Macrophages Contribute to the Elimination of Porphyromonas gingivalis More Strongly Than Neutrophils in Vivo

We previously reported that the elimination of Porphyromonas gingivalis from Fusobacterium nucleatum-immunized mice was more rapid than that of P. gingivalis-immunized mice, and that antibodies against P. gingivalis had no effect on the killing of P. gingivalis by neutrophils and macrophages. It remains unclear which phagocytes are mainly involved in the killing of P. gingivalis. To elucidate the activation of neutrophils and/or macrophages in the two types of immunized mice, we compared the elimination of P. gingivalis in a neutrophil-dominant with that in a macrophage-dominant state. Both states were induced in the mouse peritoneal cavity by casein injection. In the macrophage-dominant state, the elimination was faster in F. nucleatum-immunized than in P. gingivalis-immunized mice. Peritoneal levels of both interferon-γ (IFN-γ) and nitric oxide significantly increased in F. nucleatum-immunized mice, whereas only the IFN-γ level rose in P. gingivalis-immunized mice. In the neutrophil-dominant state, elimination was comparable between the two types of immunized mice. The peritoneal IFN-γ level slightly increased and nitric oxide level did not change in this state. No differences in the total cell number and rate of neutrophils or macrophages to the total peritoneal exudate cells in each state were comparable between the two immunized mouse types. These results suggest that macrophages, rather than neutrophils, are activated in F. nucleatum-immunized mice, and that nitric oxide produced by macrophages is important for the killing of P. gingivalis.