Abstract FDA approved molecularly targeted therapies have not significantly reduced mortality in non-small cell lung cancer (NSCLC) patients. Therefore, identification and characterization of novel targets for developing effective therapies is warranted. In this proposal, we will develop endoplasmic reticulum (ER) sensor IRE1 as a potential therapeutic target in NSCLC. Adverse conditions in the tumor microenvironment (TME) can rapidly disrupt the protein folding capacity of the ER, thereby triggering a state of cellular “ER stress”. The ER stress response arm of the unfolded protein response, particularly the conserved IRE1-XBP1 pathway has emerged as a central orchestrator of malignant progression. Activated during periods of ER stress, the IRE1 RNAse domain cleaves its downstream target X-box binding protein (Xbp1) mRNA (inactive, XBP1u), converting it to active isoform (XBP1s), which serves as a functionally active transcription factor. We have determined that increased expression of XBP1s is associated with poor survival in NSCLC, cancer cell-intrinsic deletion of IRE1 delayed malignant progression and extended survival in mouse models of NSCLC. IRE1 deficiency triggered protective type-I IFN responses associated with marked reprogramming of both the lymphoid and myeloid cell subsets. These findings have led to the hypothesis that dysregulated IRE1-XBP1s signaling in cancer cells facilitates NSCLC progression by governing key immunomodulatory programs in the tumor microenvironment (TME). Therefore, understanding the underlying mechanisms has the potential to generate unique therapeutic strategies against difficult to treat NSCLC. We will determine mechanisms by which persistent activation of ER stress sensor IRE1 drives immunosuppression in the TME (Aim 1), determine if pharmacological inhibition of IRE1α sensitizes NSCLC to PARP inhibitors, and STING agonists by enhancing type-I IFN responses (Aim 2), and assess the clinical relevance of a novel IRE1 gene signature in predicting treatment outcomes in human NSCLC (Aim 3). This proposal is conceptually and technically innovative as it seeks to investigate for the first time how cancer cell-intrinsic IRE1α activation drives immunosuppression in the TME that facilitates immune evasion by disrupting cDC1 function and blunting type-I IFN responses. We expect that the mechanistic insights from these studies will generate unique translational opportunities that may lead to the design of future clinical trials with clinical grade IRE1 inhibitors.