Osteoblasts produce monocyte chemoattractant protein-1 in a murine model of Staphylococcus aureus osteomyelitis and infected human bone tissue

Incidences of osteomyelitis caused by Staphylococcus aureus have increased dramatically in recent years, in part, due to the appearance of community-acquired antibiotic-resistant strains. Therefore, understanding the pathogenesis of this organism has become imperative. Recently, we have described the surprising ability of bone-forming osteoblasts to secrete a number of important immune mediators when exposed to S. aureus in vitro. In the present study, we provide the first evidence for the in vivo production of the pivotal inflammatory chemokine, monocyte chemoattractant protein-1 (MCP-1), by osteoblasts during S. aureus-associated bone infection. Quantitative real-time PCR was employed to determine levels of mRNA encoding MCP-1 in vivo using a mouse model that closely resembles the pathology of trauma-induced staphylococcal osteomyelitis. Expression of this inflammatory chemokine and osteoblast-specific markers was investigated by confocal laser scanning microscopy in bone tissue from organ cultures of neonatal mouse calvaria and from the in vivo mouse model. Furthermore, the clinical relevancy of these findings was investigated by performing similar studies on infected human bone tissue from patients with S. aureus-associated osteomyelitis. Here, we confirm that expression of mRNA encoding MCP-1 is elevated in bacterially infected murine bone tissue. Importantly, we show that these increases translate into marked elevations in the expression of MCP-1 protein that co-localizes with osteoblast markers in infected bone tissue. Such increases could not be attributed solely to mechanical damage as a similar response was observed in infected but otherwise undamaged organ cultures. Finally, we have demonstrated the in vivo production of MCP-1 by osteoblasts in bone specimens from patients with S. aureus-associated osteomyelitis. As such, these studies demonstrate that bacterial challenge of osteoblasts during bone diseases such as staphylococcal osteomyelitis induces cells to produce a key inflammatory chemokine that can direct appropriate host responses or may contribute to progressive inflammatory damage.