Our major goal is to develop new strategies to greatly reduce primary tumor burden while boosting immunity for the eradication of local and distal metastases. Established tumors form physical/immunological barriers that limit immune destruction. Our recent findings show that the therapeutic effect of large single dose RT targeted specifically to the primary tumor can break these tumor barriers by increasing CTL cross-priming and dramatically increasing antigen presentation, which lead to rejection of established tumors. The local RT-induced cross-priming of CTL can be further amplified by local tumor delivery of an adenovirus expressing LIGHT (Ad-LIGHT), TNF superfamily-14, which increases tumor infiltrating DCs (TIDCs) and T-cells (TIL) resulting in the clearance of metastases. Furthermore, the immune protection from metastasis can be erased by standard chemotherapy. Now our preliminary data indicate that such RT-induced tumor regression also depends on type I IFN and lymphotoxin beta receptor (LTR), a receptor for LIGHT and lymphotoxin (LT). We hypothesize that local RT induces endogenous danger signals and LTR signaling that together stimulate type I IFN production inside the tumor to recruit and functionally mature DCs for efficient priming of CTL and augmentation of the suppressive tumor microenvironment. In aim 1, we will investigate how RT controls CTLs. We will determine whether RT increases the priming of CTLs in the draining lymph node in a type I IFN dependent fashion and whether TIDCs equipped with cross-priming capacity are essential for the preservation of CTL effector function within the tumor microenvironment. In aim 2, we will determine whether and how type I IFN signaling controls TIDC. We will test whether type I IFN signaling augments the number and function of TIDCs. We will also study whether and how provision of exogenous IFN can recapitulate the effects of local RT on TIDC function. In aim 3, we will identify which cells produce type I IFN following RT. We will study how RT-mediated danger signals control type I IFN production and TIDC function. In aim 4, we will study the role of LTR signaling on TIDCs and type I IFN production for RT-mediated tumor reduction. We will test how type I IFN and LTb receptor pathways coordinate to generate functional TIDCs and mediate RT-induced tumor growth control. We will determine how local RT and immunotherapy with ad-LIGHT can synergistically link innate and adaptive immunity through endogenous danger signals and type I IFN production. Therefore, our study will reveal new mechanisms for RT-induced tumor regression and aid in the development of new strategies to initiate a cascade of innate and adaptive immunity.