Inhibition of CD95 apoptotic signaling by interferon‐γ in human osteoarthritic chondrocytes is associated with increased expression of FLICE inhibitory protein

Objective

Cartilage homeostasis dysregulation during osteoarthritis (OA) has been linked to an increased rate of apoptosis of chondrocytes, the only cell type resident in the cartilage. In addition, the CD95–CD95 ligand (the Fas system) has emerged as one of the major pathways of cell death in the cartilage. We undertook the present study to investigate the role of interferon‐γ (IFNγ) in the regulation of the Fas system by analyzing the modulation of intracellular signaling molecules (FLICE inhibitory protein [FLIP] and caspases 3 and 8) in primary cultures of human OA chondrocytes.

Methods

CD95‐induced apoptotic death of human OA chondrocytes was analyzed in the presence or absence of IFNγ using cell death immunoassay for apoptosis, real‐time polymerase chain reaction for FLIP and caspase 8 expression, Western blotting for FLIP, and proteolytic activity for caspases 3 and 8.

Results

CD95‐induced apoptotic death of human OA chondrocytes was strongly counteracted by IFNγ treatment, although the surface expression of CD95 was slightly up‐regulated by this cytokine. The messenger RNA (mRNA) expression of FLIP and caspase 8, mediators involved in CD95 signaling, revealed that FLIP expression in human OA chondrocytes was significantly up‐regulated (2‐fold increase) by IFNγ treatment. Moreover, the FLIP:caspase 8 mRNA ratio increased significantly. FLIP up‐regulation by IFNγ was confirmed at the protein level. Caspase 8 and caspase 3 proteolytic activities, both induced in these cells by stimulation with anti‐CD95, were also significantly down‐modulated by IFNγ.

Conclusion

These findings suggest that IFNγ impairs CD95‐mediated signaling and apoptotic death in human chondrocytes. Its mechanism of action involves down‐regulation of caspase 8 and caspase 3 activities and increased expression of the antiapoptotic protein FLIP, suggesting an interesting mechanism for the inhibition of chondrocyte apoptosis.

     

Inhibition of interleukin‐1β‐stimulated production of matrix metalloproteinases by hyaluronan via CD44 in human articular cartilage

Objective

To investigate the mechanism of the inhibitory action of hyaluronan (HA) on interleukin‐1β (IL‐1β)‐stimulated production of matrix metalloproteinases (MMPs) in human articular cartilage.

Methods

IL‐1β was added to normal and osteoarthritic (OA) human articular cartilage in explant culture to stimulate MMP production. Articular cartilage was incubated or preincubated with a clinically used form of 800‐kd HA to assess its effect on IL‐1β‐induced MMPs. Levels of secreted MMPs 1, 3, and 13 in conditioned media were detected by immunoblotting; intracellular MMP synthesis in chondrocytes was evaluated by immunofluorescence microscopy. Penetration of HA into cartilage tissue and its binding to CD44 were analyzed by fluorescence microscopy using fluoresceinated HA. Blocking experiments with anti‐CD44 antibody were performed to investigate the mechanism of action of HA.

Results

Treatment and pretreatment with 800‐kd HA at 1 mg/ml resulted in significant suppression of IL‐1β‐stimulated production of MMPs 1, 3, and 13 in normal and OA cartilage explant culture. Fluorescence histocytochemistry revealed that HA penetrated cartilage tissue and localized in the pericellular matrix around chondrocytes. HA‐binding blocking experiments using anti‐CD44 antibody demonstrated that the association of HA with chondrocytes was mediated by CD44. Preincubation with anti‐CD44 antibody, which suppressed IL‐1β‐stimulated MMPs, reversed the inhibitory effect of HA on MMP production that was induced by IL‐1β in normal and OA cartilage.

Conclusion

This study demonstrates that HA effectively inhibits IL‐1β‐stimulated production of MMP‐1, MMP‐3, and MMP‐13, which supports the clinical use of HA in the treatment of OA. The action of HA on IL‐1β may involve direct interaction between HA and CD44 on chondrocytes.

     

Rheumatoid arthritis synovial macrophages express the Fas‐associated death domain–like interleukin‐1β–converting enzyme–inhibitory protein and are refractory to Fas‐mediated apoptosis

Objective

The chronic inflammation and progressive joint destruction observed in rheumatoid arthritis (RA) are mediated in part by macrophages. A paucity of apoptosis has been observed in RA synovial tissues, yet the mechanism remains unknown. The present study sought to characterize the expression of Fas, Fas ligand (FasL), and Fas‐associated death domain–like interleukin‐1β–converting enzyme–inhibitory protein (FLIP), and to quantify the apoptosis induced by agonistic anti‐Fas antibody, using mononuclear cells (MNC) isolated from the peripheral blood (PB) and synovial fluid (SF) of RA patients.

Methods

The expression of Fas, FasL, and FLIP and apoptosis induced by agonistic anti‐Fas antibody in MNC from the PB and SF of RA patients were determined by flow cytometry. Immunohistochemistry employing a monospecific anti‐FLIP antibody was performed on RA and osteoarthritis (OA) synovial tissue.

Results

CD14‐positive monocyte/macrophages from normal and RA PB and from RA SF expressed equivalent levels of Fas and FasL. Furthermore, unlike the CD14‐positive PB monocytes, RA SF monocyte/macrophages were resistant to the addition of agonistic anti‐Fas antibody. In contrast, both CD14‐positive PB and SF monocyte/macrophages were sensitive to apoptosis mediated by a phosphatidylinositol 3‐kinase inhibitor. Intracellular staining of the caspase 8 inhibitor, FLIP, in CD14‐positive SF monocyte/macrophages revealed a significant up‐regulation of FLIP compared with normal and RA PB monocytes. Immunohistochemical analysis of synovial tissue from RA and OA patients revealed increased FLIP expression in the RA synovial lining compared with the OA synovial lining. Furthermore, FLIP expression was observed in the CD68‐positive population in the RA synovial lining. Forced reduction of FLIP by a chemical inhibitor resulted in RA SF macrophage apoptosis that was enhanced by agonistic anti‐Fas antibody, indicating that FLIP is necessary for SF macrophage survival.

Conclusion

These data suggest that up‐regulation of FLIP in RA macrophages may account for their persistence in the disease. Thus, the targeted suppression of FLIP may be a potential therapeutic strategy for the amelioration of RA.

     

Parathyroid hormone 1–34 inhibits terminal differentiation of human articular chondrocytes and osteoarthritis progression in rats

Objective

Parathyroid hormone 1–34 (PTH[1–34]), a parathyroid hormone analog, shares the same receptor, PTH receptor 1, with parathyroid hormone–related peptide (PTHrP). This study was undertaken to address the hypothesis that PTH(1–34) inhibits terminal differentiation of articular chondrocytes and in turn suppresses the progression of osteoarthritis (OA).

Methods

We studied the effect of PTH(1–34) on human articular chondrocytes with azacytidine (azaC)–induced terminal differentiation in vitro and on papain‐induced OA in the knee joints of rats. In the in vitro study, we measured the levels of messenger RNA for SOX9, aggrecan, type II collagen, type X collagen, alkaline phosphatase (AP), Indian hedgehog (IHH), Bcl‐2, and Bax by real‐time polymerase chain reaction, levels of glycosaminoglycan (GAG) by dimethylmethylene blue assay, and rate of apoptosis by TUNEL staining. In the in vivo study, we evaluated the histologic changes in GAG, type II collagen, type X collagen, and chondrocyte apoptosis in the articular cartilage of rat knees.

Results

AzaC induced terminal differentiation of human chondrocytes, including down‐regulation of aggrecan, type II collagen, and GAG and up‐regulation of type X collagen, alkaline phosphatase, and IHH. Apoptosis was reversed by 3–10 days of treatment with 10 nM PTH(1–34). SOX9 expression was not changed by either azaC or PTH(1–34) treatment. Bcl‐2 and Bax were up‐regulated on day 10 and day 14, respectively, after azaC induction of terminal differentiation, but PTH(1–34) treatment did not reverse this effect. Furthermore, PTH(1–34) treatment reversed papain‐induced OA changes (decreasing GAG and type II collagen, and increasing type X collagen and chondrocyte apoptosis) in the knee joints of rats.

Conclusion

Our findings indicate that PTH(1–34) inhibits the terminal differentiation of human articular chondrocytes in vitro and inhibits progression of OA in rats in vivo, and may be used to treat OA.

     

Osteoarthritic chondrocyte–secreted morphogens induce chondrogenic differentiation of human mesenchymal stem cells

Objective

The potential of stem cells to repair compromised cartilage tissue, such as in osteoarthritis (OA), depends strongly on how transplanted cells respond to factors secreted from the residing OA chondrocytes. This study was undertaken to determine the effect of morphogenetic signals from OA chondrocytes on chondrogenic differentiation of human mesenchymal stem cells (MSCs).

Methods

The effect of OA chondrocyte–secreted morphogens on chondrogenic differentiation of human MSCs was evaluated using a coculture system involving both primary and passaged OA chondrocytes. The findings were compared against findings for human MSCs cultured in OA chondrocyte–conditioned medium. Gene expression analysis, biochemical assays, and immunofluorescence staining were used to characterize the chondrogenic differentiation of human MSCs. Mass spectrometry analysis was used to identify the soluble factors. Numerical analysis was carried out to model the concentration profile of soluble factors within the human MSC–laden hydrogels.

Results

The human MSCs cocultured with primary OA chondrocytes underwent chondrogenic differentiation even in the absence of growth factors; however, the same effect could not be mimicked using OA chondrocyte–conditioned medium or expanded cells. Additionally, the cocultured environment down‐regulated hypertrophic differentiation of human MSCs. Mass spectrometry analysis demonstrated cell–cell communication and chondrocyte phenotype–dependent effects on cell‐secreted morphogens.

Conclusion

The experimental findings, along with the results of the numerical analysis, suggest a crucial role of soluble morphogens and their local concentrations in the differentiation pattern of human MSCs in a 3‐dimensional environment. The concept of using a small number of chondrocytes to promote chondrogenic differentiation of human MSCs while preventing their hypertrophic differentiation could be of great importance in formulating effective stem cell–based cartilage repair.

     

SirT1 enhances survival of human osteoarthritic chondrocytes by repressing protein tyrosine phosphatase 1B and activating the insulin‐like growth factor receptor pathway

Objective

The protein deacetylase SirT1 inhibits apoptosis in a variety of cell systems by distinct mechanisms, yet its role in chondrocyte death has not been explored. We undertook the present study to assess the role of SirT1 in the survival of osteoarthritic (OA) chondrocytes in humans.

Methods

SirT1, protein tyrosine phosphatase 1B (PTP1B), and PTP1B mutant expression plasmids as well as SirT1 small interfering RNA (siRNA) and PTP1B siRNA were transfected into primary human chondrocytes. Levels of apoptosis were determined using flow cytometry, and activation of components of the insulin‐like growth factor receptor (IGFR)/Akt pathway was assessed using immunoblotting. OA and normal knee cartilage samples were subjected to immunohistochemical analysis.

Results

Expression of SirT1 in chondrocytes led to increased chondrocyte survival in either the presence or the absence of tumor necrosis factor α/actinomycin D, while a reduction of SirT1 by siRNA led to increased chondrocyte apoptosis. Expression of SirT1 in chondrocytes led to activation of IGFR and the downstream kinases phosphatidylinositol 3‐kinase, phosphoinosite‐dependent protein kinase 1, mTOR, and Akt, which in turn phosphorylated MDM2, inhibited p53, and blocked apoptosis. Activation of IGFR occurs at least in part via SirT1‐mediated repression of PTP1B. Expression of PTP1B in chondrocytes increased apoptosis and reduced IGFR phosphorylation, while down‐regulation of PTP1B by siRNA significantly decreased apoptosis. Examination of cartilage from normal donors and OA patients revealed that PTP1B levels are elevated in OA cartilage in which SirT1 levels are decreased.

Conclusion

For the first time, it has been demonstrated that SirT1 is a mediator of human chondrocyte survival via down‐regulation of PTP1B, a potent proapoptotic protein that is elevated in OA cartilage.

     

Growth‐related oncogene α induction of apoptosis in osteoarthritis chondrocytes

Objective

To evaluate the apoptotic effect of the chemokine growth‐related oncogene α (GROα), which we recently reported to be up‐regulated in osteoarthritis (OA) chondrocytes. Chondrocyte apoptosis is considered to be a major determinant of cartilage damage in OA, a disease resulting from the aberrant production of inflammatory mediators (cytokines and chemokines) and effectors (matrix metalloproteinases and reactive oxygen and nitrogen species) by chondrocytes.

Methods

We investigated the apoptotic effect of GROα on isolated human cells and on in vitro–cultured cartilage explants by conventional methods (morphology, detection of DNA fragmentation in situ and in solution, exposure of phosphatidylserine) and by analysis of “early” biochemical events (plasma membrane depolarization, activation of caspase 3, and phosphorylation of c‐Jun N‐terminal kinase/stress‐activated protein kinase).

Results

We clearly demonstrated that GROα was able to initiate a series of morphologic, biochemical, and molecular changes that led to chondrocyte apoptosis. Moreover, we found that additional signals delivered from the extracellular matrix (ECM) were essential in the control of chondrocyte susceptibility to GROα‐induced apoptosis, since cell death was detected only when cells were stimulated after reestablishment of their proper interactions with the ECM, or in cartilage explant samples with reduced ECM, as indicated by decreased Safranin O staining.

Conclusion

GROα can induce apoptosis in articular chondrocytes, and the induction is dependent upon additional signals from the ECM. These findings are relevant to understanding the pathogenesis of OA, in view of the availability of the GROα chemokine in the joint space in the course of this rheumatic disease.

     

Role of interleukin‐8 in PiT‐1 expression and CXCR1‐mediated inorganic phosphate uptake in chondrocytes

Objective

The proinflammatory chemokine interleukin‐8 (IL‐8) induces chondrocyte hypertrophy. Moreover, chondrocyte hypertrophy develops in situ in osteoarthritic (OA) articular cartilage and promotes dysregulated matrix repair and calcification. Growth plate chondrocyte hypertrophy is associated with expression of the type III sodium‐dependent inorganic phosphate (Pi) cotransporter phosphate transporter/retrovirus receptor 1 (PiT‐1). This study was undertaken to test the hypothesis that IL‐8 promotes chondrocyte hypertrophy by modulating chondrocyte PiT‐1 expression and sodium‐dependent Pi uptake, and to assess differential roles in this activity.

Methods

The selective IL‐8 receptor CXCR1 and the promiscuous chemokine receptor CXCR2 were used. Human knee OA cartilage, cultured normal bovine knee chondrocytes, and immortalized human articular chondrocytic CH‐8 cells were transfected with CXCR1/CXCR2 chimeric receptors in which the 40–amino acid C‐terminal cytosolic tail domains were swapped and site mutants of a CXCR1‐specific region were generated.

Results

Up‐regulated PiT‐1 expression was detected in OA cartilage. IL‐8, but not IL‐1 or the CXCR2 ligand growth‐related oncogene α, induced PiT‐1 expression and increased sodium‐dependent Pi uptake by >40% in chondrocytes. The sodium/phosphate cotransport inhibitor phosphonoformic acid blocked IL‐8–induced chondrocyte hypertrophic differentiation. Signaling mediated by kinase Pyk‐2 was essential for IL‐8 induction of PitT‐1 expression and Pi uptake. Signaling through the TSYT^346–349 region of the CXCR1 cytosolic tail, a region divergent from the CXCR2 cytosolic tail, was essential for IL‐8 to induce Pi uptake.

Conclusion

Our results link low‐grade IL‐8–mediated cartilaginous inflammation in OA to altered chondrocyte differentiation and disease progression through PiT‐1 expression and sodium‐dependent Pi uptake mediated by CXCR1 signaling.

     

Water‐soluble C60 fullerene prevents degeneration of articular cartilage in osteoarthritis via down‐regulation of chondrocyte catabolic activity and inhibition of cartilage degeneration during disease development

Objective

Studies have shown the roles of oxidative stress in the pathogenesis of osteoarthritis (OA) and induction of chondrocyte senescence during OA progression. The aim of this study was to examine the potential of a strong free‐radical scavenger, water‐soluble fullerene (C60), as a protective agent against catabolic stress–induced degeneration of articular cartilage in OA, both in vitro and in vivo.

Methods

In the presence or absence of C60 (100 μM), human chondrocytes were incubated with interleukin‐1β (10 ng/ml) or H₂O₂ (100 μM), and chondrocyte activity was analyzed. An animal model of OA was produced in rabbits by resection of the medial meniscus and medial collateral ligament. Rabbits were divided into 5 subgroups: sham operation or treatment with C60 at 0.1 μM, 1 μM, 10 μM, or 40 μM. The left knee joint was injected intraarticularly with water‐soluble C60 (2 ml), while, as a control, the right knee joint received 50% polyethylene glycol (2 ml), once weekly for 4 weeks or 8 weeks. Knee bone and cartilage tissue were prepared for histologic analysis. In addition, in the OA rabbit model, the effect of C60 (10 μM) on degeneration of articular cartilage was compared with that of sodium hyaluronate (HA) (5 mg/ml).

Results

C60 (100 μM) inhibited the catabolic stress–induced production of matrix‐degrading enzymes (matrix metalloproteinases 1, 3, and 13), down‐regulation of matrix production, and apoptosis and premature senescence in human chondrocytes in vitro. In rabbits with OA, treatment with water‐soluble C60 significantly reduced articular cartilage degeneration, whereas control knee joints showed progression of cartilage degeneration with time. This inhibitory effect was dose dependent, and was superior to that of HA. Combined treatment with C60 and HA yielded a significant reduction in cartilage degeneration compared with either treatment alone.

Conclusion

The results indicate that C60 fullerene is a potential therapeutic agent for the protection of articular cartilage against progression of OA.

     

Down‐regulation of FLIP sensitizes rheumatoid synovial fibroblasts to Fas‐mediated apoptosis

Objective

Hyperplasia of fibroblast‐like synoviocytes (FLS) contributes to chronic inflammation and joint destruction in rheumatoid arthritis (RA). FLICE‐inhibitory protein (FLIP) is an antiapoptotic protein that might prevent apoptotic elimination of FLS in response to death ligands such as tumor necrosis factor α (TNFα) or Fas ligand, which are present in RA synovium. Previous studies on FLIP expression by osteoarthritis (OA) and RA FLS have shown variable results, and the specific role of FLIP as an apoptosis inhibitor in these cells remains unclear. We undertook this study to investigate the expression and antiapoptotic function of FLIP in FLS.

Methods

We studied the expression of FLIP by immunohistochemistry and immunoblotting in synovial tissues or cultured FLS from RA and OA patients. FLS apoptosis was induced by an agonistic anti‐Fas monoclonal antibody and FLS were then quantified. We studied the effects of cycloheximide (CHX), TNFα, and FLIP antisense oligonucleotide on FLIP expression and FLS apoptotic susceptibility.

Results

FLIP_L was the isoform mainly expressed in lining synoviocytes and cultured FLS. Synovial tissues and cultured FLS from OA and RA tissues displayed similar patterns and levels of expression of FLIP. Fas‐induced apoptosis was variable in different FLS lines, but differences between OA and RA groups were not detected. TNFα induced increases in FLIP_L and FLIP_S expression and protected RA FLS from apoptosis, while CHX induced the opposite effects. Down‐regulation of FLIP by antisense oligonucleotide strongly sensitized RA FLS to Fas‐mediated apoptosis.

Conclusion

Apoptosis susceptibility and FLIP expression are similar in OA and RA FLS. Down‐regulation of FLIP sensitizes RA FLS to Fas‐mediated apoptosis and may be a valuable tool for targeting RA FLS hyperplasia.

     

One of two chondrocyte‐expressed isoforms of cartilage intermediate‐layer protein functions as an insulin‐like growth factor 1 antagonist

Objective

Aging and osteoarthritic (OA) cartilage commonly demonstrate enhanced expression of the large, transforming growth factor β (TGFβ)–inducible glycoprotein cartilage intermediate‐layer protein (CILP) as well as enhanced extracellular inorganic pyrophosphate (PPi) that promotes the deposition of calcium pyrophosphate dihydrate crystals. In normal chondrocytes, TGFβ induces elevated chondrocyte extracellular PPi. Insulin‐like growth factor 1 (IGF‐1) normally blocks this response and reduces extracellular PPi. However, chondrocyte resistance to IGF‐1 is observed in OA and aging. Because CILP was reported to chromatographically fractionate with PPi‐generating nucleotide pyrophosphatase phosphodiesterase (NPP) activity, it has been broadly assumed that CILP itself has NPP activity. Our objective was to directly define CILP functions and their relationship to IGF‐1 in chondrocytes.

Methods

Using primary cultures of articular chondrocytes from the knee, we defined the function of the previously described CILP (CILP‐1) and of a recently described 50.6% identical protein that we designated the CILP‐2 isoform.

Results

Both CILP isoforms were constitutively expressed by primary cultured articular chondrocytes, but only CILP‐1 expression was detectable in cultured knee meniscal cartilage cells. Neither CILP isoform had intrinsic NPP activity. But CILP‐1 blocked the ability of IGF‐1 to decrease extracellular PPi, an activity specific for the CILP‐1 N‐terminal domain. The CILP‐1 N‐terminal domain also suppressed IGF‐1–induced (but not TGFβ‐induced) proliferation and sulfated proteoglycan synthesis, and it inhibited ligand‐induced IGF‐1 receptor autophosphorylation.

Conclusion

Two CILP isoforms are differentially expressed by chondrocytes. Neither CILP isoform exhibits PPi‐generating NPP activity. But, increased expression of CILP‐1, via N‐terminal domain–mediated inhibitory effects of CILP‐1 on chondrocyte IGF‐1 responsiveness, could impair chondrocyte growth and matrix repair and indirectly promote PPi supersaturation in aging and OA cartilage.

     

Accelerated,aging‐dependent development of osteoarthritis in α1 integrin–deficient mice

Objective

Cell–matrix interactions regulate chondrocyte differentiation and survival. The α1β1 integrin is a major collagen receptor that is expressed on chondrocytes. Mice with targeted inactivation of the integrin α1 gene (α1‐KO mice) provide a model that can be used to address the role of cell–matrix interactions in cartilage homeostasis and osteoarthritis (OA) pathogenesis.

Methods

Knee joints from α1‐KO and wild‐type (WT) BALB/c mice were harvested at ages 4–15 months. Knee joint sections were examined for inflammation, cartilage degradation, and loss of glycosaminoglycans (by Safranin O staining). Immunohistochemistry was performed to detect the distribution of α1 integrin, matrix metalloproteinases (MMPs), and chondrocyte apoptosis.

Results

In WT mice, the α1 integrin subunit was detected in hypertrophic chondrocytes in the growth plate and in a subpopulation of cells in the deep zone of articular cartilage. There was a marked increase in α1‐positive chondrocytes in the superficial and upper mid‐zones in OA‐affected areas in joints from old WT mice. The α1‐KO mice showed more severe cartilage degradation, glycosaminoglycan depletion, and synovial hyperplasia as compared with the WT mice. MMP‐2 and MMP‐3 expression was increased in the OA‐affected areas. In cartilage from α1‐KO mice, the cellularity was reduced and the frequency of apoptotic cells was increased. These results suggest that the α1 integrin subunit is involved in the early remodeling process in OA cartilage.

Conclusion

Deficiency in the α1 integrin subunit is associated with an earlier deregulation of cartilage homeostasis and an accelerated, aging‐dependent development of OA.

     

HLA–B27–restricted antigen presentation by human chondrocytes to CD8+ T cells: Potential contribution to local immunopathologic processes in ankylosing spondylitis

Objective

Analysis of the histopathologic features of hip arthritis in patients with ankylosing spondylitis (AS) has revealed accumulation of infiltrating mononuclear cells in the bone end plate and presence of hyaline articular cartilage that is not found in areas of total cartilage destruction. This study was undertaken to assess whether chondrocytes attract lymphocytes and whether cartilage chondrocytes from patients with AS have the potential to directly stimulate T cells in an HLA‐restricted manner.

Methods

Human HLA–B27+ T cell lines, specific for the Epstein‐Barr virus–derived peptide EBNA_258–266, and autologous chondrocytes, serving as nonprofessional antigen‐presenting cells (APCs), were available for use in a model system to study chondrocyte functions in femoral head joint cartilage of patients with AS. Peptide functionality of cytotoxic T cells was assessed by flow cytometry, and cellular interactions were detected by fluorescence confocal microscopy.

Results

When maintained in an alginate matrix, chondrocytes isolated from the femoral heads of patients with AS constitutively expressed type II collagen and CD80. When pulsed with the EBNA_258–266 peptide, autologous chondrocytes functioned as APCs and, specifically, induced interferon‐γ production in CD8+ T cells. In mixed chondrocyte–T cell cultures, cell–cell contacts were dependent on the presence of the EBNA_258–266 peptide. T cells adjacent to chondrocytes produced perforin and granzyme B; both molecules were found in focal aggregates, a prerequisite for antigen‐specific lysis of target cells.

Conclusion

Antigen presentation through human chondrocytes allows the stimulation of peptide‐specific CD8+ T cells. These results indicate that human chondrocytes can act as nonprofessional APCs, and suggest that there is an interferon‐γ–triggered autocrine loop of immune cell–mediated chondrocyte activation in the already inflamed environment. Thus, local HLA‐dependent activation of peptide‐specific cytotoxic CD8+ T cells by chondrocytes might contribute to inflammatory processes in the spondylarthritides.