CD34⁺THY1⁺ synovial fibroblast subset in arthritic joints has high osteoblastic and chondrogenic potentials in vitro

Synovial fibroblasts (SFs) in rheumatoid arthritis (RA) and osteoarthritis (OA) play biphasic roles in joint destruction and regeneration of bone/cartilage as mesenchymal stem cells (MSCs). Although MSCs contribute to joint homeostasis, such function is impaired in arthritic joints. We have identified functionally distinct three SF subsets characterized by the expression of CD34 and THY1 as follows: CD34⁺THY1⁺, CD34^-THY1^-, and CD34^-THY1⁺. The objective of this study was to clarify the differentiation potentials as MSCs in each SF subset since both molecules would be associated with the MSC function.

SF subsets were isolated from synovial tissues of 70 patients (RA: 18, OA: 52). Expressions of surface markers associated with MSCs (THY1, CD34, CD73, CD271, CD54, CD44, and CD29) were evaluated in fleshly isolated SF subsets by flow cytometry. The differentiation potentials of osteogenesis, chondrogenesis, and adipogenesis were evaluated with histological staining and a quantitative polymerase chain reaction of differentiation marker genes. Small interfering RNA was examined to deplete THY1 in SFs.

The expression levels of THY1⁺, CD73⁺, and CD271⁺ were highest and those of CD54⁺ and CD29⁺ were lowest in CD34⁺THY1⁺ among three subsets. Comparing three subsets, the calcified area, alkaline phosphatase (ALP)-stained area, and cartilage matrix subset were the largest in the CD34⁺THY1⁺ subset. Consistently, the expressions of differentiation markers of the osteoblasts (RUNX2, ALPL, and OCN) or chondrocytes (ACAN) were the highest in the CD34⁺THY1⁺ subset, indicating that the CD34⁺THY1⁺ subset possessed the highest osteogenic and chondrogenic potential among three subsets, while the differentiation potentials to adipocytes were comparable among the subsets regarding lipid droplet formations and the expression of LPL and PPARγ. The knockdown of THY1 in bulk SFs resulted in impaired osteoblast differentiation indicating some functional aspects in this stem-cell marker.

The CD34⁺THY1⁺ SF subset has high osteogenic and chondrogenic potentials. The preferential enhancement of MSC functions in the CD34⁺THY1⁺ subset may provide a new treatment strategy for regenerating damaged bone/cartilage in arthritic joints.