Periodontitis is one of the most common oral diseases resulting in gingival inflammation and tooth loss. Growing evidence indicates that it results from dysbiosis of the oral microbiome, which interferes with the host immune system, leading to bone destruction. Immune cells activate periodontal ligament cells to express the receptor activator of nuclear factor kappa-B (NF-κB) ligand (RANKL) and promote osteoclast activity. Osteocytes have active roles in periodontitis progression in the bone matrix. Local proteins are involved in bone regeneration through functional immunological plasticity. Here, we discuss the current knowledge of cellular and molecular mechanisms in periodontitis, the roles of local proteins, and promising synthetic compounds generating a periodontal regeneration effect. It is anticipated that this may lead to a better perception of periodontitis pathophysiology.

The macrolide erythromycin (ERM) inhibits excessive neutrophil accumulation and bone resorption in inflammatory tissues. We previously reported that the expression of developmental endothelial locus-1 (DEL-1), an endogenous anti-inflammatory factor induced by ERM, is involved in ERM action. Furthermore, DEL-1 is involved in the induction of bone regeneration. Therefore, in this study, we investigated whether ERM exerts an osteoblastogenic effect by upregulating DEL-1 under inflammatory conditions. We performed in vitro cell-based mechanistic analyses and used a model of Porphyromonas gingivalis lipopolysaccharide (LPS)-induced periodontitis to evaluate how ERM restores osteoblast activity. In vitro, P. gingivalis LPS stimulation suppressed osteoblast differentiation and bone formation. However, ERM treatment combined with P. gingivalis LPS stimulation upregulated osteoblast differentiation-related factors and Del1, indicating that osteoblast differentiation was restored. Alveolar bone resorption and gene expression were evaluated in a periodontitis model, and the results confirmed that ERM treatment increased DEL-1 expression and suppressed bone loss by increasing the expression of osteoblast-associated factors. In conclusion, ERM restores bone metabolism homeostasis in inflammatory environments possibly via the induction of DEL-1.

Pneumococcus is the main cause of bacterial pneumonia. Pneumococcal infection has been shown to cause elastase, an intracellular host defense factor, to leak from neutrophils. However, when neutrophil elastase (NE) leaks extracellularly, it can degrade host cell surface proteins such as epidermal growth factor receptor (EGFR) and potentially disrupt the alveolar epithelial barrier. In this study, we hypothesized that NE degrades the extracellular domain (ECD) of EGFR in alveolar epithelial cells and inhibits alveolar epithelial repair. Using SDS-PAGE, we showed that NE degraded the recombinant EGFR ECD and its ligand epidermal growth factor, and that the degradation of these proteins was counteracted by NE inhibitors. Furthermore, we confirmed the degradation by NE of EGFR expressed in alveolar epithelial cells in vitro. We showed that intracellular uptake of epidermal growth factor and EGFR signaling was downregulated in alveolar epithelial cells exposed to NE and found that cell proliferation was inhibited in these cells These negative effects of NE on cell proliferation were abolished by NE inhibitors. Finally, we confirmed the degradation of EGFR by NE in vivo. Fragments of EGFR ECD were detected in bronchoalveolar lavage fluid from pneumococcal pneumonia mice, and the percentage of cells positive for a cell proliferation marker Ki67 in lung tissue was reduced. In contrast, administration of an NE inhibitor decreased EGFR fragments in bronchoalveolar lavage fluid and increased the percentage of Ki67-positive cells. These findings suggest that degradation of EGFR by NE could inhibit the repair of alveolar epithelium and cause severe pneumonia.

Laminin comprises α/β/γ subunits and performs tissue-specific functions that control cellular behavior. Laminin-α2 chains are highly expressed in neural components such as glial and Schwann cells and in muscles. Macrophages play important roles in tissue homeostasis and repair, and laminins affect macrophage dynamics. Integrin α7, a transmembrane receptor crucial for regulating cell-matrix interactions, has a high affinity for laminin-α2, but its function in macrophages remains unknown. Here, we find that loss of integrin α7 signaling induces a dendritic cell (DC)-like phenotype in THP-1-derived macrophages and in primary monocytes-derived macrophages induced by granulocyte macrophage colony-stimulating factor cultured on laminin-α2 chains. Functional blocking of integrin α7 induced dendritic processes of THP-1-derived macrophages. Gene expression analysis revealed DC markers and costimulatory molecules, and coculture experiments demonstrated that the DC-like cells could stimulate T cell proliferation. Functional inhibition of integrin α7 decreased PI3K-p85α levels and activated PI3K, thereby activating AKT. Monocyte-derived macrophages cultured on laminin α2 chains decreased integrin α7 expression, exhibited dendritic-like morphology, and increased expression of DC markers and costimulatory molecules. These findings suggest that, besides the established influence of cytokine milieu, DC differentiation is regulated by laminin α2/integrin α7-mediated cell adhesion. Integrin α7 has been a therapeutic target in tumors, and antibody-based integrin α7 neutralization can be clinically useful. The results of this study suggest implications for integrin α7 and laminin-α2 chains in DC immunotherapy.

Erythromycin (ERM) has demonstrated therapeutic potential in promoting alveolar bone regeneration by enhancing developmental endothelial locus 1 (DEL-1) activity in periodontitis. Because this regenerative effect requires sustained local exposure at low, controlled doses, formulating ERM into microparticles can help deliver the drug directly to periodontal tissues while maintaining its therapeutic concentration over time. This study introduces a novel intrapalatal injection strategy for ERM-loaded microparticle delivery, to enhance bone regeneration and maintain periodontal health by activating endogenous DEL-1 signaling, supporting the potential of these formulations as pocket-injectable, bone-regenerative agents. ERM-loaded nano- and microparticles were prepared using biodegradable polymers-poly(lactic-co-glycolic acid) (PLGA) and poly(lactic acid) (PLA)-with varying lactic/glycolic ratios and molecular weights. Four formulations (SP0073-1 to SP0073-4) were produced by an oil-in-water emulsion solvent evaporation method, with ERM concentrations (12-16%). Ligature-induced periodontitis was created in 8-week-old male C57BL/6J mice, followed by a single intrapalatal injection of ERM formulations or blank particles. Mice were sacrificed after 9 or 21 days for histological evaluation using alkaline phosphatase and tartrate-resistant acid phosphatase staining. DEL-1 expression was assessed by immunofluorescence, and RNA-seq analysis of gingival tissue identified bone regeneration-related gene expression. The effects of ERM, blank particles, and ERM-loaded nano- and microparticles on osteogenic activity were assessed in MC3T3-E1, human periodontal ligament, and mouse bone marrow cells. Among ERM-loaded microparticles, SP0073-4 significantly upregulatedEdil3, Bglap2, Sp7, and Col5a1 expression, increased DEL-1 and osteocalcin expression in periodontal tissue, and preserved alveolar bone in young mice while promoting bone growth in aged mice. SP0073-4 enhanced osteoblast activity, reduced osteoclastogenesis, and increased mineralization compared with ERM alone. ERM-based microparticles (SP0073-4) promote bone regeneration and maintain periodontal integrity by activating DEL-1 signaling and modulating bone remodeling dynamics.