Background:Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease characterized by bone destruction[1]. Chemokine signaling by skeletal cells or by other cells of the bone marrow niche regulates bone formation and resorption[2]. Recent studies have found that CXCL7 enhanced the osteoclast formation in mouse bone marrow cells[3, 4]. Whether CXCL7 plays a role in human osteoclastogenesis especially in RA patients remains unclear.Objectives:To examine the functional role of CXCL7 in the induction of osteoclastogenesis in RA.Methods:The level of CXCL7 in CD14+monocyte supernatant was assessed via enzyme-linked immunosorbent assay. Osteoclastogenesis of CD14+monocyte from RA patients and healthy donors were evaluated by TRAP staining and F-actin ring immunofluorescence. Bone resorption pit was observed by scanning electron microscopy. We performed quantitative reverse transcription polymerase chain reaction (RT-PCR) to detect changes in osteoclast markers. RAW264.7 macrophages were also used to investigate specific signaling pathway by which CXCL7 stimulated during osteoclast formation.Results:CXCL7 level in CD14+monocyte supernatant from RA patients (5690 ±627.05 pg/ml) was significantly higher than that in healthy controls (2301 ±535.52 pg/ml) (n=5, P<0.001). In the presence of M-CSF and RANKL, CXCL7 promoted osteoclast formation(Figure 1A and B) and increased bone resorption area(Figure 1C) of CD14+monocyte from healthy donors in the low concentration (10ng/ml) group (n=3, p < 0.05). While in high concentration of CXCL7 (50ng/ml) group, there were no significant changes in the number of osteoclasts. Transcription level of the osteoclast markers RANK, cathepsin K, and MMP-9 was significantly increased in the CXCL7 (10 ng/mL) group after 3 days in the presence of M-CSF and sRANKL (n=3, p < 0.05). When using CD14+ monocyte from RA patients, the optimal concentration of CXCL7 was 50ng/ml, which significantly increased the number of osteoclasts (Figure 2A and B)and bone resorption area (Figure 2C) (n=3, p < 0.01). Flow cytometry analysis revealed that a higher proportion of CD14+monocytes expressed CXCR2 from healthy donors than those from RA patients (n=6, p < 0.01). Consistent with the results obtained in CD14+monocytes, the effects of exogenous CXCL7 on osteoclast formation were also observed in RAW264.7 cells (p < 0.01). The addition of CXCL7 dramatically promoted phosphorylation ERK1/2 in RAW264.7 cells, but it did not affect the phosphorylation of P65.Conclusion:CXCL7 level in CD14+monocyte supernatant was higher in RA patients than that of healthy donors. CXCL7 promoted osteoclastogenesis in CD14+monocyte both from RA patients and healthy donors. CXCL7 could be a potential therapeutic target for bone destruction in RA.References:[1] McInnes, I.B. and G. Schett, The pathogenesis of rheumatoid arthritis. N Engl J Med, 2011. 365(23): p. 2205-19.[2] Brylka, L.J. and T. Schinke, Chemokines in Physiological and Pathological Bone Remodeling. Front Immunol, 2019. 10: p. 2182.[3] Nakao, K., et al., IGF2 modulates the microenvironment for osteoclastogenesis. Biochem Biophys Res Commun, 2009. 378(3): p. 462-6.[4] Goto, Y., et al., CXCR4(+) CD45(-) Cells are Niche Forming for Osteoclastogenesis via the SDF-1, CXCL7, and CX3CL1 Signaling Pathways in Bone Marrow. Stem Cells, 2016. 34(11): p. 2733-2743.Acknowledgments :We gratefully thank the Medical Research Center of Peking University Third Hospital for providing experimental equipment and technical support.Disclosure of Interests:None declared
The role of light in osteoclast formation and pathological bone resorption in rheumatoid arthritis