Elevated levels of active Transforming Growth Factor β1 in the subchondral bone relate spatially to cartilage loss and impaired bone quality in human knee osteoarthritis

Abstract

Objective Over-activity of transforming growth factor β1 (TGFβ1) in subchondral bone has a direct causal role in rodent models of knee osteoarthritis (OA), which can be blocked by TGFβ1 neutralisation. In this study, we investigated whether the spatially distributed level of active TGFβ1 in human subchondral bone associates with the characteristic structural, cellular and molecular parameters of human knee OA. Design Subchondral bone samples (35 OA arthroplasty patients, aged 69±9 years) were obtained from regions below either macroscopically present or denuded cartilage. Bone samples were processed to determine the concentration of active TGFβ1 (ELISA) and gene-specific mRNA expression (RT-PCR). Synchrotron micro-CT imaging was utilised to assess the bone microstructure, bone mineralization, the osteocyte lacunar network and bone matrix vascularity. Finally, samples were histologically examined for cartilage OARSI grading, quantification of tartrate resistant acid phosphatase positive cells and bone marrow micro-vasculature. Results Subchondral bone below severely degenerated/depleted cartilage, characterised by impaired bone matrix quality due to sclerotic microarchitecture, disorganised collagen, high heterogeneity of the mineral distribution, contained increased concentrations of active TGFβ1, compared to adjacent areas with more intact cartilage. In addition, increased levels of active TGFβ1 related directly to increased bone volume while increased OARSI grade associated directly with morphometric characteristics (size, shape and orientation) of osteocyte lacunae. Conclusion These results indicate that increased active TGFβ1 associates spatially with impaired bone quality and the disease severity of human OA. This study therefore suggests that TGFβ1 could be a therapeutic target to prevent or reduce human disease progression.