Background Tumor necrosis factor-inducible gene 6 (TSG-6, also named TNFAPI6) is upregulated during experimental arthritis in mice (Geurts, Joosten et al. 2009). The therapeutic efficacy of TSG-6 has been shown in multiple animal models for rheumatoid arthritis, reducing both inflammation and cartilage damage. Recent studies suggest that inflammation can also play a role in early osteoarthritis (OA) pathogenesis (Ayral, Pickering et al. 2005). Therefore, TSG-6 therapy might also be an effective treatment in inflammatory OA.

Objectives In this study, we analyzed the expression of TSG-6 during experimental osteoarthritis and explored the effects of TSG-6 gene therapy in a murine model of inflammatory osteoarthritis.

Methods TSG-6 gene expression was determined in microarrays of collagenase-induced osteoarthritis at day 7 and day 14 after induction. TSG-6 coding region was isolated from murine cDNA and cloned into a SIN-lentiviral - and E1 deleted adenoviral vector. Freshly isolated bone marrow-derived dendritic cells (BMDCs) were transduced with the lentiviral vector and subsequently differentiated into osteoclasts on bone slices with M-CSF and RANKL. After 10 days, bone slices were washed and stained using Coomassie Blue and bone resorption was determined using the Leica Application Suite software.

To study the effects of TSG-6 in experimental knee osteoarthritis, mice received two intra-articular injections of collagenase. 4 days prior to and 20 days after arthritis induction, mice were injected intra-articularly with TSG-6 adenovirus. At day 7, inflammation was assessed by fluorescent imaging using ProSense 680 protease activatable probes. At day 42, mice were sacrificed and the knee joints were analyzed by X-ray and histological assessment.

Results TSG-6 gene expression was upregulated in collagenase-induced OA (2,6x at day 7 (P<0.05) and 2.5x at day 14 (P=0.55)). BMDCs transduced with TSG-6 lentivirus showed strong expression of TSG-6. Bone resorption by BMDC-derived osteoclasts was significantly reduced (20.1% surface erosion with control virus to 10.4%, p=0.01), providing a possible mechanism for the therapeutic effects in rheumatoid arthritis models.

At day 7 of collagenase-induced osteoarthritis, no difference in inflammation was detected using the Prosense probe. At day 42, no improvement on cartilage damage was seen, but X-ray analysis showed strong osteophyte formation at the femur/tibia region in the knee joint. Histological analysis showed that the osteophytes contained both bone and cartilage.

Conclusions Viral expression of TSG-6 can reduce the bone resorption activity by BMDC-derived osteoclasts. The expression of TSG-6 by synovial cells during experimental osteoarthritis results in the formation of osteophytes. These results imply a causative role for TSG-6 in osteophyte formation, supporting the recent finding that TSG-6 activity is associated with radiographic progression of OA (Wisniewski, Colon et al. 2014).

References

1. Ayral X, Pickering EH, et al. (2005), Osteoarthritis Cartilage 13(5): 361-367.

2. Geurts J, Joosten LA, et al. (2009), Mol Ther 17(11): 1877-1887.

3. Wisniewski HG, Colon E, et al. (2014), Osteoarthritis Cartilage 22(2): 235-241.

Acknowledgements This research was financially supported by the Dutch Arthritis Association (Project 11-1-409)

Disclosure of Interest None declared