IL-34 is overexpressed in the inflamed salivary glands of patients with Sjögren’s syndrome and is associated with the local expansion of pro-inflammatory CD14CD16 monocytes

Objectives. To investigate the expression of IL-34 in labial salivary glands (LSGs) of patients with primary SS (p-SS) and its role in inducing a pro-inflammatory monocyte phenotype. Methods. LSG biopsies were obtained from 20 patients with p-SS and 10 patients with non-Sjögren’s sicca syndrome (n-SS). The expression of IL-34, IL-1b, TNF-a, IL-17 and IL-23 was assessed by real-time PCR. IL-34 expression was also investigated in LSGs by immunohistochemistry. The frequencies of subpopulations of CD14 monocytes were evaluated by flow cytometry among isolated mononuclear cells from peripheral blood and salivary glands from both patients and controls. The role of recombinant IL-34 on isolated peripheral blood mononuclear cells was also evaluated. Results. IL-34 m-RNA was overexpressed in the inflamed salivary glands of p-SS and associated with increased expression of TNF-a, IL-1b, IL-17 and IL-23p19. The increased expression of IL-34 was confirmed by immunohistochemistry in paraffin-embedded salivary glands from p-SS patients. IL-34 expression was accompanied by the expansion of pro-inflammatory CD14CD16 monocytes in the salivary glands. In vitro stimulation of peripheral blood mononuclear cells with IL-34 induced the expansion of both CD14CD16 cells and CD14CD16 cells in p-SS and non-SS subjects. Conclusion. IL-34 seems to be involved in the pathogenesis of salivary gland inflammation in p-SS.