Relationship between autoantibody epitopic recognition and immunoglobulin gene usage

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.

Antibodies focused on the human autoantibody immunodominant region are induced by B lymphocytes that constitutively express thyroid peroxidase diverted to the major histocompatibility complex II pathway

We addressed the question of why naturally occurring, polyclonal thyroid peroxidase (TPO) autoantibodies have a restricted epitopic repertoire to an immunodominant region (IDR). We hypothesized that immunizing BALB/c mice with major histocompatibililty complex (MHC) class II compatible B lymphocytes (A20 cells) preferentially diverting TPO to the MHC class II pathway would produce TPO antibodies with an epitopic specificity similar to human autoantibodies, namely to the IDR. For this purpose we stably expressed in A20 cells a fusion protein of TPO sandwiched between the signal peptide and transmembrane/cytoplasmic tail of the lysosome- associated membrane protein (LAMP) 1. Expression of LAMP1-TPO in A20 B cells was confirmed by flow cytometry using a TPO monoclonal antibody. Mice injected intraperitoneally with LAMP1-TPO A20 B cells developed TPO antibodies detected by enzyme-linked immunosorbent assay (ELISA), flow cytometry and (125)I-TPO precipitation. However, TPO antibody levels were low. Most important, competition for TPO antibody binding by recombinant human TPO autoantibody Fab indicated that more than 80% of the polyclonal TPO antibodies in the immunized mice were to the human autoantibody IDR. In summary, injecting mice with B lymphocytes that constitutively express TPO diverted to the MHC class II pathway generates antibodies with epitopes similar to those of human TPO autoantibodies, namely to the autoantibody IDR. However, these antibodies are of low titer that is itself associated with this epitopic bias.

Relationship between thyroid peroxidase T cell epitope restriction and antibody recognition of the autoantibody immunodominant region in human leukocyte antigen DR3 transgenic mice

We investigated the relationship between thyroid peroxidase (TPO) antibody and T lymphocyte epitopes in TPO-adenovirus (TPO-Ad) immunized BALB/c mice and mice transgenic for the human class II molecule DR3 associated with human thyroid autoimmunity. TPO autoantibodies are largely restricted to an immunodominant region (IDR). BALB/c mice immunized with fewer (10(7) vs. 10(9)) TPO-Ad particles developed TPO antibodies with lower titers that displayed greater restriction to the IDR. However, as with higher-dose TPO-Ad immunization, T cell epitopes (assessed by splenocyte interferon-gamma response to TPO in vitro) were highly diverse and variable in different animals. In contrast, DR3 mice immunized the higher TPO-Ad dose (10(9) particles) had high TPO antibody levels that showed relative focus on the IDR. Moreover, T cell epitopes recognized by splenocytes from DR3 mice showed greater restriction than BALB/c mice. Antibody affinities for TPO were higher in DR3 than in BALB/c mice. The present study indicates that weak TPO-Ad immunization of BALB/c mice (with consequent low TPO antibody titers) is required for enhanced IDR focus yet is not associated with T cell epitopic restriction. Humanized DR3 transgenic mice, despite stronger TPO-Ad immunization, develop higher titer TPO antibodies that do focus on the autoantibody IDR with T cells that recognize a more limited range of TPO peptides. These data suggest a relationship between major histocompatibility complex class II molecules and the development of antibodies to the IDR, a feature of human thyroid autoimmunity.

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.

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.

Human autoantibodies modulate the T cell epitope repertoire but fail to unmask a pathogenic cryptic epitope

Abs can tune the responses of Ag-specific T cells by influencing the nature of the epitope repertoire displayed by APCs. We explored the interaction between human self-reactive T cells and human monoclonal autoantibodies from combinatorial Ig-gene libraries derived from autoimmune thyroiditis patients and specific for the main autoantigen thyroid peroxidase (TPO). All human mAbs extensively influenced the T cell epitope repertoire recognized by different TPO-specific T cell clones. The action of the human mAbs was complex, because sometimes the same Ab suppressed or enhanced the epitopes recognized by the 10 different TPO-specific T cell clones. The human mAbs could modulate the epitope repertoire when TPO was added exogenously and when expressed constitutively on the surface of APCs. However, they could not unmask an immunodominant cryptic TPO epitope. In this study, we show that human autoantibodies influence the activity of self-reactive T cells and prove their relevance in concealing or exposing epitopes recognized by self-reactive T cells. However, our results further stress the biological significance of the immunodominant cryptic epitope we have defined and its potential importance in the evolution of autoimmunity.

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.

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.

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.

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

Autoantibodies to thyroid peroxidase (TPO) recognize predominantly conformational epitopes, which are restricted to two distinct determinants, termed immunodominant domain region (IDR) A and B. These dominant determinants reside in the region with structural homology to myeloperoxidase (MPO)-like domain and may extend into the adjacent complement control protein (CCP) domain. We have explored the location of these determinants on the MPO-like domain of the structural model of TPO, by identifying exposed hydrophilic loops that are potential candidates for the autoantigenic sites, generating rabbit antipeptide antisera, and competing with well characterized murine monoclonal antibodies (mabs) specific for these two IDRs. We recently defined the location of IDR-B, and here report our findings on the location of IDR-A and its relationship to IDR-B, defined with a new panel of 15 antipeptide antisera. Moreover, in combination with single amino acid replacements by in vitro mutagenesis, we have defined the limits of the IDR-B region on the TPO model. The combination of antisera to peptides P12 (aa 549-563), P14 (aa 599-617) and P18 (aa 210-225) inhibited the binding of the mab specific for IDR-A (mab 2) by 75%. The same combination inhibited the binding of autoantibodies to native TPO from 67 to 94% (mean 81.5%) at autoantibody levels of 5 IU. Fabs prepared from the antipeptide IgG and pooled in this combination were also effective in competition assays, thus defining the epitopes more precisely. IDR-A was found to lie immediately adjacent to IDR-B and thus the two immunodominant epitopes form an extended patch on the surface of TPO. Finally, by single amino acid mutagenesis, we show that IDR-B extends to residue N642, thus further localizing the boundary of this autoantigenic region on the structural model.

Insight into antibody responses induced by plasmid or adenoviral vectors encoding thyroid peroxidase, a major thyroid autoantigen

Plasmid and adenoviral vectors have been used to generate antibodies in mice that resemble human autoantibodies to the thyrotrophin receptor. No such studies, however, have been performed for thyroid peroxidase (TPO), the major autoantigen in human thyroiditis. We constructed plasmid and adenovirus vectors for in vivo expression of TPO. BALB/c mice were immunized directly by intramuscular injection of TPO-plasmid or TPO-adenovirus, as well as by subcutaneous injection of dendritic cells (DC) infected previously with TPO-adenovirus. Intramuscular TPO-adenovirus induced the highest, and TPO-plasmid the lowest, TPO antibody titres. Mice injected with TPO-transfected DC developed intermediate levels. Antibodies generated by all three approaches had similar affinities (Kd approximately 10(-9)M) and recognized TPO expressed on the cell-surface. Their epitopes were analysed in competition assays using monoclonal human autoantibodies that define the TPO immunodominant region (IDR) recognized by patients with thyroid autoimmune disease. Surprisingly, high titre antibodies generated using adenovirus interacted with diverse TPO epitopes largely outside the IDR, whereas low titre antibodies induced by DNA-plasmid recognized restricted epitopes in the IDR. This inverse relationship between antibody titre and restriction to the IDR is likely to be due to epitope spreading following strong antigenic stimulation provided by the adenovirus vector. However, TPO antibody epitope spreading does not occur in Hashimoto's thyroiditis, despite high autoantibody levels. Consequently, these data support the concept that in human thyroid autoimmunity, factors besides titre must play a role in shaping an autoantibody epitopic profile.

Human thyroperoxidase folds in one complex B-cell immunodominant region

Human thyroperoxidase (TPO) ectodomain is successively made of myeloperoxidase-, complement control protein repeat-, and epidermal growth factor-like gene modules. However, the TPO immunodominant region targeted by autoantibodies from patients with an autoimmune thyroid disease has not been mapped on the molecule. Here, we used two purified recombinant TPO peptides produced in eukaryotic cells, which correspond to the major first and the further two gene modules of TPO. We compared by ELISA their respective immunoreactivity with that of the recombinant soluble TPO containing all the three gene modules. We used well-characterized murine and human TPO monoclonal antibodies and human autoantibodies affinity-purified from a large pool of patients' sera. We found that the TPO immunodominant region was susceptible to denaturation and required the integrity of the molecule to be correctly expressed. We concluded that TPO B-cell autoepitopes are made by amino acids from the three gene modules, which fold in one highly conformational immunodominant region.

Analysis of a conformational B cell epitope of human thyroid peroxidase: identification of a tyrosine residue at a strategic location for immunodominance

Thyroid peroxidase (TPO) is involved in autoimmune thyroid diseases and high titers of TPO autoantibodies directed to various conformational B cell epitopes are frequently present in patients' sera. Deciphering these epitopes is a difficult task, but can give insight into the structural basis of autoimmune recognition. TPO is a membrane-bound enzyme with the extracellular part organized in three protein domains, but of unknown three-dimensional structure. We previously localized a TPO B cell epitope within amino acid residues 742-848, a region encompassing the two C-terminal, extracellular domains of the protein. We found that at least one of the three tyrosine residues of the peptide 742-848 might be involved in autoantibody binding. In this study, we show by site-directed mutagenesis that the autoepitope contains tyrosine 772 located near the hinge area between the two protein domains, suggesting they are both involved in the epitope structure. The B cell epitopes of TPO are clustered in two overlapping immunodominant regions. To map the newly localized epitope with respect of these regions, competition experiments were performed using a reference panel of TPO mAb and a further mAb previously found to be specific for the TPO peptide 742-848 at variance with all the other ones. Here, we show that the tyrosine 772-bearing epitope in the peptide 742-848 maps in a region that partly overlaps the reported two immunodominant regions. These results are suggestive of a complex TPO folding that involves all the three TPO protein domains to form a highly conformational immunodominant region.

Recombinant anti-idiotypic antibodies inhibit human natural anti-glycoprotein (GP)IIb/IIIa autoantibodies

Anti-idiotypic antibodies (anti-Id) have been described against idiotypes expressed on various autoantibodies. Since an immunoregulatory effect has been postulated for anti-Id, modulation of the anti-Id response in autoimmune disease may be of interest. In chronic immune thrombocytopenic purpura (AITP), autoantibodies directed mainly against platelet membrane glycoprotein (GP) IIb/IIIa cause platelet destruction by Fc-mediated phagocytosis or by complement lysis. We have previously reported on the generation of two recombinant anti-GPIIb/IIIa autoantibody fragments (PDG-X, PDG-B), that are specific for conformationally intact GPIIb/IIIa and inhibit binding of autoantibodies from patients with AITP. In the present study, we show that anti-GPIIb/IIIa specificities are not limited to a single individual by isolating five additional anti-GPIIb/IIIa autoantibody fragments from a second phagemid Fab library of an unrelated healthy donor. Using soluble Fab of PDG-X and PDG-B as antigens for panning Fab phagemid libraries from healthy human individuals, we isolated anti-Id phage clones specific for PDG-X or PDG-B. In addition they inhibited the binding of PDG-X or PDG-B to GPIIb/IIIa. Amino acid sequence comparison between these specific antiId and GPIIb/IIIa was performed. Generation of these anti-Id directed against pathologically relevant anti-GPIIb/IIIa autoantibodies may represent a new suitable and specific therapeutic option for the treatment of antibody-mediated AITP.

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.

Thyroid autoimmune disease: demonstration of thyroid antigen-specific B cells and recombination-activating gene expression in chemokine-containing active intrathyroidal germinal centers

Autoimmune thyroid disease--Hashimoto thyroiditis and Graves' disease--patients produce high levels of thyroid autoantibodies and contain lymphoid tissue that resembles secondary lymphoid follicles (LFs). We compared the specificity, structure, and function of tonsil and lymph node LFs with those of the intrathyroidal LFs to assess the latter's capability to contribute to autoimmune response. Thyroglobulin and thyroperoxidase binding to LFs indicated that most intrathyroidal LFs were committed to response to thyroid self-antigens and were associated to higher levels of antibodies to thyroglobulin, thyroperoxidase, and thyroid-stimulating hormone receptor. Intrathyroidal LFs were microanatomically very similar to canonical LFs, ie, they had well-developed germinal centers with mantle, light, and dark zones and each of these zones contained B and T lymphocytes, follicular dendritic and interdigitating dendritic cells with typical phenotypes. Careful assessment of proliferation (Ki67) and apoptosis (terminal dUTP nick-end labeling) indicators and of the occurrence of secondary immunoglobulin gene rearrangements (RAG1 and RAG2) confirmed the parallelism. Unexpected high levels of RAG expression suggested that receptor revision occurs in intrathyroidal LFs and may contribute to generate high-affinity thyroid autoantibodies. Well-formed high endothelial venules and a congruent pattern of adhesion molecules and chemokine expression in intrathyroidal LFs were also detected. These data suggest that ectopic intrathyroidal LFs contain all of the elements needed to drive the autoimmune response and also that their microenvironment may favor the expansion and perpetuation of autoimmune response.

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.

Autoimmune response to the thyroid in humans: thyroid peroxidase--the common autoantigenic denominator

Autoimmunity to thyroid peroxidase (TPO), manifest as high affinity IgG class autoantibodies, is the common denominator of human thyroid autoimmunity, encompassing patients with overt hyper- or hypothyroidism as well as euthyroid individuals with subclinical disease. The identification and cloning of TPO (the "thyroid microsomal antigen") provided the critical tool for analyzing B and T cell reactivity to this major thyroid autoantigen. In particular, the availability of immunoreactive TPO permitted the isolation of essentially the entire repertoire of human monoclonal antibodies, a feat unparalled in an organ-specific autoimmune disease. These recombinant autoantibodies (expressed as Fab) provide insight into the genes encoding their H and L chains as well as the conformational epitopes on TPO with which serum autoantibodies interact. Analyses of TPO autoantibody epitopic "fingerprints" indicate a lack of epitope spreading as well as a genetic basis for their inheritance. Limited data are available for the responses and cytokine profiles of T cells to endogenously processed TPO. Moreover, the role of thyroid cells in initiating the autoimmune response to TPO, and of B cells in expanding and/or modulating the response of sensitized T cells, has yet to be established. Finally, because autoantibody (and likely T cell) responses to TPO parallel those to TSH receptor and thyroglobulin, manipulation of T and B cell responses to TPO may provide the basis for the development of immunospecific therapy for autoimmune thyroid disease in general.

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.

Human thyroid peroxidase: mapping of autoantibodies, conformational epitopes to the enzyme surface

The enzyme, thyroid peroxidase (TPO), is a dominant antigen in thyroid autoimmune diseases. Autoantibodies recognised two major dominant conformational epitopes termed A and B. The epitopes have been defined by mAbs, but the amino acid residues which constitute these determinants remain unknown. Using a model of TPO, built from the structure of myeloperoxidase (MPO), we have synthesised peptides corresponding to exposed loops and generated rabbit antibodies to the peptides. Antisera to peptide sequence 599-617 (peptide 14) representing a highly protrusive loop on the TPO, showed the highest inhibition in 65 sera from patients positive with anti-TPO antibodies. The inhibition was by 15-80% (mean 41%), and no other antibody showed any inhibition. Binding of hFabs to the B determinant on TPO was inhibited by anti-peptide 14 antibodies more then 85%, but not Fabs to the A determinant. In conclusion, the peptide 14 defines a sequence taking part in building up the B major conformational epitope. None of generated anti-peptide antibodies alone inhibited the binding of human Fabs to the A epitope, however a combination of four anti-peptide antibodies (P1, P12, P14 and P18) inhibits Fabs binding to the A determinant by more then 60% and autoantibodies binding from 65% to 94%. Combination of antibodies reacting with peptides outside the surface defined by those four antipeptide antibodies did not give any inhibition of Fabs to TPO. The inhibition of Fabs and auto Abs to TPO by this combination of anti-peptide Abs is the result of steric hindrance as none of these Abs individually inhibited auto Abs' or Fabs' binding to TPO. The four peptides define an area on the enzyme surface where the A and B major conformational epitopes are localised.

Search for the autoantibody immunodominant region on thyroid peroxidase: epitopic footprinting with a human monoclonal autoantibody locates a facet on the native antigen containing a highly conformational epitope

Autoantibodies to thyroid peroxidase (TPO) are the hallmark of the humoral autoimmune response in human autoimmune thyroiditis (Hashimoto's thyroiditis). The majority of TPO autoantibodies in individual patients' sera interact with a restricted immunodominant region on TPO. Although this region can be mapped, previous studies have failed to localize its position on the TPO molecule. We, therefore, used a footprinting approach that can localize a highly conformational, discontinuous epitope on a very large molecule. Extensive biotinylation ( approximately 15 biotins/molecule protein) of lysine residues on the surface of purified, native TPO resulted in loss of multiple tryptic cleavage sites, as determined by analysis of tryptic polypeptide fragments on reverse-phase HPLC. TPO was then complexed with a monoclonal human autoantibody Fab (TR1.9) before biotinylation. After dissociation from TR1.9, TPO was recovered by gel filtration. A trypsin site, previously observed to be lost after TPO biotinylation, was restored when biotinylation was performed on the TPO-TR1.9 complex. The epitope-protected lysine (K) was present in a 30-aa TPO fragment that, by N-terminal sequencing, was found to be K713. Altered recognition by TR1.9 of a TPO-myeloperoxidase chimeric molecule involving this region supported the epitope protection data. In conclusion, we provide the first identification of an amino acid residue (K713) comprising part of an epitope within the TPO immunodominant region. This focal residue localizes the facet on the large, highly complex TPO molecule that contains the immunodominant region and provides the basis for rational guided mutagenesis studies to more fully characterize this region.

IVIG-bound IgG and IgM cloned by phage display from a healthy individual reveal the same restricted germ-line gene origin as in autoimmune thrombocytopenia

Intravenous immunoglobulin preparations (IVIG) have shown positive effects in the treatment of immune defects and autoimmune diseases. It is not clear how IVIG interacts with the components of the immune system. To investigate this, we cloned previously a large number of phage displayed IgG Fab fragments derived from three patients with autoimmune thrombocytopenia (AITP) that were specifically bound by IVIG molecules. Many of these Fabs reacted with platelets. Sequencing revealed that the most frequently used germ-line gene segments of all IVIG-bound Fabs were identical to those observed for many other autoantibodies. Particularly, the loci 3-30 or 3-30/3-30. 5, 3-23 and 3r, 3l, and 2a2 represented the most abundant genes used for the heavy (VH) and light chain V region (VL), respectively. This suggested a specific interaction of IVIG molecules with B cells that present B cell receptors derived from these germ-line genes. In the current study we determined the genetic origin of IVIG-reactive IgG and IgM cloned from a healthy person. A favoured selection of antibodies derived from the same germ-line origins as in AITP was observed. Because 3-30 and 3-23 are the most frequently rearranged VH germ-line gene segments among human B cells, our results suggest that this favoured anti-idiotypic interaction may have an important role for the development and control of the normal B cell repertoire.

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.

Human anti-thyroid peroxidase single-chain fragment variable of Ig isolated from a combinatorial library assembled in-cell: insights into the in vivo situation

In an attempt to explore the natural variable heavy and light chain (VH/VL) pairing of autoantibodies involved in Graves' disease, we constructed a phage-displayed Ab library obtained by in-cell PCR of thyroid-infiltrating cells. We report here the molecular cloning and characterization of human single-chain fragment variable regions (scFv) specific for thyroid peroxidase (TPO) generated from this library. On the basis of the nucleotide sequences, three different scFvs were obtained (ICA1, ICB7, and ICA5). All were encoded by genes derived from the VH1 and Vlambda1 gene families. Using BIACORE for epitope mapping and kinetic analysis, we showed that these scFvs exhibited high affinity (Kd = 1 nM) for TPO and recognized three different epitopes. The biological relevance of these scFvs as compared with serum anti-TPO autoantibodies was assessed by competition studies. Sera from all the 29 Graves' disease patients tested were able to strongly inhibit (60-100%) the binding of the 3 scFvs to TPO. These data demonstrate that the in-cell PCR library generated human anti-TPO scFvs that retained the VH/VL pairing found in vivo and that the different epitope specificities defined by these scFvs overlapped with those found in the sera of patients with autoimmune thyroid disease.

Analysis of autoantibody epitopes on human thyroid peroxidase

A number of studies have indicated that the major autoantibody epitopes on human thyroid peroxidase (TPO) are conformational and are formed by two overlapping immunodominant regions on the TPO molecule. In order to investigate further autoantibody reactivity with TPO, we have studied the TPO binding characteristics of sera from patients with autoimmune thyroid disease (n = 20), autoimmune adrenal disease (Addison's disease; n = 8) and apparently healthy blood donors (n = 9) using recombinant TPO expressed with a series of truncations and internal deletions. This material was obtained using an in vitro transcription/translation system in the presence of 35S-methionine and the reactivity of TPO autoantibodies tested in an immunoprecipitation assay. In addition, we have studied the effects of denaturing purified recombinant TPO by reduction and/or sodium dodecyl sulphate on its reactivity with TPO autoantibodies by Western blotting analysis. These studies show that TPO autoantibodies can recognise TPO in Western blotting analysis when large amounts of purified TPO are run on the gels and the blotted proteins renatured prior to addition of antibody. Under these conditions TPO autoantibodies in all 20 Graves' or Hashimoto's sera tested reacted strongly with blots of non-reduced TPO but reduction of TPO had a marked effect on the ability of autoantibodies to recognise it in Western blotting analysis. Analysis of TPO autoantibody binding to 35S-labelled TPO proteins containing N-terminal, central or C-terminal deletions indicated that all modifications studied caused a statistically significant lowering of binding. In the case of some modifications, there were differences in the reactivity of TPO autoantibodies in sera from patients with Addison's disease compared to TPO autoantibodies in autoimmune thyroid disease and/or healthy blood donor sera. Overall, our results of analysis of T PO autoantibody binding in Western blotting and with modified TPO proteins in immunoprecipitation assays suggest that the main autoantibody binding sites on the TPO molecule involve extensive amino acid sequences. Our studies also suggest that TPO autoantibodies from patients with autoimmune thyroid disease, Addison's disease and apparently healthy blood donors show some differences in epitope recognition on TPO and this approach may allow differentiation between disease related and unrelated TPO autoantibodies.

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.

Somatic hypermutation in autoimmune thyroid disease

Autoimmune thyroid disease is one of the most common autoimmune diseases. There is typically patient antibody (Ab) reactivity to one or more of the antigens thyroglobulin (Tg), thyroid peroxidase (TPO) and the thyroid simulating hormone receptor (TSHr). With the advent of combinatorial library technology, there has been an enormous increase in the number of sequences from Ab to Tg and TPO. The repertoire of both Tg and TPO Ab is restricted and indicates the importance of somatic hypermutation in the development of the high affinity Ab response. However, there are still too few sequences to determine patterns in which the mutation occurs, which residues are introduced during substitution and how individual substitutions affect the affinity of the Ab. Ab to the TSHr are of far greater pathological significance than those to Tg and TPO, but the current repertoire of Ab to the TSHr has yet to include the high affinity IgG Ab characteristic of patient serum Ab. Instructive analysis of the role of somatic hypermutation in the development of TSHr Ab therefore still awaits the isolation of the pathologically active repertoire. Despite this, the Ab response in thyroid autoimmunity remains one of the best characterised of human autoimmune diseases.