Gingival lymphatic drainage protects against Porphyromonas gingivalis-induced bone loss in mice.

Periodontitis is characterized by tissue destruction and bone loss mainly due to inflammatory responses after bacterial challenge of the gingiva. Gingiva is supplied with lymphatics that drain interstitial fluid and transport immune cells to the lymph nodes for antigen presentation; yet, the role of lymphatics in periodontal disease development is unknown. To investigate the lymphatic function after periodontal infection, we used K14-VEGF receptor 3-Ig (K14) mice that lack lymphatics in gingiva. Mice were orally infected with human Porphyromonas gingivalis and observed for 42 days. The infected K14 mice developed significantly more bone loss than the wild-type mice, and were associated with an increased number of macrophages and major histocompatibility complex class II antigen-presenting cells in the bone resorptional areas. The infected transgenic mice expressed a significant higher periodontal level of several proinflammatory cytokines, whereas the plasma level of P. gingivalis IgG was significantly lower than in the wild-type mice. No differences were found in immune cell distribution in draining lymph nodes between the strains. Our results show that a strong periodontal inflammatory response and a weakened systemic humoral B-cell response took place in K14 mice after infection. We conclude that gingival lymphatics protect against P. gingivalis-induced periodontitis, and we speculate that they are critical in the protection by clearance of infection and by promotion of humoral immune responses.

Characterization of the dental lymphatic system and identification of cells immunopositive to specific lymphatic markers.

The lymphatic system is important for immune barrier function and for tissue fluid balance. During inflammation, lymphangiogenesis takes place to enhance the transport of filtered fluid, proteins, and immune cells. Dental tissue is frequently exposed to inflammatory insults, but the lymphatic system and its responses to injury have not been investigated in detail using specific lymphatic markers. We aimed to study this system and to establish whether lymphangiogenesis takes place during wound healing. Immunostaining of the lymphatic endothelial hyaluronan receptor-1 (LYVE-1) and vascular endothelial growth factor receptor-3 (VEGFR-3) demonstrated initial lymphatics in the coronal molar pulp, whereas in incisors the initial lymphatics were found only in the apical part. In molars, lymphatic vessels exit the pulp through the apex and lateral canals. In interdental bone, transverse lymphatics were found, raising the possibility that an infection can be spread from the periodontal ligament to a neighbouring tooth. LYVE-1(+) and VEGFR-3(+) immune cells were found in both molar and incisor pulps, and phenotyping of the cells showed that they are of a monocytic lineage. In inflamed pulp these cells were not observed. Macrophages are suggested to contribute directly to the formation of lymphatic vessels after pulp exposure.