In allergic asthma, an important health problem, disease is driven by allergen-specific Th2 immune responses. Differentiation of Th2 cells depends on their early interactions with antigen presenting cells, such as dendritic cells (DCs), and cytokines are crucial for this process. Osteopontin (Opn) was originally identified as an important cytokine for Th1 immunity and autoimmunity. Our group recently demonstrated that Opn is highly expressed in the lungs of asthmatic patients and of mice with Th2-mediated allergic airway inflammation. Our work revealed anti-allergic effects of Opn on airway disease during secondary pulmonary antigenic challenge mediated by regulation of DC subsets. In addition, intranasal administration of recombinant Opn during pulmonary exposure to the allergen protected mice from allergic airway disease suppressing all features of disease, recruitment of Th2 cells and allergen-specific Th2 responses. Our previous experiments, as well as preliminary studies presented in this proposal, point to an important novel immunoregulatory role for Opn in the Th2 setting. However, most aspects of the Opn-mediated immune mechanism of protection remain unclear. With this proposal, we aim at elucidating the immunoregulatory/protective mechanisms of Opn utilizing immunologic, molecular and genomic approaches as well as in vivo mouse models of allergic airway inflammation. We propose to investigate the mechanisms mediating Opn-effects on: (1) DC subsets and Treg cells that confer protection during pulmonary allergen challenge (2) recruitment and function of allergen-specific Th2 (generated during sensitization) as well as of newly-activated Th effector cells and their interactions during pulmonary allergen challenge and (3) antigenic tolerance induction in the Th2 setting. The studies proposed here will provide new insight into the biology of Opn-dependent regulation of DC subsets, Th2 responses and DC-T cell interactions opening new important questions in im