Targeting Cathepsin C in PR3-ANCA Vasculitis
Authors: Jerke Uwe , Eulenberg-Gustavus Claudia , Rousselle Anthony , Nicklin Paul , Kreideweiss Stefan , Grundl Marc A. , Eickholz Peter , Nickles Katrin , Schreiber Adrian , Korkmaz Brice , Kettritz Ralph 
Published: March 2022

Significance Statement
In ANCA–associated vasculitis (AAV), the autoantigens proteinase 3 (PR3) and myeloperoxidase (MPO) are exclusively expressed by neutrophils and monocytes. Cathepsin C (CatC), which activates the proteolytic function of neutrophil serine proteases (NSPs), provides a potential novel treatment target by controlling NSPs in both human neutrophils and monocytes. The authors show that genetic deficiency of CatC in humans and pharmacologic inhibition of CatC in a human stem cell model effectively downregulate NSPs, including PR3. Consequently, these neutrophils showed a diminished activation response to PR3-ANCA but not to MPO-ANCA. Moreover, both genetic and pharmacologic NSP depletion resulted in less neutrophil-induced injury of glomerular microvascular endothelial cells. These findings may offer encouragement for pursuing clinical studies with adjunctive CatC inhibitor administration in patients with PR3-AAV.

Background
The ANCA autoantigens proteinase 3 (PR3) and myeloperoxidase (MPO) are exclusively expressed by neutrophils and monocytes. ANCA-mediated activation of these cells is the key driver of the vascular injury process in ANCA-associated vasculitis (AAV), and neutrophil serine proteases (NSPs) are disease mediators. Cathepsin C (CatC) from zymogens activates the proteolytic function of NSPs, including PR3. Lack of NSP zymogen activation results in neutrophils with strongly reduced NSP proteins.

Methods
To explore AAV-relevant consequences of blocking NSP zymogen activation by CatC, we used myeloid cells from patients with Papillon-Lefèvre syndrome, a genetic deficiency of CatC, to assess NSPs and NSP-mediated endothelial cell injury. We also examined pharmacologic CatC inhibition in neutrophil-differentiated human hematopoietic stem cells, primary human umbilical vein cells, and primary glomerular microvascular endothelial cells.

Results
Patients with Papillon-Lefèvre syndrome showed strongly reduced NSPs in neutrophils and monocytes. Neutrophils from these patients produced a negative PR3-ANCA test, presented less PR3 on the surface of viable and apoptotic cells, and caused significantly less damage in human umbilical vein cells. These findings were recapitulated in human stem cells, in which a highly specific CatC inhibitor, but not prednisolone, reduced NSPs without affecting neutrophil differentiation, reduced membrane PR3, and diminished neutrophil activation upon PR3-ANCA but not MPO-ANCA stimulation. Compared with healthy controls, neutrophils from patients with Papillon-Lefèvre syndrome transferred less proteolytically active NSPs to glomerular microvascular endothelial cells, the cell type targeted in ANCA-induced necrotizing crescentic glomerulonephritis. Finally, both genetic CatC deficiency and pharmacologic inhibition, but not prednisolone, reduced neutrophil-induced glomerular microvascular endothelial cell damage.

Conclusions
These findings may offer encouragement for clinical studies of adjunctive CatC inhibitor in patients with PR3-AAV.