Immunoprecipitation of TSH-TSH receptor complexes

The ability of Graves' sera to interact with the TSH receptor crosslinked to a 125I-labelled photoactive derivative of TSH has been investigated. Crosslinked complexes were prepared using non-purified detergent solubilized human thyroid and guinea pig fat TSH receptors. Affinity purified porcine TSH receptor preparations were also used. After crosslinking, the crosslinked TSH-TSH receptor complexes were separated from aggregates and free TSH on Sephacryl S-300, incubated with test sera followed by immunoprecipitation using anti-IgG or Protein A. Using non-purified human TSH receptors crosslinked to TSH, a mean ± SD of 12±· 4·9% of the crosslinked complex was immunoprecipitated with Graves' sera (n= 7) compared with 10·3 · 2·6% with Hashimoto sera (n= 6; P > 0±4) and 3·8 · 1·0% with normal sera (n= 6; P < 0·004). These values were markedly reduced when TSH receptor preparations free of other thyroid autoantigens (guinea pig fat TSH receptors) were used. Under these conditions immunoprecipitation with Graves' sera (n= 24) was 1·6 · 1·3% compared with 0·8 · 0·6% for Hashimoto sera (n= 13) and 0·8 · 0·4% for normal sera (n= 2; P= 0·003). In addition complexes formed between TSH and affinity purified porcine TSH receptors gave low immunoprecipitation values for Graves' (1·44 · 0·73%; n= 20) and Hashimoto sera (1·7 · 0·94; n= 11) which were not significantly different (P= 0·4). Overall, therefore, the effects of Graves' and Hashimoto sera were similar and the amounts of material immunoprecipitated were markedly reduced when TSH receptor preparations containing reduced amounts of other autoantigens were used. Consequently the Graves' sera did not appear to interact specifically with crosslinked TSH-TSH receptor complexes. However the Graves' sera studied did contain TSH receptor antibodies which could inhibit the binding of labelled TSH to TSH receptors in the preparations used and our results suggest that the binding of TSH and these antibodies to the receptor is mutually exclusive.