Osteocytes are the most abundant cell type in bone and are distributed throughout the mineralised bone matrix forming an interconnected network that ideally positions them to sense and to respond to local biomechanical and systemic stimuli to regulate bone remodelling and adaptation. The adaptive process is dependent on the coordinated activity of osteoclasts and osteoblasts that form a so called bone multicellular unit that remodels cortical and trabecular bone through a process of osteoclast-mediated bone resorption, followed by a phase of bone formation mediated by osteoblasts. Osteocytes mediate their effects on bone remodelling via both cell–cell interactions with osteoclasts and osteoblasts, but also via signaling through the release of soluble mediators. The remodelling process provides a mechanism for adapting the skeleton to local biomechanical factors and systemic hormonal influences and for replacing bone that has undergone damage from repetitive mechanical loading.
Osteoporosis; Osteoarthritis; Bone Mineral Density; Cytokines
Simvastatin (SIM), a widely used anti-lipidaemic drug, has been identified as a bone anabolic agent. Its poor water solubility and the lack of distribution to the skeleton, however, have limited its application in the treatment of bone metabolic diseases. In this study, an amphiphilic macromolecular prodrug of SIM was designed and synthesized to overcome these limitations. The polyethylene glycol (PEG)-based prodrug can spontaneously self-assemble to form micelles. The use of SIM trimer as the prodrug’s hydrophobic segment allows easy encapsulation of additional free SIM. The in vitro studies showed that SIM/SIM-mPEG micelles were internalized by MC3T3 cells via lysosomal trafficking and consistently induced expression of both BMP2 and DKK1 mRNA, suggesting that the prodrug micelle retains the biological functions of SIM. After systemic administration, optical imaging suggests that the micelles would passively target to bone fracture sites associated with hematoma and inflammation. Furthermore, flow cytometry study revealed that SIM/SIM-mPEG micelles had preferred cellular uptake by inflammatory and resident cells within the fracture callus tissue. The treatment study using a mouse osteotomy model validated the micelles’ therapeutic efficacy in promoting bone fracture healing as demonstrated by micro-CT and histological analyses. Collectively, these data suggest that the macromolecular prodrug-based micelle formulation of SIM may have great potential for clinical management of impaired fracture healing.
Prodrug; Simvastatin; Bone Fracture; Micelle; ELVIS
Adverse local tissue reaction (ALTR), characterized by a heterogeneous cellular inflammatory infiltrate and the presence of corrosion products in the periprosthetic soft tissues, has been recognized as a mechanism of failure in total hip replacement (THA). Different histological subtypes may have unique needs for longitudinal clinical follow-up and complication rates after revision arthroplasty. The purpose of this study was to describe the histological patterns observed in the periprosthetic tissue of failed THA in three different implant classes due to ALTR and their association with clinical features of implant failure.
Consecutive patients presenting with ALTR from three major hip implant classes (N = 285 cases) were identified from our prospective Osteolysis Tissue Database and Repository. Clinical characteristics including age, sex, BMI, length of implantation, and serum metal ion levels were recorded. Retrieved synovial tissue morphology was graded using light microscopy. Clinical characteristics and features of synovial tissue analysis were compared between the three implant classes. Histological patterns of ALTR identified from our observations and the literature were used to classify each case. The association between implant class and histological patterns was compared.
Our histological analysis demonstrates that ALTR encompasses three main histological patterns: 1) macrophage predominant, 2) mixed lymphocytic and macrophagic with or without features of associated with hypersensitivity/allergy or response to particle toxicity (eosinophils/mast cells and/or lymphocytic germinal centers), and 3) predominant sarcoid-like granulomas. Implant classification was associated with histological pattern of failure, and the macrophagic predominant pattern was more common in implants with metal-on-metal bearing surfaces (MoM HRA and MoM LHTHA groups). Duration of implantation and composition of periprosthetic cellular infiltrates was significantly different amongst the three implant types examined suggesting that histopathological features of ALTR may explain the variability of clinical implant performance in these cases.
ALTR encompasses a diverse range of histological patterns, which are reflective of both the implant configuration independent of manufacturer and clinical features such as duration of implantation. The macrophagic predominant pattern and its mechanism of implant failure represent an important subgroup of ALTR which could become more prominent with increased length of implantation.
Adverse local tissue reaction; Corrosion products; Revision arthroplasty; Synovial inflammation; Metal-on-metal total hip replacement; Hip resurfacing
Concepts regarding osteoarthritis, the most common joint disease, have dramatically changed in the past decade thanks to the development of new imaging techniques and the widespread use of arthroscopy that permits direct visualisation of intra-articular tissues and structure. MRI and ultrasound allow the early detection of pre-radiographic structural changes not only in the peri-articular bone but also in the cartilage, menisci, synovial membrane, ligaments and fat pad. The significance of MRI findings such as cartilage defects, bone marrow lesions, synovial inflammation/effusions and meniscal tears in patients without radiographic signs of osteoarthritis is not fully understood. Nevertheless, early joint tissue changes are associated with symptoms and, in some cases, with progression of disease. In this short review, we discuss the emerging concept of early osteoarthritis localised to the knee based on recently updated knowledge. We highlight the need for a new definition of early osteoarthritis that will permit the identification of patients at high risk of osteoarthritis progression and to initiate early treatment interventions.
Knee Osteoarthritis; Magnetic Resonance Imaging; Osteoarthritis
This review summarises recent evidence about the interaction between bone, the immune system and cartilage in disabling conditions such as osteoarthritis, rheumatoid arthritis and spondyloarthritis. These topics have been recently discussed at the ‘OsteoRheumatology’ conference held in Genoa in October 2014. The meeting, at its 10th edition, has been conceived to bring together distinguished international experts in the fields of rheumatic and metabolic bone diseases with the aim of discussing emerging knowledge regarding the role of the bone tissue in rheumatic diseases. Moreover, this review focuses on new treatments based on underlying the pathophysiological processes in rheumatic diseases. Although, a number of issues still remain to be clarified, it seems quite clear that in clinical practice, as well as in basic and translational research, there is a need for more knowledge of the interactions between the cartilage, the immune system and the bone. In this context, ‘OsteoRheumatology’ represents a potential new discipline providing a greater insight into this interplay, in order to face the multifactorial and complex issues underlying common and disabling rheumatic diseases.
Osteoarthritis; Rheumatoid Arthritis; Spondyloarthritis; Treatment; Osteoporosis
To develop and evaluate diagnostic tools for early detection of wear particle-induced orthopaedic implant loosening.
N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer was tagged with an near infrared dye and used to detect the inflammation induced by polymethylmethacrylate (PMMA) particles in a murine peri-implant osteolysis model. It was established by inserting implant into distal femur and with routine PMMA particles challenging. The osteolysis was evaluated by micro-CT and histological analysis at different time points.
Significant peri-implant osteolysis was found three-month post PMMA particle challenge by micro-CT and histological analysis. At one-month time point, when there was no significant peri-implant bone loss, HPMA copolymer-near infrared dye conjugate was found to specifically target to the femur with PMMA particles deposition, but not the contralateral control femur with PBS infusion.
The results from this study demonstrated the feasibility of utilizing the macromolecular diagnostic agent to report particle-induced peri-implant inflammation prior to the development of detectable osteolysis. Recognition of this early pathological event would provide the window of opportunity for prevention of peri-implant osteolysis and subsequent orthopaedic implant failure.
HPMA copolymer; inflammation targeting; early diagnosis; aseptic implant loosening; ELVIS
Aseptic implant loosening related to implant wear particle-induced inflammation is the most common cause of failure after joint replacement. Modulation of the inflammatory reaction to the wear products represents a rational approach for preventing aseptic implant failure. Long-term treatment using anti-inflammatory agents, however, can be associated with significant systemic side effects due to the drugs' lack of tissue specificity. To address this issue, N-(2-hydroxypropyl) methacrylamide (HPMA) copolymer-dexamethasone conjugate (P-Dex) was developed and evaluated for prevention of wear particle-induced osteolysis and the loss of fixation in a murine prosthesis failure model. Daily administration of free dexamethasone (Dex) was able to prevent wear particle-induced osteolysis, as assessed by micro-CT and histological analysis. Remarkably, monthly P-Dex administration (dose equivalent to free Dex treatment) was equally effective as free dexamethasone, but was not associated with systemic bone loss (a major adverse side effect of glucocorticoids). The reduced systemic toxicity of P-Dex is related to preferential targeting of the sites of wear particle-induced inflammation and its subcellular sequestration and retention by local inflammatory cell populations, resulting in sustained therapeutic action. These results demonstrate the feasibility of utilizing a macromolecular prodrug with reduced systemic toxicity to prevent wear particle-induced osteolysis.
HPMA copolymer; prodrug; inflammation targeting; dexamethasone; implant loosening; ELVIS
As the only cells capable of efficiently resorbing bone, osteoclasts are central mediators of both normal bone remodeling and pathologies associates with excessive bone resorption. However, despite the clear evidence of interplay between osteoclasts and the bone surface in vivo, the role of the bone substrate in regulating osteoclast differentiation and activation at a molecular level has not been fully defined. Here, we present the first comprehensive expression profiles of osteoclasts differentiated on authentic resorbable bone substrates. This analysis has identified numerous critical pathways coordinately regulated by osteoclastogenic cytokines and bone substrate, including the transition from proliferation to differentiation, and sphingosine-1-phosphate signaling. Whilst, as expected, much of this program is dependent upon integrin beta 3, the pre-eminent mediator of osteoclast-bone interaction, a surprisingly significant portion of the bone substrate regulated expression signature is independent of this receptor. Together, these findings identify an important hitherto underappreciated role for bone substrate in osteoclastogenesis.
Adverse local tissue reaction (ALTR) is characterized by periprosthetic soft tissue inflammation composed of a mixed inflammatory cell infiltrate, extensive soft tissue necrosis, and vascular changes. Multiple hip implant classes have been reported to result in ALTR, and clinical differences may represent variation in the soft tissue response at the cellular and tissue levels. The purpose of this study was to describe similarities and differences in periprosthetic tissue structure, organization, and cellular composition by conventional histology and immunohistochemistry in ALTR resulting from two common total hip arthroplasty (THA) implant classes.
Consecutive patients presenting with ALTR from two major hip implant classes (N = 54 patients with Dual-Modular Neck implant; N = 14 patients with Metal-on-Metal implant) were identified from our prospective Osteolysis Tissue Database and Repository. Clinical characteristics including age, sex, BMI, length of implantation, and serum metal ion levels were recorded. Retrieved synovial tissue morphology was graded using light microscopy and cellular composition was assessed using immunohistochemistry.
Length of implantation was shorter in the DMN group versus MoM THA group (21.3 [8.4] months versus 43.6 [13.8] months respectively; p < 0.005) suggesting differences in implant performance. Morphologic examination revealed a common spectrum of neo-synovial proliferation and necrosis in both groups. Macrophages were more commonly present in diffuse sheets (Grade 3) in the MoM relative to DMN group (p = 0.016). Perivascular lymphocytes with germinal centers (Grade 4) were more common in the DMN group, which trended towards significance (p = 0.066). Qualitative differences in corrosion product morphology were seen between the two groups. Immunohistochemistry showed features of a CD4 and GATA-3 rich lymphocyte reaction in both implants, with increased ratios of perivascular T-cell relative to B-cell markers in the DMN relative to the MoM group (p = 0.032).
Our results demonstrate that both implant classes display common features of neo-synovial proliferation and necrosis with a CD4 and GATA-3 rich inflammatory infiltrate. Qualitative differences in corrosion product appearance, macrophage morphology, and lymphocyte distributions were seen between the two implant types. Our data suggests that ALTR represents a histological spectrum with implant-based features.
Adverse local tissue reaction; Corrosion products; Revision arthroplasty; Synovial inflammation
Synovitis is associated with pain and other symptoms in patients with knee OA, and in patients with meniscal tears even in the absence of radiographic OA. Patients undergoing arthroscopic partial meniscectomy were followed for 2 years to determine whether synovitis predicts post-operative symptoms.
Thirty-three patients scheduled for arthroscopy were recruited for this pilot study. Symptoms were assessed using a knee pain scale, the Lysholm score, and the SF-12® pre-operatively and at 16 weeks, 1 year and 2 years post-operatively. Synovial inflammation and hyperplasia were graded on surgical biopsies. Linear mixed effects models were tested to determine whether inflammation or hyperplasia is associated with outcome scores over time.
Lysholm scores and SF-12® physical component sub-scores were worse pre-operatively in patients with inflammation (Lysholm: 52.42 [95%CI 42.37,62.47] vs. 72.38 [66.03,78.72], p<0.001; SF-12: 36.81 [28.26,45.37] vs 46.23 [40.14,52.32], p<0.05). Up to two-years post- operatively, patients with inflammation achieved mean scores similar to those without inflammation. As a result, the mean improvement in Lysholm scores was 13.01 [1.48–24.53] points higher than patients without inflammation, p = 0.03. 33% (4/12) of patients with inflammation still had fair-to-poor Lysholm scores two years after surgery compared to 7% (1/15, (p=0.14) without inflammation. No association between hyperplasia and symptoms was noted.
In this pilot study of patients undergoing partial meniscectomy, synovial inflammation was associated with worse pre-operative symptoms, but not with poorer outcomes in the first two years post-arthroscopy. Larger cohorts and longer follow-up should be pursued to confirm this relationship, and determine if the initial response is sustained.
synovitis; osteoarthritis; meniscal tear; inflammation; arthroscopy
Alterations in the mechanical loading environment in joints may have both beneficial and detrimental effects on articular cartilage and subchondral bone and subsequently influence the development of osteoarthritis (OA). We used an in vivo tibial loading model to investigate the adaptive responses of cartilage and bone to mechanical loading and to assess the influence of load level and duration.
We applied cyclic compression of 4.5 and 9.0N peak loads to the left tibia via the knee joint of adult (26-week-old) C57Bl/6 male mice for 1, 2, and 6 weeks. Only 9.0N loading was utilized in young (10-week-old) mice. The changes in articular cartilage and subchondral bone were analyzed by histology and microcomputed tomography.
Loading promoted cartilage damage in both age groups, with increased damage severity dependent upon the duration of loading. Metaphyseal bone mass increased in the young mice, but not in the adult mice, whereas epiphyseal cancellous bone mass decreased with loading in both young and adult mice. Articular cartilage thickness decreased, and subchondral cortical bone thickness increased in the posterior tibial plateau in both age groups. Both age groups developed periarticular osteophytes at the tibial plateau in response to the 9.0N load, but no osteophyte formation occurred in adult mice subjected to 4.5N peak loading.
This non-invasive loading model permits dissection of temporal and topographical changes in cartilage and bone and will enable investigation of the efficacy of treatment interventions targeting joint biomechanics or biological events that promote OA onset and progression.
Mechanical loading; cartilage; subchondral bone; osteoarthritis; mice
The pathophysiology of the most common joint disease, osteoarthritis (OA), remains poorly understood. Since synovial fluid (SF) bathes joint cartilage and synovium, we reasoned that a comparative analysis of its protein constituents in health and OA could identify pathways involved in joint damage. A proteomic analysis of knee SF from OA patients and control subjects was performed and compared to microarray expression data from cartilage and synovium.
Age-matched knee SF samples from control subjects, and patients with early- and late-stage OA (n=10 per group) were compared using two-dimensional differential gel electrophoresis (2D-DIGE) and mass spectrometry (MS). A MS with multiplexed peptide selected reaction monitoring (SRM) assay was used to confirm differential expression of a subset of proteins in an independent OA patient cohort. Proteomic results were analyzed by Ingenuity pathway analysis and compared to published synovial tissue and cartilage mRNA profiles.
66 proteins were differentially present in healthy and OA SF. Three major pathways were identified among these proteins: the acute phase response, and the complement and coagulation pathways. Differential expression of 5 proteins was confirmed by SRM assay. A focused analysis of transcripts corresponding to the differentially present proteins indicates that both synovial and cartilage tissues may contribute to the OA SF proteome.
Proteins involved in the acute phase response, complement and coagulation pathways are differentially regulated in SF of OA patients suggesting they contribute to joint damage. Validation of these pathways and their utility as biomarkers or therapeutic targets in OA is warranted.
The delayed Gadolinium-Enhanced Magnetic Resonance Imaging of Cartilage (dGEMRIC) method allows for both qualitative and quantitative measurement of the spatial distribution of glycosaminoglycan [GAG] in excised cartilage. The objective of this study was to determine the effect of paraformaldehyde fixation on dGEMRIC measurements. Five bovine and seven human cartilage pieces were punched into 5-mm plugs, fixed for 18 h in 4% paraformaldehyde solution, and washed. The magnetic resonance imaging (MRI) parameter T1 was measured prior and post fixation in cartilage without (T10) and with (T1Gd), the ionically charged MRI contrast agent Gd(DTPA)2−. Images of tissue before and after fixation were qualitatively very similar. The ratios of T10, T1Gd, and calculated [GAG] after fixation, relative to before fixation, were near or slightly higher than 1 for both bovine cartilage (1.01 ± 0.01, 1.04 ± 0.02, 1.05 ± 0.03, respectively) and for human cartilage (0.96 ± 0.11, 1.03 ± 0.05, 1.09 ± 0.13). Thus, these data suggest that dGEMRIC can be used on previously fixed samples to assess the three dimensional spatial distribution of GAG.
cartilage; GAG; fixed charge; fixation; dGEMRIC
Extensive research has implicated inflammation as a necessary and causative
factor in the development of peri-implant osteolysis, suggesting that such an
inflammatory response is the sentinel event for the process. The potential to
impact the clinical course of this condition is hampered by the lack of an
effective medical therapy, as well as a limited ability for early detection prior
to radiographically evident osteolysis. Advances in nanotechnology have allowed
for the production of engineered water-soluble nanocarriers, which exploit changes
in the microvascular architecture for selective distribution to inflamed tissues.
Evaluation of the uptake of the nanocarriers in sites of inflammation has
elucidated a novel mechanism of cellular uptake and retention of these
The current review discusses the development of a novel, biocompatible,
water-soluble nanocarrier utilizing copolymers of N-(2-hydroxypropyl)methacrylamide (HPMA), conjugated to imaging and
therapeutic agents for the detection and targeted treatment of inflammatory
We performed Medline searches for the terms “periprosthetic osteolysis,”
“murine osteolysis model,” “HPMA osteolysis,” and “HPMA inflammation.” These
searches identified 631, 306, 1, and 6 articles, respectively. These were then
manually searched for articles relevant to the development of mouse models for
inflammatory osteolysis and the use of HPMA copolymer technology in mouse models
Promising results in a small animal model of osteolysis have demonstrated the
capability for detection prior to the development of bone loss, and have
highlighted the utility of nanocarriers for selective drug delivery to the
Challenges to the clinical translation of HPMA nanocarriers in peri-implant
osteolysis remain, and the future research directions necessary for human clinical
application are reviewed.
periprosthetic osteolysis; nanocarrier; HPMA; optical imaging
Negative regulation of osteoclastogenesis is important for bone homeostasis and prevention of excessive bone resorption in inflammatory and other diseases. Mechanisms that directly suppress osteoclastogenesis are not well understood. In this study we investigated regulation of osteoclast differentiation by the β2 integrin CD11b/CD18 that is expressed on myeloid lineage osteoclast precursors. CD11b-deficient mice exhibited decreased bone mass that was associated with increased osteoclast numbers and decreased bone formation. Accordingly, CD11b and β2 integrin signaling suppressed osteoclast differentiation by preventing RANKL-induced induction of the master regulator of osteoclastogenesis NFATc1 and of downstream osteoclast-related NFATc1 target genes. CD11b suppressed induction of NFATc1 by the complementary mechanisms of downregulation of RANK expression and induction of recruitment of the transcriptional repressor BCL6 to the NFATC1 gene. These findings identify CD11b as a negative regulator of the earliest stages of osteoclast differentiation, and provide an inducible mechanism by which environmental cues suppress osteoclastogenesis by activating a transcriptional repressor that makes genes refractory to osteoclastogenic signaling.
osteoclast; signaling; integrin; CD11b; BCL6
Rheumatoid arthritis (RA) is a chronic autoimmune disease that is considered to be one of the major public health problems worldwide. The development of therapies that target tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6) and co-stimulatory pathways that regulate the immune system have revolutionized the care of patients with RA. Despite these advances, many patients continue to experience symptomatic and functional impairment. To address this issue, more recent therapies that have been developed are designed to target intracellular signaling pathways involved in immunoregulation. Though this approach has been encouraging, there have been major challenges with respect to off-target organ side effects and systemic toxicities related to the widespread distribution of these signaling pathways in multiple cell types and tissues. These limitations have led to an increasing interest in the development of strategies for the macromolecularization of anti-rheumatic drugs, which could target them to the inflamed joints. This approach enhances the efficacy of the therapeutic agent with respect to synovial inflammation, while markedly reducing non-target organ adverse side effects. In this manuscript, we provide a comprehensive overview of the rational design and optimization of macromolecular prodrugs for treatment of RA. The superior and the sustained efficacy of the prodrug may be partially attributed to their Extravasation through Leaky Vasculature and subsequent Inflammatory cell-mediated Sequestration (ELVIS) in the arthritic joints. This biologic process provides a plausible mechanism, by which macromolecular prodrugs preferentially target arthritic joints and illustrates the potential benefits of applying this therapeutic strategy to the treatment of other inflammatory diseases.
Rheumatoid arthritis; Macromolecular prodrug; ELVIS; Targeting inflammation; Drug delivery
Research into the pathophysiology of osteoarthritis (OA) has focused on cartilage and peri-articular bone, but there is increasing recognition that OA affects all of the joint tissues, including the synovium (SM). Under normal physiological conditions the synovial lining consists of a thin layer of cells with phenotypic features of macrophages and fibroblasts. These cells and the underlying vascularized connective tissue stroma form a complex structure that is an important source of synovial fluid (SF) components that are essential for normal cartilage and joint function. The histological changes observed in the SM in OA generally include features indicative of an inflammatory “synovitis”; specifically they encompass a range of abnormalities, such as synovial lining hyperplasia, infiltration of macrophages and lymphocytes, neoangiogenesis and fibrosis. The pattern of synovial reaction varies with disease duration and associated metabolic and structural changes in other joint tissues. Imaging modalities including Magnetic Resonance (MRI) and ultrasound (US) have proved useful in detecting and quantifying synovial abnormalities, but individual studies have varied in their methods of evaluation. Despite these differences, most studies have concluded that the presence of synovitis in OA is associated with more severe pain and joint dysfunction. In addition, synovitis may be predictive of faster rates of cartilage loss in certain patient populations. Recent studies have provided insights into the pathogenic mechanisms underlying the development of synovitis in OA. Available evidence suggests that the inflammatory process involves engagement of Toll-like receptors and activation of the complement cascade by degradation products of extracellular matrices of cartilage and other joint tissues. The ensuing synovial reaction can lead to synthesis and release of a wide variety of cytokines and chemokines. Some of these inflammatory mediators are detected in joint tissues and SF in OA and have catabolic effects on chondrocytes. These inflammatory mediators represent potential targets for therapeutic interventions designed to reduce both symptoms and structural joint damage in OA.
Giant cell tumor of bone (GCTB) is a benign, locally destructive neoplasm, with tumors comprised of mesenchymal fibroblast-like stromal cells; monocytic, mononuclear cells of myeloid lineage; and the characteristic osteoclast-like, multinucleated giant cells. Hampering the study of the complex interaction of its constituent cell types is the propensity of longstanding, repeatedly passaged cell cultures to undergo phenotypic alteration and loss of osteoclast-inducing capacities. In this study, we employed a novel, single-step technique to purify freshly harvested stromal cells using a CD14-negative selection column. Using 9 freshly harvested GCTB specimens and the purified stromal cell component, we performed analyses for markers of osteoblast lineage and analyzed the capacity of the stromal cells to undergo osteoblastic differentiation and induce osteoclastogenesis in co-cultures with monocytic cells. Successful purification of the CD14-negative stromal cells was confirmed via flow cytometric analysis and immunocytochemistry. Osteogenic media upregulated the expression of osteocalcin, suggesting an osteoblastic lineage of the GCTB stromal cells. The effects of the Wnt pathway agonist, SB415286, and recombinant human bone morphogenetic protein (BMP)-2 on osteoblastogenesis varied among samples. Notably, osteogenic media and SB415286 reversed the receptor activator of NF-κB ligand (RANKL)/osteoprotegerin (OPG) expression ratio resulting in diminished osteoclastogenic capacity. Recombinant human BMP2 had the opposite effect, resulting in enhanced and sustained support of osteoclastogenesis. Targeting the giant cell tumor stromal cell may be an effective adjunct to existing anti-resorptive strategies.
The seronegative spondyloarthopathies (SpA) share certain common articular and peri-articular features that differ from rheumatoid arthritis (RA) and other forms of inflammatory arthritis. These include the tendency of the SpAs to involve the axial skeleton in addition to the diarthrodial joints, and the prominent involvement of the extra-articular entheses (sites of ligamentous and tendon insertion), which are not common sites of primary pathology in RA and other inflammatory arthropathies. The differential anatomic sites of bone pathology in the SpAs in comparison to the other forms of arthritis suggest that the underlying pathogenic processes and cellular and molecular mechanisms that account for the peri-articular bone pathology involve different underlying disease mechanisms. This review will highlight the molecular and cellular processes that are involved in the pathogenesis of the skeletal pathology in the SpAs, and provide evidence that many of the factors involved in regulation of bone cell function exhibit potent immune-regulatory activity, providing support for the general concept of osteoimmunology.
Osteoimmunology; Bone homeostasis; Spondyloarthritis; Ankylosing spondylitis; Psoriatic arthritis; Osteoclast; Osteoblast; Osteocyte; Bone remodeling; Inflammation; Cytokine; Bone morphogenic proteins; Wnt/βcatenin; Sclerostin
This study examined whether patients with rheumatoid arthritis (RA) demonstrate different patterns of prosthetic wear or cellular responses to implant wear debris compared to patients without inflammatory joint disease.
Thirty-eight patients who had a primary revision of a total elbow arthroplasty (TEA) for aseptic loosening between 1996 and 2008 were identified. Twenty-five had RA and 13 had no inflammatory arthritis. Clinical data, gross wear patterns of the removed prostheses, and histopathological analyses of peri-implant tissue were compared between RA and non-RA patients.
Evaluation of the retrieved prostheses showed that conformational change of the humeral polyethylene bushing was associated with the generation of polyethylene and metal particles. The amount and type of wear debris in peri-prosthetic tissues was similar in RA and non-RA patients. RA patients not on anti-tumor necrosis factor (TNF) therapy exhibited a histologic pattern of interstitial and sheet-like lymphocytic infiltrates associated with a high plasma cell composition, which was different from the predominantly perivascular infiltrates with few plasma cells seen in non-RA patients (p-value = 0.04). RA patients on anti-TNF therapy fell in between these two groups.
RA patients exhibit a distinct cellular response to implant wear debris compared with non-RA patients. This reaction was unrelated to differences in the type or amount of wear debris and was mitigated by anti-TNF therapy. These results suggest an intrinsic alteration in immunoregulation in RA and have implications for potential immunologic treatment of osteolysis in these patients.
In their recent study, Sohn and colleagues identify multiple plasma proteins in the synovial fluid of patients with osteoarthritis (OA) and demonstrate the capacity of several of the proteins to activate macrophages via the innate immune receptor Toll-like receptor-4 (TLR-4). The authors speculate that the plasma proteins transit into the synovial compartment at sites of tissue damage where the endothelial barrier may be compromised, thus bringing them into contact with the articular surface and cells within the synovium. These results demonstrate a novel mechanism by which synovial inflammation can be initiated in patients with OA and how this process may contribute to the pathogenesis of OA joint pathology.
Wear particle-induced inflammation is considered to be the major cause of aseptic implant loosening and clinical failure after total joint replacement. Due to the frequent absence of symptoms, early detection and intervention prior to implant failure presents a significant challenge. To address this issue, a N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-based optical imaging contrast agent (P-IRDye) was developed and used for the detection of wear particle-induced inflammation employing a murine calvaria osteolysis model. The particle-induced osteolysis of calvaria was evaluated by H&E, tartrate-resistant acid phosphatase (TRAP) staining and μ-CT after necropsy. One-day post particles implantation, P-IRDye was administrated to the mice via tail vein injection. Live imaging of the animals 6 days after implantation revealed the preferential distribution and sustained retention of the macromolecular contrast agent at the site of particle implantation. Immunohistochemical staining and FACS analyses of the calvaria-associated soft tissue revealed extensive uptake of the HPMA copolymer by F4/80, Ly-6G (Gr1) and CD11c positive cells, which accounts for the retention of the macromolecular probes at the inflammatory sites. To test the potential of the system for therapeutic intervention, an acid-labile HPMA copolymer-dexamethasone conjugate (P-Dex) was prepared and shown to prevent the particle-induced inflammation and bone damage in the calvaria osteolysis model.
HPMA copolymer; aseptic orthopedic implant loosening; theranostics; orthopedic wear particle; inflammation targeting
The articular cartilage and the subchondral bone form a biocomposite that is uniquely adapted to the transfer of loads across the diarthrodial joint. During the evolution of the osteoarthritic process biomechanical and biological processes result in alterations in the composition, structure and functional properties of these tissues. Given the intimate contact between the cartilage and bone, alterations of either tissue will modulate the properties and function of the other joint component. The changes in periarticular bone tend to occur very early in the development of OA. Although chondrocytes also have the capacity to modulate their functional state in response to loading, the capacity of these cells to repair and modify their surrounding extracellular matrix is relatively limited in comparison to the adjacent subchondral bone. This differential adaptive capacity likely underlies the more rapid appearance of detectable skeletal changes in OA in comparison to the articular cartilage. The OA changes in periarticular bone include increases in subchondral cortical bone thickness, gradual decreases in subchondral trabeular bone mass, formation of marginal joint osteophytes, development of bone cysts and advancement of the zone of calcified cartilage between the articular cartilage and subchondral bone. The expansion of the zone of calcified cartilage contributes to overall thinning of the articular cartilage. The mechanisms involved in this process include the release of soluble mediators from chondrocytes in the deep zones of the articular cartilage and/or the influences of microcracks that have initiated focal remodeling in the calcified cartilage and subchondral bone in an attempt to repair the microdamage. There is the need for further studies to define the pathophysiological mechanisms involved in the interaction between subchondral bone and articular cartilage and for applying this information to the development of therapeutic interventions to improve the outcomes in patients with OA.
osteoarthritis; bone remodeling; articular cartilage; biomechanics
Osteoarthritis, characterized by the breakdown of articular cartilage in synovial joints, has long been viewed as the result of “wear and tear”1. Although low-grade inflammation is detected in osteoarthritis, its role is unclear2–4. Here we identify a central role for the inflammatory complement system in the pathogenesis of osteoarthritis. Through proteomic and transcriptomic analyses of synovial fluids and membranes from individuals with osteoarthritis, we find that expression and activation of complement is abnormally high in human osteoarthritic joints. Using mice genetically deficient in C5, C6, or CD59a, we show that complement, and specifically the membrane attack complex (MAC)-mediated arm of complement, is critical to the development of arthritis in three different mouse models of osteoarthritis. Pharmacological modulation of complement in wild-type mice confirmed the results obtained with genetically deficient mice. Expression of inflammatory and degradative molecules was lower in chondrocytes from destabilized joints of C5-deficient mice than C5-sufficient mice, and MAC induced production of these molecules in cultured chondrocytes. Furthermore, MAC co-localized with matrix metalloprotease (MMP)-13 and with activated extracellular signal-regulated kinase (ERK) around chondrocytes in human osteoarthritic cartilage. Our findings indicate that dysregulation of complement in synovial joints plays a critical role in the pathogenesis of osteoarthritis.