TLR9‐dependent dendritic cell maturation promotes IL‐6‐mediated upregulation of cathepsin X.

Cysteine cathepsins are lysosomal proteases subject to dynamic regulation within antigen‐presenting cells during the immune response and associated diseases. To investigate the regulation of cathepsin X, a carboxy‐mono‐exopeptidase, during maturation of dendritic cells (DCs), we exposed immortalized mouse DCs to various Toll‐like receptor agonists. Using a cathepsin X‐selective activity‐based probe, sCy5‐Nle‐SY, we observed a significant increase in cathepsin X activation upon TLR‐9 agonism with CpG, and to a lesser extent with Pam3 (TLR1/2), FSL‐1 (TLR2/6) and LPS (TLR4). Despite clear maturation of DCs in response to Poly I:C (TLR3), cathepsin X activity was only slightly increased by this agonist, suggesting differential regulation of cathepsin X downstream of TLR activation. We demonstrated that cathepsin X was upregulated at the transcriptional level in response to CpG. This occurred at late time points and was not dampened by NF‐κB inhibition. Factors secreted from CpG‐treated cells were able to provoke cathepsin X upregulation when applied to naïve cells. Among these factors was IL‐6, which on its own was sufficient to induce transcriptional upregulation and activation of cathepsin X. IL‐6 is highly secreted by DCs in response to CpG but much less so in response to poly I:C, and inhibition of the IL‐6 receptor subunit glycoprotein 130 prevented CpG‐mediated cathepsin X upregulation. Collectively, these results demonstrate that cathepsin X is differentially transcribed during DC maturation in response to diverse stimuli, and that secreted IL‐6 is critical for its dynamic regulation. [ABSTRACT FROM AUTHOR]