GAD65 antibody epitope patterns of type 1.5 diabetic patients are consistent with slow-onset autoimmune diabetes

GAD65 Antibody Epitopes and Genetic Background in Latent Autoimmune Diabetes in Youth (LADY)

Epitope-specific GAD65Abs and HLA-DR-DQ gene assays help improve the value of risk stratification in autoimmune diabetes mellitus and protect islet function. Identification and early intervention are important for latent autoimmune diabetes in youth (LADY). The aims of this study were to investigate 1) the frequencies of the epitope-specific GAD65Abs and HLA-DR-DQ genes in LADY and 2) the association between HLA-DR-DQ genes and epitope-specific GAD65Abs. Higher frequencies of GAD65-CAb and multiepitope GAD65Abs were observed in young type 1 diabetes, LADY, and old type 1 diabetes subjects than those in latent autoimmune diabetes in adult (LADA) patients. The frequencies of the specific susceptible HLA haplotype DR3, total susceptible HLA haplotypes, and high-risk genotypes were higher in type 1 diabetes and LADY patients than those in LADA patients. In contrast, type 1 diabetes and LADY patients had lower frequencies of low/no genetic risk genotypes (DRX/X) than those of LADA patients. Logistic regression analysis suggested that the susceptible HLA haplotypes were risk factors for glutamic acid decarboxylase antibody (GADA) multiepitope positivity in autoimmune diabetes mellitus. LADY may be more severe than LADA, and LADY seemed to be a transitional type of type 1 diabetes and LADA. GADA epitope and HLA-DR-DQ gene assays are important for risk stratification in autoimmune diabetes mellitus and protection of islet function.

Past, present and future of latent autoimmune diabetes in adults

Latent autoimmune diabetes in adults (LADA) is the most common form of autoimmune diabetes diagnosed in adults. Similar to type 1 diabetes, the prevalence of LADA is impacted by ethnicity and geography. LADA is characterized by β cell loss due to autoimmunity and insulin resistance and has highly heterogeneous clinical features, autoimmunity, and genetics in a glutamic acid decarboxylase antibody (GADA) titre-dependent manner, suggesting LADA is part of a continuum spectrum between type 1 and type 2 diabetes. Although LADA is the most frequent form of autoimmune diabetes diagnosed in adults, clinical trials involving LADA are scarce. Here we review the recent advancements in LADA epidemiology, clinical features, pathogenesis, and interventions. We also highlight the environmental factors that are thought to play an important role in addition to genetics in the pathogenesis of LADA. In the future, high-throughput molecular profiles might shed light on the nature of LADA among the wide spectrum of diabetes and offer new opportunities to identify novel LADA-specific biomarkers.

Actual condition survey regarding mismatch of measurements between radioimmunoassay and enzyme-linked immunosorbent assay tests for anti-glutamic acid decarboxylase antibody in real-world clinical practice

Anti-glutamic acid decarboxylase antibody (GADA) is an important islet cell-associated autoantibody for the diagnosis of autoimmune type 1 diabetes mellitus. In Japan, the GADA assay kit was recently changed from radioimmunoassay (RIA) to enzyme-linked immunosorbent assay (ELISA). Thereafter, a mismatched measurement between the two tests became apparent in clinical situations. The present study aimed to clarify the actual extent of mismatch between the two measurements on a larger-scale real-world clinical practice. In this cross-sectional non-local/non-hospital-based study, we collected anonymized data on GADA levels of 598 participants, who were simultaneously measured with GADA-RIA and GADA enzyme-linked immunosorbent assay tests. We found that 34% of the GADA-RIA-positive participants showed negative results in the GADA enzyme-linked immunosorbent assay test; the mismatch was predominantly observed in participants with relatively low GADA-RIA levels (<32 U/mL). This considerable mismatch might lead to physicians' confusion in diagnosing type 1 diabetes mellitus.

Type 1 Diabetes-related Autoantibodies in Different Forms of Diabetes

Autoantibodies against Glutamic Acid Decarboxylase (GADA), insulinoma antigen-2 (IA- 2A), insulin (IAA) and the most recently Zinc Transporter 8 (ZnT8A) are one of the most reliable biomarkers for autoimmune diabetes in both children and adults. They are today the only biomarkers that can distinguish Latent Autoimmune Diabetes in Adults (LADA) from phenotypically type 2 diabetes. As the frequency of autoantibodies at diagnosis in childhood type 1 diabetes depends on age, GADA is by far the most common in adult onset autoimmune diabetes, especially LADA. Being multiple autoantibody positive have also shown to be more common in childhood diabetes compared to adult onset diabetes, and multiple autoantibody positivity have a high predictive value of childhood type 1 diabetes. Autoantibodies have shown inconsistent results to predict diabetes in adults. Levels of autoantibodies are reported to cause heterogeneity in LADA. Reports indicate that individuals with high levels of autoantibodies have a more type 1 diabetes like phenotype and individuals with low levels of autoantibody positivity have a more type 2 diabetes like phenotype. It is also well known that autoantibody levels can fluctuate and transient autoantibody positivity in adult onset autoimmune diabetes have been reported to affect the phenotype.

Diagnostic accuracy of the anti-glutamic acid decarboxylase antibody in type 1 diabetes mellitus: Comparison between radioimmunoassay and enzyme-linked immunosorbent assay

Aims/Introduction

The distributer of the anti‐glutamic acid decarboxylase antibody assay kit using radioimmunoassay (RIA) recently announced its discontinuation, and proposed an alternative kit using enzyme‐linked immunosorbent assay (ELISA). The aim of the present study was to investigate the diagnostic values of the anti‐glutamic acid decarboxylase antibody by RIA and ELISA among type 1 diabetes mellitus patients and control participants.

Materials and Methods

A total of 79 type 1 diabetes mellitus patients and 79 age‐matched controls were enrolled and assessed using RIA and ELISA. Sensitivity, specificity, positive predictive values and negative predictive values were calculated for cut‐off values (RIA = 1.5 U/mL and ELISA = 5.0 U/mL, respectively). Kappa coefficients were used to test for agreements between the RIA and ELISA methods regarding the diagnosis of type 1 diabetes mellitus.

Results

The sensitivity, specificity, positive predictive values, and negative predictive values for diagnosing type 1 diabetes mellitus were 57.0, 97.5, 95.7, and 69.4% by RIA, and 60.8, 100.0, 100.0 and 71.8% by ELISA, respectively. The diagnosis of type 1 diabetes mellitus using the RIA and ELISA methods showed substantial agreement with the kappa values of 0.74 for all participants, and of 0.64 for the acute type; however, there was moderate agreement with the kappa value of 0.56 for the slowly progressive type.

Conclusions

The present study suggests that both anti‐glutamic acid decarboxylase antibody by RIA and ELISA was useful for diagnosing type 1 diabetes mellitus. However, in the slowly progressive type, the degree of agreement of these two kits was poorer compared with those in all participants or in the acute type.

Detection of Antibodies Directed to the N-Terminal Region of GAD Is Dependent on Assay Format and Contributes to Differences in the Specificity of GAD Autoantibody Assays for Type 1 Diabetes

GAD autoantibodies (GADAs) are sensitive markers of islet autoimmunity and type 1 diabetes. They form the basis of robust prediction models and are widely used for the recruitment of subjects at high risk of type 1 diabetes to prevention trials. However, GADAs are also found in many individuals at low risk of diabetes progression. To identify the sources of diabetes-irrelevant GADA reactivity, we analyzed data from the 2009 and 2010 Diabetes Autoantibody Standardization Program GADA workshop and found that binding of healthy control sera varied according to assay type. The characterization of control sera found positive by radiobinding assay (RBA), but negative by ELISA, showed that many of these sera reacted to epitopes in the N-terminal region of the molecule. This finding prompted development of an N-terminally truncated GAD65 radiolabel, (35)S-GAD65(96-585), which improved the performance of most GADA RBAs participating in an Islet Autoantibody Standardization Program GADA substudy. These detailed workshop comparisons have identified a source of disease-irrelevant signals in GADA RBAs and suggest that N-terminally truncated GAD labels will enable more specific measurement of GADAs in type 1 diabetes.

Vaccination against type 1 diabetes

The clinical onset of type 1 diabetes or autoimmune diabetes occurs after a prodrome of islet autoimmunity. The warning signals for the ensuing loss of pancreatic islet beta cells are autoantibodies against insulin, GAD65, IA-2 and ZnT8, alone or in combinations. Autoantibodies against, for example, insulin alone have only a minor risk of type 1 diabetes. However, progression to clinical onset is increased by the induction of multiple islet autoantibodies. At the time of clinical onset, insulitis may be manifest, which seems to reduce the efficacy of immunosuppression. Autoantigen-specific immunotherapy with alum-formulated GAD65 (Diamyd(®)) shows promise to reduce the loss of beta-cell function after the clinical onset of type 1 diabetes. The mechanisms are unclear but may involve the induction of T regulatory cells, which may suppress islet autoantigen reactivity. Past and ongoing clinical trials have been safe. Future clinical trials, perhaps as combination autoantigen-specific immunotherapy, may increase the efficacy in preventing the clinical onset in subjects with islet autoantibodies or preserve residual beta-cell function in patients newly diagnosed with type 1 diabetes.

Epitope analysis of GAD65 autoantibodies in adult-onset type 1 diabetes and latent autoimmune diabetes in adults with thyroid autoimmunity

This study aimed at determining which GAD65 epitopes the spontaneous antibodies recognized and whether the epitope-specific GAD65Abs could be associated with the development of thyroid autoimmunity in Chinese adult-onset type 1 diabetes (T1DM) and latent autoimmune diabetes in adults (LADA). The levels of GAD65Abs and their reactivities to N-terminal (GAD65-N), middle (GAD65-M) and C-terminal (GAD65-C) regions of human GAD65 were measured by radioligand assay in 109 patients with adult-onset T1DM and 107 with LADA. TPOAb, TGAb and the genotypes of HLADQA1-DQB1 were determined. The percentage of LADA patients with GAD65-NAb was significantly higher than that of adult-onset T1DM patients (21.5% vs. 11.0%, P = 0.037), but LADA patients with GAD65-CAb less than T1DM patients (47.7% vs. 70.6%, P = 0.001). LADA patients with both GAD65-M and GAD65-CAb (GAD65-M + CAb) appeared to be at higher risk for the development of thyroid autoimmunity, lower serum C-peptide level and the requirement for insulin therapy (P < 0.05). More frequent T1DM patients with HLADQA1*03-DQB1*0303 developed GAD65-M + CAb (55.8% vs. 35.1%, P = 0.008). In comparison with those without thyroid autoimmunity, more frequent T1DM patients and LADA patients with thyroid autoimmunity displayed GAD65-M + CAbs (44.0% vs.16.9% and 53.1% vs. 17.3%, P = 0.002 and <0.001, respectively) with a diagnostic specificity of 83.1 or 82.7% for thyroid autoimmunity, respectively. LADA patients with GAD65-M + CAbs had clinical features similar to T1DM patients. Adult-onset T1DM and LADA patients with GAD65-M + CAbs are at an increased risk for the development of thyroid autoimmunity.

High-titer GAD65 autoantibodies detected in adult diabetes patients using a high efficiency expression vector and cold GAD65 displacement

Adult type 2 diabetes patients with GAD65 autoantibodies (GADA) are known as latent autoimmune diabetes in adults (LADA). It has been suggested that GADA in LADA patients preferentially bind to the N-terminal end of GAD65. Using the N-terminal end extension of ³⁵S-GAD65 generated by the pEx9 plasmid, we tested the hypothesis that GADA in LADA patients preferentially react with ³⁵S-GAD65 from the pEx9 plasmid compared to the normal length pThGAD65 plasmid. Healthy control subjects (n = 250) were compared with type 1 (n = 23), type 2 (n = 290), and unspecified (n = 57) diabetes patients. In addition, radio-binding assays for GADA with ³⁵S-GAD65 generated from both the pEx9 and pThGAD65 plasmids were used in displacement assays with an excess of recombinant human GAD65 (2 μg/mL) to correct for non-specific binding. ³⁵S-GAD65 produced by either pEx9 or pThGAD65 did not differ in binding among the healthy controls and among the type 1 diabetes patients. Among the type 2 and unspecified patients, there were 4/290 and 3/57 patients, respectively, with binding to the pEx9 but not to the pThGAD65 generated ³⁵S-GAD65. In the displacement assay, we discovered 14 patients with very high-titer GADA among the type 1 (n = 3, 12,272-29,915 U/mL), type 2 (n = 7; 12,398-334,288 U/mL), and unspecified (n = 4; 20,773-4,053,580 U/mL) patients. All samples were fully displaced following appropriate dilution. We conclude that pThGAD65 is preferred for the coupled in vitro transcription translation of ³⁵S-GAD65 and that displacement with recombinant GAD65 may detect very high-titer GADA with possible clinical relevance.

Latent autoimmune diabetes in adults

CONTEXT

Autoantibodies that are reactive to islet antigens are present at the time of diagnosis in most patients with type 1 diabetes. Additionally, approximately 10% of phenotypic type 2 diabetic patients are positive for at least one of the islet autoantibodies, and this group is often referred to as "latent autoimmune diabetes in adults (LADA)." These patients share many genetic and immunological similarities with type 1 diabetes, suggesting that LADA, like type 1 diabetes, is an autoimmune disease. However, there are differences in autoantibody clustering, T cell reactivity, and genetic susceptibility and protection between type 1 diabetes and LADA, implying important differences in the underlying disease processes.

EVIDENCE ACQUISITION AND SYNTHESIS

In this clinical review, we will summarize the current understanding of LADA based on the MEDLINE search of all peer-reviewed publications (original articles and reviews) on this topic between 1974 and 2009.

CONCLUSIONS

In LADA, diabetes occurs earlier in the beta-cell-destructive process because of the greater insulin resistance. Complexities arise also because of variable definitions of LADA and type 1 diabetes in adults. As immunomodulatory therapies that slow or halt the type 1 diabetes disease process are discovered, testing these therapies in LADA will be essential.

Low agreement between radio binding assays in analyzing glutamic acid decarboxylase (GAD65Ab) autoantibodies in patients classified with type 2 diabetes

Autoantibodies against glutamic acid decarboxylase (GAD65Ab) are used in the classification of diabetes in adults. We assessed the concordance in GAD65 autoantibody levels within subjects between three different GAD65Ab radio binding assays (RBA). Plasma samples from 112 diabetes patients (median age 50 years) initially classified with type 2 diabetes was randomly selected from a local diabetes registry. Coded samples were analyzed with two RBA employing (35)S-labeled GAD65. The first used the pEx9 plasmid (pEx9 RBA), the second employed the pThGAD65 plasmid (pThGAD65 RBA) to label GAD65 by in vitro transcription translation. We also used a commercial kit employing plasmid pGAD17 labelled with (125)I (pGAD17 RBA). Subsequent analyses followed standard procedures. Two different cut-offs for GAD65Ab positivity were used in all three assays. We calculated the correlation, concordance, and agreement between the assays. The proportion of GAD65Ab positivity differed between assays when low cut-offs were used (pEx9 RBA 25%, pThGAD65 RBA 17.9%, and pGAD17 RBA 12.5%, respectively). When high cut-offs were applied, the concordance between the pEx9 RBA and the pThGAD65 RBA was 97.3 while their concordance to the pGAD17 RBA was lower (88.4 and 87.4, respectively). There was a low agreement between both pEx9 RBA and pGAD17 RBA (0.45, 95% CI 0.20-0.70) and between pThGAD65 RBA and pGAD17 RBA (0.43, 95% CI 0.18-0.68). We found discrepancies in determining the GAD65Ab positivity, which constitutes a problem when GAD65Ab are used clinically. Further methodological GAD65Ab assays studies are warranted.

Diabetes classification: grey zones, sound and smoke: Action LADA 1

Diseases gain identity from clinical phenotype as well as genetic and environmental aetiology. The definition of type 1 diabetes is clinically exclusive, comprising patients who are considered insulin dependent at diagnosis, whilst the definition of type 2 diabetes is inclusive, only excluding those who are initially insulin dependent. Ketosis-prone diabetes (KPD) and latent autoimmune diabetes in adults (LADA) are each exclusive forms of diabetes which are, at least initially, clinically distinct from type 2 diabetes and type 1 diabetes, and each have a different natural history from these major types of diabetes.KPD can be diagnosed unequivocally as diabetes presenting with the categorical clinical feature, ketoacidosis. In contrast, LADA can be diagnosed by the co-occurrence of three traits, not one of which is categorical or exclusive to the condition: adult-onset non-insulin-requiring diabetes, an islet autoantibody such as glutamic acid decarboxylase autoantibodies (GADA) or cytoplasmic islet cell autoantibodies (ICA), and no need for insulin treatment for several months post-diagnosis. But while some would split diabetes into distinct subtypes, there is a strong case that these subtypes form a continuum of varying severity of immune and metabolic dysfunction modified by genetic and non-genetic factors. This article discusses the nature of disease classification in general, and KPD and LADA in particular, emphasizing the potential value and pitfalls in classifying diabetes and suggesting a need for more research in this area.

Autoimmune diabetes in adults: lessons from the UKPDS

Latent autoimmune diabetes in adults (LADA) is characterised by a relatively mild diabetes onset, autoantibody positivity and eventual requirement for insulin therapy. Twelve per cent of newly diagnosed, UK Prospective Diabetes Study (UKPDS) patients were positive for autoantibodies to GAD65 (GADA) and/or insulinoma-associated antigen-2A (IA-2A) and managed as if they had Type 2 diabetes according to the UKPDS protocol. Here, we compare data from UKPDS LADA patients with that from other cohorts. In common with other groups, UKPDS LADA patients required insulin therapy earlier post-diagnosis than non-LADA patients. Reduction of islet function was similar in UKPDS LADA groups randomised to oral glucose-lowering agents or insulin replacement therapy, contesting the current hypothesis of reduced decline of insulin secretion in LADA by immediate insulin therapy. Disease progression was not predicted by post-diagnosis GADA levels or epitope specificities as has been suggested. Slowly progressing insulitis and pancreatic beta-cell loss at post-mortem are consistent with sustained retention of residual C-peptide secretion in LADA. Genetic association patterns at the human leucocyte antigen (HLA) and insulin gene (INS) regions are similar in UKPDS LADA patients and individuals with adult and childhood-onset Type 1 diabetes. The combined evidence suggests that LADA is an adult-onset form of Type 1 diabetes, rather than a separate condition or an intermediate state in a continuum of phenotype from Type 1 to Type 2 diabetes.

GAD65 as a prototypic autoantigen

The repertoire of known autoantigens is limited to a very small proportion of all human proteins, and the reason why only some proteins become autoantigens is unclear, but is likely associated with structural features. The 65kDa isoform of the enzyme glutamic acid decarboxylase (GAD65) is a major autoantigen in type I diabetes, and in various neurological diseases, whereas the closely related isoform, GAD67, is rarely antigenic. Conformational epitopes of GAD65 have been mapped using human monoclonal antibodies to GAD65 and GAD mutated by GAD65/67 sequence exchanges or point mutations, but these studies have been limited by a lack of structural information. The recent publication of crystal structures for the two isoforms has shown that the N-, C- and middle domains that have been identified previously as likely epitope regions are closely associated within the GAD dimer. Two major epitope regions, ctc1 and ctc2, have been identified in the C-terminal domain of GAD65, that encompass N- and C-terminal residues, and middle and C-terminal residues respectively. These regions are highly flexible compared with the equivalent regions in GAD67, and T cell epitopes have been localized to the same surface region of GAD65. Comparative analysis of these two structurally similar isoforms, GAD65 and GAD67, only one of which is autoantigenic should provide new insights into the provocations to autoimmunity.

COOH-terminal clustering of autoantibody and T-cell determinants on the structure of GAD65 provide insights into the molecular basis of autoreactivity

OBJECTIVE

To gain structural insights into the autoantigenic properties of GAD65 in type 1 diabetes, we analyzed experimental epitope mapping data in the context of the recently determined crystal structures of GAD65 and GAD67, to allow "molecular positioning" of epitope sites for B- and T-cell reactivity.

RESEARCH DESIGN AND METHODS

Data were assembled from analysis of reported effects of mutagenesis of GAD65 on its reactivity with a panel of 11 human monoclonal antibodies (mAbs), supplemented by use of recombinant Fab to cross-inhibit reactivity with GAD65 by radioimmunoprecipitation of the same mAbs.

RESULTS

The COOH-terminal region on GAD65 was the major autoantigenic site. B-cell epitopes were distributed within two separate clusters around different faces of the COOH-terminal domain. Inclusion of epitope sites in the pyridoxal phosphate-and NH(2)-terminal domains was attributed to the juxtaposition of all three domains in the crystal structure. Epitope preferences of different mAbs to GAD65 aligned with different clinical expressions of type 1 diabetes. Epitopes for four of five known reactive T-cell sequences restricted by HLA DRB1*0401 were aligned to solvent-exposed regions of the GAD65 structure and colocalized within the two B-cell epitope clusters. The continuous COOH-terminal epitope region of GAD65 was structurally highly flexible and therefore differed markedly from the equivalent region of GAD67.

CONCLUSIONS

Structural features could explain the differing antigenicity, and perhaps immunogenicity, of GAD65 versus GAD67. The proximity of B- and T-cell epitopes within the GAD65 structure suggests that antigen-antibody complexes may influence antigen processing by accessory cells and thereby T-cell reactivity.

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.

Longitudinal changes in epitope recognition of autoantibodies against glutamate decarboxylase 65 (GAD65Ab) in prediabetic adults developing diabetes

We analysed the beta cell-specific autoimmunity reflected in autoantibodies to the smaller isoform of glutamate decarboxylase (GAD65Ab) in the prediabetic period of GAD65Ab-positive healthy adults who developed Type 2 diabetes (T2D) during a follow-up period of 10 years. We found that of the adults that tested GAD65Ab-positive at baseline (n=25), six developed T2D and one developed Type 1 diabetes (T1D). Of the subjects that tested GAD65Ab-negative at baseline (n=2209), 81 developed T2D, one developed T1D and four developed unclassified diabetes, indicating that the risk for GAD65Ab-positive healthy adults to develop diabetes is increased sixfold. The GAD65Ab epitopes were characterized in a competition radioligand binding assay using recombinant Fab derived of GAD65-specific monoclonal antibodies. We observed that the GAD65Ab epitope specificities in the prediabetic period changed dynamically. Specifically, the binding to a middle and a C-terminal epitope increased during the follow-up period (P=0 x 03), causing a significant increase in the number of epitopes recognized (P=0 x 03). These findings are similar to previous observations of dynamic changes in the prediabetic period of schoolchildren at high risk for T1D development. However, the character of the epitopes differs between the two populations, suggesting differences in the beta cell-specific autoimmune response in the prediabetic period of patients with latent autoimmune diabetes in adults (LADA) and T1D.

Association of amino-terminal-specific antiglutamate decarboxylase (GAD65) autoantibodies with beta-cell functional reserve and a milder clinical phenotype in patients with GAD65 antibodies and ketosis-prone diabetes mellitus

CONTEXT

We previously characterized patients presenting with diabetic ketoacidosis prospectively into four subgroups of ketosis-prone diabetes mellitus (KPDM), based on the presence or absence of beta-cell autoimmunity (A+ or A-) and beta-cell functional reserve (B+ or B-). The A+B- KPDM subgroup comprises patients with classic, autoimmune type 1 diabetes, whereas the A+B+ KPDM subgroup has only partial beta-cell loss and a distinct clinical phenotype.

OBJECTIVE

We hypothesized that epitope specificity of autoantibodies directed against the 65-kDa isoform of glutamate decarboxylase (GAD65) reflects differences in beta-cell destruction.

DESIGN

Sera of sequential GAD65Ab-positive KPDM patients admitted for diabetic ketoacidosis (n = 36) were analyzed for their epitope recognition using five GAD65-specific recombinant Fab and their ability to inhibit GAD65 enzymatic activity. All patients were followed longitudinally to assess beta-cell functional reserve and insulin dependence.

RESULTS

Binding to an amino-terminal epitope defined by monoclonal antibody DPD correlated positively with fasting serum C-peptide levels at baseline (P = 0.0008) and after 1 yr (P = 0.007). Binding to the DPD-defined epitope also correlated positively with area under the curve for C-peptide after glucagon stimulation (P = 0.007) and with homeostasis model assessment percent B at 1 yr (P = 0.03). Binding to the DPD-defined epitope was significantly stronger in A+B+ than in A+B- patients (P = 0.001). Sera of 16 patients (44%) significantly inhibited GAD65 enzymatic activity, but this did not correlate with beta-cell function.

CONCLUSION

DPD-defined epitope specificity is correlated directly with preserved beta-cell functional reserve in GAD65Ab-positive patients and is associated with the milder clinical phenotype of A+B+ KPDM.

Immunopathological and Genetic Features in Slowly Progressive Insulin-Dependent Diabetes Mellitus and Latent Autoimmune Diabetes in Adults

In 1982 we proposed the presence of a subtype of type 1 diabetes [slowly progressive insulin-dependent diabetes mellitus (SPIDDM)], which was characterized by persistently positive islet cell antibody, late age of onset, noninsulin-dependent diabetes, and slowly progressive beta cell failure. Since then many studies demonstrated that this subtype of type 1 diabetes is prevalent in many ethnic groups and was later called the latent autoimmune diabetes in adults (LADA). Recent epidemiological studies reported that about 10% of patients with apparent type 2 diabetes have at least one autoantibodies against islet-specific antigen with high potential to progress to insulin-dependent state. Between SPIDDM and LADA some differences are reported in terms of some genetic predispositions including HLA class II and class I genes, vitamin D receptor gene, and CTLA4 genes. Common features in SPIDDM and LADA including preserved beta cells at the onset of diabetes and weak T cell response to residual beta cells suggest that these subtypes of type 1 diabetes are suitable candidates for prevention treatment for further progression of beta cell failure.

What type of diabetes do young people have?

Although type 1 diabetes remains the most common type of diabetes in the pediatric population, there has been a dramatic rise in type 2 diabetes in youth. Clinical distinctions between these two major types have become blurred. As genetic and biochemical measures become more affordable and available, practical applications of these measures to better understand diabetes type in youth and the implications of diabetes type on the evolution of the disease are needed.

Is latent autoimmune diabetes in adults distinct from type 1 diabetes or just type 1 diabetes at an older age?

Diabetes is classified clinically into two types: type 1 and type 2 diabetes. Type 1 diabetes is an autoimmune diabetes, whereas, in contrast, type 2 diabetes is nonautoimmune. However, there is a group of phenotypic adult type 2 diabetic patients ( approximately 10%) who have islet autoantibodies similar to type 1 diabetes. These patients are said to have latent autoimmune diabetes in adults (LADA) or type 1.5 diabetes. T-cells reacting with islet proteins have been demonstrated in type 1 and type 1.5 diabetic patients. In contrast, classic autoantibody-negative type 2 diabetic patients are also negative for T-cell responses to islet proteins. Therefore, we questioned whether type 1 and type 1.5 diabetes are similar or different autoimmune diseases. We have investigated the immunological and metabolic differences between type 1, type 1.5, and classic type 2 diabetic patients. We have identified autoantibody differences, differences in islet proteins recognized by T-cells, and differences in insulin resistance. We have also identified a small group of patients who have T-cells responsive to islet proteins but who are autoantibody negative. These patients appear to be similar to type 1.5 patients in having decreased stimulated C-peptide values. These immunological differences between type 1 and type 1.5 diabetes suggest at least partially distinct disease processes.

Epitope analysis of insulin autoantibodies using recombinant Fab

Autoantibodies to insulin are often the first autoantibodies detected in young children with type 1 diabetes and can be present before the onset of clinical diabetes. These autoantibodies and their epitopes are, however, not well characterized. We explored the use of monoclonal antibodies and their recombinant Fab as reagents for epitope analysis. In this study we cloned and characterized the recombinant Fab of the insulin-specific monoclonal antibody CG7C7. We found the epitope of this antibody to be located predominantly at the A-chain loop of the insulin molecule. The recombinant Fab was then used to compete for insulin binding against insulin autoantibodies present in sera from patients with type 1 or type 1.5 diabetes. In competition experiments with sera positive for autoantibodies to insulin the recombinant Fab significantly reduced the binding to [125I]-insulin by sera of type 1 (n = 35) and type 1.5 diabetes [latent autoimmune diabetes in adults (LADA)] (n = 14) patients (P < 0.0001). We conclude that competition between insulin-specific monoclonal antibodies or their recombinant Fab and insulin autoantibodies should prove useful in the epitope analysis of autoantibodies to insulin.

Islet cell autoimmunity in a triethnic adult population of the Third National Health and Nutrition Examination Survey

Markers of humoral islet cell autoimmunity, such as autoantibodies (AAs) against the 65-kDa isoform of GAD (GAD65), serve as determinants of risk for autoimmune diabetes. Despite the high prevalence of diabetes in U.S. racial and ethnic minority adult populations, little is known concerning the prevalence of GAD65 AA in these groups. We estimated the prevalence of GAD65 AA in 1,064 diabetic and 1,036 nondiabetic participants who were 40-90 years of age from the Third National Health and Nutrition Examination Survey (NHANES III), which provides a representative ethnic sample of the U.S. diabetic population. The prevalence of GAD65 AA was higher in diabetic participants compared with nondiabetic participants in non-Hispanic whites (n = 920; 6.3% vs. 2.0%; P = 0.001) and non-Hispanic blacks (n = 534; 3.7% vs. 1.3%; P = 0.08) but not in Mexican Americans (n = 646; 1.2% vs. 2.6%; P = 0.18). Among diabetic non-Hispanic whites and non-Hispanic blacks, being GAD65 AA positive was associated with lower BMI and C-peptide (P < 0.05). These results may reflect the outcome of an autoimmune process leading to beta-cell destruction/dysfunction in non-Hispanic white and non-Hispanic black adult diabetic patients as it occurs in a similar manner in type 1 diabetes. Among diabetic Mexican Americans, the lower prevalence of GAD65 AA suggests a lower frequency of autoimmune-related 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.