T cells, particularly CD4+ T cells, play a central role in both progression and control of periodontal disease, whereas the contribution of the various CD4+ T helper subsets to periodontal destruction remains controversial, the activation, and regulation of these cells is orchestrated by dendritic cells. As sentinels of the oral mucosa, dendritic cells encounter and capture oral microbes, then migrate to the lymph node where they regulate the differentiation of CD4+ T cells. It is thus clear that dendritic cells are of major importance in the course of periodontitis, as they hold the immunological cues delivered by the pathogen and the surrounding environment, allowing them to induce destructive immunity. In recent years, advanced immunological techniques and new mouse models have facilitated in vivo studies that have provided new insights into the developmental and functional aspects of dendritic cells. This progress has also benefited the characterization of oral dendritic cells, as well as to their function in periodontitis. Here, we provide an overview of the various gingival dendritic cell subsets and their distribution, while focusing on their role in periodontal bone loss.

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In vivo studies questioned the ability of Langerhans cells (LCs) to mediate CD8+ T cell priming. To address this issue, we used intradermal immunization with plasmid DNA, a system in which activation of CD8+ T cells depends on delayed kinetics of Ag presentation. We found that dendritic cells (DCs) located in the skin at the time of immunization have limited ability to activate CD8+ T cells. This activity was mediated by a second generation of DCs that differentiated in the skin several days after immunization, as well as by lymph node–resident DCs. Intriguingly, CD8+ T cell responses were not affected following treatment with clodronate liposomes, immunization of CCR2−/− mice, or local neutralization of CCL20. This suggests that local, rather than blood-derived, DC precursors mediate CD8+ T cell priming. Analysis of DC differentiation in the immunized skin revealed a gradual increase in the number of CD11c+ cells, which reached their maximum 2 wk after immunization. A similar differentiation kinetics was observed for LCs, with the majority of differentiating LCs proliferating in situ from epidermal precursors. By using B6/Langerin–diphtheria toxin receptor chimeric mice and LC ablation, we demonstrated that epidermal LCs were crucial for the elicitation of CD8+ T cell responses in vivo. Furthermore, LCs isolated from lymph nodes 2 wk after immunization contained the immunization plasmid and directly activated Ag-specific CD8+ T cells ex vivo. Thus, these results indicate that second-generation Ag-expressing LCs differentiating from epidermal precursors directly prime CD8+ T cells and are essential for optimal cellular immune responses following immunization with plasmid DNA.

Landschütz tumour cells in the ascitic form injected subcutaneously into BALB/c or ICR mice produce solid tumours which grow progressively in most ICR mice but regress in nearly all BALB/c mice. Solid tumours in the peritoneal wall (produced by intraperitoneal inoculation of ascitic cells and treatment with normal serum) grew in both strains, but were more invasive in ICR mice. Surgical interference in BALB/c mice with these tumours, allowing adhesion of tumour to skin or subcutaneous fascia, resulted in cessation of tumour growth or regression.