Abstract
Background: Innate immune response components such as toll-like receptors (TLRs) and NLRP3-inflammasome act in concert to increase IL-1α/β secretion by synovial macrophages. Previous results suggest that IL-1α/β could be an important mediator involved in the pathogenesis of osteoarthritis (OA).Objectives: The aim of our study was to evaluate the role of NLRP3, IL-1β, and IL-1α in the menisectomy (MNX) model of murine OA.Methods: Murine chondrocytes (CHs) and bone marrow-derived machrophages (BMDM) were stimulated with hydroxyapatite (HA) crystals, a form of calcium-containing crystal found in human OA, and IL-1β and IL-6 secretion assayed by ELISA.Conversely, the ability of IL-1β and IL-6 to induce CHs calcification was assessed in vitro by Alizarin red staining. Knees from 8 to 10 weeks old C57Bl/6J wild-type (WT) (n = 7), NLRP3−/− (n = 9), IL-1α−/− (n = 5), and IL-1β−/− (n = 5) mice were menisectomized, using the sham-operated contralateral knee as control. 8 weeks later, knee cartilage degradation and synovial inflammation were evaluated by histology. In addition, apoptotic chondrocytes, metalloproteases activity, and collagen-type 2 expression were evaluated in all mice. Joint calcification and subchondral bone parameters were quantified by CT-scan in WT and IL-1β−/− menisectomized knees.Results: In vitro, HA crystals induced significant increased IL-6 secretion by CHs, while IL-1β remained undetectable.Conversely, both IL-6 and IL-1β were able to increase chondrocytes mineralization. In vivo, operated knees exhibited OA features compared to sham-operated knees as evidenced by increased cartilage degradation and synovial inflammation. In menisectomized KO mice, severity and extent of cartilage lesions were similar (IL-1α−/− mice) or exacerbated (IL-1β−/− and NLRP3−/− mice) compared to that of menisectomized WT mice. Metalloproteases activity, collagen-type 2 expression, chondrocytes apoptosis, and synovial inflammation were similar between KO and WT mice menisectomized knees. Moreover, the extent of joint calcification in osteoarthritic knees was comparable between IL-1β−/− and WT mice.Conclusions: MNX knees recapitulated features of OA, i.e, cartilage destruction, synovial inflammation, cell death, and joint calcification. Deficiency of IL-1α did not impact on the severity of these features, whereas deficiency of IL-1β or of NLRP3 led to increased cartilage erosion. Our results suggest that IL-1α and IL-1β are not key mediators in this murine OA model and may explain the inefficiency of IL-1 targeted therapies in OA.
Highlights
Osteoarthritis (OA) is a progressive disease of the joint tissues, characterized by cartilage degradation (Goldring and Goldring, 2007; Loeser et al, 2012), mild synovial inflammation (Scanzello and Goldring, 2012), subchondral bone sclerosis, osteophyte formation, and calcium crystal deposition on cartilage (McCarthy and Cheung, 2009)
We scored the histological sections and found that the severity and the extent of cartilage degradation were similar between WT and IL-1α−/−, but significantly increased in IL-1β−/− knees, both for the tibia and the femur cartilage (Figure 1B)
These results demonstrate that a single deficiency of IL-1α, IL-1β, or of NOD-like receptor protein-3 (NLRP3) does not prevent cartilage damage
Summary
Osteoarthritis (OA) is a progressive disease of the joint tissues, characterized by cartilage degradation (Goldring and Goldring, 2007; Loeser et al, 2012), mild synovial inflammation (Scanzello and Goldring, 2012), subchondral bone sclerosis, osteophyte formation, and calcium crystal deposition (calcification) on cartilage (McCarthy and Cheung, 2009). IL-6 is synthesized and secreted in an active form, which binds first to its receptor (IL-6R) and to the signaling gp130 molecule triggering STAT and ERK pathways. Both IL-1α and IL-1β exist as an intracellular proform of about 31 kDa, which can be cleaved to a mature form of 17 kDa. In particular, a first signal (such as TLR1/2 agonist PAM3Cys or TLR4 agonist LPS) is needed to trigger an NF-kB–dependent production of pro–IL-1β. Previous results suggest that IL-1α/β could be an important mediator involved in the pathogenesis of osteoarthritis (OA)
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