Low Secretory Leukocyte Protease Inhibitor (SLPI)-Level Potentiates Alternaria Extract-Induced T-Helper 2 (Th2) Airway Inflammation via the Interleukin-33 (IL-33) Pathway

Introduction Serine proteases play a critical role in the augmented release and cleavage of IL-33, leading to the expansion of group 2 innate lymphoid cells (ILC2s) and T-helper 2 (Th2) airway inflammation. However, the protective regulation of protease-dependent interleukin-33 (IL-33) activation remains poorly understood. Therefore, we investigated the role of secretory leukocyte protease inhibitor (SLPI), as a serine protease inhibitor, in this protective regulation and aimed to clarify its contribution to type 2 immunity. Methods We evaluated the role of SLPI in the <i>Alternaria</i> extract-induced expansion of ILC2s and Th2-type airway inflammation via IL-33, using three models: SLPI-deficient mice, an in vivo SLPI knockdown model with shRNA, and an in vitro model utilizing primary human bronchial epithelial cells (HBECs) exposed to <i>Alternaria</i> extract under various conditions, including plasmid transfection. Results We showed that two mouse models of downregulation of<i> SLPI </i>gene expression augmented <i>Alternaria</i> extract-induced release of IL-33 and the expansion of ILC2s, together with Th2 airway inflammation. Furthermore, two treatment models using <i>SLPI</i> KO mice, administration of a serine protease inhibitor, bovine pancreatic trypsin inhibitor, or anti-IL-33 antibody, attenuated Th2 airway inflammation. In two in vitro experiments, SLPI, as a serine protease inhibitor, prevented both the release of IL-33 from HBECs and the cleavage of full-length IL-33 to shorter mature forms by neutrophil elastase. Discussion These findings suggest that SLPI functions as a key serine protease inhibitor in regulating IL-33-mediated type 2 immune responses. Low SLPI levels may contribute to the pathogenesis of asthma by promoting Th2 inflammation, highlighting SLPI as a potential therapeutic target in patients with low SLPI expression.