Autoantigenic Epitopes Research Articles

155. Oral Delivery of Recombinant Vaccinia Viruses Expressing Adjuvanted Islet Autoantigens Protects NOD Mice from Autoimmune Diabetes

Oral administration of autoantigens can delay or suppress clinical disease in experimental autoimmune disorders. However, repeated feeding of large tolerogen amounts is required over long periods and is only partially effective in animals systemically sensitized to the ingested antigen. The cholera toxin B subunit adjuvant (CTB) when linked to autoantigen epitopes acts as a strong adjuvant to enhance suppression of inflammatory Th1 cell reactivity in both naive and immune animals. Enhanced suppression of inflammatory Th1 cell reactivity was demonstrated in both naive and immune NOD mice following oral inoculation with islet autoantigens coupled to CTB. To stimulate adjuvant-autoantigen fusion protein biosynthesis in the gut mucosae, we evaluated oral inoculation of juvenile NOD mice with recombinant vaccinia virus (rVV) expressing fusion genes encoding CTB linked to the pancreatic islet autoantigens proinsulin (INS) and a 55 kDa peptide from glutamate decarboxylase (GAD55). Oral inoculation with rVV significantly reduced pancreatic insulitis. Hyperglycemia in NOD mice inoculated with CTB::GAD fusion protein was reduced by 40% in comparison with rVV-GAD inoculated mice. Measurement of antibody isotype titers in rVV infected mice suggested activation of autoantigen specific Th2 lymphocytes. These experiments demonstrate the feasibility of vaccinia virus mediated delivery of adjuvanted autoantigens to the mucosae of prediabetic mice for suppression of autoimmune diabetes.

Autoantigenic Epitopes on Platelet Glycoproteins

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder characterized by early platelet destruction mediated by antiplatelet autoantibodies. Platelet membrane glycoproteins (GP), especially GPIIb-IIIa and GPIb-IX, contain major autoantigenic determinants in chronic ITP. Recent advances in the localization of autoantigens as well as in the detection of GP-specific antibodies have improved our understanding of the pathophysiology of the disease. The N-terminal globular head of GPIIb-IIIa, particularly the beta-propeller domain in GPIIb, seems to play an important role as a hot spot for autoantigenic epitopes in chronic ITP.

Mechanisms of Innate and Adaptive Immune Responsiveness to Bacterial Infection in Cornea

Purpose. The dysfunctional tear syndrome of Sjogren’s syndrome (SjS) is associated with focal CD4+ cell infiltration of the lacrimal glands and serum autoantibodies to intracellular proteins. To learn how lacrimal acinar cells (AC) expose or present autoantigen epitopes, we wish to identify autoantigens in amenable experimental models. Methods. Subcellular fractions from rat lacrimal glands were analyzed by SDS-PAGE. Western blots were probed with sera from NOD mice with diagnosed SjS-like disease, and IgG1 heavy chain was detected with isotype specific secondary antibody. Labeled bands were analyzed using matrix-associated laser desorption / ionization time-of-flight (MALDI-TOF) mass spectrometry. Tryptic peptides were identified by Mascot Matrix Science search software. Compartmental distributions of candidate B cell autoantigens were determined by Western blotting of isolated membrane fractions from rabbit AC cultured with and without chronic carbachol (CCh) stimulation. Results. Subcellular fractionation analysis indicated that the candidate autoantigens were primarily associated with the membrane phase; most were primarily in compartments of the biosynthetic and lysosomal pathways. Some also were in secretory vesicle-related compartments of control cells but re-distributed to the endosomes after chronic CCh stimulation. The proteomic analysis identified Grp 78, rab3D, and ribosomal protein L7 as candidate autoantigens. Western blotting with monoclonal antibodies confirmed the presence and compartmental distributions of Grp 78 and rab3D. Protein sequence analysis with TEPITOPE confirmed that an identified B cell epitope at the N-terminal end of L7 also is a likely T cell epitope. Conclusion Grp78 and L7 are known to be antigens in other autoimmune diseases but have not previously been known to be associated with SjS. Rab3D has not previously been recognized as a disease-associated autoantigen. These and other AC autoantigens are present in intracellular compartments where they may be subject to aberrant proteolytic processing that we theorize generates otherwise cryptic epitopes. Commercial Relationship(s): None; Support: EY13720, EY05801, DK48522

Early events in lupus humoral autoimmunity suggest initiation through molecular mimicry

The origins of autoimmunity in systemic lupus erythematosus (SLE) are thought to involve both genetic and environmental factors. To identify environmental agents that could potentially incite autoimmunity, we have traced the autoantibody response in human SLE back in time, prior to clinical disease onset, and identified the initial autoantigenic epitope for some lupus patients positive for antibodies to 60 kDa Ro. This initial epitope directly cross-reacts with a peptide from the latent viral protein Epstein-Barr virus nuclear antigen-1 (EBNA-1). Animals immunized with either the first epitope of 60 kDa Ro or the cross-reactive EBNA-1 epitope progressively develop autoantibodies binding multiple epitopes of Ro and spliceosomal autoantigens. They eventually acquire clinical symptoms of lupus such as leukopenia, thrombocytopenia and renal dysfunction. These data support the hypothesis that some humoral autoimmunity in human lupus arises through molecular mimicry between EBNA-1 and lupus autoantigens and provide further evidence to suspect an etiologic role for Epstein-Barr virus in SLE.

Altered traffic to the lysosome in an ex vivo lacrimal acinar cell model for chronic muscarinic receptor stimulation

Evidence suggests that lacrimal and salivary epithelial cells constitutively expose potentially pathogenic autoantigens, but that active regulatory networks normally suppress pathological autoimmune responses [Mircheff, A.K., 2003. Sjögren's syndrome as failed local immunohomeostasis: prospects for cell-based therapy. Ocular Surface 1, 160–179; Mircheff, A.K., Hamm-Alvarez, S.F., Kaslow, H.R., Schechter, J.E., Trousdale, M.D., 2004. Epithelial cells in lymphocyte regulation in the lacrimal gland. In: Zierhut, M. (Ed.), Immunology of the Ocular Surface and the Lacrimal Glands. Swets & Zeitlinger, Amsterdam; in press]. Events that potentially disrupt the regulatory networks include increased exposure of constitutive autoantigens and induced exposure of previously cryptic autoantigen epitopes. Chronic muscarinic receptor (MAChR) stimulation in an ex vivo rabbit lacrimal acinar cell model induces functional and biochemical alterations reminiscent of the functional quiescence associated with Sjögren's syndrome [Qian, L., Wang, Y., Xie, J., Rose, C.M., Yang, T., Nakamura, T., Sandberg, M., Zeng, H., Schechter, J.E., Chow, R.H., Hamm-Alvarez, S.F., Mircheff, A.K., 2003. Biochemical changes contributing to functional quiescence in lacrimal gland acinar cells after chronic ex vivo exposure to a muscarinic agonist. Scand. J. Immunol. 58, 550–565]. Chronic MAChR stimulation also elicits changes in the compartmental distribution of β-hexosaminidase, a product that normally is dually targeted into the lysosomal pathway and the regulated apical secretory pathway. Here, we use subcellular fractionation analyses to further explore the nature of the stimulation-induced traffic changes and to identify effectors that might mediate this change. Overnight stimulation of primary cultured rabbit lacrimal gland acinar cells with 10 μ m carbachol (CCh) significantly decreased the abundance of mature cathepsin B in the pre-lysosome and lysosome; decreased the abundance of preprocathepsin B in fractions containing the TGN and late endosome; increased the abundance of procathepsin B in fractions containing the basal-lateral membrane; and increased the accumulation of endocytosed [ 125I]-EGF in the recycling endosome. Alterations in distribution or abundance of traffic effectors included: increased abundances of rab5A and rab6 in the TGN; decreased overall abundance of γ-adaptin; remarkably increased relative abundance of membrane phase-associated actin; redistribution of cytoplasmic dynein from biosynthetic and proximal endocytic compartments to the lysosome; and redistribution of p150 Glued from the lysosome to biosynthetic or proximal endocytic compartments. We conclude that chronic MAChR stimulation blocks traffic from the early endosome and the TGN to the lysosome, causing lysosomal proteins to reflux to the TGN, endosomes, and basal-lateral membrane. These traffic alterations may be mediated through action on one or more of the effectors noted.

Peptides From Common Viral and Bacterial Pathogens Can Efficiently Activate Diabetogenic T-Cells

Cross-reactivity between an autoantigen and unknown microbial epitopes has been proposed as a molecular mechanism involved in the development of insulin-dependent diabetes (type 1 diabetes). Type 1 diabetes is an autoimmune disease that occurs in humans and the nonobese diabetic (NOD) mouse. BDC2.5 is an islet-specific CD4+ T-cell clone derived from the NOD mouse whose natural target antigen is unknown. A biometrical analysis of screening data from BDC2.5 T-cells and a positional scanning synthetic combinatorial library (PS-SCL) was used to analyze and rank all peptides in public viral and bacterial protein databases and identify potential molecular mimic sequences with predicted reactivity. Selected sequences were synthesized and tested for stimulatory activity with BDC2.5 T-cells. Active peptides were identified, and some of them were also able to stimulate spontaneously activated T-cells derived from young, pre-diabetic NOD mice, indicating that the reactivity of the BDC2.5 T-cell is directed at numerous mouse peptides. Our results provide evidence for their possible role as T-cell ligands involved in the activation of diabetogenic T-cells.

Key residues contributing to dominant conformational autoantigenic epitopes on thyroid peroxidase identified by mutagenesis

Thyroid peroxidase (TPO) is a major autoantigen in thyroid autoimmune disease where pathogenic autoantibodies recognise conformational epitopes restricted to two overlapping immunodominant regions (IDR) termed IDR-A and -B. Based upon our structural model of TPO, we report on the localisation of the IDRs to specific amino acids important for autoantibody binding. Using a panel of recombinant human Fabs (rhFabs) from autoimmune patients, specific for the IDR-A or -B epitopes, in combination with eukaryotic expression of 14 single amino acid mutants of TPO, we identify R225 and K627 as key components of IDR-A and -B, respectively. Moreover, each mutant specifically led to the loss of binding of three different IDR-A- or -B-specific rhFabs, without affecting the binding of autoantibodies to the other determinant. Further supportive evidence for the role of amino acids R225 and K627 was obtained with murine monoclonal antibodies that first defined the IDRs. The identification of amino acids R225 and K627 as key residues for the IDR epitopes on TPO will advance our understanding of the molecular basis of autoreactivity and facilitate the design of novel therapeutic agents.

Peptide register shifting within the MHC groove: theory becomes reality

Degeneracy in immune recognition is usually thought of in terms of the astonishing ability of the T cell receptor to recognize an enormously diverse array of peptides bound to major histocompatibility complex (MHC) molecules. However, in this essay we discuss an alternative aspect of degeneracy in T cell recognition: the notion that peptides can assume different “registers” in the groove of a single MHC molecule, as first suggested and demonstrated by Sercarz and co-workers (reviewed in [ J. Autoimmun. 16 (2001) 201]). There is now abundant evidence, derived from functional, biochemical and structural studies, that single peptides can assume alternative, unpredictable binding registers by frameshifting within the MHC groove [Nat. Immunol. 3 (2002) 175; J. Exp. Med. 170 (1989) 1609; J. Exp. Med. 187 (1998) 1505; J. Mol. Biol. 304 (2000) 177; Biochemistry 38 (1999) 16663; J. Exp. Med. 197 (2003) 1391; Eur. J. Immunol. 19 (1989) 681]. Hence, register shifting adds an additional dimension to the concept of degeneracy. In fact, the possibility of register shifting multiplies the universe of peptide–MHC (pMHC) surfaces that a TCR must recognize by an unknown, perhaps enormous factor. Register shifting also has profound implication for autoimmunity: (1) as a mechanism to “mask” autoantigenic epitopes during thymic education [Immunol. Rev. 169 (1999) 147; Immunity 17 (2002) 83]; and (2) as a possible source for pMHC complexes capable of molecular mimicry.

Autoreactive T cells can be protected from tolerance induction through competition by flanking determinants for access to class II MHC.

It is not clear why the N-terminal autoantigenic determinant of myelin basic protein (MBP), Ac1-9, is dominant in the B1O.PL (H-2(u)) mouse, given its weak I-A(u)-MHC binding affinity. Similarly, how do high-affinity T cells specific for this determinant avoid negative selection? Because the MBP:1-9 sequence is embryonically expressed uniquely in the context of Golli-MBP, determinants were sought within the contiguous N-terminal "Golli" region that could out-compete MBP:1-9 for MHC binding, and thereby prevent negative selection of the public response to Ac1-9, shown here to be comprised of a V beta 8.2J beta 2.7 and a V beta 8.2J beta 2.4 expansion. Specifically, we demonstrate that Ac1-9 itself can be an effective inducer of central tolerance induction; however, in the context of Golli-MBP, Ac1-9 is flanked by determinants which prevent its display to autoreactive T cells. Our data support competitive capture as a means of protecting high-affinity, autoreactive T cells from central tolerance induction.

Beta cell-specific CD80 (B7-1) expression disrupts tissue protection from autoantigen-specific CTL-mediated diabetes

T cell responses toward pancreatic beta cell autoantigens arise spontaneously or on immunization in many mouse strains, yet sustained islet infiltration and progressive diabetes rarely ensues. Most mouse diabetes models overcome the innocuous coexistence of anti-islet specific T cells and endogenous islets via incompletely understood mechanisms (e.g. the spontaneous disease onset of the non-obese diabetic mouse) or depend on overwhelming numbers of peripheral islet-specific T cells. We report that insulin promoter murine CD80 (RIP-CD80) transgenic mice are extraordinarily susceptible to autoantigen-induced diabetes, while spontaneous disease is rare. Autoimmunity to the pancreatic beta cell-expressed glycoprotein (GP) of the lymphocytic choriomeningitis virus (LCMV) was elicited by a single injection of syngeneic fibroblastoid cell lines (FCL) loaded with the immunodominant LCMV-GP peptide, gp33. While both RIP-GP +and RIP-CD80 +GP +mice mounted moderate CD4-independent CTL responses, only CD80 +GP +mice developed severe insulitis and diabetes due to islet-infiltration of activated, gp33-specific, CD8 +T cells. Strikingly, DNA immunization using plasmids encoding LCMV-GP or murine preproinsulin also efficiently induced Ag-specific RIP-CD80-dependent diabetes. We conclude that aberrant CD80-expression in a peripheral tissue disrupts that tissue's natural resistance to CD8 T cell-mediated autoimmune destruction. This rodent model thus represents a novel approach to identify beta cell-derived autoantigenic determinants involved in the pathogenesis of autoimmune diabetes, and may also serve as a prototype approach to uncover relevant autoantigens leading to a variety of organ-specific autoimmune disorders.

Autoimmune thyroid disease: propagation and progression

Autoimmune thyroid disease is the archetype for organ-specific autoimmune disorders. Progress in treating these disorders lies in improvements of our understanding of the predisposing factors responsible, the mechanisms responsible for progression of disease, and the interaction between thyroid antigens and the immune system at the level of the T cell and antibody. In common with other autoimmune diseases, genetic, environmental and endogenous factors are required in an appropriate combination to initiate thyroid autoimmunity. At present the only genetic factors which have been confirmed lie in the HLA complex and CTLA-4 or a closely linked gene. Identifying other predisposing genes will require large-scale family studies, or further insights into likely candidate genes. A number of environmental factors are known to predispose to autoimmune thyroid disease, including smoking, stress and iodine intake, while immunomodulatory treatments are revealing new pathways for disease emergence. The thyroid cell itself appears to play a major role in disease progression, interacting with the immune system through expression of a number of immunologically active molecules including HLA class I and II, adhesion molecules, cytokines, CD40 and complement regulatory proteins. New techniques, in particular phage display libraries, are providing the methods with which to identify autoantibody diversity in autoimmune thyroid disease and to provide tools for mapping autoantigenic epitopes. Application of these techniques is likely to lead to an understanding of how TSH receptor antibodies interact with the receptor to cause Graves' disease and also to the identification of novel orbital autoantigens in thyroid-associated ophthalmopathy.

Investigation of the reactivity patterns of antifilaggrin antibodies in sera and synovial fluids from patients with rheumatoid arthritis using citrullinated synthetic peptides

Antifilaggrin antibodies comprise a heterogeneous population of antibodies directed to citrullinated proteins. Recent studies have shown that their production is highly specific for rheumatoid arthritis (RA) and the initial antigenic trigger for these autoantibodies can be localised to the inflammed synovial tissue. The aim of our study was to compare the reactivity and specificity of antibodies in sera and synovial fluids towards citrullinated epitopes. Peptide sequence corresponding to human profilaggrin (amino acid residues 306–324) and sequences with citrulline substitution at different positions were synthetised by mutipin peptide synthesis on solid supports. Shortened versions of the peptide were also produced by removal of amino acid residues from its N and C terminals. Completely citrullinated variant of the 14-mer peptide was also prepared. Peptide with no citrulline replacement was used as a control antigen. We found significant differences in the sensitivity for RA of 19 individual peptides tested (from 5% to 68%), reflecting previous results that the surrounding amino acids play an important role in creation of an autoantigenic epitope. Further, we tested the reactivities of paired serum and synovial fluids and found very similar peptide recognition patterns in serum and synovial IgG from the same individuals. Studies on larger number of samples are in progress to evaluate the results statistically that may support further evidence of the synovial origin of antifilaggrin autoantibodies.

Application of phage display peptide library to autoimmune diabetes: identification of IA-2/ICA512bdc dominant autoantigenic epitopes.

Autoantigenic epitope mapping represents a critical issue in autoimmune diseases. The islet tyrosine phosphatase-like protein IA-2/ICA512bdc is a major autoantigen in type 1 diabetes (IDDM), but the epitopes responsible for autoantibody binding have been only partially defined. The aim of our study was to identify ICA512bdc epitopes, and in particular mini-epitopes, utilizing a novel strategy for autoimmune diseases. The study was performed in three sequential steps: (1) construction of a lambda-phage surface-displayed ICA512bdc cDNA library with the methodology of tagged random priming with peptides displayed as a fusion to the C terminus of the capsid protein D; (2) affinity selection of the resulting library, followed by immunoscreening, enzyme-linked immunosorbent assay and sequence analysis of positive clones, and (3) radioimmunoprecipitation to detect autoantibodies to the selected clones. This strategy resulted in the identification of two epitopes (IA-2 residues 761 - 964 and 929 - 979), which were recognized by 100 % and 62.9 % ICA512bdc-positive IDDM patients, respectively. Interestingly, the larger clone was detected also by a proportion (16.7 %) of new onset ICA512bdc-negative patients, thus suggesting that this region contains not only the main autoantigenic repertoire of ICA512bdc molecule, but is able to detect IA-2 autoantibodies in even higher percentages of patients. In addition, this study showed the existence of multiple epitopes located in the C-terminal domain of the IA-2 protein, one of which is formed by the 50 C-terminal amino acids, and provided evidence that the strategy used represents a valid tool for identification of epitopes within autoantigenic 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.

Epitope mapping of cytochrome P450 cholesterol side-chain cleavage enzyme by sera from patients with autoimmune polyglandular syndrome type 1.

Autoimmune polyglandular syndrome type 1 (APS-1) is a disease associated with defects of the autoimmune regulator gene and is characterized by autoimmune lesions of several tissues, predominantly endocrine glands, with multiple autoantibodies. In this study we describe autoantigenic epitopes on cholesterol side-chain cleavage enzyme (P450scc) using sera from Finnish and Sardinian patients with APS-1, and analyze the epitope reactivities during disease follow-up. A series of P450scc cDNA fragments were expressed in E. coli and tested by immunoblotting assay using the patients' sera. Epitope regions were found over the whole P450scc molecule except the last N- (amino acids (aa) 1-40) and C-termini (aa 456-521). The strongest reactivity with patients' sera was found with central and C-terminal regions of the P450scc protein. All studied patients had IgG1 subclass antibodies. The results show that Finnish and Sardinian patients with APS-1 have similar, polyclonal immune reactions against P450scc, and that epitope reactivities did not change during the disease course. These results support the opinion that autoantibodies against P450scc and their epitope reactivity pattern are formed at an early stage of steroidogenic autoimmunity.

Mapping of novel autoreactive epitopes of the diabetes-associated autoantigen IA-2.

IA-2, a member of the tyrosine phosphatase family, has been identified as a dominant autoantigen in type 1 diabetes. To define humoral IA-2 epitopes, we generated a panel of IA-2 deletion mutants and chimeric proteins using the highly homologous tyrosine phosphatase-like protein IA-2beta. Analysis of autoantibody reactivity in 111 IA-2 antibody positive sera from patients with type 1 diabetes revealed that humoral epitopes cluster to several domains of the intracytoplasmic part of IA-2 [IA-2ic, amino acid (aa) 604-979]. Immunodominant epitopes were found in the first N-terminal 73 amino acids (56% positive), in the middle domain residing between residues 699-874 (45% positive) and the C-terminus depending on the presence of aa 931-979 (at least 37% positive). Competition experiments with overlapping peptides revealed that autoantibody binding towards the N-terminus was dependent on residues 621-628. In the C-terminal domain, two novel conformation-dependent epitopes were identified. The first epitope requires the presence of the C-terminal part of IA-2 (aa 933-979) and an IA-2-specific region between residues 771-932. Reactivity against the second epitope was dependent on intact C-terminal domains as well as residues in the middle (aa 887-932) and N-terminal regions (aa 604-771) which are conserved in IA-2 and IA-2beta. We here defined novel autoantigenic determinants in the N-terminus of IA-2 and characterized conformational epitopes residing in the C-terminal region or spanning from C-terminal residues to the N-terminal domain of IA-2ic. The identification of dominant target regions of diabetes-specific autoantibodies may help to elucidate the molecular mechanisms involved in the autoimmunity towards IA-2.

Autoantibodies from patients with coeliac disease recognize distinct functional domains of the autoantigen tissue transglutaminase.

The enzyme tissue transglutaminase (tTG) has been recently identified to represent a highly sensitive and specific target of autoantibodies in coeliac disease. To characterize autoantigenic epitopes, we generated novel tTG deletion mutants by polymerase chain reaction, produced radiolabelled fragments by in vitro transcription/translation, immunoprecipitated the mutants using sera from patients with coeliac disease, and related the binding data with putative structural and functional domains of human tTG. We show that tTG antibody positive sera display a heterogeneous autoantibody response covering distinct regions of the molecule. The N-terminal and C-terminal third of tTG, comprising amino acid (aa) 1-281 and aa 473-687, harbour the dominant epitopes (67.4% and 69.4% positive), whereas the catalytic region is of minor antigenicity (22.5% positive). Autoantibodies directed to one, two and three domains were observed in 36.7%, 28.6% and 22.4% of patients, respectively. Comparative analysis revealed the presence of strictly conformational epitopes which were dependent on the N-terminus (aa 1-12) or the intact beta-barrel domains in the C-terminus (aa 473-497, aa 649-687). In conclusion, we here demonstrate for the first time that the humoral autoimmunity is directed against distinct functional tTG domains. The spectrum of autoantibodies indicates that the native folded protein may be the target of autoantibodies.

X-ray crystal structure of a monoclonal antibody that binds to a major autoantigenic epitope on thyroid peroxidase.

Thyroid peroxidase (TPO) catalyzes the production of thyroid hormones and is a major autoantigen in autoimmune thyroid disease (AITD). It is believed that the majority of TPO autoantibodies bind to an immunodominant region consisting of two overlapping domains. Precise location of these domains would help our understanding of the interaction between TPO and TPO autoantibodies. 4F5 is a mouse monoclonal antibody (IgG1, kappa) that reacts with high affinity (2.6 x 10(10) mol/L(-1)) with one of the major autoantigenic regions on TPO. Heavy chain genes of 4F5 were from the VH1 germline gene family, germline genes for the D region could not be assigned and the J region was from the JH2 germline. Light chain genes were from Vkappa4/5 and Jkappa2, germline gene families. The Fab fragment of 4F5 was prepared by papain digestion, purified, crystallized, and the structure solved to 1.9 A using molecular replacement. The refined structure had an R factor of 19.5% and a free R factor of 23.9%. Deduced amino acid sequence and amino acid sequence obtained from diffraction analysis were compared and used to finalize the 4F5 Fab model. Structural analysis indicated that the structure of 4F5 is that of a standard Fab and its combining site is flat and is rich in tyrosine residues. Comparison of the structure of 4F5 with that of a TPO autoantibody Fab, TR1.9 suggests that the two antibodies are unlikely to recognise the same structures on TPO.

Autoantibody response to islet transplantation in type 1 diabetes.

Islet allotransplantation into patients with autoimmune type 1 diabetes represents a reexposure to autoantigen. Here, measurement of antibodies to GAD and IA-2 autoantigens before and after islet transplantation in 36 patients (33 receiving islet plus kidney grafts with cyclosporin and steroid-based immunosuppression, and 3 receiving solitary islet transplants with mycophenolate but cyclosporin-free immunosuppression) demonstrated marked rises in GAD antibodies within 7 days posttransplantation in 5 patients (3 receiving islet after kidney transplants, and 2 receiving solitary islet transplants) and within 30 days in the third patient receiving solitary islet transplantation. GAD antibodies were of the IgG1 subclass, against major autoantigenic epitopes, and in cases of islet after kidney transplants, the responses were short-lived and not accompanied by HLA antibodies. Two of these patients had subsequent marked rises of IA-2 antibodies, and an additional patient had a marked rise in IgM-GAD antibodies 3 years after transplantation. Insulin independence was not achieved in patients with autoantibody elevations and was significantly less frequent in these patients. These data are consistent with a reactivation of autoimmunity that may be dependent on immunosuppression therapy and is associated with impaired graft function.

Evidence That a Peptide Spanning the B-C Junction of Proinsulin Is an Early Autoantigen Epitope in the Pathogenesis of Type 1 Diabetes

The expression of pro(insulin) in the thymus may lead to the negative selection of pro(insulin) autoreactive T cells and peripheral tolerance to this autoantigen in type 1 diabetes (T1D). We investigated whether proinsulin is expressed in the thymus of young nonobese diabetic (NOD) mice, whether T cells from naive NOD female mice at weaning are reactive to mouse proinsulin, and the role of proinsulin as a pathogenic autoantigen in T1D. Proinsulin II mRNA transcripts were detected in the thymus of 2-wk-old NOD mice at similar levels to other control strains. Despite this expression, proinsulin autoreactive T cells were detected in the periphery of 2- to 3-wk-old naive NOD mice. Peripheral T cells reactive to the insulin, glutamic acid decarboxylase 65 (GAD65), GAD67, and islet cell Ag p69 autoantigens were also detected in these mice, indicating that NOD mice are not tolerant to any of these islet autoantigens at this young age. T cell reactivities to proinsulin and islet cell Ag p69 exceeded those to GAD67, and T cell reactivity to proinsulin in the spleen and pancreatic lymph nodes was directed mainly against a p24-33 epitope that spans the B chain/C peptide junction. Intraperitoneal immunization with proinsulin perinatally beginning at 18 days of age delayed the onset and reduced the incidence of T1D. However, s.c. immunization with proinsulin initiated at 5 wk of age accelerated diabetes in female NOD mice. Our findings support the notion that proinsulin p24-33 may be a primary autoantigen epitope in the pathogenesis of T1D in NOD mice.