Normal bone remodeling depends upon a balance between the action of bone-resorbing cells, osteoclasts, and bone-forming cells, osteoblasts. When this balance is disrupted, as is seen in inflammatory diseases such as rheumatoid arthritis (RA) and ankylosing spondylitis (AS), abnormal bone loss or bone formation occurs. In RA, proinflammatory cytokines induce osteoclast differentiation and inhibit osteoblast maturation, leading to articular bone erosions. In contrast, the inflammatory milieu in AS leads to excessive osteoblast activation and bone formation at sites of entheses. While much information exists about the effects of proinflammatory cytokines on osteoclast differentiation and function, more recent studies have begun to elucidate the impact of inflammation on the osteoblast. This review will summarize the mechanisms by which inflammation perturbs bone homeostasis, with a specific focus on the osteoblast.
Bone erosion is a central feature of rheumatoid arthritis and is associated with disease severity and poor functional outcome. Erosion of periarticular cortical bone, the typical feature observed on plain radiographs in patients with rheumatoid arthritis, results from excessive local bone resorption and inadequate bone formation. The main triggers of articular bone erosion are synovitis, including the production of proinflammatory cytokines and receptor activator of nuclear factor kappaB ligand (RANKL), as well as antibodies directed against citrullinated proteins. Indeed, both cytokines and autoantibodies stimulate the differentiation of bone-resorbing osteoclasts, thereby stimulating local bone resorption. Although current antirheumatic therapy inhibits both bone erosion and inflammation, repair of existing bone lesions, albeit physiologically feasible, occurs rarely. Lack of repair is due, at least in part, to active suppression of bone formation by proinflammatory cytokines. This Review summarizes the substantial progress that has been made in understanding the pathophysiology of bone erosions and discusses the improvements in the diagnosis, monitoring and treatment of such lesions.
As very effective targeted biological therapies have become available to treat rheumatoid arthritis (RA), remission is now the goal of treatment. Since 1981, efforts have been undertaken to develop criteria for clinical remission in RA. Although several different measures of disease activity have been proposed, many issues remain unresolved. Active joint inflammation, even if involving only a few joints, negatively impacts a patient's quality of life and may ultimately result in structural damage. Thus, a low disease activity state (LDAS), which has been adopted as the target in clinical trials of 'treat to target', may not be the optimal treatment target in clinical practice. Similarly, the definitions of remission used in clinical trials may not be appropriate for use in daily clinical practice because some allow for the presence of several tender and swollen joints. Measures of disease activity do not necessarily correlate with structural remission, which implies halting progression of radiographic evidence of damage over time. Because no single measure of RA disease activity fully quantifies the global burden of disease, rheumatologists must follow multiple parameters to assess disease activity thoroughly and to adjust treatment optimally.
Repair of bone erosions in rheumatoid arthritis has been considered a difficult goal to achieve. However—with better therapies at hand to control synovial inflammation—sensitive μCT imaging techniques now available confirm that repair of bone erosion is possible, and begins at the base of erosive lesions.
In rheumatoid arthritis (RA), cells within the inflamed synovium and pannus elaborate a variety of cytokines, including tumor necrosis factor (TNF) alpha, interleukin (IL)-1, IL-6, and IL-17, that contribute to inflammation, and may directly affect bone. The receptor activator of NF-kappaB (RANK) ligand/RANK/osteoprotegerin pathway plays a critical role in regulating osteoclastogenesis in articular bone erosions in RA. Proinflammatory cytokines can modulate this pathway, and may also affect the ability of the osteoblast to repair bone at sites of articular erosion. In this review, the authors discuss the current understanding of pathogenic mechanisms of bone erosion in RA and examine current therapeutic approaches to prevent this damage.
The past decade has observed an explosion of new information regarding the impact of inflammation on bone. In rheumatic diseases, several factors that act as both immune modulators and regulators of bone homeostasis have been shown to mediate an imbalance in bone resorption and bone formation resulting in joint degeneration. In rheumatoid arthritis (RA), focal bone loss is due to excess bone resorption by osteoclasts. Resorption is mediated in part by increased local expression of the cytokine receptor activator of nuclear factor-kappaB ligand (RANKL) compared with expression of its decoy receptor osteoprotegerin (OPG). Bone formation by osteoblasts is also impaired at erosion sites in RA, and inhibitors of the canonical Wingless (Wnt) signaling pathway, including DKK1, have been implicated in the suppression of normal osteoblast function at these sites. Inhibition of DKK1 in an animal model of RA attenuated bone erosion by increasing OPG expression as well as promoting bone formation. In contrast to RA, inflammation in the spondyloarthropathies often results in excess periosteal bone formation, highlighting that the net impact of inflammation on bone is specific to the site at which inflammation occurs, and the cell types, cytokines, and factors present within the local bone microenvironment. This fertile area of research bears watching for the identification of novel targets for the prevention of abnormal bone remodeling in inflammatory diseases.
Rheumatoid arthritis, juvenile idiopathic arthritis, the seronegative spondyloarthropathies including psoriatic arthritis, and systemic lupus erythematosus are all examples of rheumatic diseases in which inflammation is associated with skeletal pathology. Although some of the mechanisms of skeletal remodeling are shared among these diseases, each disease has a unique impact on articular bone or on the axial or appendicular skeleton. Studies in human disease and in animal models of arthritis have identified the osteoclast as the predominant cell type mediating bone loss in arthritis. Many of the cytokines and growth factors implicated in the inflammatory processes in rheumatic diseases have also been demonstrated to impact osteoclast differentiation and function either directly, by acting on cells of the osteoclast-lineage, or indirectly, by acting on other cell types to modulate expression of the key osteoclastogenic factor receptor activator of nuclear factor (NF) kappaB ligand (RANKL) and/or its inhibitor osteoprotegerin (OPG). Further elucidation of the mechanisms responsible for inflammation-induced bone loss will potentially lead to the identification of novel therapeutic strategies for the prevention of bone loss in these diseases. In this review, we provide an overview of the cell types, inflammatory mediators, and mechanisms that are implicated in bone loss and new bone formation in inflammatory joint diseases.
PURPOSE OF REVIEW: Focal bone loss in inflammatory arthritis begins early in the disease process and can contribute to patient morbidity. Current treatment strategies primarily target suppression of the inflammatory cascade with varying success in limiting the progression of focal bone destruction. This review outlines the current understanding of the mechanisms mediating inflammation-induced focal bone loss in rheumatoid arthritis and other inflammatory arthritides and highlights recent studies in animal models of arthritis that have contributed to our knowledge of this process.
RECENT FINDINGS: Bone-resorbing osteoclasts have been identified as important effector cells in inflammation-induced bone loss in both experimental animal models and human rheumatoid arthritis and psoriatic arthritis. The RANK/RANKL (receptor activator of nuclear factor-kappaB and RANK ligand) pathway has been shown to be essential for osteoclast differentiation in inflammatory arthritis. In addition, in vitro and in vivo studies have demonstrated that many cytokines and growth factors elaborated by inflamed synovial tissues may contribute to osteoclast differentiation and activation.
SUMMARY: Elucidation of the mechanisms mediating osteoclast differentiation and function has identified new pathways for potential targeted therapeutic intervention for focal bone loss in inflammatory arthritis. Challenges in the application of this approach are that therapies targeting the osteoclast would need to be used in combination with effective anti-inflammatory agents, and that pathways mediating osteoclast differentiation and function would need to remain at least partially functional to allow for continued skeletal remodeling.
The molecular mechanisms underlying the putative role of osteopontin in the chronic inflammatory disease rheumatoid arthritis are unclear. A study in a murine model of arthritis now demonstrates that a specific antibody directed against the exposed osteopontin epitope SLAYGLR is capable of preventing inflammatory cell infiltration in arthritic joints.
Histopathologic characterization of bone erosions from patients with rheumatoid arthritis (RA) and studies performed in animal models of inflammatory arthritis provide strong evidence that osteoclasts play an important role in focal marginal and subchondral bone loss in inflammatory arthritis. Much has been learned concerning the factors responsible for the induction and activation of osteoclasts associated with the bone erosions in RA. Therapies that target these osteoclast-inducing factors or other aspects of osteoclast-mediated bone resorption represent potential targets for blocking or at least attenuating bone destruction in RA. The demonstration of the role of the newly described osteoclastogenic factor receptor activator of nuclear factor kappaB ligand in RA synovial tissues and the successful prevention of bone erosions in animal models of arthritis with its inhibitor osteoprotegerin provide hope that specific therapies can be developed for preventing bone and joint destruction in RA, particularly in situations in which disease-modifying agents are ineffective in controlling disease activity.
Rheumatoid arthritis represents an excellent model in which to gain insights into the local and systemic effects of joint inflammation on skeletal tissues. Three forms of bone disease have been described in rheumatoid arthritis. These include: focal bone loss affecting the immediate subchondral bone and bone at the joint margins; periarticular osteopenia adjacent to inflamed joints; and generalized osteoporosis involving the axial and appendicular skeleton. Although these three forms of bone loss have several features in common, careful histomorphometric and histopathological analysis of bone tissues from different skeletal sites, as well as the use of urinary and serum biochemical markers of bone remodeling, provide compelling evidence that different mechanisms are involved in their pathogenesis. An understanding of these distinct pathological forms of bone loss has relevance not only with respect to gaining insights into the different pathological mechanisms, but also for developing specific and effective strategies for preventing the different forms of bone loss in rheumatoid arthritis.
The cases presented illustrate some of the typical (case 1) and less common (case 2) clinical features of beta 2m amyloidosis. The accumulation of beta 2m amyloid in tissues is a potentially severe complication of dialysis-treated chronic renal failure. Beta 2m amyloidosis has been shown to have distinct clinical, radiologic, and pathologic features. The pathogenesis of this condition is not yet clearly understood, and recommendations for the clinical management of these patients at present are limited to recognition of the disease and symptomatic treatment. Further insights into the biology of this disease should lead to new strategies for prevention and treatment.