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

Autoantibodies in Pandemrix®-induced narcolepsy: Nine candidate autoantigens fail the conformational autoantibody test

Study objectives: Narcolepsy type 1 (NT1) is a chronic sleep disorder characterized by loss of hypocretin-producing neurons. Increased NT1 incidence was observed in Sweden following mass-vaccination with Pandemrix^®. Genetic association to HLA DQB1*06:02 implies an autoimmune origin, but target autoantigen remains unknown. Candidate autoantigens for NT1 have previously been identified in solid-phase immunoassays, while autoantibodies against conformation-dependent epitopes are better detected in radiobinding assays. The aims are to determine autoantibody levels against nine candidate autoantigens representing (1) proteins of the hypocretin transmitter system; Preprohypocretin (ppHypocretin), Hypocretin peptides 1 and 2 (HCRT1 and HCRT2) and Hypocretin receptor 2 (HCRTR2); (2) proteins previously associated with NT1; Tribbles homologue 2 (TRIB2), Pro-opiomelanocortin/alpha-melanocyte-stimulating-hormone (POMC/α-MSH) and Prostaglandin D2 Receptor DP1 (DP1); (3) proteins suggested as autoantigens for multiple sclerosis (another HLA DQB1*06:02-associated neurological disease); ATP-dependent Inwardly Rectifying Potassium Channel Kir4.1 (KIR4.1) and Calcium-activated chloride channel Anoctamin 2 (ANO2). Methods: Serum from post-Pandemrix^® NT1 patients (n = 31) and their healthy first-degree relatives (n = 66) were tested for autoantibody levels in radiobinding assays separating autoantibody bound from free labelled antigen with Protein A-Sepharose. ¹²⁵I-labelled HCRT1 and HCRT2 were commercially available while ³⁵S-methionine-labelled ppHypocretin, HCRTR2, TRIB2, α-MSH/POMC, DP1, KIR4.1 or ANO2 was prepared by in vitro transcription translation of respective cDNA. In-house standards were used to express data in arbitrary Units/ml (U/ml). Results: All radiolabelled autoantigens were detected in a concentration-dependent manner by respective standard sera. Levels of autoantibodies in the NT1 patients did not differ from healthy first-degree relatives in any of the nine candidate autoantigens. Conclusions: None of the nine labelled proteins proposed to be autoantigens were detected in the radiobinding assays for conformation-dependent autoantibodies. The results emphasise the need of further studies to identify autoantigen(s) and clarify the mechanisms in Pandemrix^®-induced NT1.

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.

Latent autoimmune diabetes in adults (LADA): usefulness of anti-GAD antibody titers and benefit of early insulinization

OBJECTIVE

To determine the clinical and laboratory parameters and the progression to insulin requirement in two groups of LADA patients separated according to GADA titers, and to evaluate the benefit of early insulinization in patients at high risk of premature beta-cell failure (high GADA titers).

METHODS

Among the diabetic adults seen at our service and screened for GADA at diagnosis, 54 were diagnosed with LADA and classified as having low (> 1 U/ml and < 17.2 U/ml) or high (> 17.2 U/ml) GADA titers. Fifty-four patients with type 2 diabetes (GADA-) were selected for comparison. In addition, 24 patients who had GADA titers > 20 U/ml and who were not initially insulinized were compared to 16 patients who were insulinized at diagnosis.

RESULTS

Insulin resistance was higher in the GADA- group, followed by patients with low GADA titers. BMI and the frequency of arterial hypertension, elevated triglycerides and reduced HDL cholesterol were lower in the high GADA+ group, with no difference between the GADA- or low GADA+ groups. The high GADA+ group showed a greater reduction and lower levels of C-peptide and required insulin earlier during follow-up. Patients with GADA titers > 20 U/ml and insulinized early presented no significant variation in C-peptide levels, had better glycemic control and required a lower insulin dose than patients who were insulinized later.

CONCLUSION

We agree that patients with LADA should be differentiated on the basis of GADA titers and that patients with GADA titers > 20 U/ml benefit from early insulinization.

Diabetes auto-imune latente do adulto ou diabetes melito tipo 2 magro?

A prevalência do diabetes auto-imune latente do adulto (LADA) varia em virtude da população estudada, dos critérios usados e dos anticorpos avaliados. Em 256 pacientes com menos de 25 anos, encontramos 26 (10,2%) com anticorpos anti-GAD (GADA) positivos, dos quais 16 (6,3%) evoluíram sem necessidade de insulina inicialmente. Embora exista controvérsias, sugere-se como critérios diagnósticos de LADA: idade entre 25 e 65 anos; ausência de cetoacidose ou hiperglicemia sintomática no diagnóstico ou imediatamente após, sem necessidade de insulina por 6 a 12 meses; e presença de auto-anticorpos (especialmente GADA). A auto-imunidade e a resistência insulínica coexistem no LADA, e a contribuição desses fatores parece estar refletida nos títulos de GADA. Um subgrupo similar aos diabéticos tipo 2, fenotipicamente e na progressão para necessidade de insulina, parece ser melhor identificado pela presença de baixos títulos de GADA, sobretudo isolados. Por outro lado, indivíduos com altos títulos de GADA e múltiplos anticorpos apresentam fenótipo mais próximo do diabetes melito do tipo 1 (DM1) clássico e são de maior risco para falência prematura das células-beta. Comparados aos diabéticos GADA-negativos, pacientes com LADA apresentam maior prevalência de outros auto-anticorpos (anti-TPO, anti-21-hidroxilase e associados à doença celíaca) e maior freqüência de genótipos e haplótipos de risco para DM1. Pacientes com altos títulos de GADA podem ser beneficiados, retardando a falência das células-beta, com a insulinização precoce e evitando-se o uso de sulfoniluréias. Em oposição, pacientes com baixos títulos de GADA aparentemente não teriam prejuízos em serem conduzidos da mesma forma que pacientes portadores de diabetes melito tipo 2 (DM2) (GADA-negativos).

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.

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.

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.

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.

Pathophysiology of Type 2 Diabetes Mellitus in Children and Adolescents

Type 2 diabetes mellitus was considered an exclusive disease of adulthood until the late 1970s, when reports of an increased prevalence in the pediatric age group emerged in the literature. The concerning upswing in the rate of diagnosis of type 2 diabetes mellitus in children and adolescents has continued, parallel to the increasing rates of obesity. The disease is not specific to the U.S.; it has proven to be a global problem. The current information on type 2 diabetes mellitus in children and adolescents is mostly extrapolated from studies in adults with type 2 diabetes mellitus, due to the paucity of studies conducted in youth. Obesity, family history of type 2 diabetes mellitus, minority ethnicity and race, polycystic ovary syndrome, maternal diabetes mellitus or impaired glucose tolerance during gestation, and acanthosis nigricans are the major risk factors and markers of youth-onset type 2 diabetes mellitus. The pathophysiology, which involves both an insulin secretion defect and resistance to insulin, needs further clarification in pediatric studies. Current management approaches involve lifestyle modification (nutritional and exercise) along with pharmacologic agents, such as insulin and oral antihyperglycemic medications, as indicated. A recent study on the use of metformin in childhood-onset type 2 diabetes mellitus demonstrated the drug to be effective and to have a good safety profile in this population. However, the outcomes of ongoing studies and future studies focusing on type 2 diabetes mellitus in the pediatric age group will be crucial in terms of fine-tuning management plans and setting up appropriate prevention strategies.

Type 1 diabetes islet autoantibody markers

The diagnosis of type 1 diabetes versus other forms of diabetes such as type 2 diabetes is paramount to guiding proper therapy. Several islet autoantibodies have been identified that serve to diagnose immune-mediated, type 1a diabetes in clinically ambiguous cases. These autoantibodies also serve to predict type 1 diabetes in nondiabetic individuals. The most useful islet autoantibodies include islet cell cytoplasmic autoantibodies, insulin autoantibodies, glutamic acid decarboxylase autoantibodies, and insulinoma-associated-2 autoantibodies. Once type 1 diabetes can be safely and reliably prevented, large-scale islet autoantibody screening programs of the general pediatric population may be warranted. It is controversial whether islet autoantibodies influence the course of type 1 diabetes following diagnosis.