The goals of Project 3 are to study the role of costimulatory molecules that drive the development of allergen-induced airway hyperreactivity (AHR) and asthma, and that regulate the development of immunological responses that protect against AHR and asthma. Our laboratory has extensive experience in the study of murine models of asthma, and has previously examined the specific cytokines and cell types involved in these processes. In addition, we have extensively studied the role of respiratory tolerance as an immunological mechanism that protects against the development of allergen induced AHR. These prior investigations provide the necessary groundwork and systems for the proposed studies. We will first assess the synergy between ICOS, PD-1 and OX40 costimulatory pathways in the development of allergen induced airway hyperreactivity. We propose to study the role of these costimulatory molecules in our established models of asthma using monoclonal antibodies and knockout animals already generated by the Program Cores. We will determine precisely how these costimulatory molecules function in asthma and whether they act at the initiation, at the effector stages of disease, or in the development of Th2 biased immune memory. In addition, we will examine the role of these molecules in the development of peripheral T cell tolerance induced by respiratory exposure to antigen. We recently showed, in studies performed in collaboration with Drs. Arlene Sharpe and Gordon Freeman, that the ICOS-ICOS-ligand pathway is critically involved in the development of T cell tolerance, regulatory T (TR)cells and immunological protection against asthma (Nature Medicine, 2002). We now propose to examine the role of OX40, PD-1 and CTLA-4 in this process and determine how these related pathways interact. We also propose to study after the first year of this project the role of novel costimulatory molecules identified by the Program Cores in the development of AHR and in the development of respiratory tolerance. These novel costimulatory molecules may either enhance or inhibit the development of asthma. Moreover, since Th2 and TRcells are related (both develop at mucosal sites, require ICOS signaling, and produce IL-4), we will define the specific costimulatory signals that drive the development of one but not the other. These studies will greatly enhance our understanding of asthma, and lead to new therapeutic strategies for asthma, targeting the allergic inflammatory response or the anti-inflammatory response (tolerance) in asthma.