Binding assay for thyrotropin receptor autoantibodies using the recombinant receptor protein

We have characterized a transfected Chinese hamster ovary cell line, JP09, which expresses high levels of the human TSH receptor (TSH-R). Based on a theoretical biological activity for TSH of 40 IU/mg, JP09 has approximately 90,000 receptors per cell, having a dissociation constant of 1.64 x 10(3) mU/L or 1.47 x 10(-9) mol/L. We have used JP09 to prepare solubilized TSH-Rs which have formed the basis of a binding assay for thyroid-binding inhibiting immunoglobulins in unfractionated sera. We have compared the JP09 assay with the TRAK assay (which is based on solubilized porcine TSH-R) and found a highly positive correlation between the two assays, r = 0.83 P < 0.0001, in 55 sera from patients with autoimmune thyroid disease. JP09 can be adapted to growth in suspension culture, permitting large scale production. The tracer in the assay is bovine [125I]TSH; surprisingly, despite the use of a hTSH-R, hTSH had no effect on the binding of the tracer up to 10(3) mU/L and only a minor effect at 10(4) mU/L.