Considering the Immune System during Fecal Microbiota Transplantation for Clostridioides difficile Infection

Understanding host immune responses in Clostridioides difficile infection: Implications for pathogenesis and immunotherapy

Clostridioides difficile (C. difficile) is the predominant causative agent of nosocomial diarrhea worldwide. Infection with C. difficile occurs due to the secretion of large glycosylating toxin proteins, which can lead to toxic megacolon or mortality in susceptible hosts. A critical aspect of C. difficile's biology is its ability to persist asymptomatically within the human host. Individuals harboring asymptomatic colonization or experiencing a single episode of C. difficile infection (CDI) without recurrence exhibit heightened immune responses compared to symptomatic counterparts. The significance of these immune responses cannot be overstated, as they play critical roles in the development, progression, prognosis, and outcomes of CDI. Nonetheless, our current comprehension of the immune responses implicated in CDI remains limited. Therefore, further investigation is imperative to elucidate their underlying mechanisms. This review explores recent advancements in comprehending CDI pathogenesis and how the host immune system response influences disease progression and severity, aiming to enhance our capacity to develop immunotherapy-based treatments for CDI.

Fecal microbiota transplantation stimulates type 2 and tolerogenic immune responses in a mouse model

OBJECTIVES

Clostridioides difficile infection (CDI) is the leading hospital-acquired infection in North America. While previous work on fecal microbiota transplantation (FMT), a highly effective treatment for CDI, has focused on colonization resistance mounted against C. difficile by FMT-delivered commensals, the effects of FMT on host gene expression are relatively unexplored. This study aims to identify transcriptional changes associated with FMT, particularly changes associated with protective immune responses.

METHODS

Gene expression was assessed on day 2 and day 7 after FMT in mice after antibiotic-induced dysbiosis. Flow cytometry was also performed on colon and mesenteric lymph nodes at day 7 to investigate changes in immune cell populations.

RESULTS

FMT administration after antibiotic-induced dysbiosis successfully restored microbial alpha diversity to levels of donor mice by day 7 post-FMT. Bulk RNA sequencing of cecal tissue at day 2 identified immune genes, including both pro-inflammatory and Type 2 immune pathways as upregulated after FMT. RNA sequencing was repeated on day 7 post-FMT, and expression of these immune genes was decreased along with upregulation of genes associated with restoration of intestinal homeostasis. Immunoprofiling on day 7 identified increased colonic CD45+ immune cells that exhibited dampened Type 1 and heightened regulatory and Type 2 responses. These include an increased abundance of eosinophils, alternatively activated macrophages, Th2, and T regulatory cell populations.

CONCLUSION

These results highlight the impact of FMT on host gene expression, providing evidence that FMT restores intestinal homeostasis after antibiotic treatment and facilitates tolerogenic and Type 2 immune responses.

Investigating the impact of antibiotic-induced dysbiosis on protection from Clostridium difficile colitis by mouse colonic innate lymphoid cells

Innate lymphoid cells (ILCs) play a critical role in maintaining intestinal health in homeostatic and diseased conditions. During Clostridium difficile infection (CDI), IL-33 activates ILC2 to protect from colonic damage and mortality. The function of IL-33 and ILC is tightly regulated by the intestinal microbiota. We set out to determine the impact of antibiotic-induced disruption of the microbiome on ILC function. Our goal was to understand antibiotic-induced changes in ILC function on susceptibility to C. difficile colitis in a mouse model. We utilized high-throughput single-cell RNAseq to investigate the phenotypic features of colonic ILC at baseline, after antibiotic administration with or without IL-33 treatment. We identified a heterogeneous landscape of colonic ILCs with gene signatures of inflammatory, anti-inflammatory, migratory, progenitor, plastic, and antigen-presenting ILCs. Antibiotic treatment decreased ILC2 while coordinately increasing ILC1 and ILC3 phenotypes. Notably, Ifng+, Ccl5+, and Il23r+ ILC increased after antibiotics. IL-33 treatment counteracted the antibiotic effect by downregulating ILC1 and ILC3 and activating ILC2. In addition, IL-33 treatment markedly induced the expression of type 2 genes, including Areg and Il5. Finally, we identified amphiregulin, produced by ILC2, as protective during C. difficile infection. Together, our data expand our understanding of how antibiotics induce susceptibility to C. difficile colitis through their impact on ILC subsets and function.IMPORTANCEClostridium difficile infection (CDI) accounts for around 500,000 symptomatic cases and over 20,000 deaths annually in the United States alone. A major risk factor of CDI is antibiotic-induced dysbiosis of the gut. Microbiota-regulated IL-33 and innate lymphoid cells (ILCs) are important in determining the outcomes of C. difficile infection. Understanding how antibiotic and IL-33 treatment alter the phenotype of colon ILCs is important to identify potential therapeutics. Here, we performed single-cell RNAseq of mouse colon ILCs collected at baseline, after antibiotic treatment, and after IL-33 treatment. We identified heterogeneous subpopulations of all three ILC subtypes in the mouse colon. Our analysis revealed several potential pathways of antibiotic-mediated increased susceptibility to intestinal infection. Our discovery that Areg is abundantly expressed by ILCs, and the protection of mice from CDI by amphiregulin treatment, suggests that the amphiregulin-epidermal growth factor receptor pathway is a potential therapeutic target for treating intestinal colitis.

Dynamics of inflammation-associated plasma proteins following faecal microbiota transplantation in patients with psoriatic arthritis and healthy controls: exploratory findings from the FLORA trial

OBJECTIVES

The gut microbiota can mediate both pro and anti-inflammatory responses. In patients with psoriatic arthritis (PsA), we investigated the impact of faecal microbiota transplantation (FMT), relative to sham transplantation, on 92 inflammation-associated plasma proteins.

METHODS

This study relates to the FLORA trial cohort, where 31 patients with moderate-to-high peripheral PsA disease activity, despite at least 3 months of methotrexate treatment, were included in a 26-week, double-blind, randomised, sham-controlled trial. Participants were allocated to receive either one gastroscopic-guided healthy donor FMT (n=15) or sham (n=16). Patient plasma samples were collected at baseline, week 4, 12 and 26 while samples from 31 age-matched and sex-matched healthy controls (HC) were collected at baseline. Samples were analysed using proximity extension assay technology (Olink Target-96 Inflammation panel).

RESULTS

Levels of 26 proteins differed significantly between PsA and HC pre-FMT (adjusted p<0.05), of which 10 proteins were elevated in PsA: IL-6, CCL20, CCL19, CDCP1, FGF-21, HGF, interferon-γ (IFN-γ), IL-18R1, monocyte chemotactic protein 3, and IL-2. In the FMT group, levels of 12 proteins changed significantly across all timepoints (tumour necrosis factor (TNF), CDCP1, IFN-γ, TWEAK, signalling lymphocytic activation molecule (SLAMF1), CD8A, CD5, Flt3L, CCL25, FGF-23, CD6, caspase-8). Significant differences in protein levels between FMT and sham-treated patients were observed for TNF (p=0.002), IFN-γ (p=0.011), stem cell factor (p=0.024), matrix metalloproteinase-1 (p=0.038), and SLAMF1 (p=0.042). FMT had the largest positive effect on IFN-γ, Axin-1 and CCL25 and the largest negative effect on CCL19 and IL-6.

CONCLUSIONS

Patients with active PsA have a distinct immunological plasma protein signature compared with HC pre-FMT. FMT affects several of these disease markers, including sustained elevation of IFN-γ.

TRIAL REGISTRATION NUMBER

NCT03058900.

Butyrate Protects against Clostridium difficile Infection by Regulating Bile Acid Metabolism

Clostridium difficile infection (CDI) is caused by a prevalent nosocomial enteric pathogen, leading to high morbidity and mortality. CDI recurrence after antibiotic treatment is high; therefore, it is necessary to develop novel therapeutics against this enteric pathogen. Butyrate is used to treat many diseases because it provides energy, has anti-inflammatory properties, and maintains intestinal barrier function. An anti-CDI effect for butyrate has been reported; however, the specific mechanism remains elusive. This study aimed to explore the potential role and mechanism of butyrate in the treatment of CDI. Using a CDI mouse model, we found that butyrate significantly inhibited CDI development by regulating bile acid metabolism. Dysregulation of fecal bile acid was significantly higher, and levels of short-chain fatty acids were significantly lower in patients with CDI than those in controls. In CDI mice, butyrate exhibited a protective role by enhancing barrier protection, exerting anti-inflammatory effects, and regulating bile acid metabolism. Butyrate treatment also regulated the production of bile salt hydrolase (BSH) flora and activated farnesoid X receptor (FXR), and its therapeutic effects were reduced in CDI mice treated with BSH or FXR inhibitors. Thus, butyrate treatment may serve as a novel therapeutic approach for patients with CDI. IMPORTANCE Here, we show that levels of fecal short-chain fatty acids (SCFAs), particularly butyrate, are reduced, and normal colon structure is damaged in patients with CDI compared with those in healthy individuals. Bile acid (BA) metabolic disorder in patients with CDI is characterized by increased primary BA levels and decreased secondary BAs. In mice, butyrate alters BA metabolism in CDI and may play a vital role in CDI treatment by promoting secondary BA metabolism. Lastly, butyrate-mediated therapeutic effects in CDI require FXR. Our findings demonstrate that butyrate treatment significantly decreases the severity of CDI-induced colitis in mice and affects BA metabolism and FXR activation, which provides a potential alternative treatment for CDI.

Host Immunity and Immunization Strategies for Clostridioides difficile Infection

Clostridioides difficile infection (CDI) represents a significant challenge to public health. C. difficile-associated mortality and morbidity have led the U.S. CDC to designate it as an urgent threat. Moreover, recurrence or relapses can occur in up to a third of CDI patients, due in part to antibiotics being the primary treatment for CDI and the major cause of the disease. In this review, we summarize the current knowledge of innate immune responses, adaptive immune responses, and the link between innate and adaptive immune responses of the host against CDI. The other major determinants of CDI, such as C. difficile toxins, the host microbiota, and related treatments, are also described. Finally, we discuss the known therapeutic approaches and the current status of immunization strategies for CDI, which might help to bridge the knowledge gap in the generation of therapy against CDI.

Fecal Microbiota Transplantation in Inflammatory Bowel Disease

Inflammatory bowel diseases represent a complex array of diseases of incompletely known etiology that led to gastrointestinal tract chronic inflammation. In inflammatory bowel disease, a promising method of treatment is represented by fecal microbiota transplantation (FMT), FMT has shown its increasing effectiveness and safety in recent years for recurrent CDI; moreover, it showed real clinical benefits in treating SARS-CoV-2 and CDI co-infection. Crohn’s disease and ulcerative colitis are characterized by immune dysregulation, resulting in digestive tract damage caused by immune responses. Most current therapeutic strategies are associated with high costs and many adverse effects by directly targeting the immune response, so modifying the microbial environment by FMT offers an alternative approach that could indirectly influence the host’s immune system in a safe way. Studies outline the endoscopic and clinical improvements in UC and CD in FMT patients versus control groups. This review outlines the multiple benefits of FMT in the case of IBD by improving patients unbalanced gut, therefore improving endoscopic and clinical symptomatology. We aim to emphasize the clinical importance and benefits of FMT in order to prevent flares or complications of IBD and to highlight that further validation is needed for establishing a clinical protocol for FMT in IBD.

Efficacy of Faecal Microbiota Transplantation for the Treatment of Autism in Children: Meta-Analysis of Randomised Controlled Trials

Objective

Evidence-based research methods were applied to assess the efficacy of faecal microbiota transplantation (FMT) for the treatment of autism in children.

Methods

We searched the Chinese Biomedical Literature, CNKI, Wanfang, PubMed, Embase, Web of Science, and the Cochrane Library databases to collect randomised controlled trials on faecal microbiota transplantation for the treatment of autism in children. The search included studies published from the creation of the respective database to 5 April 2022. Literature screening, data extraction, and quality evaluation were implemented by three investigators according to the inclusion and exclusion criteria. The meta-analysis was performed using the RevMan 5.1 software.

Results

Nine studies with population-based subjects and four studies with animal-based subjects were included. Five papers were screened for the meta-analysis. The results showed that FMT markedly reduced Autism Behaviour Checklist (ABC) scores in children with autism spectrum disorder (weighted mean difference (WMD) = -14.96; 95% confidence intervals (CI), -21.68 to -8.24; P < 0.001; I ² = 0%). FMT also reduced Childhood Autism Rating Scale (CARS) scores (WMD = -6.95; 95% CI, -8.76 to -5.14; P < 0.001; I ² = 28.1%).

Conclusion

Our results indicate that FMT can benefit children with autism by reducing ABC and CARS scores, but more high-quality studies are needed to verify these results.

Bibliometric and visual analysis of fecal microbiota transplantation research from 2012 to 2021

BACKGROUND

Fecal microbiota transplantation (FMT) is an emerging therapy for diseases associated with intestinal flora imbalance that has attracted increasing attention in recent years. This study aims to provide an overview of research trends in the field, and act as a reference point for future scientific research by analyzing the state of current research, identifying hotspots, and potential frontiers of FMT.

METHODS

Articles relating to FMT that were published between the years 2012 and 2021 were retrieved from the Web of Science Core Collection. Bibliometric analysis was performed using Microsoft Excel and CiteSpace.

RESULTS

A total of 2,403 English language articles relating to FMT research were published over the last ten years. Most of this research was carried out in the United States of America, with Harvard Medical school being the most productive institution. Much of the research was published in the PLoS One journal. Alexander Khoruts was identified as a prominent, productive researcher in the field. Keyword analysis revealed that research hot spots included gut microbiota, Clostridium difficile infection (CDI), and diseases. Burst detection indicated that future research frontiers include clinical practice guidelines and strategies.

CONCLUSION

Our analysis explored hot spots and emerging trends in the FMT field. Indications for use of FMT extended from digestive system diseases to other systemic diseases. Additionally, areas such as risk assessment and control, along with application methods were also a focus of current research. Moreover, research relating to optimization of clinical practice has excellent prospects.

Immunological mechanisms of fecal microbiota transplantation in recurrent Clostridioides difficile infection

Fecal microbiota transplantation (FMT) is a successful method for treating recurrent Clostridioides difficile (C. difficile) infection (rCDI) with around 90% efficacy. Due to the relative simplicity of this approach, it is being widely used and currently, thousands of patients have been treated with FMT worldwide. Nonetheless, the mechanisms underlying its effects are just beginning to be understood. Data indicate that FMT effectiveness is due to a combination of microbiological direct mechanisms against C. difficile, but also through indirect mechanisms including the production of microbiota-derived metabolites as secondary bile acids and short chain fatty acids. Moreover, the modulation of the strong inflammatory response triggered by C. difficile after FMT seems to rely on a pivotal role of regulatory T cells, which would be responsible for the reduction of several cells and soluble inflammatory mediators, ensuing normalization of the intestinal mucosal immune system. In this minireview, we analyze recent advances in these immunological aspects associated with the efficacy of FMT.

Different Alterations in Gut Microbiota between Bifidobacterium longum and Fecal Microbiota Transplantation Treatments in Propionic Acid Rat Model of Autism

Autism spectrum disorders (ASD) consist of a range of neurodevelopmental conditions accompanied by dysbiosis of gut microbiota. Therefore, a number of microbiota manipulation strategies were developed to restore their balance. However, a comprehensive comparison of the various methods on gut microbiota is still lacking. Here, we evaluated the effect of Bifidobacterium (BF) treatment and fecal microbiota transplantation (FT) on gut microbiota in a propionic acid (PPA) rat model of autism using 16S rRNA sequencing. Following PPA treatment, gut microbiota showed depletion of Bacteroidia and Akkermansia accompanied by a concomitant increase of Streptococcus, Lachnospiraceae, and Paraeggerthella. The dysbiosis was predicted to cause increased levels of porphyrin metabolism and impairments of acyl-CoA thioesterase and ubiquinone biosynthesis. On the contrary, BF and FT treatments resulted in a distinct increase of Clostridium, Bifidobacterium, Marvinbryantia, Butyricicoccus, and Dorea. The taxa in BF group positively correlated with vitamin B12 and flagella biosynthesis, while FT mainly enriched flagella biosynthesis. In contrast, BF and FT treatments negatively correlated with succinate biosynthesis, pyruvate metabolism, nitrogen metabolism, beta-Lactam resistance, and peptidoglycan biosynthesis. Therefore, the present study demonstrated that BF and FT treatments restored the PPA-induced dysbiosis in a treatment-specific manner.

Changes in IgA-targeted microbiota following fecal transplantation for recurrent Clostridioides difficile infection

Secretory immunoglobulin A (IgA) interacts with intestinal microbiota and promotes mucosal homeostasis. IgA-bacteria interactions are altered during inflammatory diseases, but how these interactions are shaped by bacterial, host, and environmental factors remains unclear. In this study, we utilized IgA-SEQ to profile IgA-bound fecal bacteria in 48 recurrent Clostridioides difficile patients before and after successful fecal microbiota transplantation (FMT) to gain further insight. Prior to FMT, Escherichia coli was the most highly IgA-targeted taxon; following restoration of the microbiota by FMT, highly IgA-targeted taxa included multiple Firmicutes species. Post-FMT IgA-targeting was unaffected by the route of FMT delivery (colonoscopy versus capsule), suggesting that both methods lead to the establishment of healthy immune-bacterial interactions in the gut. Interestingly, IgA-targeting in FMT recipients closely resembled the IgA-targeting patterns of the donors, and fecal donor identity was significantly associated with IgA-targeting of the recipient microbiota. These data support the concept that intrinsic bacterial properties drive IgA recognition across genetically distinct human hosts. Together, this study suggests that IgA-bacterial interactions are reestablished in human FMT recipients to resemble that of the healthy fecal donor.

Fecal Microbiota Transplant in Severe and Non-Severe Clostridioides difficile Infection. Is There a Role of FMT in Primary Severe CDI?

Background: Faecal microbiota transplant (FMT) is a highly effective therapy for recurrent Clostridioides difficile infection (rCDI) with cure rates ranging between 85 and 92%. The FMT role for primary Clostridioides difficile infection (CDI) has yet to be settled because of limited data and small-sample studies presented in the current literature. Our study goals were to report the risk factors and the risk of recurrence after FMT for each CDI episode (first, second, and third) and to explore if there is a role of FMT in primary severe CDI. Methods: We conducted a retrospective study to analyze the clinical characteristics and the outcomes of 96 FMT patients with a prior 10 day course of antibiotic treatment in the medical records, of which 71 patients with recurrent CDI and 25 patients with a primary CDI. Results: The overall primary cure rate in our study was 88.5% and the primary cure rate for the severe forms was 85.7%. The data analysis revealed 5.25%, 15.15%, and 27.3% FMT recurrence rates for primary, secondary, and tertiary severe CDI. The risk of recurrence was significantly associated with FMT after the second and the third CDI severe episodes (p < 0.05), but not with FMT after the first severe CDI episode. Conclusions: This study brings new data in supporting the FMT role in CDI treatment, including the primary severe CDI, however, further prospective and controlled studies on larger cohorts should be performed in this respect.

Translational Aspects of the Immunology of Clostridioides difficile Infection: Implications for Pediatric Populations

Clostridioides difficile has become the most common healthcare-associated pathogen in the United States, leading the US Centers for Disease Control and Prevention (CDC) to classify C. difficile as an "urgent" public health threat that requires "urgent and aggressive action." This call to action has led to new discoveries that have advanced our understanding of Clostridioides difficile infection (CDI) immunology and clinical development of immunologic-based therapies for CDI prevention. However, CDI immunology research has been limited in pediatric populations, and several unanswered questions remain regarding the function of host immune response in pediatric CDI pathogenesis and the potential role of immunologic-based therapies in children. This review summarizes the innate and adaptive immune responses previously characterized in animals and humans and provides a current update on clinical development of immunologic-based therapies for CDI prevention in adults and children. These data inform the future research needs for children.

Fecal Microbiota Transplantation Increases Colonic IL-25 and Dampens Tissue Inflammation in Patients with Recurrent Clostridioides difficile

Clostridioides difficile infection (CDI) is the most common hospital-acquired infection in the United States. Antibiotic-induced dysbiosis is the primary cause of susceptibility, and fecal microbiota transplantation (FMT) has emerged as an effective therapy for recurrence. We previously demonstrated in the mouse model of CDI that antibiotic-induced dysbiosis reduced colonic expression of interleukin 25 (IL-25) and that FMT protected in part by restoring IL-25 signaling. Here, we conducted a prospective study in humans to test if FMT induced IL-25 expression in the colons of patients with recurrent CDI (rCDI). Colonic biopsy specimens and blood were collected at the time of FMT and 60 days later. Colon biopsy specimens were analyzed for IL-25 protein levels, total tissue transcriptome, and epithelium-associated microbiota before and after FMT, and peripheral immune cells were immunophenotyped. FMT increased alpha diversity of the colonic microbiota and levels of IL-25 in colonic tissue. In addition, FMT increased expression of homeostatic genes and repressed inflammatory genes. Finally, circulating Th17 cells were decreased post-FMT. The increase in levels of the cytokine IL-25 accompanied by decreased inflammation is consistent with FMT acting in part to protect from recurrent CDI via restoration of commensal activation of type 2 immunity.

IMPORTANCE Fecal microbiota transplantation (FMT) is an effective treatment for C. difficile infection for most patients; however, introducing a complex mixture of microbes also has had unintended consequences for some patients. Attempts to create a standardized probiotic therapeutic that recapitulates the efficacy of FMT have been unsuccessful to date. We sought to understand what immune markers are changed in patients undergoing FMT to treat recurrent C. difficile infection and identified an immune signaling molecule, IL-25, that was restored by FMT. This finding indicates that adjunctive therapy with IL-25 could be useful in treating C. difficile infection.

Fecal Microbiota Transplantation for Recurrent Clostridioides difficile Infection Associates With Functional Alterations in Circulating microRNAs

BACKGROUND AND AIMS

The molecular mechanisms underlying successful fecal microbiota transplantation (FMT) for recurrent Clostridioides difficile infection (rCDI) remain poorly understood. The primary objective of this study was to characterize alterations in microRNAs (miRs) following FMT for rCDI.

METHODS

Sera from 2 prospective multicenter randomized controlled trials were analyzed for miRNA levels with the use of the Nanostring nCounter platform and quantitative reverse-transcription (RT) polymerase chain reaction (PCR). In addition, rCDI-FMT and toxin-treated animals and ex vivo human colonoids were used to compare intestinal tissue and circulating miRs. miR inflammatory gene targets in colonic epithelial and peripheral blood mononuclear cells were evaluated by quantitative PCR (qPCR) and 3'UTR reporter assays. Colonic epithelial cells were used for mechanistic, cytoskeleton, cell growth, and apoptosis studies.

RESULTS

miRNA profiling revealed up-regulation of 64 circulating miRs 4 and 12 weeks after FMT compared with screening, of which the top 6 were validated in the discovery cohort by means of RT-qPCR. In a murine model of relapsing-CDI, RT-qPCR analyses of sera and cecal RNA extracts demonstrated suppression of these miRs, an effect reversed by FMT. In mouse colon and human colonoids, C difficile toxin B (TcdB) mediated the suppressive effects of CDI on miRs. CDI dysregulated DROSHA, an effect reversed by FMT. Correlation analyses, qPCR ,and 3'UTR reporter assays revealed that miR-23a, miR-150, miR-26b, and miR-28 target directly the 3'UTRs of IL12B, IL18, FGF21, and TNFRSF9, respectively. miR-23a and miR-150 demonstrated cytoprotective effects against TcdB.

CONCLUSIONS

These results provide novel and provocative evidence that modulation of the gut microbiome via FMT induces alterations in circulating and intestinal tissue miRs. These findings contribute to a greater understanding of the molecular mechanisms underlying FMT and identify new potential targets for therapeutic intervention in rCDI.

The role of faecal microbiota transplantation: looking beyond Clostridioides difficile infection

Faecal microbiota transplantation (FMT) is the transfer of screened and minimally processed faecal material from a 'healthy' donor to 'diseased' recipient. It has an established role, and is recommended as a therapeutic strategy, in the management of recurrent Clostridioides difficile infection (CDI). Recognition that gut dysbiosis is associated with, and may contribute to, numerous disease states has led to interest in exploiting FMT to 'correct' this microbial imbalance. Conditions for which it is proposed to be beneficial include inflammatory bowel disease, irritable bowel syndrome, liver disease and hepatic encephalopathy, neuropsychiatric conditions such as depression and anxiety, systemic inflammatory states like sepsis, and even coronavirus disease 2019. To understand what role, if any, FMT may play in the management of these conditions, it is important to consider the potential risks and benefits of the therapy. Regardless, there are several barriers to its more widespread adoption, which include incompletely understood mechanism of action (especially outside of CDI), inability to standardise treatment, disagreement on its active ingredients and how it should be regulated, and lack of long-term outcome and safety data. Whilst the transfer of faecal material from one individual to another to treat ailments or improve health has a history dating back thousands of years, there are fewer than 10 randomised controlled trials supporting its use. Moving forward, it will be imperative to gather as much data from FMT donors and recipients over as long a timeframe as possible, and for trials to be conducted with rigorous methodology, including appropriate control groups, in order to best understand the utility of FMT for indications beyond CDI. This review discusses the history of FMT, its appreciable mechanisms of action with reference to CDI, indications for FMT with an emerging evidence base above and beyond CDI, and future perspectives on the field.

Perspectives on how mucosal immune responses, infections and gut microbiome shape IgA nephropathy and future therapies

Infections have been considered to play a critical role in the pathogenesis of IgA nephropathy (IgAN) because synpharyngitic hematuria is a common feature in IgAN. However, how infections participate in this process is still debated. More recent studies have also revealed that the alteration of the gut microbiome exerts a profound effect on host immune responses, contributing to the etiology or progression of autoimmunity. Considering IgA as the first line of defense against bacterial and viral antigens, this review evaluates the relationships among intestinal infections, gut microbiome, and IgA for a better understanding of the pathogenesis of IgAN. Moreover, as a prototype of IgA immunity, we provide detailed clarification of IgAN pathogenesis to shed light on other diseases in which IgA plays a role. Finally, we discuss potential therapies focusing on microbes and mucosal immune responses in IgAN.

Mechanisms underpinning the efficacy of faecal microbiota transplantation in treating gastrointestinal disease

Faecal microbiota transplantation (FMT) is currently a recommended therapy for recurrent/refractory Clostridioides difficile infection (CDI). The success of FMT for CDI has led to interest in its therapeutic potential in many other disorders. The mechanisms that underpin the efficacy of FMT are not fully understood. Importantly, FMT remains a crucial treatment in managing CDI and understanding the mechanisms that underpin its success will be critical to improve its clinical efficacy, safety and usability. Furthermore, a deeper understanding of this may allow us to expose FMT's full potential as a therapeutic tool for other disease states. This review will explore the current understanding of the mechanisms underlying the efficacy of FMT across a variety of diseases.

The importance of airway and lung microbiome in the critically ill

During critical illness, there are a multitude of forces such as antibiotic use, mechanical ventilation, diet changes and inflammatory responses that could bring the microbiome out of balance. This so-called dysbiosis of the microbiome seems to be involved in immunological responses and may influence outcomes even in individuals who are not as vulnerable as a critically ill ICU population. It is therefore probable that dysbiosis of the microbiome is a consequence of critical illness and may, subsequently, shape an inadequate response to these circumstances.Bronchoscopic studies have revealed that the carina represents the densest site of bacterial DNA along healthy airways, with a tapering density with further bifurcations. This likely reflects the influence of micro-aspiration as the primary route of microbial immigration in healthy adults. Though bacterial DNA density grows extremely sparse at smaller airways, bacterial signal is still consistently detectable in bronchoalveolar lavage fluid, likely reflecting the fact that lavage via a wedged bronchoscope samples an enormous surface area of small airways and alveoli. The dogma of lung sterility also violated numerous observations that long predated culture-independent microbiology.The body's resident microbial consortia (gut and/or respiratory microbiota) affect normal host inflammatory and immune response mechanisms. Disruptions in these host-pathogen interactions have been associated with infection and altered innate immunity.In this narrative review, we will focus on the rationale and current evidence for a pathogenic role of the lung microbiome in the exacerbation of complications of critical illness, such as acute respiratory distress syndrome and ventilator-associated pneumonia.

The adaptive immune response to Clostridioides difficile: A tricky balance between immunoprotection and immunopathogenesis

Clostridioides difficile (C. difficile) is the major cause of hospital-acquired gastrointestinal infections in individuals following antibiotics treatment. The pathogenesis of C. difficile infection (CDI) is mediated mainly by the production of toxins that induce tissue damage and host inflammatory responses. While innate immunity is well characterized in human and animal models of CDI, adaptive immune responses remain poorly understood. In this review, the current understanding of adaptive immunity is summarized and its influence on pathogenesis and disease outcome is discussed. The perspectives on what we believe to be the main pending questions and the focus of future research are also provided. There is no doubt that the innate immune response provides a first line of defense to CDI. But, is the adaptive immune response a friend or a foe? Probably it depends on the course of the disease. Adaptive immunity is essential for pathogen eradication, but may also trigger uncontrolled or pathological inflammation. Most of the understanding of the role of T cells is based on findings from experimental models. While they are a very valuable tool for research studies, more studies in human are needed to translate these findings into human disease. Another main challenge is to unravel the role of the different T cell populations on protection or induction of immunopathogenesis.