PROJECT SUMMARY This study investigates the ‘reactive’ immune system in tumors, namely the cellular and molecular allies that allow immunotherapies to work. In the previous cycle of this funding, we discovered that one set of intratumoral dendritic cells type 1 (cDC1) present tumor antigens to T cells and that natural killer cells provide critical Flt3L to cDC1 cells within reactive tumor microenvironments. The overall hypothesis of this proposal is that reactive tumor immunity, based on CD8 T cells and those cDC1, has an archetype and a niche—a collection of cell types that organize and support it. By defining those cells, their associated genes and their relationships, we will be in a position to better create this niche and to thus engineer tumor clearance. This program is unique in applying spatial transcriptomics together with genetic tools and conventional cellular immunoassay methods to understand the critical phenotype-biology relationship between critical cDC populations and their partners. The resultant discoveries will be formative for designing new ways to boost anti- tumor immunity.