Comparative analysis of epitope recognition of glutamic acid decarboxylase (GAD) by autoantibodies from different autoimmune disorders

Consensus Paper: Latent Autoimmune Cerebellar Ataxia (LACA)

Immune-mediated cerebellar ataxias (IMCAs) have diverse etiologies. Patients with IMCAs develop cerebellar symptoms, characterized mainly by gait ataxia, showing an acute or subacute clinical course. We present a novel concept of latent autoimmune cerebellar ataxia (LACA), analogous to latent autoimmune diabetes in adults (LADA). LADA is a slowly progressive form of autoimmune diabetes where patients are often initially diagnosed with type 2 diabetes. The sole biomarker (serum anti-GAD antibody) is not always present or can fluctuate. However, the disease progresses to pancreatic beta-cell failure and insulin dependency within about 5 years. Due to the unclear autoimmune profile, clinicians often struggle to reach an early diagnosis during the period when insulin production is not severely compromised. LACA is also characterized by a slowly progressive course, lack of obvious autoimmune background, and difficulties in reaching a diagnosis in the absence of clear markers for IMCAs. The authors discuss two aspects of LACA: (1) the not manifestly evident autoimmunity and (2) the prodromal stage of IMCA's characterized by a period of partial neuronal dysfunction where non-specific symptoms may occur. In order to achieve an early intervention and prevent cell death in the cerebellum, identification of the time-window before irreversible neuronal loss is critical. LACA occurs during this time-window when possible preservation of neural plasticity exists. Efforts should be devoted to the early identification of biological, neurophysiological, neuropsychological, morphological (brain morphometry), and multimodal biomarkers allowing early diagnosis and therapeutic intervention and to avoid irreversible neuronal loss.

Reduced display of conformational epitopes in the N-terminal truncated GAD65 isoform: relevance for people with stiff person syndrome or DQ8/8-positive Type 1 diabetes mellitus

AIMS

To investigate whether the N-terminal truncated glutamic acid decarboxylase 65 (GAD65) isoform is as well recognized by people with stiff person syndrome as it is by people with Type 1 diabetes, and whether conformational GAD65 antibody epitopes are displayed properly by the isoform.

METHODS

GAD65 antibody-positive healthy individuals (n=13), people with stiff-person syndrome (n=15) and children with new-onset Type 1 diabetes (n=654) were analysed to determine binding to full-length GAD65 and the N-terminal truncated GAD65 isoform in each of these settings. GAD65 autoantibody epitope specificity was correlated with binding ratios of full-length GAD65/N-terminal truncated GAD65.

RESULTS

The N-terminal truncated GAD65 isoform was significantly less recognized in GAD65Ab-positive people with stiff-person syndrome (P=0.002) and in healthy individuals (P=0.0001) than in people with Type 1 diabetes. Moreover, at least two specific conformational GAD65Ab epitopes were not, or were only partially, presented by the N-terminal truncated GAD65 isoform compared to full-length GAD65. Finally, an N-terminal conformational GAD65Ab epitope was significantly less recognized in DQ8/8 positive individuals with Type 1 diabetes (P=0.02).

CONCLUSIONS

In people with stiff person syndrome preferred binding to the full-length GAD65 isoform over the N-terminal truncated molecule was observed. This binding characteristic is probably attributable to reduced presentation of two conformational epitopes by the N-terminal truncated molecule. These findings support the notion of disease-specific GAD65Ab epitope specificities and emphasize the need to evaluate the applicability of novel assays for different medical conditions.

Fine Mapping of Glutamate Decarboxylase 65 Epitopes Reveals Dependency on Hydrophobic Amino Acids for Specific Interactions

Characterization of multiple antibody epitopes has revealed the necessity of specific groups of amino acid residues for reactivity. This applies to the majority of antibody-antigen interactions, where especially charged and hydrophilic amino acids have been reported to be essential for antibody reactivity. This study describes thorough characterization of glutamic acid decarboxylase (GAD) 65 antigenic epitopes, an immunodominant autoantigen in type 1 diabetes (T1D). As linear epitopes are sparsely described for GAD65 in T1D, we aimed to identify and thoroughly characterize two GAD65 antibodies using immunoassays. A monoclonal antibody recognized an epitope in the N-terminal domain of GAD65, ⁸FWSFGSE¹⁴, whereas a polyclonal antibody recognized two continuous epitopes in the C-terminal domain, corresponding to amino acids ⁵¹⁴RTLED⁵¹⁸ and ⁵⁴⁹PLGDKVNF⁵⁵⁶. Hydrophobic amino acids were essential for antibody reactivity, which was verified by competitive inhibition assays. Moreover, the epitopes were located in flexible linker regions and turn structures. These findings confirm the versatile nature of antibody-antigen interactions and describe potential continuous epitopes related to T1D, which predominantly have been proposed to be of discontinuous nature.

Epilepsy and behavioral changes, type 1 diabetes mellitus and a high titer of glutamic acid decarboxylase antibodies

Autoantibodies to the 65 kDa isoform of glutamate acid decarboxylase (GAD65Ab) are associated with a range of clinical disorders, including type 1 diabetes (T1D) and stiff-person syndrome (SPS). In this article we describe a young girl who was diagnosed with T1D at the end of her first year of life and developed drug-resistant epilepsy 18 months later, followed by behavioral disturbances. She was admitted to our center at the age of 5 yr, at which time high GAD65Ab titers were detected in the patient's serum and cerebrospinal fluid (CSF). The titer remained elevated during 19 months of follow-up. Furthermore, GAD65Ab in both serum and CSF showed epitope binding characteristics similar to those observed for GAD65Ab in SPS patients, and GAD65Ab in the serum reduced GAD65 enzyme activity. Our results suggest an association between high GAD65Ab titers and epilepsy in children with T1D. Careful titration and characterization of GAD65Ab regarding inhibition of enzyme activity and epitope specificity may be helpful in identifying T1D patients at risk for neurological complications.

Optimized purification strategies for the elimination of non-specific products in the isolation of GAD65-specific monoclonal autoantibodies

Autoantibodies against antigens expressed by insulin-producing β cells are circulating in both healthy individuals and patients at risk of developing Type 1 diabetes. Recent studies suggest that another set of antibodies (anti-idiotypic antibodies) exists in this antibody/antigen interacting network to regulate auto-reactive responses. Anti-idiotypic antibodies may block the antigen-binding site of autoantibodies or inhibit autoantibody expression and secretion. The equilibrium between autoantibodies and anti-idiotypic antibodies plays a critical role in mediating or preventing autoimmunity. In order to investigate the molecular mechanisms underlying such a network in autoimmunity and potentially develop neutralizing reagents to prevent or treat Type 1 diabetes, we need to produce autoantibodies and autoantigens with high quality and purity. Herein, using GAD65/anti-GAD65 autoantibodies as a model system, we aimed to establish reliable approaches for the preparation of highly pure autoantibodies suitable for downstream investigation.

GAD1 is a biomarker for benign and malignant prostatic tissue

OBJECTIVE

Tissue-specific markers are useful for identification of tumour type in advanced cancers of unknown origin. This study investigated the expression of glutamate decarboxylase 1 (GAD1) in prostate and control tissue compared with the established prostate-specific markers prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA).

MATERIAL AND METHODS

A tissue microarray was constructed of 36 prostate adenocarcinomas, eight benign prostate samples and benign and malignant control tissues from urinary bladder, lung and rectum. Immunohistochemistry for GAD1, PSA and PSMA was performed. The products of staining intensity and extent were analysed. The GAD1 antibody was validated by Western blot. Real-time polymerase chain reaction (RT-PCR) was performed on malignant and benign samples from each tissue type.

RESULTS

GAD1 and PSA immunostains were significantly stronger in malignant and benign prostatic tissue than in controls. PSMA was stronger in prostate cancer than in urothelial and rectal cancer but had a lower specificity than GAD1 and PSA. GAD1 expression decreased with increasing Gleason score. RT-PCR confirmed the presence of mRNA for GAD1, PSA and PSMA in prostate samples.

CONCLUSION

GAD1 is expressed in benign and malignant prostatic tissue and may serve as a highly prostate-specific tissue biomarker.

COOH-terminal clustering of autoantibody and T-cell determinants on the structure of GAD65 provide insights into the molecular basis of autoreactivity

OBJECTIVE

To gain structural insights into the autoantigenic properties of GAD65 in type 1 diabetes, we analyzed experimental epitope mapping data in the context of the recently determined crystal structures of GAD65 and GAD67, to allow "molecular positioning" of epitope sites for B- and T-cell reactivity.

RESEARCH DESIGN AND METHODS

Data were assembled from analysis of reported effects of mutagenesis of GAD65 on its reactivity with a panel of 11 human monoclonal antibodies (mAbs), supplemented by use of recombinant Fab to cross-inhibit reactivity with GAD65 by radioimmunoprecipitation of the same mAbs.

RESULTS

The COOH-terminal region on GAD65 was the major autoantigenic site. B-cell epitopes were distributed within two separate clusters around different faces of the COOH-terminal domain. Inclusion of epitope sites in the pyridoxal phosphate-and NH(2)-terminal domains was attributed to the juxtaposition of all three domains in the crystal structure. Epitope preferences of different mAbs to GAD65 aligned with different clinical expressions of type 1 diabetes. Epitopes for four of five known reactive T-cell sequences restricted by HLA DRB1*0401 were aligned to solvent-exposed regions of the GAD65 structure and colocalized within the two B-cell epitope clusters. The continuous COOH-terminal epitope region of GAD65 was structurally highly flexible and therefore differed markedly from the equivalent region of GAD67.

CONCLUSIONS

Structural features could explain the differing antigenicity, and perhaps immunogenicity, of GAD65 versus GAD67. The proximity of B- and T-cell epitopes within the GAD65 structure suggests that antigen-antibody complexes may influence antigen processing by accessory cells and thereby T-cell reactivity.

Autoantibodies to tumor-associated antigens: reporters from the immune system

Although autoantibodies have been recognized as participants in pathogenesis of tissue injury, the collateral role of autoantibodies as reporters from the immune system identifying cellular participants in tumorigenesis has not been fully appreciated. The immune system appears to be capable of sensing aberrant structure, distribution, and function of certain cellular components involved in tumorigenesis and making autoantibody responses to the tumor-associated antigens (TAAs). Autoantibodies to TAAs can report malignant transformation before standard clinical studies and may be useful as early detection biomarkers. The autoantibody response also provides insights into factors related to how cellular components may be rendered immunogenic. As diagnostic biomarkers, specific TAA miniarrays for identifying autoantibody profiles could have sufficient sensitivity in differentiating between types of tumors. Such anti-TAA profiles could also be used to monitor response to therapy. The immune system of cancer patients reveals the immune interactive sites or the autoepitopes of participants in tumorigenesis, and this information should be used in the design of immunotherapy.

Characterisation of an autoreactive conformational epitope on GAD65 recognised by the human monoclonal antibody b78 using a combination of phage display, in vitro mutagenesis and molecular modelling

Autoantibodies to the diabetes autoantigen, the 65kDa isoform of glutamic acid decarboxylase (GAD65), react with conformational epitopes defined according to linear sequences but not according to structural information, or contact sites with the antibody paratope. To ascertain such information for an exemplary human monoclonal antibody (mAb) to GAD65, b78, we combined antibody screening of phage-displayed peptide libraries, alanine mutagenesis of selected motifs, homology modelling of the PLP and C-terminal regions of GAD65, and molecular dynamics to examine for structural effects of mutagenesis. By phage display, mAb b78 selected phagotopes containing acidic residues (D, E), hydrophobic residues (Y, F or W) and LRS that localised to a possible surface-exposed conformational epitope on the combined homology model. Alanine mutants of GAD65 based on deduced contact residues were examined for binding with b78 and control sera. Mutation of (524)SRL(526), (572)DF(573) and (498)KPQ(500) reduced reactivity of b78 with mutant GAD65 > 50%. Molecular dynamics indicated that mutation of (498)KPQ(500) caused structural changes that could account for effects of this mutation. Thus phage display in combination with molecular modelling identified contact residues within a highly conformational epitope for mAb b78 in the C-terminus of GAD65. These techniques should have broad applicability to definition of epitope structure.

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.

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).

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.

Endocrine autoantibodies

The autoantibody assays that exist and that are being refined are of increasing importance to a broad spectrum of endocrine disorders. This is particularly true for type IA diabetes, which is one of the best-studied organ-specific autoimmune diseases. Autoantibodies are used as valuable markers in prediction and prevention studies of type IA diabetes. Autoantibodies related to other endocrine organs are also important because multiple related autoimmune endocrine and non-endocrine disorders are increased in frequency in patients and their families with autoimmunity. The availability of highly sensitive and specific autoantibody assays for the various endocrine disorders can allow physicians to better diagnose and promptly treat these conditions.

Modulation of antigen presentation by autoreactive B cell clones specific for GAD65 from a type I diabetic patient

We used a GAD65-specific human B-T cell line cognate system in vitro to investigate the modulation of GAD65 presentation by autoantibody, assessed in a proliferation assay. Generally, if the T cell determinant overlaps or resides within the antibody epitope, effects of presentation are blunted while if they are distant can lead to potent presentation. For three different autoreactive B-T cell line cognate pairs, the modulation of GAD65 presentation followed the mode of overlapping or distant epitopes with resultant potent or undetectable presentation. However, other cognate pairs elicited variability in this pattern of presentation. Notably, one B cell line, DPC, whose antibody epitope did not overlap with the T cell determinants, was consistently poor in presenting GAD65. Using the fluorescent dye Alexa Fluor 647 conjugated to GAD65 to study receptor-mediated antigen endocytosis showed that all the antigen-specific B cell clones were efficient in intracellular accumulation of the antigen. Additionally, multicolour immunofluorescence microscopy showed that the internalized GAD65/surface IgG complexes were rapidly targeted to a perinuclear compartment in all GAD-specific B cell clones. This analysis also demonstrated that HLA-DM expression was reduced strongly in DPC compared to the stimulatory B cell clones. Thus the capability of antigen-specific B cells to capture and present antigen to human T cell lines is dependent on the spatial relationship of B and T cell epitopes as well other factors which contribute to the efficiency of presentation.

Phage display for epitope determination: a paradigm for identifying receptor-ligand interactions

Antibodies that react with many different molecular species of protein and non-protein nature are widely studied in biology and have particular utilities, but the precise epitopes recognized are seldom well defined. The definition of epitopes by X-ray crystallography of the antigen-antibody complex, the gold standard procedure, has shown that most antibody epitopes are conformational and specified by interactions with topographic determinants on the surface of the antigenic molecule. Techniques available for the definition of such epitopes are limited. Phage display using either gene-specific libraries, or random peptide libraries, provides a powerful technique for an approach to epitope identification. The technique can identify amino acids on protein antigens that are critical for antibody binding and, further, the isolation of peptide motifs that are both structural and functional mimotopes of both protein and non-protein antigens. This review discusses techniques used to isolate such mimotopes, to confirm their specificity, and to characterize peptide epitopes. Moreover there are direct practical applications to deriving epitopes or mimotopes by sequence, notably the development of new diagnostic reagents, or therapeutic agonist or antagonist molecules. The techniques developed for mapping of antibody epitopes are applicable to probing the origins of autoimmune diseases and certain cancers by identifying "immunofootprints" of unknown initiating agents, as we discuss herein, and are directly applicable to examination of a wider range of receptor-ligand interactions.

Engineered variants of human glutamic acid decarboxylase (GAD) and autoantibody epitope recognition

Of the two homologous forms of glutamic acid decarboxylase, GAD65 and GAD67, only GAD65 is a common target of autoimmunity. Epitope profiles of autoantibodies to GAD65 (GADA) in 140 type 1 diabetes, adult-onset diabetes mellitus (AODM), and thyroid diseases (TD) were studied. Probes were GAD65, GAD65/67 hybrids (displaying separately GAD65 residues 1-95, 96-444, and 445-585), delta GAD65 (a truncated GAD65 spanning residues 69-585), and GAD67. delta GAD65 and GAD65 detected 137 and 125 positive patients, respectively. The hybrids reacted with 113 sera and in 3 cases disclosed cryptic epitopes. Eighteen patients reacted with GAD67, indicating GAD65-GAD67 cross-reactivity. Most patients recognized both middle and C-terminal epitopes, had low reactivity against N-terminal epitopes, and seldom displayed reactivity limited to the N or C terminus. Compared with type 1 and AODM, TD patients showed a greater prevalence of multiple reactivity and higher incidence of GAD67 positivity.

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.

Anti-U1A monoclonal antibodies recognize unique epitope targets of U1A which are involved in the binding of U1 RNA

The U1A (or nRNP A) protein is known to play a critical role in eukaryotic pre-mRNA splicing and polyadenylation. Previous studies revealed that several mouse monoclonal antibodies (MAbs) recognized U1A as part of the U1snRNP, while MAb 12E12 was unique in that it recognized an epitope that is masked when U1A is bound to U1 RNA. In order to further characterize and understand the antigenic targets of these MAbs, we undertook fine specificity epitope mapping studies. Anti-U1A MAbs 12E12 and 10E3 each recognize unique peptides from the U1A protein. Interestingly, these MAbs recognize epitopes which have been shown to be antigenic in human autoimmune diseases. When superimposed on structures of U1A derived from crystal and NMR data, the major epitope recognized by 12E12 (amino acids 103-108) localizes to the surface of the U1A molecule. The 12E12 epitope is immediately adjacent to a helix which probably reacts to U1 RNA binding by undergoing a conformational change. This modification of structure effectively masks the 12E12 epitope, thus preventing binding of the monoclonal to U1A/U1 RNA complexes. These findings suggest that the structure of the U1A protein may be different when not part of the U1snRNP.

Conformational epitopes on the diabetes autoantigen GAD65 identified by peptide phage display and molecular modeling

The major diabetes autoantigen, glutamic acid decarboxylase (GAD65), contains a region of sequence similarity, including six identical residues PEVKEK, to the P2C protein of coxsackie B virus, suggesting that cross-reactivity between coxsackie B virus and GAD65 can initiate autoimmune diabetes. We used the human islet cell mAbs MICA3 and MICA4 to identify the Ab epitopes of GAD65 by screening phage-displayed random peptide libraries. The identified peptide sequences could be mapped to a homology model of the pyridoxal phosphate (PLP) binding domain of GAD65. For MICA3, a surface loop containing the sequence PEVKEK and two adjacent exposed helixes were identified in the PLP binding domain as well as a region of the C terminus of GAD65 that has previously been identified as critical for MICA3 binding. To confirm that the loop containing the PEVKEK sequence contributes to the MICA3 epitope, this loop was deleted by mutagenesis. This reduced binding of MICA3 by 70%. Peptide sequences selected using MICA4 were rich in basic or hydroxyl-containing amino acids, and the surface of the GAD65 PLP-binding domain surrounding Lys358, which is known to be critical for MICA4 binding, was likewise rich in these amino acids. Also, the two phage most reactive with MICA4 encoded the motif VALxG, and the reverse of this sequence, LAV, was located in this same region. Thus, we have defined the MICA3 and MICA4 epitopes on GAD65 using the combination of phage display, molecular modeling, and mutagenesis and have provided compelling evidence for the involvement of the PEVKEK loop in the MICA3 epitope.