Synovial Fibroblasts Infected with Salmonella enterica Serovar Typhimurium Mediate Osteoclast Differentiation and Activation

Abstract

The mechanisms whereby arthritogenic organisms may induce cartilage and bone erosions in infectiontriggered arthritis remain unknown. In this study, we asked whether an arthritogenic organism could contribute to osteoclast differentiation and activation through regulation of the receptor activator of NF-B ligand (RANKL) in synovial fibroblasts. Rat synovial fibroblasts were infected in vitro with Salmonella enterica serovar Typhimurium and monitored over time. The expression of RANKL in resting and infected synovial fibroblasts was quantified by reverse transcription-PCR and Western blotting. Osteoclast progenitors, isolated from femurs of 8-week-old rats and cultured in the presence of macrophage colony-stimulating factor, were cocultured with either infected or noninfected synovial fibroblasts for 2 to 4 days. Differentiation and maturation of osteoclasts were determined by morphology and tartrate-resistant acid phosphatase (TRAP) staining and by a bone resorption bioassay. RANKL expression was undetectable in resting synovial fibroblasts but was dose-dependently upregulated in cells after Salmonella infection. Osteoprotegerin was constitutively expressed by synovial fibroblasts and was not upregulated by infection. Further, we observed the formation of multinucleated TRAP-positive cells and formation of bone resorption pits in cocultures of bone marrow-derived osteoclast precursors with synovial fibroblasts infected with Salmonella but not with heat-killed Salmonella or noninfected cells. Arthritogenic bacteria may alter bone structure via synovial fibroblast intermediaries, since infected synovial fibroblasts (i) upregulate RANKL expression and (ii) enhance osteoclast precursor maturation into multinucleated, TRAP-positive, bone-resorbing, osteoclast-like cells. These data provide a link between infection and osteoclastogenesis. A better understanding of infection-mediated osteoclast differentiation and activation may provide new therapeutic strategies for inflammatory joint disease.

Gastrointestinal Microbes Interact with Canine Adipose-Derived Mesenchymal Stem Cells In Vitro and Enhance Immunomodulatory Functions

“…These findings are compatible with a previous report that describes prolonged culture of synovial fibroblasts after ST infection [54]. These findings suggest that while ST may invade into stromal-type cells without causing significant cell death, invasion does elicit marked and distinct cellular responses.…”

Receptor Activator of NF-κB Ligand and Osteoprotegerin Regulate Proinflammatory Cytokine Production in Mice

“…Upon LPS injection or Salmonella oral infection, we observed that serum RANKL levels were down-regulated within 5 h, while serum OPG levels were up-regulated over 24 h. Down-regulation of RANKL in response to LPS or infection was unexpected, because bacterial infection increases RANKL expression at both the mRNA and protein level in osteoblasts and synovial fibroblasts (33,34). Therefore, down-regulation of serum RANKL could require posttranslational regulation, such as suppression of RANKL shedding (35,36).…”

Receptor activator of NF-κB ligand enhances the activity of macrophages as antigen presenting cells

“…RANKL expressed on the surface of osteoblasts and stromal cells or soluble recombinant RANKL, supports osteoclast differentiation and activates mature osteoclasts to resorb bone in vitro (Lacey et al, 1998;Yasuda et al, 1998;Shin et al, 2003). Recently, several groups examined that bacteria or bacterial components induce the expression of RANKL from gingival fibroblasts, periodontal ligament cells, and synovial fibroblasts (Tiranathanagul et al, 2004;Zhang et al, 2004;Belibasakis et al, 2005). Monocytes and macrophages can differentiate into osteoclasts under a suitable condition prepared in vitro (Udagawa et al, 1990;Shin et al, 1995;Yasuda et al, 1998).…”