Diagnostic sensitivity of immunodominant epitopes of glutamic acid decarboxylase (GAD65) autoantibodies in childhood IDDM

Anti-GAD Antibodies and the Cerebellum: Where Do We Stand?

Anti-GAD65 antibodies (anti-GAD65 Abs) are associated with cerebellar ataxia (CA). The significance of anti-GAD65 Abs has been a focus of debates. Since GAD65 is intracellularly located and associated with type 1 diabetes mellitus and different clinical neurological phenotypes such as CA, stiff-person syndrome, and epilepsy, some researchers have argued that anti-GAD65 Abs have no pathogenic roles. On the other hand, recent physiological studies in vitro and in vivo have elucidated that binding of GAD65 by anti-GAD65 Abs elicits loss of GAD65 functions pertaining GABA release with an epitope dependence, leading to the development of CA. Internalization of autoantibodies has been also clarified. These studies provide substantial evidence of the pathogenesis of anti-GAD65 Abs in CA. We also discuss methodological problems in the identification of anti-GAD65 Abs.

GAD65 autoantibody characteristics in patients with co-occurring type 1 diabetes and epilepsy may help identify underlying epilepsy etiologies

BACKGROUND

Autoantibodies against the smaller isoform of glutamate decarboxylase (GAD65Ab) reflect autoimmune etiologies in Type 1 diabetes (T1D) and several neurological disorders, including Stiff Person Syndrome (SPS). GAD65Ab are also reported in cases of epilepsy, indicating an autoimmune component. GAD65Ab in patients with co-occurring T1D, epilepsy or SPS may be part of either autoimmune pathogenesis. To dissect the etiologies associated with GAD65Ab, we analyzed GAD65Ab titer, epitope specificity and enzyme inhibition in GAD65Ab-positive patients diagnosed with epilepsy (n = 28), patients with epilepsy and T1D (n = 10), patients with SPS (n = 20), and patients with T1D (n = 42).

RESULTS

GAD65Ab epitope pattern in epilepsy differed from T1D and SPS patients. Four of 10 patients with co-occurring T1D and epilepsy showed GAD65Ab profiles similar to T1D patients, while lacking GAD65Ab characteristics found in GAD65Ab-positive epilepsy patients. One of these patients responded well to anti-epileptic drugs (AEDs), while another patient did not require medication for seizure control. The third patient was refractory due to a diagnosis of meningioma. The response of the remaining patient to AEDs was unknown. GAD65Ab in the remaining six patients with T1D and epilepsy showed profiles similar to those in epilepsy patients.

CONCLUSIONS

Different autoimmune responses associated with T1D, epilepsy and SPS are reflected by disease-specific GAD65Ab patterns. Moreover, the epileptic etiology in patients diagnosed with both T1D and epilepsy may present two different etiologies regarding their epileptic condition. In one group T1D co-occurs with non-autoimmune epilepsy. In the other group GAD65Ab are part of an autoimmune epileptic condition.

DPD epitope-specific glutamic acid decarboxylase (GAD)65 autoantibodies in children with Type 1 diabetes

AIM

To study whether DPD epitope-specific glutamate decarboxylase autoantibodies are found more frequently in children with milder forms of Type 1 diabetes.

METHODS

We prospectively evaluated 75 children with new-onset autoimmune Type 1 diabetes, in whom we collected demographic, anthropometric and clinical data and measured islet autoantibodies. Glutamate decarboxylase 65 autoantibody-positive samples were analysed for epitope specificities using recombinant Fab against the DPD-defined epitope of glutamate decarboxylase 65.

RESULTS

After adjustment for age, positive DPD epitope recognition was significantly associated with higher C-peptide levels at onset (P = 0.02, r² =0.21, n = 35), and high DPD recognition in the highest quartile tended to be associated with HbA_1c ≤ 53 mmol/mol (7%) at the last follow-up [mean (sd) follow-up 1.3 (0.4) years; P = 0.07; for the model, P = 0.044, n = 30)]. Age- and sex-adjusted BMI percentile was significantly correlated with recognition of the DPD-defined epitope (P < 0.03, r² =0.14, n = 34), but this correlation was driven by the older age group (age ≥ 10 years; P = 0.016, r² =0.27, n = 21) and was not significant in younger children (P = 0.93, n = 13). There were no independent associations with sex, race/ethnicity, diabetic ketoacidosis, HbA_1c , HLA DR3-DQ2/DR4-DQ8 or autoantibody number.

CONCLUSIONS

Our findings suggest that recognition of the DPD-defined glutamate decarboxylase 65 autoantibody epitope at Type 1 diabetes onset is directly associated with β-cell function, BMI and age, which supports the hypothesis that immunological factors contribute to the clinical heterogeneity of Type 1 diabetes. Larger studies relating epitope-specific glutamate decarboxylase 65 autoantibody to clinical phenotype in children with Type 1 diabetes are warranted.

Role of humoral beta-cell autoimmunity in type 1 diabetes

Islet cell antibodies (ICA) were found for the first time more than 40 yr ago in patients with autoimmune endocrine deficiencies, including type 1 diabetes (T1D). ICA detected by indirect immunofluorescence represent a heterogeneous group of autoantibodies targeting a series of biochemical autoantigens, such as the protein tyrosine phosphatase related islet antigen 2 (IA-2), the 65 kD isoform of glutamic acid decarboxylase (GA65), and zinc transporter 8 (ZnT8) as well as currently unidentified autoantigens. The general view is that the diabetes-associated autoantibodies are not directly involved in beta-cell destruction but function as biomarkers of an ongoing destructive process. The diabetes-associated autoantibodies remain the strongest predictive marker for future development of T1D. Positivity for multiple (≥2) autoantibodies is highly predictive of clinical disease both among first-degree relatives and in the general population. Autoantibody titers are highly variable during the preclinical phase, but in many cases the titers tend to decrease before diagnosis. The first signs of beta-cell autoimmunity may appear early during the first months of life. The majority of those individuals diagnosed with T1D before puberty seroconvert to autoantibody positivity before the age of 3 yr. The natural course and duration of preclinical diabetes vary substantially from one individual to another. The characteristics of the isotype-specific response during preclinical diabetes appear to be antigen-specific. Diabetes-associated autoantibodies may be useful surrogate markers of the subsequent development of T1D in primary prevention trials. T1D may occur, albeit rarely, in the absence of any signs of humoral autoimmunity at diagnosis.

Autoantibodies to insulin, insulinoma-associated antigen-2, and zinc transporter 8 improve the prediction of early insulin requirement in adult-onset autoimmune diabetes

OBJECTIVE

The aim of this study was to identify the predictive marker for early insulin requirement in adult-onset autoimmune diabetes in the Japanese populations.

DESIGN/PATIENTS

We analyzed insulin autoantibodies (IAA), insulinoma-associated antigen-2 (IA-2) autoantibodies (IA-2icA), and zinc transporter 8 (ZnT8) autoantibodies (ZnT8A) by radioimmunoassay in 47 Japanese patients with adult-onset autoimmune diabetes who were identified by native GAD autoantibody (nGADA) screening of approximately 3000 non-insulin-requiring diabetes patients and 302 nGADA-negative type 2 diabetes patients. Furthermore, GAD65 autoantibody-specific epitopes were also analyzed using GAD65/GAD67 chimeric constructs.

RESULTS

The prevalence of IAA, IA-2icA, and ZnT8A in nGADA-positive patients was 26, 15, and 19%, respectively, which was significantly higher than that in nGADA-negative type 2 diabetes (2, 2, and 2%; P < 0.0001). Among nGADA-positive patients, 38% had one or more of IAA, IA-2icA, or ZnT8A, and 15% had two or more of these autoantibodies, compared with none of the nGADA-negative patients (P < 0.0001). Thirty-six percent of nGADA-positive patients subsequently required insulin therapy; and high nGADA titer (log-rank P = 0.003), middle epitope recognition of GAD65A (P = 0.002), and the presence of one or more of IAA, IA-2icA, or ZnT8A (P = 0.002) at diagnosis marked the risk for early requirement of insulin therapy. Multivariate logistic regression analysis showed the multiple islet autoantibodies to be independently associated with the risk for insulin requirement (odds ratio = 13.77; 95% confidence interval, 2.77-68.45; P = 0.001).

CONCLUSIONS

These results indicate that the determination of IAA, IA-2icA, and ZnT8A improves the prediction of a future insulin insufficiency in adult-onset autoimmune diabetes, which appears to be superior to GADA titer and GAD65A-specific epitopes.

Antigen presentation of detergent-free glutamate decarboxylase (GAD65) is affected by human serum albumin as carrier protein

The smaller isoform of glutamate decarboxylase (GAD65) is a major autoantigen in type 1 diabetes (TID). Its hydrophobic character requires detergent to keep the protein in solution, which complicates studies of antigen processing and presentation. In this study an attempt was made to replace detergent with human serum albumin (HSA) for in vitro antigen presentation. Different preparations of recombinant human GAD65 solubilized by HSA were incubated with Priess B cells (HLA DRB1*0401) and antigen presentation was tested with HLA DRB1*0401-restricted and epitope-specific T33.1 (GAD65 epitope 274-286) and T35 (GAD65 epitope 115-127) T-cell hybridomas. Specific epitope recognition by T33.1 (274-286) and T35 (115-127) cells varied between the different GAD65/HSA preparations, and a reverse pattern of antigen presentation was detected by the two hybridoma. The HSA-specific T-cell hybridoma 17.9 response to the different GAD65/HSA preparations followed the same pattern as that observed for the T33.1 cells. The content of immunoreactive GAD65 measured with four GAD65 antibodies indicated that the lowest GAD65 concentration resulted in the highest 274-286, but the lowest 115-127 presentation. This suggests that HSA-GAD65 interactions qualitatively affect the epitope specificity of GAD65 presentation. HSA may enhance the 274-286 epitope presentation, while suppressing the 115-127 epitope.

GAD autoantibody affinity and epitope specificity identify distinct immunization profiles in children at risk for type 1 diabetes

OBJECTIVE

Autoantibodies to insulin and GAD are features of preclinical type 1 diabetes in children. For insulin autoantibodies, the antibody affinity and epitope specificity predict which children progress to diabetes. We asked whether autoantibodies to GAD (GADAs) are heterogeneous in affinity and epitope recognition and whether diabetes-related GADA are restricted to high-affinity responses.

RESEARCH DESIGN AND METHODS

GADA affinity was measured by competitive binding experiments with [(125)I]-labeled and -unlabeled recombinant human GAD65 in the first GADA-positive sample from 95 children with a type 1 diabetes family history who were prospectively followed from birth and in follow-up samples from 65 of these children.

RESULTS

At first GADA appearance, affinity ranged from 10(7) to 10(10) l/mol. Affinity was higher in multiple islet autoantibody-positive children (P < 0.0001) and in HLA DR3-positive children (P = 0.006). GADA affinities were >10(9) l/mol in 52 of 53 multiple autoantibody-positive children. In contrast, children who were single GADA positive often had lower affinity GADA and/or GADA with specificities that were restricted to minor NH(2)-terminal GAD65 epitopes. At follow-up, affinity increased from low to high in 3 of 65 children. All 24 children who developed diabetes had high-affinity GADAs before diabetes onset.

CONCLUSIONS

Children develop discrete, heterogeneous antibody responses to GAD that could arise from distinct immunization events, only some of which are diabetes relevant. Subtyping the GADA responses using affinity measurement will improve type 1 diabetes risk assessment.

Development of 2 alternative enzyme-linked immunosorbent assays for routine screening of glutamic acid decarboxylase autoantibodies

BACKGROUND

Antibodies to GAD65 (GADA) are considered highly predictive humoral markers of the type 1 diabetes mellitus and also of the insulin requirement in adult-onset patients presumptively classified as type 2 diabetics or LADA.

METHODS

We present 2 methods for GADA assessment. The first one (fluid phase, ELISA Protocol A) is carried out in a 2-step procedure in which serum GADA are first allowed to react with a fixed dose of GAD65-biotin in solution and the residual free antigen is later assayed by a conventional ELISA. In the second test (solid phase, ELISA Protocol B) GADA are measured in an ELISA that depends on the ability of divalent autoantibodies to form a bridge between immobilized TrxGAD65 and liquid-phase biotinylated TrxGAD65.

RESULTS

All normal control samples scored negative in both variants of ELISA and RBA, hence specificity was 100% for all methods; the relative sensitivity of ELISA Protocol A respect of the RBA was 94% and that of ELISA Protocol B was 76%.

CONCLUSIONS

Although ELISA Protocol A exhibited a better performance in terms of relative sensitivity than ELISA Protocol B, the simplicity of execution and the intended use of the assay must also be taken in consideration for the final choice.

Molecular characterization of a disease associated conformational epitope on GAD65 recognised by a human monoclonal antibody b96.11

Autoantibodies to the 65kDa isoform of glutamate decarboxylase (GAD65) are associated with type I diabetes and recognise highly conformational epitope(s) that remain to be defined. The human recombinant Fab from mAb b96.11 inhibits binding of most GAD65 antibody positive sera from patients and its epitope has previously been localized to the middle region of GAD65. Recent studies indicate that b96.11 antibody specificity predicts the risk of developing type 1 diabetes in prediabetic individuals. We describe the use homology modelling, protein-protein docking simulations and biopanning of random peptide phage displayed libraries with b96.11 to predict contact amino acids on the interface of GAD65/Fab b96.11 complex. Further analysis by in vitro mutagenesis of GAD65 followed by radioimmunoprecipitation refined the amino acids contributing to the b96.11 epitope. Our studies show an interface characterized by a protruding antibody-combining site centered on the long heavy chain CDR3 loop of Fab b96.11 establishing interactions with the critical residue Phe(344) in the core of the epitope on GAD65, surrounded by charged sites within (375)RK(376) and (305)DER(307). The epitope requires residues from both middle and the C-terminal domains, and is the first precise definition of an epitope on GAD65. The nature of the b96.11 epitope leads to considerations of potential structural variations for differences in antigenicity between the isoforms GAD65 and GAD67. The study shows the utility of using a combination of in silico techniques and experimental data for molecular characterization and localization of conformational epitopes for which crystal structures are lacking.

Expression of GAD67 and novel GAD67 splice variants during human fetal pancreas development: GAD67 expression in the fetal pancreas

Glutamic acid decarboxylase (GAD) is a major inhibitory neurotransmitter in the brain, which catalyses the reaction of L-glutamate to gamma-aminobutyric acid. There are two isoforms of GAD, a 65-kDa form and a 67-kDa form, which are encoded by two different genes. As previous studies suggested a role for GAD67 splice variants during fetal pancreas development, we have investigated the mRNA expression of GAD67 and GAD67 splice variants in a series of 14 human fetal pancreases between 14 weeks gestation and term and in adult control pancreases by RT-PCR. In this study, we demonstrate mRNA expression of GAD67 and four GAD67 splice variants, including GAD25, in human fetal and adult specimens. Some of the splice variants, including various proportions of exon 7 or a new exon between exons 6 and 7, have not been described before in the human pancreas. We speculate that the expression of these GAD67 splice variants might be related to human fetal pancreas development.

A highly sensitive immunoassay resistant to autoantibody interference for detection of the diabetes-associated autoantigen glutamic acid decarboxylase 65 in blood and other biological samples

BACKGROUND

Glutamic acid decarboxylase-65 (GAD65) is a major autoantigen in autoimmune diabetes and is discharged from injured islet beta cells. GAD65 may also be released by transplanted islets undergoing immunological rejection. To test hypotheses regarding the utility of GAD65 as a biomarker for transplant rejection or diabetes-associated islet damage and also regarding the timing and instigators of GAD65 release in humans or animal models, a sensitive assay capable of measuring GAD65 in serum or plasma will be necessary. Ideally, this assay would also be resistant to interference by anti-GAD65 autoantibodies.

METHODS

A novel, magnetic bead-based assay was developed based on GAD65 capture by a monoclonal antibody directed to the only region of the protein known not to be significantly targeted by autoantibodies. A subsequent denaturation step allows sensitive immunodetection to proceed using anti-GAD65 polyclonal antibodies that would otherwise potentially be blocked by bound autoantibodies.

RESULTS

The GAD65 assay worked equally well with serum and plasma as with a solution of bovine serum albumin (BSA). The limit of blank was 31 pg/mL and did not differ significantly in the BSA solution (27 pg/mL). Mean recovery of GAD65 from the plasma of control subjects and GAD65 autoantibody-positive and -negative subjects with type 1 diabetes was 101 +/- 4.6%, 88 +/- 7.8%, and 99 +/- 7.0% (+/- SEM), respectively. The assay was used to quantify both recombinant GAD65 and the GAD65 content of human and rodent islets and other tissue extracts that were added to human plasma samples.

CONCLUSIONS

A sensitive, autoantibody-resistant GAD65 assay has been developed that is compatible with detection in serum and plasma and therefore will likely also work with a variety of other biologic fluids. This assay may enable the use of circulating GAD65 as a biomarker of islet damage or transplant rejection and will facilitate in vivo studies of the pathogenesis of anti-GAD65 autoreactivity.

Dynamic changes of GAD65 autoantibody epitope specificities in individuals at risk of developing type 1 diabetes

AIMS/HYPOTHESIS

Progression to type 1 diabetes is associated with intramolecular epitope spreading to disease-specific antibody epitopes located in the middle region of glutamic acid decarboxylase 65 (GAD65).

METHODS

The relationship between intramolecular epitope spreading of autoantibodies specific to GAD65 in relation to the risk of developing type 1 diabetes was tested in 22 high-risk individuals and 38 low-risk individuals. We determined the conformational epitopes in this longitudinal study by means of competition experiments using recombinant Fab of four GAD65-specific monoclonal antibodies.

RESULTS

Sera from high-risk children in the preclinical stage recognise a specific combination of GAD65 antibody epitopes located in the middle and the C-terminus of GAD65. High risk of progressing to disease is associated with the emergence of antibodies specific for conformational epitopes at the N-terminus and the middle region. Binding to already established antibody epitopes located in the middle and at the N-terminus increases and shows a significant relation (p=0.005) with HLA, which confers risk of developing diabetes.

CONCLUSIONS/INTERPRETATION

In type 1 diabetes, GAD65 antibodies are initially generated against the middle and C-terminal regions of GAD65. In genetically predisposed subjects the autoimmune response may then undergo intramolecular epitope spreading towards epitopes on the N-terminus and further epitopes located in the middle. These findings clearly demonstrate that the GAD65 autoantibody response in the preclinical stage of type 1 diabetes is dynamic and related to the HLA genotypes that confer risk of diabetes. GAD65-specific Fab should prove useful in predicting progression from islet autoimmunity to clinical onset of type 1 diabetes.

Diabetes-related antibodies in adult diabetic patients

Islet autoimmunity is made evident by the appearance of islet-cell antibodies directed against insulin (IAA), glutamic acid decarboxylase (GADA), protein tyrosine phosphatase IA-2 (IA-2Ab) and other autoantigens. IAA and IA-2Ab are predominantly detected in childhood type 1 diabetes mellitus (T1DM), while frequency of GADA is not affected by age. In adult-onset T1DM patients, GADA is the immune marker of higher diagnostic sensitivity. In adult diabetic patients not requiring insulin treatment for at least 6 months after diagnosis, GADA identifies the so-called latent autoimmune diabetes in adults (LADA). In over 80% of cases, LADA patients develop insulin dependency within a few years after the diagnosis and have an increased risk for the development of other organ-specific autoimmune diseases. High GADA titers identify a subgroup of LADA patients with low body mass index (BMI), low C-peptide levels and increased frequency of T1DM-related HLA class II haplotypes. GADA assay should be offered to every diabetic patient, and in cases of positivity screening for other autoimmune diseases should be carried out.

Multiplicity of the antibody response to GAD65 in Type I diabetes

Type I diabetes (TID) is an autoimmune disease characterized in part by the presence of autoantibodies directed against glutamic acid decarboxylase 65 (GAD65), among other pancreatic islet antigens. We investigated the independent epitope specificities of these GAD65 antibodies (GAD65Ab) and their combinations in the sera of new onset TID patients and first-degree relatives positive for GAD65Ab. For our analysis, we used four GAD65-specific recombinant Fabs (rFabs) that recognize different conformational determinants of GAD65 located throughout the molecule, including the N-terminal, the middle and the C-terminal regions. We used these epitope-specific rFabs in competition assays to determine the binding specificity of the autoantibodies found in patient sera. Among the 61 sera from newly diagnosed GAD65Ab-positive TID patients GAD65 binding was competed for 23 sera by all four rFabs, 29 by at least two rFabs, and in nine sera were displaced by one or no rFab. In contrast, none of the 24 sera from GAD65Ab-positive first-degree relatives of TID patients were displaced by all four rFabs. When using all four rFabs simultaneously to compete with GAD65Ab binding, binding of sera from TID patients was reduced by an average of 70%. A significantly weaker competition was observed when evaluating sera of GAD65Ab-positive first-degree relatives (P < 0.0001).

Epitope analysis of GAD65Ab using fusion proteins and rFab

The identification of disease-specific autoantibodies to the 65-kDa isoform of glutamate decarboxylase (GAD65Ab) epitopes in type 1 diabetes has been hampered by their conformational nature. Here, we compared two methods of GAD65Ab epitope analysis: GAD65/67 fusion proteins and competition assays using GAD65-specific recombinant fraction antigen binding (rFab). Sera from newly diagnosed type 1 diabetes patients (n=61) were studied using both approaches. Competition of GAD65 binding by an rFab to a specific epitope did not correlate with binding to the fusion protein that represented this epitope. Conversely, samples that bound to specific fusion proteins were not necessarily competed with rFab specific to determinants in the same region. We conclude that epitopes of different characteristics are detected by fusion proteins and by competition with rFab. Fusion proteins allow the definition of large epitope regions; however, some conformational GAD65Ab epitopes, especially those residing in the middle region, are destroyed or distorted in the fusion proteins. Competition studies using rFab allow the identification of conformational epitopes. However, monoclonal rFab may only reflect a limited proportion of the epitopes recognized by polyclonal sera. A combined analysis using both approaches may therefore be necessary to gain best understanding of autoantibody characteristics and affinity maturation.

Conformation-dependent GAD65 autoantibodies in diabetes

AIMS/HYPOTHESIS

Conformation-dependent autoantibodies directed against GAD65 are markers of Type 1 diabetes. In this study we aimed to determine whether the substitution of GAD65 with GAD67 amino acids would affect the binding of conformation-dependent GAD65 autoantibodies.

METHODS

We used PCR-based site-directed mutagenesis to generate a series of mutated GAD65 cDNA constructs in which specific GAD65 coding sequences for regions of the protein critical for autoantibody binding were replaced with GAD67 coding sequences.

RESULTS

The introduction of a point mutation at position 517, substituting glutamic acid with proline, markedly reduced the binding of disease-associated GAD65 antibodies. The binding of GAD65 antibodies to the E517P mutant was reduced in the sera of all newly diagnosed Type 1 diabetes patients ( n=85) by a mean of 72% ( p<0.0001) compared with binding to wild-type GAD65. Patients with latent autoimmune diabetes in adults ( n=24) showed a similar reduction in binding (79% reduction, p<0.0001). First-degree relatives who subsequently progressed to Type 1 diabetes ( n=12) showed a reduction in binding of 80% compared with a reduction of only 65% among relatives who had not progressed to disease ( n=38; p=0.025). In healthy GAD65Ab-positive individuals who did not progress to diabetes during a 9-year follow-up period ( n=51), binding to GAD65-E517P was reduced by only 28% compared with binding to wild-type GAD65.

CONCLUSIONS/INTERPRETATION

Differences in autoantibody binding to wild-type GAD65 versus GAD65-E517P may provide predictive information about Type 1 diabetes risk beyond that provided by the presence or absence of GAD65 autoantibodies. Lack of binding to mutant GAD65-E517P defines GAD65-positive individuals who are at higher risk of developing diabetes.

Development of glutamic acid decarboxylase 65 (GAD65) autoantibody assay using biotin–GAD65 fusion protein

We evaluated a biotin-glutamic acid decarboxylase 65 (GAD65)-based enzyme-linked immunosorbent assay (B-ELISA) to detect GAD65 autoantibodies (GAD65Ab) in 78 sera from individuals with newly diagnosed type 1 diabetes. The GAD65Ab index of patients with type 1 diabetes (mean value of GAD65Ab index of 1.891) was significantly higher than those in 50 sera from healthy control group (mean value of 0.068). The intra- and inter-assay coefficients of variation (CV) were calculated to be 1.042 and 10.703%, respectively. The specificity of the B-GAD65 ELISA was comparable to the standard radioimmunoassay (RIA) which is routinely used in the laboratory. We describe the optimal conditions of the binding kinetics from each assay-step for the detection of GAD65Ab using the WHO standard serum 97/550 as a model autoantibody serum. We concluded that incubation times of 15, 90, and 90 min for step 1 (pre-incubation of Biotin14-GAD65 with serum), step 2 (binding the Ab/Ag complex to NeutrAvidin plate), and step 3 (incubation with HRPO-anti-human IgG), respectively, along with human serum dilutions of 1:50, would provide an optimal assay signal within a relatively short timeframe.

GAD65 antibody isotypes and epitope recognition during the prediabetic process in siblings of children with type I diabetes

We observed 42 initially non-diabetic siblings of affected children to characterize the humoral immune response to the 65 kDa isoform of glutamic acid decarboxylase (GAD65) in preclinical type I diabetes. During the observation period with a mean duration of 9.6 years 21 siblings progressed to type I diabetes. The humoral immune response to GAD65 was observed initially as a simultaneous response to the middle (M) and carboxy (C)-terminal regions of the GAD65 molecule in most cases, and if the response was restricted initially to the middle region, it spread rapidly to the C-terminal domain and in a few cases later to the amino (N)-terminal domain. There was some heterogeneity in the GAD65 isotype response, but it was composed mainly of antibodies of immunoglobulin (Ig) G1 subclass. Responses of IgG2-, IgG4-, IgM- and IgA-GAD65Ab were observed frequently, whereas IgE- and IgG3-GAD65Ab responses were seen more rarely. Initially, the non-progressors tended more often to have IgG2- and IgG4-GAD65Ab than the progressors. As a sign of a dynamic process a significant isotype spreading was seen for IgG2-GAD65Ab (P < 0.05) and close to significant for IgM (P = 0.06) among progressors and for IgM-GAD65Ab (P < 0.05) among non-progressors during the observation period. This study failed to identify any GAD65 epitope- or isotype-specific antibody reactivity that could be used as a marker for progression to disease, as such progression was not associated with any specific changes in reactivity over time. Our findings indicate that epitope- and isotype-specific GAD65 antibodies are hardly capable of separating progressors from non-progressors among GAD65Ab-positive first-degree relatives of children with type I diabetes.

Recombinant Fabs of human monoclonal antibodies specific to the middle epitope of GAD65 inhibit type 1 diabetes-specific GAD65Abs

Autoantibodies to the 65-kDa isoform of GAD (GAD65Abs) are associated with type 1 diabetes development, but the conformational nature of the GAD65Ab epitopes complicates the evaluation of disease risk. Six GAD65-specific recombinant Fabs (rFabs) were cloned from monoclonal antibodies b96.11, DP-C, DP-A, DPD, 144, and 221-442. The binding of GAD65Abs in 61 type 1 diabetic patients to GAD65 was analyzed by competitive radioimmunoassays with the six rFabs to ascertain disease-specific GAD65Ab binding specificities. The median binding was reduced significantly by rFab b96.11 (72%) (P < 0.0001), DP-A (84%) (P < 0.0001), DP-C (84%) (P < 0.0001), 221-442 (79%) (P < 0.0001), and DP-D (80%) (P < 0.0001). The competition pattern in type 1 diabetic patients differed from that in GAD65Ab-positive late autoimmune diabetes in adults (LADA) patients (n = 44), first-degree relatives (n = 38), and healthy individuals (n = 14). Whereas 87 and 72% of the type 1 diabetic sera were competed by rFab b96.11 and DP-C, respectively, only 34 and 26% of LADA patients, 18 and 25% of first-degree relatives, and 7 and 28% of healthy individuals showed competition (P < 0.0001). These findings support the view that type 1 diabetes is associated with disease- and epitope-specific GAD65Abs and supports the notion that the middle epitope is disease associated. These GAD65-specific rFabs should prove useful in predicting type 1 diabetes and in the study of conformational GAD65Ab epitopes.

Engineered variants of human glutamic acid decarboxylase (GAD) and autoantibody epitope recognition

Of the two homologous forms of glutamic acid decarboxylase, GAD65 and GAD67, only GAD65 is a common target of autoimmunity. Epitope profiles of autoantibodies to GAD65 (GADA) in 140 type 1 diabetes, adult-onset diabetes mellitus (AODM), and thyroid diseases (TD) were studied. Probes were GAD65, GAD65/67 hybrids (displaying separately GAD65 residues 1-95, 96-444, and 445-585), delta GAD65 (a truncated GAD65 spanning residues 69-585), and GAD67. delta GAD65 and GAD65 detected 137 and 125 positive patients, respectively. The hybrids reacted with 113 sera and in 3 cases disclosed cryptic epitopes. Eighteen patients reacted with GAD67, indicating GAD65-GAD67 cross-reactivity. Most patients recognized both middle and C-terminal epitopes, had low reactivity against N-terminal epitopes, and seldom displayed reactivity limited to the N or C terminus. Compared with type 1 and AODM, TD patients showed a greater prevalence of multiple reactivity and higher incidence of GAD67 positivity.

Improved in planta expression of the human islet autoantigen glutamic acid decarboxylase (GAD65)

The smaller isoform of the enzyme glutamic acid decarboxylase (GAD65) is a major islet autoantigen in autoimmune type 1 diabetes mellitus (T1DM). Transgenic plants expressing human GAD65 (hGAD65) are a potential means of direct oral administration of the islet autoantigen in order to induce tolerance and prevent clinical onset of disease. We have previously reported the successful generation of transgenic tobacco and carrot that express immunoreactive, full-length hGAD65. In the present study, we tested the hypothesis that the expression levels of recombinant hGAD65 in transgenic plants can be increased by targeting the enzyme to the plant cell cytosol and by mediating expression through the potato virus X (PVX) vector. By substituting the NH2-terminal region of hGAD65 with a homologous region of rat GAD67, a chimeric GAD67(1-87)/GAD65(88-585) molecule was expressed in transgenic tobacco plants. Immunolocalization analysis showed that immunoreactive GAD67/65 was found in the plant cell cytosol. By using a radio-immuno assay with human serum from a GAD65 autoantibody-positive T1DM patient, the highest expression level of the recombinant GAD67/65 protein was estimated to be 0.19% of the total soluble protein, compared to only 0.04% of wild-type hGAD65. Transient expression of wild-type, full-length hGAD65 in N. benthamiana mediated by PVX infection was associated with expression levels of immunoreactive protein as high as 2.2% of total soluble protein. This substantial improvement of the expression of hGAD65 in plants paves the way for immunoprevention studies of oral administration of GAD65-containing transgenic plant material in animal models of spontaneous autoimmune diabetes.

Distinct antigenic features of linear epitopes at the N-terminus and C-terminus of 65 kDa glutamic acid decarboxylase (GAD65): implications for autoantigen modification during pathogenesis

Autoantibodies to 65 kDa glutamic acid decarboxylase (GAD65) are produced in many patients with autoimmune polyendocrine syndrome type II (APS-II) or stiff-man syndrome (SMS) and are heterogeneous in their epitope specificities, recognizing both conformational and linear determinants. Major linear epitopes of GAD, which are recognized by autoantibodies in a minority of these patients, occur in the N-terminal and C-terminal regions. We have investigated antibody recognition of the N- and C-termini of GAD65 in relation to their structural features as an approach to understanding what modifications to the native GAD structure may occur that facilitate the generation of antibodies specific to linear epitopes in these regions during the autoimmune pathogenesis. A monoclonal antibody specific to the N-terminus of GAD65 bound both native and denatured GAD in ELISA, whereas monoclonal and polyclonal antibodies specific to the C-terminus of GAD bound only denatured GAD. These antibodies were epitope mapped using random peptide phage-display libraries and the epitopes related to a previously proposed structural model of GAD65. This has led us to propose that the alpha-helical secondary structure of the C-terminus of GAD65 must be denatured to generate linear epitopes. In contrast, the N-terminus is both surface exposed and linear in the native structure, but may be masked by membrane interactions, which must be broken to facilitate recognition by B cells.

Complexity of human immune response profiles for CD4+ T cell epitopes from the diabetes autoantigen GAD65

Complex protein antigens contain multiple potential T cell recognition epitopes, which are generated through a processing pathway involving partial antigen degradation via proteases, binding to MHC molecules, and display on the APC surface, followed by recognition via the T cell receptor. We have investigated recognition of the GAD65 protein, one of the well-characterized autoantigens in type I diabetes, among individuals carrying the HLA-DR4 haplotypes characteristic of susceptibility to IDDM. Using sets of 20-mer peptides spanning the GAD65 molecule, multiple immunostimulatory epitopes were identified, with diverse class II DR molecules functioning as the restriction element. The majority of T cell responses were restricted by DRB1 molecules; however, DRB4 restricted responses were also observed. Antigen-specific T cell clones and lines were derived from peripheral blood samples of pre-diabetic and IDDM patients and T cell recognition and response were measured. Highly variable proliferative and cytokine release profiles were observed, even among T cells specific for a single GAD65 epitope.

Antibodies to new beta cell antigen ICA12 in Latvian diabetes patients

In Latvia diabetes mellitus is diagnosed using the WHO's clinical criteria, and assays for the detection of autoantibodies are not available. In consequence, slowly progressive autoimmune diabetes or LADA is likely to be missed. Antibodies to GAD65 and IA-2 are the major immunological markers in autoimmune diabetes. Recently, a new beta cell antigen, called ICA12, has been identified, which has a homology to the SOX family of transcription factors. The aim of the study was to analyze the prevalence of ICA12 antibodies in diabetes mellitus patients and controls from Latvia and to see whether this antigen is important in revealing autoimmunity when antibodies against major antigens are not present. We studied 88 IDDM patients and 100 NIDDM patients as well as controls for the prevalence of GAD65, IA-2, and ICA12 antibodies by radioligand binding assay (RIA) using (35)S-labeled islet antigens. We found ICA12Abs in 26 of 88 IDDM patients (30%) vs. 4% in healthy controls (4/100) and in 9 of 100 NIDDM patients (9%) vs. 2% controls (2/100). ICA12Abs alone are present in only 3% (3/88) of the patients with IDDM and 1% (1/100) of the NIDDM patients. We conclude that ICA12 represents the minor antigens in autoimmune diabetes and that, as a minor antigen, ICA12 alone does not contribute significantly in revealing new cases of autoimmunity.

Stable GAD65 autoantibody epitope patterns in type 1 diabetes children five years after onset

Autoantibodies to GAD65 (GAD65Ab) are prominent in type 1 diabetes. These autoantibodies may be present both years before and after the clinical diagnosis of type 1 diabetes and are widely used as a marker for the disease. Recently it has been demonstrated that progression to type 1 diabetes is accompanied by GAD65Ab epitope maturation. Here we examine whether autoantibody maturation processes also progress after the clinical diagnosis of type 1 diabetes. Antibody reactivity to GAD65, GAD67 and GAD65/67 fusion proteins was measured by radioimmunoassays in 62 children with type 1 diabetes. Samples were taken at diagnosis and five years later. While the overall GAD65Ab level declined over time, the epitope pattern was remarkably stable with no significant changes in binding pattern. Loss of GAD65Ab-positivity was associated with significantly lower GAD65Ab indices at diagnosis compared to patients' sera that remained GAD65Ab-positive. The decrease in GAD65Ab levels did not correlate to residual C-peptide levels. Our data suggest that processes controlling GAD65Ab levels and epitope binding patterns remain stable during the first five years of type 1 diabetes.

Site-directed mutagenesis of K396R of the 65 kDa glutamic acid decarboxylase active site obliterates enzyme activity but not antibody binding

The role of K396 in the enzymatic catalysis and the antigenicity of the 65 kDa isoform of glutamate decarboxylase (GAD65) was analyzed using the K396R GAD65 mutant. GAD65 is a major autoantigen in Type 1 diabetes and autoantibodies directed to GAD65 are widely used markers for this disease. We found that (1) recombinant human GAD65 is fully enzymatically active; (2) the K396R mutation abolished GAD65 activity; and (3) the K396R mutant retained full antigenicity to GAD65 autoantibodies in serum from Type 1 diabetes patients, but not to polyclonal antibodies raised to the catalytic domain.

Recognition of glutamic acid decarboxylase (GAD) by autoantibodies from different GAD antibody-positive phenotypes

Autoantibodies against the smaller isoform of glutamic acid decarboxylase (GAD) are markers for Type 1 diabetes. GAD65 autoantibody (GAD65Ab)-positive individuals in the general population are, however, mostly at low risk of developing Type 1 diabetes, suggesting that GAD65Ab phenotypes may be associated with different underlying pathogenic processes. The aim of this study was to test the hypothesis that Type 1 diabetes patients (n = 243; group I), GAD65Ab-positive healthy individuals (n = 28; group II), and healthy first-degree relatives of Type 1 diabetes patients (n = 41; group III) have antibody phenotypes that recognize different GAD65 epitopes. Sera from groups I-III were tested for their binding to GAD65 and GAD67, as well as six different GAD65/67 fusion proteins. Regardless of group, sera reactive to both GAD65 and GAD67 showed broader epitope reactivity than GAD65-specific sera. Furthermore, Type 1 diabetes patients showed a more restricted epitope binding than healthy individuals and first-degree relatives, demonstrating significantly less binding to the N-terminal part of GAD65 and to GAD67. Our analysis demonstrates that the N-terminal part is essential for full antibody binding to GAD65, in particular, to the middle epitope. It is suggested that Type 1 diabetes is associated with restricted GAD65Ab epitope specificity.

Alternative splicing of GAD67 results in the synthesis of a third form of glutamic-acid decarboxylase in human islets and other non-neural tissues

Two forms of glutamic-acid decarboxylase (GAD) have been identified in mammalian tissues: a 65-kDa form (GAD65) and a 67-kDa form (GAD67). Alternate splicing produces one or two smaller variants of GAD67 in the brain of embryonic mice and rats. Additionally, a short, heretofore unidentified transcript homologous to GAD67 has been detected in human testis RNA. Because GAD, the enzyme responsible for gamma-aminobutyric acid production and a key autoantigen in type I diabetes, has unclear function in non-neural tissue, it is important to understand its pattern of expression. Unlike GAD65, GAD67 is not produced in human pancreatic islets. Here, we describe a novel splice variant of GAD67 that is produced in human islets, testis, adrenal cortex, and perhaps other endocrine tissues, but not in brain. This transcript directs the synthesis of a protein without GAD enzymatic activity: GAD25. A unique peptide sequence at the carboxyl terminus of GAD25 is highly conserved between mice, rats, and humans. We conclude that humans produce a third form of GAD in non-neural tissues and that human islets, although they do not synthesize full-length GAD67, do express this shortened variant.

GAD65 and insulin B chain peptide (9-23) are not primary autoantigens in the type 1 diabetes syndrome of the BB rat

To investigate whether GAD65 whole molecule, GAD65 p35 or insulin B chain peptide (amino acids 9-23) play an essential role in the pathogenesis of type 1 diabetes in the BioBreeding (BB) rat, we gave serial injections of GAD65, p35 or insulin B chain (9-23) to six groups of BB/Worcester rats. The individual antigens were administered either intrathymically on day 2 and intraperitoneally in MF 59-0 adjuvant 5 times during the first 5 weeks, or by intranasal instillation once neonatally and 5 days/week for the following 6 weeks. Control groups were injected with vehicle only. Age of onset of diabetes and degree of insulitis were not different between controls and antigen-treated rats. Rats that received GAD65 intrathymically and intraperitoneally developed high GAD65-antibody titers without altering diabetes development. In GAD65-treated animals, serum antibodies recognized epitopes at 3 sites on GAD65 in diabetic animals but only at 1 site in non-diabetic animals. GAD65-injected animals also showed a significant reduction of IFN-gamma mRNA expression in the thymus. This study provides evidence against the hypothesis that GAD65 and insulin B chain peptide (9-23) are primary diabetogenic autoantigens in BB rats because immunizations with these antigens and GAD65-induced immune deviation did not alter the development of diabetes.

Comparative analysis of epitope recognition of glutamic acid decarboxylase (GAD) by autoantibodies from different autoimmune disorders

Autoantibodies to GAD, an important marker of the autoimmune process in type I or insulin-dependent diabetes mellitus (IDDM), are also found in non-diabetic individuals with autoimmune polyendocrine syndrome type 1 (APS1), APS2, and stiff man syndrome (SMS). Most IDDM sera contain two distinct GAD antibody specificities, one of which targets an epitope region in the middle-third of GAD65 (IDDM-E1; amino acids 221-359) and one of which targets the carboxy-third of GAD65 (IDDM-E2; amino acids 453-569). Using 11 chimeric GAD65/GAD67 proteins to maintain conformation-dependent epitopes of GAD65, we compared the humoral repertoire of IgG antibodies from an individual with APS2-like disease (b35, b78, and b96) and MoAbs from an IDDM patient (MICA-2, MICA-3, and MICA-4). Neither the APS2 IgG antibodies nor the IDDM MoAbs bind the amino-terminal third of GAD65, but instead target the carboxy-terminal two-thirds of GAD65. Amino acids 270-359 (IDDM-E1) are targeted by one APS2 IgG antibody and MICA-4, while two other APS2 IgG antibodies, MICA-2 and MICA-3, target amino acids 443-585 (IDDM-E2). Using GAD65/67 chimera that span the IDDM-E2 region, we found that MICA-2 binds amino acids 514-528 of GAD65, but two APS2 IgG antibodies require this region and amino acids 529-570. In contrast, the binding of MICA-3 requires two discontinuous amino acid segments of GAD65 (452-513 and 528-569), but not amino acids 514-528. These results indicate that there are both similarities and differences in the humoral response to GAD65 in APS2 and IDDM.

A Conformation-Dependent Epitope in Addison’s Disease and Other Endocrinological Autoimmune Diseases Maps to a Carboxyl-Terminal Functional Domain of Human Steroid 21-Hydroxylase

Idiopathic Addison’s disease develops as a consequence of autoimmune destruction of steroid-producing cells in the adrenal gland. A major autoantigen is 21-hydroxylase (21OH; P450c21), which is involved in the biosynthesis of cortisol and aldosterone in the adrenal cortex. We selected a number of functionally important 21OH amino acid substitutions, found in patients with congenital adrenal hyperplasia, to study their effects on the binding of 21OH autoantibodies (21OHAb) to 21OH. The ability of 21OHAb to bind in vitro transcribed and translated wild-type 21OH and five different 21OH mutant proteins was quantified by liquid-phase assays. Sera from 21OHAb-positive patients with idiopathic Addison’s disease (n = 24), Graves’ disease (n = 3), and insulin-dependent diabetes mellitus (n = 1) were used. While the P105L, delE196, and G291S mutations had no effect on autoantibody binding, the P453S mutation had a considerable effect, and the R483P mutation almost completely abolished binding. Synthetic peptides corresponding to linear epitopes defined by amino acids 447–461 and 477–491 were unable to compete with wild-type 21OH for binding to autoantibodies. Direct 21OH DNA sequencing could not reveal any specific genetic variation in alleles found in 21OHAb-positive patients. We conclude that the region involving R483 plays a key role in the formation of a three-dimensional epitope in a functionally important C-terminal domain of the enzyme.

Species-specific autoantibodies in type 1 diabetes

GAD65 autoantibodies (GAD65Ab) are important markers for type 1 (insulin-dependent) diabetes mellitus. Although most patients have GAD65Ab at the time of clinical diagnosis, there are also GAD65Ab-positive individuals in the population at low risk of developing type 1 diabetes. The aim of this study was to test the hypothesis that the GAD65Ab reactivity to GAD65 cloned from human, mouse, and rat in newly diagnosed type 1 diabetic patients differ from antibody-positive healthy individuals. Sera from 254 new-onset 0- to 34-yr-old type 1 diabetic patients and 270 controls were assayed for their reactivity to human, mouse, and rat GAD65. Among the type 1 diabetic patients there was a significant better binding of human GAD65 compared to either mouse (P = 0.03) or rat GAD65 (P = 0.0005). The preference for human GAD65 increased with increasing age at onset (P = 0.0002). This differentiation was not observed in 88 GAD65Ab-positive control subjects. Our data indicate that recognition of epitopes by GAD65Ab in type 1 diabetes is different from that in nontype 1 diabetes, GAD65Ab-positive individuals.

Autoantigenic reactivity of diabetes sera with a hybrid glutamic acid decarboxylase GAD67-65 molecule GAD67(1-101)/GAD65(96-585)

Glutamic acid decarboxylase (GAD) is a major autoantigen in insulin-dependent diabetes mellitus (IDDM). Two GAD isoforms exist, GAD65 and GAD67, which differ mostly in the first 100 amino acids of the amino terminus. IDDM sera are predominantly reactive with GAD65 but autoepitopes have been localised only to regions of GAD65 highly homologous with GAD67. In this study we investigated the contribution of the amino terminus to the IDDM epitope on GAD65, in order to test whether this region of GAD could explain the difference in reactivity between GAD65 and GAD67. A recombinant hybrid GAD molecule consisting of amino acids 1-101 of GAD67 and 96-585 of GAD65 was constructed and a truncated GAD65 was also constructed consisting of amino acids 98-585 of GAD65. The reactivity with the hybrid GAD molecule, GAD65 and GAD67, and truncated GAD65 was examined by radioimmunoprecipitation using 50 IDDM sera with known reactivity to purified porcine brain GAD. Over 90% of the IDDM sera were reactive with the hybrid GAD molecule confirming that the amino terminus of GAD65 does not contribute to the autoepitope and that the IDDM epitope is localised to the middle and carboxyl terminal domains of GAD65. Furthermore, evidence is presented that autoantibodies to GAD65 in IDDM sera react with an epitope formed on a dimeric configuration of the molecule.

A monoclonal antibody-based characterization of autoantibodies against glutamic acid decarboxylase in adults with latent autoimmune diabetes

Autoantibodies to glutamic acid decarboxylase (GAD) are an important marker of the autoimmune-mediated beta-cell destruction in insulin-dependent (Type I) diabetes. However, these autoantibodies are also found in patients with Stiff-man syndrome (SMS) without onset of diabetes and some diabetic patients who initially present as non-insulin dependent (Type II) diabetes later becoming insulin-dependent, called as latent autoimmune diabetes in adults (LADA). To study the immune response to GAD in these LADA patients a competitive radiobinding assay based on murine monoclonal antibodies recognizing three different GAD regions was performed. The monoclonal antibodies against GAD recognize two different linear epitopes localized at the N- (amino acids 4-17) and C-terminus (amino acids 572-585) and one conformation-dependent epitope region (amino acids 221-442 IDDM-E1) known to be immunodominant for diabetes-associated autoantibodies. All LADA sera (20/20) reduced substantially the 125I-GAD binding of the monoclonal antibodies reactive with the conformation-dependent epitope region IDDM-E1 and only 20% of these sera additionally diminished the 125I-GAD65 binding by those monoclonals reactive with the both linear epitopes. The SMS sera completely abolished the GAD binding of all three monoclonals, reflecting a broader repertoire including an immune response against the IDDM-E1, a conformation-dependent GAD65 epitope region, also revealed if the SMS sera are diluted to equivalent antibody concentrations. In summary, our results show that diabetes-associated GAD autoantibodies even in adult patients with a late autoimmune process preferentially recognize a conformation-dependent middle GAD65 region. An immune response to all three GAD epitope regions is seldom in these LADA patients and only detectable in association with high antibody titres.

Humoral and cellular immune parameters before and during immunosuppressive therapy of a patient with stiff-man syndrome and insulin dependent diabetes mellitus

OBJECTIVES

Humoral and cellular immune reactivity are reported for two neuroendocrine autoantigens-glutamic acid decarboxylase (GAD) and the protein tyrosine phosphatase IA-2-in a patient with the autoimmune type of stiff-man syndrome and insulin dependent diabetes (IDDM).

METHODS

Antibodies and T cell proliferation against GAD and IA-2 and cytokine release of antigen stimulated T cells (IFN-gamma) were determined before and several times during immunosuppressive therapy with prednisolone.

RESULTS

Raised GAD antibodies against full length GAD65 or chimeric constructs were detected before therapy and they remained at a high concentration despite a marked clinical improvement during cortisone treatment. Antibodies to IA-2 were undetectable, but weak T cell responses to both GAD and IA-2 were seen before therapy and once on reduction of high cortisone dosages when the patient showed signs of clinical deterioration. Cytokine profiles showed increased IFN-gamma production after stimulation with GAD or IA-2 suggesting increased activation of TH1 cells.

CONCLUSION

Immunosuppressive therapy --even with extremely high doses of 500 mg a day--does not lead to the reduction of antibody concentrations in the periphery nor to a switch in epitope recognition of such antibodies despite clinical improvement. The amount of T cell reactivity to various antigens, however, may be a useful marker to monitor the effectiveness of immunotherapy.

Peptide specificity of high-titer anti-glutamic acid decarboxylase (GAD)65 autoantibodies

To study systematically the linear epitope specificity of anti-glutamic acid decarboxylase (GAD) autoantibodies associated with insulin-dependent diabetes mellitus (IDDM), we produced 93 overlapping 12-residue synthetic peptides derived from the sequence of the human GAD65 protein and covering the entire length of the protein. These peptides were used as antigens in an enzyme immunoassay to screen the sera from 10 IDDM patients, all of which contained at high level autoantibodies directed against GAD65. Three out of ten (30%) IDDM patients had antibodies that reacted with one or more of the synthetic peptides. Two of the peptide-reactive IDDM sera, which also bound denatured recombinant GAD65 on western blots, had the highest titers of anti-GAD antibodies in ELISA assay. Moreover, the anti-GAD antibodies-GAD complexes formed with these sera were characterized by low dissociation rates, indicative of their good stability. A fine specificity analysis, using analogs of antigen peptide 1 (residues 1-12), allowed us to identify the residues at positions 5-9 (GSGFW) as critical for antibody recognition.

Heterogeneity in the occurrence of a subset of autoantibodies to glutamic acid decarboxylase in autoimmune polyendocrine patients with islet cell antibodies

Glutamic acid decarboxylase-65 (GAD-65) is a major target for autoantibodies and autoreactive T cells in patients with insulin-dependent diabetes mellitus (IDDM). Autoantibodies to GAD are also found in patients with stiff man syndrome (SMS) or polyendocrine autoimmunity (PE). The epitope specificities of autoantibodies to GAD in IDDM and SMS have been well documented, but the locations of autoantibody epitopes of GAD in PE patients have not been mapped. Thus, the properties of anti-GAD antibodies in PE patients (with or without diabetes) were investigated. The ability of PE serum antibodies to inhibit the binding of the mouse monoclonal antibody, GAD-6, to native GAD in ELISA was determined. For PE patients without diabetes, levels of inhibition of GAD-6 binding ranged from 0% to almost 70% and were unrelated to the level of binding of serum antibodies to GAD (P = 0.351) or to the functional affinities of these antibodies. This suggests differences in the epitope specificities of anti-GAD antibodies in different patients. Levels of inhibition were also unrelated to clinical condition. SMS antibodies showed similar levels of inhibition of GAD-6 binding. Similar analysis was applied to PE patients with diabetes and levels of inhibition of GAD-6 binding to GAD were determined. These ranged from 0% to 80%, and levels of inhibition were similar in samples taken before or after diabetes onset. There was no significant difference between anti-GAD antibodies from PE patients with or without diabetes in the range of abilities to inhibit GAD-6 binding to GAD, although the highest levels of inhibition were given by sera from non-diabetic patients. This raises the possibility of differential expression of subsets of anti-GAD antibodies in progressive versus slow or non-progressive anti-islet autoimmune responses. Serum antibodies of PE and SMS patients did not inhibit the binding of antibodies specific for the extreme C-terminus of GAD, indicating that this is not the site of the epitopes for the patients' antibodies or for GAD-6.

A study of the epitopes on steroid 21-hydroxylase recognized by autoantibodies in patients with or without Addison's disease

Steroid 21-hydroxylase (21-OH) autoantibodies are found in patients with autoimmune Addison's disease (AAD), either isolated or associated with autoimmune polyglandular syndrome (APS) type I and II and in adrenal-cortex autoantibody (ACA)-positive patients without AAD. In order to assess any differences in the 21-OH autoantibodies in these different patient groups, we have studied their reactivity with different epitopes on 21-OH using full length and modified 35S-labelled 21-OH proteins produced in an in vitro transcription/translation system. There were no major differences in the pattern of autoantibody reactivity with the different modified 21-OH proteins in patients with isolated AAD or with APS types I and II, and in 21-OH autoantibody-positive patients with clinical AAD, subclinical AAD and those maintaining a normal adrenal function. Our studies also indicate that the main epitopes for 21-OH autoantibodies in patients with different forms of autoimmune adrenal disease are located in the C-terminal end and in a central region of 21-OH.

Highly-sensitive and specific enzyme-linked immunosorbent assays for GAD65 autoantibodies using a thioredoxin-GAD65 fusion antigen

Autoantibodies against glutamic acid decarboxylase (GAD65) are present in the sera of most patients with recently diagnosed insulin-dependent diabetes mellitus (IDDM). These antibodies appear years before the clinical symptoms, and they are considered to be early markers of the disease. To detect GAD65 autoantibodies (GADA), we developed new enzyme-linked immunosorbent assays (ELISA) with a fusion protein thioredoxin-GAD65 (Trx-GAD65) produced in E. coli as the antigen. These assays were compared with the reference radiobinding assay (RBA). Since most GADA are directed against native epitopes, and adsorption of GAD65 to plastic may cause disruption of its native conformation, the new assays rely on the following immobilization procedures: (a) capture ELISA (c-ELISA) with Trx-GAD65 (protocol A) or biotin-Trx-GAD (protocol B) indirectly immobilized by a non-adsorptive process; (b) ELISA with antigen-antibody preincubation in solution (p-ELISA) in which GADA were reacted first with Trx-GAD65 (protocol C) or biotin-Trx-GAD (protocol D) and the free antigen was determined by conventional ELISA. The results obtained with 42 newly diagnosed IDDM patients and 30 normal individuals were as follows: RBA had 79% sensitivity (percentage of IDDM patients detected) and 97% specificity (100% minus the percentage of false positives). c-ELISA showed low sensitivity (36 and 50%, respectively for protocols A and B), and high specificity (100 and 97%, respectively). p-ELISA were highly-sensitive (74 and 79%, respectively) and specific (97 and 93% for protocols C and D, respectively). Thus, protocols C and D had a performance similar to the reference method. The results reported here provide the basis for simple, highly-sensitive, specific, and widely-applicable tests for GADA that eliminate many of the drawbacks of the radioactive methods.