Thyroglobulin-thyroperoxidase autoantibodies are polyreactive, not bispecific: analysis using human monoclonal autoantibodies

Viral Related Tools against SARS-CoV-2

At the end of 2019, a new disease appeared and spread all over the world, the COVID-19, produced by the coronavirus SARS-CoV-2. As a consequence of this worldwide health crisis, the scientific community began to redirect their knowledge and resources to fight against it. Here we summarize the recent research on viruses employed as therapy and diagnostic of COVID-19: (i) viral-vector vaccines both in clinical trials and pre-clinical phases; (ii) the use of bacteriophages to find antibodies specific to this virus and some studies of how to use the bacteriophages themselves as a treatment against viral diseases; and finally, (iii) the use of CRISPR-Cas technology both to obtain a fast precise diagnose of the patient and also the possible use of this technology as a cure.

Adaptive immune receptor repertoires, an overview of this exciting field

The adaptive immune response in jawed vertebrates relies on the huge diversity and specificity of the B cell and T cell antigen receptors, the immunoglobulins (IG) or antibodies and the T cell receptors (TR), respectively. The high level of diversity has represented a barrier to a comprehensive analysis of the adaptive immune response before the emergence of high-throughput sequencing (HTS) technologies. The size and complexity of HTS data requires the generation of novel computational and analytical approaches, which are transforming how the adaptive immune responses are deciphered to understand the clonal dynamics and properties of antigen-specific B and T cells in response to different kind of antigens. This exciting and rapidly evolving field is not only impacting human and clinical immunology but also comparative immunology. We are now closer to understanding the evolution of adaptive immune response in jawed vertebrates. This review provides an overview about classical and current strategies developed to assess the IG/TR diversity and their applications in basic and clinical immunology.

Effect of Thyroglobulin Autoantibodies on the Metabolic Clearance of Serum Thyroglobulin

BACKGROUND

In order to establish whether thyroglobulin autoantibodies (TgAb) influence the metabolic clearance of thyroglobulin (Tg) in humans, serum Tg and TgAb were correlated shortly after radioiodine (¹³¹I) treatment.

METHODS

Samples were collected from 30 consecutive patients undergoing ¹³¹I activity for Graves' hyperthyroidism at the time of treatment and every 15 days thereafter, up to 90 days. Tg and TgAb were measured by immunometric assays (functional sensitivities: 0.1 ng/mL and 8 IU/mL).

RESULTS

Tg was detectable in all patients at day 0. Tg concentrations rose from a mean of 33.2 ng/mL [confidence interval (CI) 17.8-61.0 ng/mL] at day 0 to a mean of 214.6 ng/mL [CI 116.9-393.4 ng/mL] at day 30 and then steadily decreased, reaching the lowest concentration at day 90 (M = 10.9 ng/mL [CI 5.5-20.9 ng/mL]). Compared to their levels at day 0 (M = 23.6 IU/mL [CI 10.5-52.9 IU/mL]), TgAb remained stable through day 15 and then gradually increased up to a mean of 116.6 IU/mL [CI 51.9-262.2 IU/mL] at day 90. Patients were then split into two groups according to their TgAb status at day 0: undetectable (<8 IU/mL; 9 patients) or detectable (≥8 IU/mL; 21 patients) TgAb. Compared to the other cohort, patients with detectable TgAb showed significantly lower Tg concentrations at day 0 (M = 20.3 ng/mL [CI 10.1-40.2 ng/mL] vs. M = 101.8 ng/mL [CI 36.6-279.8 ng/mL]), similar at day 15, lower levels at day 30 (M = 146.5 ng/mL [CI 74.3-287.8 ng/mL] vs. M = 514.8 ng/mL [CI 187.8-1407.9 ng/mL]), at day 45 (M = 87.5 ng/mL [CI 43.1-176.6 ng/mL] vs. M = 337.9 ng/mL [CI 120.1-947.0 ng/mL]), at day 60 (M = 61.6 ng/mL [CI 31.0-121.4 ng/mL] vs. M = 255.8 ng/mL [CI 79.0-823.8 ng/mL]), and at day 75 (M = 24.5 ng/mL [CI 11.9-49.2 ng/mL] vs. M = 249.5 ng/mL [CI 63.5-971.1 ng/mL]), and similar levels at day 90. Patients with detectable TgAb showed a lower (M = 182.5 ng/mL [CI 92.0-361.0 ng/mL] vs. M = 514.8 ng/mL [CI 187.8-1407.9 ng/mL]) and an earlier (day 15 vs. day 30) peak of Tg. The mean Tg concentration was lower in patients with detectable TgAb than in those with undetectable TgAb (area under the curve: 17,340 ± 16,481 ng/mL vs. 36,883 ± 44,625 ng/mL; p = 0.02).

CONCLUSIONS

TgAb influence the changes in Tg concentrations observed immediately after ¹³¹I treatment, inducing lower levels and an earlier peak of Tg. These observations indicate that TgAb significantly influence the metabolic clearance of Tg, supporting the concept that their interference in the measurement of Tg is mainly due to an in vivo effect.

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.

Antibody Engineering for Pursuing a Healthier Future

Since the development of antibody-production techniques, a number of immunoglobulins have been developed on a large scale using conventional methods. Hybridoma technology opened a new horizon in the production of antibodies against target antigens of infectious pathogens, malignant diseases including autoimmune disorders, and numerous potent toxins. However, these clinical humanized or chimeric murine antibodies have several limitations and complexities. Therefore, to overcome these difficulties, recent advances in genetic engineering techniques and phage display technique have allowed the production of highly specific recombinant antibodies. These engineered antibodies have been constructed in the hunt for novel therapeutic drugs equipped with enhanced immunoprotective abilities, such as engaging immune effector functions, effective development of fusion proteins, efficient tumor and tissue penetration, and high-affinity antibodies directed against conserved targets. Advanced antibody engineering techniques have extensive applications in the fields of immunology, biotechnology, diagnostics, and therapeutic medicines. However, there is limited knowledge regarding dynamic antibody development approaches. Therefore, this review extends beyond our understanding of conventional polyclonal and monoclonal antibodies. Furthermore, recent advances in antibody engineering techniques together with antibody fragments, display technologies, immunomodulation, and broad applications of antibodies are discussed to enhance innovative antibody production in pursuit of a healthier future for humans.

Significance of Low Levels of Thyroglobulin Autoantibodies Associated with Undetectable Thyroglobulin After Thyroidectomy for Differentiated Thyroid Carcinoma

BACKGROUND

The management of patients with differentiated thyroid carcinoma (DTC) showing low levels of serum thyroglobulin autoantibodies (TgAb) and undetectable Tg after thyroidectomy is unsettled. This study sought to elucidate the clinical significance of low levels of TgAb and to evaluate their interference with Tg measurement in vitro.

METHODS

Tg and TgAb levels were correlated with the post-thyroidectomy staging of 177 consecutive DTC patients undergoing (131)I ablation after total thyroidectomy (clinical study). Tg was measured by an immunometric assay (functional sensitivity: 0.1 ng/mL), and TgAb were evaluated by six assays (functional sensitivities: 1.2-96 IU/mL; positive cutoffs: 4-150 IU/mL). The changes in Tg concentration (Tg recovery) of diluted specimens from DTC patients were also measured after incubation with 67 sera from DTC patients with undetectable Tg and low levels of TgAb (in vitro study). DTC sera containing Tg were diluted serially (from 330 to 0.1 ng/mL) and incubated with TgAb samples; Tg was then measured.

RESULTS

In the clinical study: all patients had residual thyroid tissue, and 10 had metastatic disease. Depending on the TgAb assay, median Tg values were 7.0-10.9, 0.0-5.3, and 0.0-0.0 ng/mL in patients with undetectable, borderline (between functional sensitivities and positive cutoffs), and positive TgAb, respectively (p < 0.001). An undetectable Tg value was associated with borderline levels of TgAb in five assays. Only two patients with metastatic disease had undetectable Tg; both were TgAb positive by three or more assays. Conversely, no patient with undetectable Tg and undetectable or borderline TgAb by sensitive assays had metastatic disease. In the in vitro study, TgAb interfered significantly with Tg recovery (p < 0.001), but low levels of TgAb did not abolish Tg recovery.

CONCLUSIONS

While low levels of TgAb do not preclude Tg measurement in vitro, they can be associated with an undetectable Tg in DTC patients with residual thyroid tissue after thyroidectomy. However, the finding of low levels of TgAb by sensitive assays associated with an undetectable Tg rules out metastatic disease.

Iodine, thyroid autoimmunity and cancer

This review focuses on two different topics: (a) iodine and autoimmune thyroid disease (AITD) and (b) AITD and papillary thyroid carcinoma (PTC). Iodine intake modifies the expression of thyroid diseases and has been associated with induction of AITD. Thyroglobulin (Tg) is an important target in iodine-induced autoimmune response due to post-translational modifications of iodinated Tg, as suggested in animal models. We have shown that the unmasking of a cryptic epitope on Tg contributes to iodine-induced thyroid autoimmunity in humans. The relationship between AITD and PTC has been suggested in many studies. The presence of two different mechanisms has been hypothesized, one typical of AITD and the other of an immune reaction to PTC. We have shown that in AITD, the pattern of Tg recognition by anti-Tg antibodies (TgAb) is 'restricted' to the immunodominant regions of Tg, while in patients with non-AITD, such as nodular goiter and PTC devoid of thyroid lymphocytic infiltration at histology, TgAb show a less restricted epitopic pattern and bind also to other regions of Tg. Thyroid function may also affect the frequency of PTC, the risk of cancer increasing with serum TSH levels. We have shown that this mechanism, rather than thyroiditis per se, plays a major role in the association of PTC with Hashimoto's thyroiditis, as a consequence of the autoimmune process leading to a progressive increase of serum TSH in these patients.

Thyroglobulin autoantibodies switch to immunoglobulin (Ig)G1 and IgG3 subclasses and preserve their restricted epitope pattern after 131I treatment for Graves' hyperthyroidism: the activity of autoimmune disease influences subclass distribution but not epitope pattern of autoantibodies

The subclass distribution of thyroglobulin autoantibodies (TgAb) is debated, whereas their epitope pattern is restricted. Radioidine ((131)I) treatment for Graves' disease (GD) induces a rise in TgAb levels, but it is unknown whether it modifies subclass distribution and epitope pattern of TgAb as well. We collected sera from GD patients before (131) I treatment and 3 and 6 months thereafter. We measured total TgAb, TgAb light chains and TgAb subclasses by enzyme-linked immunosorbent assay (ELISA) in 25 patients. We characterized the TgAb epitope pattern in 30 patients by inhibiting their binding to (125-) (I) Tg by a pool of four TgAb-Fab (recognizing Tg epitope regions A, B, C and D) and to Tg in ELISA by each TgAb-Fab. Total TgAb immunoglobulin (Ig)G rose significantly (P = 0.024). TgAb κ chains did not change (P = 0.052), whereas TgAb λ chains increased significantly (P = 0.001) and persistently. We observed a significant rise in IgG1 and IgG3 levels after (131)I (P = 0.008 and P = 0.006, respectively), while IgG2 and IgG4 levels did not change. The rise of IgG1 was persistent, that of IgG3 transient. The levels of inhibition of TgAb binding to Tg by the TgAb-Fab pool were comparable. A slight, non-significant reduction of the inhibition by the immune-dominant TgAb-Fab A was observed 3 and 6 months after (131)I. We conclude that (131)I treatment for GD increases the levels of the complement-activating IgG1 and IgG3 subclasses and does not influence significantly the epitope pattern of TgAb. In autoimmune thyroid disease subclass distribution of autoantibodies is dynamic in spite of a stable epitope pattern.

Breaking tolerance to thyroid antigens: changing concepts in thyroid autoimmunity

Thyroid autoimmunity involves loss of tolerance to thyroid proteins in genetically susceptible individuals in association with environmental factors. In central tolerance, intrathymic autoantigen presentation deletes immature T cells with high affinity for autoantigen-derived peptides. Regulatory T cells provide an alternative mechanism to silence autoimmune T cells in the periphery. The TSH receptor (TSHR), thyroid peroxidase (TPO), and thyroglobulin (Tg) have unusual properties ("immunogenicity") that contribute to breaking tolerance, including size, abundance, membrane association, glycosylation, and polymorphisms. Insight into loss of tolerance to thyroid proteins comes from spontaneous and induced animal models: 1) intrathymic expression controls self-tolerance to the TSHR, not TPO or Tg; 2) regulatory T cells are not involved in TSHR self-tolerance and instead control the balance between Graves' disease and thyroiditis; 3) breaking TSHR tolerance involves contributions from major histocompatibility complex molecules (humans and induced mouse models), TSHR polymorphism(s) (humans), and alternative splicing (mice); 4) loss of tolerance to Tg before TPO indicates that greater Tg immunogenicity vs TPO dominates central tolerance expectations; 5) tolerance is induced by thyroid autoantigen administration before autoimmunity is established; 6) interferon-α therapy for hepatitis C infection enhances thyroid autoimmunity in patients with intact immunity; Graves' disease developing after T-cell depletion reflects reconstitution autoimmunity; and 7) most environmental factors (including excess iodine) "reveal," but do not induce, thyroid autoimmunity. Micro-organisms likely exert their effects via bystander stimulation. Finally, no single mechanism explains the loss of tolerance to thyroid proteins. The goal of inducing self-tolerance to prevent autoimmune thyroid disease will require accurate prediction of at-risk individuals together with an antigen-specific, not blanket, therapeutic approach.

Delineating the autoimmune mechanisms in Graves' disease

The immunologic processes involved in autoimmune thyroid disease (AITD), particularly Graves' disease (GD), are similar to other autoimmune diseases with the emphasis on the antibodies as the most unique aspect. These characteristics include a lymphocytic infiltrate at the target organs, the presence of antigen-reactive T and B cells and antibodies, and the establishment of animal models of GD by antibody transfer or immunization with antigen. Similar to other autoimmune diseases, risk factors for GD include the presence of multiple susceptibility genes, including certain HLA alleles, and the TSHR gene itself. In addition, a variety of known risk factors and precipitators have been characterized including the influence of sex and sex hormones, pregnancy, stress, infection, iodine and other potential environmental factors. The pathogenesis of GD is likely the result of a breakdown in the tolerance mechanisms, both at central and peripheral levels. Different subsets of T and B cells together with their regulatory populations play important roles in the propagation and maintenance of the disease process. Understanding different mechanistic in the complex system biology interplay will help to identify unique factors contributing to the AITD pathogenesis.

Thyroglobulin autoantibodies in patients with papillary thyroid carcinoma: comparison of different assays and evaluation of causes of discrepancies

CONTEXT

Thyroglobulin autoantibodies (TgAb) have been proposed as a surrogate marker of thyroglobulin in the follow-up of differentiated thyroid carcinoma. Commercially available TgAb assays are often discordant. We investigated the causes of discrepancy.

DESIGN

TgAb were measured by three noncompetitive immunometric assays and three competitive RIA in 72 patients with papillary thyroid carcinoma and associated lymphocytic thyroiditis (PTC-T), 105 with papillary thyroid carcinoma and no lymphocytic thyroiditis (PTC), 160 with Hashimoto's thyroiditis, and in 150 normal subjects. The results of the six assays were correlated. TgAb epitope pattern, evaluated by inhibition of serum TgAb binding to thyroglobulin by TgAb-Fab regions A, B, C, and D, were compared in sera which were positive in all six assays (concordant sera) and positive in only one to five assays (discordant sera) were compared. TgAb International Reference Preparation (IRP) was measured in 2007 and 2009.

RESULTS

The correlations of the six assays ranged from -0.01 to 0.93 and were higher in PTC-T and Hashimoto's thyroiditis than in PTC and normal subjects. Two uncorrelated components, one including the three immunometric assays, the other the three RIA, explained 40 and 37% of the total variance of the results of the six assays. The levels of inhibition were higher in concordant sera than in discordant sera by TgAb-Fab region B (27.0%, 21.2-34.0 vs. 6.0%, and 2.7-12.7%) and region C (30.5%, 21.3-37.7 vs. 4.0%, and 1.0-6.5%); thus, the epitope pattern was more homogeneous in concordant sera than in discordant sera. TgAb IRP ranged from 157 to 1088 (expected 1000) IU/ml in 2009; results in 2007 were similar in all but two assays.

CONCLUSIONS

TgAb assays are highly discordant. Discrepancy is lower when comparing assays with similar methodology. Results of TgAb from PTC-T are more concordant than those from PTC because their epitope pattern is more restricted. The internal standardization of TgAb is generally, but not completely, satisfactory.

Thyroglobulin autoantibodies of patients with subacute thyroiditis are restricted to a major B cell epitope

BACKGROUND

Thyroglobulin autoantibodies (TgAb) can develop in patients with subacute thyroiditis (SAT).

AIM

Comparison of the epitope pattern of TgAb of patients with SAT, Hashimoto's thyroiditis (HT) [autoimmune thyroid disease (AITD)] and non-toxic multinodular goiter (NTMG) (non-AITD).

SUBJECTS AND METHODS

Serum TgAb from 10 patients with SAT, 45 with HT, and 19 with NTMG were evaluated. Serum TgAb binding to Tg was inhibited by 4 recombinant human TgAb-Fab, recognizing Tg epitope regions A, B, C, and D. The ability of single TgAb-Fab to inhibit the binding of serum TgAb to Tg was evaluated in enzymelinked immunosorbent assay.

RESULTS

Levels of inhibition were different for all TgAb-Fab in the 3 groups of patients. Inhibition by region A TgAb-Fab in SAT [50.5 (30.3-62.5)%] (median and 25th to 75th percentiles) was similar to HT [49.0 (38.0-69.5)%] and significantly higher than in NTMG [25.0 (14.0-37.0)%]; by region B TgAb-Fab in SAT [0.0 (0.0-12.5)%] was significantly lower than in HT [28.0 (9.5-48.0)%] and similar to NTMG [9.0 (4.8-20.5)%]; by region C TgAb-Fab in SAT [9.5 (0.0-25.8)%] were similar to HT [23.0 (9.5-41)%] and NTMG [6.5 (1.7-21.5)%]; and by region D TgAb-Fab in SAT [0.0 (0.0-8.0)%] were lower than in HT [12.0 (1.0-28.5)%] and similar to NTMG [1.0 (0.0-5.0)%].

CONCLUSIONS

The epitope pattern of TgAb of SAT is restricted to the A region that is immunodominant in AITD and non-AITD. In the majority of patients with SAT, the autoimmune phenomena represent a non-specific and transient response to the release of thyroid antigens, rather than the expression of thyroid autoimmunity.

Phage display--a powerful technique for immunotherapy: 1. Introduction and potential of therapeutic applications

One of the most effective molecular diversity techniques is phage display. This technology is based on a direct linkage between phage phenotype and its encapsulated genotype, which leads to presentation of molecule libraries on the phage surface. Phage display is utilized in studying protein-ligand interactions, receptor binding sites and in improving or modifying the affinity of proteins for their binding partners. Generating monoclonal antibodies and improving their affinity, cloning antibodies from unstable hybridoma cells and identifying epitopes, mimotopes and functional or accessible sites from antigens are also important advantages of this technology. Techniques originating from phage display have been applied to transfusion medicine, neurological disorders, mapping vascular addresses and tissue homing of peptides. Phages have been applicable to immunization therapies, which may lead to development of new tools used for treating autoimmune and cancer diseases. This review describes the phage display technology and presents the recent advancements in therapeutic applications of phage display.

Characterization of thyroglobulin epitopes in Sardinian adults and juveniles with Hashimoto's thyroiditis: evidence against a major effect of age and genetic background on B-cell epitopes

BACKGROUND

Using recombinant human monoclonal thyroglobulin antibodies expressed as Fab molecules (TgAb-Fab), we have recently confirmed the restriction of Tg epitopes in Hashimoto's thyroiditis (HT).

OBJECTIVE

To investigate Tg epitopes of serum TgAb in HT adults and HT juveniles from a geographically isolated area (Sardinia).

DESIGN AND PATIENTS

Serum TgAb of 39 Sardinian HT adults, 53 Sardinian HT juveniles and 45 non-Sardinian HT adults were evaluated. The binding of serum TgAb to Tg in ELISA was inhibited by four recombinant human TgAb-Fab, identifying Tg epitopic regions A-D. The percentage of Tg binding inhibition was calculated comparing the binding of serum TgAb in presence of each TgAb-Fab with that in its absence.

RESULTS

In the whole cohort of 137 patients, A region TgAb-Fab induced the highest levels of inhibition (55.3 +/- 17.8%) (mean +/- SD). Lower levels of inhibition were induced by TgAb-Fab of regions B (27.8 +/- 25.8%), C (26.8 +/- 24.6%) and D (17.5 +/- 18.4%). In Sardinian HT adults inhibition by TgAb-Fab of regions A, B and C were comparable to Sardinian HT juveniles; the marginal D region TgAb-Fab induced a slightly higher inhibition (22.1 vs. 13.8%; P = 0.034) in the former than in the latter group. In Sardinian and non-Sardinian HT adults inhibitions by the four TgAb-Fab were similar.

CONCLUSIONS

In HT, the Tg epitope pattern of serum TgAb was similar in juveniles and adults from a geographically restricted area and in two adult populations from different geographical areas. Thus, in HT, neither age nor genetic background appear to influence B-cell epitopes.

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.

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.

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.

Antibodies cross-reacting with thyroglobulin and thyroid peroxidase are induced by immunization of rabbits with an immunogenic thyroglobulin 20mer peptide

Thyroglobulin (Tg) and thyroid peroxidase (TPO) are two major autoantigens in autoimmune thyroid diseases (AITD). Cross-reactive anti-Tg/TPO antibodies have been identified in patients with AITD and in mice immunized with Tg or TPO. In the present study, we investigated the production of anti-Tg/TPO antibodies in rabbits immunized with human Tg and with a highly immunogenic Tg peptide (namely TgP41, sequence 2651-2670 of human Tg), by noncompetitive and competitive ELISA. TgP41 was found previously to induce intramolecular epitope spreading. We found that Tg-immunized rabbits developed a serological immune response to TPO due to cross-reactivity with Tg, since serum TPO reactivity was inhibited by soluble Tg and affinity-purified anti-Tg antibodies cross-reacted with TPO. Moreover, TgP41-immunized rabbits responded to Tg and TPO. This serological response was attributed to anti-Tg/TPO antibodies, based on the observation that serum TPO reactivity was again inhibited by soluble Tg, and affinity-purified anti-Tg antibodies, induced by TgP41-immunization, cross-reacted with TPO. Purified anti-TgP41 antibodies did not react with TPO, suggesting that a putative common antigenic determinant is not included in the peptide sequence. We propose that intermolecular spreading of reactivity to TPO observed after administration of the Tg-peptide is a result of intramolecular epitope spreading to determinant(s) responsible for Tg/TPO cross-reactivity.

Homologies between proteins of Borrelia burgdorferi and thyroid autoantigens

Subclinical exposure to microbic antigens that share amino acid sequence homology with self antigens might trigger autoimmune diseases in genetically predisposed individuals via molecular mimicry. Genetic predisposition to Graves' disease (GD) or Hashimoto's thyroiditis (HT) is conferred by HLA loci DR3 or DR5, respectively. Yersinia enterocolitica (YE) outer proteins (YOPs) are candidate triggers based on the high prevalence of serum antibodies (Ab) against YOPs in autoimmune thyroid diseases (AITD) and reactivity of these Ab with hTSH-R, suggesting homology between YOPs and hTSH-R. We have reported previously that the spirochete Borrelia burgdorferi (Bb) could be another trigger. We have explored further the homology of hTSH-R with YE and Bb. Using the Basic Local Alignment Search Tool (BLAST), we found four matches with YE and five matches with Bb . Residues 22-272, 186-330, 319-363 and 684-749 of hTSH-R matched YopM, Ysp, exopolygalacturonase and SpyA of YE (identity 23-31%, similarity 40-48%). Residues 112-205, 127-150, 141-260, 299-383 and 620-697 of hTSH-R matched outer surface protein A, flagellar motor rotation protein A, two hypothetical proteins (BBG02 and BBJ08) and DNA recombinase/ATP dependent helicase of Borrelia (identity 27-50%, similarity 40-75%). Interestingly, the above hTSH-R regions coincide with (or include) known human T-cell epitopes: aa 52-71, 140-176, 240-270, 340-380 and 441-661. Our data strengthen the hypothesis of Bb and YE as environmental triggers of AITD in genetically predisposed persons through molecular mimicry mechanisms.

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.