Heterogeneity of autoantibodies against thyroid peroxidase in autoimmune thyroid disease: evidence against antibodies directly inhibiting peroxidase activity as regulatory factors in thyroid hormone metabolism

Concordance of eight kits for antithyroid peroxidase autoantibodies determination

Numerous methods are proposed to quantify antithyroid peroxidase autoantibodies. No standardization exists but most assays use the standard MRC 66/387 with a calibration factor. Costs of the tests vary between the different kits. We evaluated the concordance of eight peroxidase autoantibodies assay kits in two centres, using a panel of sera from 269 subjects: controls (n=100), patients with autoimmune thyroid disease (n=77; Graves' disease, Hashimoto's thyroiditis), patients with non-autoimmune thyroid disease (n=69; nodular goiter, differentiated thyroid carcinoma) and individual sera with thyroglobulin antibodies only (n=23). The concordance between the eight methods was high, ranging from 88.3% to 98.8% with the total panel of sera. The majority of assays demonstrated high diagnostic performance. We encountered some false-positive results at borderline positive levels, and the nonrecognition of some sera by competitive assays.

In old age the majority of thyroid peroxidase autoantibodies are directed to a single TPO domain irrespective of thyroid function and iodine intake

OBJECTIVE

We have examined (1) which epitopes on thyroid peroxidase (TPO) are recognized by TPO autoantibodies (TPO-Aab) in old age and to what extent? (2) Does the TPO-Aab pattern differ in euthyroid and hypothyroid elderly subjects or does it depend on their iodine intake?

DESIGN

TPO-Aab positive sera obtained from a screening study of nursing-home residents living in areas of varying iodine intake were tested by competition studies with monoclonal antibodies (mAbs) recognizing different epitopes on TPO.

SUBJECTS

The nursing-home residents with TPO-Aab positivity were from (A) an iodine abundant area (Eastern Hungary, median iodine excretion -MIE-: 0.462 mumol/mmol creatinine, N = 13); (B) an area of obligatory iodinated salt prophylaxis since the 1950s (Slovakia, MIE: 0.090 mumol/mmol creatinine, N = 11); (C) a moderately iodine-deficient area (Northern Hungary, MIE: 0.065 mumol/mmol creatinine, N = 13).

MEASUREMENTS

Thirteen murine TPO antibodies generated against several epitopes of the four (A, B, C, D) antigenic domains on the TPO were co-incubated with the TPO-Aab positive sera on TPO coated microtitre plates. The amount of mAb bound was estimated after further incubation with goat anti-mouse antibodies, conjugated with horseradish peroxidase and tetramethylbenzidine as chromogen. The TPO-Aab positive sera were characterized by the pattern of percentage of inhibition of mAb binding caused by the TPO-Aabs.

RESULTS

TPO-Aabs inhibited only the binding of mAbs raised against the antigenic domains A (mAb9, mAb2, mAb60) and B (mAb64, mAb59, mAb18, mAb15). The extent of inhibition depended upon the TPO-Aab titre but in all cases the binding of mAb9 was inhibited to the highest degree. The percentage inhibition of mAb9 was (a) 34 +/- 17% (M +/- SD) caused by sera (N = 8) with TPO-Aab titre 1/100-1/200 (higher than that of all mAbs recognizing domain B, P < 0.01-P < 0.001), (b) 76 +/- 18% caused by sera (N = 14) with TPO-Aab titre 1/1000 (higher than that of all other mAbs -P < 0.01-P < 0.001, except mAb64), (c) 99 +/- 4% caused by sera (N = 15) with TPO-Aab titre 1/4000-1/16,000 (higher than that of all other mAbs, P < 0.01-P < 0.001). Thus, only mAb9 was inhibited completely by high titres of TPO-Aabs. The qualitative and quantitative distribution pattern of mAb inhibition was similar in the subgroups of elderly hypothyroid and euthyroid subjects with comparable TPO-Aab levels, as well as in the subgroups with varying iodine intake.

CONCLUSIONS

(1) In old age, there is a polyclonal TPO autoantibody response but the majority of the autoantibodies are directed to the TPO region mapped by or close to mAb9 (domain A); (2) the autoantibody response does not differ in elderly subjects with or without the clinical manifestations of autoimmune thyroid disease and does not depend on the iodine supply of the elderly subjects.

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

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

Thyroid function and immune profile in rheumatoid arthritis. A controlled study

The aim of our study was to determine the prevalence of thyroid dysfunction and autoimmune abnormalities in rheumatoid arthritis (RA) and to further investigate the possible association between D-penicillamine and autoimmune thyroiditis. For this purpose, one hundred and one unselected consecutive patients with RA and 70 age and sex matched controls were studied prospectively. Evaluation included a complete history and physical examination with special attention to symptoms suggestive of thyroid pathology, routine laboratory and serologic immune profile, plus determination of serum levels of thyroxine (T4), triiodothyronine (T3), thyroid stimulating hormone (TSH), antibodies to thyroid peroxidase (AbTPO) and TSH receptor antibodies (TRAB). Serum thyroxine binding globulin (TBG) was measured in all subjects with high thyroid hormone levels, whereas free T3 and T4 concentrations were determined in all individuals with abnormal T3, T4, TSH or TBG. Six patients with hyperhyroidism, 3 with hypothyroidism and 1 with the euthyroid hyperthyroxinemia (EH) syndrome were found, whereas four of the controls had hyperthyroidism. Thirteen patients and 6 controls had high AbTPO levels whereas no one had high TRAB. No association was detected between thyroid abnormalities and any serologic RA finding. Furthermore, no correlation between thyroid dysfunction and elevated AbTPO's was found. A relatively high prevalence of thyroid dysfunction (9,9%) and subclinical autoimmune thyroiditis (12,9%), the latter indicated by elevated AbTPO's, was found in our RA patients. These figures were higher than those in the control group (5,7% and 8,6% respectively), but the difference did not reach statistical significance. Of further interest may be our finding that, despite anecdotal reports blaming D-penicillamine for cases of autoimmune thyroiditis, the incidence of the latter was similar among recipients and nonrecipients of the drug. Similarly, TRAB were not detected in any patient treated with D-penicillamine.

The nature of autoantigens targeted in autoimmune endocrine diseases

Autoimmune endocrine diseases result from autoimmune processes involving autoreactive T cells and/or autoantibodies. Considerable progress has been made in elucidating the self- (auto-) antigens involved in these processes. These include tissue-specific membrane receptors, enzymes and secreted hormones. As discussed here by Yao-Hua Song and colleagues, diagnostic and therapeutic applications should now result from these discoveries.

Thyroid peroxidase autoantibodies and their effects on enzyme activity in patients with systemic lupus erythematosus

Thyroid microsomal antibodies (Ms-Ab) are recently proved to be directed to thyroid peroxidase (TPO). The aim of this study was to investigate whether the sera of patients with systemic lupus erythematosus (SLE) contain anti-TPO antibodies (TPO-Ab) and whether these antibodies influence enzyme activity. Sera from patient with Hashimoto's thyroiditis was also studied. Serum samples were obtained from 37 patients with SLE, 20 patients with Hashimoto's thyroiditis and 20 healthy subjects. TPO-Ab were detected by immunoprecipitation using crude microsomal preparations or enzyme-linked immunoabsorbent assay (ELISA) with recombinant TPO. Positive TPO-Ab by ELISA were found in 11 (61%) of 18 patients with lupus whose serum contained Ms-Ab. Low levels of TPO-Ab also were found in three (16%) of 19 lupus sera that did not contain Ms-Ab. All patients with Hashimoto's thyroiditis had high levels of TPO-Ab in serum. When measured by ELISA, TPO-Ab were highly correlated with the results of a TPO immunoprecipitation assay and with the titers of Ms-Ab in patients with lupus (r = 0.83, n = 18, P < 0.01; r = 0.63, n = 18, P < 0.01) and in Hashimoto's thyroiditis (r = 0.89, n = 20, P < 0.01; r = 0.75, n = 20, P < 0.01). When evaluating the direct influence of TPO-Ab on the activity of TPO, we found no significant inhibition of enzymatic activity in both guaiacol and iodide assays by lupus sera in contrast with sera from Hashimoto's thyroiditis.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid autoantibodies in hospitalized chronic geriatric patients: prevalence, effects of age, nonthyroidal clinical state, and thyroid function

OBJECTIVE

To investigate the effect of age, clinical condition, and thyroid function on the prevalence of thyroid autoantibody positivity in hospitalized chronic geriatric patients.

DESIGN

A screening study of hospitalized chronic geriatric patients.

PARTICIPANTS

249 non-selected, hospitalized, chronic geriatric patients more than 60 years of age and 81 20 to 40-year-old healthy persons.

MEASUREMENTS

Thyrotropin (TSH); thyroxine (T4) and free thyroxine (FT4); and triiodothyronine (T3), thyroglobulin (Tg), antibodies against thyroid peroxidase (AbTPO) and antibodies against thyroglobulin (AbTg) estimation in a screening study.

RESULTS

AbTPO positivity (AbTPO+) was found more often than AbTg positivity (AbTg+) (15.3% vs 9.2%, P = .04), one being positive (Ab+) in 19.3%. The occurrence was higher in females than males (Ab+:27.1% vs 7.1%, P < .001; AbTPO+:21.9% vs 5.1%, P < .001; AbTg+:13.2% vs 3.1%, P = .0052). Among the Ab+ patients, AbTPO was more often positive than AbTg (40/48 vs 21/48, P < .001). The sensitivity, specificity, and positive predictive value of Ab positivity to detect a thyroid disorder were 0.35, 0.85 and 0.38, respectively. Within the population of euthyroid geriatric patients, the occurrence of AbTg+ (chi 2(2) = 8.65, P = .013) and Ab+ (chi 2(2) = 8.02, P = .018) correlated positively with the age of the patients, and there was also a female predominance (AbTPO+ 18% vs 3.7% in the males; AbTg+ 13% vs 2.4%; Ab+ 25.8% vs 6.1%). When compared with 20 to 40-year-old subjects, only the euthyroid > or = 80-year-old patients showed a significantly higher occurrence of Ab+ (26.2% vs 9.9% chi 2(1) = 5.64, P = .017). In the euthyroid > or = 80-year-old females, AbTPO+ was 25%, AbTg:22.2%, and Ab+: 36.1%!. The nonthyroidal clinical state of the euthyroid patients did not correlate with the antibody prevalence.

CONCLUSIONS

In hospitalized chronic geriatric patients, AbTg and especially AbTPO positivity is frequent, even in euthyroid patients without goiter. This aspecific Ab positivity in the euthyroid state correlates to the age, but not to the severity of the nonthyroidal clinical condition of the patients and explains why the Ab positivity is not predictive enough for thyroid dysfunction in this subpopulation. Thus, in hospitalized chronic geriatric patients the AbTg and AbTPO titers should be examined only in cases where thyroid screening (TSH) reveals abnormal results.

Anti-thyroid peroxidase (anti-TPO) antibodies in thyroid diseases, non-thyroidal illness and controls. Clinical validity of a new commercial method for detection of anti-TPO (thyroid microsomal) autoantibodies

The identification of the thyroid peroxidase (TPO) as the main antigen of the thyroid microsomal fraction has enabled the development of a sensitive and specific assay for detection of the corresponding autoantibodies. We evaluated the diagnostic validity of the anti-TPO assay in 303 patients with different types of thyroid disease and in controls. Clearly elevated anti-TPO values (anti-TPO > 500 units/ml) were found in 59% of patients with thyroiditis but in none of the controls or the patients with non-thyroidal illness. The mean anti-TPO levels in these two control groups were 26 +/- 31 units/ml (mean +/- S.D.) and 39 +/- 34 units/ml, respectively. The highest frequency of positive results (88%) was obtained in patients with auto-immune hypothyroidism (clinical diagnosis: Hashimoto's thyroiditis) followed by patients with Graves' disease (53%). With a cut-off point of 200 units/ml, a sensitivity of 96% was obtained for Hashimoto's thyroiditis and of 59% for Graves' disease with a specificity of 100% (50 cases). The new method (anti-TPO, Dynotest) was compared with three conventional methods (35 samples). The results for all measurements were in general agreement. In two cases the results were clearly discordant: one sample contained high anti-thyroglobulin antibody concentrations, the other was obtained from a patient with non-thyroidal illness. In both instances the 'classical' assays yielded false-positive results. Treatment of autoimmune hyperthyroidism resulted in a median decrease in anti-TPO levels of over 50% after reaching the euthyroid state (P < 0.05), whereas in persistent hyperthyroidism no consistent changes were observed. In autoimmune hypothyroidism a marked variability in anti-TPO levels was noted. Some patients showed a clear decrease in anti-TPO levels during T4 substitution whereas in others no consistent changes were observed.

Thyroid autoimmunity

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

Sensitive enzyme-linked immunosorbent assay for measurement of autoantibodies to human thyroid peroxidase

The development of a sensitive assay for detection of autoantibodies against one of the major thyroid antigens, thyroid peroxidase (TPO), is described. TPO was purified from human thyroid tissue by: (1) isolation of thyroid microsomes using homogenization and differential centrifugation, (2) solubilization of membrane proteins by Zwittergent 3-14, and (3) anion exchange liquid chromatography on a FPLC Mono Q column. Autoantibodies against TPO (TPO-Ab) were measured using an enzyme-linked immunosorbent assay (ELISA) with serum samples diluted 1:100. Standards containing 70, 7, 0.7, 0.02 and 0 U ml-1 TPO-Ab were employed (reference standard code 66/387 NIBSC, London, UK). The detection limit was 0.02 U ml-1 corresponding to 2 U ml-1 in undiluted serum. The inter- and intra-assay coefficients of variation were 8.6% and 5.3%. In 109 healthy control subjects TPO-Ab was found in 9 (8.3%), while 43 (97.7%) out of 44 patients with newly diagnosed untreated Graves' disease had detectable TPO-Ab in serum. All of 16 patients with newly diagnosed spontaneously developing primary hypothyroidism had circulating TPO-Ab (range 16-7000 U ml-1). The new assay is a valuable tool for evaluation of thyroid autoimmunity in individual patients and for studying the epidemiology of thyroid autoimmunity.