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

A mutant α1antitrypsin in complex with heat shock proteins as the primary antigen in type 1 diabetes in silico investigation

Based on previous results demonstrating that complexes of a mutant α1-antitrypsin with the heat shock proteins (HSP)70 and glucose-regulated protein94 (Grp94) circulate in the blood of patients with type 1 diabetes, we raised the hypothesis that these complexes could represent the primary antigen capable of triggering the autoimmune reactions leading to overt diabetes. As a first approach to this issue, we searched whether A1AT and HSPs had a sequence similarity to major islet antigen proteins so as to identify among the similar sequences those with potential relevance for the pathogenesis of diabetes. A thorough in silico analysis was performed to establish the score of similarity of the human proteins: A1AT, pro-insulin (INS), GAD65, IAPP, IA-2, ICA69, Grp94, HSP70 and HSP60. The sequences of A1AT and HSPs with the highest score of similarity to the islet peptides reported in the literature as the main autoantigens in human diabetes were recorded. At variance with other HSPs, also including HSP90 and Grp78, Grp94 contained the highest number and the longest sequences with structural similarity to A1AT and to well-known immunogenic peptides/epitopes of INS, GAD65, and IA-2. The similarity of A1AT with Grp94 and that of Grp94 with INS also suggested a functional relationship among the proteins. Specific sequences were identified in A1AT, Grp94 and HSP70, with the highest score of cross-similarity to a pattern of eight different islet protein epitopes. The similarity also involved recently discovered autoantigens in type 1 diabetes such as a hybrid peptides of insulin and the defective ribosomal insulin gene product. The significant similarity displayed by specific sequences of Grp94 and A1AT to the islet peptides considered main antigens in human diabetes, is a strong indication for testing these sequences as new peptides of immunogenic relevance in diabetes.

Early Onset of Autoimmune Diabetes in Children with Down Syndrome-Two Separate Aetiologies or an Immune System Pre-Programmed for Autoimmunity?

PURPOSE OF REVIEW

An increased frequency of autoimmunity in children with Down syndrome (DS) is well described but few studies have investigated the underlying mechanisms. Recent immune system investigation of individuals with DS may shed light on the increased risk of autoimmune conditions including type 1 diabetes.

RECENT FINDINGS

Diagnosis of type 1 diabetes is accelerated in children with DS with 17% diagnosed at, or under, the age of 2 years compared with only 4% in the same age group in the general population. Counterintuitively, children with DS and diabetes have less human leukocyte antigen (HLA)-mediated susceptibility than age-matched children with autoimmune diabetes from the general population. Early onset of diabetes in DS is further highlighted by the recent description of neonatal cases of diabetes which is autoimmune but not HLA associated. There are two potential explanations for this accelerated onset: (1) an additional chromosome 21 increases the genetic and immunological risk of autoimmune diabetes or (2) there are two separate aetiologies in children with DS and diabetes. Autoimmunity in DS is an under-investigated area. In this review, we will draw on recent mechanistic studies in individuals with DS which shed some light on the increased risk of autoimmunity in children with DS and consider the current support for and against two aetiologies underlying diabetes in children with DS.

What Have Slow Progressors Taught Us About T1D-Mind the Gap!

PURPOSE OF REVIEW

Progression rate from islet autoimmunity to clinical diabetes is unpredictable. In this review, we focus on an intriguing group of slow progressors who have high-risk islet autoantibody profiles but some remain diabetes free for decades.

RECENT FINDINGS

Birth cohort studies show that islet autoimmunity presents early in life and approximately 70% of individuals with multiple islet autoantibodies develop clinical symptoms of diabetes within 10 years. Some "at risk" individuals however progress very slowly. Recent genetic studies confirm that approximately half of type 1 diabetes (T1D) is diagnosed in adulthood. This creates a conundrum; slow progressors cannot account for the number of cases diagnosed in the adult population. There is a large "gap" in our understanding of the pathogenesis of adult onset T1D and a need for longitudinal studies to determine whether there are "at risk" adults in the general population; some of whom are rapid and some slow adult progressors.

Islet Autoantibodies

Islet autoantibodies are the main markers of pancreatic autoimmunity in type 1 diabetes (T1D). Islet autoantibodies recognize insulin (IAA), glutamic acid decarboxylase (GADA), protein phosphatase-like IA-2 (IA-2A), and ZnT8 (ZnT8A), all antigens that are found on secretory granules within pancreatic beta cells. Islet antibodies, measured by sensitive and specific liquid phase assays, are the key parameters of the autoimmune response monitored for diagnostics or prognostics in patients with T1D or for disease prediction in at-risk individuals before T1D onset. Islet autoantibodies have been the main tool used to explore the natural history of T1D; this review summarizes the current knowledge about the autoantigens and the phenotype of islets autoantibodies acquired in large prospective studies from birth in children at risk of developing T1D.

Molecular Methods and Protein Synthesis for Definition of Autoantibody Epitopes

Epitope mapping is the process of experimentally identifying the binding sites, or "epitopes," of antibodies on their target antigens. Understanding the antibody-epitope interaction provides a basis for the rational design of potential preventative vaccines. Islet autoantibodies are currently the best available biomarkers for predicting future type 1 diabetes. These include autoantibodies to the islet beta cell proteins, insulin and the tyrosine phosphatase islet antigen-2 (IA-2) which selectively bind to a small number of dominant epitopes associated with increased risk of disease progression. The major epitope regions of insulin and IA-2 autoantibodies have been identified, but need to be mapped more precisely. In order to characterize these epitopes more accurately, this article describes the methods of cloning and mutagenesis of insulin and IA-2 and subsequent purification of the proteins that can be tested in displacement analysis and used to monitor immune responses, in vivo, to native and mutated proteins in a humanized mouse model carrying the high-risk HLA class II susceptibility haplotype DRB1*04-DQ8.

Influence of HLA-DR and -DQ alleles on autoantibody recognition of distinct epitopes within the juxtamembrane domain of the IA-2 autoantigen in type 1 diabetes

AIMS/HYPOTHESIS

Insulinoma-associated protein 2 (IA-2) is a major target of autoimmunity in type 1 diabetes. When first detected, IA-2-autoantibodies commonly bind epitopes in the juxtamembrane (JM) domain of IA-2 and antibody responses subsequently spread to the tyrosine phosphatase domain. Definition of structures of epitopes in the JM domain, and genetic requirements for autoimmunity to these epitopes, is important for our understanding of initiation and progression of autoimmunity. The aims of this study were to investigate the contribution of individual amino acids in the IA-2 JM domain to antibody binding to these epitopes and the role of HLA genotypes in determining epitope specificity.

METHODS

Regions of the JM domain recognised by autoantibodies were identified by peptide competition and inhibitory effects of alanine substitutions of residues within the JM region. Antibody binding was determined by radioligand binding assays using sera from patients genotyped for HLA-DRB1 and -DQB1 alleles.

RESULTS

Patients were categorised into two distinct groups of JM antibody reactivity according to peptide inhibition. Inhibition by substitutions of individual amino acids within the JM domain differed between patients, indicating heterogeneity in epitope recognition. Cluster analysis defined six groups of residues having similar inhibitory effects on antibody binding, with three clusters showing differences in patients affected or unaffected by peptide. One cluster demonstrated significant differences in antibody binding between HLA-DRB1*04 and HLA-DRB1*07 patients and within DRB1*04 individuals; antibody recognition of a second cluster depended on expression of HLA-DQB1*0302.

CONCLUSIONS/INTERPRETATION

The results identify amino acids contributing to distinct epitopes on IA-2, with both HLA-DR and HLA-DQ alleles influencing epitope specificity.

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.

HLA-DR4-associated T and B cell responses to specific determinants on the IA-2 autoantigen in type 1 diabetes

Autoantibodies to IA-2 in type 1 diabetes are associated with HLA-DR4, suggesting influences of HLA-DR4-restricted T cells on IA-2-specific B cell responses. The aim of this study was to investigate possible T-B cell collaboration by determining whether autoantibodies to IA-2 epitopes are associated with T cell responses to IA-2 peptides presented by DR4. T cells secreting the cytokines IFN-γ and IL-10 in response to seven peptides known to elicit T cell responses in type 1 diabetes were quantified by cytokine ELISPOT in HLA-typed patients characterized for Abs to IA-2 epitopes. T cell responses were detected to all peptides tested, but only IL-10 responses to 841-860 and 853-872 peptides were associated with DR4. Phenotyping by RT-PCR of FACS-sorted CD45RO(hi) T cells secreting IL-10 in response to these two peptides indicated that these expressed GATA-3 or T-bet, but not FOXP3, consistent with these being Th2 or Th1 memory T cells rather than of regulatory phenotype. T cell responses to the same two peptides were also associated with specific Abs: those to 841-860 peptide with Abs to juxtamembrane epitopes, which appear early in prediabetes, and those to peptide 853-872 with Abs to an epitope located in the 831-862 central region of the IA-2 tyrosine phosphatase domain. Abs to juxtamembrane and central region constructs were both DR4 associated. This study identifies a region of focus for B and T cell responses to IA-2 in HLA-DR4 diabetic patients that may explain HLA associations of IA-2 autoantibodies, and this region may provide a target for future immune intervention to prevent disease.

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.

Azide and Tween-20 reduce binding to autoantibody epitopes of islet antigen-2; implications for assay performance and reproducibility

Autoantibodies to islet antigen 2 (IA-2A) are important markers for predicting diabetes in children and young adults. Harmonization of IA-2A assay measurement is essential if results from different laboratories are to be compared. We investigated whether sodium azide, a bacteriostatic agent added to some assays, could affect IA-2A binding and thereby contribute to differences in IA-2A measurement between laboratories. Addition of 0.1% azide to assay buffer was found to reduce median IA-2A binding of 18 selected sera from IA-2A positive patients with type 1 diabetes and their relatives by 41% (range, 78 to -33%, p<0.001). The effect on binding was epitope specific; median IA-2A binding by 14 sera with antibodies to the protein tyrosine phosphatase region of IA-2 was reduced by 48% (range, 11 to 78%, p<0.001), while binding by 4 sera with antibodies specific to only the juxtamembrane region of IA-2 showed no change (median increase 16% (range 6 to 33%, p=0.125). When the Tween-20 concentration was reduced from 1% to 0.15% the median reduction in IA-2A binding with azide by the 18 sera was only 10% (range, -12 to 41%, p<0.001). Tween-20 also exerted an independent effect, since median IA-2A binding increased by 23% (range 3% to 86%, p<0.001) when Tween-20 concentration was reduced from 1% to 0.15% in the absence of azide. We conclude that common assay reagents such as azide and Tween-20 can strongly influence IA-2A binding in an epitope-related manner, and their use may explain some of the differences between laboratories in IA-2A measurement.

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.

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.

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.

Sensitive non-isotopic assays for autoantibodies to IA-2 and to a combination of both IA-2 and GAD65

BACKGROUND

A sensitive ELISA for measurement of IA-2 autoantibodies has been developed and assessed. Also, a combination ELISA for detection of both GAD65 autoantibodies and IA-2 autoantibodies is described.

METHODS

The IA-2 autoantibody assay is based on the ability of IA-2 autoantibodies to form a bridge between IA-2 intracellular fragment coated onto ELISA plate wells and liquid-phase IA-2 labelled with biotin. The combination ELISA uses plates coated with both IA-2 and GAD65 and a mixture of IA-2-biotin and GAD65-biotin. Assay sensitivity was assessed using the WHO reference (NIBSC 97/550) for islet cell antibodies. IA-2 autoantibody measurements by ELISA were compared with measurements in immunoprecipitation assays (IPAs) based on 125I or 35S labelled IA-2. Combination ELISA results were compared with results obtained for individual autoantibodies.

RESULTS

As little as 15 units/mL of NIBSC 97/550 was detectable in the IA-2 autoantibody ELISA compared to 125 units/mL by 125I-IA-2 IPA. 110/216(51%) sera from patients with type 1 DM were positive in the IA-2 autoantibody ELISA while 97/216 (45%) and 91/216 (42%) were positive in the 125I-IA2 and 35S-IA-2 IPAs, respectively. The IA-2 autoantibody ELISA showed 100% specificity for type 1 DM. The combination ELISA was able to detect GAD65 and/or IA-2 autoantibodies in 183/216 (85%) diabetic sera and 183/216 (85%) were also found positive for autoantibodies to IA-2 and/or to GAD65 in the assays for individual antibodies. Autoantibody measurements in the individual autoantibody assays and in the combination ELISA showed good agreement by Pearson correlation (r=0.92, n=216, p<0.001) and by Bland and Altman analysis.

CONCLUSIONS

Sensitive and specific ELISAs for measurement of autoantibodies to IA-2 and to a combination of IA-2 and GAD65 have been developed. These assays are suitable for screening large numbers of samples in diabetes prediction and prevention trials.

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.

Diabetes-related antibodies in adult diabetic patients

Islet autoimmunity is made evident by the appearance of islet-cell antibodies directed against insulin (IAA), glutamic acid decarboxylase (GADA), protein tyrosine phosphatase IA-2 (IA-2Ab) and other autoantigens. IAA and IA-2Ab are predominantly detected in childhood type 1 diabetes mellitus (T1DM), while frequency of GADA is not affected by age. In adult-onset T1DM patients, GADA is the immune marker of higher diagnostic sensitivity. In adult diabetic patients not requiring insulin treatment for at least 6 months after diagnosis, GADA identifies the so-called latent autoimmune diabetes in adults (LADA). In over 80% of cases, LADA patients develop insulin dependency within a few years after the diagnosis and have an increased risk for the development of other organ-specific autoimmune diseases. High GADA titers identify a subgroup of LADA patients with low body mass index (BMI), low C-peptide levels and increased frequency of T1DM-related HLA class II haplotypes. GADA assay should be offered to every diabetic patient, and in cases of positivity screening for other autoimmune diseases should be carried out.

IA-2 antibody epitopes and isotypes during the prediabetic process in siblings of children with type 1 diabetes

To characterize the humoral immune response to the protein tyrosine phosphatase (PTP)-like autoantigen (IA-2) in preclinical type 1 diabetes (T1D), and to assess the utility of epitope and isotype-specific IA-2 antibody responses as surrogate markers for disease development, we analyzed these antibodies in 34 initially non-diabetic siblings of affected children derived from the "Childhood Diabetes in Finland" (DiMe) Study. Half of them presented with T1D during an average observation period of 8.7 years. Radiobinding assays were used to determine IA-2/IA-2 beta epitope-specific (the juxtamembrane region, JM; the PTP-like and the beta PTP-like domain) antibodies and isotype-specific (IgG1-4, IgA, IgE and IgM) IA-2 antibodies. Initially, 30 of the 34 siblings tested positive for epitope-specific antibodies. The siblings who progressed to clinical diabetes had IA-2 JM antibodies more often (P<0.05) but IgE-IA-2 antibodies less frequently (P<0.05) than the siblings who did not progress to T1D. During the identical follow-up time, the non-progressors had higher integrated titers of IgE-IA-2 antibodies (P=0.05). The occurrence of IgE-IA-2 antibodies was protective, since despite IA-2 JM antibodies, children with IgE-IA-2 antibodies did rarely progress to T1D. This study demonstrates that JM-reactive IA-2 antibodies are associated with an increased risk of progression to overt T1D, whereas an IgE response to IA-2 confers relative protection against clinical disease.

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.

Peripheral antigen-expressing cells in type 1 diabetes

Studies in both humans and rodent models provide new insight into key mechanisms regulating tolerance to self-molecules. There is evidence that tissue-specific molecules are expressed in the thymus and peripheral lymphoid tissues (PLTs) by specialized antigen-presenting cells (APCs), and that such expression is critical for self-tolerance. Insulin, a key hormone exclusively produced by pancreatic beta cells and a critical autoantigen in type 1 diabetes, provides an excellent example of a molecule with tissue-restricted expression that is ectopically expressed by APCs in both thymus and PLTs. APCs may play a role in insulin presentation in both the central and peripheral immune system. Functional data from several transgenic and knockout mouse models, some specific for the expression of insulin, help dissect the significance of self-molecule presentation by APCs and its role in autoimmune diabetes.

Directed evolution of a single-chain class II MHC product by yeast display

Many autoimmune diseases have been linked to the class II region of the major histocompatibility complex (MHC). The linkage is thought to be a result of autoreactive T cells that recognize self-peptides bound to a product of this locus. For example, T cells from non-obese diabetic mice recognize specific 'diabetogenic' peptides bound to a class II MHC allele called I-A(g7). The I-A(g7) molecule is noted for being unstable and difficult to work with, especially in soluble form. In this work, yeast surface display combined with fluorescence-activated cell sorting was used as a means of directed evolution to engineer stabilized variants of a single-chain form of I-A(g7). A library containing mutations at two residues (positions 56 and 57 of the I-A(g7) beta-chain) that are important in the class II disease associations yielded stabilized mutants with preferences for a glutamic acid at residue 56 and a leucine at residue 57. Random mutation of I-A(g7) followed by selection with an anti-I-A(g7) antibody also yielded stabilized variants with mutations in other residues. The methods described here allow the discovery of novel MHC complexes that could facilitate structural studies and provide new opportunities in the development of diagnostics or antagonists of class II MHC-associated diseases.