Intestinal Microbiota Influence Immune Tolerance Post Allogeneic Hematopoietic Cell Transplantation and Intestinal GVHD

Metabolic instruction of the graft-versus-leukemia immunity

Allogeneic hematopoietic cell transplantation (allo-HCT) is frequently performed to cure hematological malignancies, such as acute myeloid leukemia (AML), through the graft-versus-leukemia (GVL) effect. In this immunological process, donor immune cells eliminate residual cancer cells in the patient and exert tumor control through immunosurveillance. However, GVL failure and subsequent leukemia relapse are frequent and associated with a dismal prognosis. A better understanding of the mechanisms underlying AML immune evasion is essential for developing novel therapeutic strategies to boost the GVL effect. Cellular metabolism has emerged as an essential regulator of survival and cell fate for both cancer and immune cells. Leukemia and T cells utilize specific metabolic programs, including the orchestrated use of glucose, amino acids, and fatty acids, to support their growth and function. Besides regulating cell-intrinsic processes, metabolism shapes the extracellular environment and plays an important role in cell-cell communication. This review focuses on recent advances in the understanding of how metabolism might affect the anti-leukemia immune response. First, we provide a general overview of the mechanisms of immune escape after allo-HCT and an introduction to leukemia and T cell metabolism. Further, we discuss how leukemia and myeloid cell metabolism contribute to an altered microenvironment that impairs T cell function. Next, we review the literature linking metabolic processes in AML cells with their inhibitory checkpoint ligand expression. Finally, we focus on recent findings concerning the role of systemic metabolism in sustained GVL efficacy. While the majority of evidence in the field still stems from basic and preclinical studies, we discuss translational findings and propose further avenues for bridging the gap between bench and bedside.

The distribution of intestinal flora after hematopoietic stem cell transplantation in children

BACKGROUND

This prospective study aimed to comprehensively understand the changes in intestinal flora at different stages after hematopoietic stem cell transplantation (HSCT) in pediatric patients and to analyze the effect of intestinal flora on acute graft versus host disease (aGVHD), especially on gastrointestinal graft versus host disease (GI GVHD).

METHODS

A total of 32 children with primary diseases of primary immunodeficiency disease (PID) and thalassemia were included. 16S sequencing was used to characterize the microbiota layout at three time points peri-transplant including pre-transplant, Day +3, and Day +30.

RESULTS

By comparing the intestinal flora of children with GI GVHD and those without GI GVHD, it suggests that in children with GI GVHD, the distribution of intestinal flora after transplantation was more variable and more chaotic (chao1 index, Friedman test, p = .029). Besides, Veillonella and Ruminococcaceae were more abundant before transplantation, Bifidobacteriaceae and Bacillales were more abundant after transplantation. Comparing children with PID and thalassemia, it was found that the destruction of gut microbiota diversity was more significant in children with thalassemia after transplantation. The comparison of children with 0-I° aGVHD and II-III° aGVHD indicates that children with II-III° aGVHD had more Bilophila before transplantation than children with 0-I° aGVHD. Additionally, exploratory analyses to evaluate correlations between clinical characteristics (medications, immune cell recovery, etc.) and microbiome features were also performed.

CONCLUSIONS

This study has synthetically shown the distribution of intestinal flora after allo-HSCT, and some characteristic bacteria at different stages that may serve as potential biomarkers were screened out additionally, perhaps providing clues for the prevention and treatment of the disease.

Oral and Gut Microbiota Dysbiosis is Associated with Mucositis Severity in Autologous Hematopoietic Stem Cell Transplantation: Evidence from an Asian Population

Mucositis is a debilitating complication of hematopoietic stem cell transplantation (HSCT). It is unclear how changes in the composition of microbiota, which are modulated by geographical location and ethnicity, may influence immune regulation leading to the development of mucositis, and the study of both oral and gut microbiota in a single population of autologous HSCT in the Asian region is lacking. The present study aimed to characterize the oral and gut microbiota changes, and the impact on both oral and lower gastrointestinal (GI) mucositis, with associated temporal changes in a population of adult recipients of autologous HSCT. Autologous HSCT recipients age ≥18 years were recruited from Hospital Ampang, Malaysia, between April 2019 and December 2020. Mucositis assessments were conducted daily, and blood, saliva, and fecal samples were collected prior to conditioning, on day 0, and at 7 days and 6 months post-transplantation. Longitudinal differences in alpha diversity and beta diversity were determined using the Wilcoxon signed-rank test and permutational multivariate analysis of variance, respectively. Changes in relative abundances of bacteria across time points were assessed using the microbiome multivariate analysis by linear models function. The combined longitudinal effects of clinical, inflammatory, and microbiota variables on mucositis severity were measured using the generalized estimating equation. Among the 96 patients analyzed, oral mucositis and diarrhea (representing lower GI mucositis) occurred in 58.3% and 95.8%, respectively. Alpha and beta diversities were significantly different between sample types (P < .001) and across time points, with alpha diversity reaching statistical significance at day 0 in fecal samples (P < .001) and at day +7 in saliva samples (P < .001). Diversities normalized to baseline by 6 months post-transplantation. Significant microbiota, clinical, and immunologic factors were associated with increasing mucositis grades. Increasing relative abundances of saliva Paludibacter, Leuconostoc, and Proteus were associated with higher oral mucositis grades, whereas increasing relative abundances of fecal Rothia and Parabacteroides were associated with higher GI mucositis grades. Meanwhile, increasing relative abundances of saliva Lactococcus and Acidaminococcus and fecal Bifidobacterium were associated with protective effects against worsening oral and GI mucositis grades, respectively. This study provides real-world evidence and insights into the dysbiosis of the microbiota in patients exposed to conditioning regimen during HSCT. Independent of clinical and immunologic factors, we demonstrated significant associations between relative bacteria abundances with the increasing severity of oral and lower GI mucositis. Our findings offer a potential rationale to consider the inclusion of preventive and restorative measures targeting oral and lower GI dysbiosis as interventional strategies to ameliorate mucositis outcome in HSCT recipients.

The Role of Fecal Microbiota Transplantation in the Allogeneic Stem Cell Transplant Setting

Microbiota changes during allogeneic hematopoietic stem cell transplantation has several known causes: conditioning chemotherapy and radiation, broad-spectrum antibiotic administration, modification in nutrition status and diet, and graft-versus-host disease. This article aims to review the current knowledge about the close link between microbiota and allogeneic stem cell transplantation setting. The PubMed search engine was used to perform this review. We analyzed data on microbiota dysbiosis related to the above-mentioned affecting factors. We also looked at treatments aimed at modifying gut dysbiosis and applications of fecal microbiota transplantation in the allogeneic stem cell transplant field, with particular interest in fecal microbiota transplantation for graft-versus-host disease (GvHD), multidrug-resistant and clostridium difficile infections, and microbiota restoration after chemotherapy and antibiotic therapy.

Gut microbiota and their metabolite profiles following peripheral nerve xenotransplantation

BACKGROUND

Intestinal pathogens are associated with xenotransplantation tolerance and rejection. However, changes in the gut microbiota in patients who have undergone peripheral nerve xenotransplantation and their association with immune rejection have not yet been reported.

OBJECTIVE

We aimed to explore intestinal microbes and their metabolites at different time points after peripheral nerve transplantation to provide new insight into improving transplant tolerance.

METHODS

A peripheral nerve xenotransplantation model was constructed by suturing the segmented nerves of Sprague Dawley rats to those of C57 male mice using xenotransplantation nerve bridging. Fecal samples and intestinal contents were collected at three time points: before surgery (Pre group; n = 10), 1 month after transplantation (Pos1 m group; n = 10), and 3 months after transplantation (Pos3 m group; n = 10) for 16S DNA sequencing and nontargeted metabolome detection.

RESULTS

Alpha diversity results suggested that species diversity was significantly downregulated after peripheral nerve xenotransplantation. There were six gut flora genera with significantly different expression levels after xenotransplantation: four were downregulated and two were upregulated. A comparison of the Pre vs. Pos1 m groups and the Pos1 m vs. Pos3 m groups revealed that the most significant differentially expressed Kyoto Encyclopedia of Genes and Genomes metabolite pathways were involved in phenylalanine, tyrosine, and tryptophan biosynthesis, as well as histidine metabolism. Metabolites with a strong relationship to the differentially expressed microbial flora were identified.

CONCLUSION

Our study found lower gut microbiome diversity, with increased short-chain fatty acid (SCFA)-producing and sulfate-reducing bacteria at 1 month post peripheral nerve xenotransplantation, and these were decreased at 3 months post-transplantation. The identification of specific bacterial metabolites is essential for recognizing potential diagnostic markers of xenotransplantation rejection or characterizing therapeutic targets to prevent post-transplant infection.

Impact of Allogeneic Hematopoietic Stem Cell Transplantation on Nutritional Status and Intake in Children

OBJECTIVES

This study aimed to describe the impact of allogeneic/haploidentical hematopoietic stem cell transplantation on nutritional status and intake in a group of children aged 2 to 18 years.

METHODS

In an observational study, data were collected prospectively. Patients were prescribed individual nutritional support by hospital routines. Anthropometrics were measured pre-transplant at hospital admission and weekly from the day of transplant (day 0) until day +28. z scores for weight, height, and BMI were calculated using Norwegian growth references to assess nutritional status. Pre-transplant diet was assessed on the day of hospitalization. Nutrient provision from enteral nutrition (EN = oral and tube) and parenteral nutrition (PN) was assessed by daily records from day +1 until day +28, or previous discharge, and compared with recommendations (RI) from the Nordic Nutrition Recommendations and ESPGHAN guidelines. Total energy intake was presented as the percentage (%) of basal metabolic rate (BMR) calculated by the Schofield equation. Macro- and micronutrient provisions were presented as medians (interquartile range) and the % of RI.

RESULTS

Twenty-eight patients, mean age 10.3 years (range 3.5-16.6), were included. Two-thirds (n = 18) had malignant diseases. At admission, mean weight Z-score was -0.3, height z scores -0.7, and BMI Z-score 0.1. Eighteen percent (n = 5) were stunted and 25% (n = 7) had overweight. At admission, 25% (n = 7) had established tube feeding, and 7% (n = 2) also had PN. No significant changes in weight z scores were detected during the studied weeks ( P = 0.454). The median daily energy provision was 115% (110-123) of BMR and proteins 1.5 (1.3-1.8) g/kg. EN was provided during a median of 93% of the studied days and provided 21% of the energy. PN was given on a median of 96% of the studied days and provided 79% of energy. RI for vitamins, magnesium, and zinc was met. Provision of copper, iodine, selenium, calcium, and phosphate was below RI.

CONCLUSIONS

Combined EN and PN providing 115% of BMR and 1.5 g/kg protein ensured stable weight by day +28 and covered RI, except for trace elements and minerals.

Outcome and Determinants of Neutropenic Enterocolitis in Pediatric Cancer Patients

BACKGROUND

Neutropenic enterocolitis (NEC) is a dreaded complication of chemotherapy. There is scant literature regarding incidence, clinical features, and determinants. The understanding of gut dysbiosis in NEC and pediatric cancer is evolving.

METHODS

Pediatric cancer patients with neutropenia and gastrointestinal symptoms were evaluated for NEC with contrast-enhanced computed tomography abdomen. Clinical, imaging, and laboratory features were analyzed. Fecal samples were analyzed for fecal calprotectin by sandwich enzyme-linked immunoassay and gut microbiota by conventional culture and compared with healthy controls and children without NEC.

RESULTS

NEC was diagnosed in 44 children based on clinical and imaging features with incidence of 7.4% (4 had recurrent episodes). Common manifestations included fever (98%), pain abdomen (88%), and diarrhea (83%). Hypoalbuminemia was observed in 78% of patients. Large bowel involvement (94%) with diffuse bowel involvement (63%) and pancolitis (64%) were common. Fecal calprotectin was significantly elevated in NEC group than non-NEC group and healthy controls (median: 87, 53, and 42 µg/g, respectively). A higher degree of gut dysbiosis was observed in children with NEC with higher isolation of Bacteroides and infrequent isolation of Lactobacilli. Mortality rate of 23% was observed. Only the presence of free fluid predicted higher mortality. Though levels of fecal calprotectin and gut dysbiosis were higher in NEC, they did not increase mortality. Isolation of Bacteroides and absence of Lactobacilli predicted a longer duration of intravenous alimentation.

CONCLUSIONS

NEC caused significant morbidity and mortality in pediatric cancer patients. Gut dysbiosis was significantly higher in NEC group suggesting a role in pathogenesis and influencing outcome. This highlights the role of targeted interventions towards gut dysbiosis like prebiotics and probiotics.

A Promising Insight: The Potential Influence and Therapeutic Value of the Gut Microbiota in GI GVHD

Allogeneic hematopoietic cell transplantation (allo-HSCT) is a reconstruction process of hematopoietic and immune functions that can be curative in patients with hematologic malignancies, but it carries risks of graft-versus-host disease (GVHD), thrombotic microangiopathy (TMA), Epstein–Barr virus (EBV) infection, cytomegalovirus infection, secondary hemophagocytic lymphohistiocytosis (sHLH), macrophage activation syndrome (MAS), bronchiolitis obliterans, and posterior reversible encephalopathy syndrome (PRES). Gastrointestinal graft-versus-host disease (GI GVHD), a common complication of allo-HSCT, is one of the leading causes of transplant-related death because of its high treatment difficulty, which is affected by preimplantation, antibiotic use, dietary changes, and intestinal inflammation. At present, human trials and animal studies have proven that a decrease in intestinal bacterial diversity is associated with the occurrence of GI GVHD. Metabolites produced by intestinal bacteria, such as lipopolysaccharides, short-chain fatty acids, and secondary bile acids, can affect the development of GVHD through direct or indirect interactions with immune cells. The targeted damage of GVHD on intestinal stem cells (ISCs) and Paneth cells results in intestinal dysbiosis or dysbacteriosis. Based on the effect of microbiota metabolites on the gastrointestinal tract, the clinical treatment of GI GVHD can be further optimized. In this review, we describe the mechanisms of GI GVHD and the damage it causes to intestinal cells and we summarize recent studies on the relationship between intestinal microbiota and GVHD in the gastrointestinal tract, highlighting the role of intestinal microbiota metabolites in GI GVHD. We hope to elucidate strategies for immunomodulatory combined microbiota targeting in the clinical treatment of GI GVHD.

Intestinal Microbiome in Hematopoietic Stem Cell Transplantation For Autoimmune Diseases: Considerations and Perspectives on Behalf of Autoimmune Diseases Working Party (ADWP) of the EBMT

Over the past decades, hematopoietic stem cell transplantation (HSCT) has been evolving as specific treatment for patients with severe and refractory autoimmune diseases (ADs), where mechanistic studies have provided evidence for a profound immune renewal facilitating the observed beneficial responses. The intestinal microbiome plays an important role in host physiology including shaping the immune repertoire. The relationships between intestinal microbiota composition and outcomes after HSCT for hematologic diseases have been identified, particularly for predicting the mortality from infectious and non-infectious causes. Furthermore, therapeutic manipulations of the gut microbiota, such as fecal microbiota transplant (FMT), have emerged as promising therapeutic approaches for restoring the functional and anatomical integrity of the intestinal microbiota post-transplantation. Although changes in the intestinal microbiome have been linked to various ADs, studies investigating the effect of intestinal dysbiosis on HSCT outcomes for ADs are scarce and require further attention. Herein, we describe some of the landmark microbiome studies in HSCT recipients and patients with chronic ADs, and discuss the challenges and opportunities of microbiome research for diagnostic and therapeutic purposes in the context of HSCT for ADs.

Crosstalk Between Intestinal Microbiota Derived Metabolites and Tissues in Allogeneic Hematopoietic Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an evidence based- cellular immunotherapy for hematological malignancies. Immune reactions not only promote graft-versus-tumor effects that kill hematological malignant cells but also graft-versus-host disease (GVHD) that is the primary complication characterized by systemic organ damages consisting of T-cells and antigen presenting cells (APCs) activation. GVHD has long been recognized as an immunological reaction that requires an immunosuppressive treatment targeting immune cells. However immune suppression cannot always prevent GVHD or effectively treat it once it has developed. Recent studies using high-throughput sequencing technology investigated the impact of microbial flora on GVHD and provided profound insights of the mechanism of GVHD other than immune cells. Allo-HSCT affects the intestinal microbiota and microbiome-metabolome axis that can alter intestinal homeostasis and the severity of experimental GVHD. This axis can potentially be manipulated via dietary intervention or metabolites produced by intestinal bacteria affected post-allo-HSCT. In this review, we discuss the mechanism of experimental GVHD regulation by the complex microbial community-metabolites-host tissue axis. Furthermore, we summarize the major findings of microbiome-based immunotherapeutic approaches that protect tissues from experimental GVHD. Understanding the complex relationships between gut microbiota-metabolites-host tissues axis provides crucial insight into the pathogenesis of GVHD and advances the development of new therapeutic approaches.

Safety and Efficacy of Fecal Microbiota Transplantation for Grade IV Steroid Refractory GI-GvHD Patients: Interim Results From FMT2017002 Trial

Gastrointestinal (GI) tract graft-versus-host disease (GvHD) is a major cause of post-allo-HSCT (hematopoietic stem cell transplantation) morbidity and mortality. Patients with steroid-refractory GI-GvHD have a poor prognosis and limited therapeutic options. FMT2017002 trial (#NCT03148743) was a non-randomized, open-label, phase I/II clinical study of FMT for treating patients with grade IV steroid-refractory GI-GvHD. A total of 55 patients with steroid-refractory GI-GvHD were enrolled in this study. Forty-one patients with grade IV steroid-refractory GI-GvHD were included in the final statistical analysis. Of them, 23 patients and 18 patients were assigned to the FMT group and the control group, respectively. On days 14 and 21 after FMT, clinical remission was significantly greater in the FMT group than in the control group. Within a follow-up period of 90 days, the FMT group showed a better overall survival (OS). At the end of the study, the median survival time was >539 days in the FMT group and 107 days in the control group (HR=3.51; 95% CI, 1.21–10.17; p=0.021). Both the event-free survival time (EFS) (HR=2.3, 95% CI, 0.99–5.4; p=0.08) and OS (HR=4.4, 95% CI, 1.5–13.04; p=0.008) were higher in the FMT group during the follow-up period. Overall, the mortality rate was lower in the FMT group (HR=3.97; 95% CI, 1.34–11.75; p=0.013). No differences in the occurrence of any other side effects were observed. Our data suggest that the diversity of the intestinal microbiota could be affected by allo-HSCT. Although its effectiveness and safety need further evaluation, FMT may serve as a therapeutic option for grade IV steroid-refractory GI-GvHD.

National Institutes of Health Consensus Development Project on Criteria for Clinical Trials in Chronic Graft-versus-Host Disease: IV. The 2020 Highly morbid forms report

Chronic graft-versus-host disease (GVHD) can be associated with significant morbidity, in part because of nonreversible fibrosis, which impacts physical functioning (eye, skin, lung manifestations) and mortality (lung, gastrointestinal manifestations). Progress in preventing severe morbidity and mortality associated with chronic GVHD is limited by a complex and incompletely understood disease biology and a lack of prognostic biomarkers. Likewise, treatment advances for highly morbid manifestations remain hindered by the absence of effective organ-specific approaches targeting "irreversible" fibrotic sequelae and difficulties in conducting clinical trials in a heterogeneous disease with small patient numbers. The purpose of this document is to identify current gaps, to outline a roadmap of research goals for highly morbid forms of chronic GVHD including advanced skin sclerosis, fasciitis, lung, ocular and gastrointestinal involvement, and to propose strategies for effective trial design. The working group made the following recommendations: (1) Phenotype chronic GVHD clinically and biologically in future cohorts, to describe the incidence, prognostic factors, mechanisms of organ damage, and clinical evolution of highly morbid conditions including long-term effects in children; (2) Conduct longitudinal multicenter studies with common definitions and research sample collections; (3) Develop new approaches for early identification and treatment of highly morbid forms of chronic GVHD, especially biologically targeted treatments, with a special focus on fibrotic changes; and (4) Establish primary endpoints for clinical trials addressing each highly morbid manifestation in relationship to the time point of intervention (early versus late). Alternative endpoints, such as lack of progression and improvement in physical functioning or quality of life, may be suitable for clinical trials in patients with highly morbid manifestations. Finally, new approaches for objective response assessment and exploration of novel trial designs for small populations are required.

Prolonged suppression of butyrate producing bacteria is associated with acute gastrointestinal graft-versus-host disease and transplant related mortality after allogeneic stem cell transplantation

BACKGROUND

Butyrogenic bacteria play an important role in gut microbiome homeostasis and intestinal epithelial integrity. Previous studies have demonstrated an association between administration of short chain fatty acids like butyrate and protection from acute graft-versus-host disease (aGvHD) after allogeneic stem cell transplantation (ASCT).

METHODS

Here we examined the abundance and butyrogenic capacity of butyrate producing bacteria in 28 healthy donors and 201 patients after ASCT. We prospectively collected serial stool samples and performed PCR analysis of the butyrate producing bacterial enzyme butyryl-CoA:acetate CoA-transferase (BCoAT) in fecal nucleic acid extracts.

RESULTS

Our data demonstrate a strong and prolonged suppression of butyrogenic bacteria early in the course of ASCT. In a multivariable analysis, early use of broad-spectrum antibiotics before day 0 (d 0, day of transplantation) was identified as independent factor associated with low BCoAT copies (odds ratio 0.370 (0.175-0.783), p=0.009). Diminished butyrogens correlated with other biomarkers of microbial diversity such as low 3-indoxyl sulfate (3-IS) levels, reduced abundance of Clostridiales and low inverse Simpson and effective Shannon indices (p<0.001, respectively). Low BCoAT copies at GvHD-onset correlated with GI-GvHD severity (p=0.002) and were associated with significantly higher GvHD associated mortality (p=0.040). Furthermore, low BCoAT copies at d 30 were associated with significantly higher transplant related mortality (p=0.017).

CONCLUSIONS

Our results are consistent with the hypothesis that alterations in the microbiome play an important role in GvHD pathogenesis and that microbial parameters such as BCoAT might serve as biomarkers to identify patients at high risk for developing lethal GI-GvHD.

Is it time to reconsider prophylactic antimicrobial use for hematopoietic stem cell transplantation? a narrative review of antimicrobials in stem cell transplantation

INTRODUCTION

Hematopoietic Stem Cell Transplantation (HSCT) is a life-saving procedure for multiple types of hematological cancer, autoimmune diseases, and genetic-linked metabolic diseases in humans. Recipients of HSCT transplant are at high risk of microbial infections that significantly correlate with the presence of graft-versus-host disease (GVHD) and the degree of immunosuppression. Infection in HSCT patients is a leading cause of life-threatening complications and mortality.

AREAS COVERED

This review covers issues pertinent to infection in the HSCT patient, including bacterial and viral infection; strategies to reduce GVHD; infection patterns; resistance and treatment options; adverse drug reactions to antimicrobials, problems of antimicrobial resistance; perturbation of the microbiome; the role of prebiotics, probiotics, and antimicrobial peptides. We highlight potential strategies to minimize the use of antimicrobials.

EXPERT OPINION

Measures to control infection and its transmission remain significant HSCT management policy and planning issues. Transplant centers need to consider carefully prophylactic use of antimicrobials for neutropenic patients. The judicious use of appropriate antimicrobials remains a crucial part of the treatment protocol. However, antimicrobials' adverse effects cause microbiome diversity and dysbiosis and have been shown to increase morbidity and mortality.

Eosinophilic gastroenteritis and graft-versus-host disease induced by transmission of Norovirus with fecal microbiota transplant

Fecal microbiota transplantation (FMT) was performed to decolonize gastrointestinal tract from antibiotic-resistant bacteria before allogeneic hematopoietic cells transplantation (alloHCT). AlloHCT was complicated by norovirus gastroenteritis, acute graft-versus-host disease, and eosinophilic pancolitis. Norovirus was identified in samples from FMT material. Symptoms resolved after steroids course and second norovirus-free FMT from another donor.

A single strain of Bacteroides fragilis protects gut integrity and reduces GVHD

Graft-versus-host disease (GVHD) is a pathological process caused by an exaggerated donor lymphocyte response to host antigens after allogeneic hematopoietic cell transplantation (allo-HCT). Donor T cells undergo extensive clonal expansion and differentiation, which culminate in damage to recipient target organs. Damage to the gastrointestinal tract is a main contributor to morbidity and mortality. The loss of diversity among intestinal bacteria caused by pretransplant conditioning regimens leads to an outgrowth of opportunistic pathogens and exacerbated GVHD after allo-HCT. Using murine models of allo-HCT, we found that an increase of Bacteroides in the intestinal microbiota of the recipients was associated with reduced GVHD in mice given fecal microbial transplantation. Administration of Bacteroides fragilis through oral gavage increased gut microbiota diversity and beneficial commensal bacteria and significantly ameliorated acute and chronic GVHD development. Preservation of gut integrity following B. fragilis exposure was likely attributed to increased short chain fatty acids, IL-22, and regulatory T cells, which in turn improved gut tight junction integrity and reduced inflammatory cytokine production of pathogenic T cells. The current study provides a proof of concept that a single strain of commensal bacteria can be a safe and effective means to protect gut integrity and ameliorate GVHD after allo-HCT.

Microbiome-Derived Metabolites in Allogeneic Hematopoietic Stem Cell Transplantation

The gut microbiome has emerged as a major character in the context of hematopoietic stem cell transplantation. The biology underpinning this relationship is still to be defined. Recently, mounting evidence has suggested a role for microbiome-derived metabolites in mediating crosstalk between intestinal microbial communities and the host. Some of these metabolites, such as fiber-derived short-chain fatty acids or amino acid-derived compounds, were found to have a role also in the transplant setting. New interesting data have been published on this topic, posing a new intriguing perspective on comprehension and treatment. This review provides an updated comprehensive overview of the available evidence in the field of gut microbiome-derived metabolites and hematopoietic stem cell transplantation.

Gut Microbiota Influence in Hematological Malignancies: From Genesis to Cure

Hematological malignancies, including multiple myeloma, lymphoma, and leukemia, are a heterogeneous group of neoplasms that affect the blood, bone marrow, and lymph nodes. They originate from uncontrolled growth of hematopoietic and lymphoid cells from different stages in their maturation/differentiation and account for 6.5% of all cancers around the world. During the last decade, it has been proven that the gut microbiota, more specifically the gastrointestinal commensal bacteria, is implicated in the genesis and progression of many diseases. The immune-modulating effects of the human microbiota extend well beyond the gut, mostly through the small molecules they produce. This review aims to summarize the current knowledge of the role of the microbiota in modulating the immune system, its role in hematological malignancies, and its influence on different therapies for these diseases, including autologous and allogeneic stem cell transplantation, chemotherapy, and chimeric antigen receptor T cells.

Current Concepts and Advances in Graft-Versus-Host Disease Immunology

Worldwide, each year over 30,000 patients undergo an allogeneic hema-topoietic stem cell transplantation with the intent to cure high-risk hematologic malignancy, immunodeficiency, metabolic disease, or a life-threatening bone marrow failure syndrome. Despite substantial advances in donor selection and conditioning regimens and greater availability of allograft sources, transplant recipients still endure the morbidity and mortality of graft-versus-host disease (GVHD). Herein, we identify key aspects of acute and chronic GVHD pathophysiology, including host/donor cell effectors, gut dysbiosis, immune system and cytokine imbalance, and the interface between inflammation and tissue fibrosis. In particular, we also summarize the translational application of this heightened understanding of immune dysregulation in the design of novel therapies to prevent and treat GVHD.

Current Prophylaxis and Treatment Approaches for Acute Graft-Versus-Host Disease in Haematopoietic Stem Cell Transplantation for Children With Acute Lymphoblastic Leukaemia

Acute graft-versus-host disease (aGvHD) continues to be a leading cause of morbidity and mortality following allogeneic haematopoietic stem cell transplantation (HSCT). However, higher event-free survival (EFS) was observed in patients with acute lymphoblastic leukaemia (ALL) and grade II aGvHD vs. patients with no or grade I GvHD in the randomised, controlled, open-label, international, multicentre Phase III For Omitting Radiation Under Majority age (FORUM) trial. This finding suggests that moderate-severity aGvHD is associated with a graft-versus-leukaemia effect which protects against leukaemia recurrence. In order to optimise the benefits of HSCT for leukaemia patients, reduction of non-relapse mortality-which is predominantly caused by severe GvHD-is of utmost importance. Herein, we review contemporary prophylaxis and treatment options for aGvHD in children with ALL and the key challenges of aGvHD management, focusing on maintaining the graft-versus-leukaemia effect without increasing the severity of GvHD.

Targeting the gut microbiome: An emerging trend in hematopoietic stem cell transplantation

Mounting evidence has demonstrated the critical role of the gut microbiome in different cancer treatment modalities showing intensive crosstalk between microbiota and the host immune system. In cancer patients receiving hematopoietic stem cell transplantation (HSCT), conditioning regimens including chemotherapy, radiotherapy, and immunosuppressive therapy, as well as antimicrobial prophylaxis, result in intestinal barrier disruption and massive changes in microbiota composition. According to clinical studies, a drastic loss of microbial diversity during HSCT is associated with enhanced pro-inflammatory immune response and an increased risk of transplant-related complications such as graft-versus-host disease (GvHD) and mortality. In this review, we outline the current understanding of the role of microbiota diversity in the patient response to cancer therapies and highlight the impact of changes in the gut microbiome on clinical outcomes in post-HSCT patients. Moreover, the therapeutic implications of microbiota modulation by probiotics, prebiotics, and fecal microbiota transplantation (FMT) in hematologic cancer patients receiving HSCT are discussed.

Broad-Spectrum Antibiotics and Risk of Graft-versus-Host Disease in Pediatric Patients Undergoing Transplantation for Acute Leukemia: Association of Carbapenem Use with the Risk of Acute Graft-versus-Host Disease

Variation in the gastrointestinal (GI) microbiota after hematopoietic cell transplantation (HCT) has been associated with acute graft-versus-host disease (aGVHD). Because antibiotics induce dysbiosis, we examined the association of broad-spectrum antibiotics with subsequent aGVHD risk in pediatric patients undergoing HCT for acute leukemia. We performed a retrospective analysis in a dataset merged from 2 sources: (1) the Center for International Blood and Marrow Transplant Research, an observational transplantation registry, and (2) the Pediatric Health Information Services, an administrative database from freestanding children's hospitals. We captured exposure to 3 classes of antibiotics used for empiric treatment of febrile neutropenia: (1) broad-spectrum cephalosporins, (2) antipseudomonal penicillins, and (3) carbapenems. The primary outcome was grade II-IV aGVHD; secondary outcomes were grade III-IV aGVHD and lower GI GVHD. The adjusted logistic regression model (full cohort) and time-to-event analysis (subcohort) included transplantation characteristics, GVHD risk factors, and adjunctive antibiotic exposures as covariates. The full cohort included 2550 patients at 36 centers; the subcohort included 1174 patients. In adjusted models, carbapenems were associated with an increased risk of grade II-IV aGVHD in the full cohort (adjusted odds ratio [aOR], 1.24; 95% confidence interval [CI], 1.02 to 1.51) and subcohort (sub hazard ratio [HR], 1.31; 95% CI, 0.99 to 1.72), as well as with an increased risk of grade III-IV aGVHD (subHR, 1.77; 95% CI, 1.25 to 2.52). Early carbapenem exposure (before day 0) especially impacted aGVHD risk. For antipseudomonal penicillins, the associations with aGVHD were in the direction of increased risk but were not statistically significant. There was no identified association between broad-spectrum cephalosporins and aGVHD. Carbapenems, more than other broad-spectrum antibiotics, should be used judiciously in pediatric HCT recipients to minimize aGVHD risk. Further research is needed to clarify the mechanism underlying this association.

Microbiome-based interventions: therapeutic strategies in cancer immunotherapy

The composition of the commensal microbiota has recently emerged as a key element influencing the efficacy of cancer treatments. It has become apparent that the interplay between the microbiome and immune system within the host influences the response to immunotherapy, particularly immune checkpoint inhibitor therapy. Identifying the key components of the gut microbiota that influence this response is paramount for designing therapeutic interventions to enhance the response to cancer therapy. This review will discuss strategies being considered to modulate the gut microbiota, including fecal microbiota transplantation, administration of defined bacterial isolates as well as bacterial consortia, supplementation with probiotics, and lifestyle modifications such as dietary changes. Understanding the influence of the complex variables of the human microbiota on the effectiveness of cancer therapy will help drive the clinical design of microbial-based interventions in the field of oncology.

Patterns of infection and infectious-related mortality in patients receiving post-transplant high dose cyclophosphamide as graft-versus-host-disease prophylaxis: impact of HLA donor matching

Post-transplant cyclophosphamide (PTCy) has become a promising option after allo-SCT, but infections may be more common than in traditional protocols. We herein report 117 consecutive adults who received PTCy-based alloSCT in our hospital: HaploSCT (34%), MRD (19%), and VUD (47%), respectively. The 18-month incidence of severe bacterial, viral, and IFI was 56%, 69%, and 8.7%, without differences between donor type, except for CMV infection and viral hemorrhagic cystitis, which had a higher incidence in the haploSCT cohort (58% vs. 43% and 30% vs. 8% on day +90, p < 0.05). Late infections by conventional respiratory viruses were common in all groups [33/87 (38%)]. The 2-year survival was 72% and did not differ by donor type. IRM at day 30, day 100, and 18 months was 1.7%, 4.4%, and 12%, without differences by donor type (p = 0.7). The primary cause of IRM was bacterial infection (42%). Grade 2–4 acute GvHD was the only independent predictor of IRM. Donor type had no impact on IRM or on survival. In our study, severe infections were common in all donor types using PTCy, with higher rates of early post-engraftment CMV-I and viral HC in haploSCT recipients, although lethal infections were uncommon and similar in all donor types.

Allogenic stem cell transplant-associated acute graft versus host disease: a computational drug discovery text mining approach using oral and gut microbiome signatures

PURPOSE

Acute graft versus host disease (aGVHD) is a major cause of non-relapse morbidity and mortality post-allogenic hematopoietic stem cell transplant (HSCT). Using conventional literature search and computational approaches, our objective was to identify oral and gut bacterial species associated with aGVHD, potentially affecting drug treatment via lipopolysaccharide (LPS) pathways.

METHODS

Medline, PubMed, PubMed Central, and Google Scholar were searched using MeSH terms. The top 100 hits per database were curated, and 25 research articles were selected to examine oral and gut microbiomes associated with health, HSCT, and aGVHD. Literature search validation, aGVHD drug targets, and microbial metabolic pathway identification were completed using BioReader, MACADAM, KEGG, and STRING programs.

RESULTS

Our review determined that (1) oral genera Rothia, Solobacterium, and Veillonella were identified in HSCT patients' stool and associated with aGVHD; (2) shifts in gut enterococci profiles were determined in HSCT-associated aGVHD; (3) gut microbiome dysbiosis prior or during HSCT and lower Shannon diversity index at time of HSCT were also associated with increased risk of aGVHD and transplant related death; and (4) Coriobacteriaceae family was negatively correlated with gut aGVHD, whereas Eubacterium limosum was associated with decreased risk of chronic GVHD relapse. Additionally, we identified molecular pathways related to TLR4/ LPS, including candidate aGVHD drug targets, impacted by oral and gut bacterial taxa.

CONCLUSION

Reduced microbial diversity reflects higher severity and mortality rate in HSCT patients with aGVHD. Multi-omics approaches to decipher oral and gut microbiome associations will be critical for developing aGVHD preventive therapies.

How to Make an Immune System and a Foreign Host Quickly Cohabit in Peace? The Challenge of Acute Graft-Versus-Host Disease Prevention After Allogeneic Hematopoietic Cell Transplantation

Allogeneic hematopoietic cell transplantation (alloHCT) has been used as cellular immunotherapy against hematological cancers for more than six decades. Its therapeutic efficacy relies on the cytoreductive effects of the conditioning regimen but also on potent graft-versus-tumor (GVT) reactions mediated by donor-derived immune cells. However, beneficial GVT effects may be counterbalanced by acute GVHD (aGVHD), a systemic syndrome in which donor immune cells attack healthy tissues of the recipient, resulting in severe inflammatory lesions mainly of the skin, gut, and liver. Despite standard prophylaxis regimens, aGVHD still occurs in approximately 20–50% of alloHCT recipients and remains a leading cause of transplant-related mortality. Over the past two decades, advances in the understanding its pathophysiology have helped to redefine aGVHD reactions and clinical presentations as well as developing novel strategies to optimize its prevention. In this review, we provide a brief overview of current knowledge on aGVHD immunopathology and discuss current approaches and novel strategies being developed and evaluated in clinical trials for aGVHD prevention. Optimal prophylaxis of aGVHD would prevent the development of clinically significant aGVHD, while preserving sufficient immune responsiveness to maintain beneficial GVT effects and immune defenses against pathogens.

Pre-transplant hepatic steatosis (fatty liver) is associated with chronic graft-vs-host disease but not mortality

Allogeneic-HCT (allo-HCT), while potentially curative, can result in significant complications including graft versus host disease (GVHD). Prior studies suggest that metabolic syndrome may be one risk factor for GVHD. We hypothesized that hepatic steatosis on pre-HCT computed tomography (CT) scans may be a marker for development of GVHD and poor outcomes in allo-HCT. In this retrospective study, we reviewed the pre-HCT CT scans and transplant outcome data of patients who underwent allo-HCT at Duke University Medical Center from 2009 to 2017. The presence of steatosis was confirmed using CT attenuation measurements. We then assessed the association between pre-HCT hepatic steatosis and HCT-related outcomes including GVHD. 80 patients who had pre-HCT CT scans were included in the study. Pre-transplant hepatic steatosis was associated with the development of chronic GVHD (OR 4.2, p = 0.02), but was not associated with acute GVHD (OR 1.3, p = 0.7), non-relapse mortality (p = 0.81) or overall survival (p = 0.74). Based on this single center retrospective study, pre-transplant hepatic steatosis is associated with development of chronic GVHD. Further, prospective study with other imaging modalities including non-contrasted CT scans is needed to determine if this association is reproducible.

Treating From the Inside Out: Relevance of Fecal Microbiota Transplantation to Counteract Gut Damage in GVHD and HIV Infection

The gastrointestinal (GI) tract is a complex and well-balanced milieu of anatomic and immunological barriers. The epithelial surface of the GI tract is colonized by trillions of microorganisms, known as the gut microbiota, which is considered an “organ” with distinctive endocrine and immunoregulatory functions. Dysregulation of the gut microbiota composition, termed dysbiosis, has been associated with epithelial damage and translocation of microbial products into the circulating blood. Dysbiosis, increased gut permeability and chronic inflammation play a major role on the clinical outcome of inflammatory bowel diseases, graft-vs.-host disease (GVHD) and HIV infection. In this review, we focus on GVHD and HIV infection, conditions sharing gut immune damage leading to dysbiosis. The degree of dysbiosis and level of epithelial gut damage predict poor clinical outcome in both conditions. Emerging interventions are therefore warranted to promote gut microbiota homeostasis and improve intestinal barrier function. Interventions such as anti-inflammatory medications, and probiotics have toxicity and/or limited transitory effects, justifying innovative approaches. Fecal microbiota transplantation (FMT) is one such approach where fecal microorganisms are transferred from healthy donors into the GI tract of the recipient to restore microbiota composition in patients with Clostridium difficile-induced colitis or inflammatory bowel diseases. Preliminary findings point toward a beneficial effect of FMT to improve GVHD and HIV-related outcomes through the engraftment of beneficial donor bacteria, notably those producing anti-inflammatory metabolites. Herein, we critically review the potential for FMT in alleviating dysbiosis and gut damage in patients with GVHD or HIV-infection. Understanding the underlying mechanism by which FMT restores gut function will pave the way toward novel scalable and targeted interventions.

The Butyrogenic and Lactic Bacteria of the Gut Microbiota Determine the Outcome of Allogenic Hematopoietic Cell Transplant

Graft versus host disease (GVHD) is a post-transplant pathology in which donor-derived T cells present in the Peyer's patches target the cell-surface alloantigens of the recipient, causing host tissue damages. Therefore, the GVHD has long been considered only a purely immunological process whose prevention requires an immunosuppressive treatment. However, since the early 2010s, the impact of gut microbiota on GVHD has received increased attention. Both a surprising fall in gut microbiota diversity and a shift toward Enterobacteriaceae were described in this disease. Recently, unexpected results were reported that further link GVHD with changes in bacterial composition in the gut and disruption of intestinal epithelial tight junctions leading to abnormal intestinal barrier permeability. Patients receiving allogenic hematopoietic stem cell transplant (allo-HCT) as treatment of hematologic malignancies showed a decrease of the overall diversity of the gut microbiota that affects Clostridia and Blautia spp. and a predominance of lactic acid bacteria (LAB) of the Enterococcus genus, in particular the lactose auxotroph Enterococcus faecium. The reduced microbiota diversity (likely including Actinobacteria, such as Bifidobacterium adolescentis that cross feed butyrogenic bacteria) deprives the butyrogenic bacteria (such as Roseburia intestinalis or Eubacterium) of their capacity to metabolize acetate to butyrate. Indeed, administration of butyrate protects against the GVHD. Here, we review the data highlighting the possible link between GVHD and lactase defect, accumulation of lactose in the gut lumen, reduction of Reg3 antimicrobial peptides, narrower enzyme equipment of bacteria that predominate post-transplant, proliferation of En. faecium that use lactose as metabolic fuels, induction of innate and adaptive immune response against these bacteria which maintains an inflammatory process, elevated expression of myosin light chain kinase 210 (MLCK210) and subsequent disruption of intestinal barrier, and translocation of microbial products (lactate) or transmigration of LAB within the liver. The analysis of data from the literature confirms that the gut microbiota plays a major role in the GVHD. Moreover, the most recent publications uncover that the LAB, butyrogenic bacteria and bacterial cross feeding were the missing pieces in the puzzle. This opens new bacteria-based strategies in the treatment of GVHD.

Breath analysis in gastrointestinal graft-versus-host disease after allogeneic hematopoietic cell transplantation

Volatile organic compounds (VOCs) are generated during pathologic processes, and their assessment can be used to diagnose and monitor a variety of diseases. Given the role of the microbiome in graft-versus-host disease (GVHD), we hypothesized that microorganisms producing volatile metabolites may alter VOCs expelled in breath in patients with gastrointestinal (GI) GVHD. In this pilot study, exhaled breath samples were obtained from 19 patients with grade 2 to 4 acute GI GVHD, 10 patients with no GVHD at day 100, and 10 healthy control subjects; the samples were analyzed by using mass spectrometry. Overall, nine (47%) patients had grade 2 GVHD, eight (42%) patients had grade 3 GVHD, and two (11%) patients had grade 4 GVHD; 26% had upper GI, 21% had lower GI, and 53% had both upper and lower GI manifestations. Stepwise canonical discriminant analysis identified 5 VOCs distinguishing patients with and without GI GVHD: 2-propanol, acetaldehyde, dimethyl sulfide, isoprene, and 1-decene (Wilks' Λ, 0.43; F statistic, 6.08; P = .001). The model correctly classified 89% (17 of 19) and 90% (9 of 10) of patients with and without GI GVHD, respectively. Breath analysis is a feasible and promising noninvasive method to detect acute GI GVHD. Further study of serial breath analysis and the gut microbiome in a larger cohort are ongoing to validate these findings.

Fecal Microbiota Transplantation for Treatment of Acute Graft-versus-Host Disease

The growing understanding of the bidirectional relationship between the gastrointestinal (GI) microbiome and the immune system has opened up new avenues for treatment of graft-versus-host disease (GVHD). Fecal microbiota transplantation (FMT) is the transfer of stool from a donor to a recipient who harbors a perturbed GI microbiome resulting in disease. We review the rationale for performing FMT for the treatment of acute GVHD, and summarize data on the safety and efficacy of the procedure among allogeneic hematopoietic stem cell transplantation (HSCT) recipients. Overall, FMT is a promising strategy in treating and preventing HSCT-related complications. However, caution should be exerted as HSCT recipients are highly immunosuppressed and unanticipated infectious adverse events may appear with the increasing application of FMT.