Purpose: To better understand the pathogenesis of thyroid-associated orbitopathy (TAO) through elucidating the role of thyrotropin receptor (TSHR) and CD40 in the expression of interleukin-8 (IL-8) in peripheral blood fibrocytes. Fibrocytes infiltrate the orbit of patients with TAO, where they differentiate into fibroblasts. Fibrocyte precursors occur with increased frequency in the peripheral blood expressing TSHR and CD40 in TAO patients. We hypothesize that in vitro derived fibrocytes and peripheral blood fibrocyte precursors express proinflammatory chemoattractant molecules including IL-8 initiated by TSHR and CD40 signaling. Since nearly all TAO patients express activating antibodies to TSHR, this is particularly relevant for activation of peripheral blood fibrocytes.
Methods: TSHR and CD40 expression on peripheral blood fibrocytes was determined by flow cytometry. IL-8 RNA was quantitated by real-time polymerase chain reaction. IL-8 protein production was measured by Luminex and flow cytometry. Thyroid-stimulating hormone and CD40 ligand-stimulated phosphorylation of Akt in peripheral blood fibrocytes was studied by flow cytometry.
Results: Both TSHR- and CD40-mediated signaling lead to IL-8 expression in mature fibrocytes. Fibrocyte precursors assayed directly from circulating peripheral blood demonstrate intracellular IL-8 expression with addition of thyroid-stimulating hormone or CD40 ligand. TSHR- and CD40-induced IL-8 production is mediated by Akt phosphorylation.
Conclusions: Peripheral blood TSHR(+) and CD40(+) fibrocytes express IL-8 and may promote the recruitment of inflammatory cells, mitogenesis, and tissue remodeling in TAO. TSHR- and CD40-mediated IL-8 signaling is mediated by Akt. Delineating the molecular mechanisms of fibrocyte immune function may provide potential therapeutic targets for TAO.