Retarded bone regeneration is characteristic to periodontitis. Even after successful conventional periodontal treatment, periodontal bone regeneration rarely, if ever, occurs, while molecular mechanism underlying retarded bone regeneration is largely unknown. This RO1 application proposes to exploit the osteoimmunological roles of osteoclast (OC)-specific cell membrane receptor, Osteoclast Stimulatory Transmembrane Protein (OCSTAMP) and Dendritic Cell-Specific Transmembrane Protein (DC-STAMP) in retarded bone regeneration in periodontitis. During osteoclastic bone resorption, osteoblast (OB)-activation molecules, such as insulin-like growth factor (IGF), act as ?coupling? factors released from demineralized bone matrix to ensure that the same amount of bone resorbed by OC is replaced by differentiation and activity of OB. Strong evidence suggests that this coupling mechanism is interrupted (?uncoupled?) in periodontitis where pathogenic bone resorption exceeds reparative bone formation, resulting in retardation of bone regeneration. Our preliminary results showed that P. gingivalis may be engaged in retarded bone regeneration in periodontitis. A recent study reported that Semaphorin4D (Sema4D) produced by OC inhibits IGF-mediated osteogenesis by OB. The upstream molecular event(s) that induce(s)/upregulate(s) Sema4D expression by RANKL-activated osteoclast precursors (OCp), as well as the mechanism of Sema4D action on OB in the context of periodontitis, are unknown. We preliminary identify the ligand for OCSTAMP is produced by activated OCp, and the binding of this ligand with OCSTAMP elicits signals for Sema4D-expression. Furthermore, periodontal pathogen, P. gingivalis, appears to upregulate the Sema4D production from OCp by upregulating the generation of OCSTAMP ligand. Based on these preliminary findings and published evidence, we hypothesized that pathogenic activation of OCSTAMP by its ligand upregulate the production of Sema4D form OCp which, in turn, inhibits osteogenesis in periodontitis. To test our hypothesis, the following two Specific Aims are proposed. Aim 1: To elucidate the molecular mechanism underlying the generation of ligand for OC-STAMP, Aim 2: To assess the impact of OC-STAMP-activation on retarded bone regeneration in a mouse model of periodontitis induced by the combination of ligature attachment and P. gingivalis infection. This study will, for the first time, elucidate the pathologic osteoimmunological mechanism that interrupts new bone formation in alveolar bone affected by periodontitis, thus, representing a potential paradigm shift in the development of novel periodontitis therapies.