CTSS contributes to airway neutrophilic inflammation in mixed granulocytic asthma.

Background: Mixed granulocytic asthma (MGA) is usually associated with poor response to corticosteroid therapy and a high risk of severe asthma. Cathepsin S (CTSS) has been found to play an important role in various inflammatory diseases. This study was aimed to investigate the role of CTSS in MGA.

Methods: Induced sputum was obtained from healthy subjects and asthma patients. Two murine models of MGA were established using either TDI (toluene diisocyanate) alone or OVA emulsified in CFA. LY3000328, a specific antagonist of CTSS, was therapeutically given to BALB/c mice after airway challenge with TDI or OVA. The effects of recombinant CTSS was tested in vivo, and Akt inhibition was used to explore a possible mechanism for CTSS-induced airway inflammation.

Results: MGA patients have a significant higher sputum CTSS level than the health and subjects with other inflammatory phenotypes, which was positively correlated with sputum level of soluble E-cadherin (sE-cadherin), sputum neutrophils, FeNO, FEF25-75% and glucocorticoid dosage. Allergen exposure markedly increased CTSS level and pharmacological antagonism of CTSS with LY3000328 decreased airway hyperresponsiveness, airway neutrophil accumulation, as well as the release of IL-17 and sE-cadherin in murine models of MGA, yet had no effects on eosinophilic inflammation nor type 2 inflammatory cytokines (IL-4 and IL-5). In addition, intratracheal instillation of recombinant CTSS leads to neutrophil recruitment and overproduction of sE-cadherin in the lung tissues, which could be attenuated by inhibition of Akt signaling.

Conclusion: Our data suggested that CTSS contributes to airway neutrophilic inflammation in MGA through an Akt-dependent pathway.