Project Summary/Abstract Dental caries represents a common public health problem. Caries involves bacterial invasion, physicochemical dissolution and proteolysis of the mineral and protein components of teeth. Direct bacterial and byproduct interaction with dental pulp cells and odontoblasts in dentinal caries activates a protective repair process of ‘tertiary’ dentin formation. This process requires the recruitment and differentiation of dental pulp stem cells (DPSCs). Clinical therapies such as pulp capping aim to promote dentin regeneration and sustain pulp vitality and consequently endurance of the natural dentition. However, therapeutic dentin regeneration is elusive. The cellular and molecular mechanisms orchestrating dentin-pulp regeneration following infection are not fully elucidated, especially the role of inflammation on dentinogenesis. We have demonstrated that the complement system, which is an important mediator of inflammation and tissue regeneration, is activated in the caries process. A major role for complement and C5a binding to its receptor C5aR in responses to injury is well established. The C5a receptor- like 2 (C5L2) also participated in inflammatory reactions of several pathological conditions, yet, to date no investigation has explored the role of this enigmatic receptor in tissue regeneration and stem cell biology. Here, we propose a significant role for the complement system and C5L2 in DPSC odontoblastic differentiation and reparative dentin formation. Preliminary studies demonstrate that C5L2 expression by DPSCs is quickly increased during odontogenic differentiation, and this expression is potentiated by the inflammatory cytokine TNFα. Moreover, siRNA silencing of C5L2 expression in DPSCs significantly increases the expression of dentinogenic markers like DMP1 and DSPP during odontogenic differentiation. We provide further evidence that p38 map kinase (p38a) plays a key role in DPSC-mediated dentinogenesis. Here, we explore ways of enhancing this odontoblastic function of DPSCs via a novel C5L2 pathway involving p38a signaling. We will define the role of C5L2 in the odontoblastic differentiation of DPSC and characterize the mechanism of action of C5L2 during dentinogenesis. In vivo dentin formation will be evaluated using the mouse pulp-capping/caries model combined with the C5aR, C5L2 and DSPP/p38a knockout mice. The results obtained from this project will shed new light onto cellular and molecular events that orchestrate the initial steps of dentinogenesis by linking the inflammation to DPSC function through C5L2 and p38a pathways. These studies will provide the basis for future potential therapeutic interventions of dentin-pulp complex regeneration and vital tooth preservation.