PROJECT SUMMARY/ABSTRACT Hematopoietic cell transplant (HCT) is an important treatment modality for patients with hematologic malignancy (HM). Its success depends on the ability of donor hematopoietic cells to establish long-term hematopoiesis and immunologically mediated elimination of residual malignant cells. Clonal hematopoiesis (CH) is an age-related condition in which detectable somatic mutations alter the biologic function and inflammatory output of hematopoietic cells, and in non-transplant populations is uniformly associated with adverse outcomes. By studying 1727 HCT donor-recipient pairs, I found that CH in donors is common and that inactivating mutations in the gene DNMT3A are the most frequent alterations. My preliminary data shows that donor DNMT3A-CH is associated with improved global recipient outcomes, mediated by a reduced risk of HM relapse, but also increases the risk of graft failure in a subset of recipients. Based on these data and the known function of DNMT3A in hematopoietic cells, I hypothesize that the effects of donor DNMT3A-CH I observe in HCT recipients are due to engraftment of DNMT3A-mutated long-term hematopoietic stem cells and subsequent altered function in mature DNMT3A-mutant leukocyte subsets, particularly T cells. To test this hypothesis, I propose the following two aims: (1) Determine the effect of DNMT3A-CH on normal and impaired hematopoiesis following HCT. I will use genomic, immunophenotypic, and single-cell technologies to define the characteristics, including cellular compartment, of donor DNMT3A mutations that engraft in recipients. I will then focus on elucidating the biology of graft failure developing in recipients of donor DNMT3A-CH. (2) Define the effect of DNMT3A-CH in donor- engrafted T-cells after transplantation. I will use genomic, immunophenotypic, and single-cell techniques to determine the effect of donor DNMT3A mutations on T-cell composition and function in recipients after transplantation, focusing specifically on how DNMT3A mutations modulate the development of T-cell exhaustion. I will then specifically assess how donor DNMT3A mutations in T cells affect the pathways of immune evasion utilized by relapsing cases of acute myeloid leukemia. The information gained from these studies will provide new insights into the biology of post-transplant hematopoiesis and immune surveillance that could have profound implications for donor selection and strategies to augment the graft-versus-leukemia effect. In concert with the proposed experiments, I have outlined a five-year career development plan aimed at the goal of becoming an independent investigator in translational transplant research. I have assembled an advisory committee of globally recognized experts in hematopoiesis, transplant immunology, and biostatistics, to provide experimental input and specific training in these fields. Dana-Farber Cancer Institute, which harbors an outstanding research community and has a long track record for...