Perinatal Interactions between the Microbiome, Immunity, and Neurodevelopment.

The microbiome modulates host immune function across the gastrointestinal tract, peripheral lymphoid organs, and central nervous system. In this review, we highlight emerging evidence that microbial effects on select immune phenotypes arise developmentally, where the maternal and neonatal microbiome influence immune cell ontogeny in the offspring during gestation and early postnatal life. We further discuss roles for the perinatal microbiome and early-life immunity in regulating normal neurodevelopmental processes. In addition, we examine evidence that abnormalities in microbiota-neuroimmune interactions during early life are associated with altered risk of neurological disorders in humans. Finally, we conclude by evaluating the potential implications of microbiota-immune interventions for neurological conditions. Continued progress toward dissecting mechanistic interactions between the perinatal microbiota, immune system, and nervous system might uncover fundamental insights into how developmental interactions across physiological systems inform later-life health and disease.

miRiad roles for the miR-17-92 cluster in development and disease.

MicroRNAs (miRNAs) encoded by the miR-17-92 cluster and its paralogs are known to act as oncogenes. Expression of these miRNAs promotes cell proliferation, suppresses apoptosis of cancer cells, and induces tumor angiogenesis. New work reveals essential functions for these miRNAs not only in tumor formation but also during normal development of the heart, lungs, and immune system.

The morphological basis of the development of the chick embryo immune system.

The chick immune system is a fundamental model in basic immunology. In birds, the bone marrow derived pluripotent stem cells after entering the circulation, migrate to bursa of Fabricius to benefit from a microenvironment which supports the differentiation and maturation of B lymphocytes by the help of its resident cells and tissues. Delivering sufficient functional B cells is required to maintain their peripheral population and normal peripheral humoral responses. Additionally, bursa acts as an active site for the generation of antibody diversity through gene conversion. Being consisted of 98% B lymphocytes, the organ is occupied by other cell types including T cells, macrophages, eosinophils and mast cells. Thymus, which is an epithelial organ is the main site of T cell development where positive and negative selections contribute to the development of functional and not autoreactive T cell repertoire. Bursectomy and thymectomy are surgical exercises through which the involvement of cells of specific immunity including B cells and T cells can be determined.

Cannabinoid exposure during pregnancy and its impact on immune function.

Cannabinoids are the most commonly abused illicit drugs worldwide. While cannabis can be beneficial for certain heath conditions, abuse of potent synthetic cannabinoids has been on the rise. Exposure to cannabinoids is also prevalent in women of child-bearing age and pregnant women. These compounds can cross the placental barrier and directly affect the fetus. They mediate their effects primarily through G-protein coupled cannabinoid receptors, CB1 and CB2. In addition to significant neurological effects, cannabinoids can trigger robust immunomodulation by altering cytokine levels, causing apoptosis of lymphoid cells and inducing suppressor cells of the immune system. Profound effects of cannabinoids on the immune system as discussed in this review, suggest that maternal exposure during pregnancy could lead to dysregulation of innate and adaptive immune system of developing fetus and offspring potentially leading to weakening of immune defenses against infections and cancer later in life. Emerging evidence also indicates the underlying role of epigenetic mechanisms causing long-lasting impact following cannabinoid exposure in utero.

Control of the thymic medulla and its influence on αßT-cell development.

The thymus is a primary lymphoid tissue that supports the generation of αßT cells. In this review, we describe the processes that give rise to the thymus medulla, a site that nurtures self-tolerant T-cell generation following positive selection events that take place in the cortex. To summarize the developmental pathways that generate medullary thymic epithelial cells (mTEC) from their immature progenitors, we describe work on both the initial emergence of the medulla during embryogenesis, and the maintenance of the medulla during postnatal stages. We also investigate the varying roles that receptors belonging to the tumor necrosis factor receptor superfamily have on thymus medulla development and formation, and highlight the impact that T-cell development has on thymus medulla formation. Finally, we examine the evidence that the thymic medulla plays an important role during the intrathymic generation of distinct αßT-cell subtypes. Collectively, these studies provide new insight into the development and functional importance of medullary microenvironments during self-tolerant T-cell production in the thymus.

Hematopoiesis and T-cell specification as a model developmental system.

The pathway to generate T cells from hematopoietic stem cells guides progenitors through a succession of fate choices while balancing differentiation progression against proliferation, stage to stage. Many elements of the regulatory system that controls this process are known, but the requirement for multiple, functionally distinct transcription factors needs clarification in terms of gene network architecture. Here, we compare the features of the T-cell specification system with the rule sets underlying two other influential types of gene network models: first, the combinatorial, hierarchical regulatory systems that generate the orderly, synchronized increases in complexity in most invertebrate embryos; second, the dueling 'master regulator' systems that are commonly used to explain bistability in microbial systems and in many fate choices in terminal differentiation. The T-cell specification process shares certain features with each of these prevalent models but differs from both of them in central respects. The T-cell system is highly combinatorial but also highly dose-sensitive in its use of crucial regulatory factors. The roles of these factors are not always T-lineage-specific, but they balance and modulate each other's activities long before any mutually exclusive silencing occurs. T-cell specification may provide a new hybrid model for gene networks in vertebrate developmental systems.

Neonatal gut and immune maturation is determined more by postnatal age than by postconceptional age in moderately preterm pigs.

Preterm infants have immature organ functions that predispose them to gut and immune disorders. Developmental delays at preterm birth may affect various organs differently at term-corrected age. We hypothesized that gut and immune maturation in moderately preterm neonates depends more on birth and postnatal factors than on advancing postconceptional age (PCA). Using preterm pigs as models, we investigated how gut and immune parameters develop until term-corrected age and how these differ from those in term counterparts. Preterm ( n = 43, 106 days of gestation) and term pigs ( n = 41, 116 days of gestation) were delivered by caesarean section and euthanized at birth ( day 1) or postnatal day 11 (term-corrected age for preterm pigs) using identical rearing conditions. Relative to term pigs, preterm pigs had lower blood oxygenation, glucose, and cortisol levels, lower gut lactase activity, villus height, and goblet cell density, and lower blood neutrophil, helper T, and cytotoxic T cell numbers at birth. Despite slower growth in preterm pigs, most intestinal and immune parameters increased markedly after birth in both groups. However, some parameters remained negatively affected by preterm birth until postnatal day 11 (goblet cells, gut permeability, and cytotoxic T cells). The colon microbiota showed limited differences between preterm and term pigs at this time. At the same PCA, preterm 11-day-old pigs had higher blood leukocyte numbers and gut enzyme activities but lower villus height and blood cytotoxic T cell numbers relative to newborn term pigs. Birth and postnatal factors, not advancing PCA, are key determinants of gut and immune maturation in moderately preterm neonates. NEW & NOTEWORTHY Postnatally, preterm infants are often considered to reach a physiological maturation similar to that in term infants when they reach term-corrected postconceptional age (PCA). Using preterm pigs as models, we show that PCA may be a poor measure of gut and immune maturation because environmental triggers (regardless of PCA at birth) are critical. Possibly, PCA is only relevant to evaluate physiological maturation of organs that develop relatively independent of the external environment (e.g., the brain).

Environmental Immunology: Lessons Learned from Exposure to a Select Panel of Immunotoxicants.

Exposure to environmental contaminants can produce profound effects on the immune system. Many classes of xenobiotics can significantly suppress or enhance immune responsiveness depending on the levels (i.e., dose) and context (i.e., timing, route) of exposure. Although defining the effects that toxicants can have on the immune system is a valuable component to improving public health, environmental immunology has greatly enhanced our understanding of how the immune system functions and has provided innovative avenues to explore new immunotherapies. This Brief Review focuses on three examples of how immunotoxicology has benefitted the field of immunology, presenting information on the aryl hydrocarbon receptor signaling pathway, the immunomodulatory effects of nanomaterials, and the impact of xenobiotic exposure on the developing immune system. Collectively, contributions from immunotoxicology have significantly enhanced public health and spurred seminal advances in both basic and applied immunology.

Innate immunity of the newborn: basic mechanisms and clinical correlates.

The fetus and newborn face a complex set of immunological demands, including protection against infection, avoidance of harmful inflammatory immune responses that can lead to pre-term delivery, and balancing the transition from a sterile intra-uterine environment to a world that is rich in foreign antigens. These demands shape a distinct neonatal innate immune system that is biased against the production of pro-inflammatory cytokines. This bias renders newborns at risk of infection and impairs responses to many vaccines. This Review describes innate immunity in newborns and discusses how this knowledge might be used to prevent and treat infection in this vulnerable population.

Ontogeny of early life immunity.

The human immune system comprises cellular and molecular components designed to coordinately prevent infection while avoiding potentially harmful inflammation and autoimmunity. Immunity varies with age, reflecting unique age-dependent challenges including fetal gestation, the neonatal phase, and infancy. Here, we review novel mechanistic insights into early life immunity, with an emphasis on emerging models of human immune ontogeny, which may inform age-specific translational development of novel anti-infectives, immunomodulators, and vaccines.

Different immunological responses to early-life antibiotic exposure affecting autoimmune diabetes development in NOD mice.

Environmental factors clearly influence the pathogenesis of Type 1 diabetes, an autoimmune disease. We have studied gut microbiota as important environmental agents that could affect the initiation or progression of type 1 diabetes especially in the prenatal period. We used neomycin, targeting mainly Gram negative or vancomycin, targeting mainly Gram positive bacteria, to treat pregnant NOD mothers and to study autoimmune diabetes development in their offspring. Neomycin-treated offspring were protected from diabetes, while vancomycin-treated offspring had accelerated diabetes development, and both antibiotics caused distinctly different shifts in gut microbiota composition compared with the offspring from untreated control mice. Our study demonstrated that neomycin treatment of pregnant mothers leads to generation of immune-tolerogenic antigen-presenting cells (APCs) in the offspring and these APCs had reduced specific autoantigen-presenting function both in vitro and in vivo. Moreover, the protection from diabetes mediated by tolerogenic APCs was vertically transmissible to the second generation. In contrast, more diabetogenic inflammatory T cells were found in the lymphoid organs of the offspring from the vancomycin-treated pregnant mothers. This change however was not transmitted to the second generation. Our results suggested that prenatal exposure to antibiotic influenced gut bacterial composition at the earliest time point in life and is critical for consequent education of the immune system. As different bacteria can induce different immune responses, understanding these differences and how to generate self-tolerogenic APCs could be important for developing new therapy for type 1 diabetes.

Monocyte and macrophage heterogeneity.

Heterogeneity of the macrophage lineage has long been recognized and, in part, is a result of the specialization of tissue macrophages in particular microenvironments. Circulating monocytes give rise to mature macrophages and are also heterogeneous themselves, although the physiological relevance of this is not completely understood. However, as we discuss here, recent studies have shown that monocyte heterogeneity is conserved in humans and mice, allowing dissection of its functional relevance: the different monocyte subsets seem to reflect developmental stages with distinct physiological roles, such as recruitment to inflammatory lesions or entry to normal tissues. These advances in our understanding have implications for the development of therapeutic strategies that are targeted to modify particular subpopulations of monocytes.

Learning to roll before you stop and drop.

In this issue of Blood, Sperandio and colleagues report the use of a unique intravital microscopic system to characterize an ontogenic process of blood cell and yolk sac endothelial maturation that is required to display full adult-type inflammation-induced leukocyte recruitment.(1) They report that murine fetal blood neutrophil rolling, adhesion, and extravasation from inflamed yolk sac vessels is apparent late in development, but that before embryonic day (E) 15, fetal blood neutrophils display little ability to roll or adhere to inflamed vascular endothelial cells. Similar behavior was displayed when fetal blood cells were tested in vitro on immobilized recombinant adhesion molecules.

An evolutionarily conserved program of B-cell development and activation in zebrafish.

Teleost fish are among the most ancient vertebrates possessing an adaptive immune system with B and T lymphocytes that produce memory responses to pathogens. Most bony fish, however, have only 2 types of B lymphocytes, in contrast to the 4 types available to mammals. To better understand the evolution of adaptive immunity, we generated transgenic zebrafish in which the major immunoglobulin M (IgM(+)) B-cell subset expresses green fluorescence protein (GFP) (IgM1:eGFP). We discovered that the earliest IgM(+) B cells appear between the dorsal aorta and posterior cardinal vein and also in the kidney around 20 days postfertilization. We also examined B-cell ontogeny in adult IgM1:eGFP;rag2:DsRed animals, where we defined pro-B, pre-B, and immature/mature B cells in the adult kidney. Sites of B-cell development that shift between the embryo and adult have previously been described in birds and mammals. Our results suggest that this developmental shift occurs in all jawed vertebrates. Finally, we used IgM1:eGFP and cd45DsRed;blimp1:eGFP zebrafish to characterize plasma B cells and investigate B-cell function. The IgM1:eGFP reporter fish are the first nonmammalian B-cell reporter animals to be described. They will be important for further investigation of immune cell evolution and development and host-pathogen interactions in zebrafish.

Ontogenetic regulation of leukocyte recruitment in mouse yolk sac vessels.

In adult mammals, leukocyte recruitment follows a well-defined cascade of adhesion events enabling leukocytes to leave the circulatory system and transmigrate into tissue. Currently, it is unclear whether leukocyte recruitment proceeds in a similar fashion during fetal development. Considering the fact that the incidence of neonatal sepsis increases dramatically with decreasing gestational age in humans, we hypothesized that leukocyte recruitment may be acquired only late during fetal ontogeny. To test this, we developed a fetal intravital microscopy model in pregnant mice and, using LysEGFP (neutrophil reporter) mice, investigated leukocyte recruitment during fetal development. We show that fetal blood neutrophils acquire the ability to roll and adhere on inflamed yolk sac vessels during late fetal development, whereas at earlier embryonic stages (before day E15), rolling and adhesion were essentially absent. Accordingly, flow chamber experiments showed that fetal EGFP(+) blood cells underwent efficient adhesion only when they were harvested on or after E15. Fluorescence-activated cell sorter analysis on EGFP(+) fetal blood cells revealed that surface expression of CXCR2 and less pronounced P-selectin glycoprotein ligand-1 (PSGL-1) begin to increase only late in fetal life. Taken together, our findings demonstrate that inflammation-induced leukocyte recruitment is ontogenetically regulated and enables efficient neutrophil trafficking only during late fetal life.

Maternal influences on fetal microbial colonization and immune development.

While critical for normal development, the exact timing of establishment of the intestinal microbiome is unknown. For example, although preterm labor and birth have been associated with bacterial colonization of the amniotic cavity and fetal membranes for many years, the prevailing dogma of a sterile intrauterine environment during normal term pregnancies has been challenged more recently. While found to be a key contributor of evolution in the animal kingdom, maternal transmission of commensal bacteria may also constitute a critical process during healthy pregnancies in humans with yet unclear developmental importance. Metagenomic sequencing has elucidated a rich placental microbiome in normal term pregnancies likely providing important metabolic and immune contributions to the growing fetus. Conversely, an altered microbial composition during pregnancy may produce aberrant metabolites impairing fetal brain development and life-long neurological outcomes. Here we review the current understanding of microbial colonization at the feto-maternal interface and explain how normal gut colonization drives a balanced neonatal mucosal immune system, while dysbiosis contributes to aberrant immune function early in life and beyond. We discuss how maternal genetics, diet, medications, and probiotics inform the fetal microbiome in preparation for perinatal and postnatal bacterial colonization.

Mode of delivery and risk for development of atopic diseases in children.

Atopic diseases are a major public health problem worldwide, and several factors are thought to contribute to this rapid increase. The observed association between mode of delivery and risk of atopy in childhood has had a great deal of interest during the past few decades. In fact, even during delivery, exposure to antigens can index immune system in newborn, which induces the release of biologically active molecules, which are polarizing immune responses toward the T-helper 2 atopic profile. However, to date, studies on the relationship between mode of delivery and atopy have produced conflicting findings. The aim of this review was to summarize what is known about the relationship between mode of delivery and risk of atopic diseases in children. A literature search of electronic databases was undertaken for the major studies published from 1994 to today. The databases searched were PubMed, EMBASE, Medline, and Cochrane Library. The following key words were used: mode of delivery, cesarean section, vaginal delivery, atopy, and atopic diseases.

Control of central and peripheral tolerance by Aire.

The negative selection of self-reactive thymocytes depends on the expression of tissue-specific antigens by medullary thymic epithelial cells. The autoimmune regulator (Aire) protein plays an important role in turning on these antigens, and the absence of even one Aire-induced tissue-specific antigen in the thymus can lead to autoimmunity in the antigen-expressing target organ. Recently, Aire protein has been detected in peripheral lymphoid organs, suggesting that peripheral Aire plays a complementary role here. In these peripheral sites, Aire was found to regulate the expression of a group of tissue-specific antigens that is distinct from those expressed in the thymus. Furthermore, transgenic antigen expression in extrathymic Aire-expressing cells (eTACs) can mediate deletional tolerance, but the immunological relevance of Aire-dependent, endogenous tissue-specific antigens remains to be determined.

Identification and characterization of microRNAs in the spleen of common carp immune organ.

MicroRNAs (miRNAs) play an important role in the regulation of many fundamental biological processes in eukaryotes; however, miRNAs associated with immune functions in the common carp have not been reported. In this study, a small-RNA cDNA library was constructed from the spleen of the common carp. A total of 10,603,456 high-quality clean reads, representing 293,603 unique sequences, were obtained from the small-cDNA library using the Solexa sequencing. By the bioinformatic analysis, 194 conserved miRNAs and 12 novel miRNAs were identified in the carp spleen. The abundant miRNAs principally belong to 30 miRNA gene families such as let-7, mir-10, mir-15, mir-30, and so on. The conservation analysis showed that 23 families were present both in protostomes and deuterostomes, 46 families were conserved only in vertebrates, and 5 families (mir-430, mir-722, mir-724, mir-734, and mir-738) were identified only in fish species. Furthermore, GO enrichment analysis and KEGG pathway analysis suggested that miRNAs expressed in the spleen of common carp are involved in immune system development, lymphoid organ development, lymphocyte activation, immune response, B cell receptor signaling pathway, T cell receptor signaling pathway, Fc gamma R-mediated phagocytosis, Toll-like receptor signaling pathway, and so on. This study described the miRNA transcriptome in spleen tissue for the first time in the common carp. The results expand the number of known common carp miRNAs and provides a meaningful framework to understand the common carp immune system and defense mechanisms.