The expression of cytokine activity by fracture callus

Cytokines, a group of proteins known to regulate hemopoietic and immune functions, are also involved in inflammation, angiogenesis, and bone and cartilage metabolism. Since all of these processes occur following bone injury, or are known to contribute to wound repair mechanisms, this investigation sought to test the hypothesis that cytokines are involved in fracture healing. Two sets of 60 male Sprague‐Dawley rats underwent the production of standard closed femoral fractures. The animals were then euthanized in groups of 15 on days 3, 7, 14, and 21 postfracture. A separate control group was also used for the harvesting of intact unfractured bone. At the time of euthanasia, calluses or bone specimens were explanted to organ culture and treated with either media alone or media containing the inducing agents lipopolysaccharide or concanavalin A. A titration of conditioned medium from these cultures was then added to factor‐dependent clonal cell lines that are known to be specifically responsive to interleukin‐1, interleukin‐6, granulocyte‐macrophage colony stimulating factor or macrophage‐colony stimulating factor. To confirm the identities of each of these cytokines, neutralizing antibody studies were performed. The results showed that interleukin‐1 is expressed at very low constitutive levels throughout the period of fracture healing but can be induced to high activities in the early inflammatory phase (day 3). Granulocyte‐macrophage colony stimulating factor showed no constitutive activity but could also be induced to high activities with lipopolysaccharide. The ability of these two cytokines to be induced declined progressively as fracture healing proceeded. Interleukin‐6 showed high constitutive activity early in the healing process (day 3), and treatment with inducing agent did not increase the activity of this cytokine at this timepoint. Lipopolysaccharide did increase interleukin‐6 activity in day 7 and 14 fracture calluses. Although macrophage‐colony stimulating factor is thought to be involved in a variety of metabolic bone conditions, it could not be detected or induced from any of the callus samples. Moreover, none of the samples of unfractured bone showed constitutive or inducible activities for any of these cytokines. A separate experiment in which calluses and samples of unfractured bone from similar cultures were examined histologically and tested for DNA or protein synthesis at two timepoints in the culture period (days 1 and 4) showed that tissue viability was maintained. Thus the inability to detect macrophage colony‐stimulating factor in fracture callus or any cytokine activity in unfractured bones was not due to cell death. These findings suggest that the occurrence of a fracture results in a cascade of events that includes the production of cytokines. The time‐dependent expression of specific cytokine activities, which are differentially regulated, supports the concept that fracture healing may be partially mediated by these factors.