Thyroid peroxidase is the organ-specific 'microsomal' autoantigen involved in thyroid autoimmunity

Abstract

Abstract. Autoantibodies (aAb) in serum of patients with autoimmune thyroid diseases (AITD) are directed to an antigen associated with thyroid microsomes. Although it has been investigated over almost three decades, the nature of this autoantigen remained unknown. Taking advantage of monoclonal antibodies (mAb) produced in our laboratory, we have demonstrated that thyroid peroxidase (TPO) is the 'microsomal' antigen. Sera of patients with AITD strongly inhibited the binding of only one of 19 mAb raised against human thyroid plasma membranes. This mAb did not react with thyroglobulin but achieved significant binding to preparations of human, bovine and porcine TPO, bovine lactoperoxidase and human myeloperoxidase without altering the enzyme activity. The mAb has been used to immunopurify the human TPO from solubilized thyroid microsomes. The procedure allowed high purification (∼ 3500-fold) of the native enzyme with a reasonable yield (∼ 10 mg TPO/kg thyroid tissue). Human TPO exhibited a specific activity of 350–400 guaiacol U/mg, a peak in the Soret region and a ratio of A411 nm to A280 nm of 0.20–0.25. Upon SDS-polyacrylamide gel electrophoresis, the purified enzyme gave two contiguous bands in the 100 kDa region. Performed in non-reducing conditions, electrophoresis of TPO showed one band in the same 100 kDa region. Sera with aAb to the microsomal antigen immunoprecipitated purified TPO to an extent ranging from 80 to 100% of the initial enzyme amount while sera from normal subjects or from patients with undectable level of anti-microsomal aAb elicit a decrease of less than 30% of the total TPO activity. IgG from sera of patients with AITD were also shown to bind to purified TPO and to inhibit, in a dose-dependent manner the mAb binding to TPO. Furthermore anti-microsomal aAb were specifically adsorbed onto TPO-sepharose column and eluted only with high ionic strength buffers. Taking into account that TPO shares almost all the characteristics attributed to the microsomal antigen and that anti-TPO and anti-microsomal aAb are undistinguishable, it can be concluded that TPO is the thyroid autoantigen termed to date the 'microsomal' antigen.