Langerhans cells down-regulate inflammation-driven alveolar bone loss

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.