Phagocyte-specific S100A8/A9 is upregulated in primary Sjögren's syndrome and triggers the secretion of pro-inflammatory cytokines in vitro

OBJECTIVES: To determine the role of S100A8/A9 in the pathogenesis of primary Sjögren's syndrome (pSS).

METHODS: The serum levels of S100A8/A9 were determined in pSS patients and healthy controls by ELISA. The expression of S100A8/A9 in salivary glands was assessed by immunohistochemistry. The phenotype of S100A8+ and S100A9+ cells was identified using double immunofluorescence. The effects of S100A8/A9 on cytokine production by peripheral blood mononuclear cells (PBMCs) from pSS patients were determined in vitro by flow cytometry. The effects of pro-inflammatory cytokines on S100A8/A9 secretion were additionally investigated in vitro by ELISA in PBMCs from pSS patients and control subjects.

RESULTS: Serum levels of S100A8/A9 were significantly increased in pSS patients compared to healthy controls. The tissular expression of S100A8 and S100A9, identified in professional phagocytes (neutrophils, monocytes and plasmacytoid dendritic cells), was increased in the salivary glands of pSS patients and correlated with focus score. In vitro, recombinant S100A8/A9 increased the production of IL-1β, IL-6, TNF-α, IFN-γ, IL-10, IL-17A and IL-22 by PBMCs. The S100A8/A9-induced increase in TNF-α production in pSS patients was significant relative to controls. Furthermore, IL-1β, TNF-α, IL-6, and IL-17A stimulated release of S100A8/A9 from PBMCs in pSS patients.

CONCLUSIONS: S100A8/A9 is increased in pSS patients contributing to the in vitro increased production of pro-inflammatory cytokines. As such, S100A8/A9 in concert with other cytokines might contribute to the pathogenesis of pSS.