This application is being submitted in response to the Notice of Special Interest (NOSI) identified as NOT- CA-21-083. Recently, radiation therapy (RT) has been combined with immune checkpoint inhibitors (ICIs) and other immunomodulatory agents in an effort to enhance the immune-mediated anti-tumor effects and multiple clinical trials are ongoing to test this combination. However, while designing these trials, dose of RT, fractionation, scheduling of the immunotherapy with respect to RT, the immunostimulatory and immunosuppressive potential of RT that would either lead to response or resistance are not considered. Moreover, influence of status of tumor, whether “cold” or “hot” on RT-mediated immune response and effect of RT on the immune cells in the normal tissues are not adequately addressed. However, different RT regimens would serve as distinct immunomodulatory ‘drugs’, generating diverse immune responses. Therefore, there is an urgent need to develop an immune radiation response indicator (iRRi) that will consider all these factors and thereby would allow an informed clinical trial design to maximize benefit and reduce the risk to patients, especially when hypofractionation with high doses per fraction are used. Based on the above, we hypothesize that specific RT characteristics will influence the intrinsic radiation sensitivity of various types of immune cells in normal tissues and the TME, leading to distinct immune modulatory effects, thereby altering the immune response. To test this hypothesis, we propose the following two Specific Aims: 1) To determine the intrinsic radiation sensitivity of different cell types of the immune compartment from normal and tumor tissues to various radiation doses and fractionation schedules; 2) To generate an iRRi after analysis of data in aim 1. RT can cause significant immunomodulation via multiple mechanisms such as by altering antigen presentation and immunogenic cell killing. Depending on the dose and fractionation, RT can augment immune activation or cause immune suppression. Hence, there is a need to develop an iRRi that based on the tumor characteristics and effects on immune cells in the TME and normal tissues would help in selection of a RT regimen that induces immune stimulatory effects. The results from this study will help in understanding the influence of radiation characteristics on immunomodulation of the TME and normal tissues and will generate a definitive iRRi that will be helpful in informing the design of future clinical trials on combined RT plus immunotherapy.