Bone resorption by osteoclasts and bone formation by osteoblasts are tightly coupled processes implicating factors in TNF, bone morphogenetic protein, and Wnt families. In osteoimmunology, macrophages were described as another critical cell population regulating bone formation by osteoblasts but the coupling factors were not identified. Using a high-throughput approach, we identified here Oncostatin M (OSM), a cytokine of the IL-6 family, as a major coupling factor produced by activated circulating CD14 1 or bone marrow CD11b 1 monocytes/macrophages that induce osteoblast differentiation and matrix mineralization from human mesenchymal stem cells while inhibiting adipogenesis. Upon activation of toll-like receptors (TLRs) by lipopolysaccharide or endogenous ligands, OSM was produced in classically activated inflammatory M1 and not M2 macrophages, through a cyclooxygenase-2 and prostaglandin-E2 regulatory loop. Stimulation of osteogenesis by activated monocytes/macrophages was prevented using neutralizing antibodies or siRNA to OSM, OSM receptor subunits gp130 and OSMR, or to the downstream transcription factor STAT3. The induced osteoblast differentiation program culminated with enhanced expression of CCAAT-enhancer-binding protein d, Cbfa1, and alkaline phosphatase. Overexpression of OSM in the tibia of mice has led to new bone apposition with no sign of bone resorption. Two other cytokines have also a potent role in bone formation induced by monocytes/macrophages and activation of TLRs: IL-6 and leukemia inhibitory factor. We propose that during bone inflammation, infection, or injury, the IL-6 family signaling network activated by macrophages and TLR ligands stimulates bone formation that is largely uncoupled from bone resorption and is thus an important target for anabolic bone therapies. STEM CELLS 2012;30:762-772 Disclosure of potential conflicts of interest is found at the end of this article.

SummaryInterleukin (IL)-36a, IL-36b and IL-36g are expressed highly in skin and are involved in the pathogenesis of psoriasis, while the antagonists IL-36Ra or IL-38, another potential IL-36 inhibitor, limit uncontrolled inflammation. The expression and role of IL-36 cytokines in rheumatoid arthritis (RA) and Crohn's disease (CD) is currently debated. Here, we observed that during imiquimod-induced mouse skin inflammation and in human psoriasis, expression of IL-36a, g and IL-36Ra, but not IL-36b and IL-38 mRNA, was induced and correlated with IL-1b and T helper type 17 (Th17) cytokines (IL-17A, IL-22, IL-23, CCL20). In mice with collageninduced arthritis and in the synovium of patients with RA, IL-36a, b, g, IL36Ra and IL-38 were all elevated and correlated with IL-1b, CCL3, CCL4 and macrophage colony-stimulating factor (M-CSF), but not with Th17 cytokines. In the colon of mice with dextran sulphate sodium-induced colitis and in patients with CD, only IL-36a, g and IL-38 were induced at relatively low levels and correlated with IL-1b and IL-17A. We suggest that only a minor subgroup of patients with RA (17-29%) or CD (25%) had an elevated IL-36 agonists/antagonists ratio, versus 93% of patients with psoriasis. By immunohistochemistry, IL-36 cytokines were produced by various cell types in skin, synovium and colonic mucosa such as keratinocytes, CD68 1 macrophages, dendritic/Langerhans cells and CD79a 1 plasma cells. In primary cultures of monocytes or inflammatory macrophages (M1), IL-36b and IL-36Ra were produced constitutively, but IL-36a, g and IL-38 were produced after lipopolysaccharide stimulation. These distinct expression profiles may help to explain why only subgroups of RA and CD patients have a potentially elevated IL-36 agonists/ antagonists ratio.

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The emergence of the molecular triad osteoprotegerin (OPG)/Receptor Activator of NF-kB (RANK)/RANK Ligand (RANKL) has helped elucidate a key signalling pathway between stromal cells and osteoclasts. The interaction between RANK and RANKL plays a critical role in promoting osteoclast differentiation and activation leading to bone resorption. OPG is a soluble decoy receptor for RANKL that blocks osteoclast formation by inhibiting RANKL binding to RANK. The OPG/RANK/RANKL system has been shown to be abnormally regulated in several malignant osteolytic pathologies such as multiple myeloma [MM, where enhanced RANKL expression (directly by tumour cells or indirectly by stromal bone cells or T-lymphocytes)] plays an important role in associated bone destruction. By contrast, production of its endogenous counteracting decoy receptor OPG is either inhibited or too low to compensate for the increase in RANKL production. Therefore, targeting the OPG/RANK/RANKL axis may offer a novel therapeutic approach to malignant osteolytic pathologies. In animal models, OPG or soluble RANK was shown both to control hypercalcaemia of malignancy and the establishment and progression of osteolytic metastases caused by various malignant tumours. To this day, only one phase I study has been performed using a recombinant OPG construct that suppressed bone resorption in patients with multiple myeloma or breast carcinoma with radiologically confirmed bone lesions. RANK-Fc also exhibits promising therapeutic effects, as revealed in animal models of prostate cancer and multiple myeloma. If the animal results translate to similar clinical benefits in humans, using RANK-Fc or OPG may yield novel and potent strategies for treating patients with established or imminent malignant bone diseases and where standard therapeutic regimens have failed.

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Osteosarcoma is the most frequent primary bone tumor that develops mainly in the young, the median age of diagnosis being 18 years. Despite improvement in osteosarcoma treatment, survival rate is only 30% at 5 years for patients with pulmonary metastases at diagnosis. This warrants exploration of new therapeutic options, and among them, osteoprotegerin (OPG), a naturally occurring protein that inhibits bone resorption, is very promising in blocking the vicious cycle between bone resorption and tumor proliferation that takes place during tumor development in bone site. As OPG binds and inhibits the activity of tumor necrosis factorrelated apoptosis-inducing ligand, the truncated form of murine OPG 1-194 was used. The cDNA encoding OPG was administered by gene transfer using replication-defective adenoviral vector or was associated with an amphiphilic polymer in two models of rodent osteosarcoma. In both models, OPG gene transfer was effective in preventing the formation of osteolytic lesions associated with osteosarcoma development, in reducing the tumor incidence and the local tumor growth, leading to a 4-fold augmentation of mice survival 28 days postimplantation. On the contrary, OPG did not prevent the development of pulmonary metastasis alone, suggesting that bone environment is necessary for OPG therapeutic efficacy. Because OPG has no direct activity on osteosarcoma cells in vitro (cell binding, cell proliferation, apoptosis, or cell cycle distribution), we show that OPG exerts indirect inhibitory effect on tumor progression through the inhibition of RANKL whose production is enhanced in bone tumor environment, leading to osteolysis inhibition as reflected by osteoclast number decrease. [Cancer Res 2007; 67(15):7308-18]

This work for the first time identifies IL-34 expression in the synovial tissue of patients with arthritis. This cytokine, as a downstream effector of TNFα and IL-1β, may contribute to inflammation and bone erosions in RA.

This study shows for the first time that IL-38 overexpression attenuates the severity of experimental arthritis. IL-38 may exert its anti-inflammatory effects by decreasing the production of proinflammatory cytokines by macrophages and synovial fibroblasts. This effect can lead to the development of novel treatment strategies in arthritis.