Conclusions
Our findings identify CTSL as a protein protective against lung fibrosis via its activation of antifibrotic ES, and whose expression in SSc pLFs and lung tissues is suppressed. Identifying strategies to boost CTSL endogenous levels in SSc patients could serve as a viable therapeutic strategy.
Methods
Fibrosis was induced experimentally using TGF-β in vitro, in primary human lung fibroblasts (pLFs), and ex vivo, in human lung tissues. ES and CTSL expression was quantified using ELISA, RT-qPCR, immunoblotting or immunofluorescence. Recombinant NC1-FLAG peptide was used to assess CTSL cleavage activity. CTSL expression was also compared between SSc vs normal (NL)-derived pLFs and lung tissues.
Results
ES levels were significantly reduced in media conditioned by TGF-β-induced pLFs. TGF-β-stimulated pLFs significantly reduced expression and secretion of CTSL into the extracellular matrix (ECM). CTSL was also sequestered in its inactive form into extracellular vesicles, further reducing its availability in the ECM. Media conditioned by TGF-β-induced pLFs showed reduced cleavage of NC1-Flag and reduced release of the antifibrotic ES fragment. SSc-derived pLFs and lung tissues expressed significantly lower levels of CTSL compared with NL. Conclusions: Our findings identify CTSL as a protein protective against lung fibrosis via its activation of antifibrotic ES, and whose expression in SSc pLFs and lung tissues is suppressed. Identifying strategies to boost CTSL endogenous levels in SSc patients could serve as a viable therapeutic strategy.