Infection-stimulated infraosseus inflammation and bone destruction is increased in P-/E-selectin knockout mice

Abstract

Infections of the dental pulp commonly result in infraosseus inflammation and bone destruction. However, the role of phagocytic leucocytes in the pathogenesis of pulpal infections has been uncertain. In this work we used P/E-/- selectin-deficient mice, which lack rolling adhesion of leucocytes to endothelium and mimic the human syndrome, leucocyte adhesion deficiency II (LAD-II), to test the hypothesis that phagocytic leucocytes protect against pulpal infection and subsequent periapical infraosseus bone resorption. P/E-/- mice and P/E+/+ wild-type controls were subjected to surgical pulp exposure, and both groups were infected with a mixture of pulpal pathogens including Prevotella intermedia, Fusobacterium nucleatum, Peptostreptococcus micros and Streptococcus intermedius. Animals were killed after 20 days, and the extent of infraosseus bone destruction was quantified by histomorphometry. In two separate experiments, P/E-/- mice had significantly greater bone resorption than P/E+/+ controls. The increased bone destruction correlated with a twofold decrease in polymorphonuclear (PMN) infiltration into periapical inflammatory tissues of P/E-/- mice. P/E-/- mice had higher tissue levels of the bone resorptive cytokine, interleukin (IL)-1alpha. Tissue levels of IL-2, IL-4, IL-10, tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) were all higher in P/E-/- mice, but the increases were not statistically significant. Only IL-12 was higher in P/E+/+ mice, possibly reflecting a greater number of infiltrating monocytes in wild-type mice. These findings demonstrate that phagocytic leucocytes are protective in this model, and suggest that elevated expression of inflammatory cytokines is responsible for the observed bone destruction.

Figure 1

Periapical bone destruction in P/E^−/− and P/E^+/+ mice. Vertical bars: standard deviation. (a) Exp. 1, 20 days postinfection: wild type (n = 9); P/E^−/− (n = 7). (b) Exp. 2, 14 days postinfection: wild type (n = 5); P/E^−/− (n = 7). Statistical significance was determined by the Student’s t-test. **P < 0·0001; *P < 0·05.

Figure 2

Periapical bone destruction and polymorphonuclear (PMN) infiltration surrounding the distal root of the first molar in P/E^−/− and P/E^+/+ mice after 20 days. (a) Periapical region, non-infected control tooth from P/E^+/+ mice; (b) infected P/E^+/+ mice; note infiltrated granulation tissue and area of bone resorption (50× magnification); (c) higher magnification (400×) of infiltrated tissue from (b); (d) infected P/E^−/− mice; note decreased infiltrate and increased bone resorption; (e) higher magnification (400×) of infiltrated tissue from (d). B, bone; T, tooth root; P, periodontal ligament space.

Figure 3

Polymorphonuclear (PMN) infiltration of periapical granulomatous tissue in P/E^−/− and P/E^+/+ mice 20 days after pulp exposure and infection. P/E^−/−, n = 9; P/E^+/+, n = 7. HPF: high-power fields; vertical bars: standard deviation. *P < 0·05, P/E^−/− versus P/E^+/+ mice.

Figure 4

Periapical tissue cytokine concentrations in P/E^−/− and P/E^+/+ mice. Cytokine concentrations were measured by enzyme-linked immunosorbent assay (ELISA) on day 20 and values were normalized to the weight of periapical granuloma tissue. Shaded bar: P/E^−/− (n = 9); open bars: P/E^+/+ (n = 7). E, pulp exposure and infection; C, non-exposed, uninfected controls (n = 3 for both P/E^−/− and P/E^+/+). Vertical line: standard deviation. **P < 0·01; *P < 0·05.