Correction to: Proteoglycan-4 is an essential regulator of synovial macrophage polarization and inflammatory macrophage joint infiltration

An amendment to this paper has been published and can be accessed via the original article.

Proteoglycan-4 is an essential regulator of synovial macrophage polarization and inflammatory macrophage joint infiltration

Background Synovial macrophages perform a multitude of functions that include clearance of cell debris and foreign bodies, tissue immune surveillance, and resolution of inflammation. The functional diversity of macrophages is enabled by distinct subpopulations that express unique surface markers. Proteoglycan-4 (PRG4) is an important regulator of synovial hyperplasia and fibrotic remodeling, and the involvement of macrophages in PRG4’s synovial role is yet to be defined. Our objectives were to study the PRG4’s importance to macrophage homeostatic regulation in the synovium and infiltration of pro-inflammatory macrophages in acute synovitis and investigate whether macrophages mediated synovial fibrosis in Prg4 gene-trap (Prg4GT/GT) murine knee joints. Methods Macrophage phenotyping in Prg4GT/GT and Prg4+/+ joints was performed by flow cytometry using pan-macrophage markers, e.g., CD11b, F4/80, and surface markers of M1 macrophages (CD86) and M2 macrophages (CD206). Characterizations of the various macrophage subpopulations were performed in 2- and 6-month-old animals. The expression of inflammatory markers, IL-6, and iNOS in macrophages that are CD86+ and/or CD206+ was studied. The impact of Prg4 recombination on synovial macrophage populations of 2- and 6-month-old animals and infiltration of pro-inflammatory macrophages in response to a TLR2 agonist challenge was determined. Macrophages were depleted using liposomal clodronate and synovial membrane thickness, and the expression of fibrotic markers α-SMA, PLOD2, and collagen type I (COL-I) was assessed using immunohistochemistry. Results Total macrophages in Prg4GT/GT joints were higher than Prg4+/+ joints (p<0.0001) at 2 and 6 months, and the percentages of CD86+/CD206− and CD86+/CD206+ macrophages increased in Prg4GT/GT joints at 6 months (p<0.0001), whereas the percentage of CD86−/CD206+ macrophages decreased (p<0.001). CD86+/CD206− and CD86+/CD206+ macrophages expressed iNOS and IL-6 compared to CD86−/CD206+ macrophages (p<0.0001). Prg4 re-expression limited the accumulation of CD86+ macrophages (p<0.05) and increased CD86−/CD206+ macrophages (p<0.001) at 6 months. Prg4 recombination attenuated synovial recruitment of pro-inflammatory macrophages in 2-month-old animals (p<0.001). Clodronate-mediated macrophage depletion reduced synovial hyperplasia, α-SMA, PLOD2, and COL-I expressions in the synovium (p<0.0001). Conclusions PRG4 regulates the accumulation and homeostatic balance of macrophages in the synovium. In its absence, the synovium becomes populated with M1 macrophages. Furthermore, macrophages exert an effector role in synovial fibrosis in Prg4GT/GT animals.

Inhibition of Synovial Macrophage Pyroptosis Alleviates Synovitis and Fibrosis in Knee Osteoarthritis

Increasing evidence has shown that macrophage pyroptosis in different tissues participates in chronic aseptic inflammation and is related to tissue fibrosis. Our last studies also revealed the vital role of synovial fibroblast pyroptosis in the onset and development of knee osteoarthritis (KOA). In this study, we aimed to investigate whether synovial macrophage pyroptosis did occur and whether this form of cell death should be related to synovitis and fibrosis of KOA. In the synovial tissue of KOA model rats, we observed a decrease of caspase1, NLRP3, ASC, and GSDMD caused by macrophage depletion in both the mRNA and protein expressions. Besides, rats treated with the specific caspase1 inhibitor Ac-YVAD-CMK showed less inflammatory reaction and fibrosis, not only in the expression of proinflammatory factors IL-1β, IL-18, and HMGB1 and fibrosis markers TGF-β, PLOD2, COL1A1, and TIMP1 but also in the observation of HE staining, Sirius Red staining, and the transverse diameters of the right knees. Subsequently, we established an LPS+ATP-induced model in macrophages mimicking the inflammatory environment of KOA and inducing macrophage pyroptosis. Macrophages transfected with caspase1 siRNA showed reduced cell death; meanwhile, the relative expression of pyroptosis-related proteins were also downregulated. In addition, the level of fibrotic markers in synovial fibroblasts were significantly decreased after coculture with siRNA GSDMD-transfected macrophages. To conclude, synovial macrophage pyroptosis may occur in the pathological processes of KOA and inhibition of synovial macrophage pyroptosis alleviates synovitis and fibrosis in KOA model rats.

High-fat dietary consumption promotes disease progression in a surgical joint destabilisation murine model of osteoarthritis: role of systemic eicosanoids and circulating monocytes

Background In this study we investigated the contribution of high-fat diet-induced metabolic overload to osteoarthritis (OA) progression originally caused by mild mechanical trauma to the mouse knee joint. We hypothesized that metabolic stress would induce a proinflammatory environment by altering systemic lipid levels and immune cell populations. Methods Twelve-week-old male C57BL/6J mice (n=20) were given a low-fat diet (LFD, 10%kcal from fat) or high-fat diet (HFD, 45%kcal from fat) for 18 weeks. OA was initiated by transecting the medial meniscotibial ligament of the right knee joint at t=10 weeks. OA severity and changes in M1/M2 polarization of synovial macrophage populations were determined in serial coronal FFPE-mounted sections. Eicosanoid levels and monocyte populations were evaluated before and after ligament transection. Results Diet-induced metabolic stress assessed by body weight, systemic cholesterol levels, and insulin resistance index was significantly higher in HFD mice. This group also showed increased cartilage damage, synovitis, and osteophyte formation compared with LFD controls. High-fat feeding elevated systemic arachidonic acid levels, which mainly resulted in increased levels of its cytochrome P450-catalysed diol metabolites 5,6-dihydroxyeicosatrienoic acid (DHET) and 8,9-DHET. High-fat feeding also triggered an increase in pro-inflammatory intermediate CD43++Ly6Cint monocytes after ligament resection. Ligament resection in addition to high-fat feeding induced increased expression of the activation marker CD11c selectively on non-classical CD43++Ly6Clow monocytes. No significant changes in synovial macrophage polarization were observed. Conclusions Metabolic stress resulted in a proinflammatory environment and aggravated injury-induced OA progression. Our results suggest that a CYP450-focused eicosanoid metabolism and activated circulating monocytes may be drivers of this metabolic stress-induced OA progression, contributing to the mechanistic understanding and potentially serving as future diagnostic and prognostic biomarkers for metabolic OA.