Early events in human T cell ontogeny. Phenotypic characterization and immunohistologic localization of T cell precursors in early human fetal tissues

Human erythroid progenitors express antigen presentation machinery

Early-life immune exposures can profoundly impact lifelong health. However, functional mechanisms underlying fetal immune development remain incomplete. Erythrocytes are not typically considered active immune mediators, primarily because erythroid precursors discard their organelles as they mature, thus losing the ability to alter gene expression in response to stimuli. Erythroid progenitors and precursors circulate in human fetuses and neonates. Although there is limited evidence that erythroid precursors are immunomodulatory, our understanding of the underlying mechanisms remains inadequate. To define the immunobiological role of fetal and perinatal erythroid progenitors and precursors, we analyzed single cell RNA-sequencing data and found that transcriptomics support erythroid progenitors as putative immune mediators. Unexpectedly, we discovered that human erythroid progenitors constitutively express Major Histocompatibility Complex (MHC) class II antigen processing and presentation machinery, which are hallmarks of specialized antigen presenting immune cells. Furthermore, we demonstrate that erythroid progenitors internalize and cleave foreign proteins into peptide antigens. Unlike conventional antigen presenting cells, erythroid progenitors express atypical costimulatory molecules and immunoregulatory cytokines that direct the development of regulatory T cells, which are critical for establishing maternal-fetal tolerance. Expression of MHC II in definitive erythroid progenitors begins during the second trimester, coinciding with the appearance of mature T cells in the fetus, and is absent in primitive progenitors. Lastly, we demonstrate physical and molecular interaction potential of erythroid progenitors and T cells in the fetal liver. Our findings shed light on a unique orchestrator of fetal immunity and provide insight into the mechanisms by which erythroid cells contribute to host defense.

Premature skewing of T cell receptor clonality and delayed memory expansion in HIV-exposed infants

While preventing vertical HIV transmission has been very successful, HIV-exposed uninfected infants (iHEU) experience an elevated risk to infections compared to HIV-unexposed and uninfected infants (iHUU). Here we present a longitudinal multimodal analysis of infant immune ontogeny that highlights the impact of HIV/ARV exposure. Using mass cytometry, we show alterations in T cell memory differentiation between iHEU and iHUU being significant from week 15 of life. The altered memory T cell differentiation in iHEU was preceded by lower TCR Vβ clonotypic diversity and linked to TCR clonal depletion within the naïve T cell compartment. Compared to iHUU, iHEU had elevated CD56loCD16loPerforin+CD38+CD45RA+FcεRIγ+ NK cells at 1 month postpartum and whose abundance pre-vaccination were predictive of vaccine-induced pertussis and rotavirus antibody responses post 3 months of life. Collectively, HIV/ARV exposure disrupted the trajectory of innate and adaptive immunity from birth which may underlie relative vulnerability to infections in iHEU.

Disrupted memory T cell expansion in HIV-exposed uninfected infants is preceded by premature skewing of T cell receptor clonality

While preventing vertical HIV transmission has been very successful, the increasing number of HIV-exposed uninfected infants (iHEU) experience an elevated risk to infections compared to HIV-unexposed and uninfected infants (iHUU). Immune developmental differences between iHEU and iHUU remains poorly understood and here we present a longitudinal multimodal analysis of infant immune ontogeny that highlights the impact of HIV/ARV exposure. Using mass cytometry, we show alterations and differences in the emergence of NK cell populations and T cell memory differentiation between iHEU and iHUU. Specific NK cells observed at birth were also predictive of acellular pertussis and rotavirus vaccine-induced IgG and IgA responses, respectively, at 3 and 9 months of life. T cell receptor Vβ clonotypic diversity was significantly and persistently lower in iHEU preceding the expansion of T cell memory. Our findings show that HIV/ARV exposure disrupts innate and adaptive immunity from birth which may underlie relative vulnerability to infections.

New perspectives on the origins and heterogeneity of mast cells

Mast cells are immune cells of the haematopoietic lineage that are now thought to have multifaceted functions during homeostasis and in various disease states. Furthermore, while mast cells have been known for a long time to contribute to allergic disease in adults, recent studies, mainly in mice, have highlighted their early origins during fetal development and potential for immune functions, including allergic responses, in early life. Our understanding of the imprinting of mast cells by particular tissues of residence and their potential for regulatory interactions with organ systems such as the peripheral immune, nervous and vascular systems is also rapidly evolving. Here, we discuss the origins of mast cells and their diverse and plastic phenotypes that are influenced by tissue residence. We explore how divergent phenotypes and functions might result from both their hard-wired 'nature' defined by their ontogeny and the 'nurture' they receive within specialized tissue microenvironments.

Effects of early postnatal life nutritional interventions on immune-microbiome interactions in the gastrointestinal tract and implications for brain development and function

The gastrointestinal (GI) microbiota has co-evolved with the host in an intricate relationship for mutual benefit, however, inappropriate development of this relationship can have detrimental effects. The developing GI microbiota plays a vital role during the first 1,000 days of postnatal life, during which occurs parallel development and maturation of the GI tract, immune system, and brain. Several factors such as mode of delivery, gestational age at birth, exposure to antibiotics, host genetics, and nutrition affect the establishment and resultant composition of the GI microbiota, and therefore play a role in shaping host development. Nutrition during the first 1,000 days is considered to have the most potential in shaping microbiota structure and function, influencing its interactions with the immune system in the GI tract and consequent impact on brain development. The importance of the microbiota-GI-brain (MGB) axis is also increasingly recognized for its importance in these developmental changes. This narrative review focuses on the importance of the GI microbiota and the impact of nutrition on MGB axis during the immune system and brain developmental period in early postnatal life of infants.

Development of the immune system in the human embryo

The fetal immune system is highly specialized which is to generate both tolerogenic and protective immune responses to tolerate both self- and maternal-antigens. Fetal T cells with pro-inflammatory potential are born in a tolerogenic environment and are tightly controlled by both cell-intrinsic and -extrinsic mechanisms. Fetal B-1 and B-2 B cells involved in innate and adaptive immune responses, respectively, arise in staggered waves of development from distinct progenitors. Innate immune responses are the key to the protection against infection and adaptive immunity creates memory after an initial response to a specific pathogen. This review aims to discuss the recent advances in understanding the development of immune system in fetus. IMPACT: During gestation, essential developmental changes occur to survive the neonates. At early stage, developmental signals and changes may be influenced due to immune deficiencies.

Establishment of tissue-resident immune populations in the fetus

The immune system establishes during the prenatal period from distinct waves of stem and progenitor cells and continuously adapts to the needs and challenges of early postnatal and adult life. Fetal immune development not only lays the foundation for postnatal immunity but establishes functional populations of tissue-resident immune cells that are instrumental for fetal immune responses amidst organ growth and maturation. This review aims to discuss current knowledge about the development and function of tissue-resident immune populations during fetal life, focusing on the brain, lung, and gastrointestinal tract as sites with distinct developmental trajectories. While recent progress using system-level approaches has shed light on the fetal immune landscape, further work is required to describe precise roles of prenatal immune populations and their migration and adaptation to respective organ environments. Defining points of prenatal susceptibility to environmental challenges will support the search for potential therapeutic targets to positively impact postnatal health.

Self-Centered Function of Adaptive Immunity in Regulation of Immune Responses and in Tolerance

The search for common mechanisms underlying the pathogenesis of chronic inflammatory conditions has crystalized the concept of continuous dual resetting of the immune repertoire (CDR) as a basic principle of the immune system function. Consequently, outlined was the first dynamic comprehensive picture of the immune system function. The goal of this study is to elaborate on regulation of immune responses and mechanisms of tolerance, particularly focusing on adaptive immunity. It is well established that the T/B cell repertoire is selected and maintained based on interactions with self. However, their activation also requires interaction with a self-specific major histocompatibility complex (MHC) “code,” i.e., the context of MHC molecules. Therefore, not only repertoire selection and maintenance but also the T/B cell activation and function are self-centered. Thus, adaptive effectors may be primarily focused on the state of self and maintenance of integrity of the self, and only to a certain degree on elimination of the foreign. As examples of such function are used immunologically poorly understood MHC-disparate settings typical for transplantation and pregnancy. Transplantation represents an extreme setting of strong systemic compartment-level adaptive/MHC-restricted immune responses. Described are clinically identified conditions for operational tolerance of MHC-disparate tissues/living systems in allotransplantation, which are in line with the CDR-proposed self-centered regulatory role of T/B cells. In contrast, normal pregnancy is coexistence of semiallogeneic or entirely allogeneic mother and fetus, but without alloreactivity akin to transplantation settings. Presented data support the notion that maintenance of pregnancy is a process that relies predominantly on innate/MHC-independent immune mechanisms. By the inception of hemotrophic stage of pregnancy (second and third trimester), both mother and child are individual living systems, with established adaptive immune repertoires. Although mother-fetus interactions at that point become indirect systemic compartment-level communications, their interactions throughout gestation remain within the innate realm of molecular-level adaptations.

The Leukemic Phase of ALK-Negative Anaplastic Large Cell Lymphoma Is Associated with CD7 Positivity, Complex Karyotype, TP53 Deletion, and a Poor Prognosis

Simple Summary

Anaplastic large cell lymphoma (ALCL) is a systemic peripheral T-cell neoplasm characterized by strong and uniform CD30 expression and, usually, the aberrant loss of one or more T-cell antigens. ALCL is further classified into anaplastic lymphoma kinase (ALK)-positive and ALK-negative types. ALCL rarely involves the peripheral blood. The reported leukemic phase ALCL cases are almost all pediatric patients with ALK-positive ALCL, which are frequently associated with the small cell morphology, t(2;5)(p23;q35), and a poorer prognosis. Leukemic phase ALK-negative ALCL is extremely rare, with approximately ten cases reported in the literature to date, mostly as single case reports. Here we report on nine patients with leukemic ALK-negative ALCL—the largest case series to date—and we compare these cases with 39 non-leukemic cases of ALK-negative ALCL. We show that the patients with leukemic ALK-negative ALCL have a greater frequency of absolute lymphocytosis, thrombocytopenia, bone marrow involvement, CD7 positivity, complex karyotype, TP53 deletion, and a dismal outcome. These data suggest that leukemic phase ALK-negative ALCL is associated with a number of poor prognostic factors and affected patients may need more aggressive treatment.

Abstract

Patients with anaplastic large cell lymphoma (ALCL) rarely develop a leukemic phase of the disease. The reported leukemic ALCL cases are almost all ALK-positive, which are frequently associated with small cell morphology, t(2;5)(p23;q35), and a poorer prognosis. Rare leukemic ALK-negative ALCL cases have been reported. In the present study, we investigated the clinical and pathologic features and outcomes of nine patients with leukemic ALK-negative ALCL and compared these features with 39 patients without leukemic disease. Compared with the non-leukemic ALK-negative ALCL group, patients with leukemic disease more often had absolute lymphocytosis (50% vs. 0%, p = 0.008), thrombocytopenia (60% vs. 11%, p = 0.03), bone marrow involvement (50% vs. 14%, p = 0.04), and CD7 positivity (71% vs. 19%, p = 0.02). Four of five (80%) patients with leukemic ALK-negative ALCL had a complex karyotype, which was significantly higher than that of the patients in the non-leukemic group. A fluorescence in situ hybridization for TP53 was performed on six leukemic ALK-negative ALCL cases and all (100%) had TP53 deletion. There were no significant differences in the other clinicopathologic features, treatment, and complete remission rates between patients in the leukemic versus non-leukemic group (all p > 0.05). The median follow-up of this cohort was 18 months with a range of 0.3–140 months. Eight of nine (90%) patients with leukemic ALK-negative ALCL died, and their overall survival was significantly shorter than that of the patients with non-leukemic disease (median 15.5 vs. 60 months, p = 0.001). In conclusion, we show that the leukemic phase of ALK-negative ALCL is associated with high-risk biologic features and, in particular, a complex karyotype and TP53 deletion. Compared with the non-leukemic ALK-negative ALCL patients, the patients with a leukemic phase of disease have poorer survival and may require more aggressive treatment.

Single-cell transcriptome analysis reveals the dynamics of human immune cells during early fetal skin development

The immune system of skin develops in stages in mice. However, the developmental dynamics of immune cells in human skin remains elusive. Here, we perform transcriptome profiling of CD45⁺ hematopoietic cells in human fetal skin at an estimated gestational age of 10-17 weeks by single-cell RNA sequencing. A total of 13 immune cell types are identified. Skin macrophages show dynamic heterogeneity over the course of skin development. A major shift in lymphoid cell developmental states occurs from the first to the second trimester that implies an in situ differentiation process. Gene expression analysis reveals a typical developmental program in immune cells in accordance with their functional maturation, possibly involving metabolic reprogramming. Finally, we identify transcription factors (TFs) that potentially regulate cellular transitions by comparing TFs and TF target gene networks. These findings provide detailed insight into how the immune system of the human skin is established during development.

Microbial exposure during early human development primes fetal immune cells

The human fetal immune system begins to develop early during gestation; however, factors responsible for fetal immune-priming remain elusive. We explored potential exposure to microbial agents in utero and their contribution toward activation of memory T cells in fetal tissues. We profiled microbes across fetal organs using 16S rRNA gene sequencing and detected low but consistent microbial signal in fetal gut, skin, placenta, and lungs in the 2^nd trimester of gestation. We identified several live bacterial strains including Staphylococcus and Lactobacillus in fetal tissues, which induced in vitro activation of memory T cells in fetal mesenteric lymph node, supporting the role of microbial exposure in fetal immune-priming. Finally, using SEM and RNA-ISH, we visualized discrete localization of bacteria-like structures and eubacterial-RNA within 14^th weeks fetal gut lumen. These findings indicate selective presence of live microbes in fetal organs during the 2^nd trimester of gestation and have broader implications toward the establishment of immune competency and priming before birth.

Tipping the Scales With Zebrafish to Understand Adaptive Tumor Immunity

The future of improved immunotherapy against cancer depends on an in-depth understanding of the dynamic interactions between the immune system and tumors. Over the past two decades, the zebrafish has served as a valuable model system to provide fresh insights into both the development of the immune system and the etiologies of many different cancers. This well-established foundation of knowledge combined with the imaging and genetic capacities of the zebrafish provides a new frontier in cancer immunology research. In this review, we provide an overview of the development of the zebrafish immune system along with a side-by-side comparison of its human counterpart. We then introduce components of the adaptive immune system with a focus on their roles in the tumor microenvironment (TME) of teleosts. In addition, we summarize zebrafish models developed for the study of cancer and adaptive immunity along with other available tools and technology afforded by this experimental system. Finally, we discuss some recent research conducted using the zebrafish to investigate adaptive immune cell-tumor interactions. Without a doubt, the zebrafish will arise as one of the driving forces to help expand the knowledge of tumor immunity and facilitate the development of improved anti-cancer immunotherapy in the foreseeable future.

CD2 and CD7 are sensitive flow cytometry screening markers for T-lineage acute leukemia(s): a study of 465 acute leukemia cases

T-lymphoblastic leukemia/lymphoma (T-ALL/LBL) is a rare acute leukemia that expresses cytoplasmic CD3 (cCD3) and frequently lacks surface CD3. Given that routine flow cytometric testing for cCD3 may not be feasible and cCD3 interpretation may be difficult, we investigate if surface CD2 and/or CD7 expression on blasts can be used by flow cytometry to screen for T-lineage acute leukemia. We retrospectively reviewed flow cytometric data from 233 acute leukemias (36 T-ALL/LBL, 8 mixed-phenotype acute leukemia T/myeloid, 80 acute myeloid leukemia, 97 B-ALL/LBL, 8 mixed-phenotype acute leukemia B/myeloid, and 4 acute undifferentiated leukemia cases). Uniform expression (≥75% of blasts) of CD2 and/or CD7 was seen in all 44 cCD3-positive cases but in only 11% (20/189) of cCD3-negative acute leukemias, thus demonstrating 100% sensitivity and 89% specificity in the identification of cCD3-positive (T-lineage) acute leukemia. To avoid selection bias, we prospectively studied 232 consecutive acute leukemias for which cCD3, CD2, and CD7 were automatically performed in all cases. Similar to the retrospective study, uniform expression of CD2 and/or CD7 on blasts showed 100% sensitivity and 88% specificity in the screening for cCD3-positive (T-lineage) acute leukemia. Therefore, acute leukemias with uniform expression of CD2 and/or CD7 warrant further testing for cCD3 to evaluate for T-lineage acute leukemia. Blasts that lack both uniform CD2 and CD7 expression do not require additional cCD3 testing. We propose that CD2 and CD7 could be utilized in a limited antibody flow cytometry panel as a sensitive, robust, and cost-effective way to screen for T-lineage acute leukemia.

Amphibian regeneration and mammalian cancer: Similarities and contrasts from an evolutionary biology perspective: Comparing the regenerative potential of mammalian embryos and urodeles to develop effective strategies against human cancer

Here we review and discuss the link between regeneration capacity and tumor suppression comparing mammals (embryos versus adults) with highly regenerative vertebrates. Similar to mammal embryo morphogenesis, in amphibians (essentially newts and salamanders) the reparative process relies on a precise molecular and cellular machinery capable of sensing abnormal signals and actively reprograming or eliminating them. As the embryo's evil twin, tumor also retains common functional attributes. The immune system plays a pivotal role in maintaining a physiological balance to provide surveillance against tumor initiation or to support its initiation and progression. We speculate that susceptibility to cancer development in adult mammals may be determined by the loss of an advanced regenerative capability during evolution and believe that gaining mechanistic insights into how regenerative capacity linked to tumor suppression is postnatally lost in mammals might illuminate an as yet unrecognized route to cancer treatment.

Malaria and Early Life Immunity: Competence in Context

Childhood vaccines have been the cornerstone tool of public health over the past century. A major barrier to neonatal vaccination is the “immaturity” of the infant immune system and the inefficiency of conventional vaccine approaches at inducing immunity at birth. While much of the literature on fetal and neonatal immunity has focused on the early life propensity toward immune tolerance, recent studies indicate that the fetus is more immunologically capable than previously thought, and can, in some circumstances, mount adaptive B and T cell responses to perinatal pathogens in utero. Although significant hurdles remain before these findings can be translated into vaccines and other protective strategies, they should lend optimism to the prospect that neonatal and even fetal vaccination is achievable. Next steps toward this goal should include efforts to define the conditions for optimal stimulation of infant immune responses, including antigen timing, dose, and route of delivery, as well as antigen presentation pathways and co-stimulatory requirements. A better understanding of these factors will enable optimal deployment of vaccines against malaria and other pathogens to protect infants during their period of greatest vulnerability.

Understanding Early-Life Adaptive Immunity to Guide Interventions for Pediatric Health

Infants are capable of mounting adaptive immune responses, but their ability to develop long-lasting immunity is limited. Understanding the particularities of the neonatal adaptive immune system is therefore critical to guide the design of immune-based interventions, including vaccines, in early life. In this review, we present a thorough summary of T cell, B cell, and humoral immunity in early life and discuss infant adaptive immune responses to pathogens and vaccines. We focus on the differences between T and B cell responses in early life and adulthood, which hinder the generation of long-lasting adaptive immune responses in infancy. We discuss how knowledge of early life adaptive immunity can be applied when developing vaccine strategies for this unique period of immune development. In particular, we emphasize the use of novel vaccine adjuvants and optimization of infant vaccine schedules. We also propose integrating maternal and infant immunization strategies to ensure optimal neonatal protection through passive maternal antibody transfer while avoiding hindering infant vaccine responses. Our review highlights that the infant adaptive immune system is functionally distinct and uniquely regulated compared to later life and that these particularities should be considered when designing interventions to promote pediatric health.

Leaving no one behind: tracing every human thymocyte by single-cell RNA-sequencing

The thymus is the primary organ for T-cell development, providing an essential microenvironment consisting of the appropriate cytokine milieu and specialized stromal cells. Thymus-seeding progenitors from circulation immigrate into the thymus and undergo the stepwise T-cell specification, commitment, and selection processes. The transcriptional factors, epigenetic regulators, and signaling pathways involved in the T-cell development have been intensively studied using mouse models. Despite our growing knowledge of T-cell development, major questions remain unanswered regarding the ontogeny and early events of T-cell development at the fetal stage, especially in humans. The recently developed single-cell RNA-sequencing technique provides an ideal tool to investigate the heterogeneity of T-cell precursors and the molecular mechanisms underlying the divergent fates of certain T-cell precursors at the single-cell level. In this review, we aim to summarize the current progress of the study on human thymus organogenesis and thymocyte and thymic epithelial cell development, which is to shed new lights on developing novel strategies for in vitro T-cell regeneration and thymus rejuvenation.

T Cell Development: Old Tales Retold By Single-Cell RNA Sequencing

Mammalian T cell development initiates from the migration of hematopoietic progenitors to the thymus, which undergo cell proliferation, T-lineage specification and commitment, as well as positive and negative selection. These processes are precisely controlled at multiple levels and have been intensively studied using gene-modified animal models and in vitro coculture systems. However, several long-standing questions, including the characterization of the rare but crucial progenitors/precursors and the molecular mechanisms underlying their fate decision, have been dampened because of cell scarcity and lack of appropriate techniques. Single-cell RNA sequencing (scRNA-seq) makes it possible to investigate and resolve some of these questions, leading to new remarkable progress in identifying and characterizing early thymic progenitors and delineating the refined developmental trajectories of conventional and unconventional T cells.

Emerging role of Fli1 in autoimmune diseases

The Ets transcription factor family exerts crucial role in cell proliferation, apoptosis, differentiation and migration. Friend leukemia integration 1 (Fli1), a member of the Ets family, is expressed in fibroblasts, endothelial cells and immune cells. Fli1 gene is participated in the development, proliferation, activation, migration and other processes of immune cells. Fli1 can also affect the function of immune cells by regulating cytokines and chemokines. Emerging evidence has shown that Fli1 is implicated in the etiology of several autoimmune diseases, including systemic sclerosis (SSc) and systemic lupus erythematosus (SLE). In this review, we mainly discuss the current evidence for the role of Fli1 in these diseases.

Generation and function of progenitor T cells from StemRegenin-1-expanded CD34+ human hematopoietic progenitor cells

Broader clinical application of umbilical cord blood (UCB), as a source of hematopoietic stem/progenitor cells (HSPCs), is limited by low CD34+ and T-cell numbers, contributing to slow lymphohematopoietic recovery, infection, and relapse. Studies have evaluated the safety, feasibility, and expedited neutrophil recovery associated with the transplantation of CD34+ HSPCs from ex vivo expansion cultures using the aryl hydrocarbon receptor antagonist StemRegenin-1 (SR1). In a phase 1/2 study of 17 patients who received combined SR1-expanded and unexpanded UCB units, a considerable advantage for enhancing T-cell chimerism was not observed. We previously showed that progenitor T (proT) cells generated in vitro from HSPCs accelerated T-cell reconstitution and restored immunity after hematopoietic stem cell transplantation (HSCT). To expedite immune recovery, we hypothesized that SR1-expanded HSPCs together with proT cells could overcome the known T-cell immune deficiency that occurs post-HSCT. Here, we show that SR1-expanded UCB can induce >250-fold expansion of CD34+ HSPCs, which can generate large numbers of proT cells upon in vitro differentiation. When compared with nonexpanded naive proT cells, SR1 proT cells also showed effective thymus-seeding and peripheral T-cell functional capabilities in vivo despite having an altered phenotype. In a competitive transfer approach, both naive and SR1 proT cells showed comparable thymus-engrafting capacities. Single-cell RNA sequencing of peripheral CD3+ T cells from mice injected with either naive or SR1 proT cells revealed functional subsets of T cells with polyclonal T-cell receptor repertoires. Our findings support the use of SR1-expanded UCB grafts combined with proT-cell generation for decreasing T-cell immunodeficiency post-HSCT.

Physioxia enhances T-cell development ex vivo from human hematopoietic stem and progenitor cells

Understanding physiologic T-cell development from hematopoietic stem (HSCs) and progenitor cells (HPCs) is essential for development of improved hematopoietic cell transplantation (HCT) and emerging T-cell therapies. Factors in the thymic niche, including Notch 1 receptor ligand, guide HSCs and HPCs through T-cell development in vitro. We report that physiologically relevant oxygen concentration (5% O₂ , physioxia), an important environmental thymic factor, promotes differentiation of cord blood CD34+ cells into progenitor T (proT) cells in serum-free and feeder-free culture system. This effect is enhanced by a potent reducing and antioxidant agent, ascorbic acid. Human CD34+ cell-derived proT cells in suspension cultures maturate into CD3+ T cells in an artificial thymic organoid (ATO) culture system more efficiently when maintained under physioxia, compared to ambient air. Low oxygen tension acts as a positive regulator of HSC commitment and HPC differentiation toward proT cells in the feeder-free culture system and for further maturation into T cells in the ATO. Culturing HSCs/HPCs in physioxia is an enhanced method of effective progenitor T and mature T-cell production ex vivo and may be of future use for HCT and T-cell immunotherapies.

Prenatal development of human immunity

The blood and immune systems develop in parallel during early prenatal life. Waves of hematopoiesis separated in anatomical space and time give rise to circulating and tissue-resident immune cells. Previous observations have relied on animal models, which differ from humans in both their developmental timeline and exposure to microorganisms. Decoding the composition of the human immune system is now tractable using single-cell multi-omics approaches. Large-scale single-cell genomics, imaging technologies, and the Human Cell Atlas initiative have together enabled a systems-level mapping of the developing human immune system and its emergent properties. Although the precise roles of specific immune cells during development require further investigation, the system as a whole displays malleable and responsive properties according to developmental need and environmental challenge.

CRISPR/Cas9-targeting of CD40 in hematopoietic stem cells limits immune activation mediated by anti-CD40

Inflammatory bowel diseases (IBD) are complex, multifactorial disorders characterized by chronic relapsing intestinal inflammation. IBD is diagnosed around 1 in 1000 individuals in Western countries with globally increasing incident rates. Association studies have identified hundreds of genes that are linked to IBD and potentially regulate its pathology. The further dissection of the genetic network underlining IBD pathogenesis and pathophysiology is hindered by the limited capacity to functionally characterize each genetic association, including generating knockout animal models for every associated gene. Cutting-edge CRISPR/Cas9-based technology may transform the field of IBD research by efficiently and effectively introducing genetic alterations. In the present study, we used CRISPR/Cas9-based technologies to genetically modify hematopoietic stem cells. Through cell sorting and bone marrow transplantation, we established a system to knock out target gene expression by over 90% in the immune system of reconstituted animals. Using a CD40-mediated colitis model, we further validated our CRISPR/Cas9-based platform for investigating gene function in experimental IBD. In doing so, we developed a model system that delivers genetically modified mice in a manner much faster than conventional methodology, significantly reducing the time from target identification to in vivo target validation and expediting drug development.

Immunomodulatory effects of interferon-γ on human fetal cardiac mesenchymal stromal cells

BACKGROUND

Mesenchymal stromal cells (MSCs), due to their regenerative and immunomodulatory properties, are therapeutically used for diseases, including heart failure. As early gestational-phase embryonic tissues exhibit extraordinary regenerative potential, fetal MSCs exposed to inflammation offer a unique opportunity to evaluate molecular mechanisms underlying preferential healing, and investigate their inherent abilities to communicate with the immune system during development. The principal aim of this study was to evaluate the effects of interferon-γ (IFNγ) on the immunomodulatory effects of first-trimester human fetal cardiac (hfc)-MSCs.

METHODS

hfcMSCs (gestational week 8) were exposed to IFNγ, with subsequent analysis of the whole transcriptome, based on RNA sequencing. Exploration of surface-expressed immunoregulatory mediators and modulation of T cell responses were performed by flow cytometry. Presence and activity of soluble mediators were assessed by ELISA or high-performance liquid chromatography.

RESULTS

Stimulation of hfcMSCs with IFNγ revealed significant transcriptional changes, particularly in respect to the expression of genes belonging to antigen presentation pathways, cell cycle control, and interferon signaling. Expression of immunomodulatory genes and associated functional changes, including indoleamine 2,3-dioxygenase activity, and regulation of T cell activation and proliferation via programmed cell death protein (PD)-1 and its ligands PD-L1 and PD-L2, were significantly upregulated. These immunoregulatory molecules diminished rapidly upon withdrawal of inflammatory stimulus, indicating a high degree of plasticity by hfcMSCs.

CONCLUSIONS

To our knowledge, this is the first study performing a systematic evaluation of inflammatory responses and immunoregulatory properties of first-trimester cardiac tissue. In summary, our study demonstrates the dynamic responsiveness of hfcMSCs to inflammatory stimuli. Further understanding as to the immunoregulatory properties of hfcMSCs may be of benefit in the development of novel stromal cell therapeutics for cardiovascular disease.

Histoarchitectural Deterioration of Lymphoid Tissues in HIV-1 Infection and in Aging

Impaired immunity is a common symptom of aging and advanced Human Immunodeficiency Virus type 1 (HIV-1) disease. In both diseases, a decline in lymphocytic function and cellularity leads to ineffective adaptive immune responses to opportunistic infections and vaccinations. Furthermore, despite sustained myeloid cellularity there is a background of chronic immune activation and a decrease in innate immune function in aging. In HIV-1 disease, myeloid cellularity is often more skewed than in normal aging, but similar chronic activation and innate immune dysfunction typically arise. Similarities between aging and HIV-1 infection have led to several investigations into HIV-1-mediated aging of the immune system. In this article, we review various studies that report alterations of leukocyte number and function during aging, and compare those alterations with those observed during progressive HIV-1 disease. We pay particular attention to changes within lymphoid tissue microenvironments and how histoarchitectural changes seen in these two diseases affect immunity. As we review various immune compartments including peripheral blood as well as primary and secondary lymphoid organs, common themes arise that help explain the decline of immunity in the elderly and in HIV-1-infected individuals with advanced disease. In both conditions, lymphoid tissues often show signs of histoarchitectural deterioration through fat accumulation and/or fibrosis. These structural changes can be attributed to a loss of communication between leukocytes and the surrounding stromal cells that produce the extracellular matrix components and growth factors necessary for cell migration, cell proliferation, and lymphoid tissue function. Despite the common general impairment of immunity in aging and HIV-1 progression, deterioration of immunity is caused by distinct mechanisms at the cellular and tissue levels in these two diseases.

The immune response to malaria in utero

Malaria causes tremendous early childhood morbidity and mortality, providing an urgent impetus for the development of a vaccine that is effective in neonates. However, the infant immune response to malaria may be influenced by events that occur well before birth. Placental malaria infection complicates one quarter of all pregnancies in Africa and frequently results in exposure of the fetus to malaria antigens in utero, while the immune system is still developing. Some data suggest that in utero exposure to malaria may induce immunologic tolerance that interferes with the development of protective immunity during childhood. More recently, however, a growing body of evidence suggests that fetal malaria exposure can prime highly functional malaria-specific T- and B-cells, which may contribute to postnatal protection from malaria. In utero exposure to malaria also impacts the activation and maturation of fetal antigen presenting cells and innate lymphocytes, which could have implications for global immunity in the infant. Here, we review recent advances in our understanding of how various components of the fetal immune system are altered by in utero exposure to malaria, discuss factors that may tilt the critical balance between tolerance and adaptive immunity, and consider the implications of these findings for malaria prevention strategies.

Influence of forkhead box protein 3 polymorphisms (rs2232365, rs3761548) with the outcome of pregnancy: A meta-analysis

Dysfunction of regulatory T cells (Tregs) may contribute to certain immune-related pregnancy complications. Forkhead box protein 3 (FOXP3) is the key transcription factor of Treg. We performed a systematic review and meta-analysis to evaluate the possible association between FOXP3 polymorphisms -924A/G (rs2232365) and -3279C/A (rs3761548) and immune-related pregnancy complications. After reviewing 78 fully published studies, 10 studies fulfilled previously defined eligibility criteria and were used for meta-analysis. Two single nucleotide polymorphisms showed a significant correlation with increased or reduced risk for immune-related pregnancy complications. For rs3761548, women with allele A were significantly at a higher risk than women carrying allele C (odds ratio = 1.29, 95% confidence interval: 1.20-1.38; p = 0.001). For rs2232365, women with GG or AG genotype were at a higher risk than women with genotype AA, thereby, allele G was significantly associated with a higher risk than allele A. Our meta-analysis supports the notion that immune-related pregnancy complications might be linked to genetic variations in the FOXP3 gene.

Comparative Transcriptomics Reveals the Role of the Toll-Like Receptor Signaling Pathway in Fluoride-Induced Cardiotoxicity

Many studies have shown that fluorosis due to long-term fluoride intake has damaging effects on the heart. However, the mechanisms underlying cardiac fluorosis have not been illuminated in detail. We performed high-throughput transcriptome sequencing (RNA-Seq) on rat cardiac tissue to explore the molecular effects of NaF exposure. In total, 372 and 254 differentially expressed genes (DEGs) were identified between a group given 30 mg/L NaF and control and between a group given 90 mg/L NaF and control, respectively. The transcript levels of most of these genes were significantly down-regulated and many were distributed in the Toll-like receptor signaling pathway. Transcriptome analysis revealed that herpes simplex infection, ECM-receptor interaction, influenza A, cytokine-cytokine receptor interaction, apoptosis, and Toll-like receptor signaling pathway were significantly affected. IL-6 and IL-10 may play a crucial role in the cardiac damage caused by NaF as external stimuli according to protein-protein interaction (PPI) network analysis. The results of qRT-PCR and Western blotting showed a marked decreased mRNA and protein levels of IL-1, IL-6, and IL-10 in the low concentration fluoride (LF) and high concentration fluoride (HF) groups, which was in agreement with RNA-Seq results. This is the first study to investigate NaF-induced cardiotoxicity at a transcriptome level.

Human T Cell Development, Localization, and Function throughout Life

Throughout life, T cells coordinate multiple aspects of adaptive immunity, including responses to pathogens, allergens, and tumors. In mouse models, the role of T cells is studied in the context of a specific type of pathogen, antigen, or disease condition over a limited time frame, whereas in humans, T cells control multiple insults simultaneously throughout the body and maintain immune homeostasis over decades. In this review, we discuss how human T cells develop and provide essential immune protection at different life stages and highlight tissue localization and subset delineation as key determinants of the T cell functional role in immune responses. We also discuss how anatomic compartments undergo distinct age-associated changes in T cell subset composition and function over a lifetime. It is important to consider age and tissue influences on human T cells when developing targeted strategies to modulate T cell-mediated immunity in vaccines and immunotherapies.

Lymphatic cannulation models in sheep: Recent advances for immunological and biomedical research

Lymphatic cannulation models are useful tools for studying the immunobiology of the lymphatic system and the immunopathology of specific tissues in diseases. Sheep cannulations have been used extensively, as models for human physiology, fetal and neonatal development, human diseases, and for studies of ruminant pathobiology. The development of new and improved cannulation techniques in recent years has meant that difficult to access sites, such as mucosal associated tissues, are now more readily available to researchers. This review highlights the new approaches to cannulation and how these, in combination with advanced omics technologies, will direct future research using the sheep model.

The mother-offspring dyad: microbial transmission, immune interactions and allergy development

The increasing prevalence of allergy in affluent countries may be caused by reduced intensity and diversity of microbial stimulation, resulting in abnormal postnatal immune maturation. Most studies investigating the underlying immunomodulatory mechanisms have focused on postnatal microbial exposure, for example demonstrating that the gut microbiota differs in composition and diversity during the first months of life in children who later do or do not develop allergic disease. However, it is also becoming increasingly evident that the maternal microbial environment during pregnancy is important in childhood immune programming, and the first microbial encounters may occur already in utero. During pregnancy, there is a close immunological interaction between the mother and her offspring, which provides important opportunities for the maternal microbial environment to influence the immune development of the child. In support of this theory, combined pre- and postnatal supplementations seem to be crucial for the preventive effect of probiotics on infant eczema. Here, the influence of microbial and immune interactions within the mother-offspring dyad on childhood allergy development will be discussed. In addition, how perinatal transmission of microbes and immunomodulatory factors from mother to offspring may shape appropriate immune maturation during infancy and beyond, potentially via epigenetic mechanisms, will be examined. Deeper understanding of these interactions between the maternal and offspring microbiome and immunity is needed to identify efficacious preventive measures to combat the allergy epidemic.

Reduced generation of lung tissue-resident memory T cells during infancy

Zens et al. demonstrate a deficiency in the establishment of protective lung tissue-resident memory T cells following respiratory infection during infancy that is T cell intrinsic and can be ameliorated by reduced expression of T-bet during infection. These findings reveal a potential mechanism for increased susceptibility to infection in infancy and identify T-bet as a mediator of TRM generation in early life.

Infants suffer disproportionately from respiratory infections and generate reduced vaccine responses compared with adults, although the underlying mechanisms remain unclear. In adult mice, lung-localized, tissue-resident memory T cells (TRMs) mediate optimal protection to respiratory pathogens, and we hypothesized that reduced protection in infancy could be due to impaired establishment of lung TRM. Using an infant mouse model, we demonstrate generation of lung-homing, virus-specific T effectors after influenza infection or live-attenuated vaccination, similar to adults. However, infection during infancy generated markedly fewer lung TRMs, and heterosubtypic protection was reduced compared with adults. Impaired TRM establishment was infant–T cell intrinsic, and infant effectors displayed distinct transcriptional profiles enriched for T-bet–regulated genes. Notably, mouse and human infant T cells exhibited increased T-bet expression after activation, and reduction of T-bet levels in infant mice enhanced lung TRM establishment. Our findings reveal that infant T cells are intrinsically programmed for short-term responses, and targeting key regulators could promote long-term, tissue-targeted protection at this critical life stage.

Stem Cell Transplantation in the Fetus

BACKGROUND

In utero transplantation (IUT) of hematopoietic stem cells has the potential to treat a large number of hematologic and metabolic diseases amenable to partial replacement of the hematopoietic system.

METHODS

A review of the literature was conducted that focused on the clinical and experimental experience with IUT and, in this context, the development of the hematopoietic and immune systems.

RESULTS

Successful application of IUT has been limited to the treatment of various types of immunodeficiencies that affect lymphocyte development and function. Other congenital defects such as the thalassemias have not resulted in clinically significant engraftment. Recent efforts at understanding and overcoming the barriers to engraftment in the fetus have focused on providing a selective advantage to donor stem cells and fostering immune tolerance toward the donor cells. The critical cellular components of the graft that promote engraftment and tolerance induction are being evaluated in animal models. Improvements in engraftment have resulted from the inclusion of T cells and/or dendritic cells in the graft, as well as a strategy of combined prenatal and postnatal transplantation.

CONCLUSIONS

The advantages, necessity, and benefits of early treatment will continue to encourage development of IUT as a means to treat hematopoietic and other types of birth defects.

Mucosal Immune Development in Early Life: Setting the Stage

Our environment poses a constant threat to our health. To survive, all organisms must be able to discriminate between good (food ingredients and microbes that help digest our food) and bad (pathogenic microbes, viruses and toxins). In vertebrates, discrimination between beneficial and harmful antigens mainly occurs at the mucosal surfaces of the respiratory, digestive, urinary and genital tract. Here, an extensive network of cells and organs form the basis of what we have come to know as the mucosal immune system. The mucosal immune system is composed of a single epithelial cell layer protected by a mucus layer. Different immune cells monitor the baso-lateral side of the epithelial cells and dispersed secondary lymphoid organs, such as Peyer’s patches and isolated lymphoid follicles are equipped with immune cells able to mount appropriate and specific responses. This review will focus on the current knowledge on host, dietary and bacterial-derived factors that shape the mucosal immune system before and after birth. We will discuss current knowledge on fetal immunity (both responsiveness and lymphoid organ development) as well as the impact of diet and microbial colonization on neonatal immunity and disease susceptibility. Lastly, inflammatory bowel disease will be discussed as an example of how the composition of the microbiota might predispose to disease later in life. A fundamental understanding of the mechanisms involved in mucosal immune development and tolerance will aid nutritional intervention strategies to improve health in neonatal and adult life.

Dendritic cells in humans--from fetus to adult

The human immune system evolves continuously during development from the embryo into the adult, reflecting the ever-changing environment and demands of our body. This ability of our immune system to sense external cues and adapt as we develop is just as important in the early tolerogenic environment of the fetus, as it is in the constantly pathogen-challenged adult. Dendritic cells (DCs), the professional antigen-sensing and antigen-presenting components of the immune system, play a crucial role in this process where they act as sentinels, both initiating and regulating immune responses. Here, we provide an overview of the human immune system in the developing fetus and the adult, with a focus on DC ontogeny and function during these discrete but intimately linked life stages.

Technical advance: ascorbic acid induces development of double-positive T cells from human hematopoietic stem cells in the absence of stromal cells

The efficacy of donor HSCT is partly reduced as a result of slow post-transplantation immune recovery. In particular, T cell regeneration is generally delayed, resulting in high infection-related mortality in the first years post-transplantation. Adoptive transfer of in vitro-generated human T cell progenitors seems a promising approach to accelerate T cell recovery in immunocompromised patients. AA may enhance T cell proliferation and differentiation in a controlled, feeder-free environment containing Notch ligands and defined growth factors. Our experiments show a pivotal role for AA during human in vitro T cell development. The blocking of NOS diminished this effect, indicating a role for the citrulline/NO cycle. AA promotes the transition of proT1 to proT2 cells and of preT to DP T cells. Furthermore, the addition of AA to feeder cocultures resulted in development of DP and SP T cells, whereas without AA, a preT cell-stage arrest occurred. We conclude that neither DLL4-expressing feeder cells nor feeder cell conditioned media are required for generating DP T cells from CB and G-CSF-mobilized HSCs and that generation and proliferation of proT and DP T cells are greatly improved by AA. This technology could potentially be used to generate T cell progenitors for adoptive therapy.

Myeloid-lymphoid ontogeny in the rhesus monkey (Macaca mulatta)

Establishment of a functional immune system has important implications for health and disease, yet questions remain regarding the mechanism, location, and timing of development of myeloid and lymphoid cell compartments. The goal of this study was to characterize the ontogeny of the myeloid-lymphoid system in rhesus monkeys to enhance current knowledge of the developmental sequence of B-cell (CD20, CD79), T-cell (CD3, CD4, CD8, FoxP3), dendritic cell (CD205), and macrophage (CD68) lineages in the fetus and infant. Immunohistochemical assessments addressed the temporal and spatial expression of select phenotypic markers in the developing liver, thymus, spleen, lymph nodes, gut-associated lymphoid tissue (GALT), and bone marrow with antibodies known to cross-react with rhesus cells. CD3 was the earliest lymphoid marker identified in the first trimester thymus and, to a lesser extent, in the spleen. T-cell markers were also expressed midgestation on cells of the liver, spleen, thymus, and in Peyer's patches of the small and large intestine, and where CCR5 expression was noted. A myeloid marker, CD68, was found on hepatic cells near blood islands in the late first trimester. B-cell markers were observed mid-second trimester in the liver, spleen, thymus, lymph nodes, bone marrow spaces, and occasionally in GALT. By the late third trimester and postnatally, secondary follicles with germinal centers were present in the thymus, spleen, and lymph nodes. These results suggest that immune ontogeny in monkeys is similar in temporal and anatomical sequence when compared to humans, providing important insights for translational studies.

Monitoring fetal immune development in human pregnancies: current concepts and future goals

The vast majority of the current knowledge on immune development in the fetal period has been gained from animal studies, particularly from mouse models. This has led to a great improvement in our current understanding of immune ontogeny. However, it has also become clear that in many ways the mouse model of pregnancy differs from the situation in human pregnancy, such as the degree and importance of trophoblast invasion, the kind of MHC class repertoire of the extravillous trophoblast cells, and differences concerning the development and regulation of T-cells. It will be of paramount importance to develop non-invasive screening methods to assess fetal immune development in humans. The focus of this mini-review is to discuss how prenatal ultrasound evaluation can be used as a tool to monitor fetal immune development in human pregnancies. To identify the fetuses at risk of immune disorders could be the first step to developing prevention strategies in the future.

Scar-free wound healing and regeneration in amphibians: immunological influences on regenerative success

Salamanders and frogs are distinct orders of Amphibians with very different immune systems during adult life, exhibiting varying potential for scar free repair and regeneration. While salamanders can regenerate a range of body parts throughout all stages of life, regeneration is restricted to early stages of frog development. Comparison of these two closely related amphibian orders provides insights into the immunological influences on wound repair, and the different strategies that have evolved either to limit infection or to facilitate efficient regeneration. After injury, cells of the immune system are responsible for the removal of damaged cells and providing a cohort of important growth factors and signaling molecules. Immune cells not only regulate new vessel growth important for supplying essential nutrients to damaged tissue but, modulate the extracellular matrix environment by regulating fibroblasts and the scarring response. The profile of immune cell infiltration and their interaction with local tissue immune cells directly influences many aspects of the wound healing outcomes and can facilitate or prevent regeneration. Evidence is emerging that the transition from wound healing to regeneration is reliant on immune cell engagement and that the success of regeneration in amphibians may depend on complex interactions between stem cell progenitors and immune cell subsets. The potential immunological barriers to mammalian regeneration are discussed with implications for the successful delivery of stem cell therapeutic strategies in patients.

Mother and child T cell receptor repertoires: deep profiling study

The relationship between maternal and child immunity has been actively studied in the context of complications during pregnancy, autoimmune diseases, and haploidentical transplantation of hematopoietic stem cells and solid organs. Here, we have for the first time used high-throughput Illumina HiSeq sequencing to perform deep quantitative profiling of T cell receptor (TCR) repertoires for peripheral blood samples of three mothers and their six children. Advanced technology allowed accurate identification of 5 × 10⁵ to 2 × 10⁶ TCR beta clonotypes per individual. We performed comparative analysis of these TCR repertoires with the aim of revealing characteristic features that distinguish related mother-child pairs, such as relative TCR beta variable segment usage frequency and relative overlap of TCR beta complementarity-determining region 3 (CDR3) repertoires. We show that thymic selection essentially and similarly shapes the initial output of the TCR recombination machinery in both related and unrelated pairs, with minor effect from inherited differences. The achieved depth of TCR profiling also allowed us to test the hypothesis that mature T cells transferred across the placenta during pregnancy can expand and persist as functional microchimeric clones in their new host, using characteristic TCR beta CDR3 variants as clonal identifiers.

Fetal regulatory T cells and peripheral immune tolerance in utero: implications for development and disease

The developing fetus must actively learn to tolerate benign antigens or suffer the consequences of broken tolerance. Tolerance of self-antigens prevents development of autoimmune diseases and is achieved by both deletion of autoreactive T cell clones in the thymus (central tolerance) and by the suppressive influence of CD4(+)  CD25(+)  FoxP3(+) regulatory T cells (Tregs) in the periphery. Fetal CD4(+) T cells have a strong predisposition to differentiate into tolerogenic Tregs that actively promote self-tolerance, as well as tolerance to non-inherited antigens on chimeric maternal cells that reside in fetal tissues. As the fetus nears birth, a crucial transition must occur between the tolerogenic fetal immune system and a more defensive adult-type immune system that is able to combat pathogens. This paper will review the unique tolerogenic nature of fetal T cells and will examine evidence for a novel model of fetal immune development: the layered immune system hypothesis.

Animal models for prenatal gene therapy: rodent models for prenatal gene therapy

Fetal gene transfer has been studied in various animal models, including rabbits, guinea pigs, cats, dogs, and nonhuman primate; however, the most common model is the rodent, particularly the mouse. There are numerous advantages to mouse models, including a short gestation time of around 20 days, large litter size usually of more than six pups, ease of colony maintenance due to the small physical size, and the relatively low expense of doing so. Moreover, the mouse genome is well defined, there are many transgenic models particularly of human monogenetic disorders, and mouse-specific biological reagents are readily available. One criticism has been that it is difficult to perform procedures on the fetal mouse with suitable accuracy. Over the past decade, accumulation of technical expertise and development of technology such as high-frequency ultrasound have permitted accurate vector delivery to organs and tissues. Here, we describe our experiences of gene transfer to the fetal mouse with and without ultrasound guidance from mid to late gestation. Depending upon the vector type, the route of delivery and the age of the fetus, specific or widespread gene transfer can be achieved, making fetal mice excellent models for exploratory biodistribution studies.

Immunological tolerance during fetal development: from mouse to man

The development of the adaptive immune system has been studied in the mouse primarily because it is easier to access fetal tissues and because there exists a rich array of probes for analysis of various components of the immune system. While much has been learned from this exercise, it is also clear that different species show substantial temporal variation in the development of the immune system during early life. In mice, for instance, mature α/β T cells first appear in the periphery during the final stages of fetal gestation and only increase in number after birth (Friedberg and Weissman, 1974); in humans, on the other hand, the first mature α/β T cells are seen in peripheral tissues at 10-12 gestational weeks (g.w.) and are circulating in significant numbers by the end of the second trimester (Ceppellini et al., 1971; Haynes et al., 1988; Hayward and Ezer, 1974; Kay et al., 1970). Although the functional implications of these differences remain unclear, it is likely that there are significant biological consequences associated with the relatively early development of the peripheral adaptive immune system in humans, for example, with respect to the development of peripheral tolerance as well as to the response to antigens that might cross the placenta from the mother (e.g., cells bearing noninherited maternal alloantigens, infectious agents, food antigens, and the like). Here, we will review studies of immune system ontogeny in the mouse and in humans, and then focus on the possible functional roles of fetal T cell populations during development and later in life in humans.

Nutritionally mediated programming of the developing immune system

A growing body of evidence highlights the importance of a mother's nutrition from preconception through lactation in programming the emerging organ systems and homeostatic pathways of her offspring. The developing immune system may be particularly vulnerable. Indeed, examples of nutrition-mediated immune programming can be found in the literature on intra-uterine growth retardation, maternal micronutrient deficiencies, and infant feeding. Current models of immune ontogeny depict a "layered" expansion of increasingly complex defenses, which may be permanently altered by maternal malnutrition. One programming mechanism involves activation of the maternal hypothalamic-pituitary-adrenal axis in response to nutritional stress. Fetal or neonatal exposure to elevated stress hormones is linked in animal studies to permanent changes in neuroendocrine-immune interactions, with diverse manifestations such as an attenuated inflammatory response or reduced resistance to tumor colonization. Maternal malnutrition may also have a direct influence, as evidenced by nutrient-driven epigenetic changes to developing T regulatory cells and subsequent risk of allergy or asthma. A 3rd programming pathway involves placental or breast milk transfer of maternal immune factors with immunomodulatory functions (e.g. cytokines). Maternal malnutrition can directly affect transfer mechanisms or influence the quality or quantity of transferred factors. The public health implications of nutrition-mediated immune programming are of particular importance in the developing world, where prevalent maternal undernutrition is coupled with persistent infectious challenges. However, early alterations to the immune system, resulting from either nutritional deficiencies or excesses, have broad relevance for immune-mediated diseases, such as asthma, and chronic inflammatory conditions like cardiovascular disease.

Fetal and adult hematopoietic stem cells give rise to distinct T cell lineages in humans

Although the mammalian immune system is generally thought to develop in a linear fashion, findings in avian and murine species argue instead for the developmentally ordered appearance (or "layering") of distinct hematopoietic stem cells (HSCs) that give rise to distinct lymphocyte lineages at different stages of development. Here we provide evidence of an analogous layered immune system in humans. Our results suggest that fetal and adult T cells are distinct populations that arise from different populations of HSCs that are present at different stages of development. We also provide evidence that the fetal T cell lineage is biased toward immune tolerance. These observations offer a mechanistic explanation for the tolerogenic properties of the developing fetus and for variable degrees of immune responsiveness at birth.