Impact of dipeptidyl peptidase I and neutrophil serine proteases on neutrophil functional responses.

Dipeptidyl peptidase-1 (DPP1) is a lysosomal cysteine protease essential for activating neutrophil serine proteases (NSPs), including neutrophil elastase, cathepsin G, and proteinase 3, during neutrophil differentiation in the bone marrow. Because NSP-mediated tissue damage contributes to chronic inflammatory and autoimmune diseases, targeting NSPs has emerged as a therapeutic strategy. In this regard, small molecule drugs have been developed such as brensocatib-a competitive, reversible DPP1 inhibitor. Active DPP1 is expressed in mature neutrophils, which raises the possibility that DPP1 inhibition might affect more than just NSP activities. Here, we investigated the effects of DPP1 and NSP inhibition or ablation on neutrophil function using brensocatib, as well as DPP1 -/- and triple NSP knockout (NE -/- CatG -/- Pr3 -/- ) mice. As expected, DPP1 inhibition with brensocatib during mouse bone marrow hematopoietic stem cell differentiation dose-dependently reduced NSP activities. While DPP1 inhibition or ablation suppressed neutrophil extracellular trap (NET) formation in mice, it had no significant impact on granulocytic differentiation, migration, phagocytosis, reactive oxygen species production, or bacterial killing. A similar outcome was observed in triple NSP KO mice. In mature human neutrophils, brensocatib had no effect on any of these responses, including NET formation. These findings suggest that while DPP1 is crucial for NSP activation during early neutrophilic differentiation, it does not substantially influence differentiation itself or core neutrophil functions, except for NET formation in mice. This study advances our understanding of the roles of DPP1 and NSPs in neutrophil biology and further emphasizes the high selectivity of brensocatib in the main target cells.