population response were dependent on an isoleucine residue at position 129. A variant IFN-beta molecule was constructed containing the single amino acid modification, I129V, in the immunodominant epitope. The variant displayed 100% of control antiproliferation activity. Mice immunized with unmodified IFN-beta responded weakly in vitro to the I129V variant. However, BALB/cByJ mice immunized with the I129V variant were unable to respond to either the I129V variant or the unmodified IFN-beta molecule by either T cell proliferation or Ag-specific IgG1 Ab production. This demonstrates that a single amino acid change in an immunodominant epitope can eliminate an immune response to an otherwise intact therapeutic protein. The elimination of the immunodominant epitope response also eliminated the response to the subdominant epitope in the protein. Modifying functionally immunodominant T cell epitopes within proteins may obviate the need for additional subdominant epitope modifications.

Mechanisms of genetic susceptibility to type I diabetes: beyond HLA

An individual's predisposition to Type I diabetes (T1D) is largely determined by complex interactions between several genetic loci and other, nonheritable factors. In T1D, the HLA locus has been known for decades to contribute 50% of the inherited risk. Outside the HLA are many proposed candidate loci with smaller effects, but only two confirmed candidate genes, the INS-VNTR and the CTLA-4 genes, which together do not contribute more than 15% of the risk. Because of the high frequency of the disease-associated DNA variants of these genes, understanding the biological mechanisms of such DNA variation in the context of T1D can have tremendous impact on the development of preventive therapeutics. However, establishing a causal relationship between common DNA variations and disease-predisposing functional effects is not trivial and remains difficult, as the effects are expected to be subtle. The variable-number tandem-repeat (VNTR) region upstream of the insulin gene is known to mediate expression in the thymus and pancreas, whereas various polymorphisms in the 5' and 3' regulatory regions of CTLA-4 are thought to alter gene expression and a coding A49G polymorphism exerts effects on post-translational processing. This review details the latest efforts in elucidating the functional mechanisms that explain the genetic association of the INS-VNTR and CTLA-4 genes with T1D.

Association of common T cell activation gene polymorphisms with multiple sclerosis in Australian patients

Susceptibility to multiple sclerosis (MS) may be influenced by the interaction of several genes within a biological pathway. T cell activation and costimulation may be potentially important in MS pathogenesis. We have therefore investigated associations between MS and polymorphisms in the CD152 (CTLA-4), CD28, CD80 and CD86 genes in Australian patients. We found no significant MS association with CTLA-4 exon 1 +49 alleles, and meta-analysis showed no significant association across nine comparable datasets (OR=1.04, p=0.54), nor with primary progressive MS across seven datasets (OR=1.19, p=0.21). Haplotype analysis showed a trend towards a decrease of the CTLA-4-1722C, -1577G, +49G haplotype in +49 G positive MS patients compared with controls (p=0.06). Screening of CD28, CD80 and CD86 genes identified novel polymorphisms in the putative promoter regions of CD28 (-372 G/A) and CD86 (exon 2 -359 deletionAAG). There was a significant increase of the CD28 -372 G allele frequency in MS patients vs. controls (p=0.045) and a trend towards a significant interaction between this allele and the CTLA-4 +49 G allele (OR=4.00, p=0.058). Our results suggest that the CTLA-4 +49 alone is not associated with overall susceptibility to MS, but may be important in clinical subsets of patients and/or may interact epistatically with other gene polymorphisms.

Critical role of the programmed death-1 (PD-1) pathway in regulation of experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is mediated by autoantigen-specific T cells dependent on critical costimulatory signals for their full activation and regulation. We report that the programmed death-1 (PD-1) costimulatory pathway plays a critical role in regulating peripheral tolerance in murine EAE and appears to be a major contributor to the resistance of disease induction in CD28-deficient mice. After immunization with myelin oligodendrocyte glycoprotein (MOG) there was a progressive increase in expression of PD-1 and its ligand PD-L1 but not PD-L2 within the central nervous system (CNS) of mice with EAE, peaking after 3 wk. In both wild-type (WT) and CD28-deficient mice, PD-1 blockade resulted in accelerated and more severe disease with increased CNS lymphocyte infiltration. Worsening of disease after PD-1 blockade was associated with a heightened autoimmune response to MOG, manifested by increased frequency of interferon gamma-producing T cells, increased delayed-type hypersensitivity responses, and higher serum levels of anti-MOG antibody. In vivo blockade of PD-1 resulted in increased antigen-specific T cell expansion, activation, and cytokine production. Interestingly, PD-L2 but not PD-L1 blockade in WT animals also resulted in disease augmentation. Our data are the first demonstration that the PD-1 pathway plays a critical role in regulating EAE.

A common autoimmunity predisposing signal peptide variant of the cytotoxic T-lymphocyte antigen 4 results in inefficient glycosylation of the susceptibility allele

A common T17A polymorphism in the signal peptide of the cytotoxic T-lymphocyte antigen 4 (CTLA-4), a T-cell receptor that negatively regulates immune responses, is associated with risk for autoimmune disease. Because the polymorphism is absent from the mature protein, we hypothesized that its biological effect must involve early stages of protein processing, prior to signal peptide cleavage. Constructs representing the two alleles were compared by in vitro translation, in the presence of endoplasmic reticulum membranes. We studied glycosylation by endoglycosidase H digestion and glycosylation mutant constructs, cleavage of peptide with inhibitors, and membrane integration by ultracentrifugation and proteinase K sensitivity. A major cleaved and glycosylated product was seen for both alleles of the protein but a band representing incomplete glycosylation was markedly more abundant in the predisposing Ala allele (32.7 +/- 1.0 versus 10.6% +/- 1.2 for Thr, p < 10(-9)). In addition, differential intracellular/surface partitioning was studied with co-transfection of the alleles fused to distinct fluorescent proteins in COS-1 cells. By quantitative confocal microscopy we found a higher ratio of cell surface/total CTLAThr(17) versus CTLAAla(17) (p = 0.01). Our findings corroborate observations, in other proteins, that the signal peptide can determine the efficiency of post-translational modifications other than cleavage and suggest inefficient processing of the autoimmunity predisposing Ala allele as the explanation for the genetic effect.

Reduced CTLA-4 protein and messenger RNA expression in umbilical cord blood T lymphocytes

OBJECTIVE

A favorable incidence and severity of graft-vs-host disease is observed in patients transplanted with banked, unrelated, HLA-mismatched umbilical cord blood (UCB) grafts, while the incidence of malignant relapse remains low. CTLA-4 mediates negative T-cell signaling and may contribute to the development of allogeneic tolerance. In this study, we compared protein and mRNA expression of CTLA-4 in stimulated UCB and adult peripheral blood T cells.

MATERIALS AND METHODS

T cells were isolated from UCB and adult peripheral blood and stimulated with anti-CD3 and anti-CD28 monoclonal antibodies. Cells were immunostained and analyzed by flow cytometry for both surface and intracellular expression of CTLA-4 in the presence and absence of cyclosporin A, and kinetics of CTLA-4 expression compared. CTLA-4 mRNA expression was measured using quantitative real-time polymerase chain reaction. NFAT1 protein levels were measured by Western blot analysis.

RESULTS

These studies demonstrate reduced surface and intracellular expression of CTLA-4 in stimulated UCB T cells compared to adult controls. Furthermore, reduced CTLA-4 protein expression in UCB T cells was noted to be in part transcriptionally regulated, as CTLA-4 mRNA levels also were significantly lower. Reduced CLTA-4 expression by UCB T cells followed the kinetics of delayed and reduced expression of the transcription factor NFAT1 by UCB T lymphocytes during primary stimulation. Moreover, cyclosporin A, which is known to modulate NFAT activation, reduced CTLA-4 protein expression in adult and UCB T cells.

CONCLUSION

Reduced expression of the key regulatory proteins CTLA-4 and NFAT-1 may contribute to favorable UCB T lymphocyte allogeneic responses.

Expression of the tyrosine phosphatase SRC homology 2 domain-containing protein tyrosine phosphatase 1 determines T cell activation threshold and severity of experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is a CD4 Th1-mediated inflammatory demyelinating disorder of the CNS and a well-established animal model for multiple sclerosis. Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP-1) is a cytosolic tyrosine phosphatase that is involved in regulating the T cell activation cascade from signals initiated through the TCR. To study the role of SHP-1 in EAE pathogenesis, we immunized B10.PL mice heterozygous for deletion of the SHP-1 gene (me(v+/-)) and B10.PL wild-type mice with the immunodominant epitope of myelin basic protein (MBP Ac1-11). T cell proliferation and IFN-gamma production were significantly increased in me(v+/-) mice after immunization with MBP Ac1-11. The frequency of MBP Ac1-11-specific CD4 T cells, analyzed by staining with fluorescently labeled tetramers (MBP1-11[4Y]: I-A(u) complexes), was increased in the draining lymph node cells of me(v+/-) mice compared with wild-type mice. In addition, me(v+/-) mice developed a more severe course of EAE with epitope spreading to proteolipid protein peptide 43-64. Finally, expansion of MBP Ac1-11-specific T cells in response to Ag was enhanced in me(v+/-) T cells, particularly at lower Ag concentrations. These data demonstrate that the level of SHP-1 plays an important role in regulating the activation threshold of autoreactive T cells.

The role of CTLA-4 in induction and maintenance of peripheral T cell tolerance

T cell receptor engagement and the B7-CD28 / CTLA-4 signaling pathways play critical roles in T cell activation and regulation. CD28 engagement results in T cell activation, differentiation and survival while CTLA-4 signals block IL-2 production, cell cycle progression and T cell differentiation. We explored the role of CTLA-4 in peripheral tolerance induced by intravenous administration of ethylene carbodiimide-fixed, antigen-coupled splenocytes in the PLP139 - 151-induced relapsing experimental autoimmune encephalomyelitis system. Tolerance induction with PLP139 - 151-coupled splenocytes correlates with low B7 expression on the fixed antigen-presenting cells, conditions that would favor CTLA-4-mediated inhibition. Administration of CTLA-4Ig or anti-CTLA-4 concomitant with the 'tolerogenic' stimulus, however, failed to reverse tolerance induction. In contrast, blocking CTLA-4 at the time of secondary 'immunogenic' encounter with antigen reversed the tolerant state. These findings indicate that CTLA-4 is required to maintain the unresponsive state of the tolerized T cells upon antigenic stimulation under inflammatory conditions and, therefore, have important implications for therapeutic regulation of autoimmune disease.

Epitope spreading in immune-mediated diseases: implications for immunotherapy

Evidence continues to accumulate supporting the hypothesis that tissue damage during an immune response can lead to the priming of self-reactive T and/or B lymphocytes, regardless of the specificity of the initial insult. This review will focus primarily on epitope spreading at the T-cell level. Understanding the cellular and molecular basis of epitope spreading in various chronic immune-mediated human diseases and their animal models is crucial to understanding the pathogenesis of these diseases and to the ultimate goal of designing antigen-specific treatments.

Targeted delivery of anti-CTLA-4 antibody downregulates T cell function in vitro and in vivo

CTLA-4 is a T cell surface molecule that binds to the costimulatory molecules CD80 and CD86 on antigen-presenting cells and downregulates T cell function. Therefore, we wanted to test whether antigen-specific activated T cells could be inhibited through directed CTLA-4 signaling using a bispecific antibody (BiAb) capable of simultaneously binding to CTLA-4 and a tissue-specific antigen. The BiAb was prepared by linking two separate monoclonal antibodies against CTLA-4 and the thyroid-stimulating hormone receptor (TSHR). The mouse B cell lymphoma line M12 (H2(d)) was used to induce alloreactive T cells in CBA/J mice (H2(k)); M12 cells stably transfected with the cDNA encoding murine TSHR (mM12) were used to restimulate the alloresponse in vitro. Results of assays for in vitro T cell proliferation, IL-2 production, and cytotoxicity in the presence of BiAb demonstrated that the BiAb could inhibit the T cell alloresponse when stimulated with mM12 cells but not with M12 cells. This effect was dependent on binding of TSHR-bound BiAb to CTLA-4, since the addition of soluble CTLA-4-Ig blocked the inhibitory effect. Injection of mM12 cells, along with the BiAb, not with antibodies against TSHR or CTLA-4 either separately or together, into CBA/J mice (H2(k)) downregulated alloreactive T cell responses. Our study demonstrated that the presence of CTLA-4 signaling molecules on the surface of target cells can protect those cells from immune attack by antigen-specific T cells and suggested that a similar approach could have potential therapeutic value in transplant rejection and tissue-specific autoimmune diseases.

Complexities of CD28/B7: CTLA-4 costimulatory pathways in autoimmunity and transplantation

Recent advances in the understanding of T cell activation have led to new therapeutic approaches in the treatment of immunological disorders. One attractive target of intervention has been the blockade of T cell costimulatory pathways, which result in more selective effects on only those T cells that have encountered specific antigen. In fact, in some instances, costimulatory pathway antagonists can induce antigen-specific tolerance that prevents the progression of autoimmune diseases and organ graft rejection. In this review, we summarize the current understanding of these complex costimulatory pathways including the individual roles of the CD28, CTLA-4, B7-1 (CD80), and B7-2 (CD86) molecules. We present evidence that suggests that multiple mechanisms contribute to CD28/B7-mediated T cell costimulation in disease settings that include expansion of activated pathogenic T cells, differentiation of Th1/Th2 cells, and the migration of T cells into target tissues. Additionally, the negative regulatory role of CTLA-4 in autoimmune diseases and graft rejection supports a dynamic but complex process of immune regulation that is prominent in the control of self-reactivity. This is most apparent in regulation of the CD4(+)CD25(+)CTLA-4(+) immunoregulatory T cells that control multiple autoimmune diseases. The implications of these complexities and the potential for use of these therapies in clinical immune intervention are discussed.

CTLA-4-mediated inhibition in regulation of T cell responses: mechanisms and manipulation in tumor immunotherapy

The T cell compartment of adaptive immunity provides vertebrates with the potential to survey for and respond specifically to an incredible diversity of antigens. The T cell repertoire must be carefully regulated to prevent unwanted responses to self. In the periphery, one important level of regulation is the action of costimulatory signals in concert with T cell antigen-receptor (TCR) signals to promote full T cell activation. The past few years have revealed that costimulation is quite complex, involving an integration of activating signals and inhibitory signals from CD28 and CTLA-4 molecules, respectively, with TCR signals to determine the outcome of a T cell's encounter with antigen. Newly emerging data suggest that inhibitory signals mediated by CTLA-4 not only can determine whether T cells become activated, but also can play a role in regulating the clonal representation in a polyclonal response. This review primarily focuses on the cellular and molecular mechanisms of regulation by CTLA-4 and its manipulation as a strategy for tumor immunotherapy.

Anti-CTLA-4 antibody treatment triggers determinant spreading and enhances murine myasthenia gravis

CTLA-4 appears to be a negative regulator of T cell activation and is implicated in T cell-mediated autoimmune diseases. Experimental autoimmune myasthenia gravis (EAMG), induced by immunization of C57BL/6 mice with acetylcholine receptor (AChR) in adjuvant, is an autoantibody-mediated disease model for human myasthenia gravis (MG). The production of anti-AChR Abs in MG and EAMG is T cell dependent. In the present study, we demonstrate that anti-CTLA-4 Ab treatment enhances T cell responses to AChR, increases anti-AChR Ab production, and provokes a rapid onset and severe EAMG. To address possible mechanisms underlying the enhanced autoreactive T cell responses after anti-CTLA-4 Ab treatment, mice were immunized with the immunodominant peptide alpha(146-162) representing an extracellular sequence of the ACHR: Anti-CTLA-4 Ab, but not control Ab, treatment subsequent to peptide immunization results in clinical EAMG with diversification of the autoantibody repertoire as well as enhanced T cell proliferation against not only the immunizing alpha(146-162) peptide, but also against other subdominant epitopes. Thus, treatment with anti-CTLA-4 Ab appears to induce determinant spreading, diversify the autoantibody repertoire, and enhance B cell-mediated autoimmune disease in this murine model of MG.