Project Summary and Abstract Immunotherapy has achieved promising outcomes in cancer treatments; however, most patients still do not respond to the current therapies. There are several critical gaps in our current knowledge of what types of immune cells exist in the tumor microenvironment and how their presence (or absence) impacts the tumor immune surveillance. My long-term goal is to develop new cancer prevention and treatment approaches through dissecting the diverse immune cells and their roles in the tumor microenvironment. My preliminary studies have identified an enhanced antitumor effect for metastatic cancers by combining radiotherapy with the innate immune sensing pathway activation (the cGAS-cGAMP-STING pathway). This proposal aims to identify novel immune cell populations that contribute to the effective antitumor treatment strategy. The central hypothesis is that specific subsets of immune cells induced by this treatment strategy are the major factors controlling tumor treatment effects. Successful identification of these immune cell populations will improve our understanding of antitumor immunity and cancer immunotherapies. I will test the central hypothesis by pursuing two specific aims: 1) Characterize the immune cell heterogeneity in tumors in response to STING activation (F99 phase); 2) Elucidate the mechanism of abscopal antitumor effects achieved through the combination of cGAMP and radiotherapy (K00 phase). I will pursue these aims using an innovative combination of experimental and computational techniques. I will use unbiased single-cell transcriptome data analysis to identify novel cell populations critical for the tumor treatments and investigate their functions in mouse models. The proposed research is significant because the unbiased genomics data allows me to identify new mechanisms that may have a broad implication in other tumor types and aid in developing new therapeutic strategies. This work will also develop resources that can be used by other researchers in the cancer immunology field. The expected outcome of this work is: 1) identifying novel immune cell populations with antitumor function, 2) offering rational combination strategies to improve antitumor effects, 3) informing the design of cancer vaccines. This project will advance cancer immunology fields by providing new therapeutic strategies for metastatic cancers. In addition, the skills and expertise obtained from both phases will prepare me to become an independent researcher in cancer immunology.