An understanding of the role of microbiome alterations, namely dysbiosis, in influencing the development and the clinical course of neoplastic and non-neoplastic hematologic diseases, as well as in affecting treatment response, is emerging. Most data concern hematopoietic cell transplantation (HCT), where dysbiosis has been linked to post-transplant mortality and graft-versus-host disease (GVHD) and where fecal transplant is under investigation.1 Other possible implications include the role of dysbiosis in the development of hematologic malignancies (acute leukemia, lymphoma, and multiple myeloma), iron deficiency anemia, autoimmune cytopenias, and aplastic anemia. Chronic inflammatory triggering, epithelial barrier alteration, failure of antigen sequestration, and molecular mimicry are the most likely pathophysiologic mechanisms by which microbiome alteration may lead to hematologic diseases. Additionally, chemotherapy, immunosuppression, biologics, and cellular therapies used for hematologic conditions may all lead to dysbiosis and promote disease progression and infectious complications.
What Is the Microbiome?
To give an overview on how to study the microbiome in hematology, let’s first clarify the terminology: “microbiota” refers to an ecologic community of commensal, symbiotic, and pathogenic microorganisms colonizing various body districts.2 This includes the gastrointestinal and respiratory tracts, the oral cavity, the skin, and the reproductive systems, encompassing more than 1 trillion microbial cells of bacteria, fungi, viruses, and archaea.3 The term “microbiome” identifies the entire genetic asset of such microorganisms and includes about 3.3 million genes.4 The microbiota contributes to several homeostatic processes of the human body, from the regulation of metabolic pathways to the synthesis of vitamins and of short-chain fatty acids,5 and constitutes a natural defense against pathogens. Antigenic exposure to the microbiota is also fundamental for immune response maturation and inflammation regulation.6 The microbiota primarily develops during infancy and early childhood and may dynamically change later in life. Antibiotics and diet represent the most impactful environmental factors.7
Studying Microbiome Alterations
Techniques for studying the composition of microbiota were based traditionally on bacteria cultures, and then enriched with the advent of modern high-throughput sequencing methods. The goal of the Human Microbiome Project (HMP) was to characterize the human microbiota in five different body sites (the oropharynx, skin, vagina, gut, and nasal cavity), with a major focus on the intestinal microbiome.8 The HMP efforts led to the study of entire microbiota communities and their ecologies. Modern molecular techniques are based on polymerase chain reaction assays that target the 16S ribosomal RNA gene, a highly conserved region contained in bacterial genomes.9 Next-generation sequencing (NGS) allows the simultaneous identification of the entire community, with greater cost-effectiveness.10 Sequences with similarity greater than 97% are grouped into the so-called “operational taxonomic unit” (OTU). The relative abundances of different OTUs are then analyzed and the diversity is reported. More recently, metagenomic sequencing of the entire microbial genome (beyond the bacterial genome) and RNA sequencing (or transcriptome analysis) are being developed, though their high complexity and cost limit their availability.11
Examples of Dysbiosis and the Impact on Hematologic Diseases
In hematologic diseases, dysbiosis mainly presents as the loss of diversity (changes in the relative abundances of different OTUs compared to the normal homeostasis) and domination of low-diversity communities, which may become pathogenic in a certain body district. For example, in myeloid and lymphoid malignancies, dysbiosis is either provoked by the microenvironmental alterations induced by the neoplastic cells or due to chemotherapy and the widespread use of antibiotics.12,13
Dysbiosis may contribute to reduced immune defense and favor systemic infections. This relationship has been reported for gut dysbiosis in acute myeloid leukemia, which favors Enterococcus-related sepsis as well as mucositis, as shown in pediatric patients with acute lymphoblastic leukemia.14,15 More selective use of antibiotics could prevent this effect, as could the use of probiotics, as reported in some experiences.16 Moreover, dysbiosis may have a detrimental late-onset effect in leukemia survivors, where a relationship with obesity and type 2 diabetes has been shown.17
It has been known for decades that lymphomagenesis may be associated with chronic microorganism infections and colonization, such as Epstein-Barr virus, human T-lymphotropic virus 1, human herpesvirus-8, HIV, hepatitis B virus, hepatitis C virus, and H. pylori. Microbiome technologies showed that dysbiosis may contribute as well, as reported in the development of the rare conjunctival mucosa-associated lymphoid tissue (MALT) lymphoma and the increased abundance of genus Delftia versus a lower abundance of Bacteroides and Clostridium.18
In multiple myeloma, patients’ gut microbiomes have been shown to display imbalanced composition and diversity compared with those of healthy controls, and certain microbes are associated with the depth of response to therapy.19
Regarding benign conditions, different composition of gut microbiota has been reported in patients with aplastic anemia versus healthy controls.20 The relative abundance of subspecies changed during clinical course and depended on treatment with immunosuppressive agents versus transplant. Similarly, in immune thrombocytopenia, Chanjuan Liu and colleagues found a skewed gut microbiota in patients versus healthy controls,21 with some associations with disease severity and response to steroid treatment.
Finally, loss of microbiota diversity has been clearly associated with poor outcomes in the allogeneic HCT setting. This observation may influence the choice of antibiotic prophylaxis during the pre-engraftment phase, as demonstrated in a recent study where antibiotic prophylaxis with ciprofloxacin led to better survival than rifaximin.22 The impact of dietary intervention on gastrointestinal microbiota and metabolites is also being investigated in this setting.23 Probiotics and prebiotics can also reconstitute the gut microbiota and increase bacterial metabolites such as short-chain fatty acids that have immunomodulatory effects preventing acute GVHD.24 In a recent placebo-controlled trial, third-party fecal microbiota transplant proved safe and ameliorated intestinal dysbiosis in allogeneic HCT recipients, but did not decrease infections.25
Conclusions
The impact of environmental factors, particularly infections, in the development of hematologic diseases has long been recognized. The advent of NGS techniques deepens the analysis of the microbiota and allows some associations with disease onset and outcome in terms of disease severity, response to therapies, and long-term complications. For example, gut and conjunctival microbiota have been implicated in lymphomagenesis and autoimmunity; oral and gut dysbiosis have been linked to infectious risk in patients with leukemia and the development of mucositis and GVHD; and the use of chemotherapy, immunosuppressive agents, and the abuse of antibiotics may affect the composition of patients’ microbiome.
In the near future, limitations on the unscrupulous use of antibiotics and immunosuppressants in hematology, the development of specific probiotics, and the optimization of microbiota transplant may help to recover microbiota homeostasis in hematologic patients and limit short- and long-term complications of dysbiosis. This information may be of interest to candidates for chemotherapies and biologics, as well as for patients receiving novel immunotherapies.
Competing Interests
Dr. Fattizzo has received consultancy and speaker fees from Alexion, Janssen, Novartis, Roche, Samsung, Sanofi, and Sobi.
