SUMMARY Background Graft-versus-host disease (GVHD) is the major cause of non-relapse mortality (NRM) after allogeneic hematopoietic stem-cell transplantation (HCT). The severity of symptoms at the onset of GVHD does not accurately define risk, and thus most patients are treated alike with high dose systemic corticosteroids. We aimed to define clinically meaningful risk strata for patients with newly diagnosed acute GVHD using plasma biomarkers. Methods We prospectively collected plasma from 492 HCT patients with newly diagnosed acute GVHD and randomly divided them into training (n=328) and test (n=164) sets. We used the concentrations of three recently validated biomarkers (TNFR1, ST2, and REG3α) to create an algorithm that computed the probability of NRM six months after GVHD onset for individual patients in the training set alone. We rank ordered the probabilities and identified thresholds that created three distinct NRM scores. We evaluated the algorithm in the testset, and again in an independent validation set of 300 additional HCT patients enrolled on multicenter clinical trials of primary therapy for acute GVHD. Findings In all three datasets, the cumulative incidence of twelve month NRM significantly increased as the GVHD score increased (8% [95% confidence interval (CI); 3%, 16%], 27% [95% CI; 20%%, 34%], and 46% [95% CI; 33%, 58%], for scores 1, 2 and 3 respectively in the multicenter validation set, p<0 · 0001). Conversely, the response rates to primary GVHD treatment decreased as the GVHD score increased (86%, 67%, and 46%, for scores 1, 2 and 3 respectively in the multicenter validation set, p<0 · 0001). Interpretation Biomarker-based scores can be used to guide risk-adapted therapy at the onset of acute GVHD.

Microbiome-derived metabolites influence intestinal homeostasis and regulate graft-versus-host disease (GVHD), but the molecular mechanisms remain unknown. Here we show the metabolite sensor G-protein-coupled receptor 43 (GPR43) is important for attenuation of gastrointestinal GVHD in multiple clinically relevant murine models. GPR43 is critical for the protective effects of short-chain fatty acids (SCFAs), butyrate and propionate. Increased severity of GVHD in the absence of GPR43 is not due to baseline differences in the endogenous microbiota of the hosts. We confirm the ability of microbiome-derived metabolites to reduce GVHD by several methods, including co-housing, antibiotic treatment, and administration of exogenous SCFAs. The GVHD protective effect of SCFAs requires GPR43-mediated ERK phosphorylation and activation of the NLRP3 inflammasome in non-hematopoietic target tissues of the host. These data provide insight into mechanisms of microbial metabolite-mediated protection of target tissues from the damage caused allogeneic T cells.

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