RecruitingEarly Phase 1Drug trialPrevention of Graft Rejection in Hematopoietic Stem Cell Transplant (HSCT) RecipientsThe investigators hypothesize that graft rejection after hematopoietic stem cell transplant (HSCT) is primarily driven by interferon gamma, and prophylactic interferon gamma inhibition in high-risk patients will prevent graft rejection. Additionally, knowledge of emapalumab PK/PD and in vitro mechanistic effects of emapalumab in this novel setting will guide optimization of dosing regimens and treatment approaches in future studies.