Bacterial infection secondary to caries, trauma or operative dental procedures results in pulpal necrosis, and ultimately in the formation of a periapical lesion with the destruction of bone. Cytokines, expressed by both infiltrating leukocytes and resident connective tissue cells, are produced in response to infection and other antigenic challenges to the host. Previous studies from this laboratory have shown that interleukin- 1alpha (IL-1alpha) is a key mediator of periapical bone destruction in vivo. In the proposed studies, we will extend this work to a determination of the regulation of the periapical bone resorptive response, using a recently-established model system in mice. The central hypotheses are that infection-stimulated bone resorption is regulated, both positively and negatively, by a network of cytokines and antagonists, and that these regulatory pathways can be manipulated therapeutically to reduce resorption in osteolytic diseases. Initial studies (Aim l) will characterize the cytokine network in pulp and periapical tissues during lesion development, at the level of gene and protein expression. Cytokines analyzed will include those with (1) bone resorptive (IL-1alpha, TNFalpha, IL-6), up-regulatory (IFNgamma, IL-12), down-regulatory (IL-4, IL-10, IL- 13), and antagonistic (IL-1 receptor antagonist, soluble type II IL-1 receptors) properties. Cytokines which comprise the network will be studied for their direct and synergistic effects on bone resorption, alone and in combination with IL-1a, using an in vitro organ culture assay (Aim 2). These studies will distinguish their regulatory effects from direct effects on bone resorption.
In Aim 3, the role of the key regulatory cytokine IL-4 will be established using genetically-engineered transgenic overexpressor and knockout mice. In complementary approaches, the effect of administration of exogenous IL-4, and neutralization of endogenous IL-4 with anticytokine antibodies will be determined, and compared to results with transgenic overexpressor and knockout animals, respectively. These studies will establish the role of IL-4 in periapical bone destruction, and will indicate whether these experimental approaches can serve as surrogates for genetically-engineered animals. These same methodologies will be used to determine the roles of other regulatory cytokines in Aim 4. The goals of these studies are to define the cytokine network which is activated in infection-stimulated periapical bone resorption, and to determine whether these regulatory modalities can be manipulated to reduce bone destruction in vivo. Ultimately, these strategies may have utility in preventing resorption in osteolytic diseases, including periodontitis, osteomyelitis, arthritis, osteoporosis, and bone-destructive malignancies.
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