Evaluation of conformational epitopes on thyroid peroxidase by antipeptide antibody binding and mutagenesis

Exposure of Akwesasne Mohawk women to polychlorinated biphenyls and hexachlorobenzene is associated with increased serum levels of thyroid peroxidase autoantibodies

Persistent organic pollutants (POPs) including polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB), and dichlorodiphenyltrichloroethane (p,p'-DDT) were reported to influence immunological activity. As endocrine-disrupting chemicals (EDC), these pollutants may disrupt normal thyroid function and act as catalysts for development of autoimmune thyroid disease by directly and indirectly affecting levels of thyroid peroxidase antibodies (TPOAbs). Native American communities are disproportionately exposed to harmful toxicants and are at an increased risk of developing an autoimmune disease. The aim of this study was to determine the association between POPs and TPOAbs in serum obtained from Native American women. This assessment was used to measure whether increased risk of autoimmune thyroid disease occurred as a result of exposure to POPs. Data were collected from 183 Akwesasne Mohawk women, 21-38 years of age, between 2009 and 2013. Multivariate analyses were conducted to determine the association between toxicant exposure and levels of TPOAbs. In multiple logistic regression analyses, exposure to PCB congener 33 was related to elevated risk of individuals possessing above normal levels of TPOAbs. Further, HCB was associated with more than 2-fold higher risk of possessing above normal levels of TPOAbs compared to women with normal levels of TPOAbs. p,p'-DDE was not associated with TPOAb levels within this study. Exposure to PCB congener 33 and HCB was correlated with above normal levels of TPOAbs, a marker of autoimmune thyroid disease. Additional investigations are needed to establish the causes and factors surrounding autoimmune thyroid disease which are multiple and complex.

Prevalence of Thyroid Peroxidase and Thyroglobulin Autoantibodies in the Swedish Population

Autoimmune thyroid disease (AITD) may be detected prior to clinical symptoms through the presence of autoantibodies against thyroid peroxidase (TPOab), thyroglobulin (TGab), or both.The present study aimed to develop a novel radiobinding assay (RBA) for TPOab and to determine the prevalence of TPOab and TGab in the Swedish population.Patient samples from 27 newly diagnosed Graves' disease patients in longitudinal follow-up and 124 AITD autoantibody-positive children in prospective follow-up for increased risk of type 1 diabetes were included to validate the novel RBA for TPO. The results of RBA were compared with those obtained by commercial radioimmunoassay (RIA) and electrochemiluminescence (ECL). Furthermore, 476 serum samples from adult blood donors and 297 from 13-year-old school children were analyzed for the presence of TPOab and TGab.Receiver operating characteristics analysis for the novel TPOab resulted in an area under curve (AUC) value of 0.82 (p<0.0001), a sensitivity of 77.8%, and a specificity of 91.9% in adult blood donors, and an AUC value of 0.70 (p<0.0001), a sensitivity of 53.2% and a specificity of 95.3% in the 13-year-old school children, respectively. TPOab levels in RBA correlated with both ECL (r=0.8950, p<0.0001) and RIA (r=0.9295, p<0.0001). The prevalence of TPOab and TGab was 6.3% and 7.6% in adult blood donors and 2.9 and 3.7% in 13-year-old school children.In conclusion, a novel RBA for the determination of TPOab was developed and validated with current methodologies. This study also reports an increasing prevalence of thyroid autoantibodies from adolescence to adulthood.

Associations of organophosphate metabolites with thyroid hormone and antibody levels: findings from U.S. National Health and Nutrition Examination Survey (NHANES)

Studies have shown that organophosphate pesticides (OPs) exposure may disrupt thyroid endocrine functions in animal models, agricultural population, occupational workers, and work-related population. However, the relationships between OPs exposure and thyroid hormone levels in the general population are unclear. This study aimed to explore the relationships of OPs exposure with thyroid hormone and antibody levels in the general population. We analyzed a sample of 1089 US adults from the National Health and Nutrition Examination Survey (NHANES) 2001-2002. OPs exposure was estimated using measures of six non-specific dialkyl phosphate metabolites (DAPs), e.g., dimethylphosphate (DMP). Multiple linear regression models were used to examine the associations of OPs exposure with thyroid hormone and antibody levels. The medians of urinary ∑DAPs detected in males and females were 32.98 nmol/g creatinine and 40.77 nmol/g creatinine, with statistical significance (p = 0.001). After controlling for sociodemographic factors, we found that concentrations of urinary OPs metabolites were positively associated with the serum thyroid stimulating hormone (TSH) in the general US population, particularly in males; OPs metabolites were associated with the serum TgAb, tT3, fT3, and TSH. These findings showed that thyroid hormone and antibody disruption are probably associated with OPs exposure in the general population; more studies are needed to confirm our findings.

Epitopes specificity of antibodies to thyroid peroxidase in patients with Graves’ disease, Hashimoto’s thyroiditis and overlap-syndrome

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Further research that may shed light on specificity of MAbs in various TAIDs is strongly needed.

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Heterogeneity of epitopes recognized by anti-TPO in patients with HT, GD and overlap-syndrome may help to clarify specificity of MAbs in various TAIDs.

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Various MAbs in HT, GD and overlap-syndrome is different and this might be influenced by concentration of TPO in blood serum.

Background

Antibodies against thyroid peroxidase (anti-TPO) serve as clinical markers of thyroid autoimmune diseases (TAIDs). By trying to elucidate the causes of heterogeneity in autoantibody levels among patients with different TAIDs it becomes possible to clarify the pathophysiology of GD and HT.

Objective

To investigate the heterogeneity of epitopes recognized by anti-TPO in patients with Hashimoto’s thyroiditis (HT), Graves’ disease (GD) and overlap-syndrome.

Methods

We carried out a cross-sectional study on 398 patients with GD, HT and overlap syndrome and analyzed the specificity of epitopes and binding constants of TPO with monoclonal antibodies (MAbs). Ten MAbs to TPO were used, of which five were reactive with native TPO and the rest were reactive with denaturated TPO.

Results

The autoantibodies in blood serum of HT patients inhibited the binding of MAb63 more significantly than those in serum of GD patients: 59.62 % versus 54.02 %, respectively (p = 0.001). The anti-TPOs in serum of GD patients inhibited the binding of MAb77 more significantly than those in serum of HT patients: 54.36 % versus 51.13 %, respectively (p = 0.047). The binding of MAb45 was more inhibited in serum of patients with anti-TPO concentration over 1000 IU/ml (58.36 %). The blood serum of patients with overlap-syndrome showed less significant inhibition of MAb63 binding than that of patients with no overlap-syndrome: 52.47 % versus 58.81 %, respectively (p = 0.043).

Conclusion

Mapping the epitopes to TPO with the help of MAbs may improve the differential diagnosis between different thyroid autoimmunities.

Thyroid peroxidase in human endometrium and placenta: a potential target for anti-TPO antibodies

Autoimmune thyroid disease is the most common endocrine disorder during pregnancy. Thyroid autoantibodies (TAs) have been suggested to serve a role in implantation failure and spontaneous abortion. Until now, there are no data on the potential interaction of TAs with human reproductive organs. Here, we set out for the first time to test this hypothesis by studying the expression of thyroid peroxidase (TPO) at gene and protein level in human reproductive organs. Endometrial samples were taken from normal women, and placenta tissues were collected after full-term caesarian section. Expression of TPO messenger RNA (mRNA) was investigated by qRT-PCR. In addition, polyclonal anti-TPO antibodies were produced and the expression of TPO protein in mentioned tissues was evaluated by immunohistochemistry and Western blot analysis. The reactivity of anti-TPO antibody in human embryos was evaluated by immunofluorescent staining. For the first time, our study showed that TPO is expressed at gene and protein levels in endometrium and placenta. TPO expression was mainly localized to glandular and luminal epithelial cells in the endometrium. In placenta, the syncytiotrophoblasts and invasive trophoblast cells were the main cell types that expressed TPO protein. Specific band of approximately 110 kDa was observed in all endometrial and placental tissues by Western blot analysis. However, no expression of TPO protein was observed in human embryo. TPO expression in endometrium and placenta may explain higher frequency of abortion and infertility in patients with thyroid autoimmunity.

Biochemical properties of thyroid peroxidase (TPO) expressed in human breast and mammary-derived cell lines

Thyroid peroxidase (TPO) is an enzyme and autoantigen expressed in thyroid and breast tissues. Thyroid TPO undergoes a complex maturation process however, nothing is known about post-translational modifications of breast-expressed TPO. In this study, we have investigated the biochemical properties of TPO expressed in normal and cancerous human breast tissues, and the maturation process and antigenicity of TPO present in a panel of human breast tissue-derived cell lines. We found that the molecular weight of breast TPO was slightly lower than that of thyroid TPO due to decreased glycosylation and as suggest results of Western blot also shorter amino acid chain. Breast TPO exhibit enzymatic activity and isoelectric point comparable to that of thyroid TPO. The biochemical properties of TPO expressed in mammary cell lines and normal thyrocytes are similar regarding glycan content, molecular weight and isoelectric point. However, no peroxidase activity and dimer formation was detected in any of these cell lines since the majority of TPO protein was localized in the cytoplasmic compartment, and the TPO expression at the cell surface was too low to detect its enzymatic activity. Lactoperoxidase, a protein highly homologous to TPO expressed also in breast tissues, does not influence the obtained data. TPO expressed in the cell lines was recognized by a broad panel of TPO-specific antibodies. Although some differences in biochemical properties between thyroid and breast TPO were observed, they do not seem to be critical for the overall three-dimensional structure. This conclusion is supported by the fact that TPO expressed in breast tissues and cell lines reacts well with conformation-sensitive antibodies. Taking into account a close resemblance between both proteins, especially high antigenicity, future studies should investigate the potential immunotherapies directed against breast-expressed TPO and its specific epitopes.

Thyroid Autoantibodies Display both "Original Antigenic Sin" and Epitope Spreading

Evidence for original antigenic sin in spontaneous thyroid autoimmunity is revealed by autoantibody interactions with immunodominant regions on thyroid autoantigens, thyroglobulin (Tg), thyroid peroxidase (TPO), and the thyrotropin receptor (TSHR) A-subunit. In contrast, antibodies induced by immunization of rabbits or mice recognize diverse epitopes. Recognition of immunodominant regions persists despite fluctuations in autoantibody levels following treatment or over time. The enhancement of spontaneously arising pathogenic TSHR antibodies in transgenic human thyrotropin receptor/NOD.H2^h4 mice by injecting a non-pathogenic form of TSHR A-subunit protein also provides evidence for original antigenic sin. From other studies, antigen presentation by B cells, not dendritic cells, is likely responsible for original antigenic sin. Recognition of restricted epitopes on the large glycosylated thyroid autoantigens (60-kDa A-subunit, 100-kDa TPO, and 600-kDa Tg) facilitates exploring the amino acid locations in the immunodominant regions. Epitope spreading has also been revealed by autoantibodies in thyroid autoimmunity. In humans, and in mice that spontaneously develop autoimmunity to all three thyroid autoantigens, autoantibodies develop first to Tg and later to TPO and the TSHR A-subunit. The pattern of intermolecular epitope spreading is related in part to the thyroidal content of Tg, TPO and TSHR A-subunit and to the molecular sizes of these proteins. Importantly, the epitope spreading pattern provides a rationale for future antigen-specific manipulation to block the development of all thyroid autoantibodies by inducing tolerance to Tg, first in the autoantigen cascade. Because of its abundance, Tg may be the autoantigen of choice to explore antigen-specific treatment, preventing the development of pathogenic TSHR antibodies.

Thyroid peroxidase (TPO) expressed in thyroid and breast tissues shows similar antigenic properties

BACKGROUND

Thyroid peroxidase (TPO) is essential for physiological function of the thyroid gland. The high prevalence of thyroid peroxidase antibodies (TPOAbs) in patients with breast cancer and their protective role had previously been demonstrated, indicating a link between breast cancer and thyroid autoimmunity. Recently, TPO was shown to be present in breast cancer tissue samples but its antigenicity has not been analyzed.

METHODS

In this study, we investigated TPO expression levels in a series of fifty-six breast cancer samples paired with normal (peri-tumoral) tissue and its antigenic activity using a panel of well-characterized murine anti-human TPOAbs.

RESULTS

We have shown that TPO transcripts were present in both normal and cancer tissue samples, although the amounts in the latter were reduced. Additionally, we observed that TPO levels are lower in more advanced cancers. TPO protein expression was confirmed in all tissue samples, both normal and cancerous. We also found that the antigenicity of the immunodominant regions (IDRs) in breast TPO resembles that of thyroid TPO, which is crucial for effective interactions with human TPOAbs.

CONCLUSIONS

Expression of TPO in breast cancer together with its antigenic activity may have beneficial effects in TPOAb-positive breast cancer patients. However, further studies are needed to confirm the beneficial role of TPOAbs and to better understand the underlying mechanism.

A redundant role of human thyroid peroxidase propeptide for cellular, enzymatic, and immunological activity

BACKGROUND

Thyroid peroxidase (TPO) is a dimeric membrane-bound enzyme of thyroid follicular cells, responsible for thyroid hormone biosynthesis. TPO is also a common target antigen in autoimmune thyroid disease (AITD). With two active sites, TPO is an unusual enzyme, and thus there is much interest in understanding its structure and role in AITD. Homology modeling has shown TPO to be composed of different structural modules, as well as a propeptide sequence. During the course of studies to obtain homogeneous preparations of recombinant TPO for structural studies, we investigated the role of the large propeptide sequence in TPO.

METHODS

An engineered recombinant human TPO preparation expressed in Chinese hamster ovary (CHO) cells lacking the propeptide (TPOΔpro; amino acid residues 21-108) was characterized and its properties compared to wild-type TPO. Plasma membrane localization was determined by cell surface protein biotinylation, and biochemical studies were performed to evaluate enzymatic activity and the effect of deglycosylation. Immunological investigations using autoantibodies from AITD patients and other epitope-specific antibodies that recognize conformational determinants on TPO were evaluated for binding to TPOΔpro by flow cytometry, immunocytochemistry, and capture enzyme-linked immunosorbent assay. Molecular modeling and dynamics simulation of TPOΔpro comprising a dimer of myeloperoxidase-like domains was performed in order to investigate the impact of propeptide removal and the role of glycosylation.

RESULTS

The TPOΔpro was expressed on the cell surface at comparable levels to wild-type TPO. The TPOΔpro was enzymatically active and recognized by patients' autoantibodies and a panel of epitope-specific antibodies, confirming structural integrity of the two major conformational determinants recognized by autoantibodies. Faithful intracellular trafficking and N-glycosylation of TPOΔpro was also maintained. Molecular modeling and dynamics simulations were consistent with these observations.

CONCLUSIONS

Our results point to a redundant role for the propeptide sequence in TPO. The successful expression of TPOΔpro in a membrane-anchored, enzymatically active form that is insensitive to intramolecular proteolysis, and importantly is recognized by patients' autoantibodies, is a key advance for purification of substantial quantities of homogeneous preparation of TPO for crystallization, structural, and immunological studies.

Thyroid peroxidase forms thionamide-sensitive homodimers: relevance for immunomodulation of thyroid autoimmunity

Thyroid peroxidase (TPO) is the key enzyme in thyroid hormone production and a universal autoantigen in Graves’ and other autoimmune thyroid diseases. We wished to explore the expression of TPO and whether it was affected by thionamide antithyroid drugs. We studied recombinant TPO, stably expressed by a Chinese hamster ovary cell line (CHO-TPO) and transiently expressed TPO-enhanced green fluorescent protein (eGFP) and -FLAG fusion proteins. Immunoblotting of CHO-TPO cell extracts showed high-molecular weight (HMW) TPO isoforms that were resistant to reduction, as well as 110 kDa monomeric TPO. Co-immunoprecipitation and enzyme-linked-immunosorbent assay (ELISA) binding studies of FLAG- and eGFP-tagged TPO demonstrated TPO dimerisation. CHO-TPO cells cultured in methimazole (MMI) for 10 days showed a significant reduction in HMW-TPO isoforms at MMI concentrations of 1 µM and above (p < 0.01), whereas monomeric TPO expression was unchanged. We observed a similar reduction in HMW-TPO in CHO-TPO cells cultured in propylthiouracil (10 µM and above). Binding of Graves’ disease patient sera and TPO-Fabs to enzymatically active TPO that was captured onto solid phase was not abrogated by MMI. The cellular localisation of TPO in CHO-TPO cells was unchanged by MMI treatment. Our demonstration of homodimeric TPO and the reduction in HMW-TPO isoforms during thionamide treatment of CHO-TPO cells shows, for the first time, an effect of thionamides on TPO structure. This suggests a structural correlate to the effect of thionamides on TPO enzymatic activity and opens up a novel potential mechanism for thionamide immunomodulation of autoimmune thyroid disease.

Thyroid peroxidase as an autoantigen

Thyroid peroxidase (TPO) evokes high-affinity, IgG-class autoantibodies [TPO autoantibodies (TPOAbs)] and TPO-specific T cells that are markers of thyroid infiltration or implicated in thyroid destruction, respectively. A diverse repertoire of human monoclonal TPOAbs, unparalleled in other autoimmune diseases, provides invaluable probes for investigating antibody epitopes. Human TPOAbs recognize an immunodominant region comprising overlapping A and B domains on conformationally intact TPO. Amino acids recognized by TPOAbs are located in the regions with homology to myeloperoxidase (MPO) and the complement control protein (CCP) but not in the epidermal growth factor (EGF)-like region. T cells recognize epitopes in the MPO-like region but not in the CCP- or EGF-like regions in humans. Monoclonal human TPOAbs modulate processing of TPO protein to provide peptides for some T cells. A human T cell clone expressed transgenically in mice induces lymphocytic infiltration and hypothyroidism. This T cell's epitope is only generated by thyrocyte processing of endogenous TPO. Further, intact TPO expressed in vivo is also required for induction of TPOAbs in mice that resemble human autoantibodies. Overall, some TPO-specific T cells and the majority of autoantibodies in humans develop in response to TPO presented by thyroid cells, rather than to TPO released by damaged thyrocytes.

Induction of autoimmune thyroiditis and hypothyroidism by immunization of immunoactive T cell epitope of thyroid peroxidase

Autoimmune thyroiditis (AT) is characterized by a continuous inflammatory self-destructive process that eventually leads to chronic progressive dysfunction of the thyroid. In a previously established experimental AT model, C57bl/6 mice immunized with recombinant mouse thyroid peroxidase (TPO) (rmTPO) developed lymphocytic thyroiditis and anti-TPO antibody but not chronic hypothyroidism. To determine the immunodominant epitope(s) of TPO, T cell proliferation assays were performed in which rmTPO-primed lymph nodes cells were reacted with recombinant mTPO fragments or short overlapping synthetic TPO peptides. Within residue 405-849, peptide 540-559 gave the maximum proliferation response with a stimulation index more than 12. Mice immunized with peptide 540-559 developed antibody against rmTPO and native mouse TPO protein, lymphocytic thyroiditis, and hypothyroidism. In conclusion, this study demonstrated that TPO is the autoantigen for the development of lymphocyte thyroiditis and thyroid dysfunction, and peptide 540-559 is the immunodominant T cell epitope of TPO. Identification of T cell epitopes of TPO may enable the development of immunotherapy to prevent chronic hypothyroidism in AT.

The key residues in the immunodominant region 353–363 of human thyroid peroxidase were identified

Auto-antibodies (aAbs) to thyroid peroxidase (TPO) interact with a restricted immunodominant region (IDR) divided into two overlapping regions A and B. Among the five major regions structuring the IDR/B, regions 210-225, 353-363, 549-563, 713-720 and 766-775, region 353-363 constitutes an important anchor point for the binding of TPO-specific aAbs in sera from Hashimoto's and Graves' patients. We combined site-directed mutagenesis and expression of TPO mutants in stably transfected CHO cells to precisely define the critical residues in that region. By using flow cytometry and ELISA, we identified four amino acid residues, H353, D358, S359 and R361, that contribute to the interaction between human TPO and anti-TPO aAbs. This identification of these contributing amino acid residues in the IDR allowed us to more precisely depict contours of the IDR.

Human thyroid autoantigens and proteins of Yersinia and Borrelia share amino acid sequence homology that includes binding motifs to HLA-DR molecules and T-cell receptor

We previously reported that the spirochete Borrelia burgdorferi could trigger autoimmune thyroid diseases (AITD). Subsequently, we showed local amino acid sequence homology between all human thyroid autoantigens (human thyrotropin receptor [hTSH-R], human thyroglobulin [hTg], human thyroperoxidase [hTPO], human sodium iodide symporter [hNIS]) and Borrelia proteins (n = 6,606), and between hTSH-R and Yersinia enterocolitica (n = 1,153). We have now updated our search of homology with Borrelia (n = 11,198 proteins) and extended our search on Yersinia to the entire species (n = 40,964 proteins). We also searched the homologous human and microbial sequences for peptide-binding motifs of HLA-DR molecules, because a number of these class II major histocompatibility complex (MHC) molecules (DR3, DR4, DR5, DR8, and DR9) are associated with AITD. Significant homologies were found for only 16 Borrelia proteins (5 with hTSH-R, 2 with hTg, 3 with hTPO, and 6 with hNIS) and only 19 Yersinia proteins (4 with hTSH-R, 2 with hTg, 2 with hTPO, and 11 with hNIS). Noteworthy, segments of thyroid autoantigens homologous to these microbial proteins are known to be autoantigenic. Also, the hTSH-R homologous region of one Borrelia protein (OspA) contains an immunodominant epitope that others have found to be homologous to hLFA-1. This is of interest, as the hLFA-1/ICAM-1 ligand/receptor pair is aberrantly expressed in the follicular cells of thyroids affected by Hashimoto's thyroiditis. A computer-assisted search detected antigenic peptide binding motifs to the DR molecules implicated in AITD. In conclusion, our in silico data do not directly demonstrate that Borrelia and Yersinia proteins trigger AITD but suggest that a restricted number of them might have the potential to, at least in persons with certain HLA-DR alleles.

Structural and functional aspects of thyroid peroxidase

Thyroperoxidase (TPO) is the enzyme involved in thyroid hormone synthesis. Although many studies have been carried out on TPO since it was first identified as being the thyroid microsomal antigen involved in autoimmune thyroid disease, previous authors have focused more on the immunological than on the biochemical aspects of TPO during the last few years. Here, we review the latest contributions in the field of TPO research and provide a large reference list of original publications. Given this promising background, scientists and clinicians will certainly continue in the future to investigate the mechanisms whereby TPO contributes to hormone synthesis and constitutes an important autoantigen involved in autoimmune thyroid disease, and the circumstances under which the normal physiological function of this enzyme takes on a pathological role.

New insights into the conformational dominant epitopes on thyroid peroxidase recognized by human autoantibodies

Human anti-thyroperoxidase (TPO) autoantibodies (aAbs) are a major hallmark of autoimmune thyroid diseases. Their epitopes are discontinuous and mainly restricted to an immunodominant region (IDR) consisting of two overlapping regions (IDR/A and B). To shed light on the relationship between these regions, we first performed competitive studies using all available reference anti-TPO antibodies. Interestingly, we showed that human IDR/A- and B-specific anti-TPO aAbs recognized essentially the same regions on the TPO molecule. However, our data also indicated that IDR/A-specific human aAbs strongly recognized the region containing residues 599-617, whereas the IDR/B-specific aAbs bind to several regions as well as region 599-617. Next, we scanned this key region to identify the residues involved in the immunodominant autoepitope. Using peptide spot technology together with competitive ELISA experiments, we demonstrated that residues (604)ETP-DL(609) play a major role in the anti-peptide P14 epitope and that IDR/A-specific human anti-TPO aAbs, either expressed as recombinant Fab or obtained from Graves' disease patients, specifically recognize the sequences (597)FCGLPRLE(604) and (611)TAIASRSV(618). All together our data emphasize that both the IDRs involve the same surface area on human TPO, but the differential usage of one or the other regions leads to different inhibition patterns in competitive experiments. In conclusion, our data help to resolve the long-sought issue on the molecular immunology of the two IDRs on TPO and provide new clues to design efficient peptides that may be part of a combinatorial treatment aiming at delaying development of autoimmune thyroiditis when used prophylactically.

Localization of the immunodominant region on human thyroid peroxidase in autoimmune thyroid diseases: an update

Recent studies in the field of autoimmune thyroid diseases have largely focused on the delineation of B-cell auto-epitopes recognized by the main autoantigens to improve our understanding of how these molecules are seen by the immune system. Among these autoantigens which are targeted by autoantibodies during the development of autoimmune thyroid diseases, thyroid peroxidase is a major player. Indeed, high amounts of anti-thyroid peroxidase autoantibodies are found in the sera of patients suffering from Graves' disease and Hashimoto's thyroiditis, respectively hyper and hypothyroidism. Since anti-thyroid peroxidase autoantibodies from patients'sera mainly recognize a discontinuous immunodominant region on thyroid peroxidase and due to the complexity of the three dimensional structure of human thyroid peroxidase, numerous investigations have been necessary to closely localize this immunodominant region. The aim of the present review is to summarize the current knowledge regarding the localization of the immunodominant region recognized by human thyroid peroxidase-specific autoantibodies generated during the development of autoimmune thyroid diseases.

Superiority of thyroid peroxidase DNA over protein immunization in replicating human thyroid autoimmunity in HLA-DRB1*0301 (DR3) transgenic mice

Murine experimental autoimmune thyroiditis (EAT), characterized by thyroid destruction after immunization with thyroglobulin (Tg), has long been a useful model of organ-specific autoimmune disease. More recently, porcine thyroid peroxidase (pTPO) has also been shown to induce thyroiditis, but these results have not been confirmed. When (C57BL/6 x CBA)F(1) mice, recently shown to be susceptible to mouse TPO-induced EAT, were immunized with plasmid DNA to human TPO (hTPO) and cytokines IL-12 or GM-CSF, significant antibody (Ab) titres were generated, but minimal thyroiditis was detected in one mouse only from the TPO + GM-CSF immunized group. However, after TPO DNA immunization of HLA-DR3 transgenic class II-deficient NOD mice, thyroiditis was present in 23% of mice injected with TPO + IL-12 or GM-CSF. We also used another marker for assessing the closeness of the model to human thyroid autoimmunity by examining the epitope profile of the anti-TPO Abs to immunodominant determinants on TPO. Remarkably, the majority of the anti-TPO Abs was directed to immunodominant regions A and B, demonstrating the close replication of the model to human autoimmunity. TPO protein immunizations of HLA-DR3 transgenic mice with recombinant hTPO did not result in thyroiditis, nor did immunization of other mice expressing HLA class II transgenes HLA-DR4 or HLA-DQ8, with differential susceptibility to Tg-induced EAT. Moreover, our efforts to duplicate exactly the experimental procedures used with pTPO also failed to induce thyroiditis. The success of hTPO plasmid DNA immunization of DR3(+) mice, similar to our reports on Tg-induced thyroiditis and thyrotropin receptor DNA-induced Graves' hyperthyroidism, underscores the importance of DR3 genes for all three major thyroid antigens, and provides another humanized model to study autoimmune thyroid disease.

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.