Exosomes Expressing Thyrotropin Receptor Attenuate Autoantibody-Mediated Stimulation of Cyclic Adenosine Monophosphate Production

Background: Exosomes or small extracellular vesicles secreted from cells are nanovesicles with a diameter of 40-150 nm, which play a number of roles in both physiologic and pathologic processes. In Graves' disease (GD), autoantibodies bind to the thyrotropin receptor (TSHR) on the surface of thyroid follicular epithelial cells and stimulate thyroid growth and thyroid hormone synthesis and secretion via cyclic adenosine monophosphate (cAMP) production. The present study aimed to confirm the existence of TSHR in exosomes secreted from thyroid cells and to define the role of TSHR exosomes in GD. Methods: Exosomes were isolated by differential centrifugation from the culture medium of the human thyroid follicular epithelial cell line (NTHY-ori 3-1) and thyroid carcinoma cell lines (8305C, 8505C, and FTC-133). TSHR expression in cell lysates and exosomes was evaluated by Western blot analysis. In order to study the function of TSHR exosomes, human embryonic kidney (HEK) 293 cells stably expressing TSHR (HEK/TSHR) were established. Using exosomes isolated from both HEK and HEK/TSHR cells, the binding capacity of the M22 human monoclonal autoantibody to TSHR exosomes and their effect on M22-mediated stimulation of cAMP production in HEK/TSHR cells were evaluated. As a positive control for the functional assay, human recombinant TSHR chimera protein capable of binding to TSH was used. Results: TSHR was detected in exosomes from cancer cells as well as normal epithelial cells. An in vitro binding assay showed that alkaline phosphatase-labeled M22 bound to TSHR exosomes in a dose-dependent manner. M22 dose-dependently stimulated intracellular cAMP production in HEK/TSHR cells. The addition of exosomes from HEK/TSHR cells but not those from parental HEK cells significantly ameliorated cAMP production stimulated by treatment with M22 in HEK/TSHR cells. A decoy effect similar to TSHR exosomes was observed for human recombinant TSHR chimera. Conclusions: The results suggest that exosomes expressing TSHR may be secreted from normal and cancerous thyroid cells. In the thyroid gland of patients with GD, TSHR exosomes may exert a decoy effect by sequestering autoantibody, thereby ameliorating autoantibody-mediated activation of thyroid function.