Thyroid cells in Graves' disease and Hashimoto's thyroiditis stimulate allogeneic T cells when pretreated with phorbol ester

OBJECTIVE To assess the capacity of thyroid follicular cells to function as antigen presenting cells, we have examined their ability to stimulate allogeneic T cells. DESIGN Thyroid follicular cells were pretreated with interferon‐γ or phorbol myristate acetate, washed thoroughly, and their capacity to induce allogeneic T cell proliferation was determined. PATIENTS Thyroid cells were prepared using thyroidectomy specimens from eight patients with Graves' disease, one with Hashimoto's thyroiditis and two with non‐toxic multinodular goitre. MEASUREMENTS T cell 3 H‐thymidine incorporation was measured after a 16‐hour incubation period with the isotope, 3 days after co‐culture of T cells and thyroid cells. RESULTS Four of the eight thyroid cell preparations from thyroid autoimmunity patients failed to stimulate T cells, although there was a significant, weak stimulation for the whole group (P <0.05): interferon‐γ pretreatment had no effect on this. Phorbol myristate acetate pretreatment significantly increased the ability of thyroid cells (from both autoimmune and multinodular glands) to stimulate T cells (P <0.05); this was time and concentration dependent. Cell fixation after PMA treatment did not abolish this stimulatory activity, which could be transferred by supernatants from unfixed cells; exogenous interleukin‐1 did not mimic the activity. CONCLUSIONS Thyroid cells expressing major histocompatibility complex class II molecules only weakly and inconsistently stimulate allogeneic T cells, compared to thyroid cells pretreated with phorbol myristate acetate, a difference which may be due to the expression of an unidentified co‐stimulatory signal induced by phorbol ester.