Bacterial Infection of Osteoblasts Induces Interleukin-1 β and Interleukin-18 Transcription but Not Protein Synthesis

Abstract

A growing body of evidence has shown that bacterially challenged bone-forming osteoblasts are a significant source of an array of cytokines and chemokines that can support immune responses during bone disease. In the present study, Staphylococcus aureus and Salmonella, two common pathogens of bone, were investigated for their ability to induce production of two related inflammatory cytokines, interleukin-1β(IL-1β) and IL18, in osteoblasts. Cultured mouse osteoblasts were found to respond rapidly to either bacterial challenge by upregulation in the levels of mRNA encoding both IL-1β and IL-18. Surprisingly, this mRNA expression did not translate into intracellular accumulation of IL-1β or IL-18 precursor proteins or secretion of mature cytokines, despite the presence of detectable caspase-1 activity in these cells. These studies demonstrate that although osteoblasts can secrete a number of key proinflammatory mediators in response to bacterial pathogens, IL-1β and IL-18 are not among this number. We suggest that osteoblasts are an unlikely source of these cytokines during the progression of bacterial infection of bone.