Background Rheumatoid arthritis (RA) is an inflammatory autoimmune disease that is characterised by the infiltration of activated immune cells in the synovial joint, followed by progressive destruction of cartilage and bone. Even though there are many effective drug-based treatment options available not all patients do respond satisfactorily. In those cases treatment with low dose radiotherapy (LD-RT) might be an option as previous studies demonstrated that LD-RT attenuates inflammation and pain. In previous experiments using the hTNF-α tg mouse model we discovered that fibroblast-like synoviocytes (FLS) can be altered into an anti-inflammatory phenotype and numbers of differentiated osteoclasts are being reduced by LD-RT. Our in vivo experiments further showed reduced inflammation and bone loss in locally irradiated (1 × 0.5Gy) paws. As factors secreted by inflammatory FLS modulate other immune cells, we now aimed to investigate their impact on macrophages (MPHs) that are key players in inflammation and also major producers of inflammatory cytokines. Further, as bone turnover is a tightly linked process between bone resorption and build-up, we subsequently aimed to investigate whether and how LD-RT impacts on osteoblasts.

Material and methods Bone marrow-derived MPHs of hTNF-α tg mice were differentiated using M-CSF and supernatants of X-ray treated FLS. Consecutively, MPH phenotypes were analysed by flow cytometry and ELISA. hTNF-α tg-derived osteoblasts were irradiated with various doses of X-rays and the impact of radiation was analysed using alizarin-red, qPCR and ELISA.

Results Treatment of MPHs with supernatants of irradiated FLS lead to decreased CD86 (0.5-2Gy) and MHCII (0.1–0.5Gy) surface expression, while secreted IL-10 levels were increased at 0.5Gy. LD-RT of osteoblasts led to increased mineralization at a dose of 0.5Gy, a dose dependent increase in expression levels of osteoblast maturation markers, and reduced Rank-L expression, alongside with increased secretion of osteoprotegerin.

Conclusion We conclude that LD-RT is able to positively modulate bone-turnover and exerts anti-inflammatory effects on FLS and MPHs. In accordance with our previous results, these modulations might contribute to improvements in TNF-driven RA. In the future, placebo controlled studies are desirable in order to investigate the effects of LD-RT in RA and osteoimmunological events in the clinics.

Acknowledgement Supported by the German Federal Ministry of Education and Research (GREWIS, 02NUK017G).