GAD65 as a prototypic autoantigen

The CD318/CD6 axis limits type 1 diabetes islet autoantigen-specific human T cell activation

CD6 is a glycoprotein expressed on CD4 and CD8 T cells involved in immunoregulation. CD318 has been identified as a CD6 ligand. The role of CD318 in T cell immunity is restricted as it has only been investigated in a few mice autoimmune models but not in human diseases. CD318 expression was thought to be limited to mesenchymal-epithelial cells and, therefore, contribute to CD6-mediated T cell activation in the CD318-expressing tissue rather than through interaction with antigen-presenting cells. Here, we report CD318 expression in a subpopulation of CD318+ myeloid dendritic (mDC), whereas the other peripheral blood populations were CD318 negative. However, CD318 can be induced by activation: a subset of monocytes treated with LPS and IFNγ and in vitro monocyte derived DCs were CD318+. We also showed that recombinant CD318 inhibited T cell function. Strikingly, CD318+ DCs suppressed the proliferation of autoreactive T cells specific for GAD65, a well-known targeted self-antigen in Type 1 Diabetes (T1D). Our study provides new insight into the role of the CD318/CD6 axis in the immunopathogenesis of inflammation, suggesting a novel immunoregulatory role of CD318 in T cell-mediated autoimmune diseases and identifying a potential novel immune checkpoint inhibitor as a target for intervention in T1D which is an unmet therapeutic need.

The role of the GABAergic system on insomnia

Sleep is an essential activity for the survival of mammals. Good sleep quality helps promote the performance of daily functions. In contrast, insufficient sleep reduces the efficiency of daily activities, causes various chronic diseases like Alzheimer's disease, and increases the risk of having accidents. The GABAergic system is the primary inhibitory neurotransmitter system in the central nervous system. It transits the gamma-aminobutyric acid (GABA) neurotransmitter via GABAA and GABAB receptors to counterbalance excitatory neurotransmitters, such as glutamate, noradrenaline, serotonin, acetylcholine, orexin, and dopamine, which release and increase arousal activities during sleep. Several studies emphasized that dysfunction of the GABAergic system is related to insomnia, the most prevalent sleep-related disorder. The GABAergic system comprises the GABA neurotransmitter, GABA receptors, GABA synthesis, and degradation. Many studies have demonstrated that GABA levels correlate with sleep quality, suggesting that modulating the GABAergic system may be a promising therapeutic approach for insomnia. In this article, we highlight the significance of sleep, the classification and pathology of insomnia, and the impact of the GABAergic system changes on sleep. In addition, we also review the medications that target the GABAergic systems for insomnia, including benzodiazepines (BZDs), non-BZDs, barbiturates, GABA supplements, and Chinese herbal medicines.

Immune-mediated epilepsy with GAD65 antibodies

Anti-GAD65 antibodies have been identified in both acute/subacute seizures (limbic encephalitis and extralimbic encephalitis) and chronic isolated epilepsy. The evidence of high serum titers and intrathecal synthesis play a fundamental role in diagnosis but poorly correlate with disease severity or response to therapies. It remains controversial whether anti-GAD65 Abs are the pathogenic entity or only serve as a surrogate marker for autoimmune disorders mediated by cytotoxic T cells. Unlike other immune-mediated epilepsy, although multiple combinations of therapeutics are used, the efficacy and prognosis of patients with GAD65-epilepsy patients are poor. Besides, GAD65-epilepsy is more prone to relapse and potentially evolve into a more widespread CNS inflammatory disorder. This article reviews the recent advances of GAD65-epilepsy, focusing on the diagnosis, epidemiology, pathophysiology, clinical features, and treatment, to better promote the recognition and provide proper therapy for this condition.

Gut microbiome in type 1 diabetes: A comprehensive review

Type 1 diabetes (T1D) is an autoimmune disease, which is characterized by the destruction of islet β cells in the pancreas triggered by genetic and environmental factors. In past decades, extensive familial and genome-wide association studies have revealed more than 50 risk loci in the genome. However, genetic susceptibility cannot explain the increased incidence of T1D worldwide, which is very likely attributed by the growing impact of environmental factors, especially gut microbiome. Recently, the role of gut microbiome in the pathogenesis of T1D has been uncovered by the increasing evidence from both human subjects and animal models, strongly indicating that gut microbiome might be a pivotal hub of T1D-triggering factors, especially environmental factors. In this review, we summarize the current aetiological and mechanism studies of gut microbiome in T1D. A better understanding of the role of gut microbiome in T1D may provide us with powerful prognostic and therapeutic tools in the near future.

Treatment strategies for autoimmune encephalitis

Autoimmune encephalitis is one of the most rapidly growing research topics in neurology. Along with discoveries of novel antibodies associated with the disease, clinical experience and outcomes with diverse immunotherapeutic agents in the treatment of autoimmune encephalitis are accumulating. Retrospective observations indicate that early aggressive treatment is associated with better functional outcomes and fewer relapses. Immune response to first-line immunotherapeutic agents (corticosteroids, intravenous immunoglobulin, plasma exchange, and immunoadsorption) is fair, but approximately half or more of patients are administered second-line immunotherapy (rituximab and cyclophosphamide). A small but significant proportion of patients are refractory to all first- and second-line therapies and require further treatment. Although several investigations have shown promising alternatives, the low absolute number of patients involved necessitates more evidence to establish further treatment strategies. In this review, the agents used for first- and second-line immunotherapy are discussed and recent attempts at finding new treatment options are introduced.

The Diagnosis and Treatment of Autoimmune Encephalitis

Autoimmune encephalitis causes subacute deficits of memory and cognition, often followed by suppressed level of consciousness or coma. A careful history and examination may show early clues to particular autoimmune causes, such as neuromyotonia, hyperekplexia, psychosis, dystonia, or the presence of particular tumors. Ancillary testing with MRI and EEG may be helpful for excluding other causes, managing seizures, and, rarely, for identifying characteristic findings. Appropriate autoantibody testing can confirm specific diagnoses, although this is often done in parallel with exclusion of infectious and other causes. Autoimmune encephalitis may be divided into several groups of diseases: those with pathogenic antibodies to cell surface proteins, those with antibodies to intracellular synaptic proteins, T-cell diseases associated with antibodies to intracellular antigens, and those associated with other autoimmune disorders. Many forms of autoimmune encephalitis are paraneoplastic, and each of these conveys a distinct risk profile for various tumors. Tumor screening and, if necessary, treatment is essential to proper management. Most forms of autoimmune encephalitis respond to immune therapies, although powerful immune suppression for weeks or months may be needed in difficult cases. Autoimmune encephalitis may relapse, so follow-up care is important.

Binding of autoantibodies to the core region of tissue transglutaminase is a feature of paediatric coeliac disease

OBJECTIVES

Production of autoantibodies to the enzyme tissue transglutaminase (tTG) is a hallmark of coeliac disease (CD). We have previously demonstrated that the immumoglobulin (Ig) A response to tTG in adult CD specifically targets its catalytic core region, containing the active-site triad of amino acids. The aim of the present study was to investigate this phenomenon in paediatric patients with CD, and to elucidate the contribution of each active-site residue to epitopes recognised. The specificity of the IgG anti-tTG response was also investigated and compared with that of the IgA anti-tTG response, in both paediatric and adult patients with CD.

METHODS

Wild-type and novel variants of tTG were generated via site-directed mutagenesis and expressed as glutathione-S-transferase-fusion proteins in Escherichia coli BL-21. The mutagenic variants of tTG had substitutions of 1, 1, or all of the 3 of the catalytic triad amino acids. All of the recombinant tTGs were tested for their antigenicity in IgA and IgG enzyme-linked immunosorbent assays with cohorts of paediatric (n=63) and adult (n=30) CD sera.

RESULTS

Substitution of even 1 amino acid in the catalytic triad resulted in a significant reduction of CD IgA and IgG anti-tTG binding, with all of the mutant proteins displaying diminished antigenicity compared with the wild-type protein.

CONCLUSIONS

The core region of tTG is specifically targeted from early on in disease course by CD patient autoantibodies of both the IgA and IgG class.

An analysis of the cross-reactivity of autoantibodies to GAD65 and GAD67 in diabetes

Background

Autoantibodies to GAD65 (anti-GAD65) are present in the sera of 70–80% of patients with type 1 diabetes (T1D), but antibodies to the structurally similar 67 kDa isoform GAD67 are rare. Antibodies to GAD67 may represent a cross-reactive population of anti-GAD65, but this has not been formally tested.

Methodology/Principal Findings

In this study we examined the frequency, levels and affinity of anti-GAD67 in diabetes sera that contained anti-GAD65, and compared the specificity of GAD65 and GAD67 reactivity. Anti-GAD65 and anti-GAD67 were measured by radioimmunoprecipitation (RIP) using ¹²⁵I labeled recombinant GAD65 and GAD67. For each antibody population, the specificity of the binding was measured by incubation with 100-fold excess of unlabeled GAD in homologous and heterologous inhibition assays, and the affinity of binding with GAD65 and GAD67 was measured in selected sera. Sera were also tested for reactivity to GAD65 and GAD67 by immunoblotting. Of the 85 sera that contained antibodies to GAD65, 28 contained anti–GAD67 measured by RIP. Inhibition with unlabeled GAD65 substantially or completely reduced antibody reactivity with both ¹²⁵I GAD65 and with ¹²⁵I GAD67. In contrast, unlabeled GAD67 reduced autoantibody reactivity with ¹²⁵I GAD67 but not with ¹²⁵I GAD65. Both populations of antibodies were of high affinity (>10¹⁰ l/mol).

Conclusions

Our findings show that autoantibodies to GAD67 represent a minor population of anti-GAD65 that are reactive with a cross-reactive epitope found also on GAD67. Experimental results confirm that GAD65 is the major autoantigen in T1D, and that GAD67 per se has very low immunogenicity. We discuss our findings in light of the known similarities between the structures of the GAD isoforms, in particular the location of a minor cross-reactive epitope that could be induced by epitope spreading.

Glutamatergic Excitation and GABA Release from a Transplantable Cell Line

The cell line M213-2O CL-4 was derived from cell line M213-2O and further modified to express human glutamate decarboxylase (hGAD-67), the enzyme that synthesizes GABA. Brain transplants of this cell line in animal models of epilepsy have been shown to modulate seizures. However, the mechanisms that underlie such actions are unknown. The purpose of the present study was to characterize this cell line and its responsiveness to several depolarizing conditions, in order to better understand how these cells exert their effects. Intracellular GABA levels were 34-fold higher and GAD activity was 16-fold higher in clone M213-2O CL-4 than in M213-2O. Both cell lines could take up [3H]GABA in vitro, and this uptake was prevented by nipecotic acid. By combining GABA release measurements and calcium imaging in vitro, we found that high extracellular K+, zero Mg2+, or glutamate activated M213-2O CL-4 cells and resulted in GABA release. The response to glutamate appeared to be mediated by AMPA/NMDA-like receptors. High KCl-induced GABA release was prevented when a Ca2+-free Krebs solution was used, suggesting an exocytotic-like mechanism. These results indicate that the cell line M213-2O CL-4 synthesizes, releases, and takes up GABA in vitro, and can be activated by depolarizing stimuli.

Characterization of CD4+ T cells specific for glutamic acid decarboxylase (GAD65) and proinsulin in a patient with stiff-person syndrome but without type 1 diabetes

BACKGROUND

Glutamic acid decarboxylase (GAD) is a rate-limiting enzyme in the synthesis of gamma-amino butyric acid (GABA) and an important autoantigen both in patients with type 1 diabetes (T1D) and stiff-person syndrome (SPS). Autoantibodies (GADA) to the 65-kDa isoform of GAD are a characteristic feature in both diseases. Approximately 30% of patients with SPS develop diabetes, yet, it is unclear to which extent co-existing autoimmunity to GAD65 and other islet autoantigens determines the risk of developing T1D.

METHODS

In this study, we monitored CD4+ T-cell responses to GAD65 and proinsulin in a patient with SPS who remained normoglycaemic during the 46-month follow-up.

RESULTS

Fluctuating but persistent T-cell reactivity to GAD65 was identified, as well as T-cell reactivity to proinsulin at one time point. The majority of the T-cell clones isolated from the patient with SPS produced high levels of Th2 cytokines (IL-13, IL-5 and IL-4). We also examined levels of GADA, insulin and IA-2 autoantibodies, and epitope specificity of GADA. In both serum and cerebrospinal fluid (CSF), GADA levels were high, and GADA persisted throughout the follow-up. Despite T-cell reactivity to both GAD65 and proinsulin, autoantibodies to other islet autoantigens did not develop.

CONCLUSIONS

Further follow-up will determine whether the beta-cell autoimmunity observed in this patient will eventually lead to T1D.

Protease-resistant human GAD-derived altered peptide ligands decrease TNF-alpha and IL-17 production in peripheral blood cells from patients with type 1 diabetes mellitus

Glutamic acid decarboxylase 65 (GAD) and proinsulin are major diabetes-associated autoantigens that drive autoreactive T cells. Altered peptide ligands (APL) have been proposed as reagents for the modification of autoimmune reactions. Here, we have prepared GAD-derived protease-resistant APL (prAPL) by cleavage site-directed modification. The resulting prAPL are resistant to lysosomal and serum proteases, bind with high-affinity to HLA-DRB1(*)0401 and have a prolonged half-life in the serum. GAD-derived prAPL significantly decreased the secretion of proinflammatory cytokines by a GAD-specific human T cell clone. Likewise, the production of IL-17, TNF-alpha, and secretion of IL-6 by peripheral blood lymphocytes from patients with type 1 diabetes mellitus (T1D) was reduced, when stimulated with both GAD and GAD-derived prAPL. Thus, prAPL with high affinity for HLA-DRB1(*)0401 mitigate the response of GAD-reactive human Th17 cells. The strategy of designing specific immunomodulatory protease-resistant altered peptide ligands provides the basis for novel avenues of therapeutic intervention.

Therapy with GAD in diabetes

The enzyme glutamic acid decarboxylase (GAD) is of great importance for the neurotransmission in the central nervous system, and therefore of interest for treatment of pain and neurological disease. However, it is also released in pancreas although its role is not quite clear. GAD is a major auto-antigen in the process leading to type 1 diabetes with both a clear cell-mediated immune response to GAD and auto-antibodies to GAD (GADA), which can be used as a predictor of diabetes. Administration of the isoform GAD65 can prevent autoimmune destruction of pancreatic beta cells in non-obese diabetic (NOD) mice and the subsequent need for exogenous insulin replacement. In Phase I and II studies an alum-formulated vaccine (Diamyd) has shown to be safe, and in a dose-finding study in Latent Autoimmune Diabetes in Adults (LADA) patients 20-microg was given subcutaneously one month apart indicating preservation of residual insulin secretion. A double-blind randomized Phase II trial in 70 patients (10-18 years old) with recent-onset type 1 diabetes showed significant preservation of residual insulin secretion and a GAD-specific immune response, both humoral and cell-mediated, but no treatment-related adverse events. With this promising background further studies are on their way, both intervention in newly diagnosed type 1 diabetic patients, and trials to prevent the disease.

Evolutionarily conserved antigens in autoimmune disease: implications for an infective aetiology

The immune system has evolved to eliminate or inactivate infectious organisms. An inappropriate response against self-components (autoantigens) can result in autoimmune disease. Here we examine the hypothesis that some evolutionarily conserved proteins, present in pathogenic and commensal organisms and their hosts, provide the stimulus that initiates autoimmune disease in susceptible individuals. We focus on seven autoantigens, of which at least four, glutamate decarboxylase, pyruvate dehydrogenase, histidyl-tRNA synthetase and alpha enolase, have orthologs in bacteria. Citrullinated alpha-enolase, a target for autoantibodies in 40% of patients with rheumatoid arthritis, is our main example. The major epitope is highly conserved, with over 90% identity to human in some bacteria. We propose that this reactivity of autoantibodies to shared sequences provides a model of autoimmunity in rheumatoid arthritis, which may well extend to other autoimmune disease in humans.