Excessive bone resorption is frequently associated with chronic infections and inflammatory diseases. Whereas T cells were demonstrated to facilitate osteoclastogenesis in such diseases, the role of dendritic cells, the most potent activators of naive T cells, remains unclear. Using a model involving inflammation-driven alveolar bone loss attributable to infection, we showed that in vivo ablation of Langerhans cells (LCs) resulted in enhanced bone loss. An increased infiltration of B and T lymphocytes into the tissue surrounding the bone was observed in LC-ablated mice, including receptor activator of NF-κB ligand (RANKL)-expressing CD4 + T cells with known capabilities of altering bone homeostasis. In addition, the absence of LCs significantly reduced the numbers of CD4 + Foxp3 + T-regulatory cells in the tissue. Further investigation revealed that LCs were not directly involved in presenting antigens to T cells. Nevertheless, despite their low numbers in the tissue, the absence of LCs resulted in an elevated activation of CD4 + but not CD8 + T cells. This activation involved elevated production of IFN-γ but not IL-17 or IL-10 cytokines. Our data, thus, reveal a protective immunoregulatory role for LCs in inflammation-induced alveolar bone resorption, by inhibiting IFN-γ secretion and excessive activation of RANKL + CD4 + T cells with a capability of promoting osteoclastogenesis.

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Although oral dendritic cells (DCs) were shown to induce cell-mediated immunity, the identity and function of the various oral DC subsets involved in this process is unclear. In this study, we examined the mechanisms used by DCs of the buccal mucosa and of the lining mucosa to elicit immunity. After plasmid DNA immunization, buccally immunized mice generated robust local and systemic CD8+ T cell responses, whereas lower responses were seen by lining immunization. A delayed Ag presentation was monitored in vivo in both groups; yet, a more efficient presentation was mediated by buccal-derived DCs. Restricting transgene expression to CD11c+ cells resulted in diminished CD8+ T cell responses in both oral tissues, suggesting that immune induction is mediated mainly by cross-presentation. We then identified, in addition to the previously characterized Langerhans cells (LCs) and interstitial dendritic cells (iDCs), a third DC subset expressing the CD103+ molecule, which represents an uncharacterized subset of oral iDCs expressing the langerin receptor (Ln+iDCs). Using Langerin-DTR mice, we demonstrated that whereas LCs and Ln+iDCs were dispensable for T cell induction in lining-immunized mice, LCs were essential for optimal CD8+ T cell priming in the buccal mucosa. Buccal LCs, however, failed to directly present Ag to CD8+ T cells, an activity that was mediated by buccal iDCs and Ln+iDCs. Taken together, our findings suggest that the mechanisms engaged by oral DCs to prime T cells vary between oral mucosal tissues, thus emphasizing the complexity of the oral immune network. Furthermore, we found a novel regulatory role for buccal LCs in potentiating CD8+ T cell responses.