T-lymphocyte lines specific for glutamic acid decarboxylase (GAD) the 64K beta-cell antigen of IDDM

No specific reactivity to E. coli glutamic acid decarboxylase from sera of newly-diagnosed insulin dependent diabetic patients

The 65 kD isoform of Glutamic Acid Decarboxylase (GAD), is one of the major autoantigens in human type 1 diabetes mellitus. This enzyme shares amino acid identity, in select regions already determined as antigenic with its counterpart from E. coli. We tested the reactivity of diabetic and normal sera and an E. coli GAD-specific monoclonal antibody (2D9) to E. coli GAD by solid phase and competition ELISA, as well as immunoblotting to check for cross-reactivity of autoantibodies to the two antigens. Specific antibodies for E. coli GAD are present in diabetics and normal subjects without any differences in frequency and titer. The reactivity of such antibodies in ELISA could be blocked in a dose-dependent manner by the addition of excess antigen in the liquid phase. Furthermore, the monoclonal antibody against E. coli GAD does not recognise human recombinant GAD65 in an ELISA. We conclude that there is no basis for cross-reactivity between the two antigens, and antibody reactivity to GAD65 in man cannot arise from cross-reactivity to the E. coli enzyme.

T cell reactivity to human insulinoma cell line (CM) antigens in patients with type 1 diabetes

Autoimmune (type 1) diabetes mellitus results from a progressive destruction of insulin secreting beta cells operated by T lymphocytes in pancreatic islets. Circulating autoreactive T cells to specific beta cell antigens are detected in patients with type 1 diabetes. To date, several beta cell autoantigens have been identified in this disease (GAD, IA-2, 38kD secretory protein, insulin, ICA69 etc.), however, it is possible that also other unidentified self molecules contribute to trigger beta cell autoimmunity. In this study we used the human insulinoma cell line CM as source of beta cell antigens to detect reactive T lymphocytes in patients with type 1 diabetes mellitus. This cell line has been previously shown to express a number of recognized beta cell antigens. Since the expression of several beta cell antigens is affected by glucose stimulation we tested two preparations of CM cells cultured under different conditions containing low (0.8 mM) and high glucose concentration (11 mM). T cell proliferation was measured using cells from 32 patients with type 1 diabetes (19 of recent onset and 13 at 3 to 22 months from diagnosis) and 27 age-matched control subjects. A significant increase in T cell proliferation to CM cells grown in high glucose conditions (11 mM) (p < 0.05) was found in type 1 diabetic patients compared to controls. No significant differences were observed when using CM cells cultured at the low glucose concentration. Furthermore, the response to both extracts of CM cells was independent of disease duration (p = 0.6 for both CM cells cultured at 0.8 and 11 mM glucose). These data indicate that T cell reactivity to homogenates of CM cells is detectable in patients with type 1 diabetes and suggest that this human insulinoma cell line is an interesting potential source of beta cell material for immunological studies of autoimmune diabetes.

Anti-GAD monoclonal antibody delays the onset of diabetes mellitus in NOD mice

Insulin Dependent Diabetes Mellitus (IDDM type I) is the result of autoimmune destruction of insulin producing pancreatic beta-cells by the cellular immune system, specifically, autoreactive T cells. Disease progression is evident by multiple autoantibodies responding to self-antigens in a cascade mechanism, wherein the first self-antigen induces the activation of the immune system, leading to the destruction of beta-cells and consequently, exposure of other antigens. Glutamic Acid Decarboxylase (GAD) is recognized in the literature as a primary autoantigen involved in the cascade. We questioned the immunological involvement of this autoantigen in the overall progression of the disease, specifically if antigen recognition by the cellular immune system (T cells) is necessary for organ specific autoimmunity and cellular toxicity. We tested this hypothesis by isolating, purifying and injecting monoclonal antibodies against GAD (anti-GAD Ab; 0.1 mg or 0.3 mg) into non-obese diabetic (NOD) mice on a weekly basis. We suggest that the anti-GAD Ab will bind to the GAD antigen, or perhaps bind to the epitope presented in association with APC-MHC and prevent T cell recognition, thereby delaying disease onset. Our results demonstrate a delay in the onset of diabetes and a decrease in the severity of insulitis in our test animals, when compared to controls. The mechanism of action of the anti-GAD Ab may be associated with a passive protection mechanism, as evidenced by the fact that splenocytes transferred from anti-GAD Ab treated mice did not prevent or delay diabetes in syngeneic irradiated NOD mice. The mechanism of diabetes prevention by administration of anti-GAD antibody could be associated with an interference in recognition of GAD by T cells, and continuing research will be perform to investigate this hypothesis.

Enzyme-linked immunosorbent assay of autoantibodies against mitochondrial glycerophosphate dehydrogenase in insulin-dependent and non-insulin-dependent diabetic subjects

The mitochondrial enzyme FAD-linked glycerophosphate dehydrogenase (mGDH) plays a key role in the recognition of glucose as a stimulus for insulin release from the pancreatic islet B-cell. In the present study, an ELISA procedure was used for the measurement of mGDH antibodies in both insulin-dependent (IDDM) and non-insulin-dependent (NIDDM) diabetic patients. Positive readings, exceeding the upper limit of the normal range, were recorded in 7 out of 12 IDDM patients, as distinct (P < 0.01) from 2 out of 12 nondiabetic subjects of comparable age. The study conducted in 41 NIDDM patients and 15 control subjects of similar age indicated that the incidence of mGDH-positive cases was not significantly different in the diabetic (4/41) and control (1/15) groups, the measurement of optical density in the positive cases barely exceeding the upper limit of the normal range. These findings indicate that the mitochondrial enzyme mGDH often acts as an antigenic determinant in IDDM, but not in NIDDM, patients.

Mononuclear cytotoxicity and proliferation towards glucose stimulated rodent pancreatic islet cells

Diabetes is due to an autoimmune cellular immunologic destruction of the pancreatic beta cells. By the use of a chromium release assay and a proliferation assay we have investigated the possible role of beta cell activity for this destruction. Results show that in vitro glucose stimulated pancreatic islet cells are subjects to a slight but significantly higher cellular immunologic destruction by mononuclear spleen cells than unstimulated islet cells. The functional dependency of the islet cell destruction must be a product of both a mononuclear cell dysfunction and a specific islet cell pattern. This is due to the fact that all combinations of mononuclear cells and islet cells from diabetes prone BB rats and non-diabetes prone WF rats tested against each other, results in functional dependent cytotoxicity, except for the assay in which both effector cells and target cells are of WF rat origin. Additional observations indicate, that the diabetes prone BB rat mononuclear cells need previous in vivo activation as only cells from diabetic individuals, and not normoglycemic ones, display the reaction in question. Functional dependent cytotoxicity is validated in an other IDDM animal model--the NOD mouse. NOD mononuclear cells towards the murine MIN-6 beta cell line results in increased cellular cytotoxicity when the latter is glucose stimulated. Also the proliferative response of BB rat mononuclear cells to whole islets tend to show function dependency.

Imogen 38: a novel 38-kD islet mitochondrial autoantigen recognized by T cells from a newly diagnosed type 1 diabetic patient

Cell-mediated autoimmune attack directed against islet proteins of approximately 38 kD in size has been associated with type 1 diabetes. A novel murine cDNA encoding an antigen of this size was cloned using a screening procedure based on the proliferative response of a human diabetic T cell clone (1C6) to a recombinant antigen epitope library. Membrane preparations from COS 7 cells transfected with the full-length 1,267-bp cDNA elicited a proliferative response from the reporter T cells comparable to that of the defined peptide epitope and native insulinoma antigen. In vitro translation and transfection experiments suggested that the protein is initially synthesized as a 44-kD protein and then processed to the native 38-kD form through the proteolytic removal of a 54-aa NH2-terminal mitochondrial targeting sequence. Differential centrifugation, Percoll density gradient centrifugation, and immunofluorescence studies confirmed localization of the antigen to mitochondria. Northern blot, Western blot, and 1C6 T cell proliferation assays showed that, although imogen 38 was more highly expressed in beta cell than alpha cell lines, it was also present in other tissues. It is concluded that imogen 38 may be a target for bystander autoimmune attack in diabetes rather than a primary autoantigen.

The pathogenicity of spontaneously-occurring thyroglobulin-reactive T lymphocytes from BB/WOR rats

Autoantigen-reactive T lymphocytes have been implicated in the pathogenesis organ-specific autoimmune disease. Thyroglobulin (Tg) is one of the primary autoantigens associated with autoimmune lymphocytic thyroiditis (LT). These experiments investigated the pathogenicity of a lymphocyte line derived from spontaneously-occurring Tg-reactive T lymphocytes isolated from unprimed NB line BB/Wor rats which have nearly a 100% incidence of spontaneous LT. Adoptive transfer of LT was accomplished by injecting 1.0 x 10(5) Tg-reactive lymphocytes into the tail vein of MHC compatible, non LT-prone BB line BB/Wor rats. All of the Tg-reactive cell line recipients (5/5) developed LT compared to only 20% (1/5) of the control rats given a parallel tetanus toxoid-reactive T cell line (p < 0.05, Fisher's exact test). Furthermore, despite the presence of LT, only one Tg-reactive cell line recipient developed insulitis. When Tg-reactive lymphocytes were incubated with an MHC compatible Wistar rat thyrocyte line at increasing effector: target ratios, the T cell line lysed thyrocytes in a dose-response fashion (r = 0.99, p < 0.05, linear regression), but did not lyse smooth muscle cell targets. FACS analysis established that this cell line is CD8 predominant. This is the first study to demonstrate that spontaneously-occurring Tg-reactive T lymphocytes from a nonimmunized animal model for LT are pathogenic. Further investigations into the repertoire of Tg-reactive lymphocytes in BB/Wor rats should provide insight into the pathogenesis of autoimmune thyroid disease and provide a basis for targeted immunotherapy.

GAD65 is recognized by T-cells, but not by antibodies from NOD-mice

Since the 64kDa-protein glutamic acid decarboxylase (GAD) is one of the major autoantigens in T-cell mediated Type 1 diabetes, its relevance as a T-cell antigen needs to be clarified. After isolation of splenic T-cells from non-obese diabetic (NOD) mice, a useful model for human Type 1 diabetes, we found that these T-cells proliferate spontaneously when incubated with human GAD65, but only marginally after incubation with GAD67, both recombinated in the baculovirus system. No effect was observed with non-diabetic NOD mice or with T-cells from H-2 identical NON-NOD-H-2g7 control mice. It has been published previously that NOD mice develop autoantibodies against a 64kDa protein detected with mouse beta cells. In immunoprecipitation experiments with sera from the same NOD mice and 35S-methionine-labelled GAD, no autoantibody binding could be detected. We conclude firstly that GAD65 is an important T-cell antigen which is relevant early in the development of Type 1 diabetes and secondly that there is an antigenic epitope in the human GAD65 molecule recognized by NOD T-cells, but not by NOD autoantibodies precipitating conformational epitopes. Our results therefore provide further evidence that GAD65 is a T-cell antigen in NOD mice, being possibly also involved in very early processes leading to the development of human Type 1 diabetes.

Autoreactive epitopes defined by diabetes-associated human monoclonal antibodies are localized in the middle and C-terminal domains of the smaller form of glutamate decarboxylase

The gamma-aminobutyrate-synthesizing enzyme glutamate decarboxylase (GAD; L-glutamate 1-carboxy-lyase, EC 4.1.1.15) is a major target of autoantibodies associated with both early and late stages of pancreatic beta-cell destruction and development of type 1 diabetes. We have used five monoclonal anti-islet-cell antibodies (MICAs 1,2,3,4, and 6) derived from a newly diagnosed diabetic patient to probe the autoimmune epitopes in the enzyme. All the MICAs specifically recognized the smaller GAD protein, GAD65, and did not recognize the nonallelic GAD67 protein. A series of N-terminal, C-terminal, and internal deletion mutants, as well as protein footprinting, were used to identify the target regions in GAD65. Immunoprecipitation revealed two major native epitope areas in the GAD65 molecule. The first, defined by MICAs 1 and 3, is destroyed by deleting 41 amino acids at the C terminus but is also dependent on intact amino acids 244-295. This epitope (or epitopes) may span both middle and C-terminal domains of the protein. The second conformational epitope region, defined by MICAs 4 and 6, is dependent on intact amino acids 245-295 but is not affected by deletion of 110 amino acids at the C terminus and is therefore confined to domain(s) in the middle of the molecule. MICA 2 recognizes a linear epitope close to the C terminus. Thus, the N-terminal domain of GAD65, which differs most significantly from GAD67, does not harbor the MICA epitopes. Rather subtle amino acid differences in the middle and C-terminal domains define the GAD65-specific autoimmune epitopes. Analysis of sera from 10 type 1 diabetic patients suggests that MICAs 1, 3, 4, and 6 represent a common epitope recognition in this disease, whereas the MICA 2 epitope is rare. Furthermore, autoantibodies in some sera are restricted to the MICA 1/3 epitope, suggesting that this epitope may represent a single dominant epitope in the early phases of beta-cell autoimmunity.