Endogenous TGF-β activity limits TSLP expression in the intervertebral disc tissue by suppressing NF-κB activation

Thymic stromal lymphopoietin (TSLP), an IL-7-like cytokine, is highly expressed in herniated disc (HD) tissue and may act as a key molecule for the initiation of macrophage recruitment into the tissue and natural resorption of HD. However, it remains unclear how TSLP expression is regulated in the intervertebral discs. This study showed that expression of TSLP and phosphorylated NF-κB in HD tissue samples was inversely correlated with expression of phosphorylated Smad2/3 (an indicator of active TGF-β signaling) and vice versa in posterior lumbar spinal fusion samples. The pharmacological blockades of endogenous TGF-β activity induced TSLP expression in mouse intervertebral disc tissue culture, which was inhibited by NF-κB inhibitors. Additionally, phosphorylation of Smad2/3 was constitutively detected in mouse intervertebral disc tissue in the steady states. Collectively, these results suggest that endogenous TGF-β activity limits TSLP expression in intervertebral disc tissue in the steady states by suppressing NF-κB activation. The findings reveal a regulatory mechanism how TSLP expression is induced in the intervertebral disc tissue and suggest a novel role of TGF-β in maintaining the homeostasis of intervertebral disc tissue.