DESCRIPTION (provided by applicant): Periodontitis is an inflammatory disease that causes destruction of the tooth-supporting tissues (periodontium) and typically affects adults. However, individuals with disorders affecting neutrophil recruitment to the periodontium, such as leukocyte adhesion deficiency Type I (LAD-I), develop severe periodontitis early in life. Understanding the pathogenic mechanisms of rare monogenic diseases, such as LAD-I, is important for two reasons: First, for effective treatment of patients with these specific disorders; second, these disorders represent real-life models to understand human biology and provide critical insights into common diseases. Periodontitis associated with LAD-I has been historically attributed to impaired neutrophil surveillance of the periodontal infection; however, it is unresponsive to antibiotics and/or mechanical removal of the tooth- associated biofilm, leads to premature loss of primary and permanent teeth, and can have serious adverse psychological and functional consequences in affected children. The overall objective of this project is to identify the fundamental underlying mechanism(s) of periodontitis associated with LAD-I ("LAD-I periodontitis") and to propose rational novel treatment options. The overarching hypothesis is that LAD-I periodontitis is caused by dysregulated overexpression of the bone-resorptive cytokine IL-17 due to the disruption of a homeostatic mechanism known as the "neutrostat". This is a regulatory feedback loop involving the IL-23-IL-17 axis that coordinates neutrophil recruitment and efferocytosis in tissues with neutrophil production in the bone marrow. The overarching hypothesis is supported by ample preliminary evidence, e.g., data revealing profound elevation of IL-23 and IL-17 in human LAD-I patients and in relevant mouse models of LAD-I, whereas considerably lower levels of these cytokines are detected in adult-type chronic periodontitis. This proposal comprises four specific aims and focuses on mouse model-based mechanistic and intervention studies. In Aim 1, it is proposed that that reconstitution of LAD-I mice with transmigration-competent neutrophils restores normal IL-23 and IL-17 levels and inhibits bone loss. Aim 2 investigates whether inhibition of apoptotic neutrophil phagocytosis (efferocytosis) replicates the LAD-I periodontitis phenotype. Aim 3 is to determine whether synthetic agonist-induced activation of liver X receptor (normally activated in macrophages downstream of neutrophil efferocytosis) compensates for the lack of apoptotic neutrophils and restores IL- 23/IL-17 regulation in the gingiva of LAD-I mice. Aim 4 involves the development of novel therapeutic approaches to LAD-I periodontitis through the use of small-molecule compounds targeting the IL-23-IL-17 pathway. This application, therefore, offers a fundamentally new insight into the mechanism of LAD-I periodontitis (and perhaps other disorders with impaired neutrophil recru...