Use of recombinant epitopes to study the heterogeneous nature of the autoantibodies against thyroid peroxidase in autoimmune thyroid disease

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.

Mapping of melanin-concentrating hormone receptor 1 B cell epitopes predicts two major binding sites for vitiligo patient autoantibodies

The melanin-concentrating hormone receptor 1 (MCHR1) has been identified as a B cell autoantigen in vitiligo with antibodies to the receptor detectable in binding and function-blocking assays. Two epitope domains (amino acids 1-138 and 139-298) have been previously identified. In this study, we aimed to further define the epitope specificity of MCHR1 antibodies using phage-display technology and to identify the epitopes recognised by receptor antibodies detected in MCHR1 function-blocking assays. Antibody reactivity to MCHR1 peptides 51-80, 85-98, 154-158 and 254-260 was identified by phage-display and subsequently confirmed in phage ELISA in 2/12, 5/12, 3/12 and 6/12 of vitiligo patients, respectively. The results suggest that major autoantibody epitopes are localised in the 85-98 and 254-260 amino acid regions of MCHR1 with minor epitopes in amino acid sequences 51-80 and 154-158. Antibodies with MCHR1 function-blocking activity were determined to recognise epitope 254-260, this being the first epitope to be reported as a target site for antibodies that block the function of the receptor.

Directed Mutagenesis in Region 713-720 of Human Thyroperoxidase Assigns 713KFPED717 Residues as Being Involved in the B Domain of the Discontinuous Immunodominant Region Recognized by Human Autoantibodies

Autoantibodies (aAbs) to thyroid peroxidase (TPO), the hallmark of autoimmune thyroid disease (AITD), recognize conformational epitopes restricted to an immunodominant region (IDR), divided into two overlapping domains A and B. Despite numerous efforts aimed at localizing the IDR and identifying aAb-interacting residues on TPO, only two critical amino acids, Lys(713) and Tyr(772), have been characterized. Precise and complete delineation of the other residues involved in the IDR remains to be defined. By using a recombinant anti-TPO aAb T13, we demonstrated that four regions on TPO are part of the IDR/B; one of them, located between amino acids 713 and 720, is particularly important for the binding of sera from patients suffering from AITD. To precisely define critical residues implicated in the binding of aAb to human TPO, we used directed mutagenesis and expressed the mutants in stably transfected CHO cells. Then we assessed the kinetic parameters involved in the interactions between anti-TPO aAbs and mutants by real-time analysis. We identified (i) the minimal epitope 713-717 recognized by mAb 47 (a reference antibody) and (ii) the amino acids used as contact points for two IDR-specific human monoclonal aAbs TR1.9 (Pro(715) and Asp(717)) and T13 (Lys(713), Phe(714), Pro(715), and Glu(716)). Using a rational strategy to identify complex epitopes on proteins showing a highly convoluted architecture, this study definitively identifies the amino acids Lys(713)-Asp(717) as being the key residues recognized by IDR/B-specific anti-TPO aAbs in AITD.

Autoantibodies in vitiligo patients recognize multiple domains of the melanin-concentrating hormone receptor

Previously, we reported the identification of autoantibodies against the melanin-concentrating hormone receptor 1 in patients with vitiligo. In this study, the B cell epitopes on melanin-concentrating hormone receptor 1 that are recognized by these autoantibodies have been identified. Deletion derivatives of melanin-concentrating hormone receptor 1 complementary DNA were constructed and then translated in vitro with the concomitant incorporation of [35S]-methionine into the protein products. The [35S]-labeled melanin-concentrating hormone receptor 1 derivatives were subsequently used in radio-binding assays to investigate the reactivity of sera from nine vitiligo patients that were known to contain antibodies to the receptor. Analysis of the results obtained in the radio-binding assays suggested the existence of multiple antibody binding sites on melanin-concentrating hormone receptor 1, including regions between amino acids 1 to 138 and 139 to 298. Several patients exhibited autoantibodies to more than one melanin-concentrating hormone receptor 1 epitope indicating a heterogeneous humoral response to the receptor. Computer prediction of the potential B cell epitopes on melanin-concentrating hormone receptor 1 revealed that the epitope domains identified overlapped, at least in part, with regions predicted to be highly antigenic.

Localization of the discontinuous immunodominant region recognized by human anti-thyroperoxidase autoantibodies in autoimmune thyroid diseases

The discontinuous immunodominant region (IDR) recognized by autoantibodies directed against the thyroperoxidase (TPO) molecule, a major autoantigen in autoimmune thyroid diseases, has not yet been completely localized. By using peptide phage-displayed technology, we identified three critical motifs, LXPEXD, QSYP, and EX(E/D)PPV, within selected mimotopes which interacted with the human recombinant anti-TPO autoantibody (aAb) T13, derived from an antibody phage-displayed library obtained from thyroid-infiltrating TPO-selected B cells of Graves' disease patients. Mimotope sequence alignment on the TPO molecule, together with the binding analysis of the T13 aAb on TPO mutants expressed by Chinese hamster ovary cells, demonstrated that regions 353-363, 377-386, and 713-720 from the myeloperoxidase-like domain and region 766-775 from the complement control protein-like domain are a part of the IDR recognized by the recombinant aAb T13. Furthermore, we demonstrated that these regions were involved in the binding to TPO of sera containing TPO-specific autoantibodies from patients suffering from Hashimoto's and Graves' autoimmune diseases. Identification of the IDR could lead to improved diagnosis of thyroid autoimmune diseases by engineering "mini-TPO" as a target autoantigen or designing therapeutic peptides able to block undesired autoimmune responses.

Localization of the thyroid peroxidase autoantibody immunodominant region to a junctional region containing portions of the domains homologous to complement control protein and myeloperoxidase

Thyroid peroxidase (TPO) autoantibody epitopes are largely restricted to an immunodominant region (IDR) on the extracellular region of the native molecule. Localization of the IDR has been a longstanding and difficult goal. The TPO extracellular region comprises a large myeloperoxidase-like domain, linked to the plasma membrane by two smaller domains with homology to complement control protein (CCP) and epidermal growth factor (EGF), respectively. Recent studies have focused on the CCP- and EGF-like domains as the putative location of the TPO autoantibody IDR. To address this issue, we attempted to express on the surface of transfected cells native TPO in which the CCP- and EGF-like domains were deleted, either together or individually. We used a quartet of human monoclonal autoantibodies that define the TPO IDR, as well as polyclonal TPO autoantibodies in patients' sera, to detect these mutated TPO molecules by flow cytometry. The combined CCP/EGF-like domain deletion did not produce a signal with TPO autoantibodies but did not traffic to the cell surface. In contrast, both monoclonal and polyclonal autoantibodies recognized TPO with the juxtamembrane EGF-like domain deleted equally as well as the wild-type TPO on the cell surface. TPO with the CCP-like domain deleted expressed normally on the cell surface, as determined using the polyclonal mouse antiserum. Nevertheless, this modified TPO molecule was recognized very poorly by both the human monoclonal autoantibodies and the polyclonal autoantibodies in patients' sera. In conclusion, we have clearly excluded the juxtamembrane EGF-like domain as being part of the IDR. In contrast, a component of the CCP-like domain does contribute to the IDR. These data, together with findings from other studies, localize the TPO autoantibody IDR to the junction of the CCP-like domain and the much larger myeloperoxidase-like domain on TPO.

Identification of antigenic domains on the human sodium-iodide symporter which are recognized by autoantibodies from patients with autoimmune thyroid disease

The sodium-iodide symporter (NIS) is a novel autoantigen in autoimmune thyroid disease. In the present study we have characterized the antigenic domains on the human symporter which are recognized by autoantibodies from patients with either Graves' disease (GD) or autoimmune hypothyroidism (AH). Deletion derivatives of complementary DNA (cDNA) encoding the Na(+)/I(-) symporter were constructed using polymerase chain reaction (PCR) amplification. These deletion constructs were translated in vitro with the concomitant incorporation of [(35)S]methionine into the protein products. The reactivity of seven GD and six AH sera, which were known to contain symporter-binding antibodies, to each of the radiolabelled modified symporters was then determined in immunoprecipitation experiments. Analyses of the results obtained in the radiobinding assays suggest the existence of multiple antibody binding sites on human NIS (hNIS), including regions between amino acids (aa) 1--134, 191--286, 290--411, 411--520 and 520--588. Computer prediction of the potential B cell epitopes on the symporter revealed that, apart from aa 134--191, all the epitope domains identified overlapped, at least in part, with areas predicted to be highly antigenic. Interestingly, the antigenic domains represented by aa 191--286, 290--411 and 411--520 include regions of the polypeptide which form putative extracellular domains in the secondary structure model of the rat symporter. No correlation between the recognition of specific epitopes on the human symporter and the type of autoimmune thyroid disease was demonstrated.

Human monoclonal autoantibodies to B-cell epitopes outside the thyroid peroxidase autoantibody immunodominant region

Human autoantibodies to thyroid peroxidase (TPO) interact with a restricted or immunodominant region (IDR) on intact TPO. However, a smaller proportion of polyclonal serum TPO autoantibodies bind outside this region. To isolate monoclonal nonimmunodominant region (non-IDR) TPO autoantibodies, we screened a thyroid-derived immunoglobulin gene phage display library while "epitope masking" the TPO IDR with four human TPO monoclonal autoantibodies that define the IDR. Among 31 non-IDR autoantibodies obtained (expressed as Fab), 8 representatives were analyzed further based on their restriction digestion profiles. All are encoded by almost identical H chains (VH3 family), with extremely long D regions, paired with three different types of light chains. In contrast, IDR TPO Fab from the same patient utilize seven different heavy chains (VH1 and VH5 families) paired nonpromiscuously with different light chains. Use of VH5 genes has not been reported previously for TPO autoantibodies. Both non-IDR and IDR Fab bind specifically to TPO and not to other proteins. The non-IDR Fab affinities for TPO are moderately high (Kd 1-2 x 10(-9) M), somewhat lower than those for most IDR Fab (Kd 1-4 x 10(-10) M). The epitopes of the three types of non-IDR Fab overlap with each other, indicating a major role for their heavy chain in TPO binding. Most importantly, the epitopes of non-IDR Fab are recognized by patients' serum autoantibodies. In summary, we provide the first insight into the immunoglobulin genes, affinities and epitopes of human monoclonal autoantibodies that bind outside the TPO-immunodominant region.

Does the autoantibody immunodominant region on thyroid peroxidase include amino acid residues 742-771?

Identification of the thyroid peroxidase (TPO) amino acid residues that comprise the autoantibody immunodominant region is an important goal that has proven difficult because of the conformational nature of the epitopes involved. Recent data suggest that the immunodominant region has been located. Thus, by autoantibody recognition of tryptic fragments of native TPO, as well as of conformational portions of TPO expressed as cell-free translates, the autoantibody immunodominant region appears to include amino acid residues 742-771, near the C terminus of the ectodomain. To evaluate this deduction, we expressed as cell-free translates the full TPO ectodomain, as well as TPO truncated after residues 741 and 771. The epitopic integrity of these molecules was first confirmed by immunoprecipitation by patient sera containing TPO autoantibodies. However, autoantibody recognition could involve a minority of TPO autoantibodies with the individual sera, not fulfilling the strict criteria for immunodominance. In order to obtain definitive data, we performed immunoprecipitations on these TPO variants with four recombinant human monoclonal autoantibodies that define the immunodominant region. All four monoclonal autoantibodies immunoprecipitated TPO 1-741 to the same extent as they did TPO 1-771 and the full TPO ectodomain, indicating that the immunodominant region comprises (at least in large part) amino acid residues upstream of residue 741.

Identification of an immunodominant region recognized by human autoantibodies in a three-dimensional model of thyroid peroxidase

Autoimmune thyroid diseases (AITD) are characterized by the presence of autoantibodies to thyroid peroxidase (TPO). This response is dominated by autoantibodies to two conformational determinants, termed A and B, that have been defined by monoclonal antibodies but whose structures and location within TPO are unknown. We have modeled the three-dimensional structure of the extracellular region of TPO, raised antisera to prominent surface structures, and identified an epitope that we show to be a critical part of the B determinant. Antibodies to this epitope inhibit the binding to TPO of human autoantibodies in virtually all serum samples from 65 patients with AITD that were tested. This first description of a model of the three-dimensional structure and location of a major autoantigenic determinant within the TPO molecule may provide structural clues for identifying causative agents or developing novel therapeutic strategies.

Distinct immunological and biochemical properties of thyroid peroxidase purified from human thyroid glands and recombinant protein produced in insect cells

The biosynthesis of thyroid hormone from thyroglobulin is catalysed by thyroid peroxidase (TPO), an integral membrane protein. TPO is also a major autoantigen in autoimmune thyroid disease and autoantibodies to TPO are markers for disease activity. Large quantities of purified TPO are essential for elucidating its structure and understanding its role in disease activity. We describe the high yield purification of full-length recombinant human TPO from baculovirus infected insect cells and compare it to purified native TPO from human thyroid glands. In contrast to native human TPO, the human TPO produced in insect cells as a recombinant protein was insoluble and resistant to solubilisation in detergents. Reversible substitution of lysine residues with citraconic anhydride led to increased solubility of the recombinant TPO, allowing high-yield purification by monoclonal antibody chromatography. The purified enzyme preparation was shown to be TPO by its reactivity with monoclonal and polyclonal antibodies by enzyme linked immunosorbent assay and Western blotting. Both the human and recombinant purified TPO preparations also react with sera from patients with autoimmune thyroid disease, although the binding of conformational dependent autoantibodies was considerably lower to the recombinant TPO than to the native TPO. This suggests that the recombinant TPO may differ in some aspects of its tertiary structure. The purified recombinant TPO was devoid of enzyme activity, in contrast to the enzymatically active, purified human TPO preparations. Both preparations contained comparable amounts of haem (R(z)=0.269), but a shift in the Soret band of recombinant TPO (402 nm) from that of natural TPO (409 nm) indicates that the lack of enzymatic activity of the recombinant enzyme may be due to changes in the protein backbone surrounding the haem. Both the purified native and recombinant TPO, under non-denaturing conditions, show evidence of high molecular mass oligomers, although the latter preparation is prone to a greater degree of aggregation. In conclusion, our studies indicate that recombinant TPO generated in insect cells is conformationally distinct from the native TPO, is insoluble and enzymatically inactive, consistent with the difficulties associated with its purification and crystallisation.

Rarity of autoantibodies to a major autoantigen, thyroid peroxidase, that interact with denatured antigen or with epitopes outside the immunodominant region

The nature of the autoantibody repertoire to the dominant autoantigen in human autoimmune thyroid disease is controversial. There is evidence that autoantibodies to thyroid peroxidase (TPO) interact with overlapping conformational epitopes in an immunodominant region and binding to denatured (DN) protein is decreased. Contrary data demonstrate TPO autoantibody reactivity with DN-TPO or polypeptide fragments. However, none of the TPO-specific, human monoclonal autoantibodies isolated to date preferentially recognize denatured autoantigen. We therefore searched an immunoglobulin gene phage display library for human autoantibodies that bind TPO denatured by reduction and alkylation (DN-TPO). Thyroid-infiltrating B cells from a typical TPO autoantibody-positive patient were the source of mRNA for library construction. Surprisingly, the library enriched after panning on DN-TPO, as well as a panel of individual clones, preferentially bound native (N)-TPO. Of 13 clones selected using DN-TPO or N-TPO, 12 clones recognized the TPO immunodominant region. Moreover, regardless of selection with N-TPO or DN-TPO, their heavy and light chains were encoded by similar VDJ and Vkappa combinations. One clone (DN4), isolated using DN-TPO, did not interact with the TPO immunodominant region and its H chain derives from a different VH gene. Although DN4 binds specifically to TPO, its affinity is low, unlike the high affinities of other human TPO autoantibodies. In conclusion, human monoclonal autoantibodies that preferentially recognize denatured TPO could not be isolated from an immunoglobulin gene library despite selection with denatured protein. Our findings demonstrate the bias of the human B cell repertoire towards recognition of an immunodominant region on the conformationally intact form of a major thyroid autoantigen.

A conformational B-cell epitope on the C-terminal end of the extracellular part of human thyroid peroxidase

To investigate the B-cell autoimmune epitopes on human thyroid peroxidase (TPO), we generated proteolytic peptides by enzymatic hydrolysis of TPO in nondenaturing and nonreducing conditions. The hydrolysate was chromatographed on a reverse phase column. We eluted a material immunoreactive with both a TPO monoclonal antibody recognizing a linear epitope (mAb47, amino acid 713-721) and TPO autoantibodies (aAb) from patients. The aAb immunoreactivity, but not that of mAb47, was lost after reduction. Western blots after electrophoresis without reduction showed that the aAb and mAb47 were immunoreactive with a 66-kDa band and that aAb identified a doublet at 20 kDa. For electrophoresis under reducing conditions, the 66-kDa band resolved into two peptides of 40 and 26 kDa, whereas the doublet at 20 kDa remained unchanged. None of these reduced peptides was immunoreactive with aAb, whereas the 40-kDa peptide was immunoreactive with mAb47. The 40-kDa peptide extends from amino acid 549 to 933 of TPO, and its last 192 amino acids overlap the autoimmune 20-kDa peptide. After iodine labeling, the 20-kDa peptide lost its immunoreactivity. We conclude that the C-terminal end of the extracellular part of TPO, which includes all the tyrosine residues of the 20-kDa peptide, contains at least one conformational B-cell epitope involved in autoimmune thyroid diseases.

Mapping of B cell epitopes on steroid 17 alpha-hydroxylase, an autoantigen in autoimmune polyglandular syndrome type I

Earlier studies have shown that 17 alpha-hydroxylase (P450c17), steroid 21-hydroxylase (P450c21) and side-chain cleavage enzyme (P450scc) are the main autoantigens recognized by sera from patients with Addison's disease associated with autoimmune polyglandular syndrome type I (APS I). In this study we tried to identify the autoantigenic epitopes on P450c17 and compared the identified sequences with corresponding regions in two other adrenal autoantigens, P450scc and P450c21. A series of P450c17 cDNA fragments was expressed in Escherichia coli and the recognition of the corresponding protein fragments by patient sera was tested by immunoblotting. Four distinct epitope regions (ER) were found: ER1 (amino acids 122-148), ER2 (280-304), ER3 (396-432) and ER4 (466-508). B cell epitope prediction analysis showed that the four identified ERs were all located in regions of high predicted antigenicity. Homology search revealed that ER3 and ER4 but not ER1 and ER2 were related to similar sequences on P450c21. No significant homologies with P450scc in these regions were found. Although all three P450 cytochromes are genuine autoantigens this finding suggests that the autoantibody reaction against P450c17 and P450c21 can partly be a result of immunological cross-reactivity.

Thyroid autoimmunity

Antigenic structure remains a major focus in thyroid immunology. The genes for three major thyroid antigens--thyroglobulin, thyroid peroxidase and the thyrotropin receptor--were sequenced in the late 1980's, and epitopes for antibody and T cells have been reported within the last year. In addition, new evidence for selective use of T-cell receptor V gene segments in human thyroid infiltrates may point the way to specific immunotherapy.