Thyrotropin Binding to Cultured Lymphocytes and Thyroid Cells*

Abstract

The TSH [thyrotropin]-binding properties of human lymphocytes in continuous culture were studied and compared to those of bovine and human thyroid cells in primary culture. Both lymphocytes and thyroid cells had maximal TSH-binding capacity at pH 5.2. At pH 7.4, thyroid cells bound 15% but lymphocytes bound only 3% of the amount bound at pH 5.2. At 37.degree. C, maximal binding of [125I]iodo-TSH to lymphocytes was reached within 60-90 min and maximal binding to thyroid cells was reached within 15-20 min. TSH binding to lymphocytes was salt sensitive, being inhibited to 50% by 0.2 mM MgCl and 0.4 mM CaCl2 and by 20 mM KI, KCl, and NaCl. The saturable binding of bovine [b] TSH to thyroid cells at pH 5.2 and 7.4 was above 90% of the total binding. Saturable binding to lymphocytes at pH 5.2 was also above 90%, but at pH 7.4 was 75% of total. At pH 5.2, both cell types displayed identical displacement curves of [125I]iodo-bTSH by unlabeled bTSH. Pure hCG [human chorionic gonadotropin] human placental lactogen, human GH [Somatotropin] and insulin cross-reacted to less than 1% with [125I]iodo-bTSH binding to lymphocytes at pH 5.2, whereas a crude preparation of hCG and human FSH [follitropin] plus human LH [lutropin] showed a strong cross-reaction. Nonhormone glycoproteins, including mucin, normal human .gamma.-globulin, and b-thyroglobulin showed intermediate cross-reactivity. At pH 7.4, the cross-reactivity of normal human .gamma.-globulin, thyroglobulin, and pure hCG with bTSH binding to both lymphocytes and thyroid cells was below 1%. The TSH-binding properties of lymphocytes and thyroid cells show many similiarities but differ in kinetics and the relative binding capacity at neutral pH. Although the physiological significance of these differences is not yet clear, cultured cells provide a convenient system for studies of TSH-receptor interaction.