Differential effects of specific cathepsin S inhibition in biocompartments from patients with primary Sjögren syndrome

Abstract Objective Primary Sjögren syndrome (pSS) is characterized by T and B cell infiltration of exocrine glands. The cysteine protease cathepsin S (CatS) is crucially involved in MHCII processing and T cell stimulation, and elevated levels have been found in patients with RA, psoriasis and pSS. However, little is known about the functional characteristics and mechanisms of SS-A- and SS-B-specific T cells in pSS patients. We herein investigated the inhibition of CatS activity in different biocompartments of pSS patients including antigen-specific T cell responses. Methods Ex vivo CatS activity was assessed in tears, plasma and saliva of 15 pSS patients and 13 healthy controls (HC) and in the presence or absence of the specific CatS inhibitor RO5459072. In addition, antigen (SS-A (60kD), SS-B, influenza H3N2, tetanus toxoid and SEB)-specific T cell responses were examined using ex vivo IFN-γ/IL-17 Dual ELISPOT and Bromdesoxyuridin (BrdU) proliferation assays in the presence or absence of RO5459072. Supernatants were analysed for IL-1β, IL-6, IL-10, TNF-α, IL-21, IL-22 and IL-23, using conventional ELISA. Results CatS activity was significantly elevated in tear fluid, but not other biocompartments, was inversely associated with exocrinic function in pSS patients and could significantly be suppressed by RO5459072. Moreover, CatS inhibition by RO5459072 led to strong and dose-dependent suppression of SS-A/SS-B-specific T cell effector functions and cytokine secretion by CD14+ monocytes. However, RO5459072 was incapable of suppressing SS-A/SS-B-induced secretion of cytokines in CD14+ monocytes when T cells were absent, confirming a CatS/MHCII-mediated mechanism of suppression. Conclusion CatS activity in tear fluid seems to be a relevant biomarker for pSS disease activity. Conversely, CatS inhibition diminishes T cell and associated monokine responses towards relevant autoantigens in pSS. Thus, CatS inhibition may represent a promising novel treatment strategy in pSS.