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.     

Anti-idiotypic antibody specific to GAD65 autoantibody prevents type 1 diabetes in the NOD mouse

Overt autoantibodies to the smaller isoform of glutamate decarboxylase (GAD65Ab) are a characteristic in patients with Type 1 diabetes (T1D). Anti-idiotypic antibodies (anti-Id) directed to GAD65Ab effectively prevent the binding of GAD65 to GAD65Ab in healthy individuals. Levels of GAD65Ab-specific anti-Id are significantly lower in patients with T1D, leading to overt GAD65Ab in these patients. To determine the possible protective role of GAD65Ab-specific anti-Id in T1D pathogenesis, we developed the monoclonal anti-Id MAb 8E6G4 specifically targeting human monoclonal GAD65Ab b96.11. MAb 8E6G4 was demonstrated as a specific anti-Id directed to the antigen binding site of b96.11. MAb 8E6G4 recognized human antibodies in sera from healthy individuals, T2D patients, and T1D patients as established by ELISA. We confirmed these MAb 8E6G4-bound human antibodies to contain GAD65Ab by testing the eluted antibodies for binding to GAD65 in radioligand binding assays. These findings confirm that GAD65Ab are present in sera of individuals, who test GAD65Ab-negative in conventional detection assays. To test our hypothesis that GAD65Ab-specific anti-Id have an immune modulatory role in T1D, we injected young Non Obese Diabetic (NOD) mice with MAb 8E6G4. The animals were carefully monitored for development of T1D for 40 weeks. Infiltration of pancreatic islets by mononuclear cells (insulitis) was determined to establish the extent of an autoimmune attack on the pancreatic islets. Administration of MAb 8E6G4 significantly reduced the cumulative incidence rate of T1D and delayed the time of onset. Insulitis was significantly less severe in animals that received MAb 8E6G4 as compared to control animals. These results support our hypothesis that anti-Id specific to GAD65Ab have a protective role in T1D.     

Structural and functional analysis of cysteine residues in human glutamate decarboxylase 65 (GAD65) and GAD67

Previously, we have shown that brain glutamate decarboxylase (GAD) is greatly inhibited by sulfhydryl reactive reagent suggesting cysteine residue(s) may play an important role in GAD function. In this report, we determined the role of cysteine residues in the recombinant human 65-kDa GAD isoform (hGAD65) and 67-kDa GAD isoform (hGAD67), using a combination of matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry and site-directed mutagenesis. Here, we report that cysteine 446 (C446) in hGAD65 is important for its activity and is present as free sulfhydryl group. This conclusion is based on the following observations: (i) mutation of C446 in hGAD65 to alanine reduced hGAD65 activity by more than 90%, (ii) MALDI-TOF analysis of the non-reduced, trypsin-digested GAD65 revealed that C446 is present as a free sulfhydryl group as indicated by a peak at m/z (mass/charge) 647.3446 (peptide 443-448) and, when GAD65 was treated with sulfhydryl reagent, N-ethylmaleimide (NEM), the peak is shifted to m/z 772.3702,a mass increase of 125.1 daltons (Da) as a result of modification of cysteine by NEM. Parallel studies have also been conducted with hGAD67. Cysteine 455 was found to be important for GAD67 activity.     

Decline in Titers of Anti-Idiotypic Antibodies Specific to Autoantibodies to GAD65 (GAD65Ab) Precedes Development of GAD65Ab and Type 1 Diabetes

The humoral Idiotypic Network consisting of antibodies and their anti-idiotypic antibodies (anti-Id) can be temporarily upset by antigen exposure. In the healthy immune response the original equilibrium is eventually restored through counter-regulatory mechanisms. In certain autoimmune diseases however, autoantibody levels exceed those of their respective anti-Id, indicating a permanent disturbance in the respective humoral Idiotypic Network. We investigated anti-Id directed to a major Type 1 diabetes (T1D)-associated autoantibody (GAD65Ab) in two independent cohorts during progression to disease. Samples taken from participants of the Natural History Study showed significantly lower anti-Id levels in individuals that later progressed to T1D compared to non-progressors (anti-Id antibody index of 0.06 vs. 0.08, respectively, p = 0.02). We also observed a significant inverse correlation between anti-Id levels and age at sampling, but only in progressors (p = 0.014). Finally, anti-Id levels in progressors showed a significant decline during progression as compared to longitudinal anti-Id levels in non-progressors (median rate of change: −0.0004 vs. +0.0004, respectively, p = 0.003), suggesting a loss of anti-Id during progression. Our analysis of the Diabetes Prediction in Skåne cohort showed that early in life (age 2) individuals at risk have anti-Id levels indistinguishable from those in healthy controls, indicating that low anti-Id levels are not an innate characteristic of the immune response in individuals at risk. Notably, anti-Id levels declined significantly in individuals that later developed GAD65Ab suggesting that the decline in anti-Id levels precedes the emergence of GAD65Ab (median rate of change: −0.005) compared to matched controls (median rate of change: +0.001) (p = 0.0016). We conclude that while anti-Id are present early in life, their levels decrease prior to the appearance of GAD65Ab and to the development of T1D.     

Influence of sex and age at onset on autoantibodies against insulin, GAD65 and IA2 in recent onset type 1 diabetic patients.

METHODS: Autoantibodies against insulin (IAA), glutamic acid decarboxylase (GADA) and tyrosine phosphatase IA2 (IA2A) were measured in sera from 448 recent onset patients with type 1 diabetes mellitus (DM) subdivided according to sex (194 female and 254 male) and age at onset (134 patients diagnosed before 10 years, 187 between 10 and 20 years, 66 between 20 and 30 years and 61 over 30 years. RESULTS: Autoantibodies were more frequent in female DM patients (93.8 vs. 86.6%, p = 0.013) due to an increased prevalence of both GADA (86.1 vs. 70.1%) and IA2A (59.3 vs. 49.2%), with GADA levels also significantly higher in women (0.24 vs. 0.18 U, p = 0.0003). When age groups were compared, there was a reduction in prevalence in patients over 20 years for both IAA (70% for patients diagnosed under 20 and 36% for older patients) and IA2A (65 and 25%, respectively). These differences also affected IAA levels, with the highest antibody titres in the youngest group (1,214.1 nU/ml in children under 10 compared to 546.9, 345.6 and 341.1 nU/ml in the subsequent groups; p < 10(-4)). GADA prevalence did not differ significantly between age groups but, nevertheless, autoantibody levels were highest among the oldest type 1 DM patients (0.327 U compared to 0.216, 0.197 and 0.176 U in the decreasing age groups; p < 10(-4)). CONCLUSION: There are sex- and age-related differences affecting the presence and/or titres of beta cell autoantibodies. We speculate that these differences could reflect the severity and specificity of the autoimmune attack against the endocrine pancreas and might influence the rate of progression to type 1 DM or the risk of developing other autoimmune diseases.     

Glutamate decarboxylase (GAD) autoantibody epitope shift during the first year of type 1 diabetes.

Autoantibodies in Type 1 diabetes patients may differentiate between glutamate decarboxylase (GAD65) cloned from human, mouse and rat with a significant better binding to the human antigen. A subgroup of 15% (27/183) patients showed significantly better binding to rodent than to human GAD65. The aim of this study was to determine whether the autoantibody specificity would remain anti-rodent during longitudinal follow-up for one year. We observed 1) that the average slope of the difference between human and mouse GAD65 autoantibodies binding increased between onset and after one year, which demonstrates reduced binding to rodent GAD65 and 2) that, in a group followed every third month, 9/11 (80%) children with rodent specific GAD65 autoantibodies at onset converted within one year to preference against human GAD65. This shift in preference was confirmed by significantly lower EC50 values in the initially anti-rodent GAD65 autoantibodies compared to samples taken one year after clinical diagnosis as determined in displacement studies with unlabeled human GAD65. We speculate that the evolution of GAD65 autoantibodies in Type 1 diabetes includes reactivity to a non-human GAD65 N-terminal end conformation. Progression towards Type 1 diabetes is, however, associated with a maturation of the immune response towards human GAD65 autoreactivity.     

Epitope mapping reveals the binding mechanism of a functional antibody cross-reactive to both human and murine programmed death 1

Of the inhibitory checkpoints in the immune system, programmed death 1 (PD-1) is one of the most promising targets for cancer immunotherapy. The anti-PD-1 antibodies currently approved for clinical use or under development bind to human PD-1 (hPD-1), but not murine PD-1. To facilitate studies in murine models, we developed a functional antibody against both human and murine PD-1, and compared the epitopes of such antibody to a counterpart that only bound to hPD-1. To quickly identify the epitopes of the 2 antibodies, we used alanine scanning and mammalian cell expression cassette. The epitope identification was based on PD-1-binding ELISA and supported by affinity ranking of surface plasmon resonance results. The hPD-1 epitopes of the 2 functional antibodies were also compared with the binding region on hPD-1 that is responsible for PD-L1 interaction. In silico modeling were conducted to explain the different binding modes of the 2 antibodies, suggesting a potential mechanism of the antibody cross-species binding.     

Molecular and electrophysiological characterization of GFP-expressing CA1 interneurons in GAD65-GFP mice

The use of transgenic mice in which subtypes of neurons are labeled with a fluorescent protein has greatly facilitated modern neuroscience research. GAD65-GFP mice, which have GABAergic interneurons labeled with GFP, are widely used in many research laboratories, although the properties of the labeled cells have not been studied in detail. Here we investigate these cells in the hippocampal area CA1 and show that they constitute ～20% of interneurons in this area. The majority of them expresses either reelin (70±2%) or vasoactive intestinal peptide (VIP; 15±2%), while expression of parvalbumin and somatostatin is virtually absent. This strongly suggests they originate from the caudal, and not the medial, ganglionic eminence. GFP-labeled interneurons can be subdivided according to the (partially overlapping) expression of neuropeptide Y (42±3%), cholecystokinin (25±3%), calbindin (20±2%) or calretinin (20±2%). Most of these subtypes (with the exception of calretinin-expressing interneurons) target the dendrites of CA1 pyramidal cells. GFP-labeled interneurons mostly show delayed onset of firing around threshold, and regular firing with moderate frequency adaptation at more depolarized potentials.     

Lipid peroxidation, antioxidant defence and acid-base status in cord blood at birth: the influence of diabetes.

Pregnancy complicated by poor control of diabetes is associated with a higher risk of embryopathies, spontaneous abortions and perinatal mortality. A number of authors suggest an involvement of reactive oxygen species (ROS) in diabetic pregnancy. Determining lipid peroxidation products (LP), scavenging enzyme activities and the umbilical cord blood's acid-base balance may contribute to an adequate diagnosis of the neonate at birth. Nevertheless, such measurements seem to have limited value in practical clinical routine. The present study evaluates LP, antioxidant defence and acid-base status related to diabetic pregnancy. Twenty-eight women with type 1 diabetes (PGDM), 19 with gestational diabetes (GDM) and 13 control cases were investigated. An additional control group consisted of 15 healthy patients with negative diabetic history; all women underwent vaginal delivery. Immediately after delivery cord blood samples and placental tissue were collected for malondialdehyde (MDA), superoxide dismutase (SOD) and glutathione (GSH) determination. Additionally, pH, pCO2, pO2 and base excess were measured in both vessels and compared to identify and exclude double venous samples. MDA levels in both cord blood and placental homogenates were significantly higher in both pregestational and gestational diabetic groups, but SOD activity was significantly diminished. Cord blood GSH was markedly elevated in PGDM and GDM. We have also shown significant differences in acid-base parameters in infants of PGDM group. Statistical analysis was performed using the Mann-Whitney U-test.These findings indicate an excessive oxidative stress in pregnancy complicated by diabetes mellitus. Evaluating LP products and scavenging enzyme activities may be valuable, sensitive indexes of fetal/neonatal threat in diabetic pregnancy in humans. Since oxidative stress is an important pathway for fetal injury, we believe that obtaining adequate measurements at the time of birth would contribute to clarifying the fetal/neonatal status in a medical and legal context and might be of value in altering therapy in newborn infants.     

Secretin-stimulated amylase release into blood is impaired in type 1 diabetes mellitus.

BACKGROUND: Prior studies have provided data indicating the existence of close interaction between pancreatic endocrine and exocrine function, but few clinical studies have explored this relationship in depth. We compared pancreatic exocrine function non-endoscopically in individuals with type 1 diabetes mellitus, type 2 diabetes mellitus, and normal glucose tolerant controls, to assess the importance of local insulin production to pancreatic exocrine function. METHODS: The plasma amylase response to intravenous secretin challenge was measured in men with type 1 diabetes mellitus (n = 5), type 2 diabetes mellitus (n = 5), and normal controls (n = 3). Patients were characterized by their urinary excretion of c-peptide and albumin over 24 hours. Autonomic neuropathy was non-invasively assessed by measuring RR variation (with deep respiration on EKG). RESULTS: Post-secretin amylase responses were generally absent with low baseline levels in the patients with type 1 diabetes mellitus. Patients with type 2 diabetes mellitus and controls showed similar twofold increases over baseline after secretin administration. When normal glucose tolerant and type 2 diabetic patients were pooled and compared against type 1 diabetes mellitus, the differences were statistically significant (p < 0.03). Total amylase response correlated positively, but weakly, with 24 h urinary C-peptide excretion (r = 0.507; p < 0.112), but not with glycemic control, duration of diabetes, or indices of autonomic neuropathy. CONCLUSIONS: Patients with type 1 diabetes mellitus, but not type 2 diabetes mellitus, have reduced pancreatic exocrine function, supporting the concept of a local paracrine effect of insulin on pancreatic acinar cells. Further studies are needed to determine the clinical impact of this deficiency, and whether such patients with type 1 diabetes mellitus would benefit from therapy with pancreatic enzyme supplementation.     

肠道菌群与儿童1型糖尿病关系的研究进展

1型糖尿病（type 1 diabetes mellitus,T1DM）是胰岛β细胞被破坏而导致胰岛素绝对缺乏所致。T1DM绝大多数属于自身免疫性疾病,由遗传及环境因素共同参与其发病过程。儿童糖尿病95%以上为T1DM。近年来儿童T1DM发病率呈现逐年上升趋势。研究表明肠道菌群与儿童T1DM的发病密切相关,T1DM儿童肠道菌群分布失调,肠道菌群可能通过影响肠道免疫系统和改变肠壁通透性来参与T1DM的发病过程。动物实验的研究中,改变肠道菌群分布和应用益生菌可以预防和延缓T1DM的发病。干预肠道菌群是否会预防、延缓儿童T1DM的发生尚需要进一步的研究,希望干预肠道菌群能为预防和治疗儿童T1DM提供新的方向。     

TH1/TH2 cytokine balance in patients with both type 1 diabetes mellitus and asthma

BACKGROUND AND OBJECTIVE: T1DM and asthma are mediated by opposite arms of the cellular immune system namely T helper (Th)1 and Th2 CD4(+) cells, respectively. Our aim was to characterize the Th1/Th2 cytokine balance in patients with both T1DM and asthma. METHODS: Forty-four patients, mean age 19 years were matched by gender and age, to 4 paired groups: T1DM and asthma, asthma only, T1DM only and healthy controls. Peripheral blood mononuclear cells (PBMC) were stimulated in vitro with disease-specific recombinant antigens; glutamic acid decarboxylase and house dust mite (Der p1 antigen) for T1DM and asthma, respectively, and non-specific mitogens; phytohemaglutinin (PHA), tetanus toxin and anti-CD3 mAb. ELISPOT and ELISA technique were used to determine INF-gamma, IL-2, IL-4, IL-13 and IL-10 expression. RESULTS: Patients with T1DM and asthma demonstrated a similar cytokine pattern but lower Th1/Th2 ratio compared to patients with T1DM only. The Th2 cytokines response to Der p1 was enhanced in patients with both diseases compared to controls. The IL-10 overall secretion was higher in patients with both diseases compared to one disease only. CONCLUSION: The Th1 and Th2 secretory pattern of patients with T1DM and asthma combines features of both diseases suggesting a unique Th1/Th2 balance.     

Identification and functional analysis of CBLB mutations in type 1 diabetes

Casitas B-lineage lymphoma b (Cblb) is a negative regulator of T-cell activation and dysfunction of Cblb in rats and mice results in autoimmunity. In particular, a nonsense mutation in Cblb has been identified in a rat model of autoimmune type 1 diabetes. To clarify the possible involvement of CBLB mutation in type 1 diabetes in humans, we performed mutation screening of CBLB and characterized functional properties of the mutations in Japanese subjects. Six missense mutations (A155V, F328L, N466D, K837R, T882A, and R968L) were identified in one diabetic subject each, excepting N466D. Of these mutations, F328L showed impaired suppression of T-cell activation and was a loss-of-function mutation. These data suggest that the F328L mutation is involved in the development of autoimmune diseases including type 1 diabetes, and also provide insight into the structure-function relationship of CBLB protein.     

HSP 10 is a new autoantigen in both autoimmune pancreatitis and fulminant type 1 diabetes

To search autoantigens in autoimmune pancreatitis (AIP), we have screened the human pancreas cDNA library with a patient’s serum and obtained 10 positive clones. Seven out of 10 clones were amylase α-2A, the autoantibody to which was specifically detected in sera from patients with AIP and fulminant type 1 diabetes (FT1DM) [T. Endo, S. Takizawa, S. Tanaka, M. Takahashi, H. Fujii, T. Kamisawa, T. Kobayashi, Amylase α-2A autoantibodies: novel marker of autoimmune pancreatitis and fulminant type 1 diabetes mellitus, Diabetes 58 (2009) 732-737]. Sequencing of 1 out of remaining 3 positive clones revealed that it was identical to heat shock protein 10 (HSP 10) cDNA. Using a recombinant HSP 10, we have developed enzyme-linked immunosorbent assay (ELISA) system for detecting autoantibodies against HSP 10. We found that autoantibody against HSP 10 was also produced with high frequency in sera from patients with AIP (92%) and FT1DM (81%), but not in chronic alcoholic pancreatitis (8%) or healthy volunteers (1.4%). These results suggest that an autoantibody against HSP 10 is also a new diagnostic marker for both AIP and FT1DM.     

Circulating immune complexes in the serum of diabetes mellitus in childhood by a modified 125I-C1q binding test.

The 125I-C1q binding test for the detection of soluble immune complexes in native unheated human serum was applied to the study of sera from 52 patients with diabetes mellitus in childhood. This radiolabeled C1q binding test is more sensitive and reproducible among the various methods proposed for the detection of immune complexes. The 125I-C1q binding activity in 52 sera from diabetes mellitus in childhood was 9.47 +/- 0.36% compared to 6.94 +/- 0.74% in normal controls. 125I-C1q binding values in diabetes mellitus in childhood were significantly higher than normal controls. Slight high values were seen in 3 patients with positive anti-DNA-antibodies in diabetes mellitus in childhood. 125I-C1q binding was not significantly increased in patients with positive antithyroid antibodies and insulin antibodies. There was no significant correlation between the duration of diabetes and 125I-C1q binding activity.     

Genome-wide analysis of copy number variation in type 1 diabetes

Type 1 diabetes (T1D) tends to cluster in families, suggesting there may be a genetic component predisposing to disease. However, a recent large-scale genome-wide association study concluded that identified genetic factors, single nucleotide polymorphisms, do not account for overall familiality. Another class of genetic variation is the amplification or deletion of >1 kilobase segments of the genome, also termed copy number variations (CNVs). We performed genome-wide CNV analysis on a cohort of 20 unrelated adults with T1D and a control (Ctrl) cohort of 20 subjects using the Affymetrix SNP Array 6.0 in combination with the Birdsuite copy number calling software. We identified 39 CNVs as enriched or depleted in T1D versus Ctrl. Additionally, we performed CNV analysis in a group of 10 monozygotic twin pairs discordant for T1D. Eleven of these 39 CNVs were also respectively enriched or depleted in the Twin cohort, suggesting that these variants may be involved in the development of islet autoimmunity, as the presently unaffected twin is at high risk for developing islet autoimmunity and T1D in his or her lifetime. These CNVs include a deletion on chromosome 6p21, near an HLA-DQ allele. CNVs were found that were both enriched or depleted in patients with or at high risk for developing T1D. These regions may represent genetic variants contributing to development of islet autoimmunity in T1D.