Activation of innate immunity by 14-3-3 ε, a new potential alarmin in osteoarthritis

Objective: The innate immune system plays a central role in osteoarthritis (OA). We identified 14-3-3 epsilon as a novel mediator that guides chondrocytes toward an inflammatory phenotype. 14-3-3 epsilon shares common characteristics with alarmins. These endogenous molecules, released into extracellular media, are increasingly incriminated in sustaining OA inflammation. Alarmins bind mainly to toll-like receptor 2 (TLR2) and TLR4 receptors and polarize macrophages in the synovium. We investigated the effects of 14-3-3 epsilon in joint cells and tissues and its interactions with TLRs to define it as a new alarmin involved in OA. Design: Chondrocyte, synoviocyte and macrophage cultures from murine or OA human samples were treated with 14-3-3 epsilon. To inhibit TLR2/4 in chondrocytes, blocking antibodies were used. Moreover, chondrocytes and bone marrow macrophage (BMM) cultures from knockout (KO) TLRs mice were stimulated with 14-3-3 epsilon. Gene expression and release of inflammatory mediators [interleukin 6 (IL-6), monocyte chemoattractant protein-1 (MCP-1), tumor necrosis factor alpha (TNF alpha)] were evaluated via reverse transcription quantitative polymerase chain reaction (RT-qPCR) and ELISA. Results: In vitro, 14-3-3 epsilon induced gene expression and release of IL6 and MCP1 in the treated cells. The inflammatory effects of 14-3-3 epsilon were significantly reduced following TLRs inhibition or in TLRs KO chondrocytes and BMM. Conclusions: 14-3-3 epsilon is able to induce an inflammatory phenotype in synoviocytes, macrophages and chondrocytes in addition to polarizing macrophages. These effects seem to involve TLR2 or TLR4 to trigger innate immunity. Our results designate 14-3-3 epsilon as a novel alarmin in OA and as a new target either for therapeutic and/or prognostic purposes. (C) 2020 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.