PROJECT SUMMARY. Despite advances in allogeneic hematopoietic cell transplantation (allo-HCT), transplant-related morbidity and mortality remain substantial barriers to improving clinical outcomes. A recent body of work introduced the concept that shifts in the composition and density of the human intestinal bacterial microbiota shapes allo-HCT outcomes, including overall survival, non-relapse mortality, acute graft-versus-host disease (GvH), and invasive bacterial infections. A major knowledge gap relates to the role of non-bacterial microbiota constituents in allo-HCT outcomes. We established a high-resolution bioinformatics pipeline to analyze the role of intestinal fungi, the mycobiota, in allo-HCT outcomes. In pilot studies, we discovered that intestinal fungal pathobionts, specifically Candida species, can dominate the microbiota and translocate across the intestinal mucosa to cause invasive fungal bloodstream infections. In a murine major antigen mismatch allo-HCT model we found that Candida intestinal colonization exacerbates transplant outcomes while drug-induced elimination of intestinal fungi from the gut ameliorates transplant outcomes, respectively. However, how the composition, diversity, and magnitude of the mycobiota relates to human allo-HCT outcomes, specifically for overall and non-relapse survival, the development of GvH, and transplant complications, remains unknown. Based on these findings, the central hypothesis of this proposal is that human allo-HCT induces shifts in the intestinal mycobiota (in density, composition, and diversity) and promotes states of fungal dysbiosis that predict the risk of acute GvH, and overall and non-relapse mortality. To test this hypothesis, we have assembled a 287 patient allo-HCT study cohort (transplanted between January 1, 2017 and December 31, 2018) that will be followed for two years. This hypothesis will be explored in the following aims. Aim 1 will define the magnitude, composition, diversity, and stability of the intestinal mycobiota during human allo-HCT and identify the impact of antifungal drugs on mycobiota dynamics. Aim 2 will define the relationship between the intestinal mycobiota and a broad range of clinical outcomes in allo-HCT. The proposed study will measure the dynamics of the intestinal mycobiota during allo-HCT, define states of dysbiosis linked to transplant-related and supportive care practices, and define the relationship between shifts in the mycobiota and transplant outcomes. Insight into the role of mycobiota in allo-HCT outcomes will open opportunities to test mycobiota-based or mycobiota- perturbing interventions for clinical benefit. This exploratory study relies on a unique patient biorepository and innovative methodologies in mycobiota analyses. Beyond allo-HCT, the results of our study may have the potential to be highly informative for understanding the impact of the mycobiota in other organ transplant and immune compromised patient populations.

PROJECT NARRATIVE. Allogeneic hematopoietic cell transplantation (allo-HCT) is a potentially curative therapy for hematologic disorders and malignancies, though graft versus host disease and transplant-related mortality remain significant threats to clinical outcomes. Recent studies have implicated changes in the intestinal bacterial microbiota during allo-HCT to clinical outcomes, though the contribution of non-bacterial constituents remains unknown. This proposal seeks to understand the dynamics of and changes in endogenous fungal communities, the mycobiota, that occur during allo-HCT and how these changes affect clinical outcomes.