Analysis of carbohydrate residues on human thyroid peroxidase (TPO) and thyroglobulin (Tg) and effects of deglycosylation, reduction and unfolding on autoantibody binding

The antigenic link between thyroid autoimmunity and breast cancer

The association between breast cancer and benign thyroid disorders, in particular thyroid autoimmunity, has been debated for decades. Autoantibodies to thyroid peroxidase, the hallmark of thyroid autoimmunity, have a higher prevalence among patients with breast cancer compared with the general population. Furthermore a correlation between their positivity and a better prognosis of breast cancer was found in several independent small-scale studies, even if such observation was not confirmed in a subsequent retrospective study conducted on the largest patient cohort to date. The thyroid and mammary glands present several biological similarities, therefore the hypothesis of an immune response to shared thyroid/breast antigens could in part explain the association between thyroid autoimmunity and breast cancer. The sodium iodide symporter is expressed in both glands, however it seems unlikely to be the key common antigen, considering that autoantibodies targeting it are rare. Instead thyroid peroxidase, one of the major thyroid autoantigens, is also expressed in breast tissue and therefore represents the main antigenic link between thyroid autoimmunity and breast cancer. Furthermore lactoperoxidase, an enzyme of the same family that shares structural similarities with thyroid peroxidase, is expressed in neoplastic breast cells and is responsible for the cross-reactivity with some autoantibodies to thyroid peroxidase. Novel strategies for the diagnosis and treatment of breast cancer might take advantage of the antigenic link between thyroid and breast tissues.

Glycosylation in the Thyroid Gland: Vital Aspects of Glycoprotein Function in Thyrocyte Physiology and Thyroid Disorders

The key proteins responsible for hormone synthesis in the thyroid are glycosylated. Oligosaccharides strongly affect the function of glycosylated proteins. Both thyroid-stimulating hormone (TSH) secreted by the pituitary gland and TSH receptors on the surface of thyrocytes contain N-glycans, which are crucial to their proper activity. Thyroglobulin (Tg), the protein backbone for synthesis of thyroid hormones, is a heavily N-glycosylated protein, containing 20 putative N-glycosylated sites. N-oligosaccharides play a role in Tg transport into the follicular lumen, where thyroid hormones are produced, and into thyrocytes, where hyposialylated Tg is degraded. N-glycans of the cell membrane transporters sodium/iodide symporter and pendrin are necessary for iodide transport. Some changes in glycosylation result in abnormal activity of the thyroid and alteration of the metabolic clearance rate of hormones. Alteration of glycan structures is a pathological process related to the progression of chronic diseases such as thyroid cancers and autoimmunity. Thyroid carcinogenesis is accompanied by changes in sialylation and fucosylation, β1,6-branching of glycans, the content and structure of poly-LacNAc chains, as well as O-GlcNAcylation, while in thyroid autoimmunity the main processes affected are sialylation and fucosylation. The glycobiology of the thyroid gland is an intensively studied field of research, providing new data helpful in understanding the role of the sugar component in thyroid protein biology and disorders.

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.

Effect of Advanced Glycation End Products on Human Thyroglobulin's Antigenicity as Identified by the Use of Sera from Patients with Hashimoto's Thyroiditis and Gestational Diabetes Mellitus

Advanced glycation end products (AGEs) are formed on proteins after exposure to high concentrations of glucose and modify protein's immunogenicity. Herein, we investigated whether the modification of thyroglobulin (Tg) by AGEs influences its antigenicity and immunogenicity. Human Tg was incubated in vitro with increasing concentrations of D-glucose-6-phosphate in order to produce Tgs with different AGE content (AGE-Tg). Native Tg and AGE-Tgs were used in ELISA to assess the serum antibody reactivity of two patient groups, pregnant women with gestational diabetes (GDM), and patients with Hashimoto's thyroiditis (HT). We produced in vitro AGE-Tg with low and high AGE content, 13 and 49 AGE units/mg Tg, respectively. All HT patients' sera presented the same antibody reactivity profile against native Tg and AGE-Tgs, indicating that the modification of Tg by AGEs did not alter its antigenicity. Similarly, the GDM patients' sera did not discriminate among the two forms of Tg, native or artificially glycated, suggesting that the modification of Tg by AGEs might not alter its immunogenicity. The modification of Tg by AGEs has no obvious effect on neither its antigenicity nor, most likely, its immunogenicity. It seems that other Tg modifications might account for the production of aTgAbs in patients with GDM.

Isolation and structural identification of glycopolymers of Bifidobacterium bifidum BIM B-733D as putative players in pathogenesis of autoimmune thyroid diseases

Bifidobacterium bifidum 791 (commercially available as B. bifidum BIM B-733D) cell-surface biopolymers (BPs) interact selectively with human serum thyroid peroxidase (TPO) and thyroglobulin (Tg) autoantibodies (anti TPO and anti Tg, respectively). BPanti-TPO and BPanti-Tg were isolated from the soluble fraction of B. bifidum BIM B-733D by affinity chromatography with anti-TPO or anti-Tg, respectively. Homogeneity of affinity eluates (AEanti-TPO and AEanti-Tg) was tested by size exclusion chromatography. For each AE, the elution profiles generated on the basis of absorbance at 280 nm do not conform to ELISA data for functional activity characteristic of BPs. Moreover, high functional activity was detected in chromatographic fractions that had significantly different molecular weights and no absorbance at 280 nm, which suggests a non-protein (carbohydrate) nature of BPanti-TPO and BPanti-Tg. The semi-preparative size exclusion chromatography of AEanti-TPO and AEanti-Tg with detection by refractometer gave 5,000-7,000 Da fractions containing substances that interact selectively with either anti TPO (BPanti-TPO) or anti-Tg (BPanti-Tg) according to ELISA data. Analysis by two-dimensional NMR spectroscopy including a 1H, 13C-heteronuclear single-quantum coherence experiment indicated that both substances are linear α-1,6-glucans. For the first time, an immunological similarity (molecular mimicry) of glycopolymers of B. bifidum BIM B-733D and human thyroid proteins, TPO and Tg, was shown. On the whole, our data point to a possible role of bifidobacteria in the pathogenesis of autoimmune thyroid diseases (ATD). The main requirements for triggering/acceleration or prevention/abrogation of ATD by bifidobacteria through molecular mimicry mechanism are hypothesised to be (1) genetic predisposition to ATD and (2) intestinal epithelium penetration by α-1,6-glucan.

Does thyroid peroxidase provide an antigenic link between thyroid autoimmunity and breast cancer?

Women with breast cancer (BC) and antithyroid peroxidase (TPO) autoantibodies (TPOAb) have a better prognosis than women lacking TPOAb. Sera from women with TPOAb displayed immunoreactivity to BC tissue by immunofluorescence that was not apparent in women without TPOAb. We hypothesize a BC/thyroid shared antigen that provides a target for humoral or cell-mediated immune activity; candidates include the sodium/iodide symporter (expressed in thyroid and BC), cross-reacting epitopes in TPO and lactoperoxidase (LPO) or TPO itself. As the association is with TPOAb, we investigated TPO expression in BC, breast peritumoral tissue (PT), other tissues (tumoral and not) and thyroid as positive control. Transcripts for known and novel TPO isoforms were detected in BC (n = 8) and PT (n = 8) but at approximately 10(4) -fold lower than in thyroid while in non-BC tumors (n = 5) they were at the limit of detection. TPO was expressed also in adipose tissue (n = 17), 10(3) -fold lower than in thyroid. Full length TPO (Mr 105-110 kDa) was detected in Western blots in the majority of examined tissues; preabsorption of the TPO antibody with recombinant TPO (but not LPO) reduced the signal, indicating specificity. The same occurred with some lower molecular weight bands, which could correspond to smaller TPO transcript isoforms, present in all samples. In conclusion, TPO is weakly expressed in BC and other tissues; this could partly explain the high frequency and protective role of TPOAb in BC patients. Further studies will investigate tissue specificity, function and immunogenicity of the novel TPO variants (some BC-specific) identified.

Glycosylation of sera thyroglobulin antibody in patients with thyroid diseases

OBJECTIVE

Thyroglobulin antibody (TgAb) is an important autoantibody in thyroid diseases, which is a glycoprotein, predominantly of IgG class. Glycosylation of the IgG-Fc contributes to many effector functions exhibited by antibodies. The aim of our study was to investigate the glycosylation of sera TgAb in patients with different thyroid diseases.

DESIGN AND METHODS

Sera from 146 patients were collected and divided into four groups: Hashimoto's thyroiditis (HT, n=90), Graves' disease (GD, n=20), papillary thyroid carcinoma (PTC, n=17), and PTC with histological lymphocytic thyroiditis (PTC-T, n=19). HT patients were further divided into euthyroidism and subclinical and overt hypothyroidism groups. Lectin-ELISAs were performed to detect the relative amount of core fucose, terminal galactose, and sialic acid on each TgAb respectively.

RESULTS

Among HT, GD, and PTC groups, HT patients had significantly lower core fucose content on TgAb than the other two groups; an increasing trend of sialylation was found in PTC sera (P=0.076) compared with HT groups. PTC-T patients had significantly higher sialylated TgAb than HT and GD patients, and no significant difference was found between PTC and PTC-T. There was no significant difference in the three carbohydrate residue contents on sera TgAb among HT subgroups. In all the patients, negative correlation was found between sialic acid content and TgAb IgG levels (r=-0.736, P<0.001).

CONCLUSIONS

Our study showed that glycosylation of sera TgAb varied in different thyroid diseases and it might be involved in pathogenesis of thyroid disorders.

Characterization of thyroglobulin epitopes in patients with autoimmune and non-autoimmune thyroid diseases using recombinant human monoclonal thyroglobulin autoantibodies

CONTEXT

Thyroglobulin (Tg) epitopes of serum Tg autoantibodies (TgAb) have been characterized using inhibition of Tg binding by human monoclonal TgAb in autoimmune thyroid diseases (AITD) [Hashimoto's thyroiditis (HT) and Graves' disease (GD)] but not in non-AITD [nontoxic multinodular goiter (NTMG) and papillary thyroid carcinoma (PTC)].

OBJECTIVE

Our objective was to compare Tg epitopes of serum TgAb from patients with AITD, non-AITD, and PTC associated with histological thyroiditis (PTC-T) using inhibition of Tg binding by four recombinant human TgAb-Fab (epitopic regions A-D).

DESIGN

Inhibition of Tg binding of 24 HT, 25 GD, 19 NTMG, 15 PTC, and 25 PTC-T TgAb-positive sera by each TgAb-Fab was evaluated in ELISA. Inhibition by the pool of the four TgAb-Fab was evaluated using labeled Tg.

RESULTS

Levels of inhibition were different for TgAb-Fab regions A (P = 0.001), B (0.007), and D (0.011). Inhibition by region A TgAb-Fab was significantly higher in HT, GD, and PTC-T than in NTMG and PTC patients. Inhibition levels by region B TgAb-Fab were significantly higher in HT compared with NTMG and PTC patients and in GD compared with NTMG patients. Inhibition by D region TgAb-Fab was significantly lower in NTMG than in the other groups. Inhibition by the pool ranged from 44% (NTMG) to 72% (GD).

CONCLUSIONS

The pattern of Tg recognition is similar when HT patients are compared to GD and NTMG to PTC patients and differs when AITD are compared with non-AITD patients. In PTC-T patients, it is similar to that of AITD patients.

Why measure thyroglobulin autoantibodies rather than thyroid peroxidase autoantibodies?

Autoantibodies to thyroglobulin (TgAb) and thyroid peroxidase (TPOAb) are of immunoglobulin G (IgG) class and have high affinities for their respective autoantigens. Both autoantibodies are markers of thyroid autoimmunity and they can be measured by a variety of assays. From the clinical perspective, TgAb are less prevalent than TPOAb and less useful than TPOAb for prediction of thyroid dysfunction. Moreover, TgAb interfere with Tg measurements to monitor metastases in thyroid cancer. However, increasing evidence suggests that these TgAb provide a surrogate for Tg. In terms of disease pathogenesis, Tg has been suggested to play a role in Graves' ophthalmopathy. Pending further studies, TgAb epitopes could distinguish between individuals who are euthyroid or who have clinical disease. A final, intriguing reason for measuring and characterizing TgAb is the interest these autoantibodies have rekindled in their autoantigen. It is conceivable that Tg polymorphisms, combined with the explosive mix of iodine, TPO and H2O2 necessary for thyroid hormone synthesis, inadvertently provide the trigger for the autoimmune thyroid response.

The melanin-concentrating hormone receptor 1, a novel target of autoantibody responses in vitiligo

Vitiligo is a common depigmenting disorder resulting from the loss of melanocytes in the skin. The pathogenesis of the disease remains obscure, although autoimmune mechanisms are thought to be involved. Indeed, autoantibodies and autoreactive T lymphocytes that target melanocytes have been reported in some vitiligo patients. The objective of this study was to identify pigment cell antigens that are recognized by autoantibodies in vitiligo. Using IgG from vitiligo patients to screen a melanocyte cDNA phage-display library, we identified the melanin-concentrating hormone receptor 1 (MCHR1) as a novel autoantigen related to this disorder. Immunoreactivity against the receptor was demonstrated in vitiligo patient sera by using radiobinding assays. Among sera from healthy controls and from patients with autoimmune disease, none exhibited immunoreactivity to MCHR1, indicating a high disease specificity for Ab's against the receptor. Inhibition of MCH binding to its receptor by IgG from vitiligo patients was also shown.

Serum thyroglobulin measurement. Utility in clinical practice

Serum thyroglobulin measurement has greatly facilitated the clinical management of patients with differentiated thyroid cancer and a variety of other thyroid disorders. Thyroglobulin autoantibodies remain a significant obstacle to the clinical use of thyroglobulin measurement. The interpretation of any given thyroglobulin value requires the careful synthesis of all pertinent clinical and laboratory data available to the clinician. The diagnostic use of rhTSH-stimulated thyroglobulin levels has greatly facilitated the follow-up of low-risk patients with thyroid cancer. Although the measurement of thyroglobulin mRNA from peripheral blood is likely to affect the future management of these patients, it is expected that serum thyroglobulin measurement will continue to have a principal role in the care of patients with differentiated thyroid cancer.

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.

Cell type-dependent differences in thyroid peroxidase cell surface expression

Recently, it has been suggested that only approximately 2% of human thyroid peroxidase (hTPO(933)) reaches the surface of stably transfected (Chinese hamster ovary) cells, most being degraded intracellularly, and this might be representative of thyroid peroxidase (TPO) behavior in thyrocytes (Fayadat, L., Siffroi-Fernandez, S., Lanet, J., and Franc, J.-L. (2000) J. Biol. Chem. 275, 15948-15954). In agreement, in stably transfected Madin-Darby canine kidney clones, nonpermeabilized cells exhibit wild-type hTPO(933) immunofluorescence (apically) on <10% of that found in permeabilized cells, where an endoplasmic reticulum pattern is observed. Further, a C-terminally truncated, membrane-anchorless hTPO(848) is also retained in the endoplasmic reticulum of stably transfected Madin-Darby canine kidney cells. However, by contrast, in Chinese hamster ovary cells after transient transfection, hTPO(933) immunofluorescence is detected equally well in nonpermeabilized and permeabilized cells, indicating that a large portion of hTPO(933) is present at the cell surface; furthermore, hTPO(848) is efficiently secreted. Further, using an antiserum not cross-reacting with rat TPO, we find by immunofluorescence that in stable clones of PC Cl3 (rat) thyrocytes, considerably more ( approximately 50%) of the cells exhibit hTPO(933) at the cell surface. However, cell surface biotinylation and endoglycosidase H digestion assays appear to under-represent the extent of hTPO(933) transport, presumably because protein folding limits both Golgi carbohydrate modification and accessibility of lysines in the extracellular domain. We conclude that cell type-specific factors may facilitate stable expression of TPO at the cell surface of thyrocytes.

Detection of binding and blocking autoantibodies to the human sodium-iodide symporter in patients with autoimmune thyroid disease

The sodium iodide symporter (NIS) is a novel autoantigen in autoimmune thyroid disease (ATD). A recent study has described the development of a bioassay for human (h) NIS antibody detection, but this will not detect antibodies that bind the symporter without modulating its activity. Therefore, the establishment of a binding assay is of importance to determine the overall prevalence of hNIS antibodies in ATD patients. An in vitro transcription and translation system was used to produce [35S]-labeled hNIS. The radiolabeled ligand reacted specifically in immunoprecipitation experiments with rabbit antiserum raised against a peptide fragment of hNIS. Subsequently, the reactivity of control and ATD sera to translated [35S]hNIS was determined using RIAs. A significant difference in the frequency of hNIS antibody-positive sera was found when patients with either Graves' disease (GD) or autoimmune hypothyroidism (AH) were compared with normal controls (P = 0.01 and P = 0.03, respectively). Of 49 GD and 29 AH sera tested, 11 (22%) and 7 (24%), respectively, were found to contain hNIS antibodies. Differences were also significant when the antibody-binding indices of the control group of sera were compared with those of both the GD and the AH patient sera (P < 0.0001 and P = 0.001, respectively). In contrast, sera from 10 patients with Addison's disease and 10 patients with vitiligo (without associated ATD) were all negative for antibody reactivity to the symporter. No differences were detected when the antibody binding indices of either the Addison's disease or the vitiligo sera were compared with those of the normal sera group (P = 0.9 and P = 0.6, respectively). Eight of the 11 (73%) GD and 3 of the 7 (43%) AH sera, which were positive for hNIS antibodies in the immunoprecipitation assay, were also found to inhibit iodide uptake in hNIS-transfected CHO-K1 cells, suggesting the existence of antibodies in some serum samples that bind to the symporter without modulating its function. Overall, a significant correlation was found between the iodide uptake inhibition and the binding assays for hNIS antibody detection (r = 0.49, P < 0.0001). In summary, we have developed a specific and quantitative assay for the detection of hNIS binding antibodies in sera of patients with ATD. This system offers the advantage of studying antibody reactivity against conformational epitopes and will be useful in understanding the role of NIS autoreactivity in the pathogenesis of ATD.

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.

Genetic and epitopic analysis of thyroid peroxidase (TPO) autoantibodies: markers of the human thyroid autoimmune response

TPO autoantibodies, the hallmark of human autoimmune thyroid disease, are of IgG class and are associated with thyroid destruction and hypothyroidism. Using the immunoglobulin gene combinatorial library approach, a panel of human monoclonal TPO autoantibodies (expressed as Fab) has been generated from thyroid tissue-infiltrating B cells. TPO-specific Fab closely resemble patients' serum autoantibodies in terms of L chain type, IgG subclass, affinities for TPO as well as epitopes recognized by > 80% of TPO autoantibodies in an individual's serum. TPO autoantibody V region genes are not unique; H chain V genes are usually mutated, while L chain V genes are sometimes in germ-line conformation. The autoantibodies recognize an immunodominant region involving conformational, overlapping epitopes in domains A and B. Finally, TPO autoantibody epitopic fingerprints are distinctive for individual sera, are not associated with hypothyroidism, but are conserved over time (indicating a lack of B cell epitope spreading). Evidence for conservation as well as inheritance of the fingerprints in some families, together with VH gene polymorphisms, may provide insight into the genetic basis of human autoimmune thyroid disease. Furthermore, monoclonal human TPO autoantibodies will be invaluable for B cell presentation of TPO to determine the T cell epitopes involved in TPO autoantibody production.

Autoimmune Addison's disease. Analysis of autoantibody binding sites on human steroid 21-hydroxylase

Human steroid 21-hydroxylase (21-OH) expressed in an in vitro translation system was found to react specifically with adrenal autoantibodies from patients with Addison's disease. The epitopes on 21-OH which reacted with autoantibodies were studied by incorporating a series of terminal and internal deletions into the 21-OH gene and analysing the expressed proteins by Western blotting. N-Terminal deletions up to amino acid 280 had no effect on autoantibody binding whereas a series of C-terminal deletions and truncations (amino acids 281-494) showed marked effects. Our results indicate that a central segment (281-379) and a C-terminal segment (380-494) of 21-OH interact to form at least one major autoantibody binding site.