Has GAD a central role in type 1 diabetes?

CD4+ T cells recognize diverse epitopes within GAD65: implications for repertoire development and diabetes monitoring

Type 1 diabetes is associated with T-cell responses to β-cell antigens such as GAD65. Single T-cell epitopes have been investigated for immune monitoring with some success, but multiple epitopes may be required to fully characterize responses in all subjects. We used a systematic approach to examine the diversity of the GAD65-specific T-cell repertoire in subjects with DRB1*04:01 haplotypes. Using class II tetramers, we observed responses to 15 GAD65 epitopes, including five novel epitopes. The majority were confirmed to be processed and presented. Upon stimulation with peptides, GAD-specific responses were equally broad in subjects with diabetes and healthy controls in the presence or absence of CD25(+) T cells, suggesting that a susceptible HLA is sufficient to generate a potentially autoreactive repertoire. Without depleting CD25(+) cells, GAD(113-132) and GAD(265-284) responses were significantly stronger in subjects with diabetes. Although nearly every individual responded to at least one GAD65 epitope, most were seen in less than half of the subjects tested, suggesting that multiple epitopes are recommended for immune monitoring.

A novel expression platform for the production of diabetes-associated autoantigen human glutamic acid decarboxylase (hGAD65)

Background

Human glutamic acid decarboxylase 65 (hGAD65) is a key autoantigen in type 1 diabetes, having much potential as an important marker for the prediction and diagnosis of type 1 diabetes, and for the development of novel antigen-specific therapies for the treatment of type 1 diabetes. However, recombinant production of hGAD65 using conventional bacterial or mammalian cell culture-based expression systems or nuclear transformed plants is limited by low yield and low efficiency. Chloroplast transformation of the unicellular eukaryotic alga Chlamydomonas reinhardtii may offer a potential solution.

Results

A DNA cassette encoding full-length hGAD65, under the control of the C. reinhardtii chloroplast rbcL promoter and 5'- and 3'-UTRs, was constructed and introduced into the chloroplast genome of C. reinhardtii by particle bombardment. Integration of hGAD65 DNA into the algal chloroplast genome was confirmed by PCR. Transcriptional expression of hGAD65 was demonstrated by RT-PCR. Immunoblotting verified the expression and accumulation of the recombinant protein. The antigenicity of algal-derived hGAD65 was demonstrated with its immunoreactivity to diabetic sera by ELISA and by its ability to induce proliferation of spleen cells from NOD mice. Recombinant hGAD65 accumulated in transgenic algae, accounts for approximately 0.25–0.3% of its total soluble protein.

Conclusion

Our results demonstrate the potential value of C. reinhardtii chloroplasts as a novel platform for rapid mass production of immunologically active hGAD65. This demonstration opens the future possibility for using algal chloroplasts as novel bioreactors for the production of many other biologically active mammalian therapeutic proteins.

T cells to a dominant epitope of GAD65 express a public CDR3 motif

Non-obese diabetic (NOD) mice spontaneously develop autoimmune diabetes, and serve as a model for type 1 diabetes (T1D) and natural autoimmunity. T cell responses to the pancreatic islet antigen glutamic acid decarboxylase 65 (GAD65) can be detected in the spleens of young prediabetic NOD mice, which display a unique MHC class II molecule. Here, we report that a distinct TcR beta chain and CDR3 motif are utilized by all NOD mice in response to a dominant determinant on GAD65, establishing a public repertoire in the spontaneous autoimmunity to an important islet cell antigen. GAD65 530-543 (p530)-reactive T cells preferentially utilize the Vbeta4, Dbeta2.1 and Jbeta2.7 gene segments, with a CDR3 that is characterized by a triad of amino acids, DWG, preceded by a polar residue. In addition, we used CDR3 length spectratyping, CDR3-specific reverse transcriptase-PCR and direct TcR sequencing to show that the TcR beta chain structural patterns associated with p530-specific T cells consistently appeared in the islets of young NOD mice with insulitis, but not in the inflamed islets of streptozotocin-treated C57BL/6 mice, or in inflamed NOD salivary glands. To our knowledge, this is the first report to demonstrate that a public T cell repertoire is used in spontaneous autoimmunity to a dominant self-determinant. These findings suggest that defined clonotypes and repertoires may be preferentially selected in haplotypes predisposed to spontaneous autoimmunity.

Autoimmune destruction of pancreatic beta cells

Type 1 diabetes results from the destruction of insulin-producing pancreatic beta cells by a beta cell-specific autoimmune process. Beta cell autoantigens, macrophages, dendritic cells, B lymphocytes, and T lymphocytes have been shown to be involved in the pathogenesis of autoimmune diabetes. Beta cell autoantigens are thought to be released from beta cells by cellular turnover or damage and are processed and presented to T helper cells by antigen-presenting cells. Macrophages and dendritic cells are the first cell types to infiltrate the pancreatic islets. Naive CD4+ T cells that circulate in the blood and lymphoid organs, including the pancreatic lymph nodes, may recognize major histocompatibility complex and beta cell peptides presented by dendritic cells and macrophages in the islets. These CD4+ T cells can be activated by interleukin (IL)-12 released from macrophages and dendritic cells. While this process takes place, beta cell antigen-specific CD8+ T cells are activated by IL-2 produced by the activated TH1 CD4+ T cells, differentiate into cytotoxic T cells and are recruited into the pancreatic islets. These activated TH1 CD4+ T cells and CD8+ cytotoxic T cells are involved in the destruction of beta cells. In addition, beta cells can also be damaged by granzymes and perforin released from CD8+ cytotoxic T cells and by soluble mediators such as cytokines and reactive oxygen molecules released from activated macrophages in the islets. Thus, activated macrophages, TH1 CD4+ T cells, and beta cell-cytotoxic CD8+ T cells act synergistically to destroy beta cells, resulting in autoimmune type 1 diabetes.

Glutamate Decarboxylase: Loss of N-terminal Segment Does Not Affect Homodimerization and Determination of the Oxidation State of Cysteine Residues

Glutamate decarboxylase (GAD) produces GABA, the main inhibitory neurotransmitter in adult mammalian brain. The physical characteristics of GAD were studied using mass spectrometry and partial protein digests. The N-termini of the two main isoforms, GAD65 and GAD67, were processed by removal of the initial methionine residues and acetylation of the penultimate alanines. Native recombinant GAD65 and GAD67 exist as homodimers that can be dissociated with non-reducing methods, indicating that homodimerization does not involve intermolecular disulfide bonds. Truncation of the N-terminal segment with trypsin digestion did not affect homodimerization but increased activity by decreasing the Km of GAD67 and increasing the Vmax of both isoforms. Of the 15 cysteines in GAD65, the six found in the N-terminal segment can form disulfide bonds and of the 13 cysteines in GAD67, cysteines 32 and 38 can form a disulfide bond. The in vitro formation of disulfide bonds in the N-termini, and the removal of the termini with relatively low amounts of trypsin, indicate that the N-terminal segments of GAD65 and GAD67 are exposed and flexible. The formation of a disulfide bridge between cysteines 30 and 45 of GAD65 suggests that alteration of normal redox conditions could affect GAD targeting.

Multiplicity of the antibody response to GAD65 in Type I diabetes

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

Medullary epithelial cells of the human thymus express a highly diverse selection of tissue-specific genes colocalized in chromosomal clusters

Promiscuous expression of tissue-specific self-antigens in the thymus imposes T cell tolerance and protects from autoimmune diseases, as shown in animal studies. Analysis of promiscuous gene expression in purified stromal cells of the human thymus at the single and global gene level documents the species conservation of this phenomenon. Medullary thymic epithelial cells overexpress a highly diverse set of genes (>400) including many tissue-specific antigens, disease-associated autoantigens, and cancer-germline genes. Although there are no apparent structural or functional commonalities among these genes and their products, they cluster along chromosomes. These findings have implications for human autoimmune diseases, immuno-therapy of tumors, and the understanding of the nature of this unorthodox regulation of gene expression.

Evidence for a primary islet autoantigen (preproinsulin 1) for insulitis and diabetes in the nonobese diabetic mouse

It has been reported that an insulin 2 gene knockout, when bred onto nonobese diabetic (NOD) mice, accelerates diabetes. We produced insulin 1 gene knockout congenic NOD mice. In contrast to insulin 2, diabetes and insulitis were markedly reduced in insulin 1 knockout mice, with decreased and delayed diabetes in heterozygous females and no insulitis and diabetes in most homozygous female mice. Lack of insulitis was found for insulin 1 female homozygous knockout mice at 8, 12, and 37 weeks of age. Despite a lack of insulitis, insulin 1 homozygous knockout mice spontaneously expressed insulin autoantibodies. Administration of insulin peptide B:9-23 of both insulin 1 and 2 to NOD mice induced insulin autoantibodies. Insulin 1 is not the only lymphocytic target of NOD mice. Insulin 1 homozygous knockout islets, when transplanted into recently diabetic wild-type NOD mice, became infiltrated with lymphocytes and only transiently reversed diabetes. These observations indicate that loss of either insulin gene can influence progression to diabetes of NOD mice and suggest that the preproinsulin 1 gene is crucial for the spontaneous development of NOD insulitis and diabetes.

Streptozotocin upregulates GAD67 expression in MIN6N8a mouse beta cells

Glutamic acid decarboxylase (GAD) is one major autoantigen involved in the pathogenesis of autoimmune insulin dependent diabetes mellitus (IDDM). Molecular mechanisms regulating GAD expression in pancreatic beta cell are still ill-defined. Here we investigated the effect of streptozotocin (STZ), a beta cell-specific toxin, on the expression of GAD67 in MIN6N8a mouse beta cell. A 5-6-fold increase in the expression GAD67 mRNA was found in cells treated with 1.25mM STZ for 12h. Addition of NAD+ to the incubation medium slightly reduced the STZ-induced upregulation of GAD67. STZ increased p53 levels that in turn up-modulated GAD67 expression. This effect was abolished upon addition of the antioxidant N-acetyl cysteine (NAC). STZ also activated NF-kappaB and blockade of NF-kappaB activation inhibited the STZ-mediated upregulation of GAD67 expression. As a whole these data show that low dose of STZ up-regulates GAD67 expression in mouse bate cell and that NF-kappaB activation through oxidative stress plays a key role in this phenomenon. They also suggest that various stimuli promoting NF-kappaB activation may up-regulate expression of GAD autoantigen in mouse beta cells.