Abstract Allogeneic hematopoietic stem cell transplantation (HSCT) is the only curative treatment for patients with high- risk acute myeloid leukemia (AML). However, HSCT is affected by graft-versus-host disease (GvHD) and graft- versus-leukemia (GvL) effects, both are mediated by donor T lymphocytes and significantly impact treatment success and thus overall outcome. AML patients commonly harbor FLT3/internal tandem duplication (FLT3-ITD), a mutation in the receptor tyrosine kinase FLT3 that is associated with poor prognosis. FLT3 targeted therapies have proven clinical benefit particularly when used in combinational approaches. Midostaurin (a kinase inhibitor) was recently approved for pre-transplant patients with FLT3-ITD in combination with standard therapy. In addition to their direct leukemia suppressive effects, FLT3 inhibitors activate leukemia antigen-specific T-cell responses. T-cell receptors (TCRs) are proteins expressed on the surface of T cells that recognize antigens presented by MHC molecules. We recently characterized the TCR repertoire in patients who underwent matched donor or haplo-cord HSCT. We demonstrated that GvHD and relapse (exclusive of each other) are associated with lower TCR repertoire diversity and expansion of certain T-cell clones. Our data suggest that individual variations in the immune repertoire significantly impact the clinical outcome in AML patients and underscore the need for comprehensive quantitative, functional, and mechanistic analyses of the TCR repertoire in a large cohort of AML patients. Here, we hypothesize: 1) TCR repertoire (diversity, clonal expansion, and V-segment utilization) affects clinical outcome (GvHD or relapse) and can therefore be used to identify GvL- and GvHD- associated clones; 2) Somatic mutations in leukemic cells (e.g., FLT3-ITD) affect the TCR repertoire and subsequent expansion of specific T-cell clones; and 3) FLT3 inhibitors (e.g., midostaurin) modulate the TCR repertoire and function and enhance GvL effects in patients undergoing HSCT. We will conduct a prospective longitudinal cohort study characterizing the TCR repertoire and mutational landscape of leukemia cells in ~250 patients (~ 60–80 with FLT3-ITD). GvHD or relapse will be predicted using a proportional hazards model for competing risks based on TCR repertoire characteristics. TCR sequences and somatic mutations will be analyzed using a structure based prediction algorithms we developed to predict candidate leukemia neoantigens and associated TCR clones. Neoantigens will be validated using in vitro and murine models. Finally, functional analyses will examine the effect of midostaurin on TCR repertoire and function. Our findings will establish the TCR repertoire as a useful tool for predicting clinical outcomes of HSCT and identify responsible TCR clones. The identification of TCR clones associated with the GvL effect against FLT3-ITD+ cells will facilitate the development of engineered T cells expressing GvL-associ...