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Osteoarthritis, characterized by the breakdown of articular cartilage in synovial joints, has long been viewed as the result of 'wear and tear'. Although low-grade inflammation is detected in osteoarthritis, its role is unclear. 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 complement component 5 (C5), C6 or the complement regulatory protein CD59a, we show that complement, specifically, the membrane attack complex (MAC)-mediated arm of complement, is crucial 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 from C5-deficient mice than C5-sufficient mice, and MAC induced production of these molecules in cultured chondrocytes. Further, MAC colocalized with matrix metalloprotease 13 (MMP13) and with activated extracellular signal-regulated kinase (ERK) around chondrocytes in human osteoarthritic cartilage. Our findings indicate that dysregulation of complement in synovial joints has a key role in the pathogenesis of osteoarthritis.

Introduction: The development of effective treatments for osteoarthritis (OA) has been hampered by a poor understanding of OA at the cellular and molecular levels. Emerging as a disease of the ‘whole joint’, the importance of the biochemical contribution of various tissues, including synovium, bone and articular cartilage, has become increasingly significant. Bathing the entire joint structure, the proteomic analysis of synovial fluid (SF) from osteoarthritic shoulders offers a valuable ‘snapshot’ of the biologic environment throughout disease progression. The purpose of this study was to identify differentially expressed proteins in early and late shoulder osteoarthritic SF in comparison to healthy SF. Methods: A quantitative 18 O labeling proteomic approach was employed to identify the dysregulated SF proteins in early (n = 5) and late (n = 4) OA patients compared to control individuals (n = 5). In addition, ELISA was used to quantify six pro-inflammatory and two anti-inflammatory cytokines. Results: Key results include a greater relative abundance of proteins related to the complement system and the extracellular matrix in SF from both early and late OA. Pathway analyses suggests dysregulation of the acute phase response, liver x receptor/retinoid x receptor (LXR/RXR), complement system and coagulation pathways in both early and late OA. The network related to lipid metabolism was down-regulated in both early and late OA. Inflammatory cytokines including interleukin (IL) 6, IL 8 and IL 18 were up-regulated in early and late OA. Conclusions: The results suggest a dysregulation of wound repair pathways in shoulder OA contributing to the presence of a ‘chronic wound’ that progresses irreversibly from early to later stages of OA. Protease inhibitors were downregulated in late OA suggesting uncontrolled proteolytic activity occurring in late OA. These results contribute to the theory that protease inhibitors represent promising therapeutic agents which could limit proteolytic activity that ultimately leads to cartilage destruction.

Diagnostic arthroscopy has long been considered the gold standard for assessment of superior labrum anterior posterior (SLAP) tears. The most widely used classification system for these lesions was described by Snyder et. al. in 1990 [1]. However, until recently, no studies had ever evaluated the interobserver and intraobserver variability amongst orthopedic surgeons using the Snyder classification in order to make a diagnosis and determine treatment of these tears. We recently conducted a study to assess the interobserver and intraobserver variability of experienced shoulder arthroscopists in the evaluation of SLAP tears. In our study design, we sent a CD with 23 brief video vignettes to more than 300 members of the Arthroscopy Association of North America, the American Shoulder and Elbow Society, and the American Orthopedic Society for Sports Medicine. Each of these surgeons were asked to view the vignettes and make a diagnosis according to the Snyder classification and recommend a treatment based on this classification in addition to various demographic data on their level of experience, case volume, and fellowship training. Seventy-three surgeons responded with completed surveys. The membership of these organizations included several prominent European shoulder surgeons. Overall, we found that the interobserver variability amongst experienced arthroscopists was considerable and the analysis of intraobserver variability showed only moderate agreement for both diagnosis and treatment. The results also showed that shoulder arthroscopists had difficulty distinguishing normal shoulders from Types I and II SLAP tears and less than half of surgeons agreed on the diagnosis and treatment of Types II and III lesions respectively. We concluded that the Snyder classification is not helpful for guiding diagnosis and treatment of Types II and III lesions. The implications of these findings are profound. Many previous studies on the predictive value of physician examination maneuvers and diagnostic imaging such as MRI have already demonstrated variability and lack of reproducibility. Thus, diagnostic arthroscopy has been designated as the gold standard in determining whether or not a SLAP lesion is present. Our data clearly demonstrate that there is a real concern about the accuracy of arthroscopic determination of SLAP lesions. Another important conclusion from our observation of variable interobserver and intraobserver determination of pathology of SLAP lesions is that the Snyder classification for SLAP lesions is not reproducible. A key observation from our study was that interobserver variability was markedly improved (in fact, “good”) when the diagnosis was made based on what treatment the surgeon would recommend for any given lesion. In short, surgeons agreed on the diagnosis when they were asked about how they would treat these lesions. This thought process is the “reverse” of what the Snyder classification proposes: make the diagnosis in order to guide the treatment. So, in the end, accurate determination of superior labrum pathology may not be possible based on arthroscopy; however, when a lesion is encountered that seems to require surgical repair, surgeons probably make the correct decision even if they cannot accurately classify the type of superior labrum pathology.

The development of increasingly high-throughput and sensitive mass spectroscopy-based proteomic techniques provides new opportunities to examine the physiology and pathophysiology of many biologic fluids and tissues. The purpose of this study was to determine protein expression profiles of high-abundance synovial fluid (SF) proteins in health and in the prevalent joint disease osteoarthritis (OA). A cross-sectional study of 62 patients with early OA (n = 21), patients with late OA (n = 21), and control individuals (n = 20) was conducted. SF proteins were separated by using one-dimensional PAGE, and the in-gel digested proteins were analyzed by electrospray ionization tandem mass spectrometry. A total of 362 spots were examined and 135 high-abundance SF proteins were identified as being expressed across all three study cohorts. A total of 135 SF proteins were identified. Eighteen proteins were found to be significantly differentially expressed between control individuals and OA patients. Two subsets of OA that are not dependent on disease duration were identified using unsupervised analysis of the data. Several novel SF proteins were also identified. Our analyses demonstrate no disease duration-dependent differences in abundant protein composition of SF in OA, and we clearly identified two previously unappreciated yet distinct subsets of protein profiles in this disease cohort. Additionally, our findings reveal novel abundant protein species in healthy SF whose functional contribution to SF physiology was not previously recognized. Finally, our studies identify candidate biomarkers for OA with potential for use as highly sensitive and specific tests for diagnostic purposes or for evaluating therapeutic response.