Monocyte chemoattractant protein-1 expression by osteoblasts following infection with Staphylococcus aureus or Salmonella

Two common pathogens of bone, Staphylococcus aureus and Salmonella, were investigated for their ability to induce chemokine expression in bone-forming osteoblasts, Cultured mouse or human osteoblasts could rapidly respond to bacterial infection by upregulating the mRNA encoding the chemokine, monocyte chemoattractant protein-1 (MCP-1). This rapid induction occurred on infection with either the gram-positive pathogen, S. aureus, or the gram-negative pathogen, Salmonella, Increased mRNA expression translated into MCP-1 secretion by cultured mouse or human osteoblasts in response to viable bacteria, whereas UV-killed bacteria were less effective in stimulating chemokine secretion. There was a dose-response relationship observed between the amount of input bacteria and increases in MCP-1 secretion. Immunohistochemical staining of infected osteoblasts indicated that the majority of cells could express MCP-1, with some osteoblasts having a higher intensity of staining than others. Organ cultures of mouse calvaria (skullcap) bone showed increases in MCP-1 immunostaining following bacterial infection. The immunoreactive MCP-1 in infected calvaria localized to areas containing active osteoblasts. Taken together, these studies demonstrate a conserved osteoblast-derived MCP-1 response to two very different pathogens of bone.