Synovial fibroblasts promote osteoclast formation by RANKL in a novel model of spontaneous erosive arthritis

Abstract

Objective. Erosion of cartilage and bone is a hallmark of rheumatoid arthritis (RA). This study was undertaken to explore the roles of hyperproliferating synovial fibroblasts and macrophages in abnormal osteoclast formation, using the recently described BXD2 mouse model of RA.Methods. Cell distribution in the joints was analyzed by immunohistochemistry, using tartrateresistant acid phosphatase (TRAP) staining to identify osteoclasts. To identify the defective cells in BXD2 mice, mouse synovial fibroblasts (MSFs) were cultured with bone marrow-derived macrophages. Osteoclast formation was assayed by TRAP staining and bone resorption pit assay, and the cytokine profiles of the MSFs and macrophages were determined by quantitative real-time polymerase chain reaction and enzyme-linked immunosorbent assay.Results. In BXD2 mice, TRAP-positive osteoclasts were found at sites of active bone erosion, in close proximity to hyperproliferating synovial fibroblasts. On coculture, MSFs from BXD2 mice, but not C57BL/6 mice, produced high levels of RANKL messenger RNA, induced macrophages to form osteoclasts, and actively eroded bone slices, through a mechanism(s) that could be blocked by pretreatment with osteoprotegerin. Although macrophages from BXD2 mice expressed higher basal levels of tumor necrosis factor ␣ (TNF␣), interleukin-1␤ (IL-1␤), and IL-6 than those from C57BL/6 mice, abnormal osteoclast formation was not due to enhanced sensitivity of the BXD2 mouse macrophages to RANKL. TNF␣, produced by both BXD2 MSFs and BXD2 mouse macrophages, had a strong stimulatory effect on RANKL expression. Conclusion. BXD2 MSFs produce RANKL and induce the development of osteoclasts from macrophages. The enhanced production of RANKL is possibly due to autocrine stimulation, together with paracrine stimulation by factors produced by macrophages.Rheumatoid arthritis (RA) is characterized by synovial hyperplasia, immune cell infiltration, cartilage destruction, and bone erosion (1,2). The presence of osteoclasts at the site of active bone erosions in patients with RA implicates these cells in the erosive disease (3). It has been shown that receptor activator of NF-B ligand (RANKL; also known as osteoclast differentiation factor, osteoprotegerin [OPG] ligand, TRANCE, and tumor necrosis factor [TNF] superfamily 11), is essential for the development of monocyte/macrophages into mature osteoclasts (4). During normal bone metabolism, RANKL is expressed by the stromal cells and osteoblast precursor cells of the bone marrow and is one of the factors that couple osteoclast formation and osteoblast formation such that bone turnover is balanced appropriately (5). In patients with RA, both the T cells and the synovial fibroblasts have been found to produce RANKL, and it has been proposed that this promotes osteoclast development (6,7). In the TNF␣-transgenic mouse model of arthritis, OPG, a decoy receptor of RANKL, protects against joint erosions, suggesting that the production of RANKL plays a pathogenic role in the development of the erosions (8).

The RANK/RANKL/osteoprotegerin system in rheumatoid arthritis: New insights from animal models

“…Similar to findings in other animal models of RA, inhibition of RANKL had no effects on inflammation but completely prevented bone loss and partially protected against cartilage loss in BXD2 mice (11). These findings are consistent with observations that inhibition of tumor necrosis factor (TNF) and interleukin-1 (IL-1) prevents inflammation and bone loss in human RA only to a limited extent (12)(13)(14), but inhibition of downstream RANKL effectors via OPG or other drugs could prevent bone destruction and reduce cartilage damage in human RA.…”

“…In the new animal model for RA described by Wu et al (11), the recombinant inbred strain BXD2 spontaneously develops features characteristic of RA. Large numbers of osteoclasts in close proximity to macrophages and synovial fibroblasts adjacent to the erosion sites can be detected.…”

“…Several animal models have been used to study the in vivo effects of these 3 molecules in experimental arthritis. In this issue of Arthritis & Rheumatism, Wu et al (11) report on the role of RANKL in osteoclast formation in a new model of spontaneously developing erosive arthritis in the mouse. BXD2 mice, a recombinant inbred strain derived from C57BL/6 and DBA/2 mice, develop arthritis with bone erosions, synovial hyperplasia with mononuclear cell infiltration, and joint deformation; these features are observed at 6 months of age in 50% of female mice.…”

Interleukin 17–producing T helper cells and interleukin 17 orchestrate autoreactive germinal center development in autoimmune BXD2 mice

“…We previously used BXD2 with 19 other recombinant inbred strains of BXD mice in a survey of genetic loci that may influence T cell senescence 12 . In those studies, we noted that the BXD2 mice develop a spontaneous erosive arthritis that progresses as the mice age 12,13 . Further analysis established that the mice show other hallmarks of autoimmune disease, including increasing titers of circulating immune complexes and the progressive development of glomerulonephritis 14,15 .…”

Suppression of inflammation and structural damage in experimental arthritis through molecular targeted therapy with PPI‐2458

“…Bone erosion is mediated by osteoclasts, highly specialized multinucleated cells which are derived from hematopoietic precursors through a sequence of processes involving proliferation, differentiation, fusion, and activation (6,7). In the inflamed joint, activated synoviocytes and T cells secrete RANKL, which, in synergy with TNF␣, ⌱L-1, and other osteoclastogenic mediators, stimulates pathologic bone resorption through osteoclast recruitment, differentiation, and activation (5,(7)(8)(9)(10).…”