Gut microbiota, microbiota‐derived metabolites, and graft‐versus‐host disease

Abstract Allogeneic hematopoietic stem cell transplantation is one of the most effective treatment strategies for leukemia, lymphoma, and other hematologic malignancies. However, graft‐versus‐host disease (GVHD) can significantly reduce the survival rate and quality of life of patients after transplantation, and is therefore the greatest obstacle to transplantation. The recent development of new technologies, including high‐throughput sequencing, metabolomics, and others, has facilitated great progress in understanding the complex interactions between gut microbiota, microbiota‐derived metabolites, and the host. Of these interactions, the relationship between gut microbiota, microbial‐associated metabolites, and GVHD has been most intensively researched. Studies have shown that GVHD patients often suffer from gut microbiota dysbiosis, which mainly manifests as decreased microbial diversity and changes in microbial composition and microbiota‐derived metabolites, both of which are significant predictors of poor prognosis in GVHD patients. Therefore, the purpose of this review is to summarize what is known regarding changes in gut microbiota and microbiota‐derived metabolites in GVHD, their relationship to GVHD prognosis, and corresponding clinical strategies designed to prevent microbial dysregulation and facilitate treatment of GVHD.

40%-60%, and the mortality rate can be as high as 15%. 4,6][8] The human body is home to trillions of microbes, most of which live within the gut.][14][15][16] Recent development of new high-throughput sequencing technologies, including metagenomics, metabolomics, and metatranscriptomics, has facilitated new research on gut microbiota, the relationship between gut microbiota or microbiota-derived metabolites and GVHD. 17eanwhile, the GI tract is the main organ targeted by GVHD.Conditioning regimens before transplantation and activation of allogeneic T cells following transplantation can damage intestinal epithelial cells (IECs) and change the composition of gut microbiota, thereby leading to gut microbiota dysbiosis and translocation. 3,18,19][20] Therefore, the purpose of this review is to summarize the relationship between changes in gut microbiota and microbiota-derived metabolites in GVHD, their relationship to GVHD prognosis, and corresponding clinical strategies for microbial prevention and treatment of GVHD.

| GVHD
GVHD is caused by the allogenic activation of T cells, which recognize host antigens as foreign, thereby causing an autoimmune-like attack in various recipient organs, including the skin, lung, liver, intestine, thymus, hematopoietic system, and central nervous system (CNS). 21In the past, GVHD was divided into acute GVHD (aGVHD) and chronic GVHD (cGVHD) according to the time of onset.The current consensus criteria of the National Institutes of Health define different GVHD syndromes according to their clinical manifestations.Here, GVHD is subdivided into classic aGVHD, defined as an occurrence within 100 days (+100 days) after transplantation, and mainly manifests as an inflammatory reaction in the skin, GI tract, and liver.Late-onset acute GVHD refers to GVHD featuring the clinical manifestations of classic aGVHD and occurs after +100 days.Further subcategories include new-onset aGVHD, which occurs after +100 days, reactivation of controlled classic aGVHD after +100 days, and classic aGVHD that lasting after +100 days.Typical cGVHD, which has definite features but lacks acute features, can occur at any time after transplantation.Finally, overlapping cGVHD is diagnosed when features of both aGVHD and cGVHD coexist. 19,22,23he pathophysiology of aGVHD is divided into three main stages.The first stage involves host tissue damage caused by the preconditioning process, which in turn causes the release of inflammatory factors, chemokines, and bacterial products (e.g., lipopolysaccharides) as well as the activation of recipient antigen-presenting cells (APCs).In the second stage, activated APCs activate donor T cells, which leads to the proliferation and differentiation of donor T cells into different subsets, including Helper T (Th) cells such as Th1, Th2, and Th17 cells, as well as CD8 + T cells.In the third stage, Th cells and cytotoxic T (Tc) cells work together to mediate damage to aGVHD target organs-including the intestine, liver, lungs, and skin-by secreting a variety of cytokines. 5,24Compared to aGVHD, the pathological physiological conditions, the gut microbiota is highly diverse and is mainly composed of Firmicutes, Bacteroidetes, Actinomycetes, and Proteobacteria.It is involved in several important physiological processes, including host nutrient absorption, material metabolism, and immune defense. 26,27The gut microbiota plays an important role in the production of bioactive metabolites. 28Specifically, the microbiota consumes undigested food eaten by the host and excretes IECs as substrates.These then carry out complex and active metabolic reactions within the intestine to produce a variety of small molecule metabolites, including short-chain fatty acids (SCFAs), bile acids (BAs), and tryptophan and its derivatives (e.g., indole and indole derivatives), among others. 29,30These bioactive metabolites can directly or indirectly affect the physiological functions of the host, including host body development, digestion and metabolism, and immune regulation. 11

| GUT MICROBIOTA AND GVHD
The delicate balance between the human host and gut microbiota must be actively maintained by both parties to achieve a healthy steady state.Disruptions to the gut microbial balance can lead to the loss of multiple host functions, including impaired intestinal barrier function, immune dysfunction, and inflammation, which are associated with various diseases. 9,11,14,15][33][34] The interaction between gut microbiota and GVHD has been studied for several decades.As early as the 1970s, it was shown that rearing mice in a sterile environment 35 or antibiotic-mediated intestinal purification 36 reduced aGVHD symptoms.However, multiple subsequent clinical sample studies confirmed that reduced gut microbial diversity in allo-HSCT patients after transplantation is associated with a significant increase in the risk of GVHD. 3 Therefore, changes in gut microbial diversity and composition play an important role in the occurrence and development of GVHD and therefore can be used as a prognostic indicator for GVHD patients.

DIVERSITY AND GVHD
Studies have shown that changes in gut microbial diversity are significantly associated with the occurrence, development, and prognosis of GVHD, 37 and have confirmed that decreased gut microbial diversity after allo-HSCT was significantly associated with shortened overall survival (OS), increased GVHD-related mortality, and increased risk of developing GVHD.
Taur et al. found that patients with lower gut microbial diversity had significantly worse mortality outcomes, with 3-year OS values of 36%, 60%, and 67% for patients with low, intermediate, and high gut microbial diversity groups after allo-HSCT, respectively (p = 0.019). 18In addition, Han et al., 38 ILETT et al., 39 and Greco et al. 40 all confirmed that aGVHD patients showed lower microbial diversity than non-aGVHD patients, as well as that lower microbial diversity was an independent risk factor for aGVHD. 40A recent multicenter study also demonstrated that the gut microbial diversity of allo-HSCT patients was lower than healthy controls and that lower gut microbial diversity was in turn associated with an increased incidence of acute intestinal GVHD (p = 0.02) and TRM (p = 0.03). 41Jenq et al. also demonstrated that decreased gut microbial diversity was associated with increased GVHD-related mortality and lower OS. 42nother study confirmed that lower gut microbial diversity was associated with decreased OS, a higher risk of TRM (82/349 vs. 52/354, HR = 0.63, 95%CI: 0.44-0.89),and a higher risk of GVHD-related mortality (17/244 vs. 26/184, HR = 0.49, 95%CI: 0.26-0.90). 43aken together, these studies confirm that allo-HSCT patients show significant decreases in gut microbial diversity, especially in patients with GVHD, and that such reductions in gut microbial diversity are associated with poor OS, higher incidence of GVHD, and higher GVHDrelated deaths.Accordingly, gut microbial diversity can be used as an important factor for predicting the prognosis of GVHD patients (Table 1).

CHANGES IN THE COMPOSITION OF THE GUT MICROBIOTA AND GVHD
Dysbiosis of the gut microbiota is manifested by altered composition of the microbiota, and is significantly associated with GVHD. 37Studies have indicated that GVHD is associated with decreased abundance of specific bacteria, including Firmicutes (Clostridium, Faecalibacterium, Lachnospiracea, Ruminococcaceae, Eubacteriaceae and Peptostreptococcaceae), Bacteroidetes (Bacteroides and Parabacteroides), and Actinobacteria, as well as an increase in the abundance of Proteobacteria (Gammaproteobacteria and Enterobacteriales), Verrucomicrobia (Akkermansia) and opportunistic pathogens belonging ][46] Related studies have shown that the abundance of Clostridia, Lachnospiraceae, Blautia, Bacteroide, and Akkermansia muciniphil significantly decreased in aGVHD patients, which was associated with lower OS, and higher GVHD-related mortality. 39,47In cGVHD patients, it was also found that reduced abundance of butyrate-producing bacteria such as Lachnoclostridium, Clostridium, and Faecalibacterium was associated with a higher incidence of cGVHD. 48In another study, reduced accumulated intestinal microbiota (AIM) scores 15 days after transplantation, which reflect the abundance of four focal bacterial clades (i.e., Lachnospiraceae, Peptostreptococcaceae, Erysipelotrichaceae, and Enterobacteriaceae), were an independent risk factor for aGVHD.Moreover, AIM scores were positively correlated with aGVHD grade and could therefore be a valuable predictor for the risk of the development of aGVHD. 491][52] Moreover, gut microbial biomarkers have been found to be able to predict the risk of organ-specific aGVHD involvement, with patients showing relative intestinal dominance of Staphylococcaceae (>40%) had a higher risk of aGVHD with liver involvement (early: HR = 7.99, p = 0.007; late: HR = 5.14, p = 0.029), early GI involvement (HR = 4.84, p = 0.037), and the occurrence of steroid-refractory aGVHD (RR = 8.40, p = 0.007). 40hanges in the composition of gut microbial may contribute to the imbalance of specific immune cell subsets, which are involved in GVHD pathogenesis.For example, Han et al. demonstrated that decreased abundance of Lachnospiraceae and Ruminococcaceae and increased abundance of Enterobacteriaceae were associated with Treg/Th17 imbalance, which might be through acetylated H3 in CD4 + T cells.Therefore, gut microbial might induce aGVHD by influencing the Treg/Th17 balance. 38n addition, using a cGVHD mouse model, bacterial extracts of gut microbiota from cGVHD patients were found to induce the murine splenic T cells to differentiate into Th1 cells and inhibit their differentiation to Treg cells, resulting in Th1/Treg imbalance, which was significantly correlated with the onset of cGVHD. 53Therefore, these studies have shown that gut microbial imbalance can lead to immune imbalance, which in turn participates in the pathogenesis of GVHD.
In conclusion, the studies listed above suggest that changes in gut microbial composition were significantly correlated with the prognosis of GVHD patients (Table 1) and could be used as biomarkers and therapeutic targets for predicting GVHD.

METABOLITES AND GVHD
Gut microbiota exert effects on tissues mainly through microbiota-derived metabolites.Accordingly, changes in the gut microbiota can lead in turn to changes in microbiota-derived metabolites.Studies have shown that these metabolites play a crucial role in regulating intestinal homeostasis and the immune response.5][56] Microbiota-derived metabolites which include SCFAs, tryptophan and its derivatives, choline metabolites, tyrosine, and BAs can affect the severity and prognosis of GVHD (Table 2, Figure 1B). 57,58

| SCFAs
SCFAs are produced by gut microbiota fermenting food that the host cannot metabolize, and include butyrate, propionate, and acetate, among others. 59SCFAs are an important energy source for gut microbes, as well as for the IECs.In addition to serving as a local substrate for energy production, SCFAs also have immunomodulatory functions, and play important roles in innate immunity, and the production of lymphocytes, neutrophils, monocytes, macrophages, and dendritic cells (DCs).1][62][63] Studies have also shown that SCFAs can promote the expression of genes such as aryl hydrocarbon receptor (AhR) and hypoxia-inducible factor 1α (HIF1α); this occurs via the activation of cell surface G-protein receptor 41 (GPR41) receptor and the inhibition of HDAC, which promotes the production of IL-22 by CD4 + T cells and innate lymphoid cells (ILCs). 64SCFAs are also known to induce the production and enhance the function of Treg cells by inhibiting HDAC, which occurs by promoting the acetylation of histone H3.6][67][68] Furthermore, SCFAs have been found to inhibit the maturation of DCs and affect the secretion of cytokine IL-23, 69 which regulates the function of macrophages and the secretion of cytokines. 70inally, SCFAs have multiple effects on IECs and intestinal immune cells, insofar as they maintain IEC barrier integrity and prevent the translocation of pathogenic bacteria. 71 Many studies have confirmed that SCFA levels were significantly reduced in GVHD patients and were associated with GVHD severity and mortality.A study of 316 patients undergoing allogeneic HSCT found a significant reduction in SCFA production at the onset of aGVHD, with variation depending on disease severity.In patients with severe aGVHD, the levels of acetate, propionate, and butyrate decreased by 75.8%, 95.8%, and 94.6%, respectively.Moreover, patients with mild aGVHD showed a decrease in butyrate of 86.0% but patients with severe aGVHD showed decreases as great as 94.6%.These data suggest that butyrate can be used as a potential diagnostic marker for aGVHD patients. 73In a clinical trial, plasma butyrate concentration was also tracked along with urinary-derived human chorionic gonadotropin (uhCG) during the treatment of steroidrefractory aGVHD.Compared to the baseline condition, uhCG responders showed higher plasma butyrate concentrations on Days 28 and 56, while no change in butyrate concentration over time was observed in nonresponders. 74SCFAs were also found to be less abundant in cGVHD patients.In addition, Markey et al. found that compared to patients without cGVHD, the plasma concentration of SCFAs (including butyrate and propionate) in cGVHD patients significantly decreased within 100 days of transplantation. 48t present, the specific mechanism by which SCFAs contribute to GVHD remains unknown.However, Mathewson et al. found that butyrate was significantly reduced in aGVHD mice model, and the reduction of butyrate in CD326 + IECs following allo-HSCT can lead to a decrease in histone acetylation.Interestingly, IEC acetylation levels can be restored by supplementation with exogenous butyrate, which directly enhances the barrier function of epithelial cells and reduces aGVHD via a Treg-independent pathway.In addition, alteration of the indigenous microbiota in the host to produce high levels of butyrate can also been found to reduce the severity of aGVHD. 75Herefore, this study suggests that local and specific alterations in microbial metabolites may confer direct beneficial effects on GVHD target tissues and can mitigate the severity of aGVHD.In another study, indoleamine-2,3-dioxygenase (IDO) expression was found to be upregulated in intestinal parenchymal cells and APCs in mouse via IFN-γ produced by alloreactive T cells.Butyrate acts as a histone deacetylase (HDAC) inhibitor and effectively reduces GVHD by inhibiting indoleamine-2,3-dioxygenase (IDO)-dependent innate immune and allo-stimulating APC functions in a STAT-3-dependent manner. 4,76,77Butyrate is also known to promote histone H3 acetylation by inhibiting HDACs, up-regulating FOXP3 expression, promoting the differentiation of Tregs, and enhancing Treg function. 68Tregs have been found to be highly efficient during the prevention and treatment of GVHD. 78SCFAs can affect the host through GPR43, which is expressed in a variety of cells, including IECs.Another study using a mouse model demonstrated that butyrate and propionate both reduce intestinal permeability and relieve GVHD by binding to the GPR43 receptor on IECs and thereby activating the NLRP3 inflammasome via ERK phosphorylation. 57owever, signal transduction inhibitors currently approved by the Food and Drug Administration (FDA) for GVHD treatment, such as ibrutinib can inhibit BTK in B cells, leading to reduced production of autoreactive antibodies or inhibit the homologous enzyme, interleukin-2-inducible T-cell kinase (ITK), leading to selective suppression of Th2 immune responses that may contribute to the pathogenesis of cGVHD, 79,80 while ruxolitinib can inhibit the production of inflammatory cytokines and regulate immune response by inhibiting JAK-STAT signaling pathway, and finally achieve the purpose of treating GVHD. 81Ruxolitinib has shown excellent efficacy with overall response rate (ORR) of 62.3 at Day 28 for steroid-refractory aGVHD in REACH2 clinical trial and with ORR of 49.7% at Week 24 regardless of the cGVHDinvolved organs in patients with steroid-refractory/intolerant cGVHD in REACH3 clinical trial. 81,82Belumosudil, a selective inhibitor of Rho-associated coiled-coilcontaining protein kinase 2 (ROCK2) has emerged as a promising novel therapeutic approach for treating steroid-refractory/intolerant cGVHD with 74%-76% ORR. 83SCFAs can improve GVHD by inhibiting HDAC activity and binding to GPR43, which differs from other known mechanisms of these signal transduction inhibitors, and suggests that SCFAs may provide a new treatment option for GVHD patients.
However, Golob et al. noted that although butyrate may help prevent the occurrence of aGVHD, once aGVHD has occurred and entered into progress, high level of butyrate may impair IEC recovery and actually exacerbate aGVHD. 84The study also found that the presence of butyrate-producing bacteria during 21 days after onset of severe aGVHD was associated with increased risk of steroid-resistant GVHD and cGVHD. 84This is primarily because butyrate inhibits the proliferation of colon stem/progenitor cells, thereby preventing colon stem cells from forming a complete monolith of the epithelium. 84,85herefore, if butyrate is persistent and active during the progress of severe intestinal aGVHD, loss of epithelial structure can lead to the exposure of colonic stem cells to microbially produced butyrate, which in turn impairs the recovery of the colonic mucosa of aGVHD patients, and thus leads to increased risk of refractory aGVHD and cGVHD. 84Therefore, supplementation with SCFAs at the time of the beginning of aGVHD may be more useful.

DERIVATIVES
Tryptophan is an aromatic essential amino acid.Studies have shown that gut microbiota metabolize 4%-6% of available tryptophan into indole and indole derivatives, including indole-3-aldehyde (IAld), indole-3-acid-acetic (IAA), indole-3-propionic acid (IPA), indole-3-acetalic acid (IAAld), and indoleacetic acid.These indoles and indole derivatives are AhR ligands, and AhRs are widely expressed in immune cells, [86][87][88] since AhR signaling is an important component of the immune response at the barrier.][91] Indoles are produced by tryptophan from commensal bacteria expressing tryptophanase.They are not only an important intercellular signal for the microbial community but also play a role in regulating intestinal immune balance, inhibiting the inflammatory response, and maintaining intestinal barrier function. 92tudies have shown that indoles and indole derivatives can also regulate the differentiation of Treg and Th17 cells, inhibit Th17 cells, and promote Treg cell differentiation. 93In addition, indole derivatives produced by Lactobacillus reuteri have been found to promote the transformation of intestinal epithelial CD4 + T cells into CD4 + CD8 + double positive intraepithelial lymphocytes, which facilitates the maintenance of intestinal immune homeostasis. 94Indoles can be further metabolized into 3-indoxyl sulfate (3-IS) by cytochrome P450 enzymes (including CYP2E1) and sulfotransferase (SULT), which is excreted in the urine. 95Holler et al. demonstrated that urinary 3-IS levels decreased in allo-HSCT patients, but were higher in patients receiving antibiotics with GI GVHD.In addition, urinary 3-IS levels were positive correlated with gut microbiota disruption in patients undergoing allo-HSCT; these disruptions saw decreased abundance of some Enterococci spp.but increased abundance of colonic commensals such as E. rectale and Clostridium phytofermentalis. 52In another study, Weber et al. found that decreased urinary excretion of 3-IS within the first 10 days after allo-HSCT was associated with significantly increased TRM (p = 0.017) and worse OS (p = 0.05) within the first year following allo-HSCT.Moreover, the composition of the gut microbiota was also found to be able to predict the level of urinary 3-IS.The abundance of Lachnospiraceae and Ruminococcaceae (class: Clostridia) has also been found to be associated with higher levels of urinary 3-IS, while the abundance of the class of Bacilli was correlated with lower levels of urinary 3-IS. 96Taken together, these findings suggest that urinary 3-IS levels may be an important marker of gut microbiota disruption and can reveal increased risk of developing GI GVHD following allo-HSCT.
Indole-3-carboxaldehyde (ICA) is specific, biologically relevant indole derivative.In one study, Swimm et al. used a mouse model to reveal that ICA treatment can limit intestinal epithelial injury, reduce epithelial bacterial translocation, reduce inflammatory cytokine F I G U R E 1 Gut microbiota, microbiota-derived metabolites, and graft-versus-host disease.(A) Gut microbiota, microbiota-derived metabolites in homeostasis.In a stable state, a large number of gut microbiota are distributed in the intestinal lumen, which can maintain intestinal homeostasis and epithelial integrity, resist pathogens, regulate immune system development and immune response.Microbiotaderived metabolites such as SCFAs, tryptophan and its derivatives, and bile acids can directly act on intestinal epithelial cells and intestinal immune cells to regulate the differentiation, recruitment and activation of immune cells.The intestinal epithelial surface maintains a complete barrier to prevent bacteria from invading host tissues, Paneth cells produce AMPs to prevent microbial invasion, goblet cells produce a mucus barrier that separates bacteria from epithelial cells and the lymphocytes (including T cells, B cells, ILC, and MAIT cell) distributed in the intestinal lamina propria resist the invasion of pathogenic microbiota and inhibit the immune response.(B) Relationship between gut microbiota and pathogenesis of GVHD.Preconditioning regimens (including radiotherapy, chemotherapy, immunotherapy, and broad-spectrum antibiotics) before allo-HSCT can damage IECs, Paneth cells, and goblet cells, thereby damaging the intestinal barrier, reducing the production of AMPs, depleting the mucus barrier, leading to the dysbiosis of gut microbiota and the translocation of gut microbiota.Gut microbiota, microbiota-derived metabolites (PAMPs, such as bacterial lipopolysaccharide) are translocated to the lamina propria of the intestine and are recognized by host APCs.APCs can activate allogeneic reactive T cells, lead to the continuous proliferation of autologous reactive T cells, release a large number of cytokines, trigger cytokine storms, and aggravate intestinal tissue damage.B cells are considered to be the target of GVHD, and their damage can lead to a decrease in sIgA secretion and aggravate intestinal inflammation and bacterial translocation.The imbalance of gut microbiota leads to the disorder of its metabolites, which is mainly manifested by the decrease of SCFAs, tryptophan and its derivatives, and the increase of bile acids, which can aggravate the damage of intestinal epithelial cells, lead to the imbalance of immune cells in intestinal lamina propria, and aggravate GVHD.AhR, aryl hydrocarbon receptor; allo-HSCT, allogeneic hematopoietic stem cell transplantation; AMPs, antimicrobial peptides; APCs, antigen-presenting cells; DC, dendritic cell; GVHD, graftversus-host disease; IECs, intestinal epithelial cells; IgA, immunoglobulin A; ILC, innate lymphoid cell; MAIT, mucosal-associated invariant T cell; PAMPs, pathogen-associated molecular patterns; SCFAs, short chain fatty acids; Th1, T helper 1 cell; Th17, T helper 17 cell; TMAO, trimethylamine N-oxide; Treg, regulatory T cell.production, reduce aGVHD pathological scores, and reduce aGVHD mortality.Moreover, this is accomplished without affecting the donor T-cell-mediated graftversus-leukemia response.Transcriptional profiling and gene ontology analysis suggest that ICA treatment can up-regulate genes associated with type I interferon (IFN1) response, which has been reported to resist radiation-induced intestinal injury and reduce aGVHD pathological scores.This is thought to result from the fact that the protective effect of ICA against radiation exposure is eliminated in mice lacking IFN1 signaling.Therefore, this study confirmed that indole metabolites produced by intestinal flora can limit intestinal inflammation and injuries associated with myeloablative chemoradiotherapy and aGVHD through type I IFNs.Moreover, this metabolite-mediated effect does not affect the anti-tumor response, or require new treatment options for BMT patients at risk of GVHD. 58n immune cells and epithelial cells, tryptophan enters the kynurenine pathway via indoleamine 2,3-dioxygenase (IDO) and generates kynurenine (Kyn) and its downstream products. 87IDO is the rate-limiting enzyme in the degradation of tryptophan to Kyn.It is mainly expressed by APCs and parenchymal cells and is further induced by inflammation. 97Studies have shown that the gut microbiota plays an important role in the activation of IDO.In addition, Jasperson et al. confirmed that IDO is a key regulator of GVHD, and that IDO expression was strongly upregulated in the colonic epithelial cells of GVHD mice.Furthermore, IDO can reduce the proliferation and survival of T cells, thereby reducing colon inflammation, GVHD severity, and GVHD-related mortality.Therefore, this study indicated that regulating the IDO pathway is an effective method for the treatment of GVHD. 76Another recent study found that aGVHD is associated with significant changes in microbial-derived metabolites, especially AhR ligands.Of these, tryptophan metabolites, including 3-indoxyl sulfate, indole acetate, indole acetylglutamine, and indole propionate, as well as host-derived compounds produced by the IDO tolerogenic pathway, were all significantly reduced by the onset of aGVHD. 98Therefore, this study suggests that the allogeneic immune response during aGVHD may be affected by the reduction of AhR ligands produced by the gut microbiota, insofar as this limits the generation of IDO and affecting the allogeneic T-cell response. 76,89,98

| TRIMETHYLAMINE N-OXIDE (TMAO)
Choline, phosphatidylcholine, carnitine, and glycerophosphocholine are highly abundant in eggs, liver, dairy products, peanuts, and other foods.In the gut, each of these compounds can be metabolized to trimethylamine (TMA), which is then converted to trimethylamine N-oxide (TMAO) by host hepatoflavin monooxygenase.The intestinal bacteria regulating TMA generation mainly include Campylobacter jejuni, Clostridium, Bifidobacterium, and Faecalibacterium prausnitzii. 99As a circulating intestinal microbial metabolite, TMAOs are capable of inducing vascular inflammation and endothelial dysfunction by forming and activating the NLRP3 inflammasome in endothelial cells. 100u et al. found that TMAO or a high choline diet could aggravate disease severity and mortality in GVHD mice, and a choline structural analog known as 3,3-dimethyl-1-butanol (DMB) 101 was found to reduce elevated serum TMAO concentrations caused by a high choline diet and reverse TMAO-induced GVHD severity and mortality.The mechanism occurs because TMAO can promote the nuclear translocation of NF-kB and increase the production of reactive oxygen species in mitochondria, activate the NLRP3 inflammasome, promote the activation of the downstream Caspase-1, and produce IL-1β inflammatory factors.Taken together, these effects promote the differentiation of macrophages into the M1 phenotype in GVHD mice, where they then secrete pro-inflammatory mediators to induce the differentiation of allogeneic T cells into Th1 and Th17 subsets; this differentiation ultimately causes worsening of GVHD. 102

| TYROSINE
Tyrosine is a non-essential amino acid that is an important precursor for the synthesis of many bioactive substances.Huang et al. found that the abundance of Lachnospiraceae subclass in aGVHD mice was significantly reduced in the intestine.This was accompanied by a decrease in tyrosine content in the intestine, and multi-component correlation analysis suggested that the presence of tyrosine was closely related to the abundance of key bacterial species in aGVHD.Moreover, by providing a safe dietary tyrosine supplement, the survival time of aGVHD mice was significantly prolonged, and intestinal rejection symptoms improved.Meanwhile, the abundance of the dominant bacterial community in the intestinal tract was found to increase following the recovery of various metabolites related to the tyrosine metabolic pathway.Thus, since mice deprived of a tyrosine diet developed more severe rejection symptoms, diet therapy may be a potential treatment for aGVHD. 103ut microbiota can metabolize tyrosine into derivatives such as p-cresol sulfate and 3-phenylpropionate. 104eikvam et al. estimated the metabolic characteristics of patients with and without aGVHD by random forest analysis of serum metabolic profiles.They found that patients with advanced aGVHD had increased levels of potential pro-inflammatory tyrosine metabolites (i.e., p-cresol sulfate and 3-phenylpropionate) formed by gut microbiota, and these could be used as candidate biomarkers for predicting the risk of GVHD. 104Reikvam et al. performed a similar analysis of cGVHD patients in another study and found that compared with patients without cGVHD, the levels of tyrosine metabolites (including phenylacetate, 3-(4-hydroxyphenyl) lactate, and tyramine o-sulfate) in the gut microbiota in serum of patients with cGVHD were higher.Thus, changes in tyrosine metabolite levels can partially reflect changes in gut microbiota composition following allo-HSCT. 105owever, the specific mechanism by which tyrosine and its metabolites regulate GVHD remains unclear.Accordingly, further investigation is required in future studies. 12| BAs AND THEIR METABOLITES BAs are metabolites affected by intestinal microbial composition, 106 and can be divided into primary and secondary BAs according to their source.Both primary and secondary BAs regulate the host metabolism and host immune response together. 107,108Studies have shown that primary and secondary BAs act as signaling molecules to target BA receptors in host immune cells (e.g., farnesoid X receptor and the G-protein-coupled receptor TGR5), 109 downregulate TNF-α and IL-12 in DCs and monocytes, and increase the production of IL-10 in macrophages.Via these functions, they disrupt the proinflammatory functions of immune cells. 110,111In addition, BAs are involved in regulating the differentiation and function of T cells, including both Th17 and Treg cells. 112Hang et al. screened a library of BA metabolites and identified that derivatives of secondary BAs including lithocholic acid (LCA), 3-oxoLCA, and isoalloLCA can regulate T-cell function in mice.Of these BA derivatives, 3-oxoLCA was found to inhibit Th17 differentiation by directly binding to the key transcription factor retinoid-related orphan receptor-γt (ROR-γt).In contrast, isoalloLCA increased the differentiation of Treg cells by inducing oxidative phosphorylation and enhancing CNS3 H3K27 acetylation, both of which promote the expression of FOXP3.Therefore, this study indicated that BA metabolites can regulate host immunity by directly regulating the balance of Th17 and Treg cells. 113urthermore, Huh et al. used both in vitro and in vivo datasets to demonstrate that the BA metabolite isoLCA inhibited the differentiation of immature CD4 + T cells into TH17 cells by inhibiting ROR-γt. 114ext, Reikvam et al. analyzed serum metabolite levels in patients with and without cGVHD.Their Random Forest Classification Analysis revealed that four BAs, including the secondary BA hyocholate, as well as three primary BAs, glycochenodeoxycholate sulfate, taurocholate, and glycocholate, ranked among the 30 highestranking metabolites that differed among cGVHD patients and control patients.These results show that differences in BA metabolism were found in cGVHD patients. 115Taurine is another BA metabolite of interest; for instance, Toubai et al. used a mouse model to reveal that taurine could activate the signal transduction of the NLRP6 inflammasome and thereby aggravate aGVHD. 116In addition, Michonneau et al. used a metabolomic analysis to reveal that taurine production was significantly reduced in the plasma of patients without GVHD after HSCT.In contrast, the levels of primary and secondary BAs in the plasma of aGVHD patients were significantly increased.Therefore, this study suggests that the allogeneic immune response during aGVHD may be affected by BAs, thereby limiting the production of indoleamine 2,3-dioxygenase and affecting allogeneic T-cell reactivity. 98n conclusion, these studies have confirmed that gut microbiota-derived metabolites such as SCFAs, tryptophan, TMAO, tyrosine, and BAs play important roles in the occurrence and development of GVHD.Moreover, they represent an important opportunity for a novel therapeutic strategy for the treatment and prevention of GVHD.However, the specific mechanisms by which gut microbiota-derived metabolites regulate GVHD is not completely clear, and further research is therefore still required. 13| GUT MICROBIOTA-RELATED

THERAPEUTIC OPTIONS FOR GVHD PATIENTS
Previous studies have demonstrated the importance of gut microbiota and microbiota-derived metabolites for patients with GVHD.Therefore, purposefully regulating the gut microbiota via interventions may improve the prognosis of allo-HSCT.Specifically, using treatments to maintain gut microbial homeostasis and preserving the advantages of beneficial bacteria may improve clinical outcomes for allo-HSCT recipients.The main strategies used to accomplish this include nutritional support therapy, fecal microbiota transplantation (FMT), probiotics, prebiotics, and antibiotics, all of which aim to supplement or regulate the biology of the intestinal microbiome (Table 3).

| NUTRITIONAL SUPPORT THERAPY
Most allo-HSCT patients develop some degree of nausea, mucositis, and anorexia due to the preconditioning regimen.Therefore, oral nutrient reduction is very common for allo-HSCT patients. 117,118In addition, the changes in gut microbiota observed during HSCT and GVHD may reflect the fact that insufficient nutrition to maintain a balanced gut microbiota is present in the intestine. 21otal parenteral nutrition (TPN) is widely used to support allo-HSCT recipients.However, an increasing body of evidence supports the use of enteral nutrition (EN) instead.Several studies have confirmed that EN can reduce the incidence and severity of GVHD and decrease T A B L E 3 Summarizing the advantages and disadvantages of gut microbiota-related therapeutic options.

Advantages Disadvantages
Nutritional support therapy 119,125,126 EN can maintain the integrity of intestinal mucosa, regulate the composition of gut microbiota, reduce the incidence and severity of GVHD, and reduce the mortality associated with GVHD Patients receiving PN are prone to intestinal microbiota dysregulation, increased intestinal microbiota translocation, and increased incidence, severity, and GVHD-related mortality Fecal microbiota transplantation 128,136,138,140,142 FMT can directly change the composition of the gut microbiota of the host to rebuild the gut microbiota balance, repair the intestinal mucosal barrier, control the inflammatory response, and regulate the body's immunity Drug-resistant bacterial colonization, intestinal infection, bloodstream infections, sepsis, and even death Probiotics 33,144,147 Maintaining the balance of gut microbiome, inhibiting intestinal inflammation, protecting intestinal mucosal barrier, regulating the body's immunity, reducing the severity of GVHD, and improving survival For patients with compromised immune function, probiotic use was associated with a higher incidence of infection Prebiotics GVHD-related mortality in allo-HSCT patients.For example, Seguy et al. demonstrated that patients receiving EN after allo-HSCT showed lower aGVHD incidence and mortality, while parenteral nutrition (PN) was associated with higher aGVHD incidence and mortality. 119vahn et al. also demonstrated that EN deficiency was associated with a higher incidence of aGVHD and poorer OS. 120 In addition, Gonzales et al. demonstrated that the incidence of aGVHD and non-relapse mortality were lower in an EN group in allo-HSCT patients undergoing myeloablative preconditioning. 121Another retrospective study also demonstrated that compared to adequate EN, inadequate nutrition and adequate PN both showed increased non-relapse mortality, lower 5-year OS, lower GVHD-free/relapse-free survival, and increased incidence of aGVHD. 122Moreover, Zama et al. conducted a metaanalysis that confirmed that EN reduced the incidence of aGVHD, especially grade III-IV and intestinal aGVHD. 123inally, another recent study showed that patients receiving PN had a higher incidence of aGVHD than those receiving EN (9.1% vs. 24.8%,p = 0.01). 124ifferent nutritional methods can also affect the composition of gut microbiota.A retrospective study confirmed that the loss of Blautia was associated with receiving TPN for more than 10 days, and the loss of Blautia can lead to increased GVHD mortality. 42D'Amico et al. confirmed that compared to patients receiving PN, the gut microbiota of patients receiving EN after transplantation recovered more rapidly.In addition, the abundance of Blautia, Dorea, and Bacteroidaceae increased in patients receiving EN, while the abundance of Faecalibacterium decreased significantly in patients receiving PN.A further metabolomic analysis showed that SCFAs, including butyrate, acetate, and propionate, were significantly enriched in patients receiving EN, while patients receiving PN showed reduced SCFA levels. 125Finally, another study confirmed that patients receiving EN had a higher abundance of SCFA-producing bacteria-including Ruminococcus bromii and several Faecalibacterium praunitzii strains-relative to patients receiving PN. 126 Taken together, the studies mentioned above support the recommendation that EN rather than PN should be used to treat allo-HSCT patients.According to experimental evidence, EN can better maintain the integrity of intestinal mucosa, regulate the composition of gut microbiota, reduce the incidence of GVHD, and improve the prognosis of GVHD patients.Patients receiving allo-HSCT should therefore be encouraged to optimize oral intake, even during nutritional support.A randomized, prospective, multicenter study is currently evaluating the effects of different nutritional regimens on GVHD and TRM (NCT01955772).This is valuable, and additional multicenter randomized controlled studies are needed to better determine the best nutrition management method for allo-HSCT patients. 15| FECAL MICROBIOTA TRANSPLANTATION FMT involves transplantation of functional bacteria found in the feces of healthy people into the intestinal tract of recipient patients. 127This procedure can directly change the composition of the gut microbiota of the host; it does so by rebuilding the intestinal microbiome balance, repairing the intestinal mucosal barrier, controlling the inflammatory response, regulating the immune response, and treating intestinal and extraintestinal diseases. 128ince the gut microbiota plays an important role in the pathogenesis of GVHD, FMT has been studied as a potential therapeutic intervention.Currently, several studies have confirmed that FMT shows good clinical efficacy and safety in steroid-resistant/refractory GVHD (Table 4).Moreover, the European Society for Blood and Marrow Transplantation included FMT as a treatment for steroidresistant/refractory GVHD patients in 2020. 129here is increasing evidence that FMT has good efficacy and safety when treating steroid-dependent/resistant aGVHD patients.Kakihana et al. first applied FMT to treat steroid-resistant/dependent intestinal aGVHD in 2016 and found that all patients (i.e., four cases) responded to FMT treatment, with three complete responses (CR), and one partial response (PR).Patients with steroid-resistance all showed improvement in GI symptoms within a few days, and no serious adverse effects were observed during treatment.Moreover, this study also confirmed that FMT can increase the number of peripheral effector regulatory T cells (eTreg), which have been reported to be prognostic cellular biomarkers for aGVHD.][133][134][135] Therefore, FMT appears to be safe and effective, and is expected to be a potential treatment option for aGVHD. 136ue to the long-term use of broad-spectrum antibiotics, as well as the destruction of the gut microbiota Although one patient experienced constipation and two patients experienced grade I diarrhea after FMT, but no other adverse events were observed. 137Bilinski et al. conducted a prospective multicenter study to assess the effectiveness and safety of FMT in patients with GVHD who were colonized by antibiotic-resistant bacteria (ARB).
According to the results, 11 of 14 patients had ARB decolonized or partially decolonized after FMT.Among aGVHD patients, the ORR was 57% (8/14 FMTs), including 42% (6/14) reaching CR.Moreover, the median OS of responders and non-responders was 332 and 66 days, respectively (HR = 0.18, 95 %CI: 0.03-0.93,p < 0.005).After FMT treatment, both patients with cGVHD showed stabilization or improvement of their organ disease and were still alive at the final follow-up. 138Therefore, FMT appears to be highly effective for the treatment of GVHD and for decolonization of the GI tract from ARB.
The combination of FMT and other protocols has also demonstrated favorable efficacy and safety outcomes in patients with steroid-refractory aGVHD.For example, a recent study by Liu et al. evaluated the effectiveness of FMT in combination with ruxolitinib as a salvage therapy for intestinal steroid-refractory aGVHD after allo-HSCT.According to their results, the ORR on Day 28 of the combined regimen was 71.4%, with 10 patients reaching CR and five patients reaching PR.The durable overall response in responders on Day 56 was 80%, the estimated 6month OS was 57.1%, and the EFS was 52.4%.In addition, the authors observed declines in the levels of inflammatory cytokines (mainly including IL-2, IL-17A, IL-4, IL-6, and IL-10), T cells, and in NK cell activation.In addition, the diversity of gut microbiota was higher in patients who responded to combination therapy, while the proportion of Lactobacillus was increased in CR patients and the abundance of Escherichia was lower following treatment.The most frequent adverse events recorded were viral reactivation (61.9%) and severe cytopenia (grades 3-4, 81.0%).Therefore, this study confirmed that FMT combined with ruxolitinib is an effective treatment for intestinal SR-aGVHD following HSCT. 139owever, the safety of FMT may need further to be fully validated.FMT involves the transfer of feces from the donor to the recipient, so there is a risk of infectious disease.Two previous independent clinical trials in which the donors had not been screened for multidrug-resistant organisms reported the spread of extended-spectrum betalactamase (ESBL) with adverse consequences.Specifically, both patients displayed ESBL-producing Escherichia coli bacteremia after undergoing FMT, and one of the patients died. 140Furthermore, a worldwide analysis of FMT-related adverse events from 129 studies performed between 2000 and 2020 showed that FMT-related serious adverse events (SAEs), including infection and deaths, occurred in 1.4% of patients who underwent FMT.Moreover, all reported FMT-related SAEs occurred in patients with mucosal barrier injury.Therefore, researchers and clinicians should pay attention to the potential risks of FMT for treating GI-aGVHD. 141Another study confirmed a high incidence of bloodstream infections (BSIs) following FMT for GVHD (i.e., in 22 events out of 33 patients). 142Therefore, it is still necessary to pay full attention to the safety of FMT in clinical practice, and donor screening and careful benefit risk assessments are required when designing FMT research protocols. 143n conclusion, although some of the above studies confirm that FMT shows good efficacy and safety in patients with GVHD, where it increases the diversity of gut microbiota and changes the composition of gut microbiota.However, most of these studies are single-center, small-sample studies, and focused mainly on applications to patients with aGVHD, while studies of patients with cGVHD were limited.In addition, the specific mechanisms by which FMT affects GVHD patients remain unclear.Therefore, more prospective, multicenter studies are required to evaluate the safety and efficacy of FMT for patients with GVHD (Table 5).

| PROBIOTICS
Probiotics refer to living microorganisms that can bring health benefits to a host when internalized.Probiotics can refer to a single strain or a mixture of strains, and can improve the composition of gut microbiota, control the regulation the immune function of the human body, and prevent or alleviate the occurrence of a variety of diseases. 144,145Current research on probiotic interventions often focuses on the biological use of Bifidobacterium and Lactobacillus. 146robiotic-based therapies have been shown to be able to regulate the composition of gut microbiota and improve the prognosis of GVHD.Gerbitz et al. used a mouse model to demonstrate that oral administration of the probiotic Lactobacillus rhamnosus GG before and after transplantation can reduce intestinal inflammation and bacterial translocation, thereby reducing the severity of aGVHD and improving survival. 147However, in a randomized controlled trial, supplementation with Lactobacillus rhamnosus GG in allo-HSCT patients did not appear to significantly alter the gut microbiome or provide protection against GVHD. 148This may be due to the fact that humans have human-specific intestinal mucosal colonization against transient probiotic colonization, which may obscure some potential benefits of probiotic treatments. 149enq et al. demonstrated that an ampicillin treatment targeting Lactobacillales can lead to aGVHD exacerbation in a mouse model, and that reintroduction of Lactobacillales following ampicillin treatment can reduce aGVHD mortality and severity.This may occur because Lactobacillales can reduce the severity of aGVHD by preventing the expansion of Enterococcus. 33In another study, Ladas et al. confirmed that the application of another probiotic, which contained Lactobacillus plantarum (LBP), is safe and reasonably tolerated in children and adolescents undergoing allo-HSCT. 150iven this evidence, probiotic therapy may be an effective treatment to improve the severity and mortality of GVHD, but requires further validation in subsequent prospective studies.Studies are currently evaluating the efficacy of probiotics such as Lactobacillus plantarum (NCT03057054) and Clostridium butyricum MIYAIRI 588 (CBM588) (NCT03922035) in GVHD patients (Table 5).

| PREBIOTICS
Prebiotics are dietary supplements that can help improve the growth and development of beneficial bacteria, inhibit harmful bacteria, and increase the production of beneficial microbiota-derived metabolites, including SCFAs, via bacterial fermentation. 3,151Prebiotics have been shown to improve the prognosis of GVHD by maintaining intestinal integrity and modulating immune response.Lyama et al. used a retrospective study to demonstrate that nutritional supplements consisting of glutamine, fiber, and oligosaccharides reduced the severity of mucosal injury following allo-HSCT. 152Galactooligosaccharides (GOS) are a widely studied prebiotic ingredient that are used as a dietary supplement to reduce inflammatory GI symptoms, promote intestinal barrier function, increase NK cell activity, and regulate cytokine activity. 153,154In a recent study using GVHD model mice, Holmes et al. confirmed that exogenous supplementation of GOS can improve the survival rate of allo-HSCT mice and reduce the severity of GVHD.Interestingly, GOS only improved the allo-HSCT prognoses of antibiotic-treated mice, which suggests that prebiotics could counter the harmful effects of antibiotics on commensal gut microbiota and clinical outcomes.In addition, this study also confirmed that the supplementation of GOS could improve the composition of gut microbiota, since it showed that the probiotic treatment was associated with reduced abundance of Bacteroidaceae and Bacteroidales_S24-7_group as well as increased abundance of Porphyromonadaceae was increased. 154,155Furthermore, is involved in butyrate production, and has been found to reduce the severity of GVHD. 156,157herefore, this study suggests that prebiotic therapy may be an adjunct therapy for the prevention of GVHD.Studies are currently evaluating the efficacy of prebiotic regimens including potato starch (NCT02763033), a gluten-free diet (NCT03102060), fructose oligosaccharides (NCT02805075), GOS (NCT04373057), inulin (NCT04111471), and 2′-fucosyllactose (NCT04263597) (Table 5) in GVHD patients.

| ANTIBIOTICS
Preconditioning regimens can lead to neutropenia, which result in oral and gastrointestinal mucositis that increases the risk of bacterial translocation, in turn leading to increased risk of various infections. 158Therefore, allo-HSCT patients are often given prophylactic or empirical antibiotics to prevent and treat bacterial infections. 159Early studies using mouse models have shown that sterile environment feeding 35 or antibioticmediated intestinal purification 36 can alleviate aGVHD symptoms.However, subsequent studies have not confirmed the benefits of intestinal purification in reducing GVHD. 1601][162] Therefore, the rational deployment of antibiotic treatments is needed in clinical practice.6][167][168][169] A retrospective study confirmed that imipenem-cilastatin and piperacillin-tazobactam antibiotic treatments for neutropenic fever were associated with increased GVHDrelated mortality after 5 years.In addition, mice treated with imipenem-cilastatin showed increased abundance of Akkermansia muciniphila and reduced colonization of Clostridiales in the gut. 163Furthermore, Tanaka et al. showed that the abundance of Bifidobacteriales and Clostridiales decreased in patients that received piperacillin-tazobactam or carbapenems between 7 days before to 28 days after HSCT.In contrast, patients who received piperacillin-tazobactam or carbapenems experienced a higher risk of acute gut/liver GVHD and GVHD-related mortality. 164pecific narrow-spectrum antibiotics such as cefepime, aztreonam, and rifaximin have been associated with reduced GVHD severity.For example, Shono et al. confirmed that treatment with cefepime and aztreonam was significantly associated with a reduced risk of GVHD-related mortality. 163Rifaximin is a rifamycin derivative, and can retain the structure of the colonic microbiota and exert anti-inflammatory activity. 170In a retrospective study, Weber et al. confirmed that the use of rifaximin reduced the negative effects of systemic antibiotics on microbial composition relative to patients taking a ciprofloxacin/metronidazole treatment.Moreover, patients treated with rifaximin often showed higher urinary 3-IS levels and a lower abundance of Enterococcus and rifaximin can reduce the incidence of intestinal GVHD, reduce GVHD-related mortality, and improve OS. 171 Another study demonstrated that patients treated with rifaximin alone or rifaximin combined with systemic antibiotics showed higher 3-IS levels, a higher abundance of Clostridium cluster XIVa (CCXIVa), a higher Shannon index, a lower incidence of severe GI GVHD, a lower TRM, and longer OS. 162Given these results, the evidence suggests that rifaximin permits a higher gut microbiome diversity, even in the presence of systemic broad-spectrum antibiotics.
In addition, the duration of antibiotic use is also associated with differences in aGVHD response.A retrospective study evaluated the effect of antibiotic use timing on gut microbiota composition and prognosis, and their results showed that early exposure to antibiotics (i.e., between Day −7 and Day 0 relative to the allo-HSCT procedure) significantly reduced the abundance of symbiotic bacteria Clostridiales and increased GVHD-related mortality. 172aken together, these studies confirm that different antibiotic regimens and timing of use treatments following allo-HSCT are associated with different clinical outcomes.Specifically, the use of broad-spectrum antibiotics is associated with increased incidence and severity of GVHD in multiple centers, while some narrow-spectrum antibiotics, such as rifaximin, are associated with decreased incidence and severity of GVHD.Therefore, rational selection of antibiotic treatments after allo-HSCT may be able to reduce the incidence and severity of GVHD.Further multicenter, prospective studies are required to optimize the use of antibiotics in allo-HSCT patients to both prevent bacteremia or sepsis while limiting intestinal microbiome damage, thereby reducing the incidence and mortality of GVHD.
In conclusion, there increasing empirical evidence that gut microbiota and microbiota-derived metabolites play important roles in the occurrence and development of GVHD.Moreover, their dysregulation appears to be significantly associated with poor prognosis.A series of therapeutic strategies to regulate the gut microbiota via nutritional support, FMT, probiotics, prebiotics, and adjustment of antibiotic use may assist in the clinical prevention and treatment of GVHD.However, the specific mechanisms involved in the regulation of gut microbiota and microbiota-derived metabolites in GVHD remain unclear.Therefore, it is still necessary to further explore the role of gut microbiota and microbiotaderived metabolites in GVHD in future research.This can elucidate the real effects of gut microbiota on GVHD, and facilitate the identification of a safe and effective treatment strategy for regulating the damaged microbiota of GVHD patients, thereby improving their prognosis.

T B L E 5 FMT 1 NCT04139577 1 NCT05094765 1 NCT04280471FMT 1 NCT04285424 1 NCT03214289 2 NCT03812705 3 NCT03819803 3 NCT04769895Probiotics
Ongoing clinical trials for the treatment and prevention of GVHD.study to compare enteral nutrition with parenteral nutrition as feeding support in patients presenting with malignant hemopathy who underwent an allogeneic HSC transplantation (NEPHA) Fecal microbiota transplantation for treatment of refractory graft-versus-host disease-a pilot study Steroid-resistant GI-related aGVHD FMT Pilot NCT03549676 Prospective study of FMT for acute intestinal GVHD After allo-HSCT Steroid-resistant/dependent intestinal aGVHD FMT NA NCT04711967 FMT in high-risk acute GVHD after ALLO HCT High-risk aGVHD FMT Fecal microbiota transplant (FMT) capsule for improving the efficacy of GI-aGVHD Glucocorticoid-refractory aGVHD FMT Fecal microbiota transplantation for the treatment of severe acute gut graftversus-host disease High-risk or steroid refractory aGVHD FMT for Steroid resistant gut acute GVHD aGVHD FMT Fecal microbiota transplantation for steroid resistant and steroid-dependent gut acute graft-versus-host disease Steroid-resistant or steroiddependent gut aGVHD FMT Efficacy and safety of FMT capsule treating steroid-refractory GI-aGvHD Steroid refractory GI-aGVHD FMT NA NCT04622475 Fecal microbiota transplant and dietary fiber supplementation for the treatment of gut graft-versus-for steroid resistant/dependent acute GI GVHD Steroid resistant/dependent acute intestinal GVHD FMT Fecal microbiota transplantation in aGVHD after ASCT Steroid refractory GI-aGVHD FMT MaaT013 as Salvage therapy in ruxolitinib refractory GI-aGVHD patients Steroid refractory GVHD MaaT013 Fecal microbiota transplantation with ruxolitinib and steroids as an upfront treatment of severe acute intestinal GVHD (JAK-FMT) Lactobacillus plantarum in preventing acute graft-versus-host disease in children undergoing donor stem cell transplant diet in preventing graft-versus-host disease in patients undergoing donor stem cell transplant HSCT recipient Gluten-free diet (GFD) NA NCT03102060 Dietary manipulation of the microbiome-metabolomic axis for mitigating GVHD in allo HCT patients of a prebiotic to promote a healthy gut microbiome in pediatric stem cell transplant vitamin A in preventing gastrointestinal GVHD in participants undergoing donor stem cell transplant best antibiotic to protect friendly gut bacteria during the course of stem cell Summary of the relationship between gut microbiota and GVHD.
T A B L E 2 Summary of recent FMT clinical trials in GVHD patients.Battipaglia et al. confirmed that FMT is safe and effective for treating allo-HSCT patients carrying MDRB; in that study, seven of ten patients were successfully decolonized after FMT.
T A B L E 4