Crucial role of synovial lining macrophages in the promotion of transforming growth factor β–mediated osteophyte formation

Abstract

Objective. To investigate in vivo and in vitro whether macrophages have an intermediate role in transforming growth factor ␤ (TGF␤)-induced osteophyte formation.Methods. In vivo, synovial lining macrophages were selectively depleted by injection of clodronateladen liposomes 7 days prior to injection of 20 ng or 200 ng of TGF␤ into murine knee joints 3 times, on alternate days. Total knee joint sections were obtained on day 7 after the last injection and stained with Safranin O. Production of bone morphogenetic protein 2 (BMP-2) and BMP-4 was determined by immunolocalization. The interaction between murine macrophages and mesenchymal cells (precursors with chondrogenic potential) was studied in vitro using a Transwell system in which RAW macrophages were cocultured with C3H10T1/2 mesenchymal cells. Spheroid neocartilage formation was quantified microscopically after staining with May-Grünwald-Giemsa.Results Osteoarthritis (OA) is a chronic disease characterized by severe cartilage damage and osteophyte formation. Its pathogenesis is thought to be multifactorial, and most likely, both metabolic and mechanical factors are involved (1,2).During OA, dysregulated chondrogenesis is observed along the margins of articular cartilage. It involves differentiation of chondrogenic cells that reside in the periosteum (periosteal cells). In addition, periosteal cells express cartilage-specific genes producing extracellular matrix proteins. These cells may mature from prechondrocytic cells into hypertrophic chondrocytes (3). Cartilage formation is followed by cartilage maturation, removal of cartilage, and replacement with bone, resulting in structures called osteophytes (4).Factors that regulate mesenchymal cells to differentiate along the chondrocytic lineage for new cartilage and bone formation include cytokines and growth fac-1 P.

Osteal macrophages promote in vivo intramembranous bone healing in a mouse tibial injury model

“…(47) In addition, depletion of macrophages in osteoarthritic models reduced osteophyte formation dramatically. (48) Interestingly, a recent publication reported that preventing the recruitment of inflammatory macrophages in a CCR2 À/À mouse nonunion fracture model resulted in reduced endochondral bone formation but did not impede intramembranous bone formation. (49) This study demonstrated that at least some of the macrophages involved in endochondral bone formation are a recruited cell population, but a remaining question is whether resident macrophages also play a role in endochondral bone formation during fracture healing.…”

The Role of Synovial Macrophages and Macrophage-Produced Mediators in Driving Inflammatory and Destructive Responses in Osteoarthritis

“…The collagenase injection causes weakening of ligaments leading to gradual onset of OA pathology within six weeks of induction, without any direct collagenase-induced cartilage damage being observed. If macrophage depletion had been achieved prior to the elicitation of experimental OA, there was potent reduction of both fibrosis and osteophyte formation (Blom et al, 2004(Blom et al, , 2007avan Lent et al, 2004). This would indicate that in this murine model of OA, synovial macrophages control the production of the growth factors that promote fibrosis and osteophyte formation, both key pathophysiological events in OA.…”

“…It can be speculated that once the macrophages are removed, the synovial fibroblasts change their phenotype and downregulate their production of both proinflammatory cytokines and destructive MMPs. An important series of papers, using injections of liposome-encapsulated clodronate to induce depletion of synovial lining macrophages, has provided some intriguing new information about the role of macrophages in driving degenerative changes in a mouse model of experimental OA induced by injection of collagenase (Blom et al, 2004(Blom et al, , 2007a. The collagenase injection causes weakening of ligaments leading to gradual onset of OA pathology within six weeks of induction, without any direct collagenase-induced cartilage damage being observed.…”

In vitro stage‐specific chondrogenesis of mesenchymal stem cells committed to chondrocytes

“…Complementary DNA was used for each real-time PCR reaction (ABI Prism 7300 system; Applied BioSystems, Foster City, CA) in an RT 2 Profiler PCR Array human osteogenesis system (APHS-026A; SuperArray, Frederick, MD) with RT 2 RealTime SYBR Green PCR master mix (PA-012; SuperArray) according to the manufacturer's instructions. Collected data were interpreted using the ⌬⌬C t method.…”