Role of crosstalk between synovial cells and chondrocytes in osteoarthritis (Review)

Osteoarthritis (OA) is a low-grade, nonspecific inflammatory disease that affects the entire joint. This condition is characterized by synovitis, cartilage erosion, subchondral bone defects, and subpatellar fat pad damage. There is mounting evidence demonstrating the significance of crosstalk between synovitis and cartilage destruction in the development of OA. To comprehensively explore the phenotypic alterations of synovitis and cartilage destruction, it is important to elucidate the crosstalk mechanisms between chondrocytes and synovial cells. Furthermore, the updated iteration of single-cell sequencing technology reveals the interaction between chondrocyte and synovial cells. In the present review, the histological and pathological alterations between cartilage and synovium during OA progression are described, and the mode of interaction and molecular mechanisms between synovial cells and chondrocytes in OA, both of which affect the OA process mainly by altering the inflammatory environment and cellular state, are elucidated. Finally, the current OA therapeutic approaches are summarized and emerging therapeutic targets are reviewed in an attempt to provide potential insights into OA treatment.

The Impact of 45S5-Bioactive Glass on Synovial Cells in Knee Osteoarthritis-An In Vitro Study

Synovial inflammation in osteoarthritis (OA) is characterized by the release of cartilage-degrading enzymes and inflammatory cytokines. 45S5-bioactive glass (45S5-BG) can modulate inflammation processes; however, its influence on OA-associated inflammation has hardly been investigated. In this study, the effects of 45S5-BG on the release of cartilage-degrading metalloproteinases and cytokines from synovial membrane cells (SM) isolated from patients with knee OA was assessed in vitro. SM were cultivated as SM monocultures in the presence or absence of 45S5-BG. On day 1 (d1) and d7 (d7), the concentrations of Matrix Metalloproteinases (MMPs) and cytokines were assessed. In 45S5-BG-treated SM cultures, MMP9 concentration was significantly reduced at d1 and d7, whilst MMP13 was significantly increased at d7. Concentrations of interleukin (IL)-1B and C-C motif chemokine ligand 2 (CCL2) in 45S5-BG-treated SM cultures were significantly increased at both time points, as were interferon gamma (IFNG) and IL-6 at d7. Our data show an effect of 45S5-BG on SM activity, which was not clearly protective, anti-inflammatory, or pro-inflammatory. The influence of 45S5-BG on MMP release was more suggestive of a cartilage protective effect, but 45S5-BG also increased the release of pro-inflammatory cytokines. Further studies are needed to analyze the effect of BGs on OA inflammation, including the anti-inflammatory modification of BG compositions.

The potential role of synovial cells in the progression and treatment of osteoarthritis

Osteoarthritis (OA), the commonest arthritis, is characterized by the progressive destruction of cartilage, leading to disability. The Current early clinical treatment strategy for OA often centers on anti-inflammatory or analgesia medication, weight loss, improved muscular function and articular cartilage repair. Although these treatments can relieve symptoms, OA tends to be progressive, and most patients require arthroplasty at the terminal stages of OA. Recent studies have shown a close correlation between joint pain, inflammation, cartilage destruction and synovial cells. Consequently, understanding the potential mechanisms associated with the action of synovial cells in OA could be beneficial for the clinical management of OA. Therefore, this review comprehensively describes the biological functions of synovial cells, the synovium, together with the pathological changes of synovial cells in OA, and the interaction between the cartilage and synovium, which is lacking in the present literature. Additionally, therapeutic approaches based on synovial cells for OA treatment are further discussed from a clinical perspective, highlighting a new direction in the treatment of OA.

[Effects of moxibustion on the ultrastructure of synovial cells of knee joint and serum cytokines in adjuvant arthritis rats]

OBJECTIVE

To observe the effects of moxibustion on the ultrastructure of synovial cells of knee joint and serum cytokines in adjuvant arthritis (AA) rats, and to explore the potential mechanism of moxibustion in treatment of rheumatoid arthritis.

METHODS

Forty-five Wistar male rats were randomly divided into a normal group, a model group and a moxibustion group, with 15 rats in each group. In the model group and the moxibustion group, the AA model was replicated under wind, cold and humid environment and by injection with complete freund's adjuvant. In the moxibustion group, moxibustion at "Zusanli" (ST 36) and "Shenshu" (BL 23) was used, 20 min each time, once daily, for consecutive 21 days. In the normal group and the model group, no intervention was processed. The scores of the knee joint swelling degree (JSD) and arthritis index (AI) were compared among groups. The ultrastructure of synovial cells of knee joint were observed under transmission electron microscope (TEM). The levels of serum cytokines such as tumor necrosis factor-α (TNF-α), interieukin (IL)-1β, IL-6 and IL-10 were detected using ELISA method.

RESULTS

Compared with the normal group, JSD and AI scores, the levels of TNF-α, IL-1β and IL-6 were increased (P<0.01), while IL-10 was reduced (P<0.01) in the model group after intervention. JSD and AI scores, and the levels of TNF-α, IL-1β and IL-6 were lower (P<0.05, P<0.01), while the level of IL-10 was higher (P<0.01) in the moxibustion group compared with the model group. Compared with the normal group, the ultrastructure of synovial cell was obviously damaged in the model group, and the damage was attenuated in the moxibustion group compared with the model group.

CONCLUSION

Moxibustion can reduce the symptoms of arthritis in AA rats, which may be related to the improvement of the ultrastructure of synovial cells and the regulation of cytokines.

Potential IFNγ Modulation of Inflammasome Pathway in Chlamydia trachomatis Infected Synovial Cells

Following a Chlamydia&nbsp;trachomatis infection, the host immune response is characterized by its recognition via Toll-like and Nod-like Receptors, and the subsequent activation of interferon (IFN)-γ-mediated signaling pathways. Recently, the inflammasome-mediated host cell response has emerged to play a role in the physiopathology of C. trachomatis infection. Here we investigated, for the first time, the interaction of IFN-γ and inflammasome in an in vitro model of C. trachomatis-infected primary human synovial cells. Chlamydial replication as well as the expression of caspase-1, IL-1β, as well as IL-18 and IL-6, were assayed. Our results demonstrated the inhibitory activity of IFN-γ by interfering with the inflammasome network through the downregulation of caspase-1 mRNA expression. In addition, the ability of C. trachomatis to hinder the inflammasome pathway favoring its intracellular survival within synovial cells, was observed. Overall, our data suggest a potential mechanism of immune evasion by C. trachomatis in synovial cells, that may be contested by IFN-γ.

Effects of isoflavone derivatives on the production of inflammatory cytokines by synovial cells

The present study investigated the effects of isoflavone derivatives (daidzein, genistein and glycitein) on the production of inflammatory cytokines (IL-6 and IL-8) by IL-1β-stimulated synovial cells. Synovial MH7A cells were stimulated with IL-1β in the absence or presence of isoflavone derivatives, and IL-6 and IL-8 production was measured by ELISA. The results of the present study indicated that daidzein significantly inhibited the production of IL-6, but not IL-8. Conversely, neither genistein nor glycitein exerted any inhibitory effects on the production of IL-6 or IL-8 by IL-1β-stimulated synovial cells. To elucidate the molecular mechanisms underlying the daidzein-mediated inhibition of IL-6 production, the present study examined the effects of daidzein on the phosphorylation (activation) of NF-κB p65, ERK1/2 and p38 MAPK. Daidzein significantly inhibited the phosphorylation of NF-κB p65 and ERK1/2, but not p38 MAPK in IL-1β-stimulated MH7A cells. The present study revealed that among the isoflavone derivatives examined (daidzein, genistein and glycitein), daidzein inhibited the production of IL-6, but not IL-8, by IL-1β-stimulated synovial MH7A cells via the suppression of NF-κB p65 and ERK1/2 activation. Collectively, these results suggested that daidzein may have potential as a therapeutic agent for the treatment of arthritic disorders through its anti-inflammatory effects via the inhibition of IL-6 production.

Impact of Isolation Procedures on the Development of a Preclinical Synovial Fibroblasts/Macrophages in an In Vitro Model of Osteoarthritis

Simple Summary

In vitro models able to represent osteoarthritis (OA) synovial tissue (ST) inflammation are lacking. Therefore, we aimed to characterize OA ST and to compare mechanical and enzymatic digestion procedures to find the one that better preserve the heterogeneity of the main OA synovial cell populations: fibroblasts and macrophages. We demonstrated that typical macrophage phenotypical markers, like CD68, CD80 and CD163, were higher expressed on cells isolated with mechanical than enzymatic procedure. Moreover, we found that typical cytokines of inflammatory macrophages (i.e., TNFα) and anti-inflammatory macrophages (i.e., IL10) were also higher on mechanically isolated cells. Synovial fibroblasts were well conserved in both procedures. The definition of an OA ST model in vitro that better preserve the heterogeneity of specific cell populations represents a prerequisite for defining the real effects of new cell therapies or drugs for OA treatment, and could contribute to the reduction or avoidance of animal models.

Abstract

There is a lack ofin vitromodels able to properly represent osteoarthritis (OA) synovial tissue (ST). We aimed to characterize OA ST and to investigate whether a mechanical or enzymatic digestion procedures influence synovial cell functional heterogeneity in vitro. Procedures using mechanical nondigested fragments (NDF), synovial digested fragments (SDF), and filtrated synovial digested cells (SDC) were compared. An immunophenotypic profile was performed to distinguish synovial fibroblasts (CD55, CD73, CD90, CD106), macrophages (CD14, CD68), M1-like (CD80, CD86), and M2-like (CD163, CD206) synovial macrophages. Pro-inflammatory (interleukin 6 IL6), tumor necrosis factor alpha (TNFα), chemokine C-C motif ligand 3 (CCL3/MIP1α), C-X- motif chemokine ligand 10 (CXCL10/IP10) and anti-inflammatory (interleukin 10 (IL10)), transforming growth factor beta 1 (TGFβ1), C-C motif chemokine ligand 18 (CCL18) cytokines were evaluated. CD68 and CD163 markers were higher in NDF and SDF compared to the SDC procedure, while CD80, CD86, and CD206 were higher only in NDF compared to the SDC procedure. Synovial fibroblast markers showed similar percentages. TNFα, CCL3/MIP1α, CXCL10/IP10, and CCL18 were higher in NDF compared to SDC, but not compared to SDF. IL10 and TGFβ1 were higher in NDF than SDC at the molecular level, while IL6 did not show differences among procedures. We demonstrated that NDF isolation procedures better preserved the heterogeneity of specific OA synovial populations (fibroblasts, macrophages), fostering their use for testing new cell therapies or drugs for OA, reducing or avoiding the use of animal models.

Effect of cadmium on the viability on monolayer cultures of synoviocytes, chondrocytes, and Hoffa: A preliminary study

Osteoarthritis (OA) is the gradual loss of articular cartilage and involves several tissues, such as the synovial membrane, meniscus, ligaments, and adipose tissue known as Hoffa fat pad. There are largely unexplored factors that lead to OA development, such as the impact of exposure to heavy metals like cadmium (Cd) on the viability of cells in the knee joint tissue. The objective of this report was to identify the cell type with the highest susceptibility to Cd toxicity with respect to cell viability and death. Our findings showed that a concentration as low as 3 μM cadmium chloride for 12 h affects the viability of synovial cells, and a concentration of 10 μM affects Hoffa cells. Our results suggest that Cd can affect the viability of synovial and chondral cells primarily. In contrast, Hoffa cells were less susceptible, likely because Cd favors the production of pro-inflammatory cytokines before triggering their death as part of its damage mechanism at the articular level.

Expression of matrix metalloproteinase-9 and tumour necrosis factor-alpha in the synovial cells of patients with meniscus tears

BACKGROUND

A meniscus tear is a serious trauma that develops during swinging motion of the fixed foot. Meniscus tears may also be accompanied by divergence of the lateral ligaments of the knee joint.

MATERIALS AND METHODS

We enrolled 45 males and 35 females with meniscal tears in the present study. Patients with local joint pain, swelling, difficulty climbing stairs, patellar creeping, difficulties with daily living activities, local pain on palpation, and walking and running complaints, were included. We performed preoperative magnetic resonance imaging. Synovial fluid (5 mL) was aspirated from the lateral suprapatellar pouch of each knee with meniscal pain with the patient in the supine position. Blood samples were taken and biochemical parameters were analysed. The Harris haematoxylin and eosin staining protocol was used to evaluate tissue samples, and the levels of anti-matrix metalloproteinase (MMP)-9 and anti-tumour necrosis factor alpha (TNF-α) antibodies were measured immunohistochemically.

RESULTS

Increased numbers of lymphocytes and neutrophils, hyperplastic erythrocytes, and fibroblasts were observed in the joint fluid of females. In males, the fibroblast cells were hyperplastic and plasma cell numbers were increased. MMP-9 expression was elevated in plasma cells, fibroblasts, and neutrophils; and TNF-α expression was observed in lymphocytes and polymorphic nucleated cells. We suggest that increased fluid levels in inflamed joints with meniscal tears, and the associated inflammation, disrupt the cartilage matrix and elevate the production of cytokines such as TNF-α and MMP-9 via release from cells such as fibroblasts that synthesise these mediators.

CONCLUSIONS

Anti-TNF-α treatment may prevent meniscal tears and prevent or slow the development of osteoarthritis.

Effects of human cyclooxygenase-2 gene silencing on synovial cells of rheumatoid arthritis mediated by lentivirus

The aim of the study is to screen the effective shRNA sequence which can silence the human COX-2 expression level in synovial cells of rheumatoid arthritis (RA) patient transfected by the lentivirus. Four pairs of hCOX-2 shRNA were designed and inserted into lentivirus to form pGPHI/GFP/Neo-shRNA vector. The reconstructed virus was transfected into synovial cells derived from RA patients, and then the expression level of hCOX-2 mRNA and the protein of the inflammatory factors including prostaglandin E2 (PGE2), vascular endothelial growth factor (VEGF), interleukin-1β (IL-1β) and tumour necrosis factor alpha (TNF-α) in the supernatants were examined with real-time PCR and ELISA, respectively. There was no obvious negative influence on cell growth and morphology after hCOX-2 shRNA gene transfection mediated by lentivirus. The hCOX-2 mRNA expression level, as well as the concentration of PGE2, VEGF, IL-1β and TNF-α, decreased significantly (p < .05). RNAi mediated by lentivirus can significantly inhibit hCOX-2 mRNA expression level in synovial cells of RA patients, so as to reduce the expression of inflammatory cytokines.

Fengshi Gutong Capsule Attenuates Osteoarthritis by Inhibiting MAPK, NF-κB, AP-1, and Akt Pathways

Background and purpose: Fengshi Gutong capsule (FSGTC), a traditional herbal formula, has been used clinically in China for the treatment of arthritis. However, the mechanism underlying the therapeutic effects of FSGTC on osteoarthritis (OA) has not been elucidated. The present study investigated the function and mechanisms of FSGTC in rat OA model and interleukin (IL)-1β-stimulated synovial cells. Materials and methods: Rat OA model was established by intra-articular injection containing 4% papain. IL-1β-induced SW982 cells were used as an OA cell model. Safranin-O-Fast green (S-O) and hematoxylin-eosin (HE) stainings were used to observe the changes in cartilage morphology. Enzyme-linked immunosorbent assay (ELISA) and real-time quantitative PCR (qPCR) detected the expression of inflammatory cytokines. In addition, molecular mechanisms were analyzed by Western blot in the OA cell model. Results: FSGTC treatment significantly relieved the degeneration of cartilage and reduced the contents of tumor necrosis factor-α (TNF-α) and IL-6 in the serum in papain-induced OA rats. FSGTC also reduced the protein and mRNA levels of IL-6 and IL-8 in IL-1β-stimulated SW982 cells. Moreover, it inhibited the phosphorylation levels of ERK (extracellular signal-related kinase), JNK (c-Jun N-terminal kinase), p38, Akt (protein kinase B), and c-Jun. It also decreased the extent of IκBα degradation and p65 protein translocation into the nucleus. Conclusion: The current data confirmed the protective effects of FSGTC in the rat and OA cell models. The results suggested that FSGTC reduced the production of inflammatory mediators via restraining the activation of mitogen-activated protein kinases (MAPK), nuclear factor kappa B (NF-κB), activator protein-1 (AP-1), and Akt.

Next Generation Mesenchymal Stem Cell (MSC)-Based Cartilage Repair Using Scaffold-Free Tissue Engineered Constructs Generated with Synovial Mesenchymal Stem Cells

Because of its limited healing capacity, treatments for articular cartilage injuries are still challenging. Since the first report by Brittberg, autologous chondrocyte implantation has been extensively studied. Recently, as an alternative for chondrocyte-based therapy, mesenchymal stem cell-based therapy has received considerable research attention because of the relative ease in handling for tissue harvest, and subsequent cell expansion and differentiation. This review summarizes latest development of stem cell therapies in cartilage repair with special attention to scaffold-free approaches.

The discovery of the synovial lymphatic stomata and lymphatic reabsorption in knee effusion

To illustrate the mechanism of lymphatic reabsorption in knee joint effusion. The current investigation employed transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques to reveal the ultrastructure of the knee synovial membrane in New Zealand rabbits and human. Ultrastructural changes of the synovial lymphatic stomata were observed by using trypan blue absorption and sodium hydroxide (NaOH) digestion methods, and the animal models of synovitis. New Zealand rabbits and human synovial membranes were composed of two types of synovial cells: type A and type B. No lymphatic stomata were found among type A synovial cells, whereas lymphatic stomata with the diameters ranging 0.74-3.26 µm were found in type B synovial cells, and some stomata were closed. After the NaOH digestion, a number of sieve pores, similar to lymphatic stomata in size and shape, were observed in the dense fibrous connective tissue underneath the type B synovial cells. After injecting trypan blue into the rabbit knee joint cavity, absorption of trypan blue through the lymphatic stomata was observed, suggesting the absorption function of the synovial lymphatic stomata. In the rabbit knee joint synovitis models, the synovial lymphatic stomata diameter enlarged. Some macrophages migrated from the lymphatic stomata, indicating that the synovial lymphatic stomata were involved in the joint effusion absorption and inflammatory response. Our study is the first to report the existence of synovial lymphatic stomata in the New Zealand rabbits and human knee joints. Lymphatic stomata may have an important role in the reabsorption of joint effusion.

Inhibition of chondrocyte and synovial cell death after exposure to commonly used anesthetics: chondrocyte apoptosis after anesthetics

BACKGROUND

An intra-articular injection of local anesthetics is a common procedure for diagnostic and therapeutic purposes. It has been shown that these agents are toxic to articular cartilage and synovial tissue in a dose- and time-dependent fashion, and in some cases, they may lead to postarthroscopic glenohumeral chondrolysis (PAGCL). However, the role of apoptosis in cell death is still unclear, and the potential role of apoptosis inhibition in minimizing chondrocyte and synovial cell death has not been reported.

PURPOSE

(1) To quantify the degree of apoptotic cell death in chondrocytes and synovial cells exposed to local anesthetics, and (2) to determine whether caspase inhibition could reduce cell death.

STUDY DESIGN

Controlled laboratory study.

METHODS

Human chondrocytes and synovial cells were expanded in vitro and exposed to normal saline, 0.5% bupivacaine, 0.5% ropivacaine, 1% lidocaine, or 1:1000 epinephrine for 90 minutes. Apoptosis was then detected at 1, 3, 5, and 7 days after exposure using terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) and immunohistochemistry. Apoptosis was then inhibited using the pan-caspase inhibitor z-vad-fmk. Results were normalized to normal saline controls and analyzed by generalized regression models and pairwise confidence intervals.

RESULTS

Analysis of cumulative chondrocyte apoptosis relative to controls after anesthetic exposure demonstrated more than 60% cell death with 0.5% bupivacaine and 1:1000 epinephrine. The greatest chondroprotective effect of caspase inhibition occurred with 0.5% ropivacaine. Similarly, in synovial cells, epinephrine was also very cytotoxic; however, 1% lidocaine induced the most apoptosis. Synovial cells exposed to 0.5% ropivacaine were again most sensitive to protective caspase inhibition.

CONCLUSION

Local anesthetics induce chondrocyte and synovial cell apoptosis in a time-dependent fashion, with peak apoptosis occurring 5 days after exposure. Both chondrocytes and synovial cells are most sensitive to caspase inhibition after exposure to 0.5% ropivacaine.

CLINICAL RELEVANCE

Apoptosis inhibition may be an effective strategy in minimizing chondrocyte and synovial cell death after exposure to anesthetics. Further investigation is clinically warranted.

Reduction of interleukin-6 expression in human synoviocytes and rheumatoid arthritis rat joints by linear polarized near infrared light (Superlizer) irradiation

BACKGROUND

Rheumatoid arthritis (RA) is a systemic autoimmune disorder that involves inflammation and pain of joints. Low-level laser irradiation is being evaluated for treating RA, however, the effectiveness of linear polarized near infrared light (SuperLizer; SL) irradiation is unclear.

AIM

It has been reported that interleukin 6 (IL-6) plays a key role in the progression of RA. In our previous study, using DNA microarray analysis, we examined the gene expression profiling of human rheumatoid fibroblast-like synoviocyte MH7A in response to IL-1ß administration and SL irradiation. As a result, IL-6 was listed in altered gene as increased by IL-1ß and decreased by SL irradiation.

MATERIAL AND METHODS

The reduction of IL-6 gene expression in MH7A by SL irradiation was confirmed by reverse transcription polymerase chain reaction (RT-PCR) and real-time PCR. Effect of SL irradiation on the RA inflammation in the collagen induced arthritis (CIA) rats was also studied by measuring temperature. IL-6 production in knee joint of rats was analyzed by immunohisto-chemistry.

RESULTS

Scatter plot analysis demonstrated that an increase in IL-6 gene expression by IL-1ß was reduced by SL irradiation. The reduction of IL-6 mRNA level by SL irradiation was successfully confirmed by RT-PCR and real-time PCR. SL irradiation treated CIA rat decreased the temperature of knee joints. The immunohistochemical analysis demonstrated a strong IL-6 staining in synovial membrane tissue of CIA rat joint, and SL irradiation significantly reduced the staining.

DISCUSSION

Since IL-6 has been identified to be an important proinflarnmatory cytokine in the pathogenesis of RA, the reduction of IL-6 expression is one of mechanisms in reduction of inflammation in RA joints by SL irradiation suggesting that SL irradiation may be useful for RA therapy.

CONCLUSION

SL irradiation reduced IL-6 gene expression in MH7A, and reduced inflammation and IL-6 protein expression in knee joint of CIA rats.

Is the spinal motion segment a diarthrodial polyaxial joint: what a nice nucleus like you doing in a joint like this?

This review challenges an earlier view that the intervertebral joint could not be classified as a diarthrodial joint and should remain as an amphiarthrosis. However, a careful analysis of the relevant literature and in light of more recent studies, it is clear that while some differences exist between the spinal articulation and the generic synovial joint, there are clear structural, functional and developmental similarities between the joints that in sum outweigh the differences. Further, since the intervertebral motion segment displays movement in three dimensions and the whole spine itself provides integrated rotatory movements, it is proposed that it should be classified not as an amphiarthrose, "a slightly moveable joint" but as a complex polyaxial joint. Hopefully, reclassification will encourage further analysis of the structure and function of the two types of overlapping joints and provide common new insights into diseases that afflict the many joints of the human skeleton.

Synoviolin/Hrd1, an E3 ubiquitin ligase, as a novel pathogenic factor for arthropathy

Rheumatoid arthritis (RA) is one of the most critical articular diseases with synovial hyperplasia followed by impairment of quality of life. However, the mechanism(s) that regulates synovial cell outgrowth is not fully understood. To clarify its mechanism(s), we carried out immunoscreening by using antirheumatoid synovial cell antibody and identified and cloned "Synoviolin/Hrd1", an E3 ubiquitin ligase. Synoviolin/Hrd1 was highly expressed in the rheumatoid synovium, and mice overexpressing this enzyme developed spontaneous arthropathy. Conversely, synoviolin/hrd1(+/-) mice were resistant to collagen-induced arthritis by enhanced apoptosis of synovial cells. We conclude that Synoviolin/Hrd1 is a novel causative factor for arthropathy by triggering synovial cell outgrowth through its antiapoptotic effects. Our findings provide a new pathogenetic model of RA and suggest that Synoviolin/Hrd1 could be targeted as a therapeutic strategy for RA.

Induction of Ia antigens on synovial cells by gamma interferon

Gamma interferon (IFN-γ) is a potent inducer of class II MHC antigens (la) in vitro. Enhanced Ia Expression is a characteristic feature of inflamed rheumatoid synovium. We investigated the potential of Ia induction upon non-inflammatory synovial cells under the influences of gamma interferon, alpha interferon and conditional medium from concanavalin A (Con-A) stimulated peripheral blood lymphocytes cultures (Con-A CM), Ia expression on enzyme dispersed cultured synovial cells decreased with time coincident with the loss of macrophage like cells. Addition of gamma interferon induced Ia molecules upon Ia negative synovial cells in a time and dose dependent manner, whereas alpha interferon treatment failed to induced Ia antigen. Con-A CM exerted the ability to induce Ia upon synovial cells. These findings suggest that: (1) Ia antigen expression by synovial tissue is the result of direct T cell influence: (2) non-inflammatory synovium has a potential to modulate Ia molecules under an altered environment.