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

Affinity purification of serum-derived anti-IA-2 autoantibodies in type 1 diabetes using a novel MBP-IA-2 fusion protein

Autoantibodies targeting epitopes contained within the intracellular domain (IC) of the protein phosphatase-like islet antigen 2 (IA-2) are a common marker of autoimmune type 1 diabetes (T1D), however the isolation of genuine, serum derived anti-IA-2 autoantibodies has proven challenging due to a lack of suitable bioassays. In the current study, an ELISA format was developed for affinity purification of human anti-IA-2ic autoantibodies utilizing a fusion protein (FP) incorporating maltose binding protein and the full-length IA-2IC domain. Using a T1D patient cohort validated for anti-IA-2ic autoantibodies by commercial ELISA, we demonstrate the MBP-IA-2ic FP ELISA detects serum anti-IA-2IC autoantibodies from 3 of 9 IA-2 positive patients. Further to this, a multi-plate MBP-IA-2ic FP ELISA protocol specifically affinity purifies IgG enriched for anti-IA-2ic autoantibodies. Interestingly, serum derived autoantibodies immobilised on the MBP-IA-2ic FP ELISA demonstrate increased Kappa light chain usage when compared to the respective total IgG derived from donor patients, suggesting a clonally restricted repertoire of anti-IA-2ic autoantigen specific B plasma cells is responsible for autoantibodies detect by the MBP-IA-2ic FP ELISA. This study is the first to demonstrate the generation of specific, genuine human derived anti-IA-2ic autoantibodies, thereby facilitating further investigation into the origin and functional significance of IA-2 autoantibodies in T1D.

Insights From Single Cell RNA Sequencing Into the Immunology of Type 1 Diabetes- Cell Phenotypes and Antigen Specificity

In the past few years, huge advances have been made in techniques to analyse cells at an individual level using RNA sequencing, and many of these have precipitated exciting discoveries in the immunology of type 1 diabetes (T1D). This review will cover the first papers to use scRNAseq to characterise human lymphocyte phenotypes in T1D in the peripheral blood, pancreatic lymph nodes and islets. These have revealed specific genes such as IL-32 that are differentially expressed in islet -specific T cells in T1D. scRNAseq has also revealed wider gene expression patterns that are involved in T1D and can predict its development even predating autoantibody production. Single cell sequencing of TCRs has revealed V genes and CDR3 motifs that are commonly used to target islet autoantigens, although truly public TCRs remain elusive. Little is known about BCR repertoires in T1D, but scRNAseq approaches have revealed that insulin binding BCRs commonly use specific J genes, share motifs between donors and frequently demonstrate poly-reactivity. This review will also summarise new developments in scRNAseq technology, the insights they have given into other diseases and how they could be leveraged to advance research in the type 1 diabetes field to identify novel biomarkers and targets for immunotherapy.

Investigating the potential impact of post translational modification of auto-antigens by tissue transglutaminase on humoral islet autoimmunity in type 1 diabetes

Background

Post-translational modification (PTM) of antigens plays a role in the pathogenesis of many autoimmune disorders. In coeliac disease (CD), tissue transglutaminase (tTG) deamidates gliadin peptides to activate the immune response against the gut endomysium. CD is six times more prevalent in type 1 diabetes (T1D) patients than in the general population.

Hypothesis

tTG also modifies auto-antigens implicated in the pathogenesis of T1D, leading to an autoimmune response to pancreatic β-cells.

Methods

tTG PTM was investigated in the following auto-antigens, which had been previously shown to have high importance in the development of T1D: glutamic acid decarboxylase isoform 65 (GAD65), full length islet antigen (IA-2), intracellular portion of IA-2 (IA-2ic), and both isoforms of zinc transporter 8 (ZnT8W and ZnT8R), on antibody binding. Radiolabelled antigen was incubated with tTG for 20 h at 37 °C in 100 mM Denver buffer, 3.33 nM CaCl₂, at pH 7.3. Antibody binding in 20 mixed samples from the Bart’s-Oxford (BOX) cohort was measured by radiobinding assay.

Results

Results varied between serum samples. Generally, tTG treatment of ZnT8W, ZnT8R and IA-2ic showed no significant change in antigen: autoantibody binding, while increases in binding were observed with tTG-treated GAD65 and full length IA-2.

Conclusion

In the case of GAD65, full length IA-2, the strength of antibody: antigen binding increased after incubation with tTG. However, the exact tTG-modification events that occurred requires further elucidation.

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Post-translational modification of antigens plays a role in the pathogenesis of many autoimmune disorders.

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Celiac disease is six times more prevalent in type 1 diabetics.

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Tissue transglutaminase modifies auto-antigens GAD65 and full-length IA-2 to increase antibody binding.

Relationships between major epitopes of the IA-2 autoantigen in Type 1 diabetes: Implications for determinant spreading

Diversification of autoimmunity to islet autoantigens is critical for progression to Type 1 diabetes. B-cells participate in diversification by modifying antigen processing, thereby influencing which peptides are presented to T-cells. In Type 1 diabetes, JM antibodies are associated with T-cell responses to PTP domain peptides. We investigated whether this is the consequence of close structural alignment of JM and PTP domain determinants on IA-2. Fab fragments of IA-2 antibodies with epitopes mapped to the JM domain blocked IA-2 binding of antibodies that recognise epitopes in the IA-2 PTP domain. Peptides from both the JM and PTP domains were protected from degradation during proteolysis of JM antibody:IA-2 complexes and included those representing major T-cell determinants in Type 1 diabetes. The results demonstrate close structural relationships between JM and PTP domain epitopes on IA-2. Stabilisation of PTP domain peptides during proteolysis in JM-specific B-cells may explain determinant spreading in IA-2 autoimmunity.

Failure to detect anti-idiotypic antibodies in the autoimmune response to IA-2 in Type 1 diabetes

The concept that immune responses to self antigens are regulated by anti-idiotypic networks has attracted renewed interest following reports of circulating factors within IgG fractions of serum that impair detection of autoantibodies with autoantigen. Thus, preclearance of sera with bead-immobilised monoclonal autoantibodies to the Type 1 diabetes autoantigen GAD65, or prebinding of serum antibodies to protein A Sepharose prior to addition of antigen, increases immunoreactivity detected in serum samples consistent with the trapping on the beads of anti-idiotypic antibodies that block antibody binding to the autoantigen. The aim of this study was to investigate the presence of anti-idiotypic antibodies to another major target of autoantibodies in Type 1 diabetes, IA-2. As previously observed for GAD65, preadsorption of serum samples with immobilised monoclonal IA-2 autoantibody, or prebinding to protein A Sepharose, resulted in substantial increases in subsequent immunoprecipitation of radiolabeled IA-2 in a proportion of samples. However, control experiments indicated that the increases seen on pre-incubation with immobilized autoantibodies were caused by displacement of the antibody by serum IgG, whereas impaired detection of immunoreactivity in liquid-phase radiobinding assays was the result of formation of insoluble complexes that bind poorly to protein A. The results emphasise the importance of direct demonstration of specific binding of antibodies to the idiotype in the study of idiotypic networks in autoimmunity. Variability between patients in formation of insoluble immune complexes has implications for the design and standardization of autoantibody assays for diabetes prediction.

Yersinia enterocolitica provides the link between thyroid-stimulating antibodies and their germline counterparts in Graves' disease

Graves' disease results from thyroid-stimulating Abs (TSAbs) activating the thyrotropin receptor (TSHR). How TSAbs arise from early precursor B cells has not been established. Genetic and environmental factors may contribute to pathogenesis, including the bacterium Yersinia enterocolitica. We developed two pathogenic monoclonal TSAbs from a single experimental mouse undergoing Graves' disease, which shared the same H and L chain germline gene rearrangements and then diversified by numerous somatic hypermutations. To address the Ag specificity of the shared germline precursor of the monoclonal TSAbs, we prepared rFab germline, which showed negligible binding to TSHR, indicating importance of somatic hypermutation in acquiring TSAb activity. Using rFab chimeras, we demonstrate the dominant role of the H chain V region in TSHR recognition. The role of microbial Ags was tested with Y. enterocolitica proteins. The monoclonal TSAbs recognize 37-kDa envelope proteins, also recognized by rFab germline. MALDI-TOF identified the proteins as outer membrane porin (Omp) A and OmpC. Using recombinant OmpA, OmpC, and related OmpF, we demonstrate cross-reactivity of monoclonal TSAbs with the heterogeneous porins. Importantly, rFab germline binds recombinant OmpA, OmpC, and OmpF confirming reactivity with Y. enterocolitica. A human monoclonal TSAb, M22 with similar properties to murine TSAbs, also binds recombinant porins, showing cross-reactivity of a spontaneously arising pathogenic Ab with Y. enterocolitica. The data provide a mechanistic framework for molecular mimicry in Graves' disease, where early precursor B cells are expanded by Y. enterocolitica porins to undergo somatic hypermutation to acquire a cross-reactive pathogenic response to TSHR.

The core cysteines, (C909) of islet antigen-2 and (C945) of islet antigen-2β, are crucial to autoantibody binding in type 1 diabetes

Cysteines are thought integral to conformational epitopes of islet antigen-2 (IA-2) autoantibodies (IA-2A), possibly through disulfide bond formation. We therefore investigated which cysteines are critical to IA-2A binding in patients with newly diagnosed type 1 diabetes. All 10 cysteines in the intracellular domain of IA-2 were modified to serine by site-directed mutagenesis, and the effects of these changes on autoantibody binding in comparison with wild-type control were investigated by radiobinding assay. Mutation of the protein tyrosine phosphatase (PTP) core cysteine (C909) in IA-2 caused large reductions in autoantibody binding. In contrast, little or no reduction in binding was seen following substitution of the other cysteines. Modification of the core cysteine (C945) in IA-2β also greatly reduced autoantibody binding. Lysine substitution of glutamate-836 in IA-2 or glutamate-872 in IA-2β resulted in modest reductions in binding and identified a second epitope region. Binding to IA-2 PTP and IA-2β PTP was almost abolished by mutation of both the core cysteine and these glutamates. The core cysteine is key to the major PTP conformational epitope, but disulfide bonding contributes little to IA-2A epitope integrity. In most patients, at disease onset, >90% of antibodies binding to the PTP domain of IA-2 recognize just two epitope regions.

Autoantibodies and associated T-cell responses to determinants within the 831-860 region of the autoantigen IA-2 in Type 1 diabetes

B-cells influence T-cell reactivity by facilitating antigen presentation, but the role of autoantibody-secreting B-cells in regulating T-cell responses in Type 1 diabetes is poorly defined. The aims of this study were to characterise epitopes on the IA-2 autoantigen for three monoclonal antibodies from diabetic patients by amino acid substitutions of selected residues of IA-2, establish contributions of these epitopes to binding of serum antibodies in Type 1 diabetes and relate B- and T-cell responses to overlapping determinants on IA-2. The monoclonal antibodies recognised overlapping epitopes, with residues within the 831-860 region of IA-2 contributing to binding; substitution of Glu836 inhibited binding of all three antibodies. Monoclonal antibody Fab fragments and substitution of residues within the 831-836 region blocked serum antibody binding to an IA-2 643-937 construct. IL-10-secreting T-cells responding to peptides within the 831-860 region were detected by cytokine-specific ELISPOT in diabetic patients and responses to 841-860 peptide were associated with antibodies to the region of IA-2 recognised by the monoclonal antibodies. The study identifies a region of IA-2 frequently recognised by antibodies in Type 1 diabetes and demonstrates that these responses are associated with T-cells secreting IL-10 in response to a neighbouring determinant.

Identification of tyrosine phosphatase 2(256-760) construct as a new, sensitive marker for the detection of islet autoimmunity in type 2 diabetic patients: the non-insulin requiring autoimmune diabetes (NIRAD) study 2

OBJECTIVE

The presence of autoantibodies to islet antigens GAD and/or tyrosine phosphatase 2 (IA-2) in type 2 diabetic patients (latent autoimmune diabetes in adults [LADA]) identifies subjects at high risk to develop insulin dependency. The aim of this study was to dissect humoral anti-IA-2 immune response in Caucasian LADA patients, identifying the most sensitive construct to evaluate IA-2 immunoreactivity and comparing LADA IA-2 epitope specificities to those found in type 1 diabetes.

RESEARCH DESIGN AND METHODS

We analyzed 177 LADA and 978 type 2 diabetic patients with different disease duration, collected in a nationwide Italian survey, the Non-Insulin Requiring Autoimmune Diabetes (NIRAD) study aimed at assessing prevalence and characteristics of autoimmune diabetes in type 2 diabetic patients and 106 newly diagnosed type 1 diabetic patients (53 children, 53 adults). By radioimmunoassay, we analyzed humoral immunoreactivity to seven IA-2 constructs: IA-2(PTP (687-979)), IA-2((761-964)), IA-2((256-760)), IA-2(JM (601-630)), IA-2(IC (605-979)), IA-2(BDC (256-556:630-979)), and IA-2(FL (1-979)).

RESULTS

IA-2((256-760)) fragment was identified as the marker with the highest sensitivity for detection of humoral IA-2 immunoreactivity in LADA patients, identifying IA-2 autoantibodies in approximately 30% of GAD antibody (GADA)-positive LADA patients and in 3.4% of GADA-negative type 2 diabetic patients. LADA IA-2((256-760))A positivity was associated with an increased frequency of autoimmune diabetes HLA-susceptible genotypes and with a higher risk for developing thyroid autoimmunity compared with autoantibody-negative type 2 diabetic patients. At disease diagnosis, adult-onset type 1 diabetic and LADA patients showed a lower IA-2 COOH-terminal immunoreactivity compared with childhood-onset type 1 diabetic patients.

CONCLUSIONS

IA-2 immunoreactivity in LADA patients has thus far been underestimated, and IA-2((256-760)) autoantibody detection may represent a novel diagnostic tool for the identification of islet autoimmunity in these patients.

Epitope mapping of series of monoclonal antibodies against the hepatocellular carcinoma-associated antigen HAb18G/CD147

The hepatocellular carcinoma-associated antigen HAb18G/CD147, a member of CD147 family, could promote tumour invasion and metastasis via inducing the secretion of matrix metalloproteinases (MMP). Anti-CD147 monoclonal antibodies (MoAb) have exhibited obvious inhibitory effect on MMP induction. However, none of the epitopes of these MoAb has been reported. We previously prepared five MoAb against HAb18G/CD147, named HAb18, 3B3, 1B3, 5A5 and 4D2. To map the epitopes of these MoAb, a series of truncated fragments of extracellular region of HAb18G/CD147 was expressed in Escherichia coli and the MoAb-binding affinity to these fragments was examined with an enzyme-linked immunosorbent assay and Western blot. The residues (39)LTCSLNDSATEV(50), (36)KILLTCS(42) and (22)AAGTVFTTVEDL(33) were determined to be the epitopes of HAb18, 3B3 and 1B3, respectively, which were further proved by a dot-blot analysis with synthesized peptides and bioinformatics epitope prediction. The binding regions of MoAb 5A5 and 4D2 were located at residues E(120)-R(203). Then we constructed and expressed full-length HAb18G/CD147 and truncated HAb18G/CD147 without residues A(22)-V(50) in COS-7 cells. Gelatin zymography and Boyden chamber assay showed that the COS-7 cells expressing truncated HAb18G/CD147 failed to induce MMP production and enhance the cells' invasive potential, compared with the cells expressing full-length HAb18G/CD147. Taken together with the obviously inhibitory effects of HAb18 on the function of full-length HAb18G/CD147, these findings suggest that residues (22)AAGTVFTTVEDLGSKILLTCSLNDSATEV(50) may play a critical role in the functions of HAb18G/CD147 on MMP secretion and tumour invasion. These key residues can be used as potential drug target in cancer therapy.

Crystal structure of the major diabetes autoantigen insulinoma-associated protein 2 reveals distinctive immune epitopes

Insulinoma-associated protein-2 (IA-2) is a major autoantigen in type 1 diabetes that occurs through autoimmune-mediated beta-cell destruction. We present here the crystal structure of the protein tyrosine phosphatase (PTP)-like domain of human IA-2. The structure reveals a canonical PTP domain with the closed WPD loop over the active site pocket, explaining the lack of enzyme activity in the native protein. The structural interpretation of previous mutagenesis studies indicates that the B-cell epitopes are concentrated on two distinctive regions on peripheral loops of the central beta-sheet surrounding T-cell epitopes within the sheet. The detailed structural information on immune epitopes provides a framework for the future development of immune intervention strategies against diabetes.

Monoclonal antibody 76F distinguishes IA-2 from IA-2beta and overlaps an autoantibody epitope

IA-2 and IA-2beta are highly related proteins that are autoantigens in type 1 diabetes, and provide a model for developing reagents and assays that distinguish similar proteins with unique autoantibody epitopes. Monoclonal antibodies (mAb) to IA-2 and IA-2beta were prepared and tested for their ability to bind to the related proteins and their ability to compete for specific autoantibody epitope binding by sera from patients with type 1 diabetes. Monoclonal antibodies that specifically bound IA-2 (76F) or bound both IA-2 and IA-2beta (A9) were isolated and characterized. 76F mAb recognized IA-2 of human, rat and mouse origin in native and denatured forms and had an epitope specificity for residues 626-630 (FEYQD) which are found in the juxtamembrane (JM) region of human and mouse IA-2, but not IA-2beta. This region overlaps with the autoantibody epitope JM2. Binding to the 76F monoclonal antibody was specifically inhibited by sera with antibodies to the JM2 epitope but not with antibodies to the adjacent JM1 epitope, indicating that unique epitopes can be distinguished by this approach. 76F mAb has the unique property to distinguish between the two closely related autoantigens IA-2 and IA-2beta by targeting an IA-2 specific epitope of the juxtamembrane region. The findings define an approach to develop assays for specific antibody epitope measurements which may be relevant for disease prognosis and monitoring intervention therapies.

Amino acids critical for binding of autoantibody to an immunodominant conformational epitope of the pyruvate dehydrogenase complex subunit E2: Identification by phage display and site-directed mutagenesis

The E2 subunit of the mitochondrial multienzyme pyruvate dehydrogenase complex (PDC-E2) is the major autoantigen in the liver disease, primary biliary cirrhosis (PBC). An epitope region which has been localized to amino acids 91-227 is believed to include the residue K173 to which is attached the lipoyl cofactor. We investigated structural features of this epitope region by screening random peptide phage-displayed libraries and identified prevalent phagotopes that contained likely contact amino acids in separate regions of the linear sequence, H132M133, and F178, V180. These were confirmed by site-directed alanine mutagenesis singly or in combination of the HM and FV residues in wild-type (wt) PDC-E2, and by immunization of rabbits with phage that expressed peptides MHLNTPP or FVLPWRI. The lipoyl lysine K173 also was mutated. Reactivities of mutants and wild-type (wt) PDC-E2, compared by ELISA using 12 PBC sera, showed decremental reactivity of mutant versus wt PDC-E2 (normalized to 100%): wt PDC-E2 (100%)>>PDC-E2(F178A,V180A) (mean+/-S.D., 59+/-17%)>PDC-E2(M133A) (50+/-13%)>PDC-E2(H132A) (36+/-13%)>PDC-E2(H132A,M133A) (28+/-8%)>PDC-E2(H132A,M133A,F178V,M180A) (18+/-13%). Notably PDC-E2(K173A) retained full reactivity (93+/-21%). Rabbits immunized with phage peptides generated antibodies reactive with entire PDC-E2. Our data convincingly validate phage library technology for defining spatially disparate contact residues for conformational epitopes. Ensuing data could be generally applicable to search for occult extrinsic agents as initiators of autoimmunity.

Autoantibodies in diabetes

Islet cell autoantibodies are strongly associated with the development of type 1 diabetes. The appearance of autoantibodies to one or several of the autoantigens-GAD65, IA-2, or insulin-signals an autoimmune pathogenesis of beta-cell killing. A beta-cell attack may be best reflected by the emergence of autoantibodies dependent on the genotype risk factors, isotype, and subtype of the autoantibodies as well as their epitope specificity. It is speculated that progression to beta-cell loss and clinical onset of type 1 diabetes is reflected in a developing pattern of epitope-specific autoantibodies. Although the appearance of autoantibodies does not follow a distinct pattern, the presence of multiple autoantibodies has the highest positive predictive value for type 1 diabetes. In the absence of reliable T-cell tests, dissection of autoantibody responses in subjects of genetic risk should prove useful in identifying triggers of islet autoimmunity by examining seroconversion and maturation of the autoantibody response that may mark time to onset of type 1 diabetes. The complexity of the disease process is exemplified by multiple clinical phenotypes, including autoimmune diabetes masquerading as type 2 diabetes in youth and adults. Autoantibodies may also provide prognostic information in clinically heterogeneous patient populations when examined longitudinally.

Epitope analysis of insulin autoantibodies using recombinant Fab

Autoantibodies to insulin are often the first autoantibodies detected in young children with type 1 diabetes and can be present before the onset of clinical diabetes. These autoantibodies and their epitopes are, however, not well characterized. We explored the use of monoclonal antibodies and their recombinant Fab as reagents for epitope analysis. In this study we cloned and characterized the recombinant Fab of the insulin-specific monoclonal antibody CG7C7. We found the epitope of this antibody to be located predominantly at the A-chain loop of the insulin molecule. The recombinant Fab was then used to compete for insulin binding against insulin autoantibodies present in sera from patients with type 1 or type 1.5 diabetes. In competition experiments with sera positive for autoantibodies to insulin the recombinant Fab significantly reduced the binding to [125I]-insulin by sera of type 1 (n = 35) and type 1.5 diabetes [latent autoimmune diabetes in adults (LADA)] (n = 14) patients (P < 0.0001). We conclude that competition between insulin-specific monoclonal antibodies or their recombinant Fab and insulin autoantibodies should prove useful in the epitope analysis of autoantibodies to insulin.

Isolation and characterisation of a human monoclonal autoantibody to the islet cell autoantigen IA-2

A hybridoma secreting a human monoclonal autoantibody to the islet cell autoantigen IA-2 was prepared from peripheral lymphocytes of a patient with type 1 diabetes and Graves' disease using EBV infection followed by fusion with a mouse/human hybrid cell line. The monoclonal antibody (M13) is an IgG1/kappa and in an immunofluorescence test M13 at 1 microg/mL showed islet cell antibody reactivity equivalent to 40 JDF units. M13 IgG bound (35)S-labelled IA-2 (26% at 100 microg/mL) and (125)I-labelled IA-2 (34% at 100 microg/mL) in an immunoprecipitation assay and reacted well with IA-2 in western blotting analysis. Amino acids 777-808 in the PTP domain of IA-2 were found to be important for M13 binding in an analysis using modified (35)S-labelled IA-2 proteins. M13 V region genes were from VH1-3, D3-22, JH4b, VKI DPK8/Vd+ and JK3 genes and showed a high replacement/silent mutation ratio for both the heavy (11.0) and the light (6.0) chain genes. Mouse monoclonal antibodies (mMAbs) reactive with at least three different epitopes within IA-2 aa 604-686 corresponding to the juxtamembrane domain were also obtained. F(ab')(2) or Fab from the mMAbs inhibited serum IA-2 autoantibody binding to IA-2 in 20/22 diabetic sera whereas M13 F(ab')(2) caused inhibition in only 6/22 sera. M13 is representative of some patient serum IA-2 autoantibodies and as such provides a useful tool to study autoimmune responses to IA-2.

Absence of avidity maturation of autoantibodies to the protein tyrosine phosphatase-like IA-2 molecule and glutamic acid decarboxylase (GAD65) during progression to type 1 diabetes

Immunoglobulin G avidity assays are used to distinguish between the acute and chronic phase of several infectious diseases, and there is evidence of autoantibody affinity maturation also in autoimmune diseases. To assess whether the analysis of the avidity of autoantibodies against the protein tyrosine phosphatase-like IA-2 molecule and glutamic acid decarboxylase (GAD65) could improve the accuracy of risk assessment of progression to clinical type 1 diabetes, we established methods for the determination of the autoantibody avidity based on our previously developed time-resolved fluorometric IA-2 and GAD65 autoantibody (IA-2A and GADA) assays. The avidity indices of sequential plasma samples from six IA-2A-positive and seven GADA-positive prediabetic children were analysed applying elution with urea and diethylamine (DEA). For comparison, corresponding avidity indices of control children, who have remained non-diabetic for at least 3 years after seroconversion to IA-2A and GADA positivity, were analysed. For most of the children, only a slight fluctuation in the avidity index values was observed over time, although the titres for IA-2A and GADA varied substantially in some cases. The avidity indices of the prediabetic children remained within the same range as those of the control group throughout the follow-up. Our results indicate that the analysis of the avidity index levels of IA-2A and GADA does not improve the accuracy of the prediction of type 1 diabetes based on autoantibody detection.

Fine mapping of diabetes-associated IA-2 specific autoantibodies

The related tyrosine phosphatase-like proteins (PTP) IA-2 and IA-2beta are autoantigens of type 1 diabetes. Autoantibodies are predominantly against IA-2. We utilized the close homology between IA-2 and IA-2beta PTP domains to design chimeras and mutants in order to identify humoral IA-2-specific epitopes. Fifteen sera with antibodies to IA-2 specific PTP domain epitopes were tested against IA-2beta(741-848)/IA-2(795-889)/IA-2beta(943-1033), IA-2beta(741-848)/IA-2(795-845)/IA-2beta(900-1033), and IA-2beta(741-898)/IA-2(845-875)/IA-2beta(930-1033)chimeras. Two sera bound IA-2beta(741-848)/IA-2(795-889)/IA-2beta(943-1033)and IA-2beta(741-848)/IA-2(795-845)/IA-2beta(900-1033)only indicating that the IA-2 specific residues 859, 862, and/or 867 were critical for antibody binding. Mutation of glutamine 862 abolished binding in one of these sera. Seven sera bound only the IA-2beta(741-848)/IA-2(795-889)/IA-2beta(943-1033)chimera, indicating that binding required IA-2 specific amino acids within both 795-845 and 846-875, or that IA-2 residues 876-888 were important for binding. Mutation of glutamine 862 abolished binding in two of these sera, and mutation of residues 876, 877, 878, and 880 markedly reduced binding in two others. Six sera bound all three chimeras indicating that they contained multiple IA-2 specific PTP domain antibodies. In three of these sera, mutation of residues at positions 876, 877, 878, 880, and/or residues 862 and 822 reduced antibody binding by more than 50%. These findings indicate that glutamine at position 862, and residues 876-880 of the WPD loop of IA-2 are important for several of the IA-2 specific PTP domain epitopes.

Mapping of epitopes for autoantibodies to the type 1 diabetes autoantigen IA-2 by peptide phage display and molecular modeling: overlap of antibody and T cell determinants

IA-2 is a major target of autoimmunity in type 1 diabetes. IA-2 responsive T cells recognize determinants within regions represented by amino acids 787-817 and 841-869 of the molecule. Epitopes for IA-2 autoantibodies are largely conformational and not well defined. In this study, we used peptide phage display and homology modeling to characterize the epitope of a monoclonal IA-2 Ab (96/3) from a human type 1 diabetic patient. This Ab competes for IA-2 binding with Abs from the majority of patients with type 1 diabetes and therefore binds a region close to common autoantibody epitopes. Alignment of peptides obtained after screening phage-displayed peptide libraries with purified 96/3 identified a consensus binding sequence of Asn-x-Glu-x-x-(aromatic)-x-x-Gly. The predicted surface on a three-dimensional homology model of the tyrosine phosphatase domain of IA-2 was analyzed for clusters of Asn, Glu, and aromatic residues and amino acids contributing to the epitope investigated using site-directed mutagenesis. Mutation of each of amino acids Asn(858), Glu(836), and Trp(799) reduced 96/3 Ab binding by >45%. Mutations of these residues also inhibited binding of serum autoantibodies from IA-2 Ab-positive type 1 diabetic patients. This study identifies a region commonly recognized by autoantibodies in type 1 diabetes that overlaps with dominant T cell determinants.

Combining EL4-B5-based B-cell stimulation and phage display technology for the successful isolation of human anti-Scl-70 autoantibody fragments

Scl-70 is the major antigen recognised by autoantibodies in the sera of patients with systemic sclerosis (SSc). The autoantibodies that specifically react with Scl-70 are highly characteristic of the disease and represent valuable markers for the diagnosis of SSc. We describe a novel strategy for cloning autoantibody fragments starting with a small blood sample from an SSc patient. B cells isolated from the collected peripheral blood mononuclear cells (PBMCs) were cultured in vitro using the EL4-B5 system. Anti-Scl-70 IgG-producing cells were pooled for RNA preparation followed by the generation of phagemid libraries of approximately 10(7) independent single-chain Fvs (scFvs). The screening of these libraries by phage display allowed us to isolate four anti-Scl-70 scFvs following three rounds of biopanning. About 10 times more starting blood material was needed to generate scFv libraries of similar size from PBMCs of an SSc patient and only two anti-Scl-70 scFvs were isolated after three rounds of phage selection. Together, this work shows that functional autoantibody fragments can be advantageously cloned after in vitro expansion of B cells. The isolated anti-Scl-70 autoantibody fragments represent useful tools for calibrating SSc diagnostic assays.

Detection of autoantibodies to protein tyrosine phosphatase-like protein IA-2 with a novel time-resolved fluorimetric assay

BACKGROUND

Circulating autoantibodies to pancreatic glutamic acid decarboxylase (GAD65; the 65-kDa isoform of glutamic acid decarboxylase), protein tyrosine phosphatase-like protein IA-2, and insulin can be used as predictive markers of type 1 diabetes. We developed a novel assay for the detection of IA-2 autoantibodies (IA-2As) in serum based on time-resolved fluorimetry, hypothesizing that this kind of assay could provide several advantages over methods described to date, including radiobinding assays (RBAs) and ELISAs.

METHODS

The intracellular part of IA-2 (IA-2ic) was biotinylated and bound to streptavidin-coated 96-well plates by simultaneous incubation with serum samples and glutathione S-transferase (GST)-IA-2ic fusion protein. GST-IA-2ic captured by autoantibodies in the serum was detected with europium-labeled anti-GST antibody, and the signal was measured in a time-resolved fluorimeter. A serum sample panel from 100 patients with newly diagnosed type 1 diabetes and 100 unaffected controls was analyzed with the new assay and a conventional RBA.

RESULTS

Among the 100 serum samples from patients with type 1 diabetes, the time-resolved fluorimetric assay identified 74 IA-2A-containing sera, whereas the RBA detected 80 IA-2A-positive samples. Five of the six samples positive in the RBA but not detected by the time-resolved fluorimetric assay were only weakly positive in the RBA. The performance time of the time-resolved fluorimetric assay was 2.5 h compared with 10-12 h required by the RBA.

CONCLUSIONS

The time-resolved fluorimetric assay provides a simple, nonradioactive analysis method for the detection of IA-2As with a specificity and a sensitivity comparable to the RBA method. This assay allows substantial reduction in performance time compared with the conventional RBA.

Two distinctly HLA-associated contiguous linear epitopes uniquely expressed within the islet antigen 2 molecule are major autoantibody epitopes of the diabetes-specific tyrosine phosphatase-like protein autoantigens

The related tyrosine phosphatase-like proteins islet Ag (IA)-2 and IA-2beta are autoantigens of type 1 diabetes in humans. Autoantibodies are predominantly against IA-2, and IA-2-specific epitopes are major autoantibody targets. We used the close homology of IA-2 and IA-2beta to design chimeras and mutants to identify humoral IA-2-specific epitopes. Two major IA-2 epitopes that are absent from the related autoantigens IA-2beta and IA-2Delta 13 splice variant ICA512.bdc were found contiguous to each other within IA-2 juxtamembrane amino acids 611-620 (epitope JM1) and 621-630 (epitope JM2). JM1 and JM2 are recognized by sera from 67% of patients with IA-2 Abs, and relatives of patients with type 1 diabetes having Abs to either JM epitope had a >50% risk for developing type 1 diabetes within 6 years, even in the absence of diabetes-associated HLA genotypes. Remarkably, the presence of Abs to one of these two epitopes was mutually exclusive of the other; JM2 Abs and not JM1 Abs were found in relatives with HLA DR3/4, DR4/13, or DR1/4 genotypes; and the binding of autoantibodies to the JM2 epitope, but not the JM1 epitope, markedly affected proteolysis of IA-2. This is a unique demonstration of HLA-associated B cell responses to epitopes within a single autoantigen in humans and is consistent with modification of Ag processing by specific Ab-influencing peptide presentation by HLA molecules.

ICA512(IA-2) epitope specific assays distinguish transient from diabetes associated autoantibodies

ICA512/IA-2, a tyrosine phosphatase-like protein, is one of the major autoantigens in type 1 diabetes. Following phage display characterization of ICA512 autoantigenic epitopes, we developed fluid phase autoantibody radioimmunoassays for a series of ICA512 fragments (F1 [amino acids (aa): 761-964], F2A [aa 256-760], F2B [aa 761-928], and F2C [aa 929-979]). With the hypothesis that 'non-diabetes associated' ICA512 autoantibodies would differ from diabetes associated ICA512 autoantibodies in terms of epitopes recognized, we analyzed ten such serum samples (two from normal control individuals, one from a general population subject with transient ICA512 autoantibodies and seven from relatives of patients with type 1 diabetes who had single transient ICA512 positivity). All but one of the 'non-diabetes associated' ICA512 positive samples (9/10) did not react with Fragment 1 which contains the major antigenic epitopes of the molecule that were recognized by almost all (51/52) ICA512 positive new onset patient samples and pre-diabetic relatives (P< 10(-6)). The great majority of samples (44/52) from the new onset patients and pre-diabetic relatives reacted with at least two fragments and 60% (31/52) with three or more fragments. In contrast, only one sample of the ICA512 'non-diabetes associated' sera reacted with multiple fragments (P< 10(-4)). Our findings suggest that diabetes associated anti-ICA512 autoantibodies react with multiple ICA512 epitopes while non-diabetes associated ICA512 autoantibodies may usually represent reactivity of antibodies with determinants of ICA512 unrelated to type 1 diabetes. The ability to distinguish diabetes associated from non-diabetes associated anti-ICA512 autoantibodies should provide prognostic information and more importantly suggests that even with highly specific radioassays positivity may occur unrelated to type 1 diabetes.