First in vivo detection and characterization of hyaluronan‐coated extracellular vesicles in human synovial fluid

Extracellular vesicles (EVs) function in intercellular signaling by transporting different membrane and cytosolic molecules, including hyaluronan (HA) and its synthesis machinery. As both EVs and HA are abundant in synovial fluid, we hypothesized that HA synthesized in synovial membrane would be carried on the surface of EVs. Synovial fluid (n = 15) and membrane samples (n = 5) were obtained from knee surgery patients. HA concentrations were analyzed in synovial fluid and HA and its synthesis machinery were examined with histochemical stainings in synovial membrane. To assess the size distribution of EVs in synovial fluid and to visualize HA on EVs, nanoparticle tracking analysis (NTA), confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM) were utilized. The average HA concentration in synovial fluid was 2.0 ± 0.21 mg/ml without significant differences between the patients with trauma/diagnostic arthroscopy and primary or post‐traumatic osteoarthritis. Positive stainings of HA synthases (HAS1–3), HA and its receptor CD44 in synovial cells indicated active HA secretion in synovial membrane. According to NTA, EVs were abundant in synovial fluid and their main populations were ≤300 nm in diameter after differential centrifugation. There were no significant differences in the EV counts between the patients with primary or post‐traumatic osteoarthritis. TEM verified that HA‐positive particles detected by CLSM were lipid membrane vesicles surrounded by a HA coat. Our results provide the first in vivo evidence that human synovial fluid contains HA‐positive EVs, one source of which presumably is the long HAS‐positive protrusions of synovial fibroblasts. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1960–1968, 2016.