Objective: To evaluate synovial fluid (SF) for the presence of mesenchymal progenitor cells (MPCs), to compare SF MPCs with bone marrow (BM) MPCs, and to enumerate these cells in both inflammatory arthritis and osteoarthritis (OA).
Methods: SF from 100 patients with arthritis (53 rheumatoid arthritis [RA], 20 OA, and 27 other arthropathies) was evaluated. To establish multipotentiality, polyclonal and single cell-derived cultures of SF fibroblasts were examined by standard and quantitative differentiation assays. Their phenotype before and after expansion was determined by multiparameter flow cytometry. A colony-forming unit-fibroblast assay was used for SF MPC enumeration.
Results: Regardless of the nature of the arthritis, both polyclonal and single cell-derived cultures of SF fibroblasts possessed trilineage mesenchymal differentiation potentials. The number of MPCs in a milliliter of SF was higher in OA (median 37) than in RA (median 2) (P < 0.00001). No significant differences in MPC numbers were found between early and established RA (median 3 and 2 cells/ml, respectively). Culture-expanded SF and BM MPCs had the same phenotype (negative for CD45 and positive for D7-FIB, CD13, CD105, CD55, and CD10). Rare, uncultured SF fibroblasts were CD45(low) and expressed low-affinity nerve growth factor receptor, similar to in vivo BM MPCs.
Conclusion: Our findings prove the presence of rare tripotential MPCs, at the single-cell level, in the SF of patients with arthritis. SF MPCs are clonogenic and multipotential fibroblasts that, despite the pathologic environment within a diseased joint, have a phenotype similar to that of uncultured BM MPCs. The higher prevalence of MPCs in OA SF suggests their likely origin from disrupted joint structures. These findings could determine the role of MPCs in the pathogenesis of inflammatory arthritis, together with their role in attempted joint regeneration in degenerative arthritis, which has yet to be established.