Modulation of allergic airway inflammation by the oral pathogen Porphyromonas gingivalis

Abstract

Accumulating evidence suggests that bacteria associated with periodontal disease may exert systemic immunomodulatory effects. Although the improvement in oral hygiene practices in recent decades correlates with the increased incidence of asthma in developed nations, it is not known whether diseases of the respiratory system might be influenced by the presence of oral pathogens. The present study sought to determine whether subcutaneous infection with the anaerobic oral pathogen Porphyromonas gingivalis exerts a regulatory effect on allergic airway inflammation. BALB/c mice sensitized and subsequently challenged with ovalbumin exhibited airway hyperresponsiveness to methacholine aerosol and increased airway inflammatory cell influx and Th2 cytokine (interleukin-4 [IL-4], IL-5, and IL-13) content relative to those in nonallergic controls. Airway inflammatory cell and cytokine contents were significantly reduced by establishment of a subcutaneous infection with P. gingivalis prior to allergen sensitization, whereas serum levels of ovalbumin-specific IgE and airway responsiveness were not altered. Conversely, subcutaneous infection initiated after allergen sensitization did not alter inflammatory end points but did reduce airway responsiveness in spite of increased serum IgE levels. These data provide the first direct evidence of a regulatory effect of an oral pathogen on allergic airway inflammation and responsiveness. Furthermore, a temporal importance of the establishment of infection relative to allergen sensitization is demonstrated for allergic outcomes.

FIG. 1.

Experimental groups and timeline of study. HK, heat killed; P.g., P. gingivalis; OVA, ovalbumin. Six-week-old female BALB/c mice were cage acclimated for 1 week prior to chamber implantation surgery and thus were approximately 7 weeks old at “day 0.”

FIG. 2.

Airway responsiveness to methacholine aerosol in allergic mice is reduced by establishment of infection with P. gingivalis after, but not prior to, sensitization to allergen. (A) Increased airway responsiveness in allergic mice was blunted when infection with P. gingivalis occurred after, but not prior to, sensitization to OVA. *, P < 0.05 versus infection only (n = 7 to 13 per group). (B) Calculated PC200 values for methacholine (y axis) were lowest for the allergic group. Infection with P. gingivalis prior to sensitization to OVA did not alter these values, but infection established after sensitization increased the PC200 value for allergic mice. *, P < 0.05 versus infection-only group and group receiving infection after allergic sensitization (n = 7 to 13 per group).

FIG. 3.

Histopathological evidence of airway inflammation is reduced in mice infected with P. gingivalis prior to but not after sensitization to allergen. (A) Representative histological sections (stained with hematoxylin and eosin) demonstrating areas of eosinophilic and lymphocytic inflammation in perivascular and peribronchiolar regions (depicted by arrows). These regions were less numerous and intense in mice infected before allergic sensitization. (B) Calculated histopathological scores revealed decreased inflammation in allergic mice infected with P. gingivalis prior to sensitization to OVA. *, P < 0.05 versus allergic group and group receiving infection after allergic sensitization (n = 9 to 16 per group).

FIG. 4.

BAL fluid inflammatory parameters and serum IgE levels in mice infected with P. gingivalis prior to or after sensitization to allergen. (A) Total cell and eosinophil populations in the airways of allergic mice were reduced when infection with P. gingivalis was established prior to sensitization to OVA. *, P < 0.05 versus allergic group and group receiving infection after allergic sensitization (n = 9 to 16 per group). (B) BAL fluid cytokine content in allergic mice was reduced when infection with P. gingivalis was established prior to sensitization to OVA. *, P < 0.05 versus allergic group and group receiving infection after allergic sensitization (n = 9 to 16 per group). (C) Serum IgE levels were higher in the infection-only group than in the allergic group, but this was not statistically significant. Infection initiated either before or after sensitization to OVA increased levels relative to those for the allergic group, with infection initiated after sensitization resulting in the highest levels. *, P < 0.05 versus allergic group; **, P < 0.05 versus all other groups (n = 9 to 15 per group). (D) OVA-specific IgE levels in serum were highest when infection was initiated after sensitization to OVA, whereas infection initiated prior to sensitization to OVA did not alter the level compared to that in the allergic group. *, P < 0.05 versus infection-only group; **, P < 0.05 versus all other groups (n = 10 to 15 per group).

FIG. 5.

Allergic airway inflammation is not altered by heat-killed P. gingivalis. BAL fluid cell counts (A) and cytokine levels (B) were not altered by intrachamber injection of heat-killed P. gingivalis either before or after sensitization to OVA (compare data to those in Fig. 3C and D) (n = 4 to 7 per group).