Synovial fluid CD34⁻ CD44⁺ CD90⁺ mesenchymal stem cell levels are associated with the severity of primary knee osteoarthritis.

To the best of our knowledge, no reports have directly compared synovial fluid (SF)- and synovial membrane (SM)-derived mesenchymal stem cells (MSCs) from primary knee osteoarthritis patients in terms of MSC proportion, either immediately after isolation or during culture. Any possible correlation between SM- and SF-MSC purity and osteoarthritis severity, also remains unclear. We therefore assessed quantitative and phenotypic differences in MSCs isolated from SF and SM. We also evaluated the correlation between sample MSC purity, and disease severity, in patients with osteoarthritis. The main result of the current study was that the mean SF-MSC proportion at passage 0 was negatively correlated with Kellgren-Lawrence (KL) grade (r = -0.565, P = 0.002). In addition, KL grade was a only significant independent negative predictor of SF-MSC proportion at passage 0 (β = -0.356, P = 0.039). Conclusively, the proportion of SF-MSCs in fresh samples, evaluated at the single cell level, was inversely correlated with osteoarthritis severity.