The aim of this study was to analyze local and systematic inflammatory status in knee osteoarthritis (KOA), focusing on intra-articular and remote adipose tissue depots, and to explore its potential association with metabolic syndrome (MetS).
Patients (n = 27) with end-stage KOA were enrolled in the study and samples from infrapatellar fat pad (IFP), synovium, subcutaneous adipose tissue (SAT), synovial fluid (SF), and serum were collected. In homogenates from the tissues, mRNA expression of developmental endothelial locus-1 (DEL-1) was determined. Interleukin 6 (IL-6) and interleukin 8 (IL-8) were measured in tissues and SF and serum samples by enzyme-linked immunosorbent assay.
Fifteen patients fulfilled MetS criteria (w-MetS group) and 12 did not (non-MetS). In the entire population, IL-6 levels were significantly higher in IFP compared to synovium (median (interquartile range), 26.05 (26.16) vs. 15.75 (14.8) pg/mg of total protein, p = 0.043), but not to SAT (17.89 (17.9) pg/mg); IL-8 levels were significantly higher in IFP (17.3 (19.3) pg/mg) and SAT (24.2 (26) pg/mg) when compared to synovium (8.45 (6.17) pg/mg) (p = 0.029 and < 0.001, respectively). Significantly higher IL-6 concentrations in SF were detected in w-MetS patients compared to non-MetS (194.8 (299) vs. 64.1 (86.9) pg/ml, p = 0.027). Finally, DEL-1 mRNA expression was higher in IFP compared to synovium (eightfold, p = 0.019).
Our findings support the critical role of IFP in knee joint homeostasis and progression of KOA. Furthermore, in KOA patients w-MetS, SAT is thought to play an important role in intra-knee inflammation via secretion of soluble inflammatory mediators, such as IL-6.