PROJECT SUMMARY/ABSTRACT The idiopathic inflammatory myopathies (IIMs) represent a group of systemic autoimmune disorders in which muscle and extra-muscular organs are targeted for immune-mediated destruction. We have previously established a model of histidyl-tRNA synthetase (HRS)-induced myositis that involves MyD88-dependent innate immune signaling pathways featuring Toll-like receptors 2 and 4 (TLR2, TLR4). Given the prominent role of these TLRs in our model of HRS-induced myositis, we hypothesize that heightened activation of the downstream transcription regulator NF-κB is ultimately responsible for various inflammatory cascades as well as non-immune pathways promoting muscle dysfunction in this system—effectively linking HRS-induced myositis/IIM with other disorders (including muscular dystrophy as well as sepsis- and trauma/ischemia- induced myopathies) in which NF-κB dysregulation contributes to muscle inflammation, muscle degeneration, and impaired regenerative potential. Through a series of in vitro culture systems and in vivo immunization strategies involving knockout mice lacking critical components of MyD88-dependent signaling pathways, we will systematically examine the impact of HRS-induced TLR signaling and NF-κB activation on T cell migration, T cell activation, and muscle weakness. While Specific Aim 1 will focus on HRS-induced changes in T cell function and TLR-mediated activation of vascular endothelium (leading to lymphocytic infiltration of target organs), Specific Aim 2 will define direct and indirect pathways of HRS-induced NF-κB activation in muscle tissue through in vitro myoblast stimulation assays as well as additional immunization studies focusing on correlations between muscle inflammation, NF-κB activation, and in vivo/ex vivo parameters of muscle weakness. Complementary in vivo assessment tools including MRI and the use of NF-κB-luciferase transgenic mice will further define the relationship between HRS-mediated NF-κB activation and muscle dysfunction, providing the foundation for experimental trials of comparative NF-κB inhibition in Specific Aim 3. Collectively, these studies will elucidate the contribution of innate immunity to the pathogenesis of IIM, supplementing more traditional paradigms of antigen-specific, adaptive immune recognition and identifying therapeutic targets that are potentially relevant to a range of human inflammatory muscle diseases.