Background Exosomes are small membrane vesicles (40–150 nm) of endocytic origin secreted by many types of cells and engage in cell-to-cell communication by transferring proteins, microRNAs, mRNA, and lipid to recipient cells. Exosomes are also identified in various biological fluids including blood, urine, amniotic fluid, saliva, malignant ascites, and synovial fluid. Until now, the role of exosomes from synovial fluid (SF) on human osteoclastogenesis is unknown.
Objectives We aimed to identify the characteristics of synovial exosomes and evaluate the effects on human osteoclastogenesis.
Methods Synovial exosomes were isolated from SF of rheumatoid arthritis (RA), osteoarthritis (OA), ankylosing spondylitis (AS), and gout patients using ExoQuick (System Biosciences, USA). Monocytes were isolated from human peripheral blood mononuclear cell and were differentiated into macrophages by treatment of 20 ng/ml recombinant human macrophage colony-stimulating factor (M-CSF) for 3–4 days. Then, macrophages were incubated with synovial exosomes in alpha-MEM supplemented with 10% FBS and 1% penicillin/streptomycin for 9–10 days. Osteoclast differentiation was evaluated by tartrate resistant acid phosphatase stain. Exosomes from RA (n=11), OA (n=5), AS (n=6), and gout (n=6) patients were quantified by measuring acetylcholinesterase activity in colorimetric assay.
Results The number of synovial exosomes isolated from same volume of SF from RA, OA, AS, and gout patients was assessed. We found that exosomes are abundant in SF of RA, AS, and gout patients compared to OA patients. (mean ± SEM 7.36×107±1.35×107, 9.91×107±2.50×107, 7.50×107±2.17×107 vs. 2.66×107±0.42×107). Interestingly, the number of exosomes from AS SF was significantly higher (by 3.72 fold, p=0.0476) than exosomes from OA SF. Exosomes from RA, OA, AS, and gout patients induced proliferation and osteoclast formation without M-CSF and receptor activator of nuclear factor kappa-B ligand (RANKL) (Figure 1), which is essential to proliferation and differentiation of osteoclast. Noticeably, exosomes from RA SF had higher potential on osteoclast differentiation than other three diseases. (20.28±14.28 fold vs. 2.38±0.42 fold).
Conclusions Our results suggest that exosomes from SF of various inflammatory diseases might have differential functional roles in osteoclast differentiation and the number of exosomes may be associated with inflammatory conditions.
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Disclosure of Interest None declared
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