Autoantibodies against GAD65 rather than GAD67 precede the onset of type 1 diabetes

Initial screening for neuronal autoantibodies and their putative impact on survival in patients with small-cell lung cancer

INTRODUCTION

Small-cell lung cancer (SCLC) may be associated with neuronal autoantibodies and paraneoplastic neurological syndromes. It has been suggested that neuronal autoantibodies, especially antineuronal nuclear antibody type 1 (Hu) autoantibodies, are associated with longer survival of patients with SCLC. The objective of this study was to determine the frequency and distribution of neuronal autoantibodies at the time of diagnosis of SCLC patients and assess survival rates in relation to autoimmunity.

METHODS

In this retrospective study, serum from 40 patients with biopsy-proven SCLC at the time of diagnosis was studied prior to treatment. The sera originated from a cancer registry at the Oncology Department, Vejle Hospital from 2007 to 2010. The sera were analyzed blindly to clinical status for the presence of neuronal autoantibodies. Medical records were reviewed for neurological symptoms.

RESULTS

Neuronal autoantibodies were detected in 22/40 (55%) of the SCLC patients. A broad range of neurological symptoms was recorded in 28/40 (70%) patients, of which 14/28 (50%) were positive for neuronal autoantibodies. The most frequently detected autoantibodies were Hu (7/40, 17.5%) followed by GAD65 (6/22, 15.0%). Striational and P/Q- or N-type voltage-gated calcium channel antibodies were less common, with each found in five patients (12.5%). Eight patients (20%) had coexisting autoantibodies. Autoantibody-positivity was not associated with survival.

CONCLUSION

Neuronal autoantibodies were at time of diagnosis found in approximately half of the treatment-naïve SCLC patients. Neither autoantibody positivity at diagnosis nor neurological manifestations correlated with survival and their clinical importance requires further studies in larger, prospective cohorts.

Evaluation of glutamic acid decarboxylase (GAD) 65 antibody detection methods for neurological and diabetic investigation in an Australian diagnostic laboratory

The role of anti-glutamic acid decarboxylase (GAD) 65 autoantibodies in autoimmune neurological conditions is evolving, but testing recommendations remain unchanged in Australia with GAD enzyme-linked immunosorbent assay (ELISA) and immunoblot as the only two Therapeutic Goods Administration approved testing methods available in Australia. Common practice is for use of ELISA in diagnosis of type 1 diabetes mellitus (T1DM) and use of immunoblot for diagnosis of GAD65-associated neurological disease. We observed a cohort of patients with negative immunoblot results and positive ELISA in the context of GAD-associated neurological disease without T1DM. In the absence of robust consensus guidelines on preferred testing modalities, we sought to determine if ELISA could have a superior role in the diagnosis of GAD-associated neurological disease when compared to immunoblot in this paper. We tested for anti-GAD65 autoantibodies on 55 patient samples, 40 samples requested for neurological disease and 15 type 1 diabetes samples with detectable anti-GAD65, using two testing platforms: Euroimmun anti-GAD enzyme-linked immunosorbent assay (ELISA) and. Euroimmun EuroLine immunoblot for paraneoplastic neurologic syndromes. These results were correlated against the clinical scenario. Positive ELISA results had a sensitivity of 100% and specificity of 91% for GAD65-related neurological disease. Immunoblot showed sensitivity of 43% and specificity of 76% for GAD65-related neurological disease. ELISA proved more sensitive and specific for GAD65-related neurological disease compared to immunoblot, raising questions about the role of this testing modality in neurological disease. We propose that ELISA should be used as a sole diagnostic method for all GAD65 antibody-related neurological disease over immunoblot. The presence of anti-GAD65 antibody on immunoblot is of doubtful clinical significance.

Peptide Antibody Reactivity to Homologous Regions in Glutamate Decarboxylase Isoforms and Coxsackievirus B4 P2C

Two isoforms of the glutamate decarboxylase (GAD) enzyme exist, GAD65 and GAD67, which are associated with type 1 diabetes (T1D) and stiff-person syndrome (SPS), respectively. Interestingly, it has been reported that T1D patients seldom develop SPS, whereas patients with SPS occasionally develop T1D. In addition, coxsackievirus B4 (CVB4) has previously been proposed to be involved in the onset of T1D through molecular mimicry. On this basis, we aimed to examine antibody cross-reactivity between a specific region of GAD65 and GAD67, which has high sequence homology to the nonstructural P2C protein of CVB4 to determine potential correlations at antibody level. Monoclonal peptide antibodies generated in mice specific for a region with high similarity in all three proteins were screened for reactivity along with human sera in immunoassays. In total, six antibodies were generated. Two of the antibodies reacted to both GAD isoforms. However, none of the antibodies were cross-reactive to CVB, suggesting that antibody cross-reactivity between GAD65 and CVB, and GAD67 and CVB may not contribute to the onset of T1D and SPS, respectively.

New antigenic targets in type 1 diabetes

PURPOSE OF REVIEW

The beta-cell-specific zinc transporter isoform 8 (SLC30A8) has recently emerged both as a major autoantigenic target of type 1 diabetes and also as a genetic marker for type 2 diabetes. We examine the hypothesis that the cell specificity and cellular localization of this granule membrane protein are significant factors in its contribution to the pathogenesis of these diseases.

RECENT FINDINGS

Both type 1 diabetes and type 2 diabetes are associated with islet functional failure and both diseases may be linked to stress responses and changes in the secretory pathway, which lead to cell apoptosis and thus directly to reduction of beta-cell mass or activation of underlying autoimmunity. In both cases, the common polymorphism at aa 325 has been implicated in disease, in type 1 diabetes by determining the autoantibody epitope specificity and in type 2 diabetes through association with altered beta-cell mass and impaired secretion.

SUMMARY

Functional studies of the transporter will be key to understanding the role of ZnT8 in type 2 diabetes. Investigation of the cellular immune response to ZnT8 will be essential in evaluating its contribution to type 1 diabetes. Measurement of autoantibodies to ZnT8 takes us a step closer to detection of prediabetes in the general population.

The cation efflux transporter ZnT8 (Slc30A8) is a major autoantigen in human type 1 diabetes

Type 1 diabetes (T1D) results from progressive loss of pancreatic islet mass through autoimmunity targeted at a diverse, yet limited, series of molecules that are expressed in the pancreatic beta cell. Identification of these molecular targets provides insight into the pathogenic process, diagnostic assays, and potential therapeutic agents. Autoantigen candidates were identified from microarray expression profiling of human and rodent pancreas and islet cells and screened with radioimmunoprecipitation assays using new-onset T1D and prediabetic sera. A high-ranking candidate, the zinc transporter ZnT8 (Slc30A8), was targeted by autoantibodies in 60-80% of new-onset T1D compared with <2% of controls and <3% type 2 diabetic and in up to 30% of patients with other autoimmune disorders with a T1D association. ZnT8 antibodies (ZnTA) were found in 26% of T1D subjects classified as autoantibody-negative on the basis of existing markers [glutamate decarboxylase (GADA), protein tyrosine phosphatase IA2 (IA2A), antibodies to insulin (IAA), and islet cytoplasmic autoantibodies (ICA)]. Individuals followed from birth to T1D showed ZnT8A as early as 2 years of age and increasing levels and prevalence persisting to disease onset. ZnT8A generally emerged later than GADA and IAA in prediabetes, although not in a strict order. The combined measurement of ZnT8A, GADA, IA2A, and IAA raised autoimmunity detection rates to 98% at disease onset, a level that approaches that needed to detect prediabetes in a general pediatric population. The combination of bioinformatics and molecular engineering used here will potentially generate other diabetes autoimmunity markers and is also broadly applicable to other autoimmune disorders.

Autoimmune diabetes: ongoing development of immunological intervention strategies targeted directly against autoreactive T cells

It is well known that autoimmunity associated with the onset of insulin-dependent diabetes mellitus (IDDM) involves the generation of autoreactive T and B cells. The findings that diabetics mount humoral and cellular immune responses against islet cell antigens (ICAs) have led to the testing of ICAs and their analogs as candidates for therapeutic agents for better treatment of IDDM at its prediabetic and diabetic stages. Apart from this type of approach, various immunological intervention strategies aimed at direct targeting of the autoreactive T cells have also been investigated. The present review covers the ongoing aspects of these developments focusing on the preclinical findings made in NOD (nonobese diabetic) mice which have been commonly used as a disease model for human autoimmune diabetes. Other types of approaches involving the mobilization of regulatory T cells to indirectly control or modulate the pathological activity of autoreactive T cells will not be discussed within this scope.

Multiple and high-titer single autoantibodies in schoolchildren reflecting the genetic predisposition for type 1 diabetes

The study aimed to compare the HLA specificities of AAb-positive healthy schoolchildren with those of patients with type 1 diabetes (T1D). HLA-DRB1 and DQB1 alleles were determined in 178 AAb-positive and 339 AAb-negative schoolchildren aged 6-17 years without first-degree relatives with T1D and in 274 patients with T1D. AAbs against glutamic acid decarboxylase (GADA), protein tyrosine phosphatase (IA-2A), and insulin (IAA) were determined by (125)I-antigen binding, and islet cell cytoplasmic antibodies (ICAs) immunohistochemically. Here, 82.6% (147/178) of AAb-positive schoolchildren had single AAbs and 17.4% (31/178) had multiple AAbs. In both groups, GADA occurred with highest and IAA with lowest frequency. In children with single AAbs at levels between the 99th and 99.9th percentile, frequencies of the diabetes-associated DRB1 (*03, *04) and DQB1 (*02, *0302) alleles and the protective DRB1 (*15) and DQB1 (*0602) alleles did not differ from those of controls. In patients, the positive associations were confirmed for DRB1*04 (OR = 5.39) and DQB1*0302 (OR = 9.05), whereas DRB1*15 (OR = 0.05) and DQB1*0602 (OR = 0.06) were negative-associated (p < 0.001). The same association was found in schoolchildren with multiple AAbs for DRB1*04 (OR = 3.84), DQB1*0302 (OR = 4.95), and DRB1*15 (OR = 0.1; p < 0.001-0.014), and with high-titer single AAbs (>/=99.9th percentile), but none of them had DQB1*0602. The highest risk genotype DQB1*02/*0302 occurred in 36.5% of patients (OR = 21.07) and in 19.3% of children with multiple AAbs (OR = 8.8; p<0.001). It is concluded that probands with multiple and high-titer single AAbs in the general population have the same genetic predisposition for T1D as patients and are therefore at highest risk for the disease.

Diabetes mellitus-related autoantibodies in childhood autoimmune hepatitis

OBJECTIVE

To determine the frequency and significance of diabetes mellitus (DM)-related autoantibodies in children with autoimmune hepatitis (AIH).

RESEARCH DESIGN AND METHODS

Anti-islet cell antibodies (ICA), insulin autoantibodies (IAA), and anti-glutamic acid decarboxylase (GAD65) antibodies were assessed in 28 children (25 female) with AIH before and after 3-9 years of therapy with azathioprine and prednisone.

RESULTS

There was biochemical and clinical remission of AIH activity in 76% of the children after 1 year of immunosuppressive therapy. Positive ICA and IAA were found in 60.7% and 18.5% of the patients, decreasing to 38.5% and 12% after 3-9 years of therapy. Anti-GAD autoantibodies were present in only one patient who had Graves' disease, high ICA titer, and developed type 1 DM after 3 years. After 3-9 years of follow up, all had normal fasting glycemia, glycosylated hemoglobin (HbA1c), and, with a single exception, normal responses to oral glucose tolerance testing. No increase in the frequencies of HLA antigens was observed in ICA- and IAA-positive patients compared to antibody-negative patients or a control population. The majority of the patients with HLA-DRB1*03 or DRB1*04, however, were positive for ICA (7/10), and three of them had IAA. The frequency of high risk HLA DQB1*0302 or DQB1*02 alleles was low and similar to control frequencies, indicating low-risk for DM despite the presence of DM-related autoimmunity markers.

CONCLUSIONS

AIH in childhood is associated with high frequency of ICA and IAA, with less than expected rates of progression to DM. Immunosuppression reduced ICA and IAA frequency and titers.

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.

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.

Sensitive monoclonal antibody-based sandwich ELISA for determination of the diabetes-associated autoantigen glutamic acid decarboxylase GAD65

Although various methods for the detection of autoantibodies against glutamic acid decarboxylase (GAD65-AAb) are known, no sensitive method for the quantification of GAD65 as autoantigen is available. We describe a sandwich ELISA based on monoclonal GAD65 antibodies (Mc-GAD65-Ab) of different epitope specificities to quantify GAD65 in pancreatic islets and in different organ/cell extracts and during the preparation of GAD from brain extracts. GAD65 was captured via solid phase coated Mc-GAD65-Ab and detected via a second biotin-labelled Mc-GAD65-Ab recognizing a NH2-terminal epitope of the molecule. The detection limit was estimated to be 0.03 ng GAD65/ml using alkaline phosphatase (AP)-conjugated streptavidin. GAD65 contents in islets of neonatal BB/OK rats and Lewis rats amounted to 37.4 and 43.7 pg/islet, respectively. Furthermore, GAD65 was quantified in brain extracts of pig (55.1 ng/mg protein), mouse (39.5 ng/mg), rat (243.8 ng/mg) and pig cerebellum (514.8 ng/mg) and in different organ extracts of Lewis rat.

A new solid-phase radioimmunoassay to detect anti-GAD65 autoantibodies

This paper describes a simple, rapid, routine method to detect anti-GAD65 autoantibodies by a solid-phase radioimmunoassay using human recombinant GAD65 coated microwells and 125I-protein A to reveal antibody binding. Both recombinant and radiolabelled proteins are commercially available. This new method was validated by investigating the presence of GAD65 autoantibodies in two different studies (A and B); the first including subjects originating from our own case histories (group A sera), the second made up of recoded subjects and standards sent to our lab by the Second International GAD Antibody Workshop organizers (group B sera). In study A we tested sera from 52 normal subjects, 25 newly diagnosed type 1 diabetics and 3 stiff man syndrome (SMS) subjects detecting GAD65 autoantibodies in 72% of IDDM and 100% of SMS patients. In study B we tested (in blind fashion) 89 recoded sample sera or standards that were part of the larger group used in the Second International GAD Antibody Workshop, finding GAD65 autoantibodies in 3.3% of healthy control subjects (1/30), 60% of IDDM patients (18/30), 100% of ICA + nondiabetic subjects (3/3) but in none of 4 nondiabetic patients with Graves disease. Comparing our solid-phase RIA results with those published for the same sera from the Second International GAD Antibody Workshop we obtained for our method a sensitivity of 85.7%, a specificity of 93.9% and a consistency of 100%. These result indicate that our assay, which is based on commercially available reagents, should be a useful tool for the detection of GAD65 autoantibodies in large scale studies.

Glutamic acid decarboxylase autoantibody assay using 125I-labelled recombinant GAD65 produced in yeast

We describe a new method for measuring autoantibodies (Ab) to the 65 kDa isoform of glutamic acid carboxylase (GAD65). In particular, GAD65 without the hydrophobic N-terminal region has been produced in yeast, purified, labelled with 125I and reacted with GAD65 Ab. Antibody bound 125I-GAD65 is then precipitated by the addition of solid phase protein A. With the assay, GAD65 Ab were detected in 59 of 71 (83%) islet cell antibody (ICA) positive IDDM patients and in 8 of 23 (35%) ICA negative IDDM patients (overall 67 of 94 (71%) of IDDM patients). Low concentrations of GAD65 Ab were also detected in 2/98 (2%) healthy blood donors and 1/27 (4%) Graves' disease patients had a high level of antibody. GAD65 Ab were not detected in any of 10 Hashimoto's thyroiditis, 20 Addison's disease or 19 myasthenia gravis sera. There was good agreement between the 125I assay and the current reference method based on 35S-labelled full-length GAD65 (produced by in vitro transcription/translation reaction) and solid phase protein A (r = 0.91, n = 108). Overall, our 125I assay showed sensitivity, precision and disease group specificity at least as good as any assay so far described. These features, combined with a simple assay protocol and the convenience of 125I counting and handling indicate that the method is suitable for routine GAD65 Ab measurements.

Murine monoclonal glutamic acid decarboxylase (GAD)65 antibodies recognize autoimmune-associated GAD epitope regions targeted in patients with type 1 diabetes mellitus and stiff-man syndrome

To study the immune response to glutamic acid decarboxylase (GAD) in insulin-dependent diabetes mellitus, monoclonal GAD antibodies after fusion of splenocytes from a nondiabetes-susceptible BALB/c mouse immunized with human recombinant GAD65 were generated. Of the 44 monoclonals, 35 are specific for the GAD65 isoform, whereas 9 also react with GAD67. Some 37 monoclonals, including all GAD65/67 reactive antibodies, react with GAD by Western blot analysis. The remaining 7 GAD65 monoclonals bind GAD only in an immunoprecipitation assay, which implies that they target epitopes dependent on the conformation of the GAD molecule. The 125I-GAD binding of the GAD65 monoclonals reactive on Western blotting was significantly diminished by all 3 sera from Stiff-man syndrome patients but only by 3/30 (10%) sera from type 1 diabetic patients. In contrast, the 7 monoclonal antibodies reactive with a conformation-dependent GAD epitope were competitive with 83% of GAD-autoantibody-positive sera from these diabetic patients. Using chimeric GAD65/67 proteins, the epitope region targeted by these monoclonals was mapped to the middle of GAD65 (amino acids 221-442). This central conformation-dependent GAD region was also targeted by sera from patients with type 1 diabetes. In conclusion, our data show that even after common immunization of a nondiabetes-susceptible mouse strain, monoclonal were obtained which preferentially react with the GAD65 linear amino-terminus (amino acids 4-17) and a conformation-dependent region located in the middle of GAD targeted by autoantibodies, indicating that this GAD region is not restricted to the autoimmune response associated with the Stiff-man syndrome and the beta-cell destruction in type 1 diabetes mellitus.

Comparison of the islet cell antibody pattern of monoclonal glutamic acid decarboxylase antibodies recognizing linear and conformational epitopes

In order to compare the reactivity of glutamic acid decarboxylase (GAD) antibodies recognizing linear and conformational epitopes as islet cell cytoplasmic antibodies (ICA), monoclonal antibodies were generated. An ELISA displacement test using two biotinylated monoclonals recognizing a linear (M61/7E11) or a conformational GAD65 epitope (M65/6B12) was performed to identify epitope regions recognized by monoclonal GAD antibodies. The GAD binding by monoclonal GAD antibodies was tested by immunofluorescence on fixed and unfixed pancreatic sections of human, rat, and mouse, and by Dot-blot experiments. 16/23 (69.6%) of the monoclonals were specifically reactive with GAD65 and 7/23 (30.4%) were reactive with both GAD isoforms. 8/16 (50%) of monoclonal GAD65 antibodies recognized a linear GAD epitope located at the N-terminus (pattern 1). 5/16 (31.3%) displaced M65/6B12, indicating the recognition of a conformational GAD epitope (pattern 2). Monoclonals belonging to patterns 1 and 2 showed strong ICA binding. 3/16 (18.8%) of monoclonals specific for GAD65 with weak or no immunostaining of pancreatic islets (pattern 3) did not inhibit the binding of both biotinylated antibodies in the displacement test, indicating other epitope specificities. In conclusion, GAD antibodies recognizing both conformational and linear epitopes of the GAD65 molecule are involved in ICA binding with strong reactivity. Furthermore, results obtained with monoclonals of pattern 3 suggest the occurrence of GAD65 epitopes partly inaccessible on cryosections, which may result in an ICA-negative test of GAD65 autoantibody positive sera.

Antibody response to islet antigens in anti-CD4/prednisolone immune intervention of type 1 diabetes

Cytoplasmic islet cell antibodies, glutamic acid decarboxylase autoantibodies, spontaneous insulin autoantibodies, and insulin-induced antibodies were analyzed in a 1-year follow-up study of 12 newly diagnosed patients with insulin-dependent diabetes mellitus aged 14 +/- 2 years (range 7-20 years) who had been initially treated with either multiple injections of insulin alone (control group) or, in addition, anti-CD4 monoclonal antibody/prednisolone (treatment group). Despite individual variations in islet cell antibody titers, there were no significant differences in the prevalence or changes in the mean titers between the two groups. Glutamic acid decarboxylase autoantibodies remained almost unchanged, but correlated with levels of islet cell antibodies. While at initiation of treatment only 50% of the patients from both groups had spontaneous insulin autoantibodies, all patients developed insulin-induced antibodies upon conventional insulin therapy during the course of follow-up. This was not related to islet cell antibody or glutamic acid decarboxylase antibody levels. The insulin requirement was markedly reduced through the period of follow-up, but did not significantly differ between the two groups. A correlation between islet cell antibody levels and insulin requirement was observed in the control group but not in the treatment group. Plasma levels of the antibodies were not associated with changes in stimulated C-peptide or hemoglobin A1 concentrations. Activated T-lymphocytes persisted in both groups of patients, but their mean levels were not significantly different. The reason for the absence of statistically significant differences between treatment and control groups could be due to the small number of patients in the study. In conclusion, short-term immune intervention with anti-CD4 monoclonal antibody in addition to insulin therapy did not suppress autoimmune reactions towards the beta cells.