Background/Purpose: Osteoclasts (OCs) are multinucleated bone-resorbing cells playing a key role in rheumatoid arthritis (RA). Under physiological conditions, OCs are in balance with bone-forming osteoblasts, whilst in RA homeostasis is disrupted, leading to excessive bone resorption. Specialised pro-resolving mediators (SPMs), a group of lipids biosynthesised from fatty acids, have been shown to resolve inflammation and play a role in bone remodelling. Notably, the SPM Resolvin E1 (RvE1) was shown to inhibit OC differentiation and bone resorption using the RAW264.7 cell line and murine models. However, the effect of RvE1 on primary human cells and its mechanism-of-action in OCs remain unknown. Another SPM, Resolvin D1 (RvD1), has also been associated with reduced bone resorption in human OCs. Thus, this study aimed to evaluate the effects of SPMs on OCs (and their pre-cursors) differentiated from primary human cells derived from healthy individuals and those with RA; while also investigating the expression of proposed SPM receptors.

Methods: Peripheral blood mononuclear cells (PBMCs) were obtained from RA patients with active disease (DAS28 >2.8) and treated with conventional DMARDs or healthy controls (HC). CD14⁺ monocytes were isolated from PBMCs and differentiated either into macrophages with 25 ng/ml of M-CSF or into OCs with 25 ng/ml of M-CSF and RANKL. The effect of various SPMs, namely RvD1, RvE1, 17-HDHA, Maresin 1, and their combination was tested in the presence or absence of TNF to mimic inflammatory environment. The impact of SPMs on osteoclastogenesis, bone resorption, ATP production, and mitochondrial superoxide levels was assessed. SPM receptor expression was analysed by RNA sequencing and protein expression was determined by western blot (WB).

Results: Exposure of TNF-stimulated OC pre-cursors to RvE1 resulted in inhibition of osteoclastogenesis in HC individuals (15±6.7% inhibition) but not in RA. In contrast, RvD1 inhibited osteoclastogenesis under the same conditions in RA patients (16±2.4% inhibition) but not in HC. Notably, OC inhibition was associated with the presence of numerous mono- and bi-nuclear pre-cursors, which failed to differentiate into multinucleated mature OCs. The inhibition of OC differentiation by RvE1 and RvD1 was associated with reduced superoxide production in HC and RA OCs, respectively. Transcriptional profiling of human monocytes and OCs revealed that LTB4R, CMKLR1, FPR2, and GPR18 SPM receptors were expressed in both cell types. Validation of the selected SPM receptors via WB, demonstrated that LTB4R was significantly higher in HC monocytes and OCs than in RA, while CMKLR1 and FPR2 were similarly expressed.

Conclusion: In summary, this study highlights the capacity of RvE1 and RvD1 to specifically modulate OC differentiation in primary human cells in either health or RA. Combined, these findings suggest a potential for SPMs as novel treatment for RA.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-effect-of-resolvin-e1-and-resolvin-d1-specialised-pro-resolving-mediators-on-the-inhibition-of-osteoclastogenesis/