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Background 16S rRNA gene-based studies report that allogeneic hematopoietic stem cell transplant (aHSCT) recipients with low bacterial diversity and certain bacteria close to engraftment have increased risk of developing acute graft-vs.-host disease (aGvHD). Using shotgun metagenomic data, we aim to confirm and extend these observations in a larger cohort. Methods Adult aHSCT recipients with stool samples collected days −30 to +100 relative to aHSCT, but prior to aGvHD, were included. One sample was selected per patient and time point: pre-aHSCT (T1), post-aHSCT pre-engraftment (T2). Complete ascertainment of aGvHD (grades ≥ 2) until day +100 was performed. Bacterial microbiome factors (α-diversity, gene richness and 16 a priori bacteria) and clinical factors were tested for associations with aGvHD across T1:2 in univariable models. Significant factors (P < 0.1) were included in a multivariable model. Results Of 147 aHSCT recipients, 35 developed aGvHD a median of 35 days (IQR 24–51) post-aHSCT. We found that higher gene richness was significantly associated with lower aGvHD risk in T2, but not T1, samples (OR 0.65 (95% CI 0.42–1.00), P = 0.04 vs. OR 1.14 (95% CI 0.67–1.94), P = 0.64 per doubling of unique genes). A decreased abundance of Akkermansia muciniphila, Proteobacteria, Blautia and Gammaproteobacteria was observed in those that developed aGvHD, again in T2 samples only (Figure 1). Among clinical factors, donor sex, donor/recipient (related/unrelated) and conditioning regimen (adjusted OR = 0.34 for non-myeloablative vs myeloablative (95% CI 0.15–0.77)) were significantly associated with aGvHD. Conditioning regimen was also strongly associated with microbiome changes; myeloablative recipients had lower gene richness and differences in bacterial abundance, including decreased abundance of aforementioned bacteria, compared with non-myeloablative recipients at T2 (Figures 2and 3). Conclusion Post-aHSCT pre-engraftment was a crucial timepoint where microbial changes, including lower gene richness and abundance of certain bacteria, were associated with development of aGvHD. Myeloablative regimes were also associated with both aGvHD and microbiome changes, suggesting that intense conditioning may affect aGvHD risk through perturbation of the gut microbiome. Disclosures All authors: No reported disclosures.

2569. The Gut Microbiome and Acute Graft vs. Host Disease Risk in Hematopoietic Stem Cell Transplantation Recipients