Assessment of the Frequency of Mutant ( hprt - ) T Lymphocytes from Peripheral Blood of Patients with Hashimoto's Thyroiditis

A salient feature of Hashimoto's thyroiditis (HT) is the T-cell-mediated destruction of the thyroid gland leading to hypothyroidism. In HT, as in other autoimmune diseases, a central premise has been that autoreactive T cells must be dividing in response to autoantigens, accumulating random spontaneous mutations during the activation process. Here, we have examined this hypothesis by using as monitor of somatic cell mutation the hprt gene, encoding the salvage pathway enzyme hypoxanthine-guanine phosphoribosyl transferase. Eleven newly diagnosed patients with HT and 10 patients with chronic disease were selected for the study, whereas 10 healthy individuals were used as controls. Peripheral T cells were cultured under limiting dilution conditions in the presence of 6-thioguanine and the frequency(MF) of surviving mutant hprt ^- T cells was calculated by Poisson statistics. It was observed that the mean MF value of either patient group (6.6 ± 5.8 per 10⁶ cells for the newly diagnosed, and 8.8 ± 4.0 per 10⁶ cells for the patients with chronic disease) was not significantly different (p > 0.05) from that of the control group (6.8 ± 6.4 per 10⁶ cells). These data do not support the concept that patients with HT have an increased number of actively dividing T cells in the circulation compared to healthy controls. Autoreactive T cells may be activated mainly in situ or home readily to the thyroid in the early stages of the disease and reach a nonexpansion stage as the chronic disease is stabilized.