Page 15 - 2021_06-Haematologica-web

The post-HCT microbiome: effects and targeted therapy
bial translocation from the lumen of the oral cavity or gas- trointestinal tract into the systemic circulation, which has led to the widespread use of antibiotic prophylaxis in allo- geneic HCT recipients in whom prolonged mucositis and co-incident neutropenia are common. Patients also have high rates of exposure to broad-spectrum empiric antibi- otics, which are initiated in response to fever in the neu- tropenic period. Microbial sources of these neutropenic febrile episodes are rarely proven but the consequences of untreated severe infection in this group of patients are such that it is standard practice to initiate broad-spectrum empiric therapy when a fever occurs.5
Acute GvHD is classically described as a three-step process.6,7 The first step involves host tissue injury, result- ing in release of damage-associated patterns (DAMPS; uric acid, ATP, heparin sulfate, HMGB-1, interleukin [IL]- 33) and pathogen-associated patterns (PAMPS; lipopolysaccharides [LPS]), which further stimulate the production of inflammatory cytokines such as tumor necrosis factor [TNF]-α, IL-1 and IL-6. The second step comprises priming and expansion of alloreactive donor lymphocytes, predominantly T cells, which are recruited to the host tissues; hematopoietic and non-hematopoietic antigen-presenting cells interact with alloreactive lym- phocytes, skewing CD4+ T lymphocytes towards a T- helper (Th1) or Th17 phenotype that produce inflamma- tory cytokines such as interferon (IFN)-γ and IL-17, acti- vating cytotoxic CD8+ T cells, and stimulating their pro- liferation. The third step involves recruitment and activa- tion of additional (innate) effector cells, such as macrophages and neutrophils, which amplify the cytokine production (‘cytokine storm’) and orchestrate further apoptotic tissue damage.
Systemic, high-dose corticosteroids are the first-line treatment for acute GvHD and are successful in about two-thirds of the cases.8 Patients with severe or steroid- refractory GvHD have a dismal prognosis, with long- term survival rates reported between 5-30%.9 This has led to persistent efforts to identify new therapies for GvHD.
Our current knowledge of the composition of the intes- tinal microbial community, in both health and disease, is largely based on the analysis of fecal samples using an amplicon-based sequencing approach that targets the variable regions of the prokaryotic 16S ribosomal RNA gene. 16S rRNA sequencing allows us to measure micro- bial composition (i.e., the presence of certain taxa) and compute the summary measure α-diversity (commonly measured by the Simpson or Shannon index), which has been used in a large number of transplant studies that have associated microbiota abnormalities with transplant outcome.10 A second method is metagenomic sequencing, which involves untargeted (‘shotgun’) metagenomic sequencing of all the genes present in a given sample. Multiple technologies are available for metagenomic sequencing, but they can be broadly grouped into either short-read (e.g., Illumina) or long-read (e.g., PacBio, Nanopore) platforms.11 Independently of the sequencing method, metagenomic sequencing provides reliable species-level taxonomy, as well as profiling of the func- tional capacity of the intestinal microbial community as a whole, by identifying genes that confer microbial func- tion (e.g., fatty acid synthesis or degradation). Metagenomic sequencing can thus be a powerful hypoth- esis-generating tool and can enhance our understanding
of potential mechanisms underpinning associations between particular microbial compositions and clinical outcome.
The healthy microbiome and intestinal homeostasis
The human microbiome constitutes a diverse collection of bacteria, viruses, archaea and eukaryotic microbes that inhabit all parts of the body but predominantly reside in the gut.12 Bacteria represent the largest group within the intestinal microbiome and most species belong to the Firmicutes or Bacteroidetes phyla with smaller contribu- tions from the Proteobacteria, Actinobacteria, Fusobacteria and Verrucomicrobia phyla. The dynamic ecosystem within the intestinal tract is continuously sub- jected to environmental changes induced by diet, medica- tion and disease of the host, which can dramatically alter microbial composition.13-15 Factors important in maintain- ing homeostasis and preventing pathogen outgrowth and translocation include: an intact gut epithelium that physi- cally separates the luminal microorganisms from the underlying host tissue; an extra layer of protection, pro- vided by the mucus layer that is produced by goblet cells, to hinder bacterial translocation and prevent mucosal bar- rier injury; the anaerobic commensals that predominate in the gut microbiota community of healthy individuals and that provide resistance against colonization by pathogens. The products of anaerobic commensals fur- ther prevent pathogen outgrowth; molecules derived from commensal bacteria, which can signal to Paneth cells via toll-like receptors (TLR) 2, 4, 5 and 9, and induce production of antimicrobial peptides.16 Antimicrobial pep- tides, such as defensins and regenerating gene (REG) III proteins, shape the microbial composition by sequester- ing essential nutrients and permeabilizing bacterial mem- branes; the commensals, which in addition to mediating resistance to colonization by potential pathogens, also produce metabolites that are essential micronutrients for the human host.
Microbial metabolites include vitamins (K, B12), short- chain fatty acids (SCFA) such as butyrate, propionate and acetate, which are the products of indigestible dietary fiber fermentation, aryl hydrocarbon receptor ligands called indoles generated through tryptophan metabolism and secondary bile acids.17-20 These microbial metabolites are locally important for maintenance of epithelial integri- ty and stimulation of the mucosal immune system, but can also exert effects on peripheral organs via secretion into the systemic circulation.21-24 SCFA act via several pathways to maintain homeostasis: they serve as an ener- gy source for colonocytes, promote mucus secretion, enhance expression of tight junction proteins to improve epithelial integrity, induce anti-inflammatory cytokines such as IL-10 and IL-18, and promote differentiation of anti-inflammatory regulatory T cells via engagement with G-protein coupled receptors (GPR) 43, 41 and 109a on both epithelial cells and immune cells.25 Indoles also have immune-modulating potential. For example, they can induce IL-22 production by different aryl hydrocarbon receptor-expressing lymphocytes, such as Th17 cells, nat- ural killer T cells, γδ T cells and innate lymphoid cells, which in turn enhances epithelial barrier function and promotes antimicrobial peptide production.26
haematologica | 2021; 106(8)
2043