Innate immunity in human bone

Bone has exceptional regenerative properties. Oral bone appears to be particularly resistant to infection despite exposure to oral flora, even in circumstances such as oral surgery where the thin mucosal layer covering the bone is disrupted. The goal of this study was to determine whether the innate immune system of antimicrobial peptides exists inside bone. Biopsies of non-infected and chronically infected mandibular bone were harvested from patients during maxillofacial surgical procedures. Bone biopsies from the iliac crest and fibula served as controls. Immunohistochemical staining was performed, directed against the human beta-defensin antimicrobial peptides (hBD) -1, -2 and -3. In addition, cultures of osteoblast-like cells were examined for the presence of each of the three beta-defensins and their mRNA transcripts. All three human beta-defensins were detected within the mineralized bone matrix of chronically infected mandibular bone in the vicinity of the endosteum and osteocytes. hBD-1, -2 and -3 were also found in the cytoplasm of osteocytes. Expression of all three beta-defensins was detected in each of the non-infected bone types including the controls, however, to a lesser degree than that found in the chronically infected mandibular bone. This may reflect upregulation of antimicrobial peptide expression in the presence of chronic infection. Cultures of non-infected osteoblast-like cells were found to express mRNA for each of hBD-1, -2 and -3. Immunohistochemical staining of the cultures was positive for hBD-1 and -2, but not for hBD-3. We provide the first evidence of a previously unrecognized innate immunological function of bone through the demonstration of the presence of the human beta-defensins hBD-1, -2 and -3 in bone.