The leading causes of death following serious burn injury are sepsis and multiple system organ failure secondary to profound innate and adaptive immune dysfunction, though the controlling mechanism for the response is unknown. Major determinants of immune activity are the innate Toll-like receptor (TLR) and nucleotide binding oligomerization domain (NOD)-like receptor (NLR) families. These are expressed by many immune cells and their primary function is to sense microbial molecules leading to cytokine release or cell death. Our overall hypothesis is that impairment of both the innate and adaptive immune systems after burn injury is mediated directly by TLR and NLR signaling. The proposed experiments are designed to test this hypothesis during both immunopathogenic phases after burn injury and investigate innate receptor-blockade as a possible immunotherapeutic approach for clinically relevant issues facing burn patients. First, to determine the mechanism by which TLR and NLR signaling controls the innate immune system after injury: We will determine phenotypic and functional (cytokine and cell death) changes of MF and DC using an animal model of burn injury. We will correlate TLR and NLR expression with changes in their function during early and late after burn injury. We will then use TLR and NLR deficient mice to validate that the altered burn injury-mediated innate immune response is mediated through innate-receptor signaling. Second, to determine the mechanism by which innate signaling controls the adaptive immune response after burn injury: We will examine the effect of burn injury on the expression and activation of TLR and NLR on CD4+ and CD8+ T cells and relevant T cell subsets. We will correlate changes in TLR and NLR expression with the ability of T cells to elicit an adaptive immune response. To determine the relative importance of both direct (expression of innate receptors on T cells) and indirect (expression on MF or DC) innate receptor expression in mediating the adaptive response to burn injury, we will transfer T cells or MF and DC, deficient in innate signaling receptors, into burn or sham recipients and test T cell immune function and survival. Third, to demonstrate that innate receptors are critical for the dysfunctional immunologic response to burn injury in vivo: We will use mice deficient in TLR and NLR innate signaling to examine the role of innate receptors in mediating skin allograft rejection and preventing establishment of effective immune tolerance after burn. We will examine the importance of innate signaling pathways in mediating Pseudomonas aeruginosa susceptibility early and late after burn injury using mice deficient in TLR and NLR signaling.