Synovial fluid analysis of two groups of proteoglycan epitopes distinguishes early and late cartilage lesions.

OBJECTIVE. To investigate whether fragmentation of proteoglycans in arthritis results in domains that have different levels of release from cartilage at different stages of the disease. METHODS. Two regions of the proteoglycan, the hyaluronan-binding region and the glycosaminoglycan-rich region of the core protein, were measured, by immunoassay, in knee joint synovial fluids of patients with rheumatoid arthritis or reactive arthritis. RESULTS. Synovial fluid concentrations of the glycosaminoglycan-rich region were highest in rheumatoid arthritis patients who had little cartilage damage as determined by radiography, whereas release of the hyaluronan-binding region predominated in patients with advanced cartilage destruction. In reactive arthritis, release of the glycosaminoglycan-rich region predominated. CONCLUSION. These findings indicate that the hyaluronan-binding region is initially retained in the tissue during the development of cartilage destruction. The combined analysis of these markers offers a new avenue for assessment of the degree of cartilage damage in arthritis.     

Release of cartilage macromolecules into the synovial fluid in patients with acute and prolonged phases of reactive arthritis

Objective. Extensive changes in articular cartilage metabolism occur during the acute phase of reactive arthritis, as indicated by altered release of cartilage macromolecules into synovial fluid (SF) demonstrated immunochemically. Nevertheless, permanent cartilage lesions are rare in this disease. To monitor specific events during the evolution of reactive arthritis, we investigated the content of cartilage macromolecules in sequentially obtained SF samples from 22 patients. Methods. Two groups of proteoglycan epitopes, the glycosaminoglycan-rich region of aggrecan (referred to as proteoglycan) and its hyaluronan-binding region (HABr), as well as one matrix protein, cartilage oligomeric matrix protein (COMP), were quantified by immunoassay. Results. SF proteoglycan concentrations, which were initially elevated, decreased significantly with prolonged arthritis, whereas COMP levels changed less markedly and levels of HABr remained stable. There was a positive correlation between SF and serum concentrations of COMP in samples obtained during the early phase of the disease. Conclusion. Cartilage involvement in reactive arthritis is transient, in contrast to findings in rheumatoid arthritis. Reactive arthritis should therefore be a suitable model for studies of repair processes in cartilage, which will facilitate understanding of the pathophysiology of cartilage involvement in arthritis.     

Suppression of human cartilage proteoglycan synthesis by rheumatoid synovial fluid mononuclear cells activated with mycobacterial 60-kd heat-shock protein

Objective. To examine whether T cell reactivity toward heat-shock proteins (HSP) contributes to cartilage destruction in rheumatoid arthritis (RA). Methods. An in vitro system was used, in which human cartilage explants were cocultured with hsp60-activated synovial fluid mononuclear cells (SFMC) from patients with RA, and proteoglycan (PG) synthesis was measured. Results. The hsp60-activated SFMC suppressed cartilage PG synthesis. This effect was dependent on the production of interleukin-1 (IL-1) and tumor necrosis factor α (TNFα). Conclusion. Mycobacterial 60-kd heat-shock protein can activate rheumatoid SFMC to suppress human cartilage PG synthesis. This suppression is mediated by IL-1 and TNFα.     

Serum matrix metalloproteinase activity relating to cartilage destruction in rheumatoid arthritis

Aims: The aim of this study was to determine which matrix metalloproteinases (MMPs) are involved with C‐reactive protein (CRP) and which stage of rheumatoid arthritis (RA) correlated with MMP‐9 or MMP‐13 level. Methods: In the present clinical trial, we analysed MMP‐9 activity and MMP‐13 activity in 53 RA patients. We examined the presence of MMP‐9 and MMP‐13 in the synovium tissue and articular cartilage in RA patients by immunohistochemistry. In addition, we compared these factors and clinical Steinbrocker staging. Results: We found that MMP‐9 in blood serum correlates significantly with CRP. MMP‐13 also correlates with CRP but the coefficiency with CRP is much higher in MMP‐9 (r = 0.6694) than in MMP‐13 (r = 0.4037). MMP‐9 and MMP‐13 did not correlated with rheumatoid factor (RF). MMP‐9 level is increased in stages II and III in RA. On the other hand, MMP‐13 is increased in stages III and IV. Our results indicated that early stage RA shows high MMP‐9 release in serum while late stage RA shows high MMP‐13 release. Conclusion: MMP‐9 activity may correlate with synovium proliferation with vascularization, and serum MMP‐13 activity may correlate with the grade of joint destruction in rheumatoid arthritis.     

Cartilage destruction by matrix degradation products

Abstract

The progressive destruction of articular cartilage is one of the hallmarks of osteoarthritis and rheumatoid arthritis. Cartilage degradation is attributed to different classes of catabolic factors, including proinflammatory cytokines, aggrecanases, matrix metalloproteinases, and nitric oxide. Recently, matrix degradation products generated by excessive proteolysis in arthritis have been found to mediate cartilage destruction. These proteolytic fragments activate chondrocytes and synovial fibroblasts via specific cell surface receptors that can stimulate catabolic intracellular signaling pathways, leading to the induction of such catalysts. This review describes the catabolic activities of matrix degradation products, especially fibronectin fragments, and discusses the pathologic implication in cartilage destruction in osteoarthritis and rheumatoid arthritis. Increased levels of these degradation products, found in diseased joints, may stimulate cartilage breakdown by mechanisms of the kind demonstrated in the review.     

Increased release of bone sialoprotein into synovial fluid reflects tissue destruction in rheumatoid arthritis

Objective. Bone sialoprotein (BSP) was quantified in synovial fluids and sera from rheumatoid arthritis (RA) patients to elucidate whether its release from bone relates to the degree of joint tissue destruction. Osteocalcin was assayed for comparison. Methods. BSP and osteocalcin levels were determined by immunoassays of knee synovial fluids and of sera from RA patients who were selected on the basis of radiographic knee joint tissue damage. Results. Synovial fluid concentrations of BSP increased with increasing degrees of knee joint damage (r_s = 0.6848, P < 0.001). Synovial fluid concentrations of osteocalcin did not relate to the degree of joint damage. Serum concentrations of BSP were increased, but did not relate to the degree of joint damage. Serum concentrations of osteocalcin were normal, but increased within the range of normal during progression of joint destruction (r_s = 0.4567, P < 0.001). Conclusion. Quantification of BSP in synovial fluid holds promise as a useful means of assessing the degree of tissue damage at the molecular level in patients with RA.     

Cysteine Proteinase Inhibitors Decrease Articular Cartilage and Bone Destruction in Chronic Inflammatory Arthritis

Objective. To determine the effects of peptidyl fluoromethyl ketones on the in vitro activity of purified cathepsins B and L, on tissue cysteine proteinase activity, and on cartilage and bone destruction in experimental arthritis. Methods. The effects of the fluoroketones on cathepsins B and L in vitro and the effects of oral administration of fluoroketones on ex vivo cysteine proteinase activity in tissue homogenates were determined by measuring the inhibition of fluorogenic substrate cleavage. To determine the effects on arthritis, animals were injected with adjuvant or type II collagen, treated orally with the fluoroketones, and the severity of arthritis was assessed by clinical, histologic, and radiologic methods. Results. All of the fluoroketones tested were potent inhibitors of purified cathepsins B and L activity. Oral administration of the fluoroketones reduced tissue cysteine proteinase activity by up to 77%. In addition, fluoroketone treatment significantly reduced the severity of clinical joint disease and decreased the destruction of articular cartilage and bone. Quantitative analysis of radiographic images indicated that treatment significantly reduced soft tissue changes, periosteal proliferation, and bone erosion, but only partially reduced juxtaarticular osteoporosis. Conclusion. These studies suggest that cysteine proteinase inhibitors may limit tissue destruction in diseases such as rheumatoid arthritis.     

Interference of cartilage surface with interaction of granulocyte elastase with α1-proteinase inhibitor

Summary Synovial fluids of patients suffering from rheumatoid arthritis contain elevated levels of granulocyte (PMN) elastase in complex with alpha ₁-proteinase inhibitor (alpha ₁-PI), whereas free-elastase activity is usually not detectable. This absence of free enzymatic activity in joint effusions has cast some doubt on the pathophysiological relevance of PMN elastase in inflammatory joint destruction. Our in vitro experiments using bovine nasal cartilage demonstrate that incubation with elastase and alpha ₁-PI in equimolar concentrations to or even in excess of the serum proteinase inhibitor resulted in significant tissue destruction as assessed by histological staining for proteoglycans, release of uronic acid from the matrix and loss of mechanical stability. Though in the supernatants containing alpha ₁-PI, free-elastase activity was not detectable, immunofluorescent staining for elastase evidenced penetration of the enzyme into the matrix. Simultaneous measurements of the incubation media employing a sandwich enzyme-linked immunoadsorption assay (ELISA) revealed PMN elastase in complex with alpha ₁-PI but without correlation to the parameters of tissue degradation. In comparison with the results obtained using the chromogenic substrate Suc-Ala-Ala-Ala-pNA (SAPA) for titration of alpha ₁-PI against elastase, the employment of cartilage matrix showed that a fourfold increase in inhibitor concentration was necessary to achieve 100% enzyme inhibition. Hence, cartilage surface obviously interferes with the interaction between alpha ₁-PI and elastase. Measurements of elastase-inhibitor concentrations or free enzymatic activity in synovial fluid seem to have limited value in predicting cartilage destruction.     

Wnt signaling in rheumatoid synoviocyte activation

Abstract

Rheumatoid arthritis (RA) is a joint-specific disease with complex pathogenesis. It is characterized by synovial inflammation, cartilage loss, and joint destruction. The reasons why joint damage recurs when therapy is discontinued are not clearly understood. Several lines of evidence suggest that cartilage damage is promoted by the transformed and invasive fibroblast-like synoviocytes (FLS) of the rheumatoid joint. It has been demonstrated in several systems that aberrant wnt-mediated signaling causes blockade of cartilage differentiation and malformation of joints. In this review, we have discussed the importance of wnt–frizzled-mediated signaling in the autonomous activation of FLS in patients with RA. Anti-wnt/anti-frizzled antibodies, frizzled receptor antagonists, or small molecule inhibitors of wnt–frizzled signaling might be useful for therapeutic interventions in RA.     

Rheumatoid arthritis synovial fibroblast and U937 macrophage/monocyte cell line interaction in cartilage degradation

Objective. To examine the interaction between synovial fibroblasts and macrophages in the context of cartilage degradation. Methods. An in vitro model of human cartilage degradation was used, in which purified populations of fibroblasts and macrophages were added to a radio-labeled cartilage disc. Cartilage destruction was measured by the percentage of radiolabel release. Results. Fibroblasts, obtained from either rheumatoid arthritis (RA) or osteoarthritis synovial tissue, could mediate cartilage degradation if cocultured with the U937 macrophage cell line. Skin and RA bone marrow fibroblasts had no degradative effect on cartilage. Fibroblast—macrophage contact was not required for cartilage degradation. Cartilage degradation by synovial fibroblasts was inhibited by antibodies to tumor necrosis factor α (TNFα), interleukin-1β (IL-1β), and IL-6. Cartilage degradation was almost completely abrogated by a combination of antibodies to TNFα and IL-1β. Contact between fibroblasts and cartilage was shown to be essential. Antibodies to CD44, but not to intercellular adhesion molecule 1, markedly inhibited cartilage degradation. Conclusion. TNFα, IL-1β, and IL-6 were involved in the activation of synovial fibroblasts to cause cartilage degradation. Cartilage degradation occurred only when fibroblasts were in contact with cartilage. CD44 was demonstrated to be involved in the fibroblast—cartilage interaction.     

Enhanced chondrocyte destruction by lymphokine-activated killer cells. possible role in rheumatoid arthritis

Objective. The lysis of chondrocytes, the parenchymal cells of cartilage, by lymphocytes may provide a potent mechanism by which the immune system participates in sustaining joint damage in rheumatoid arthritis (RA). We studied the capability of lymphocytes from healthy individuals and patients with arthritis to lyse chondrocytes. Methods. Peripheral blood mononuclear cells (PBMC) were tested for their ability to lyse chondrocytes in a ⁵¹Cr-release assay. Enhancement of the chondrolytic activity was determined by preincubating the cells with T cell growth factor (TCGF) or recombinant interleukin-2 (rIL-2) before cytotoxic testing. Results. PBMC from healthy individuals possessed a low ability to lyse chondrocytes, whereas cells from the synovial fluid of patients with RA displayed higher chondrolytic activity. In RA, modulating factors must come into play because not all synovial fluid sample cells showed high chondrolytic activity and cells from synovial tissue had little or no lytic action on chondrocytes. Chondrolytic activities of cells from all sources, including PBMC from healthy subjects and patients with arthritis and cells isolated from synovial fluid or from the synovial tissue of RA patients, were greatly increased by incubating the cells with TCGF or rIL-2. In contrast, treatment of chondrocytes with interferon-γ, which enhances major histocompatibility complex gene expression, decreased the susceptibility of chondrocytes to lysis. Conclusion. These observations suggest a mechanism for joint damage in which the destruction of chondrocytes by lymphocytes is controlled by cytokines released during the inflammatory process in arthritic diseases.     

Common carotid intima-media thickness and von Willebrand factor serum levels in rheumatoid arthritis female patients without cardiovascular risk factors

High atherosclerosis prevalence was found in rheumatoid arthritis (RA), and the von Willebrand factor (vWF) was shown to be a marker for endothelial damage. The aim of this study was to evaluate the association of intima-media thickness of the left common carotid artery with vWF serum levels in rheumatoid arthritis patients without cardiovascular risk factors. We included 55 RA female patients, each with at least 5 years of duration of the disease, and 20 healthy female subjects as members of the control group. The vWF, cholesterol, triglycerides, and the immune variables—rheumatoid factor and reactive C protein—were evaluated. The media thickness and intima-media thickness (IMT) in patients and in the control subjects were assessed by Doppler ultrasound of the left common carotid artery. Although the ages for RA patients and healthy female controls were not different, the IMT of the left common carotid artery (IMT CCA) in rheumatoid arthritis patients was increased in comparison with healthy control measurements, the mean being 0.67 mm (SD 0.18) vs 0.58 mm (SD 0.10) with a p value 0.01. The vWF serum levels showed differences in RA patients from those in control patients, 145.6 (SD 30.08) vs 121.8 (SD 37.17), respectively, with p=0.007. A correlation was also found between vWF with IMT CCA in the RA patients: r=0.390 and p<0.05. We concluded that the measurements of the left common carotid artery intima-media thickness together with the von Willebrand factor serum levels could give valuable information about the artery status and the atherosclerosis process in early stages in patients with rheumatoid arthritis without cardiovascular risk factors.     

Host metalloproteinases in Lyme arthritis

Objective

: To assess the role of matrix metalloproteinases (MMPs) in cartilage and bone erosions in Lyme arthritis

Methods

We examined synovial fluid from 10 patients with Lyme arthritis for the presence of MMP‐2, MMP‐3, MMP‐9, and “aggrecanase” activity using gelatinolytic zymography and immunoblot analysis. We developed an in vitro model of Lyme arthritis using cartilage explants and observed changes in cartilage degradation in the presence of Borrelia burgdorferi and/or various protease inhibitors.

Results

Synovial fluid from patients with Lyme arthritis was found to contain at least 3 MMPs: gelatinase A (MMP‐2), stromelysin (MMP‐3), and gelatinase B (MMP‐9). In addition, there was evidence in 2 patients of “aggrecanase” activity not accounted for by the above enzymes. Infection of cartilage explants with B burgdorferi resulted in induction of MMP‐3, MMP‐9, and “aggrecanase” activity. Increased induction of these enzymes by B burgdorferi alone was not sufficient to cause cartilage destruction in the explants as measured by glycosaminoglycan (GAG) and hydroxyproline release. However, addition of plasminogen, which can act as an MMP activator, to cultures resulted in significant GAG and hydroxyproline release in the presence of B burgdorferi. The MMP inhibitor batimastat significantly reduced the GAG release and completely inhibited the collagen degradation.

Conclusion

MMPs are found in synovial fluids from patients with Lyme arthritis and are induced from cartilage tissue by the presence of B burgdorferi. Inhibition of MMP activity prevents B burgdorferi–induced cartilage degradation in vitro.

     

Mammalian target of rapamycin signaling is crucial for joint destruction in experimental arthritis and is activated in osteoclasts from patients with rheumatoid arthritis

Objective

Activation of the mammalian target of rapamycin (mTOR) pathway is important for immune cell activation and bone metabolism. To date, the contribution of mTOR signaling to joint inflammation and structural bone and cartilage damage is unknown. The aim of this study was to investigate the potential of inhibiting mTOR as a treatment of inflammatory arthritis.

Methods

Human tumor necrosis factor–transgenic mice in which inflammatory arthritis was developing were treated with 2 different mTOR inhibitors, sirolimus or everolimus. The effects of treatment on clinical disease activity, inflammation, and localized joint and cartilage destruction were studied. In addition, the effects of mTOR inhibition on osteoclast survival and expression of key molecules of osteoclast function were analyzed in vitro. Moreover, synovial tissue from patients with rheumatoid arthritis (RA) was assessed for activation of the mTOR pathway.

Results

Inhibition of mTOR by sirolimus or everolimus reduced synovial osteoclast formation and protected against local bone erosions and cartilage loss. Clinical signs of arthritis improved after mTOR inhibition, and histologic evaluation showed a decrease in synovitis. In vitro, mTOR inhibition down‐regulated the expression of digestive enzymes and led to osteoclast apoptosis. Moreover, mTOR signaling was shown to be active in the synovial membrane of patients with RA, particularly in synovial osteoclasts.

Conclusion

Signaling through mTOR is an important link between synovitis and structural damage in inflammatory arthritis. Current pharmacologic inhibitors of mTOR could be effective in protecting joints against structural damage.

     

Cartilage oligomeric matrix protein in serum and synovial fluid of rheumatoid arthritis: potential use as a marker for joint cartilage damage

Abstract

This study examined the serum and synovial fluid concentrations of cartilage oligomeric matrix protein (COMP) in relation to the evolution of joint cartilage damage and the requirement for surgery in 125 patients with rheumatoid arthritis (RA). We compared the erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) level, and matrix metalloproteinase-3 (MMP-3) levels with COMP levels determined by specific enzyme-linked immunosorbent assay (ELISA). Patients were divided into three groups: (1) patients with least erosive disease (LES); (2) patients with more erosive disease (MES); and (3) patients with mutilating disease (MUD). In addition, synovial fluid samples were collected from patients undergoing arthroscopic synovectomy of the knee joint (ASS) and total knee arthroplasty (TKA). Serum COMP levels correlated with the ESR (P < 0.0001, r = 0.374, n = 125) and the CRP level (P = 0.0014, r = 0.281, n = 125). COMP levels did not correlate with the MMP-3 level (P = 0.182, r = 0.114, n = 125). The COMP levels of the LES group were significantly lower than those of the MES or MUD groups. Lastly, synovial fluid COMP levels in the TKA group were higher than in the ASS group. Therefore, these findings suggest that serum and synovial fluid COMP levels in patients with RA may reflect cartilage destruction and are correlated with the ESR and the CRP level, which are indicators of the acute-phase response.     

Involvement of nonarticular cartilage, as demonstrated by release of a cartilage-specific protein, in rheumatoid arthritis

Analysis of human cartilage extracts by radioimmunoassay showed that the noncollagenous 148-kd cartilage matrix protein was present in extracts of tracheal cartilage but was undetectable in normal or arthritic joint cartilage, corroborating previous results with bovine cartilage samples. Concentrations of the protein in the circulation, as studied by radioimmunoassay, were greatly elevated in patients with rheumatoid arthritis and polyarticular juvenile rheumatoid arthritis. In contrast, patients with reactive arthritis and oligoarticular juvenile rheumatoid arthritis, as well as rheumatoid arthritis patients treated with low-dose glucocorticoids, had levels similar to those in healthy controls. The serum concentrations were not related to age. A patient with polychondritis and tracheal involvement had a high serum concentration of the protein, which decreased during plasma exchange and cyclophosphamide treatment. Studies of the release of this cartilage matrix protein, which is present in nonarticular cartilage but not in articular cartilage, should aid in the understanding of the mechanisms of cartilage involvement in disease, and the protein may become a clinically useful marker.     

Monoclonal antibodies that detect biochemical markers of arthritis in humans

Objective. To evaluate the potential of using monoclonal antibodies (MAb) 3-B-3(–) and 7-D-4 to detect biochemical markers of altered cartilage metabolism in human arthritides. Methods. Fifty-five samples of normal articular cartilage (subjects' age range 18 weeks of gestation to 83 years of age) and 89 samples of arthritic cartilage (patients' age range 20–81 years) were collected, and their proteoglycans were extracted and analyzed for the presence of the epitopes recognized by MAb 3-B-3 and 7-D-4. Results. Native 3-B-3(–) mimotope was expressed at a high incidence in proteoglycans extracted from the cartilage of patients with most of the arthritic diseases examined (osteoarthritis, juvenile rheumatoid arthritis, rheumatoid arthritis, avascular necrosis, and degenerative meniscal tears). Its expression in normal cartilage specimens was very low or absent, occurring mainly in the young, skeletally immature individuals. In contrast, expression of the 7-D-4 epitope was more variable in patients with different arthritides and was also frequently found in normal cartilage specimens. Immunohistochemical analyses with both 3-B-3(–) and 7-D-4 showed strong focal positive staining in superficial areas, where cartilage degeneration, remodeling, and repair were greatest. Conclusion. The biochemical markers recognized by MAb 3-B-3(–) and 7-D-4 are indicative of altered proteoglycan synthesis and metabolism in human articular cartilage. The data suggest that in human cartilage, the 3-B-3(–) epitope might be a better marker of biochemical changes than the 7-D-4 epitope.     

Cartilage oligomeric matrix protein in serum and synovial fluid of rheumatoid arthritis: potential use as a marker for joint cartilage damage

This study examined the serum and synovial fluid concentrations of cartilage oligomeric matrix protein (COMP) in relation to the evolution of joint cartilage damage and the requirement for surgery in 125 patients with rheumatoid arthritis (RA). We compared the erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) level, and matrix metalloproteinase-3 (MMP-3) levels with COMP levels determined by specific enzyme-linked immunosorbent assay (ELISA). Patients were divided into three groups: (1) patients with least erosive disease (LES); (2) patients with more erosive disease (MES); and (3) patients with mutilating disease (MUD). In addition, synovial fluid samples were collected from patients undergoing arthroscopic synovectomy of the knee joint (ASS) and total knee arthroplasty (TKA). Serum COMP levels correlated with the ESR (P < 0.0001, r = 0.374, n = 125) and the CRP level (P = 0.0014, r = 0.281, n = 125). COMP levels did not correlate with the MMP-3 level (P = 0.182, r = 0.114, n = 125). The COMP levels of the LES group were significantly lower than those of the MES or MUD groups. Lastly, synovial fluid COMP levels in the TKA group were higher than in the ASS group. Therefore, these findings suggest that serum and synovial fluid COMP levels in patients with RA may reflect cartilage destruction and are correlated with the ESR and the CRP level, which are indicators of the acute-phase response.     

Co-existent sickle cell disease and juvenile rheumatoid arthritis. Two cases with delayed diagnosis and severe destructive arthropathy

We describe 2 pediatric patients with sickle cell disease (SCD) who developed seropositive juvenile rheumatoid arthritis (JRA). Both patients have severe joint damage, the compound effect of both disease processes. The bone and cartilage destruction, which poses serious therapeutic challenges, highlights the difficulty of making a diagnosis of chronic inflammatory disease in the setting of SCD. There may be a correlation between increased levels of tumor necrosis factor-alpha in the synovial tissue of joints damaged by arthritis and local sickling. The resultant ischemia and corresponding inflammatory infiltrates could in turn worsen existing synovial proliferation and cartilage destruction as well as trigger further sickling.     

Interactions of synovial fluid immunoglobulins with chondrocytes

Objective. To study the interaction of synovial fluid (SF) immunoglobulins with living chondrocytes, and to evaluate the relative contribution of type II collagen (CII) antibodies. Methods. SF of patients with rheumatoid arthritis (RA), osteoarthritis (OA), and gout were incubated with isolated bovine articular chondrocytes. Ig binding was measured by flow cytometry and by quantitation with ¹²⁵I-labeled anti-IgG and anti-IgM. Complement-dependent cytotoxicity was determined by ⁵¹Cr release. Immunoglobulin binding and cytotoxicity were compared between chondrocytes obtained from the superficial and from the deep cartilage zones. Results. Significantly greater IgG and IgM binding was found with RA SF compared with OA or gout SF. Chondrocytes bound more Ig than did fibroblasts. The relative contribution of anti-CII antibodies to Ig binding was studied following absorption of the SF with bovine CII, and by incubation with bacterial collagenase-treated chondrocytes. There was a small but significant reduction in IgG and IgM binding with SF samples that were positive for anti-CII. RA SF exhibited modest, but significantly greater complement-dependent cytotoxicity than OA SF. Gel chromatography fractionation indicated that IgM antibodies were responsible for the cytotoxic activity. Additional studies showed that SF IgM antibodies bound preferentially to, and killed chondrocytes obtained from, the superficial layers of cartilage. Conclusion. Anti-CII antibodies contained in RA SF represent one of many antibody specificities reacting with chondrocyte membrane antigens. Chondrocyte-reactive SF antibodies may play an important pathogenic role in the processes leading to irreversible cartilage damage in RA. These deleterious effects appear to be exerted particularly on chondrocytes located near the articular surface of cartilage.