Figure 2. Interaction of CD4⁺ T-cells and alveolar bone remodeling in the periodontium. After being challenged by periodontal pathogens through antigen processing and presentation (via APC-dendritic cells, macrophages, activated B-cells, etc.), these micro-organisms-specific periodontal CD4⁺ T-cells become activated and produce RANK-L molecules. RANK-L can activate osteoclasts (OC) directly to induce bone resorption and also induce the differentiation, survival, and activation of OC precursors. OPG, the natural decoy receptor of RANK-L produced by the stromal cells (i.e., osteoblasts, chondrocytes), can compete for the binding to RANK-L molecules, thereby counteracting the effects of RANK-L/RANK signaling on OC and OC precursors.