Synovial fluid proteins are required for the induction of interleukin-1β production by monosodium urate crystals

Objectives: Monosodium urate (MSU) crystal deposition in gouty joints promotes the release of inflammatory mediators, in particular interleukin (IL)-1 beta. The induction of IL-1 beta production by MSU crystals requires a co-stimulus. The objective of this study was to determine which part of the synovial fluid (SF) provides co-stimulation to MSU crystals to induce IL-1 beta in macrophages. Method: The lipidic fraction (LF) and the protein fraction (PF) were isolated from the SF of patients with arthropathies. The PF was subfractionated according to different molecular weight (MW) ranges. THP-1 cells or human primary monocytes were stimulated with MSU crystals in the presence or absence of SF or SF fractions. IL-1 beta and IL-8 production and IL-1 beta mRNA expression were assessed by an enzyme-linked immunosorbent assay (ELISA) and quantitative real-time polymerase chain reaction (qPCR). Results: Exposure of monocytes/macrophages to MSU crystals alone induced the moderate release of IL-8 but not of IL-1 beta. The production of IL-1 beta required the presence of both SF from patients with inflammatory arthritis (SFi) and MSU crystals. SF from patients with non-inflammatory arthritis, that is patients with osteoarthritis (OA), did not affect the IL-1 beta production but slightly enhanced the secretion of IL-8. Both MSU crystals and SFi were required for the induction of the IL-1 beta transcript, which was not expressed in the presence of either stimulus alone. SFi fractionation demonstrated that the MSU crystal co-stimulus was contained in the PF of SFi with MW > 50 kDa but not in the LF. Conclusions: This study shows that the SF of inflammatory arthritis patients, including gout patients, contains proteins required for the induction of IL-1 beta by MSU crystals in macrophages whereas lipids are not involved.