Gamma interferon (IFN-gamma) and IFN-gamma-inducing cytokines interleukin-12 (IL-12) and IL-18 do not augment infection-stimulated bone resorption in vivo

Abstract

Periapical granulomas are induced by bacterial infection of the dental pulp and result in destruction of the surrounding alveolar bone. In previous studies we have reported that the bone resorption in this model is primarily mediated by macrophage-expressed interleukin-1 (IL-1). The expression and activity of IL-1 is in turn modulated by a network of Th1 and Th2 regulatory cytokines. In the present study, the functional roles of the Th1 cytokine gamma interferon (IFN-gamma) and IFN-gamma-inducing cytokines IL-12 and IL-18 were determined in a murine model of periapical bone destruction. IL-12-/-, IL-18-/-, and IFN-gamma-/- mice were subjected to surgical pulp exposure and infection with a mixture of four endodontic pathogens, and bone destruction was determined by microcomputed tomography on day 21. The results indicated that all IL-12-/-, IL-18-/-, and IFN-gamma-/- mice had similar infection-stimulated bone resorption in vivo as wild-type control mice. Mice infused with recombinant IL-12 also had resorption similar to controls. IFN-gamma-/- mice exhibited significant elevations in IL-6, IL-10, IL-12, and tumor necrosis factor alpha in lesions compared to wild-type mice, but these modulations had no net effect on IL-1alpha levels. Recombinant IL-12, IL-18, and IFN-gamma individually failed to consistently modulate macrophage IL-1alpha production in vitro. We conclude that, at least individually, endogenous IL-12, IL-18, and IFN-gamma do not have a significant effect on the pathogenesis of infection-stimulated bone resorption in vivo, suggesting possible functional redundancy in proinflammatory pathways.

FIG. 1.

Effect of Th1 cytokine deficiency on periapical bone destruction. Hemi-mandibles were scanned by micro-CT on day 21, and the area of the normal periodontal ligament space or infection-stimulated bone resorption was measured. Open columns, normal periodontal ligament space; closed columns, periapical bone destruction; vertical bars, standard deviations. No significant differences were present in knockout mice versus wild-type controls.

FIG. 2.

Effect of IL-12 infusion on periapical bone destruction. The area of the normal periodontal ligament space or infection-stimulated bone resorption was determined by micro-CT. Open column, uninfected control with normal periodontal ligament space; closed columns, periapical bone destruction; vertical bars, standard deviations. No significant difference was observed between the IL-12- and PBS-infused groups.

FIG. 3.

Profile of cytokine expression in periapical inflammatory lesions. Cytokine concentrations in inflammatory tissues were determined by ELISA on day 21, and values were normalized to the weight of periapical tissue. Vertical bar, standard deviation by two-way fractorial analysis of variance. *, P < 0.05 for exposure effect; †, P < 0.05 for genotype effect.

FIG. 4.

Modulation of pathogen-stimulated macrophage IL-1α production by Th1 cytokines. Resident peritoneal macrophages were harvested from IL-12^−/−, IL-18^−/−, and IFNγ^−/− mice and challenged with bacterial pathogens in the presence or absence of recombinant Th1 cytokines. IL-1α concentrations were determined by ELISA, and values were normalized to the volume of culture supernatant. Each plot indicates the bacterial stimulant, and the x axis indicates the modulating cytokine. Vertical bar, standard deviation. Statistical differences were determined by Dunnett's two-tailed t test versus controls preincubated without cytokines. *, P < 0.01.