Sampling of synovial fluid

Knee SF was obtained by arthrocentesis as described elsewhere [19]. In brief, SF samples that were contaminated with blood or excessively turbid were excluded by visual inspection. Human SF samples were diluted by addition of 2.0 ml 0.9% NaCl. Human and equine SF were incubated at 37°C for 15 min before filtering with a 1.2-μm filter. After addition of a 10% (v/v) cocktail containing proteases and phospholipase inhibitors, cellular particles were eliminated by centrifugation (16,100 x g, 45 min, RT), and subsequently frozen at -86°C until further analysis [19]. For ELISA assays, equine SF samples were incubated with 20 μl hyaluronidase (1U/μl; STEMCELL Technologies, Cologne, Germany) for 15 min. to reduce their viscosity before assaying.