Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by synovial hyperplasia, cartilage damage, and bone erosions, leading to progressive disability. Information on the character of the initial destructive events in arthritis is limited, since the affected structures are not directly accessible in patients with very early disease, and imaging techniques such as radiography and magnetic resonance imaging (MRI) do not provide insight into the cellular processes involved in the initial destructive process. Thus, it is unclear whether joint destruction accompanies inflammation of the joints right from disease onset or whether there is a certain lag period between onset of inflammation and structural damage. The latter has been supported by evidence of synovial inflammation occurring even in the absence of clinically apparent symptoms of arthritis (1).
Chronic arthritic diseases such as RA usually lead to joint damage after a short period of time. Even comparatively insensitive techniques such as plain radiography show destructive changes within 6 months from disease onset in more than 50% of patients with RA (2). The destructive processes in RA depend on a complex interplay of synovial fibroblasts, lymphocytes, macrophages, and monocyte-derived osteoclasts (3, 4). The documented invasive properties of synovial fibroblasts include production of tissue-degrading enzymes such as cathepsins and matrix metalloproteinases (5). In addition, macrophages, activated synovial fibroblasts, and lymphocytes contribute to this process by synthesizing proinflammatory cytokines and chemokines. Tumor necrosis factor (TNF) is a key cytokine in RA (6), and selective overexpression of TNF in mice is sufficient for the development of destructive arthritis (7). Of note, synovial fibroblasts and lymphocytes drive the differentiation of osteoclasts from monocyte precursor cells via the expression of RANKL (8–11). Osteoclasts are cells that are specifically designed to resorb bone (12).
The purpose of this study was to investigate the nature and timing of joint destruction in chronic arthritis. We hypothesized that microscopic signs of joint destruction are present as early as the onset of clinically apparent disease. To address this question, we used human TNF-transgenic (HuTNF-Tg) mice as an experimental arthritis model. This model is characterized by 1) a chronic progressive course of disease, 2) a symmetric polyarthritis with predominant involvement of the small joints, and 3) a destructive phenotype, all of which are features that closely resemble those of human RA. Structural changes in the joints were assessed at the stage of preclinical disease, at clinical onset of disease, and during early disease. Alterations in the periarticular tissue, such as the tendons, as well as in the articular cartilage and bone were assessed during these 3 initial phases of arthritis. Our results show that tenosynovitis is the first pathologic event manifest in arthritis. Moreover, tenosynovitis precipitates the rapid destruction of the adjacent juxtaarticular bone even in these very early stages of disease.
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- MATERIALS AND METHODS
- AUTHOR CONTRIBUTIONS
The early phases of RA have not been studied extensively. Although it is known that the pattern of disease onset may differ between patients, in the majority of patients with RA the disease starts insidiously (14). Not infrequently, tenosynovitis accompanies the early steps of RA (15–17). The sequence of the earliest pathologic events is still unknown. It has been postulated that an influx of inflammatory cells into the synovium precedes the development of the signs and symptoms of the disease, since frank synovitis could be found in the joints of RA patients who were clinically unaffected (1). It is also evident that immune system abnormalities precede the development of RA, since autoantibodies can be detected months to years before onset of the disease (18–20). Moreover, ∼10% of RA patients have evidence of bone erosions on plain radiographs as early as 8 weeks after disease onset (Machold K: personal communication). These and other observations have led to the concept of prearthritis (21).
However, the events governing the changes that occur before arthritis, i.e., at the time that inflammatory joint disease becomes manifest, cannot be easily studied in humans, since at the preclinical stage, tissue is not easily accessible and standard imaging techniques, including MRI, have an insufficient resolution to discern changes on the cellular level. To address this question, we therefore studied an animal model that closely resembles human RA in terms of overexpression of TNF and the characteristics of inflammation and bone and cartilage destruction.
With the aim of studying the first pathologic events of TNF-mediated inflammatory arthritis in the joint, we used the HuTNF-Tg mouse model of arthritis and screened for the onset of inflammation and structural damage before, at the time of, and shortly after disease onset. In fact, even in animal models, the knowledge regarding these initial phases of arthritis is scarce. Both collagen-induced and adjuvant-induced arthritis have a very rapid disease onset and progress quickly, making the dissection of these initial phases of arthritis difficult (3, 22). However, it is evident from these models that highly active arthritis needs only a few days to induce destruction of the affected joint (3). The disease in HuTNF-Tg mice has a mild and rather unspectacular onset, but it continuously progresses over time, leading to accumulation of an increasing amount of joint damage. Thus, with the use of this model, we were able to better study these initial phases of disease and relate them to the onset of clinical symptoms in more detail.
Interestingly, our study revealed that tendons are the earliest structures affected by inflammation (Figure 5). These structures are subject to the most prominent shear forces and are under prolonged mechanical load. Tendons are surrounded by synovial tissue, and the tendon sheaths have a cell composition similar to that of the synovial lining. Accumulation of fluid as well as effusion of inflammatory cells, primarily granulocytes and macrophages, into these structures is the first pathologic event induced by overexpression of TNF. Many investigators have speculated as to why these tendon sheaths are the first structures affected by chronic inflammation, but mechanical factors are likely to play a key role in this process. Ultimately, a complete remodeling of the tendon sheaths to pannus-like synovial tissue occurs, thereby completely filling the synovial space with inflamed tissue. This obviously leads to massive disturbance of the tendon and ultimately impedes joint function.
Figure 5. Model of arthritis onset in mice. Arthritis starts before onset of clinical disease, with infiltration of tendon sheaths (purple) by granulocytes and monocytes, leading to tenosynovitis. Tenosynovitis precipitates local osteoclast formation (red), in which tendons pass by the joint edges, thus inducing resorption of mineralized cartilage and bone. As tenosynovitis becomes more severe, the synovial membrane of the joint also becomes inflamed and hyperplastic (green), which is reflected in the onset of clinical disease. Structural damage further increases (light yellow). In the next step, tendon sheaths become further infiltrated and more pronounced inflammation of the synovial membrane occurs, which leads to an increase in clinical symptoms and acceleration of joint damage.
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Moreover, the tendons use the bony ends and their edges as anchors to increase function, thus bringing these structures into close contact with sites predisposed to the development of bone erosions. These areas in which tendon sheaths, mineralized cartilage, and subchondral bone are in close contact are very distinct. In these contact regions, the first osteoclasts appear and start to resorb subchondral bone and mineralized cartilage (10, 23). Thus, even before the onset of synovial inflammation, osteoclasts start their destructive work and pave the way for early B cell accumulation in the adjacent bone marrow, which is closely linked to breaking of the cortical bone barrier (24, 25). Since osteoclasts appear even before the first clinical signs of arthritis, destructive changes begin to occur even before arthritis is clinically apparent in the patients. This finding further strengthens the concept of a prearthritic state and confirms that there is a very narrow window of opportunity for achieving full remission of RA without any sequelae (26).
The activity of TNF leads to activation of MAPK cascades, such as p38 MAPK and ERK, very early in the disease. This is consistent with the well-documented activation of MAPK families in human RA (27–30). Thus, even in the preclinical disease phase, the joints of the HuTNF-Tg mice showed an activation of these 2 MAPK members, suggesting that p38 MAPK and ERK mediate the cellular effects of TNF right from the start of molecular onset of arthritis. As a matter of fact, p38 MAPK plays a major role in facilitating the cytokine activation induced by TNF. Induction of proinflammatory mediators such as IL-1 and IL-6 by TNF is mediated by activation of p38 MAPK (31). Of note, these cytokines are inducibly expressed in these initial stages of arthritis and appear to be selectively linked to the areas of onset of inflammation, such as the cellular infiltration of the tendon sheaths. IL-1 and IL-6 are subsequently expressed extensively throughout the newly formed inflamed synovial tissue. We found that this increased expression of IL-1 and IL-6 was paralleled by a rise in the serum levels of these 2 cytokines. Early activation of ERK, in contrast, might influence the survival of cells in the synovial membrane. ERK activation by TNF plays an important role in the maintenance of cell survival and the inhibition of apoptosis (32). In fact, apoptosis is low in the synovial membrane, suggesting that cytokines like TNF act on intracellular survival factors rather than apoptosis-inducing signaling molecules (33, 34).
In the present study, we also observed that damage of unmineralized surface cartilage occurred subsequent to, and not before or concomitant with, tenosynovitis and osteoclast formation. In contrast to osteoclast-mediated damage, which affects mineralized structures, breakdown of the surface cartilage depends on cytokine and enzyme production of cells in the joint cavity and the synovial lining. Cartilage damage was linked to a certain level of inflammation and structural organization of the synovial surface (lining layer), suggesting that a threshold of synovial inflammation is critical for matrix resorption. Thus, whereas destruction of mineralized tissue was linked to tenosynovitis and occurred very early in the disease, cartilage damage started later and was linked to inflammation of the synovial membrane.
In summary, we have shown that tenosynovitis is the first structural change to occur in TNF-mediated arthritis. Regions of close interaction between the tendons and mineralized tissue allow the spread of inflammatory changes to the joints, particularly leading to the resorption of mineralized cartilage and bone. Importantly, these changes occur before the first clinical signs of joint swelling are apparent. Our data suggest that newly occurring tenosynovitis should be taken seriously, since it might be followed by the development of RA. Moreover, structural changes in the joints are a very early feature of disease, starting right from its subclinical onset. Thus, the time window to protect joints from damage may, in fact, be very short, and this knowledge should give impetus to the efforts for early initiation of targeted therapy in patients with RA.
- Top of page
- MATERIALS AND METHODS
- AUTHOR CONTRIBUTIONS
Dr. Schett had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study design. Drs. Hayer, Redlich, Smolen, and Schett.
Acquisition of data. Drs. Hayer and Korb.
Analysis and interpretation of data. Drs. Hayer, Redlich, Hermann, and Schett.
Manuscript preparation. Drs. Hayer, Smolen, and Schett.
Statistical analysis. Drs. Hayer and Hermann.