The mechanism by which the thionamide antithyroid drugs produce long-term remission of Graves hyperthyroidism is unknown. We wished to explore the hypothesis that methimazole (MMI) could induce a structural change in thyroid peroxidase (TPO), such that its major (immunodominant) epitope(s) was no longer recognised by circulating anti-TPO antibodies.
An enzymatically active membrane preparation of a CHO cell-line stably transfected with human TPO was used as a source of TPO, which was bound to immunoassay plates with a monoclonal murine anti-human TPO antibody (mAb47). A membrane preparation from untransfected CHO cells was used as a negative control. Serum samples from 36 sequential Graves patients, known to be positive for TPO antibodies, and 2 healthy controls, were used in the above system to form a sandwich ELISA. Antibody binding to immobilised TPO was tested at various concentrations of MMI (500 nM to 500 mM), with TPO in its native conformation, and following irreversible denaturation, with DTT (50 mM) and iodoacetamide (40 mM), to an enzymatically inert form.
Sera from >90% of Graves samples bound the immobilized TPO in a concentration dependent fashion. Twenty out of the 36 sera showed a greater than 25% decrement in binding to the denatured TPO preparation when compared to native TPO, showing a clear structural dependency of their immunoreactivity. The effect of MMI on TPO-antibody interaction in the ELISA was studied, using either preincubation, co-incubation or adding MMI after serum-binding. There was no effect of MMI on the binding of sera at relevant in vivo concentrations (up to 100 μM), for either structurally dependent or independent sera. Binding of the monoclonal antibody mAb47 could not be abrogated by denaturation or MMI.
A change in TPO epitope conformation induced by methimazole does not seem to be the explanation for the immunomodulatory effects of antithyroid drugs in Graves disease.