Severe trauma leads to the activation a systemic inflammatory response that when excessive, contributes to both end-organ dysfunction and immune dysregulation. The goal of this project is to define molecular mechanisms for the initiation and propagation of inflammation following severe injury. We hypothesize that tissue damage and/or ischemia leads to the release of endogenous molecules that then trigger inflammatory signaling through pattern recognition receptors of the innate immune system. We have compelling evidence that two pattern recognition receptors, toll-like receptors 4 and 9 (TLR4 and 9) and the nuclear protein and TLR4 ligand, high mobility group box-1 (HMGB1), play critical roles in initiating inflammation following injury. We will more fully characterize the function of TLR4 and 9 and HMGB1 in the injury response in three Aims. Under Aim 1, we will define the relative roles of TLR4 and TLR9 to the systemic and organ-specific responses. We will also use a TLR4-loxP mouse recently developed in our lab to define cell-type specific roles for TLR4. Under Aim II, we will use gene knockout mice to establish the major TLR-dependent signaling pathways involved in trauma-induced inflammatory response. Under Aim 111, we will define the cell types that mobilize and release HMGB1 following systemic injury and the mechanisms leading to HMGB1 release. To accomplish these aims, we will utilize our well-characterized mouse models of hemorrhagic shock and peripheral tissue trauma. It is expected that the completion of this work will both advance our understanding of how the immune system becomes activated after injury and define potential targets for therapeutic intervention to limit the excessive inflammation in severely injured patients. RELEVANCE (See instructions): Trauma is the most common cause of death and morbidity in peopl under the age of 50 in the USA. Much of the morbidity and mortality are due to an excessive inflammatory response. The research described in this proposal is aimed at understanding how trauma induces an inflammatory response at the molecular level with the long-term goal identifying strategies to modify this response and improve survival.