S100A8/A9 drives monocytes towards M2-like macrophage differentiation and associates with M2-like macrophages in osteoarthritic synovium.

Objectives: Macrophages are key orchestrators of the osteoarthritis (OA)-associated inflammatory response. Macrophage phenotype is dependent on environmental cues like the inflammatory factor S100A8/A9. Here, we investigated how S100A9 exposure during monocyte-to-macrophage differentiation affects macrophage phenotype and function.

Intensive cholesterol-lowering treatment reduces synovial inflammation during early collagenase-induced osteoarthritis, but not pathology at end-stage disease in female dyslipidemic E3L.CETP mice.

Introduction: The association between metabolic syndrome (MetS) and osteoarthritis (OA) development has become increasingly recognized. In this context, the exact role of cholesterol and cholesterol-lowering therapies in OA development has remained elusive. Recently, we did not observe beneficial effects of intensive cholesterol-lowering treatments on spontaneous OA development in E3L.

2 Deficiency Reduces Cartilage Damage and Ectopic Bone Formation in an Experimental Model for Osteoarthritis.

Osteoarthritis (OA) is a destructive disease of the joint with age and obesity being its most important risk factors. Around 50% of OA patients suffer from inflammation of the synovial joint capsule, which is characterized by increased abundance and activation of synovial macrophages that produce reactive oxygen species (ROS) via NADPH-oxidase 2 (NOX2). Both ROS and high blood levels of low-density lipoprotein (LDL) are implicated in OA pathophysiology, which may interact to form oxidized LDL (oxLDL) and thereby promote disease.

A single dose of anti-IL-1β antibodies prevents Western diet-induced immune activation during early stage collagenase-induced osteoarthritis, but does not ameliorate end-stage pathology.

Objective: Metabolic dysfunction can cause IL-1β mediated activation of the innate immune system, which could have important implications for the therapeutic efficacy of IL-1β neutralizing drugs as treatment for OA in the context of metabolic syndrome (MetS). In the present study, we investigated whether early treatment with a single dose of IL-1β blocking antibodies could prevent Western diet (WD) induced changes to systemic monocyte populations and their cytokine secretion profile and herewith modulate collagenase induced osteoarthritis (CiOA) pathology.

Methods: CiOA was induced in female C57Bl/6 mice fed either a standard diet (SD) or WD and treated with a single dose of either polyclonal anti-IL-1β antibodies or control.

High LDL levels lessen bone destruction during antigen-induced arthritis by inhibiting osteoclast formation and function.

Rheumatoid arthritis (RA) is a chronic inflammatory disease, characterized by severe joint inflammation and bone destruction as the result of increased numbers and activity of osteoclasts. RA is often associated with metabolic syndrome, whereby elevated levels of LDL are oxidized into oxLDL, which might affect osteoclastogenesis. In this study, we induced antigen-induced arthritis (AIA) in Apoe mice, which spontaneously develop high LDL levels, to investigate the effects of high LDL/oxLDL levels on osteoclast differentiation and bone destruction.

IL-1β-Mediated Activation of Adipose-Derived Mesenchymal Stromal Cells Results in PMN Reallocation and Enhanced Phagocytosis: A Possible Mechanism for the Reduction of Osteoarthritis Pathology.

Injection of adipose-derived mesenchymal stromal cells (ASCs) into murine knee joints after induction of inflammatory collagenase-induced osteoarthritis (CiOA) reduces development of joint pathology. This protection is only achieved when ASCs are applied in early CiOA, which is characterized by synovitis and high S100A8/A9 and IL-1β levels, suggesting that inflammation is a prerequisite for the protective effect of ASCs. Our objective was to gain more insight into the interplay between synovitis and ASC-mediated amelioration of CiOA pathology.

High LDL-C levels attenuate onset of inflammation and cartilage destruction in antigen-induced arthritis.

Objectives: In this study, we used hypercholesterolaemic apolipoprotein E-deficient (Apoe-/-) mice to investigate LDL/oxLDL effect on synovial inflammation and cartilage destruction during antigen-induced arthritis (AIA). Further, as macrophage FcγRs are crucial to immune complex-mediated AIA, we investigated in vitro the effects of high cholesterol levels on the expression of FcγRs on macrophages.

Methods: AIA was induced by intra-articular injection of mBSA into knee joints of immunised Apoe-/- and wild type (WT) control mice.

The role of NOX2-derived reactive oxygen species in collagenase-induced osteoarthritis.

Objective: Synovitis in collagenase-induced osteoarthritis (CiOA) is driven by locally released S100A8/A9 proteins and enhances joint destruction. S100A8/A9 can induce reactive oxygen species (ROS) release by phagocytes in OA synovium via neutrophil cytosolic factor-1 (Ncf1)-regulated NOX2 activation. In the present study we investigated whether NOX2-derived ROS affect joint pathology during CiOA.

Fcγ receptor-mediated influx of S100A8/A9-producing neutrophils as inducer of bone erosion during antigen-induced arthritis.

Background: Osteoclast-mediated bone erosion is a central feature of rheumatoid arthritis (RA). Immune complexes, present in a large percentage of patients, bind to Fcγ receptors (FcγRs), thereby modulating the activity of immune cells. In this study, we investigated the contribution of FcγRs, and FcγRIV in particular, during antigen-induced arthritis (AIA).

S100A8/A9 increases the mobilization of pro-inflammatory Ly6C monocytes to the synovium during experimental osteoarthritis.

Background: Monocytes are dominant cells present within the inflamed synovium during osteoarthritis (OA). In mice, two functionally distinct monocyte subsets are described: pro-inflammatory Ly6C and patrolling Ly6C monocytes. Alarmins S100A8/A9 locally released by the synovium during inflammatory OA for prolonged periods may be dominant proteins involved in stimulating recruitment of Ly6C monocytes from the circulation to the joint.

S100A8/A9, a potent serum and molecular imaging biomarker for synovial inflammation and joint destruction in seronegative experimental arthritis.

Background: Seronegative joint diseases are characterized by a lack of well-defined biomarkers since autoantibodies are not elevated. Calprotectin (S100A8/A9) is a damage-associated molecular pattern (DAMP) which is released by activated phagocytes, and high levels are found in seronegative arthritides. In this study, we investigated the biomarker potential of systemic and local levels of these S100 proteins to assess joint inflammation and joint destruction in an experimental model for seronegative arthritis.

Interleukin-1 is not involved in synovial inflammation and cartilage destruction in collagenase-induced osteoarthritis.

Objective: Interleukin-1 (IL-1) is an alleged important cytokine in osteoarthritis (OA), although the exact contribution of IL-1 to joint destruction remains unclear. Here we investigated the involvement of IL-1α and IL-1β in joint pathology during collagenase-induced OA (CiOA).

Methods: CiOA was induced in wild type (WT) and IL-1αβ mice.

High LDL levels lead to increased synovial inflammation and accelerated ectopic bone formation during experimental osteoarthritis.

Objective: A relation between osteoarthritis (OA) and increased cholesterol levels is apparent. In the present study we investigate OA pathology in apolipoprotein E (ApoE)(-)(/-) mice with and without a cholesterol-rich diet, a model for high systemic low density lipoprotein (LDL) cholesterol levels independent of weight.

Method: Wild type (WT), Apoe(-)(/-), S100a9(-/-) and Apoe(-)(/-)S100a9(-/-) mice (C57BL/6 background) received a standard or cholesterol-rich diet.

Induction of Canonical Wnt Signaling by the Alarmins S100A8/A9 in Murine Knee Joints: Implications for Osteoarthritis.

Objective: Both alarmins S100A8/A9 and canonical Wnt signaling have been found to play active roles in the development of experimental osteoarthritis (OA). However, what activates canonical Wnt signaling remains unknown. This study was undertaken to investigate whether S100A8 induces canonical Wnt signaling and whether S100 proteins exert their effects via activation of Wnt signaling.

Induction of Canonical Wnt Signaling by Synovial Overexpression of Selected Wnts Leads to Protease Activity and Early Osteoarthritis-Like Cartilage Damage.

Proteins from the Wnt signaling pathway are very important for joint development. Curiously, osteoarthritis (OA) is thought to be a recapitulation of developmental processes. Various members of the Wnt signaling pathway are overexpressed in the synovium during experimental OA.

Alarmins S100A8/S100A9 aggravate osteophyte formation in experimental osteoarthritis and predict osteophyte progression in early human symptomatic osteoarthritis.

Objective: Alarmins S100A8 and S100A9 are major products of activated macrophages regulating cartilage damage and synovial activation during murine and human osteoarthritis (OA). In the current study, we investigated whether S100A8 and S100A9 are involved in osteophyte formation during experimental OA and whether S100A8/A9 predicts osteophyte progression in early human OA.

Methods: OA was elicited in S100A9-/- mice in two experimental models that differ in degree of synovial activation.

Active involvement of alarmins S100A8 and S100A9 in the regulation of synovial activation and joint destruction during mouse and human osteoarthritis.

Objective: To investigate whether alarmins S100A8 and S100A9 are involved in mediating cartilage destruction during murine and human osteoarthritis (OA).

Methods: Two different murine models of OA that differed in terms of synovial activation were compared. Cartilage destruction was measured histologically.

Alarmins S100A8 and S100A9 elicit a catabolic effect in human osteoarthritic chondrocytes that is dependent on Toll-like receptor 4.

Objective: S100A8 and S100A9 are two Ca(2+) binding proteins classified as damage-associated molecular patterns or alarmins that are found in high amounts in the synovial fluid of osteoarthritis (OA) patients. The purpose of this study was to investigate whether S100A8 and/or S100A9 can interact with chondrocytes from OA patients to increase catabolic mediators.

Methods: Using immunohistochemistry, we stained for S100A8 and S100A9 protein, matrix metalloproteinases (MMPs), and a cartilage-breakdown epitope specific for MMPs (VDIPEN) in cartilage from OA donors.

S100A8 enhances osteoclastic bone resorption in vitro through activation of Toll-like receptor 4: implications for bone destruction in murine antigen-induced arthritis.

Objective: Rheumatoid arthritis, which is associated with elevated levels of S100A8 and S100A9, is characterized by severe bone erosions caused by enhanced osteoclast formation and activity. The aim of the present study was to investigate the role of S100A8 and S100A9 in osteoclastic bone destruction in murine antigen-induced arthritis (AIA).

Methods: Bone destruction was analyzed in the arthritic knee joints of S100A9-deficient mice in which S100A8 protein expression was also lacking, and in wild-type (WT) controls.

S100A8 causes a shift toward expression of activatory Fcγ receptors on macrophages via toll-like receptor 4 and regulates Fcγ receptor expression in synovium during chronic experimental arthritis.

Objective: The levels of both Fcγ receptor (FcγR) and the alarmins S100A8 and S100A9 are correlated with the development and progression of cartilage destruction during antigen-induced arthritis (AIA). This study was undertaken to study the active involvement of S100A8, S100A9, and S100A8/S100A9 in FcγR regulation in murine macrophages and synovium during AIA.

Methods: Recombinant murine S100A8 (rS100A8) was injected into normal mouse knee joints, and the synovium was isolated for analysis of FcγR messenger RNA (mRNA) expression by reverse transcription-polymerase chain reaction (RT-PCR).

Scavenger receptor class A type I/II determines matrix metalloproteinase-mediated cartilage destruction and chondrocyte death in antigen-induced arthritis.

Objective: Scavenger receptor class A type I (SR-AI) and SR-AII are expressed by macrophages in particular and bind and internalize a broad range of molecules (including endotoxins, apoptotic bodies, and oxidized low-density lipoprotein). This study was undertaken to investigate the role of SR-AI/II in mediating severe cartilage destruction in antigen-induced arthritis (AIA).

Methods: AIA was induced in the knee joints of SR-AI/II(-/-) mice and wild-type (WT) controls.

Selectively induced death of macrophages in the synovial lining of murine knee joints using 10B-liposomes and boron neutron capture synovectomy.

Objective: To investigate whether macrophages in the synovial lining can be selectively eliminated by local administration of an improved boron-10 ((10)B) containing liposome formulation combined with neutron irradiation (boron neutron capture synovectomy [BNCS]).

Methods: Disodium dodecahydrododecaborate (Na(2)(10)B(12)H(12)) was encapsulated into unilamellar liposomes ((10)B-liposomes). (10)B-liposomes were injected into the mouse knee joint.

Different amplifying mechanisms of interleukin-17 and interferon-gamma in Fcgamma receptor-mediated cartilage destruction in murine immune complex-mediated arthritis.

Objective: Previously, we reported that interferon-gamma (IFNgamma) aggravates cartilage destruction in immune complex (IC)-mediated arthritis via up-regulation of activating Fcgamma receptors (FcgammaR). Recently, we found that interleukin-17 (IL-17) also aggravates cartilage destruction in arthritis models in which ICs are involved, but the underlying mechanism remains unknown. This study was undertaken to determine the role of IL-17 in FcgammaR-mediated cartilage destruction in IC-mediated arthritis and to compare its effect with that of IFNgamma.

Stimulation of chondrocyte-mediated cartilage destruction by S100A8 in experimental murine arthritis.

Objective: To investigate whether S100A8 is actively involved in matrix metalloproteinase (MMP)-mediated chondrocyte activation.

Methods: S100A8 and S100A9 proteins were detected in inflamed knee joints from mice with various forms of murine arthritis, using immunolocalization. Murine chondrocyte cell line H4 was stimulated with proinflammatory cytokines or recombinant S100A8.

Myeloid-related proteins S100A8/S100A9 regulate joint inflammation and cartilage destruction during antigen-induced arthritis.

Objective: To study the active involvement of Myeloid-related proteins S100A8 and S100A9 in joint inflammation and cartilage destruction during antigen-induced arthritis (AIA).

Methods: Joint inflammation and cartilage destruction was measured with 99mTc uptake and histology. The role of S100A8/A9 was investigated by inducing AIA in S100A9-/- mice that also lack S100A8 at protein level, or after intra-articular injection of rS100A8 in mouse knee joints.