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

Abstract

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.