Abstract: Non-alcoholic steatohepatitis (NASH) is the fastest growing cause of liver cancer and liver transplant in the U.S. There is currently no approved treatment for NASH and the mechanistic understanding of why some patients with fatty liver maintain a benign condition while others progress to NASH is not clear. Macrophages play a pivotal role in NASH pathogenesis. Accumulating evidence suggest that ‘triggering receptor expressed on myeloid cells’ (TREM2) is overexpressed in NASH associated macrophages. Interestingly, even though TREM2 expression is increased during NASH, its absence exacerbates NASH and fibrosis development. This indicates that TREM2 is protective in NASH and fibrosis progression. However, the mechanism of TREM2 signaling remains poorly understood. Moreover, upon ligand engagement, TREM2 gets cleaved from the cell surface into a soluble form (sTREM2) that can be detected in the serum. We found that sTREM2 levels increase with NAFLD and NASH progression, however, the trans-signaling axis of sTREM2 is an area that has not been addressed in the field yet. sTREM2 has the ability to soak up inflammatory ligands such as LPS and lipids as well as bind to macrophages. However, whether sTREM2 can bind and influence downstream signaling in other cells remain unknown. We discovered that sTREM2 itself has NASH protective as well as therapeutic abilities. Introduction of sTREM2 fragment into mice that already had advanced NASH had profound effects on NASH and fibrosis resolution. The goal of this proposal is to better understand how TREM2 exerts its protective functions and dissect the intracellular and trans-signaling axes of TREM2. We will use mouse models of NASH and HCC that closely mimics diet induced metabolic syndrome and human NASH gene signature as well as primary cultures of both mouse and human liver cells in order to assess the following 2 Specific Aims: Aim 1 will identify the key intracellular TREM2 signaling pathways that exerts its protective actions in controlling NASH development. We will also identify the transcription factors responsible for TREM2 expression in NASH associated macrophages. Aim 2 will elucidate the trans-signaling properties of sTREM2 and whether sTREM2 is directly involved in disease progression. The proposed studies will provide key insights into the mechanisms of NASH and HCC development. The findings from this study will lay the foundation for future development and refinement of sTREM2 as a novel therapeutic agent for NASH-Fibrosis and HCC.