Objective.: Interleukin 34 (IL-34) is a recently discovered cytokine that binds macrophage colony-stimulating factor (M-CSF) receptor. Rheumatoid arthritis (RA) is characterized by increased osteoclastogenesis. To identify the significance of IL-34 in RA, the IL-34 concentration was measured in serum and synovial fluid (SF).Methods.: IL-34 concentrations were measured in serum from patients with RA (n = 113), patients with osteoarthritis (OA; n = 56), and controls (n = 36), and in SF isolated from patients with RA (n = 36) or OA (n = 24). Correlations between serum IL-34 levels and clinical features in RA were assessed. The levels of IL-1β, IL-6, IL-17α, interferon-γ-induced protein 10, receptor activator of nuclear factor κB ligand (RANKL), and Dickkopf-1 were also measured.Results.: Patients with RA had a higher mean serum level of IL-34 than did patients with OA and controls (188.0 ± 550.3, 36.6 ± 38.0, and 49.1 ± 78.5 pg/ml, respectively). Similarly, SF IL-34 concentration was higher in patients with RA than in those with OA. IL-34 levels were positively associated with IL-6 levels in serum from patients with RA and OA. SF IL-34 concentration correlated significantly with IL-6 and RANKL levels only in RA. The serum level of IL-34 was not correlated with systemic osteoporosis and radiographic joint damage in RA. IL-34 levels in the serum from patients with RA were positively correlated with rheumatoid factor and anticyclic citrullinated peptide antibody titers (r = 0.282 and 0.491, respectively).Conclusion.: Circulating IL-34 levels in RA correlated with autoantibody production. Further investigations of localized and systemic effects of IL-34 are warranted to elucidate RA pathogenesis.

Rheumatoid arthritis (RA) is a systemic inflammatory disease that mainly affects the synovial joints. Although involvement of the fibroblast growth factor (FGF) signaling pathway has been suggested as an important modulator in RA development, no clear evidence has been provided. In this study, we found that synovial fluid basic FGF (bFGF) concentration was significantly higher in RA than in osteoarthritis (OA) patients. bFGF stimulates proliferation and migration of human fibroblast-like synoviocytes (FLSs) by activation of the bFGF-FGF receptor 3 (FGFR3)-ribosomal S6 kinase 2 (RSK2) signaling axis. Moreover, a molecular docking study revealed that kaempferol inhibited FGFR3 activity by binding to the active pocket of the FGFR3 kinase domain. Kaempferol forms hydrogen bonds with the FGFR3 backbone oxygen of Glu555 and Ala558 and the side chain of Lys508. Notably, the inhibition of bFGF-FGFR3–RSK2 signaling by kaempferol suppresses the proliferation and migration of RA FLSs and the release of activated T-cell-mediated inflammatory cytokines, such as IL-17, IL-21, and TNF-α. We further found that activated phospho-FGFR3 and -RSK2 were more highly observed in RA than in OA synovium. The hyperplastic lining and sublining lymphoid aggregate layers of RA synovium showed p-RSK2-expressing CD68⁺macrophages with high frequency, while pRSK2-expressing CD4⁺T-cells was observed at a lower frequency. Notably, kaempferol administration in collagen-induced arthritis mice relieved the frequency and severity of arthritis. Kaempferol reduced osteoclast differentiation in vitro and in vivo relative to the controls and was associated with the inhibition of osteoclast markers, such as tartrate-resistant acid phosphatase, integrin β3, and MMP9. Conclusively, our data suggest that bFGF-induced FGFR3–RSK2 signaling may play a critical role during the initiation and progression of RA in terms of FLS proliferation and enhanced osteoclastogenesis, and that kaempferol may be effective as a new treatment for RA.

Interleukin (IL)-27 is a novel cytokine of the IL-6/IL-12 family that has been reported to be involved in the pathogenesis of autoimmune diseases and has a pivotal role as both a pro- and anti-inflammatory cytokine. We investigated the in vivo effects of IL-27 on arthritis severity in a murine collagen-induced arthritis (CIA) model and its mechanism of action regarding control of regulatory T (Tregs) and IL-17-producing T helper 17 (Th17) cells. IL-27-Fc-treated CIA mice showed a lower severity of arthritis. IL-17 expression in the spleens was significantly decreased in IL-27-Fc-treated CIA mice compared with that in the CIA model. The Th17 population was decreased in the spleens of IL-27-Fc-treated CIA mice, whereas the CD4⁺CD25⁺Foxp3⁺ Treg population increased. In vitro studies revealed that IL-27 inhibited IL-17 production in murine CD4⁺ T cells, and the effect was associated with retinoic acid-related orphan receptor γT and signal transducer and activator of transcription 3 inhibition. In contrast, fluorescein isothiocyanate-labeled forkhead box P3 (Foxp3) and IL-10 were profoundly augmented by IL-27 treatment. Regarding the suppressive capacity of Treg cells, the proportions of CTLA-4⁺ (cytotoxic T-lymphocyte antigen 4), PD-1⁺ (programmed cell death protein 1) and GITR⁺ (glucocorticoid-induced tumor necrosis factor receptor) Tregs increased in the spleens of IL-27-Fc-treated CIA mice. Furthermore, in vitro differentiated Treg cells with IL-27 exerted a more suppressive capacity on T-cell proliferation. We found that IL-27 acts as a reciprocal regulator of the Th17 and Treg populations in CD4⁺ cells isolated from healthy human peripheral blood mononuclear cells (PBMCs), as well as from humans with rheumatoid arthritis (RA) PBMCs. Our study suggests that IL-27 has the potential to ameliorate overwhelming inflammation in patients with RA through a reciprocal regulation of Th17 and Treg cells.

Objective: Rebamipide, a gastroprotective agent, has the ability to scavenge reactive oxygen radicals. Increased oxidative stress is implicated in the pathogenesis of rheumatoid arthritis (RA). We undertook this study to investigate the impact of rebamipide on the development of arthritis and the pathophysiologic mechanisms by which rebamipide attenuates arthritis severity in a murine model of RA.Methods: Collagen‐induced arthritis (CIA) was induced in DBA/1J mice. Anti–type II collagen antibody titers and interleukin‐17 (IL‐17) levels were determined using enzyme‐linked immunosorbent assay. The expression of transcription factors was analyzed by immunostaining and Western blotting. Frequencies of IL‐17–producing CD4+ T cells (Th17 cells) and CD4+CD25+FoxP3+ Treg cells were analyzed by flow cytometry.Results: Rebamipide reduced the clinical arthritis score and severity of histologic inflammation and cartilage destruction in a dose‐dependent manner. The joints isolated from rebamipide‐treated mice with CIA showed decreased expression of nitrotyrosine, an oxidative stress marker. Rebamipide‐treated mice showed lower circulating levels of type II collagen–specific IgG, IgG1, and IgG2a. Whereas the number of Th17 cells in spleens was decreased in rebamipide‐treated mice with CIA, a significant increase in the number of Treg cells in spleens was observed. In vitro, rebamipide inhibited Th17 cell differentiation through STAT‐3/retinoic acid receptor–related orphan nuclear receptor γt and reciprocally induced Treg cell differentiation through FoxP3. Rebamipide increased Nrf2 nuclear activities in murine CD4+ T cells and LBRM‐33 murine T lymphoma cells. Heme oxygenase 1 (HO‐1) expression in the spleens was markedly increased in rebamipide‐treated mice.Conclusion: The inhibitory effects of rebamipide on joint inflammation are associated with recovery from an imbalance between Th17 cells and Treg cells and with activation of an Nrf2/HO‐1 antioxidant pathway.

Increases in oxidative stress are thought to be associated with the development of osteoarthritis (OA). Eupatilin, one of the major compounds present in artemisia species, was shown to have both anti-oxidative and anti-inflammatory properties. Here, we investigated the in vivo effects of eupatilin on pain severity and cartilage degradation in an experimental rat model of OA, along with the mechanisms of action underlying these effects. Experimental OA was induced via an intra-articular injection of monosodium iodoacetate (MIA), with oral administration of eupatilin initiated on the day of MIA injection. Pain was assessed by measuring the paw withdrawal latency and threshold. Cartilage destruction was analyzed macroscopically and histomorphologically. The effects of eupatilin on mRNA expression were investigated in interleukin-1β (IL-1β)-stimulated human OA chondrocytes. Eupatilin treatment exhibited clear antinociceptive effects, along with an attenuation of cartilage degradation in OA rats. Additionally, the number of osteoclasts present in the subchondral bone region was significantly decreased following eupatilin treatment. Eupatilin reduced the expression of interleukin-1β (IL-1β), interleukin-6 (IL-6), nitrotyrosine and inducible nitric oxide synthase (iNOS) in cartilage. mRNA levels of matrix metalloproteinase-3 (MMP-3), MMP13, and a disintegrin and metalloproteinase with thrombospondin motifs-5 (ADAMTS-5) were reduced in IL-1β-stimulated human OA chondrocytes, while tissue inhibitor of metalloproteinases-1 (TIMP-1) was induced. Phosphorylated protein levels of the c-jun N-terminal kinase (JNK) was reduced by eupatilin. Taken together, these results suggest that eupatilin suppresses oxidative damage and reciprocally enhances extracellular matrix production in articular chondrocytes, making eupatilin a promising therapeutic option for the treatment of OA.

Ko KM, et al. Yonsei Med J. 2020 May;61(5):406-415. https://doi.org/10.3349/ymj.2020.61.5.406