The Impact of Maternal Microbes and Microbial Colonization in Early Life on Hematopoiesis

The impact of gut microbial signals on hematopoietic stem cells and the bone marrow microenvironment

Hematopoietic stem cells (HSCs) undergo self-renewal and differentiation in the bone marrow, which is tightly regulated by cues from the microenvironment. The gut microbiota, a dynamic community residing on the mucosal surface of vertebrates, plays a crucial role in maintaining host health. Recent evidence suggests that the gut microbiota influences HSCs differentiation by modulating the bone marrow microenvironment through microbial products. This paper comprehensively analyzes the impact of the gut microbiota on hematopoiesis and its effect on HSCs fate and differentiation by modifying the bone marrow microenvironment, including mechanical properties, inflammatory signals, bone marrow stromal cells, and metabolites. Furthermore, we discuss the involvement of the gut microbiota in the development of hematologic malignancies, such as leukemia, multiple myeloma, and lymphoma.

Ecological insights into hematopoiesis regulation: unraveling the influence of gut microbiota

The gut microbiota constitutes a vast ecological system within the human body, forming a mutually interdependent entity with the host. In recent years, advancements in molecular biology technologies have provided a clearer understanding of the role of the gut microbiota. They not only influence the local immune status and metabolic functions of the host's intestinal tract but also impact the functional transformation of hematopoietic stem cells (HSCs) through the gut-blood axis. In this review, we will discuss the role of the gut microbiota in influencing hematopoiesis. We analyze the interactions between HSCs and other cellular components, with a particular emphasis on the direct functional regulation of HSCs by the gut microbiota and their indirect influence through cellular components in the bone marrow microenvironment. Additionally, we propose potential control targets for signaling pathways triggered by the gut microbiota to regulate hematopoietic function, filling crucial knowledge gaps in the development of this research field.

Maternal diet modulates the infant microbiome and intestinal Flt3L necessary for dendritic cell development and immunity to respiratory infection

Poor maternal diet during pregnancy is a risk factor for severe lower respiratory infections (sLRIs) in the offspring, but the underlying mechanisms remain elusive. Here, we demonstrate that in mice a maternal low-fiber diet (LFD) led to enhanced LRI severity in infants because of delayed plasmacytoid dendritic cell (pDC) recruitment and perturbation of regulatory T cell expansion in the lungs. LFD altered the composition of the maternal milk microbiome and assembling infant gut microbiome. These microbial changes reduced the secretion of the DC growth factor Flt3L by neonatal intestinal epithelial cells and impaired downstream pDC hematopoiesis. Therapy with a propionate-producing bacteria isolated from the milk of high-fiber diet-fed mothers, or supplementation with propionate, conferred protection against sLRI by restoring gut Flt3L expression and pDC hematopoiesis. Our findings identify a microbiome-dependent Flt3L axis in the gut that promotes pDC hematopoiesis in early life and confers disease resistance against sLRIs.

Maternal effects drive intestinal development beginning in the embryonic period on the basis of maternal immune and microbial transfer in chickens

BACKGROUND

Nutrition drives immunity and health in animals, and maternal immunity benefits offspring. In our previous study, a nutritional intervention strategy was found to promote the immunity of hens, which subsequently improved immunity and growth in offspring chicks. Maternal effects clearly exist, but how are mothers' immune advantages transferred to their offspring, and how do they benefit them?

RESULTS

Here, we traced the beneficial effects back to the process of egg formation in the reproductive system, and we focused on the embryonic intestinal transcriptome and development, as well as on maternal microbial transfer in offspring. We found that maternal nutritional intervention benefits maternal immunity, egg hatching, and offspring growth. The results of protein and gene quantitative assays showed that the transfer of immune factors into egg whites and yolks depends on maternal levels. Histological observations indicated that the promotion of offspring intestinal development begins in the embryonic period. Microbiota analyses suggested that maternal microbes transfer to the embryonic gut from the magnum to the egg white. Transcriptome analyses revealed that offspring embryonic intestinal transcriptome shifts are related to development and immunity. Moreover, correlation analyses showed that the embryonic gut microbiota is correlated with the intestinal transcriptome and development.

CONCLUSIONS

This study suggests that maternal immunity positively influences offspring intestinal immunity establishment and intestinal development beginning in the embryonic period. Adaptive maternal effects might be accomplished via the transfer of relatively large amounts of maternal immune factors and by shaping of the reproductive system microbiota by strong maternal immunity. Moreover, reproductive system microbes may be useful resources for the promotion of animal health. Video Abstract.

Microbial Dysbiosis Tunes the Immune Response Towards Allergic Disease Outcomes

The hygiene hypothesis has been popularized as an explanation for the rapid increase in allergic disease observed over the past 50 years. Subsequent epidemiological studies have described the protective effects that in utero and early life exposures to an environment high in microbial diversity have in conferring protective benefits against the development of allergic diseases. The rapid advancement in next generation sequencing technology has allowed for analysis of the diverse nature of microbial communities present in the barrier organs and a determination of their role in the induction of allergic disease. Here, we discuss the recent literature describing how colonization of barrier organs during early life by the microbiota influences the development of the adaptive immune system. In parallel, mechanistic studies have delivered insight into the pathogenesis of disease, by demonstrating the comparative effects of protective T regulatory (Treg) cells, with inflammatory T helper 2 (Th2) cells in the development of immune tolerance or induction of an allergic response. More recently, a significant advancement in our understanding into how interactions between the adaptive immune system and microbially derived factors play a central role in the development of allergic disease has emerged. Providing a deeper understanding of the symbiotic relationship between our microbiome and immune system, which explains key observations made by the hygiene hypothesis. By studying how perturbations that drive dysbiosis of the microbiome can cause allergic disease, we stand to benefit by delineating the protective versus pathogenic aspects of human interactions with our microbial companions, allowing us to better harness the use of microbial agents in the design of novel prophylactic and therapeutic strategies.

From germ-free to wild: modulating microbiome complexity to understand mucosal immunology

The gut microbiota influences host responses at practically every level, and as research into host-microbe interactions expands, it is not surprising that we are uncovering similar roles for the microbiota at other barrier sites, such as the lung and skin. Using standard laboratory mice to assess host-microbe interactions, or even host intrinsic responses, can be challenging, as slight variations in the microbiota can affect experimental outcomes. When it comes to designing and selecting an appropriate level of microbial diversity and community structure for colonization of our laboratory rodents, we have more choices available to us than ever before. Here we will discuss the different approaches used to modulate microbial complexity that are available to study host-microbe interactions. We will describe how different models have been used to answer distinct biological questions, covering the entire microbial spectrum, from germ-free to wild.

NOD1 in the interplay between microbiota and gastrointestinal immune adaptations

Nucleotide-binding oligomerization domain 1 (NOD1), a pattern recognition receptor (PRR) that detects bacterial peptidoglycan fragments and other danger signals, has been linked to inflammatory pathologies. NOD1, which is expressed by immune and non-immune cells, is activated after recognizing microbe-associated molecular patterns (MAMPs). This recognition triggers host defense responses and both immune memory and tolerance can also be achieved during these processes. Since the gut microbiota is currently considered a master regulator of human physiology central in health and disease and the intestine metabolizes a wide range of nutrients, drugs and hormones, it is a fact that dysbiosis can alter tissues and organs homeostasis. These systemic alterations occur in response to gastrointestinal immune adaptations that are not yet fully understood. Even if previous evidence confirms the connection between the microbiota, the immune system and metabolic disorders, much remains to be discovered about the contribution of NOD1 to low-grade inflammatory pathologies such as obesity, diabetes and cardiovascular diseases. This review compiles the most recent findings in this area, while providing a dynamic and practical framework with future approaches for research and clinical applications on targeting NOD1. This knowledge can help to rate the consequences of the disease and to stratify the patients for therapeutic interventions.

Microbiota and Microglia Interactions in ASD

Autism spectrum disorders (ASD) are serious, highly variable neurodevelopmental disorders, commonly characterized by the manifestation of specific behavioral abnormalities, such as stereotypic behaviors and deficits in social skills, including communication. Although the neurobiological basis for ASD has attracted attention in recent decades, the role of microglial cells, which are the main resident myeloid cell population in the brain, is still controversial and underexplored. Microglia play several fundamental roles in orchestrating brain development and homeostasis. As such, alterations in the intrinsic functions of these cells could be one of the driving forces responsible for the development of various neurodevelopmental disorders, including ASD. Microglia are highly sensitive to environmental cues. Amongst the environmental factors known to influence their intrinsic functions, the gut microbiota has emerged as a central player, controlling both microglial maturation and activation. Strikingly, there is now compelling data suggesting that the intestinal microbiota can play a causative role in driving the behavioural changes associated with ASD. Not only is intestinal dysbiosis commonly reported in ASD patients, but therapies targeting the microbiome can markedly alleviate behavioral symptoms. Here we explore the emerging mechanisms by which altered microglial functions could contribute to several major etiological factors of ASD. We then demonstrate how pre- and postnatal environmental stimuli can modulate microglial cell phenotype and function, underpinning the notion that reciprocal interactions between microglia and intestinal microbes could play a crucial role in ASD aetiology.

Dirty mice join the immunologist's toolkit

The microbiota is a driving force that influences host physiological functions. In this review, we discuss some of the methods that have been used in the pursuit of relevant host-microbiota interactions that control immune fitness and disease susceptibility, with a focus on dirty mice which have been recently incorporated in the immunologist's toolkit.

Historical Perspective: Metchnikoff and the intestinal microbiome

Metchnikoff's essay, Intestinal Bacteriotherapy, was written when the study of microbiology was still in its infancy and few intestinal diseases had been ascribed to a specific bacterial infection. Metchnikoff offered perceptive ideas that have become standard in today's science. This Historical Perspectives commentary examines how Metchnikoff's article influenced our field. An accompanying editorial by Siamon Gordon explores this topic further and describes the relevance of Metchnikoff's work to the current Covid-19 infection. We also include a translation of this fundamental article by Metchnikoff, as presented by Claudine Neyen.

Differences in the gut Firmicutes to Bacteroidetes ratio across age groups in healthy Ukrainian population

BACKGROUND

Gut microbiota plays an important role in physiological and pathological processes of the host organism, including aging. Microbiota composition was shown to vary significantly throughout the life course. Age-related changes in the composition of microbiota were reported in several human studies. In present study, age-related dynamics of phylogenetic profile of gut microbiota was investigated in 1550 healthy participants from Ukrainian population.

RESULTS

Significant changes in the microbiota composition determined by qRT-PCR at the level of major microbial phyla across age groups have been observed. The relative abundance of Actinobacteria and Firmicutes phyla increased, while that of Bacteroidetes decreased from childhood to elderly age. Accordingly, the Firmicutes/Bacteroidetes (F/B) ratio was shown to significantly increase until elder age. In both sexes, odds to have F/B > 1 tended to increase with age, reaching maximum values in elder age groups [OR = 2.7 (95% CI, 1.2-6.0) and OR = 3.7 (95% CI, 1.4-9.6) for female and male 60-69-year age groups, respectively, compared to same-sex reference (0-9-year) age groups].

CONCLUSIONS

In conclusion, data from our study indicate that composition of the human intestinal microbiota at the level of major microbial phyla significantly differs across age groups. In both sexes, the F/B ratio tends to increase with age from 0-9-year to 60-69-year age groups. Further studies are needed for a better understanding of mechanisms underlying age-related dynamics of human microbiota composition.

Perturbed mitochondrial dynamics, an emerging aspect of epithelial-microbe interactions

Mitochondria exist in a complex network that is constantly remodeling via the processes of fission and fusion in response to intracellular conditions and extracellular stimuli. Excessive fragmentation of the mitochondrial network because of an imbalance between fission and fusion reduces the cells' capacity to generate ATP and can be a forerunner to cell death. Given the critical roles mitochondria play in cellular homeostasis and innate immunity, it is not surprising that many microbial pathogens can disrupt mitochondrial activity. Here we note the putative contribution of mitochondrial dysfunction to gut disease and review data showing that infection with microbial pathogens can alter the balance between mitochondrial fragmentation and fusion, preventing normal remodeling (i.e., dynamics) and can lead to cell death. Current data indicate that infection of epithelia or macrophages with microbial pathogens will ultimately result in excessive fragmentation of the mitochondrial network. Concerted research efforts are required to elucidate fully the processes that regulate mitochondrial dynamics, the mechanisms by which microbes affect epithelial mitochondrial fission and/or fusion, and the implications of this for susceptibility to infectious disease. We speculate that the commensal microbiome of the gut may be important for normal epithelial mitochondrial form and function. Drugs designed to counteract the effect of microbial pathogen interference with mitochondrial dynamics may be a new approach to infectious disease at mucosal surfaces.

Maternal Diet Alters Trained Immunity in the Pathogenesis of Pediatric NAFLD

Pediatric nonalcoholic fatty liver disease (NAFLD) affects 1 in 10 children in the US, increases risk of cirrhosis and transplantation in early adulthood, and shortens lifespan, even after transplantation. Exposure to maternal obesity and/or a diet high in fat, sugar and cholesterol is strongly associated with development of NAFLD in offspring. However, mechanisms by which "priming" of the immune system in early life increases susceptibility to NAFLD are poorly understood. Recent studies have focused on the role "non-reparative" macrophages play in accelerating inflammatory signals promoting fibrogenesis. In this Commentary, we review evidence that the pioneering gut bacteria colonizing the infant intestinal tract remodel the naïve immune system in the offspring. Epigenetic changes in hematopoietic stem and progenitor cells, induced by exposure to an obesogenic diet in utero, may skew lineage commitment of myeloid cells during gestation. Further, microbial dysbiosis in neonatal life contributes to training innate immune cell responsiveness in the gut, bone marrow, and liver, leading to developmental programming of pediatric NAFLD. Comprehensive understanding of how different gut bacteria and their byproducts shape development of the early innate immune system and microbiome will uncover early interventions to prevent NAFLD pathophysiology.

The microbiome and immune memory formation

The microbiota plays an important role in regulating both the innate and adaptive immune systems. Many studies have focused on the ability of microbes to shape the immune system by stimulating B-cell and antibody responses and the differentiation of T helper cell function. However, an important feature of the immune system is its ability to generate memory responses, which provide increased survival for the host. This review will highlight the role of the microbiota in the induction of immune memory with a focus on both adaptive and innate memory as well as vaccine efficacy.

Maternal helminth infections and the shaping of offspring immunity

Helminth infections leave a long-lasting immunological footprint on their hosts. Clinical studies have provided first evidence that maternal helminth infections can result in an altered immune profile in their offspring which can potentially shape how they respond to conditions throughout life. This can relate to changes in offspring induction of immune responses against other diseases. However, whether these changes result in actual changes in offspring ability to control disease is unclear. Our understanding of which immune mechanisms are altered and how they are changed is limited. In this review, we highlight what we know from human and mouse studies about this important context of helminth exposure. Moreover, we discuss how mechanisms such as antibody transfer, antigen exposure, maternal cell uptake, chimerism and epigenetics are all likely to be functional contributors to the striking changes that are seen in offspring born or nursed by helminth exposed mothers.

Innate Immune Influences on the Gut Microbiome: Lessons from Mouse Models

The gut microbiota is important in health and disease. Whereas the intestinal immune system has evolved to protect the mucosal barrier against pathogens, there is much interest in understanding how it influences the composition and functions of resident microbial communities. Overall, host innate immunity exerts little influence on the microbiota at homeostasis, but increases upon immune activation and the onset of inflammation, as well as in the presence of certain members of the microbiota. However, many experiments have not adequately incorporated study design to detect such immune influences, including using proper control groups, precise sampling and timing, and measures beyond broad-scale descriptions of dysbiosis for microbial analysis. We discuss these and other challenges in the context of current understanding of chronic inflammatory disease.