Predicting type 1 diabetes using autoantibodies: the latest results from the diabetes autoantibody standardization program

Serum proteomics reveals systemic dysregulation of innate immunity in type 1 diabetes

Proteomics analysis identifies human serum proteins involved with innate immune responses, complement activation, and blood coagulation that are diagnostic for type 1 diabetes.

Using global liquid chromatography-mass spectrometry (LC-MS)–based proteomics analyses, we identified 24 serum proteins that were significantly variant between those with type 1 diabetes (T1D) and healthy controls. Functionally, these proteins represent innate immune responses, the activation cascade of complement, inflammatory responses, and blood coagulation. Targeted verification analyses were performed on 52 surrogate peptides representing these proteins, with serum samples from an antibody standardization program cohort of 100 healthy control and 50 type 1 diabetic subjects. 16 peptides were verified as having very good discriminating power, with areas under the receiver operating characteristic curve ≥0.8. Further validation with blinded serum samples from an independent cohort (10 healthy control and 10 type 1 diabetics) demonstrated that peptides from platelet basic protein and C1 inhibitor achieved both 100% sensitivity and 100% specificity for classification of samples. The disease specificity of these proteins was assessed using sera from 50 age-matched type 2 diabetic individuals, and a subset of proteins, C1 inhibitor in particular, were exceptionally good discriminators between these two forms of diabetes. The panel of biomarkers distinguishing those with T1D from healthy controls and those with type 2 diabetes suggests that dysregulated innate immune responses may be associated with the development of this disorder.

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

Background

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

Methodology/Principal Findings

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

Conclusions

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

HLA genes, islet autoantibodies and residual C-peptide at the clinical onset of type 1 diabetes mellitus and the risk of retinopathy 15 years later

Aims/Hypothesis

HLA genes, islet autoantibodies and residual C-peptide were studied to determine the independent association of each exposure with diabetic retinopathy (DR), 15 years after the clinical onset of type 1 diabetes in 15–34 year old individuals.

Methods

The cohort was identified in 1992 and 1993 by the Diabetes Incidence Study in Sweden (DISS), which investigates incident cases of diabetes for patients between 15 and 34 years of age. Blood samples at diagnosis were analyzed to determine HLA genotype, islet autoantibodies and serum C-peptide. In 2009, fundus photographs were obtained from patient records. Study measures were supplemented with data from the Swedish National Diabetes Registry.

Results

The prevalence of DR was 60.2% (148/246). Autoantibodies against the 65 kD isoform of glutamate decarboxylase (GADA) at the onset of clinical diabetes increased the risk of DR 15 years later, relative risk 1.12 for each 100 WHO units/ml, [95% CI 1.02 to 1.23]. This equates to risk estimates of 1.27, [95% CI 1.04 to 1.62] and 1.43, [95% CI 1.06 to 1.94] for participants in the highest 25^th (GADA>233 WHO units/ml) and 5^th percentile (GADA>319 WHO units/ml) of GADA, respectively. These were adjusted for duration of diabetes, HbA_1c, treated hypertension, sex, age at diagnosis, HLA and C-peptide. Islet cell autoantibodies, insulinoma-antigen 2 autoantibodies, residual C-peptide and the type 1 diabetes associated haplotypes DQ2, DQ8 and DQ6 were not associated with DR.

Conclusions

Increased levels of GADA at the onset of type 1 diabetes were associated with DR 15 years later. These results, if confirmed, could provide additional insights into the pathogenesis of the most common microvascular complication of diabetes and lead to better risk stratification for both patient screenings and DR treatment trials.

Development of a fibrinogen-specific sandwich enzyme-linked immunosorbent assay microarray assay for distinguishing between blood plasma and serum samples

We have developed a fibrinogen-specific sandwich enzyme-linked immunosorbent assay (ELISA) microarray assay for use in qualitatively distinguishing between blood plasma and serum samples. Three capture antibodies (49D2, HPA001900, and F8512) were evaluated in conjunction with 1D6 as the detection antibody. The data show that 49D2 and (to a lesser extent) F8512 successfully identify previously unknown plasma and serum samples based on approximately a 28-fold difference in signal intensity between the sample types. This assay has utility in rapidly identifying previously archived clinical samples with incomplete annotation in a high-throughput manner prior to proteomic analyses.

Application of proteomics in the discovery of candidate protein biomarkers in a diabetes autoantibody standardization program sample subset

Novel biomarkers of type 1 diabetes must be identified and validated in initial, exploratory studies before they can be assessed in proficiency evaluations. Currently, untargeted "-omics" approaches are underutilized in profiling studies of clinical samples. This report describes the evaluation of capillary liquid chromatography (LC) coupled with mass spectrometry (MS) in a pilot proteomic analysis of human plasma and serum from a subset of control and type 1 diabetic individuals enrolled in the Diabetes Autoantibody Standardization Program, with the goal of identifying candidate biomarkers of type 1 diabetes. Initial high-resolution capillary LC-MS/MS experiments were performed to augment an existing plasma peptide database, while subsequent LC-FTICR studies identified quantitative differences in the abundance of plasma proteins. Analysis of LC-FTICR proteomic data identified five candidate protein biomarkers of type 1 diabetes. alpha-2-Glycoprotein 1 (zinc), corticosteroid-binding globulin, and lumican were 2-fold up-regulated in type 1 diabetic samples relative to control samples, whereas clusterin and serotransferrin were 2-fold up-regulated in control samples relative to type 1 diabetic samples. Observed perturbations in the levels of all five proteins are consistent with the metabolic aberrations found in type 1 diabetes. While the discovery of these candidate protein biomarkers of type 1 diabetes is encouraging, follow up studies are required for validation in a larger population of individuals and for determination of laboratory-defined sensitivity and specificity values using blinded samples.

Islet cell autoantibody levels after the diagnosis of young adult diabetic patients

AIMS

The aim was to determine the course of islet cell antibodies [glutamate decarboxylase (GADA), tyrosine phosphatase-like islet antigen 2 (IA-2A) and islet cell (ICA)] after the diagnosis of the diabetic patient.

METHODS

The Diabetes Incidence Study in Sweden (DISS) attempted to prospectively enrol all newly diagnosed diabetic patients aged 15-34 years during 1992 and 1993. C-peptide and autoantibody levels were determined from venous blood samples at diagnosis and again at yearly intervals for 6 years.

RESULTS

After the first year, the odds of remaining GADA positive decreased by 9% per year [odds ratio (OR) = 0.91, 95% confidence interval (CI) = 0.85-0.96] while the mean GADA index remained unchanged ( = 0.8, P = 0.37). There was no change in the percentage of subjects testing IA-2A positive after the first year ( = 0.1, P = 0.75). However, the mean index decreased 0.04 per year (95% CI: 0.03-0.05)-a 7.9% decline (95% CI: 5.4-10.4%). The odds of a subject testing positive for ICA decreased by 24% per year (OR = 0.76, 95% CI = 0.70-0.82). The mean ICA levels decreased 0.75 per year (95% CI: 0.66-0.84)-a 16.4% decline (95% CI: 14.1-18.6%). The rate of change in titres for all three autoantibodies was independent of gender, human leucocyte antigen genotype and C-peptide status.

CONCLUSIONS

GADA levels remained high while ICA levels declined. In contrast to a previous study, we found that the proportion of IA-2A subjects remaining positive did not decrease after the first year, while the average index decreased slightly.

Multiple factors affect the loss of measurable C-peptide over 6 years in newly diagnosed 15- to 35-year-old diabetic subjects

OBJECTIVE

The aim of this study is to identify risk factors for the loss of measurable plasma C-peptide in newly diagnosed 15- to 35-year-old diabetic subjects.

METHODS

This Swedish study included 778 subjects. C-peptide levels were obtained each year for 6 years after diagnosis. Loss of measurable C-peptide was defined as a level at or below the lower detection limit of the local assay (0.13 nmol/l). In addition to C-peptide, other baseline covariates included gender, age, body mass index, HLA genotype, and autoantibody levels.

RESULTS

Compared with autoantibody-negative subjects, autoantibody-positive subjects had lower median baseline C-peptide (0.27 vs. 0.50, P<.001), their levels declined over the study period, and the risk of losing measurable C-peptide was significantly higher when more than one autoantibody was present [odds ratio (OR), 4.0; 95% confidence interval (CI), 2.13-7.54]. Among autoantibody-positive individuals, the presence of GAD65Ab (OR, 1.8; 95% CI, 1.24-2.51) and islet cell antibodies (OR, 1.6; 95% CI, 1.19-2.18) conferred a higher risk for loss of measurable C-peptide as did female gender (OR, 1.6; 95% CI, 1.17-2.11) and time after diagnosis (OR, 1.5 for each additional year postdiagnosis; 95% CI, 1.41-1.57). Higher baseline C-peptide levels were protective (OR, 0.5 for each additional log(e) nanomoles per liter; 95% CI, 0.36-0.58).

CONCLUSIONS

This study identified autoantibody status, gender, and baseline C-peptide levels as factors that will be useful for predicting the disease course of 15- to 35-year-old diabetic individuals.

Analysis of T-cell assays to measure autoimmune responses in subjects with type 1 diabetes: results of a blinded controlled study

Type 1 diabetes is a chronic autoimmune disease mediated by autoreactive T-cells. Several experimental therapies targeting T-cells are in clinical trials. To understand how these therapies affect T-cell responses in vivo, assays that directly measure human T-cell function are needed. In a blinded, multicenter, case-controlled study conducted by the Immune Tolerance Network, we tested responses in an immunoblot and T-cell proliferative assay to distinguish type 1 diabetic patients from healthy control subjects. Peripheral blood cells from 39 healthy control subjects selected for DR4 and 23 subjects with recently diagnosed type 1 diabetes were studied. Autoantibody responses were measured in serum samples. Positive responses in both assays were more common in peripheral blood mononuclear cells from new-onset type 1 diabetic patients compared with control subjects. The proliferative, immunoblot, and autoantibody assays had sensitivities of 58, 91, and 78% with specificities of 94, 83, and 85%, respectively. When cellular assays were combined with autoantibody measurements, the sensitivity of the measurements was 75% with 100% specificity. We conclude that cellular assays performed on peripheral blood have a high degree of accuracy in discriminating responses in subjects with type 1 diabetes from healthy control subjects. They may be useful for assessment of cellular autoimmune responses involved in type 1 diabetes.

Endocrine autoantibodies

The autoantibody assays that exist and that are being refined are of increasing importance to a broad spectrum of endocrine disorders. This is particularly true for type IA diabetes, which is one of the best-studied organ-specific autoimmune diseases. Autoantibodies are used as valuable markers in prediction and prevention studies of type IA diabetes. Autoantibodies related to other endocrine organs are also important because multiple related autoimmune endocrine and non-endocrine disorders are increased in frequency in patients and their families with autoimmunity. The availability of highly sensitive and specific autoantibody assays for the various endocrine disorders can allow physicians to better diagnose and promptly treat these conditions.