Pos0349 Single Osteoclast Resorption Analysis Reveals an Increased Intrinsic Osteoclast Activity in Ra Patients

Background: Inflamed joints and resulting bone erosions are typical features of rheumatoid arthritis (RA). To prevent long-term bone loss in joints as well as systemically, osteoclasts (OCs) have become a main therapeutic target in RA. Despite utilizing new and improved therapeutic approaches, ongoing bone deterioration cannot always be stopped. It is still controversially debated, whether this bone loss is caused by increased OC precursor numbers in circulation, enhanced OC differentiation, or by OCs being more active. It proved difficult to investigate OC activity, since all common in vitro assays depend on OC numbers, hence the efficiency of OC differentiation. Therefore, new techniques are needed that allow the quantification of OC activity on a single cell basis. Objectives: To investigate the OC activity of RA patients and healthy controls independent from the common limitations of standard osteoclast resorption assays, by utilizing a novel single cell approach. Methods: Blood monocytes from RA patients were differentiated to OCs and seeded onto calcein-incorporated calcium phosphate-coated (calcein-CaP) plates. After 24h, OC movement and resorption were recorded for up to 24h using a time-lapse fluorescent microscope. At last, the OC nuclei were stained with Hoechst and quantified. Resorption pits of individual OCs were than tracked by utilizing the fluorescence signal of the calcein-CaP coating. From here, the resorption speed of the OCs was calculated. Together with the number of nuclei, determined from the Hoechst signal, the resorptive capacity (resorption speed/nuclei) of each individual OC was calculated. The mean resorptive capacities of the RA patients' OCs were then compared to OCs from controls. Results: The resorptive capacity of the OCs was significantly increased by 55% in one half of the RA patients in comparison to NHDs. The other half of the RA cohort displayed resorptive capacities comparable to NHDs. This effect was independent from DAS28, CRP, ACPA status, sex, age and medication strategy, but correlated with joint pain. Furthermore, we observed a significantly elevated erythrocyte sedimentation rate (ESR) in RA patients with increased resorptive capacity. Conclusion: Our results suggest, that osteoclasts from RA patients with ongoing joint inflammation reflected by joint pain display an intrinsically increased resorption capacity compared with osteoclasts from healthy donors with the same number of nuclei. This phenomenon likely contributes to the accelerated bone loss in RA patients. REFERENCES: NIL. Acknowledgements: NIL. Disclosure of Interests: None declared.