Immunoglobulin variable gene analysis of human autoantibodies reveals antigen-driven immune response to glutamate decarboxylase in type 1 diabetes mellitus

Dynamic changes of GAD65 autoantibody epitope specificities in individuals at risk of developing type 1 diabetes

AIMS/HYPOTHESIS

Progression to type 1 diabetes is associated with intramolecular epitope spreading to disease-specific antibody epitopes located in the middle region of glutamic acid decarboxylase 65 (GAD65).

METHODS

The relationship between intramolecular epitope spreading of autoantibodies specific to GAD65 in relation to the risk of developing type 1 diabetes was tested in 22 high-risk individuals and 38 low-risk individuals. We determined the conformational epitopes in this longitudinal study by means of competition experiments using recombinant Fab of four GAD65-specific monoclonal antibodies.

RESULTS

Sera from high-risk children in the preclinical stage recognise a specific combination of GAD65 antibody epitopes located in the middle and the C-terminus of GAD65. High risk of progressing to disease is associated with the emergence of antibodies specific for conformational epitopes at the N-terminus and the middle region. Binding to already established antibody epitopes located in the middle and at the N-terminus increases and shows a significant relation (p=0.005) with HLA, which confers risk of developing diabetes.

CONCLUSIONS/INTERPRETATION

In type 1 diabetes, GAD65 antibodies are initially generated against the middle and C-terminal regions of GAD65. In genetically predisposed subjects the autoimmune response may then undergo intramolecular epitope spreading towards epitopes on the N-terminus and further epitopes located in the middle. These findings clearly demonstrate that the GAD65 autoantibody response in the preclinical stage of type 1 diabetes is dynamic and related to the HLA genotypes that confer risk of diabetes. GAD65-specific Fab should prove useful in predicting progression from islet autoimmunity to clinical onset of type 1 diabetes.

Characteristics of a human monoclonal autoantibody to the thyrotropin receptor: sequence structure and function

The properties of a human monoclonal antibody to the thyrotropin receptor (TSHR) (M22) with the characteristics of patient sera thyroid stimulating autoantibodies is described. Similar concentrations (pmol/L) of M22 Fab and porcine TSH had similar stimulating effects on cyclic adenosine monophosphate (cAMP) production in TSHR-transfected Chinese hamster ovary cells whereas higher doses of intact M22 immunoglobulin G (IgG) were required to cause the same level of stimulation. Patient sera containing TSHR autoantibodies with TSH antagonist (blocking) activity inhibited M22 Fab and IgG stimulation in a similar way to their ability to block TSH stimulation. Thyroid-stimulating monoclonal antibodies (TSmAbs) produced in mice inhibited 125I-TSH binding and 125I-M22 Fab binding to the TSHR but the mouse TSmAbs were less effective inhibitors than M22. These competition studies emphasized the close relationship between the binding sites on the TSHR for TSH, TSHR autoantibodies with TSH agonist activity, and TSHR autoantibodies with TSH antagonist activity. Recombinant M22 Fab could be produced in Escherichia coli and the recombinant and hybridoma produced Fabs were similarly active in terms of inhibition of TSH binding and cAMP stimulation. The crystal structure of M22 Fab was determined to 1.65 A resolution and is that of a standard Fab although the hypervariable region of the heavy chain protrudes further from the framework than the hypervariable region of the light chain. The M22 antigen binding site is rich in aromatic residues and its surface is dominated by acidic patches on one side and basic patches on the other in agreement with an important role for charge-charge interactions in the TSHR-autoantibody interaction.

Humoral immune response directed against LEDGF in patients with VKH

Vogt-Koyanagi-Harada disease is an autoimmune systemic disorder. In Vogt-Koyanagi-Harada disease, inflammatory disorders occur in multiple organs containing melanocytes, including uvea (resulting in acute bilateral panuveitis), skin (resulting in vitiligo and alopecia), central nervous system (resulting in meningitis) and inner ears (resulting in hearing loss and tinnitus). These inflammatory aspects are attributed to the destruction of melanocytes through immunological mechanisms. Studies have been carried out to elucidate the exact etiology and target autoantigen in Vogt-Koyanagi-Harada disease, but much remains to be investigated. Identification of target autoantigen is important to understand the etiology of autoimmune diseases, and for development of antigen-specific immuno-modulation therapy. To identify the target autoantigens in Vogt-Koyanagi-Harada disease, we made use of an immunoscreening of a bovine uveal cDNA expression library with serum samples obtained from patients with Vogt-Koyanagi-Harada disease. We identified an immunoreactive cDNA clone that encodes bovine lens epithelium derived growth factor. mRNA of human lens epithelium derived growth factor was determined by reverse transcription-polymerase chain reaction and it was expressed in human uvea, retina and melanocytes. Immunoglobulin G (IgG) autoantibodies were quantitated in an enzyme-linked immunosorbent assay, using recombinant human lens epithelium derived growth factor. The prevalence of IgG anti-lens epithelium derived growth factor autoantibodies in patients with Vogt-Koyanagi-Harada disease was significantly higher than that in healthy controls (66.7% versus 21.6%, P<0.001). On the other hand, the prevalence of the autoantibody in patients with panuveitis of other etiology, Behçet's disease and sarcoidosis, was almost same as that in healthy controls. These results suggest that the humoral immune response agonist lens epithelium derived growth factor is not a mere secondary phenomena caused by uveal tissue damage.

Isolation and functional characterization of recombinant GAD65 autoantibodies derived by IgG repertoire cloning from patients with type 1 diabetes

The generation of human monoclonal autoantibodies is critical for understanding humoral immune responses in autoimmunity. In this study, we isolated the first human recombinant antibodies to glutamate decarboxylase (rGAD65ab) by IgG repertoire cloning, phage display of Fab fragments, and biopanning from two patients at onset of type 1 diabetes. We demonstrate that natural Ig heavy- and light-chain pairings of autoantibodies can be isolated by the recombinant approach and have a major selection advantage over other rGAD65ab. Among eight rGAD65ab, three (rGAD65ab A-C) displayed all functional and structural properties of known disease-related GAD65ab, including reactivity in the enzyme-linked immunosorbent assay (ELISA), radioimmunoassay (RIA), islet cell antibody (ICA) test, and variable gene usage. Dominant epitope recognition was directed to the previously defined epitope EP-1 in the middle of GAD65, corroborating its immunodominance in the molecule. New features, such as assay-dependent GAD65 reactivity and new epitope recognition, were observed in two rGAD65ab (D and E). These antibodies were positive in the GAD65 ELISA and ICA test but not in the GAD65 RIA, providing the first examples for ICA with incongruent results in solid-phase and fluid-phase assays. In conclusion, phage display-derived antibodies reflected well the natural autoantibody response in type 1 diabetes and may allow further characterization of assay-dependent features of GAD65ab and the recognition of epitopes in solid- but not fluid-phase assays.

Fusion Proteins for Combined Analysis of Autoantibodies to the 65-kDa Isoform of Glutamic Acid Decarboxylase and Islet Antigen-2 in Insulin-dependent Diabetes Mellitus

Background: Prediction, risk assessment, and diagnosis of autoimmune diseases often rely on detection of autoantibodies directed to multiple target antigens, such as the 65-kDa isoform of glutamic acid decarboxylase (GAD65-abs) and the tyrosine phosphatase-like protein islet antigen-2 (IA2-abs), the two major subspecificities of islet cell antibodies (ICAs) associated with insulin-dependent diabetes mellitus. We hypothesized that a combination of autoantigens in a fusion protein unifying the important immunodominant epitopes could provide an efficient target for cost-effective, one-step screening of sera.

Methods: Chimeric proteins composed of GAD65 and IA2 residues were constructed, analyzed for their immune reactivity with monoclonal antibodies and sera, and used in a diagnostic assay with 35S-labeled protein as antigen.

Results: Length and order of GAD65 and IA2 sequences were critical for conservation of the conformational epitopes in the fusion protein. Among four chimera tested, only IA2(606–979)/GAD65(1–585) retained wild-type-like folding of GAD65 and IA2 domains and yielded a stable protein after baculovirus expression. Reactivity of GAD65 antibody- and IA2 antibody-positive sera from patients newly diagnosed with insulin-dependent diabetes mellitus, from ICA-positive prediabetics, and from ICA-positive first-degree relatives demonstrated conservation of the relevant autoreactive epitopes. The assay based on the in vitro translated fusion antigen had a sensitivity and specificity identical to those for detection of GAD65- and IA2-abs based on the two separate GAD65 and IA2 proteins.

Conclusions: Autoantigens such as GAD65 and IA2 can be combined successfully in a fusion protein of similar immune reactivity. This allows simultaneous detection of GAD65- and IA2-abs in a one-step screening assay and cost-effective identification of positive individuals at risk of diabetes or at onset of disease.

Identification of a novel autoantigen UACA in patients with panuveitis

To identify the target autoantigens in Vogt-Koyanagi-Harada disease, we made use of an immunoscreening of a bovine uveal cDNA expression library with serum samples obtained from patients with Vogt-Koyanagi-Harada disease. We identified a novel bovine antigen and homologous human autoantigen and designated it as UACA (uveal autoantigen with coiled coil domains and ankyrin repeats). mRNA of human UACA is expressed most abundantly in skeletal muscles and in various human tissues, including choroid, retina, and epidermal melanocytes. IgG autoantibodies were quantitated in an ELISA, using recombinant C-terminal 18.0% fragment of human UACA. The prevalence of IgG anti-UACA autoantibodies in patients with panuveitis (Vogt-Koyanagi-Harada disease, Behçet's disease, sarcoidosis) was significantly higher than that in healthy controls (19.6-28.1% vs 0%, P < 0.05) indicating that autoimmunity directed against UACA is a common phenomenon in these diseases.

Human monoclonal antibodies isolated from type I diabetes patients define multiple epitopes in the protein tyrosine phosphatase-like IA-2 antigen

Protein tyrosine phosphatase-like IA-2 autoantigen is one of the major targets of humoral autoimmunity in patients with insulin-dependant diabetes mellitus (IDDM). In an effort to define the epitopes recognized by autoantibodies against IA-2, we generated five human mAbs (hAbs) from peripheral B lymphocytes isolated from patients most of whom had been recently diagnosed for IDDM. Determination and fine mapping of the critical regions for autoantibody binding was performed by RIA using mutant and chimeric constructs of IA-2- and IA-2beta-regions. Four of the five IgG autoantibodies recognized distinct epitopes within the protein tyrosine phosphatase (PTP)-like domain of IA-2. The minimal region required for binding by three of the PTP-like domain-specific hAbs could be located to aa 777-979. Two of these hAbs cross-reacted with the related IA-2beta PTP-like domain (IA-2beta aa 741-1033). A further PTP-like domain specific hAb required the entire PTP-like domain (aa 687-979) for binding, but critical amino acids clustered in the N-terminal region 687-777. An additional epitope could be localized within the juxtamembrane domain (aa 603-779). In competition experiments, the epitope recognized by one of the hAbs was shown to be targeted by 10 of 14 anti-IA-2-positive sera. Nucleotide sequence analysis of this hAb revealed that it used a V(H) germline gene (DP-71) preferably expressed in autoantibodies associated with IDDM. The presence of somatic mutations in both heavy and light chain genes and the high affinity or this Ab suggest that the immune response to IA-2 is Ag driven.

Virus-induced diabetes in a transgenic model: role of cross-reacting viruses and quantitation of effector T cells needed to cause disease

Virus-specific cytotoxic T lymphocytes (CTL) at frequencies of >1/1, 000 are sufficient to cause insulin-dependent diabetes mellitus (IDDM) in transgenic mice whose pancreatic beta cells express as "self" antigen a protein from a virus later used to initiate infection. The inability to generate sufficient effector CTL for other cross-reacting viruses that fail to cause IDDM could be mapped to point mutations in the CTL epitope or its COO(-) flanking region. These data indicate that IDDM and likely other autoimmune diseases are caused by a quantifiable number of T cells, that neither standard epidemiologic markers nor molecular analysis with nucleic acid probes reliably distinguishes between viruses that do or do not cause diabetes, and that a single-amino-acid change flanking a CTL epitope can interfere with antigen presentation and development of autoimmune disease in vivo.

Systemic levels of cytokines and GAD-specific autoantibodies isotypes in Chinese IDDM patients

It is not clear if a Th1/Th2 imbalance in Type 1 diabetes (insulin-dependent diabetes mellitus, IDDM) would lead to a particular antigen-specific IgG subclass dominant as had been shown in the mouse model. In new-onset Type 1 diabetics, an autoantibody response to glutamate decarboxylase (GADab) is frequently observed but the GADab subclass repertoire is not well-established. We determined the systemic levels of representative Th1 and Th2 cytokines and the GADab IgG subclass distribution in 41 Chinese IDDM patients of whom 26 were recently diagnosed (< or = 1 year) and 32 had GADab, to ascertain a likely association of antigen-specific antibody isotype and the Th1/Th2 dichotomy. With high-sensitivity ELISA systems that measure sub-picogram cytokine concentrations, 26 of the 41 patients (63.4%) had at least one of the pro-inflammatory Th1 cytokines (TNF-alpha, IFN-gamma and IL-12) detected. Fewer patients (4/41) had the anti-inflammatory Th2 cytokine IL-4 detected. For IL-10, all subjects had measurable quantities but only three diabetics had levels above the upper limit for healthy subjects (n = 20). Grouped according to the profile of detectable cytokines, there were 24 Th1, 2 Th2 and 2 Th0 patterns. GAD-specific IgG1 antibody was more frequently expressed; 22 of 32 GADab[+] patients. The rank order for the GADab subclasses was IgG1 > 4 > 3 > 2; IgG2 was found in 11 GADab[+] patients. Recent-onset diabetics have a similar ranking of the GAD-specific IgG subclasses. In human Type 1 diabetes, a predominance of GAD-specific IgG1 antibody response is observed together with a dominant Th1 cytokine pattern.

Inhibition of gamma-aminobutyric acid synthesis by glutamic acid decarboxylase autoantibodies in stiff-man syndrome

Stiff-man syndrome (SMS) is a rare disorder of the central nervous system thought to result from an impairment of gamma-aminobutyric acid (GABA)ergic neurotransmission. Autoantibodies to the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD), present in about 60% of SMS patients, have suggested an autoimmune pathogenesis of SMS. By using serum or cerebrospinal fluid from 25 SMS patients, we assessed the effect of GAD autoantibodies (GAD-A) on GAD enzymatic activity in vitro; 83% of GAD-A-positive SMS sera reduced GABA production in crude rat cerebellar extracts, whereas GAD-A- sera from SMS patients or healthy blood donors did not alter the enzyme activity. Inhibition of GABA synthesis by SMS sera was dose dependent and mediated by the purified IgG fraction of the sera. Human monoclonal GAD65-A and IgG purified from serum of GAD-A-positive patients with insulin-dependent diabetes or autoimmune polyendocrine syndrome did not affect GAD activity, suggesting that a specific epitope recognition of GAD-A mediates inhibition of GAD. The disease-specific detection of GAD-inhibitory antibodies is compatible with their functional involvement in the etiopathology of SMS; the relevance of such antibodies in vivo, however, remains to be determined.

Identification and characterization of glima 38, a glycosylated islet cell membrane antigen, which together with GAD65 and IA2 marks the early phases of autoimmune response in type 1 diabetes

Immunoprecipitating IgG autoantibodies to glutamic acid decarboxylase, GAD65, and/or a tyrosine phosphatase, IA2, are present in the majority of individuals experiencing pancreatic beta cell destruction and development of type 1 diabetes. Here we identify a third islet cell autoantigen, a novel 38-kD protein, which is specifically immunoprecipitated with sera from a subset of prediabetic individuals and newly diagnosed type 1 diabetic patients. The 38-kD autoantigen, named glima 38, is an amphiphilic membrane glycoprotein, specifically expressed in islet and neuronal cell lines, and thus shares the neuroendocrine expression patterns of GAD65 and IA2. Removal of N-linked carbohydrates results in a protein of 22,000 Mr. Glima 38 autoantibodies were detected in 16/86 (19%) of newly diagnosed patients, including three very young children, who had a rapid onset of disease, and in 6/44 (14%) of prediabetic individuals up to several years before clinical onset. The cumulative incidence of GAD65 and glima 38 antibodies in these two groups was 83 and 80%, respectively, and the cumulative incidence of GAD65, glima 38, and IA2 antibodies in the same groups was 91 and 84%, respectively. GAD65, IA2, and glima 38 represent three distinct targets of immunoprecipitating IgG autoantibodies associated with beta cell destruction and type 1 diabetes.