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.

The occurrence of infections following allogeneic hematopoietic stem-cell transplant (HCT) is nearly universal. However, the relationship between infections and graft-versus-host disease (GVHD) is complex and attribution of infectious-related mortality is highly inconsistent, making comparison of infectious complication rates across allogeneic HCT clinical studies difficult. We categorized infectious complications from diagnosis or 1-year prior to HCT (whichever occurred later) through 2-years post-HCT according to timing, frequency, causative organism, severity, and contribution to mortality for 431 consecutive allogeneic HCT patients transplanted from 2008–2011. We then assessed the contribution of risk factors such as the frequency of pre-HCT infections and post-HCT GVHD on post-HCT infection frequency and severity. We found that each pre-HCT bacterial infection/year leads to an additional 2.15 post-HCT bacterial infection/year (p = 0.004). Pre-HCT viral and fungal infections were not predictors for post-HCT infections. Acute GVHD (aGVHD) significantly increased the risk of developing life-threatening (HR 1.97, 95% CI 1.33–2.90) and fatal (HR 2.8, 95% CI 1.10–7.08) infections. Furthermore, patients who develop aGVHD experienced ~60% more infections than patients who never develop aGVHD. Quantification of infection frequency and severity for patients with and without GVHD may facilitate comparison of infectious outcomes across allogeneic HCT trials.

Key Points• Grade 2 to 4 acute GVHD in URD HCT patients who received vorinostat and tacrolimus/methotrexate after myeloablative conditioning was 22%.• HDAC inhibition with vorinostat shows potential efficacy for GVHD prevention and should be investigated in a randomized phase 3 trial.The oral histone deacetylase (HDAC) inhibitor (vorinostat) is safe and results in low incidence of acute graft-versus-host disease (GVHD) after reduced-intensity conditioning, related donor hematopoietic cell transplantation (HCT). However, its safety and efficacy in preventing acute GVHD in settings of heightened clinical risk that use myeloablative conditioning, unrelated donor (URD), and methotrexate are not known. We conducted a prospective, phase 2 study in this higher-risk setting. We enrolled 37 patients to provide 80% power to detect a significant difference in grade 2 to 4 acute GVHD of 50% compared with a reduction in target to 28%. Eligibility included adults with a hematological malignancy to receive myeloablative HCT from an available 8/8-HLA matched URD. Patients received GVHD prophylaxis with tacrolimus and methotrexate. Vorinostat (100 mg twice daily) was started on day 210 and continued through day 1100 post-HCT. Median age was 56 years (range, 18-69 years), and 95% had acute myelogenous leukemia or high-risk myelodysplastic syndrome. Vorinostat was safe and tolerable. The cumulative incidence of grade 2 to 4 acute GVHD at day 100 was 22%, and for grade 3 to 4 it was 8%. The cumulative incidence of chronic GVHD was 29%; relapse, nonrelapse mortality, GVHD-free relapse-free survival, and overall survival at 1 year were 19%, 16%, 47%, and 76%, respectively. Correlative analyses showed enhanced histone (H3) acetylation in peripheral blood mononuclear cells and reduced interleukin 6 (P 5 .028) and GVHD biomarkers (Reg3, P 5 .041; ST2, P 5 .002) at day 30 post-HCT in vorinostat-treated subjects compared with similarly treated patients who did not receive vorinostat. Vorinostat for GVHD prevention is an effective strategy that should be confirmed in a randomized phase 3 study. This trial was registered at www.clinicaltrials.gov as #NCT01790568. (Blood. 2017;130(15):1760-1767

Corticosteroid resistance after acute graft-versus-host disease (SR-aGVHD) results in high morbidity and mortality after allogeneic hematopoietic cell transplantation. Current immunosuppressive therapies for SR-aGVHD provide marginal effectiveness because of poor response or excessive toxicity, primarily from infection. α-Antitrypsin (AAT), a naturally abundant serine protease inhibitor, is capable of suppressing experimental GVHD by downmodulating inflammation and increasing ratios of regulatory (T) to effector T cells (Ts). In this prospective multicenter clinical study, we sought to determine the safety and response rate of AAT administration in SR-aGVHD. Forty patients with a median age of 59 years received intravenous AAT twice weekly for 4 weeks as first-line treatment of SR-aGVHD. The primary end point was overall response rate (ORR), the proportion of patients with SR-aGVHD in complete (CR) or partial response by day 28 without addition of further immunosuppression. Treatment was well tolerated without drug-related adverse events. A significant increase in serum levels of AAT was observed after treatment. The ORR and CR rates by day 28 were 65% and 35%, respectively, and included responses in all aGVHD target organs. At day 60, responses were sustained in 73% of patients without intervening immunosuppression. Infectious mortality was 10% at 6 months and 2.5% within 30 days of last AAT infusion. Consistent with preclinical data, correlative samples showed an increase in ratio of activated Ts to Ts after AAT treatment. These data suggest that AAT is safe and may be potentially efficacious in treating SR-aGVHD. This trial was registered at www.clinicaltrials.gov as #NCT01700036.

Allogeneic HCT has been increasingly used in the setting of FLT3 mutated AML. However, its role in conferring durable relapse-free intervals remains in question. Herein, we sought to investigate FLT3 mutational status on transplant outcomes. We conducted a retrospective cohort study of 262 consecutive AML patients who underwent first-time allogeneic HCT (2008-2014), of whom 171 had undergone FLT3-ITD mutational testing. FLT3 mutated AML was associated with nearly twice the relapse risk (RR) compared with those without FLT3 mutation 3 years post-HCT (63% vs. 37%, P<0.001), and with a shorter median time to relapse (100 vs. 121 days). FLT3 mutational status remained significantly associated with this outcome after controlling for patient, disease, and transplant-related risk factors (P<0.05). Multivariate analysis showed a significant association of FLT3 mutation with increased 3-year RR (HR 3.63, 95% CI: 2.13, 6.19, P<0.001), and inferior disease-free survival (HR 2.05, 95% CI: 1.29, 3.27, P<0.01) and overall survival (HR 1.92, 95% CI: 1.14, 3.24, P<0.05). These data demonstrate high risk of early relapse after allogeneic HCT for FLT3 mutated AML that translates into adverse disease-free and overall survival outcomes. Additional targeted and coordinated interventions are needed to maintain durable remission after allogeneic HCT in this high-risk population.

The health of mammals is a reflection of the diversity and composition of the intestinal microbiota. Alterations in the composition and functions of the intestinal microbiota have been implicated in multiple disease processes. The impact of the microbiota in health and disease is in part a function of the nutrient processing and release of metabolites. Recent studies have uncovered a major role for microbial metabolites in the function of the host immune system by which they influence disease processes such as acute graft versus host disease (GVHD), which is the main complication of allogeneic hematopoietic cell transplantation (allo-HCT). The mechanisms of acute GVHD regulation by the complex microbial community and the metabolites released by them are unclear. In this review we summarize major findings of how microbial metabolites interact with the immune system and discuss how these interactions could impact acute GVHD.

Host NOD-like receptor family pyrin domain-containing 6 (NLRP6) regulates innate immune responses and gastrointestinal homeostasis. Its protective role in intestinal colitis and tumorigenesis is dependent on the host microbiome. Host innate immunity and microbial diversity also play a role in the severity of allogeneic immune-mediated gastrointestinal graft-versus-host disease (GVHD), the principal toxicity after allogeneic haematopoietic cell transplantation. Here, we examined the role of host NLRP6 in multiple murine models of allogeneic bone marrow transplantation. In contrast to its role in intestinal colitis, host NLRP6 aggravated gastrointestinal Reprints and permissions information is available at www.nature.com/reprints.

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