Analysis of the first two waves of thymus homing stem cells and their T cell progeny in chick-quail chimeras

The search to understand the development of the chicken immune system: Differences in expression of MHC class I loci and waves of thymocytes as evolutionary relics?

Chickens are renowned as a model for embryogenesis but have also been responsible for crucial advances in virology, cancer research and immunology. However, chickens are best known as a major source of animal protein for human nutrition, with roughly 80 billion chickens alive each year supplying meat and eggs, the vast majority part of a global poultry industry. As a result, avian immunology been studied intensively for over 60 years, and it has become clear that a major genetic locus in chickens determining resistance to infectious disease and response to vaccines is the major histocompatibility complex (MHC). Compared to typical mammals, the chicken MHC is compact and simple, with only two classical class I genes. A dominantly-expressed class I gene, BF2, is the major ligand for cytotoxic T lymphocytes (CTLs), while the other locus, BF1, is much less well-expressed, lacking in some MHC haplotypes, and is a ligand for natural killer (NK) cells. Cell surface class I expression in neonatal chicks is far less than in adults, and one possibility is that BF2 is not well-expressed early in ontogeny. A precedent is found for amphibians: the single classical class I molecule is not expressed in tadpoles of Xenopus frogs, although non-polymorphic (and thus non-classical) class I molecules from the XNC locus are expressed, which are recognised for immune defence by non-canonical NKT lymphocytes. Indeed, three waves of different T cells are produced by the Xenopus thymus: in tadpoles, during metamorphosis and finally as adults. Three waves of thymic emigrants are also found for chickens, and reasoning by analogy, it may be that the waves of thymocytes and the expression of class I molecules during ontogeny of chickens are evolutionary relics. As well as scientific interest in the ontogeny of MHC class I expression and appearance of peripheral T cells, there are potential practical implications, given the importance of vaccination in ovo and in day-old chicks for the poultry industry.

Characterization and ontogeny of a novel lymphoid follicle inducer cell during development of the bursa of Fabricius

The avian bursa of Fabricius (BF) is a primary lymphoid organ, where B-cell development occurs within bursal follicles of epithelial origin. During embryogenesis the epithelial anlage of the BF emerges as a diverticulum of the cloaca surrounded by undifferentiated tail bud mesenchyme. While it is believed that the epithelial-mesenchymal BF primordium provides a selective microenvironment for developing B cells, the initial events inducing lymphoid follicle formation are not fully elucidated. Using wild type and CSF1R-eGFP transgenic chick embryos, we find that separate B cell, macrophage and dendritic cell precursors enter the BF mesenchyme, migrate to the surface epithelium, and colonize the lymphoid follicle buds. Detailed immunocytochemical characterization revealed a novel EIV-E12+ blood-borne cell type, colonizing the surface epithelium of the BF rudiment before the entry of myeloid and lymphoid lineages and the appearance of this cell type coincides with the onset of follicle bud formation. Chick-duck chimeras and chick-quail tissue recombination experiments suggest that EIV-E12+ cells represent a transient lymphoid inducer cell population. They are not dendritic or B cells precursors, and they are capable of follicle bud induction in both dendritic cell- and B cell-depleted bursae.

Delineation of chicken immune markers in the era of omics and multicolor flow cytometry

Multiparameter flow cytometry is a routine method in immunological studies incorporated in biomedical, veterinary, agricultural, and wildlife research and routinely used in veterinary clinical laboratories. Its use in the diagnostics of poultry diseases is still limited, but due to the continuous expansion of reagents and cost reductions, this may change in the near future. Although the structure and function of the avian immune system show commonalities with mammals, at the molecular level, there is often low homology across species. The cross-reactivity of mammalian immunological reagents is therefore low, but nevertheless, the list of reagents to study chicken immune cells is increasing. Recent improvement in multicolor antibody panels for chicken cells has resulted in more detailed analysis by flow cytometry and has allowed the discovery of novel leukocyte cell subpopulations. In this article, we present an overview of the reagents and guidance needed to perform multicolor flow cytometry using chicken samples and common pitfalls to avoid.

How to Break through the Bottlenecks of in Ovo Vaccination in Poultry Farming

Poultry farming is one of the pillar industries of global animal husbandry. In order to guarantee production, poultry are frequently vaccinated from the moment they are hatched. Even so, the initial immunity of chicks is still very poor as they are in the "window period" of immune protection. In ovo vaccination pushes the initial immunization time forward to the incubation period, thereby providing earlier immune protection for chicks. In ovo vaccination is currently a research hotspot of poultry disease prevention and control, which is in line with the intensification of poultry production. However, the vaccines currently available for in ovo vaccination are limited and cannot meet the needs of industrial development, so how to efficiently activate the adaptive immune response of chicken embryos becomes the key to restrict product development and technological progress of in ovo vaccination. Its breakthrough, to a large extent, depends on systematic illustration of the mechanism underlying the adaptive immune response post immunization. Clarification of this issue will provide us with theoretical support and potential solutions for the development of novel vaccines for in ovo vaccination, the augmentation of efficacy of current vaccines and the optimization of immune programs.

Development and characterization of a CRISPR/Cas9-mediated RAG1 knockout chicken model lacking mature B and T cells

Although birds have been used historically as a model animal for immunological research, resulting in remarkable achievements, immune cell development in birds themselves has yet to be fully elucidated. In this study, we firstly generated an immunodeficient chicken model using a CRISPR/Cas9-mediated recombination activating gene 1 (RAG1) knockout, to investigate avian-specific immune cell development. Unlike previously reported immunoglobulin (Ig) heavy chain knockout chickens, the proportion and development of B cells in both RAG1^+/- and RAG1^-/- embryos were significantly impaired during B cell proliferation (embryonic day 16 to 18). Our findings indicate that, this is likely due to disordered B cell receptor (BCR)-mediated signaling and interaction of CXC motif chemokine receptor (CXCR4) with CXCL12, resulting from disrupted Ig V(D)J recombination at the embryonic stage. Histological analysis after hatching showed that, unlike wild-type (WT) and RAG1^+/- chickens, lymphatic organs in 3-week old RAG1^-/- chickens were severely damaged. Furthermore, relative to WT chickens, RAG1^+/- and RAG1^-/- birds had reduced serum Igs, fewer mature CD4⁺ and CD8⁺ T lymphocytes. Furthermore, BCR-mediated B cell activation in RAG1^+/- chickens was insufficient, leading to decreased expression of the activation-induced deaminase (AID) gene, which is important for Ig gene conversion. Overall, this immunodeficient chicken model underlines the pivotal role of RAG1 in immature B cell development, Ig gene conversion during embryonic stages, and demonstrates the dose-dependent regulatory role of RAG1 during immune cell development. This model will provide ongoing insights for understanding chicken immune system development and applied in the fields of immunology and biomedical science.

Alteration in the Population of Intraepithelial Lymphocytes and Virus Shedding in Specific-Pathogen-Free Chickens Following Inoculation with Lentogenic and Velogenic Newcastle Disease Virus Strains

Intraepithelial lymphocytes (IELs) provide the first line of immunological defense after the invasion of the intestine by a pathogen. To understand the changes of IEL response in chickens, we measured the population of different subsets of avian IELs at different time points after primary inoculation of Newcastle disease virus (NDV) lentogenic strain (LaSota) and subsequent challenge with NDV velogenic strain- genotypes VII and VIII. Furthermore, NDV shed after each treatment was quantified. Specific-pathogen-free chickens were randomly divided into six groups of chickens, one to six, inoculated with phosphate buffered saline; NDV lentogenic strain (LaSota); genotype VII (GVII); LaSota and challenged with GVII (LSGVII); genotype VIII (GVIII); and group of LaSota and challenged with GVIII (LSGVIII). The chickens were euthanized at 12, 36, and 60 h postchallenge. Immunophenotyping of CD25⁺ IEL, CD3⁺ cells, CD4⁺ cells, and CD8⁺ cells was conducted using flow cytometer. Furthermore, virus shedding was measured using reverse transcriptase-quantitative polymerase chain reaction. Data were analyzed using a two-way analysis of variance (ANOVA). The results showed that the percentage population of IEL subsets was generally lower in the chickens inoculated with GVII or GVIII when compared with LaSota, LSGVII and LSGVIII inoculated groups. The NDV copy number was significantly higher in chickens challenged with NDV GVII or GVIII when compared with chickens inoculated with LaSota, LSGVII or LSGVIII. Taking together, NDV velogenic strain caused decrease in the population of subsets of chickens' IEL. However, inoculation of NDV LaSota may increase the population of avian IEL subsets and decrease shedding of virulent NDV.

Role of Physiology, Immunity, Microbiota, and Infectious Diseases in the Gut Health of Poultry

“Gut health” refers to the physical state and physiological function of the gastrointestinal tract and in the livestock system; this topic is often focused on the complex interacting components of the intestinal system that influence animal growth performance and host-microbial homeostasis. Regardless, there is an increasing need to better understand the complexity of the intestinal system and the various factors that influence gut health, since the intestine is the largest immune and neuroendocrine organ that interacts with the most complex microbiome population. As we face the post-antibiotic growth promoters (AGP) era in many countries of the world, livestock need more options to deal with food security, food safety, and antibiotic resilience to maintain agricultural sustainability to feed the increasing human population. Furthermore, developing novel antibiotic alternative strategies needs a comprehensive understanding of how this complex system maintains homeostasis as we face unpredictable changes in external factors like antibiotic-resistant microbes, farming practices, climate changes, and consumers’ preferences for food. In this review, we attempt to assemble and summarize all the relevant information on chicken gut health to provide deeper insights into various aspects of gut health. Due to the broad and complex nature of the concept of “gut health”, we have highlighted the most pertinent factors related to the field performance of broiler chickens.

Development of innate immunity in chicken embryos and newly hatched chicks: a disease control perspective

Newly hatched chickens are confronted by a wide array of pathogenic microbes because their adaptive immune defences have limited capabilities to control these pathogens. In such circumstances, and within this age group, innate responses provide a degree of protection. Moreover, as the adaptive immune system is relatively naïve to foreign antigens, synergy with innate defences is critical. This review presents knowledge on the ontogeny of innate immunity in chickens pre-hatch and early post-hatch and provides insights into possible interventions to modulate innate responses early in the life of the bird. As in other vertebrate species, the chicken innate immune system which include cellular mediators, cytokine and chemokine repertoires and molecules involved in antigen detection, develop early in life. Comparison of innate immune systems in newly hatched chickens and mature birds has revealed differences in magnitude and quality, but responses in younger chickens can be boosted using innate immune system modulators. Functional expression of pattern recognition receptors and several defence molecules by innate immune system cells of embryos and newly hatched chicks suggests that innate responses can be modulated at this stage of development to combat pathogens. Improved understanding of innate immune system ontogeny and functionality in chickens is critical for the implementation of sound and safe interventions to provide long-term protection against pathogens. Next-generation tools for studying genetic and epigenetic regulation of genes, functional metagenomics and gene knockouts can be used in the future to explore and dissect the contributions of signalling pathways of innate immunity and to devise more efficacious disease control strategies.

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.

Hatchery Vaccination Against Poultry Viral Diseases: Potential Mechanisms and Limitations

Commercial broiler and layer chickens are heavily vaccinated against economically important viral diseases with a view of preventing morbidity, mortality, and production impacts encountered during short production cycles. Hatchery vaccination is performed through in ovo embryo vaccination prehatch or spray and subcutaneous vaccinations performed at the day of hatch before the day-old chickens are being placed in barns with potentially contaminated environments. Commercially, multiple vaccines (e.g., live, live attenuated, and viral vectored vaccines) are available to administer through these routes within a short period (embryo day 18 prehatch to day 1 posthatch). Although the ability to mount immune response, especially the adaptive immune response, is not optimal around the hatch, it is possible that the efficacy of these vaccines depends partly on innate host responses elicited in response to replicating vaccine viruses. This review focuses on the current knowledge of hatchery vaccination in poultry and potential mechanisms of hatchery vaccine-mediated protective responses and limitations.

Avian dendritic cells: Phenotype and ontogeny in lymphoid organs

Dendritic cells (DC) are critically important accessory cells in the innate and adaptive immune systems. Avian DCs were originally identified in primary and secondary lymphoid organs by their typical morphology, displaying long cell processes with cytoplasmic granules. Several subtypes are known. Bursal secretory dendritic cells (BSDC) are elongated cells which express vimentin intermediate filaments, MHC II molecules, macrophage colony-stimulating factor 1 receptor (CSF1R), and produce 74.3+ secretory granules. Avian follicular dendritic cells (FDC) highly resemble BSDC, express the CD83, 74.3 and CSF1R molecules, and present antigen in germinal centers. Thymic dendritic cells (TDC), which express 74.3 and CD83, are concentrated in thymic medulla while interdigitating DC are found in T cell-rich areas of secondary lymphoid organs. Avian Langerhans cells are a specialized 74.3-/MHC II+ cell population found in stratified squamous epithelium and are capable of differentiating into 74.3+ migratory DCs. During organogenesis hematopoietic precursors of DC colonize the developing lymphoid organ primordia prior to immigration of lymphoid precursor cells. This review summarizes our current understanding of the ontogeny, cytoarchitecture, and immunophenotype of avian DC, and offers an antibody panel for the in vitro and in vivo identification of these heterogeneous cell types.

Analysis of antigen specific T cells in diabetes - Lessons from pre-clinical studies and early clinical trials

Antigen-specific immune tolerance promises to provide safe and effective therapies to prevent type 1 diabetes (T1D). Antigen-specific therapy requires two components: well-defined, clinically relevant autoantigens; and safe approaches to inducing tolerance in T cells specific for these antigens. Proinsulin is a critical autoantigen in both NOD mice, based on knockout mouse studies and induction of immune tolerance to proinsulin preventing disease whereas most antigens cannot, and also in human T1D based on proinsulin-specific T cells being found in the islets of affected individuals and the early appearance of insulin autoantibodies. Effective antigen-specific therapies that prevent T1D in humans have not yet been developed although doubt remains about the best molecular form of the antigen, the dose and the route of administration. Preclinical studies suggest that antigen specific therapy is most useful when administered before onset of autoimmunity but this time-window has not been tested in humans until the recent "pre-point" study. There may be a 'window of opportunity' during the neonatal period when 'vaccine' like administration of proinsulin for a short period may be sufficient to prevent diabetes. After the onset of autoimmunity, naive antigen-specific T cells have differentiated into antigen-experienced memory cells and the immune responses have spread to multiple antigens. Induction of tolerance at this stage becomes more difficult although recent studies have suggested generation of antigen-specific TR1 cells can inhibit memory T cells. Preclinical studies are required to identify additional 'help' that is required to induce tolerance to memory T cells and develop protocols for effective therapy in individuals with established autoimmunity.

Poultry coccidiosis: recent advancements in control measures and vaccine development

Coccidiosis is recognized as the major parasitic disease of poultry and is caused by the apicomplexan protozoan Eimeria. Coccidiosis seriously impairs the growth and feed utilization of infected animals resulting in loss of productivity. Conventional disease control strategies rely heavily on chemoprophylaxis and, to a certain extent, live vaccines. Combined, these factors inflict tremendous economic losses to the world poultry industry in excess of USD 3 billion annually. Increasing regulations and bans on the use of anticoccidial drugs coupled with the associated costs in developing new drugs and live vaccines increases the need for the development of novel approaches and alternative control strategies for coccidiosis. This paper aims to review the current progress in understanding the host immune response to Eimeria and discuss current and potential strategies being developed for coccidiosis control in poultry.

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.

The murine Th2 locus undergoes epigenetic modification in the thymus during fetal and postnatal ontogeny

Epigenetic modifications play a central role in the differentiation and function of immune cells in adult animals. Developmentally regulated epigenetic patterns also appear to contribute to the ontogeny of the immune system. We show here that the epigenetic profile of the T-helper (Th) 2 locus undergoes changes in T lineage cells beginning in mid-gestation and extending throughout the first week of life. In particular, regulatory regions of the Th2 locus are largely methylated at CpG residues among fetal liver common lymphoid progenitor cells. The locus subsequently becomes highly hypomethylated among the downstream progeny of these cells within the fetal thymus. This hypomethylated state is preserved until birth when the locus becomes rapidly re-methylated, achieving adult-like status by 3–6 days post birth. Notably, the capacity for rapid, high level Th2 cytokine production is lost in parallel with this re-methylation. In vitro organ culture and in vivo transplantation experiments indicate that signals from the adult environment are required to achieve the postnatal methylated state. Together, these findings indicate that the Th2 bias of neonates may be conferred, in part, by an epigenetic profile inherited from fetal life. However, the fetal program is rapidly terminated post birth by the development of signals leading to the acquisition of adult-like epigenetic patterns.

CD4+CD25+ Regulatory T cell ontogeny and preferential migration to the cecal tonsils in chickens

Thymic CD4(+)CD25(+) cells have regulatory-T-cell-like properties in chickens. This study examined the ontogeny of CD4(+)CD25(+) cells in the thymus and in peripheral compartments in chickens. CD4(+)CD25(+) cells started to appear in the thymus at day 15 of incubation (E15), although at low percentages. Expressed as a percentage of CD4(+) cells, CD4(+)CD25(+) cells increased (P<0.01) from 1.7% at E20 to 7.3% at 0 d post-hatch (D0). CD4(+)CD25(+) cells did not appear in the spleen or cecal tonsils of embryos. Expressed as a percentage of CD4(+) cells, CD4(+)CD25(+) cells increased (P<0.01) from 0% at D0 to 27% at D1 in cecal tonsils and from 0% at D0 to 11% at D1 in the spleen. Expressed as a percentage of all mononuclear cells, cecal tonsils at D1 had approximately 3.5-fold higher percentage of CD4(+)CD25(+) cells than the spleen at D1. CD4(+)CD25(+) cells from cecal tonsils of chicks at D1 were suppressive. CD4(+)CD25(+) cells from D0 thymus, when injected back into MHC-compatible chicks, migrated to cecal tonsils and lungs and were detected until 10 d post-injection. CD4(+)CD25(+) cells from cecal tonsils had a higher (P = 0.01) relative amount of CCR9 mRNA than CD4(+)CD25(+) cells from the thymus. It could be concluded that in chickens CD4(+)CD25(+) cells migrate from the thymus immediately post-hatch and preferentially colonize the gut associated lymphoid tissues. CD4(+)CD25(+) cells' preferential migration to cecal tonsils is likely directed through the CCR9 pathway in chickens.

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.

A life of adventure in immunobiology

This article outlines my early start in medicine, a late start in immunology research, and my efforts to integrate the two activities. I first describe some of the background information, excitement, and implications of the recognition of T and B cells as separate but functionally intertwined arms of the adaptive immune system. The article continues with a brief account of my colleagues' and my efforts to use the model of hematopoietic stem cell differentiation along T and B cell lines to gain a better understanding of immunodeficiency diseases and lymphoid malignancies. It concludes with the discovery of a more ancient adaptive immune system in which T-like and B-like cells in jawless vertebrates use variable lymphocyte receptors constructed with leucine-rich-repeat sequences to recognize antigens.

Early embryonic blood cells collect antigens and induce immunotolerance in the hatched chicken

Earlier experimental data in our laboratory showed that introduction of an exogenous protein into early chicken embryonic blood leads to immunotolerance of hatched chicken to that protein. However, the underlying mechanism is yet unknown. In the present study, we show that the blood cells collecting circulating antigen might contribute to the establishment of immunotolerance. In this experiment, most of the chicken embryo blood cells took up injected fluorescein isothiocyanate-BSA at approximately embryonic d 3. At the same stage, 1 microL of embryo blood was taken out and incubated with BSA. After being loaded with BSA in vitro and washed, these cells were injected back into the original embryo. The BSA-specific lymphocytes were depleted in chickens whose early embryo cells had been loaded with BSA, as evidenced by a significant decrease in anti-BSA antibody after challenge with BSA when the chickens were 3 wk old. In addition, by direct injection of BSA to embryonic d 3 embryo blood, the hatched chickens had decreased amounts of anti-trinitrophenol antibody after the chickens were challenged with trinitrophenol-BSA, indicating that the helper function of BSA-specific T cells was impaired. In conclusion, these observations suggest that some early embryo blood cells possibly collect and store antigen for the establishment of self-tolerance before the maturation of B and T cells.

Two Groups of Porcine TCRγδ+Thymocytes Behave and Diverge Differently

Developmental pathways of gammadelta T cells are still unknown, largely because of the absence of recognized lineage-specific surface markers other than the TCR. We have shown that porcine gammadelta thymocytes can be divided into 12 subsets of the following two major groups: 1) CD4(-) gammadelta thymocytes that can be further subdivided according to their CD2/CD8alphaalpha phenotype, and 2) CD4(+) gammadelta thymocytes that are always CD1(+)CD2(+)CD8alphabeta(+) and have no counterpart in the periphery. In this study, we have analyzed gammadelta thymocyte subsets with respect to behavior during cultivation, cell cycle status, and lymphocyte-specific transcripts. The group of CD4(-) gammadelta thymocytes gives rise to all gammadelta T cells found in the periphery. Proliferating CD2(+)CD8(-)CD1(+)CD45RC(-) gammadelta thymocytes are a common precursor of this group. These precursors differentiate into CD2(+)CD8alphaalpha(+), CD2(+)CD8(-), and CD2(-)CD8(-) gammadelta T cell subsets, which subsequently mature by loss of CD1 and by eventual gain of CD45RC expression. In contrast, the group of CD4(+) gammadelta thymocytes represents transient and independent subsets that are never exported from thymus as TCRgammadelta(+) T cells. In accordance with the following findings, we propose that CD4(+)CD8alphabeta(+) gammadelta thymocytes extinguish their TCRgammadelta expression and differentiate along the alphabeta T cell lineage program: 1) CD4(+) gammadelta thymocytes are actively dividing; 2) CD4(+) gammadelta thymocytes do not die, although their numbers decreased with prolonged cultivation; 3) CD4(+) gammadelta thymocytes express transcripts for RAG-1, TdT, and TCRbeta; and 4) CD4(+) gammadelta thymocytes are able to alter their phenotype to TCRalphabeta(+) thymocytes under appropriate culture conditions.

Journey through the thymus: stromal guides for T-cell development and selection

Lympho-stromal interactions in multiple microenvironments within the thymus have a crucial role in the regulation of T-cell development and selection. Recent studies have implicated that chemokines that are produced by thymic stromal cells have a pivotal role in positioning developing T cells within the thymus. In this Review, I discuss the importance of stroma-derived chemokines in guiding the traffic of developing thymocytes, with an emphasis on the processes of cortex-to-medulla migration and T-cell-repertoire selection, including central tolerance.

Cellular and molecular events during early thymus development

The thymic stromal compartment consists of several cell types that collectively enable the attraction, survival, expansion, migration, and differentiation of T-cell precursors. The thymic epithelial cells constitute the most abundant cell type of the thymic microenvironment and can be differentiated into morphologically, phenotypically, and functionally separate subpopulations of the postnatal thymus. All thymic epithelial cells are derived from the endodermal lining of the third pharyngeal pouch. Very soon after the formation of a thymus primordium and prior to its vascularization, thymic epithelial cells orchestrate the first steps of intrathymic T-cell development, including the attraction of lymphoid precursor cells to the thymic microenvironment. The correct segmentation of pharyngeal epithelial cells and their subsequent crosstalk with cells in the pharyngeal arches are critical prerequisites for the formation of a thymus anlage. Mutations in several transcription factors and their target genes have been informative to detail some of the complex mechanisms that control the development of the thymus anlage. This review highlights recent findings related to the genetic control of early thymus organogenesis and provides insight into the molecular basis by which lymphocyte precursors are attracted to the thymus.

From the hemangioblast to self-tolerance: a series of innovations gained from studies on the avian embryo

During the last decades of the 20th century, studies on the vertebrate hematopoietic and immune systems have largely been performed, on mammalian models. The mouse has been the preferred material for several cogent reasons: (i) numerous well defined genetic strains are available; (ii) this species has been and still is instrumental in the study of gene activity through transgenesis; and (iii) in vitro culture techniques and in vivo assays for blood cells together with a wide array of antibodies and nucleic acid probes have been developed to investigate the cellular interactions occurring during hematopoiesis and immune reactivity. However, important and fundamental notions have emerged from using another higher vertebrate model, the avian embryo. The distinction among small lymphocytes of two populations, the T and B lymphocytes, endowed with different roles in adaptive immunity and dependant on different environments for their specification, has relied on experiments carried out in birds. The avian model has been critical for the analysis of the origin and traffic of hematopoietic precursor cells. It allowed the demonstration that both hematopoietic and angioblastic lineages arise from a common precursor, a cell whose existence had been proposed but never undoubtedly proven, the hemangioblast. Finally a form of thymus-dependant 'dominant' tolerance was demonstrated on the basis of experiments in the avian embryo, which initiated a large current of studies on 'regulatory T-cells'. Work in this model during the last decades has relied strongly on the construction of chimeras between quail and chick embryos that allowed a refined analysis of cell behaviour during embryogenesis. The novel perception of developmental neuropoiesis and immunopoiesis that followed proved to be largely applicable to lower and higher vertebrates, notably mammals.

Commentary: the role of cell migration in the ontogeny of the lymphoid system

The foundations of experimental hematology were laid by histologists, and while their contributions were enormous, they were limited in their interpretation of very dynamic processes by the static nature of the methodology. The middle of the twentieth century saw the introduction of techniques for hematopoietic cell marking and development of in vitro and in vivo assays for primitive hematopoietic cells, allowing dynamic studies of hematopoiesis. Paralleling this was an understanding of cellular immunology with the discovery of the role of the thymus and the identification of T and B lymphocyte lineages. In the 1960s a series of ontogenetic studies in birds and subsequently in mice revealed that hematopoietic and lymphoid development involved migration streams of primitive cells that colonized developing primary lymphoid organs as well as spleen, marrow, and liver. The yolk sac was proposed as the ultimate origin of these lympho-hematopoietic precursors. Subsequent studies identified a region associated with the dorsal aorta as the primary site of "definitive" stem cells. These opposing views are currently achieving a compromise that recognizes that both sites contribute stem cells involved in seeding the developing tissues. The clear distinction between the local origin of the inducing microenvironment provided by the endoderm or by stroma derived from mesenchymal stem cells of mesodermal origin, and the immigrant origin of the hematopoietic stem cells and progenitors, raises intriguing questions in the current climate of stem cell plasticity, cell fusion, and discovery of stem cells in adult marrow with the capacity to generate hematopoiesis as well as other mesodermal, ectodermal, and endodermal lineages.

Comparison ofin ovoand post‐hatch vaccination with particular reference to infectious bursal disease. A review

In ovo vaccination is an alternative approach to post-hatch vaccination of chickens, particularly in broilers. Vaccination at embryonation day 18 helps to 'close the window' of susceptibility i.e. the time between vaccination and early exposure to infectious agents compared with post-hatch vaccination. Attempts on embryonal vaccination as a mode of vaccine delivery were approached from the observation that chickens already develop certain immunologic functions before hatching. The immune system in birds begins to develop early during embryogenesis and various immune reactions have been induced in the late stage chicken embryos. Compared with post-hatch vaccination, in ovo vaccination stimulates both the innate and adaptive immune responses with the advantage that because of the prenatal immunization, in ovo vaccinated chicks have developed an appreciable degree of protection by the time of hatch. Effects of maternal antibodies on vaccines to be used for in ovo vaccination can be prevented by developing vaccines that are insensitive to maternal antibodies. It has been described that vaccination of chicken embryos at embryonation day 18 did not significantly affect the immune competence of hatched chickens. The apparent absence of tolerance in chicks hatched from embryos exposed to an antigen at the late stage of embryonation implies the feasibility of in ovo vaccination. Investigations on in ovo vaccination to produce safe and efficient vaccines are still in progress. Currently a large number of vaccines are under investigation for viral, bacterial and protozoal diseases.

Ontogeny of gamma delta T cells in humans

T cell receptors consist either of an alpha-chain combined with a beta-chain or a gamma-chain combined with a delta-chain. alphabeta T cells constitute the majority of T cells in human blood throughout life. Flow cytometric analyses presented in this study, which focus on the representation of the developmental (naive and memory) subsets of gammadelta T cells, show by function and phenotype that this lineage contains both naive and memory cells. In addition, we show that the representation of naive T cells is higher among alphabeta than gammadelta T cells in adults and that the low frequency of naive gammadelta T cells in adults reflects ontological differences between the two major gammadelta subsets, which are distinguished by expression of Vdelta1 vs Vdelta2 delta-chains. Vdelta1 cells, which mirror alphabeta cells with respect to naive representation, predominate during fetal and early life, but represent the minority of gammadelta cells in healthy adults. In contrast, Vdelta2 cells, which constitute the majority of adult gammadelta cells, show lower frequencies of naive cells than Vdelta1 early in life and show vanishingly small naive frequencies in adults. In essence, nearly all naive Vdelta2 cells disappear from blood by 1 year of life. Importantly, even in children less than 1 year old, most of the nonnaive Vdelta2 cells stain for perforin and produce IFN-gamma after short-term in vitro stimulation. This represents the earliest immunological maturation of any lymphocyte compartment in humans and most likely indicates the importance of these cells in controlling pathology due to common environmental challenges.

Avian HSC emergence, migration, and commitment toward the T cell lineage

To date three sites of emergence of hemopoietin cells have been identified during early avian development: the yolk sac, the intraaortic clusters and recently the allantois. However, the contributions of the hematopoietic stem cell (HSC) populations generated by these different sites to definitive hematopoiesis and their migration routes are not fully unraveled. Experimental embryology as well as the establishment of the genetic cascades involved in HSC emergence help now to draw a better scheme of these processes.

Exploring lymphocyte differentiation pathways

Highlights in a 4-decade exploration of lymphocyte differentiation begin with comparative studies in birds and mammals leading to recognition of the separate T- and B-cell differentiation pathways and their cooperative interaction. The global effects of aborting IgM B-cell development with anti-mu antibodies indicated that B cells can undergo immunoglobulin isotype switching. A search for the mammalian bursa equivalent that began with an extended excursion through the gut-associated lymphoepithelial tissues ultimately led to the hematopoietic tissue origin of mammalian B cells. The identification of the precursors of B cells in hematopoietic tissues provided an expanded view of the life history of B cells. A recurring theme in this essay is the interplay between understanding normal lymphocyte differentiation and the defects that underlie immunodeficiency diseases and lymphoid malignancies.

Complex expression patterns of lymphocyte-specific genes during the development of cartilaginous fish implicate unique lymphoid tissues in generating an immune repertoire

Cartilaginous fish express canonical B and T cell recognition genes, but their lymphoid organs and lymphocyte development have been poorly defined. Here, the expression of Ig, TCR, recombination-activating gene (Rag)-1 and terminal deoxynucleosidase (TdT) genes has been used to identify roles of various lymphoid tissues throughout development in the cartilaginous fish, Raja eglanteria (clearnose skate). In embryogenesis, Ig and TCR genes are sharply up-regulated at 8 weeks of development. At this stage TCR and TdT expression is limited to the thymus; later, TCR gene expression appears in peripheral sites in hatchlings and adults, suggesting that the thymus is a source of T cells as in mammals. B cell gene expression indicates more complex roles for the spleen and two special organs of cartilaginous fish-the Leydig and epigonal (gonad-associated) organs. In the adult, the Leydig organ is the site of the highest IgM and IgX expression. However, the spleen is the first site of IgM expression, while IgX is expressed first in gonad, liver, Leydig and even thymus. Distinctive spatiotemporal patterns of Ig light chain gene expression also are seen. A subset of Ig genes is pre-rearranged in the germline of the cartilaginous fish, making expression possible without rearrangement. To assess whether this allows differential developmental regulation, IgM and IgX heavy chain cDNA sequences from specific tissues and developmental stages have been compared with known germline-joined genomic sequences. Both non-productively rearranged genes and germline-joined genes are transcribed in the embryo and hatchling, but not in the adult.

Intestinal CD8 alpha alpha and CD8 alpha beta intraepithelial lymphocytes are thymus derived and exhibit subtle differences in TCR beta repertoires

Intraepithelial lymphocytes (IEL) of the small intestine are anatomically positioned to be in the first line of cellular defense against enteric pathogens. Therefore, determining the origin of these cells has important implications for the mechanisms of T cell maturation and repertoire selection. Recent evidence suggests that murine CD8 alpha alpha intestinal IELs (iIELs) can mature and undergo selection in the absence of a thymus. We analyzed IEL origin by cell transfer, using two congenic chicken strains. Embryonic day 14 and adult thymocytes did not contain any detectable CD8 alpha alpha T cells. However, when TCR(+) thymocytes were injected into congenic animals, they migrated to the gut and developed into CD8alphaalpha iIELs, while TCR(-) T cell progenitors did not. The TCR V beta 1 repertoire of CD8 alpha alpha(+) TCR V beta 1(+) iIELs contained only part of the TCR V beta 1 repertoire of total iIELs, and it exhibited no new members compared with CD8(+) T cells in the thymus. This indicated that these T cells emigrated from the thymus at an early stage in their developmental process. In conclusion, we show that while CD8 alpha alpha iIELs originate in the thymus, T cells acquire the expression of CD8 alpha alpha homodimers in the gut microenvironment.

The role of cell traffic in the emergence of the T lymphoid system

The role of the thymus is to ensure the differentiation and selection of T lymphocytes, which are one of the major players in the immune system. Recent studies show that the establishment of the T lymphoid system requires a complex cell traffic. In this field, avian embryos yield particularly informative developmental models because they are amenable to many experimental approaches during the phases of morphogenesis, and, in addition, the immune system resembles that of mammals.

Early ontogeny of thymocytes in pigs: sequential colonization of the thymus by T cell progenitors

Successive colonization of the thymus by waves of thymocyte progenitors has been described in chicken-quail chimeras and suggested from studies in mice. In swine, we show that the first CD3epsilon-bearing thymocytes appear on day 40 of gestation (DG40). These early thymocytes were CD3epsilonhigh and belonged to the gammadelta T cell lineage. Mature CD3epsilonhigh alphabeta thymocytes were observed 15 days later (DG55), and their occurrence was preceded by the appearance of CD3epsilonlow thymocytes (DG45). Thereafter, we observed transient changes in thymocyte subset composition (DG56-DG74), which can be explained by a gap in pro-T cell delivery to the thymus. This delivery gap corresponds with the expression of the pan-leukocyte CD45 and pan-myelomonocytic SWC3a markers in fetal liver and bone marrow and is probably caused by shifting of primary lymphopoiesis between these organs. Therefore, we conclude that the embryonic thymus is colonized by at least two successive waves of hemopoietic progenitors during embryogenesis and that the influx of thymocyte progenitors is discontinuous. Surface immunophenotyping and cell cycle analysis of thymocyte subsets allowed us to compare thymocyte differentiation in pigs with that described for rodents and humans and to propose a model for T cell lymphopoiesis in swine. We also observed that the porcine IL-2Ralpha (CD25), a typical differentiation marker of pre-T cells in mice and humans, was not expressed on thymocyte precursors in pigs and could only be found on mature thymocytes. Finally, we observed a subset of TCRgammadelta+ thymocytes that were cycling late during their development in the thymus.

In vivo selection of sFv from phage display libraries

The development of phage display technology has facilitated the development of many new and sometimes novel antibody based reagents for scientific research. However, present methods for selection from phage-sFv display libraries are limited to selection against purified antigens or ex vivo cells of known origin and phenotype. Existing methods therefore preclude the isolation of sFv against unknown molecules in their natural environment, where expression is complex and subject to diverse control mechanisms. Since such a complex environment is difficult to mimic in vitro, the development of an in vivo selection procedure would greatly enhance the selection from phage display antibody libraries and lead to the development of reagents against cell surface molecules in their natural environment. This would be particularly advantageous for isolation of sFv against vascular endothelium which can readily change phenotype when cultured and is believed to express molecules in a tissue specific manner and in response to different stimuli. We describe here the development of an in vivo selection procedure in the mouse and demonstrate its potential for the selection of sFv from a phage-sFv library. The target antigen for one sFv is expressed solely on the thymic endothelium, while the second, a 165-170 kDa molecule in present on both thymic endothelium and the perivascular epithelium.

Age- and disease-related decline in immune function: an opportunity for "thymus-boosting" therapies

The thymus is the site of production of mature T lymphocytes and thus is indispensable for the development and maintenance of the T cell-mediated arm of the immune system. Thymic production of mature T cells is critically dependent on an influx of bone marrow-derived progenitor T cells that undergo replication and selection within the thymus. Thymus cellularity and thymic hormone secretion reach a peak during the first year of life and then decline gradually until the age of 50-60 years, a process known as "thymic involution." A rapid reduction of thymus cellularity occurs in young patients following injuries, chemotherapy, and other forms of stress. The mechanisms underlying the involution process appear to be dependent on factors intrinsic to the thymic tissue, such as the local production of cytokines and chemoattractants, promoting the recruitment, growth, and differentiation of bone marrow-derived T cell progenitors in the thymus, as well as extrinsic factors, such as systemic levels of endocrine hormones and mediators released by intrathymic nerves of the autonomic nervous system. Knowledge of these factors provides a rational basis for the development of an approach based on tissue engineering that could be used to provide either temporary or permanent reconstitution of thymic function.

Epitopes for chicken monoclonal antibodies in spleens of selected Japanese quail lines

A line of Japanese quail selected for high plasma cholesterol is highly susceptible to diet-induced atherosclerosis. Lymphocyte epitopes recognized by mouse anti-chicken monoclonal antibodies (c-mAb), TCR-1, TCR-2, TCR-3. CD-3, CD-4, CD-8, and BU-1a/b were reacted with spleens from quail selected for high (HL) and low (LL) plasma total cholesterol and their nonselected controls (CL). Cross reactivity to c-mAb and effect of line and gender were immunohistochemically evaluated. Chicken spleens were positive controls. Quail were immunologically stimulated with either sheep red blood cells (SRBC) or Brucella abortus 2 weeks before spleens were removed. Quail spleen epitopes of all lines recognized TCR-3 and CD-8 c-mAb, but no other c-mAb. Number of reacting cells and staining intensity to the TCR-3 c-mAb were greater in the HL than in the LL regardless of the stimulating Ag or dose used. For the CD-8 c-mAb, there were no differences among lines in birds receiving SRBC. In B. abortus-immunized birds, sex x line interactions indicated that males of the HL and CL had lower responses than females but LL males were not different than females. TCR-3 and CD8 c-mAb may be useful in studying immunological mechanisms for atherosclerosis in Japanese quail.

Quantification of T-Cell Progenitors During Ontogeny: Thymus Colonization Depends on Blood Delivery of Progenitors

An in vivo thymus reconstitution assay based on intrathymic injection of hematopoietic progenitors into irradiated chicks was used to determine the number of T-cell progenitors in peripheral blood, paraaortic foci, bone marrow (BM), and spleen during ontogeny. This study allowed us to analyze the regulation of thymus colonization occurring in three waves during embryogenesis. It confirmed that progenitors of the first wave of thymus colonization originate from the paraaortic foci, whereas progenitors of the second and the third waves originate from the BM. The analysis of the number of T-cell progenitors indicates that each wave of thymus colonization is correlated with a peak number of T-cell progenitors in peripheral blood, whereas they are almost absent during the periods defined as refractory for colonization. Moreover, injection of T-cell progenitors into the blood circulation showed that they homed into the thymus without delay during the refractory periods. Thus, thymus colonization kinetics depend mainly on the blood delivery of T-cell progenitors during embryogenesis.

Quantification of T-Cell Progenitors During Ontogeny: Thymus Colonization Depends on Blood Delivery of Progenitors

An in vivo thymus reconstitution assay based on intrathymic injection of hematopoietic progenitors into irradiated chicks was used to determine the number of T-cell progenitors in peripheral blood, paraaortic foci, bone marrow (BM), and spleen during ontogeny. This study allowed us to analyze the regulation of thymus colonization occurring in three waves during embryogenesis. It confirmed that progenitors of the first wave of thymus colonization originate from the paraaortic foci, whereas progenitors of the second and the third waves originate from the BM. The analysis of the number of T-cell progenitors indicates that each wave of thymus colonization is correlated with a peak number of T-cell progenitors in peripheral blood, whereas they are almost absent during the periods defined as refractory for colonization. Moreover, injection of T-cell progenitors into the blood circulation showed that they homed into the thymus without delay during the refractory periods. Thus, thymus colonization kinetics depend mainly on the blood delivery of T-cell progenitors during embryogenesis.

T cell receptor gene deletion circles identify recent thymic emigrants in the peripheral T cell pool

Progenitor cells undergo T cell receptor (TCR) gene rearrangements during their intrathymic differentiation to become T cells. Rearrangements of the variable (V), diversity (D), and joining (J) segments of the TCR genes result in deletion of the intervening chromosomal DNA and the formation of circular episomes as a byproduct. Detection of these extrachromosomal excision circles in T cells located in the peripheral lymphoid tissues has been viewed as evidence for the existence of extrathymic T cell generation. Because all of the T cells in chickens apparently are generated in the thymus, we have employed this avian model to determine the fate of the V(D)J deletion circles. In normal animals we identified TCR Vgamma-Jgamma and Vbeta-Dbeta deletion circles in the blood, spleen, and intestines, as well as in the thymus. Thymectomy resulted in the gradual loss of these DNA deletion circles in all of the peripheral lymphoid tissues. A quantitative PCR analysis of Vgamma1-Jgamma1 and Vbeta1-Dbeta deletion circles in splenic gamma delta and Vbeta1(+) alphabeta T cells indicated that their numbers progressively decline after thymectomy with a half-life of approximately 2 weeks. Although TCR deletion circles therefore cannot be regarded as reliable indicators of in situ V(D)J rearrangement, measuring their levels in peripheral T cell samples can provide a valuable index of newly generated T cells entering the T cell pool.

Introduction to poultry vaccines and immunity

The poultry industry constitutes a significant sector of world agriculture. In the United States, more than 8 billion birds are produced yearly with a value exceeding $20 billion. Broiler chickens are the largest segment of the industry. Birds raised under commercial conditions are vulnerable to environmental exposure to a number of pathogens. Therefore, disease prevention by vaccination is an integral part of flock health management protocols. Active immunization using live vaccines is the current industry standard. Routinely used vaccines in chickens include MDV, NDV, IBV, and IBDV, and in turkeys NDV and HEV. Newer vaccines, including molecular recombinants in which genes of immunogenic proteins from infectious agents are inserted into a live viral vector, are also being examined for commercial use. Efforts are under way to enhance vaccine efficacy by the use of adjuvants, particularly cytokines. The vaccine delivery systems include in ovo injection, aerosol, spray, drinking water, eye drop, and wing web injection. The in ovo vaccination procedure is relatively new and at the present time it is used primarily to vaccinate broiler chickens against MDV. Birds respond to vaccines by developing humoral and cellular immune responses. Bursa of Fabricius and the thymus serve as the primary lymphoid organs of the immune system. B cells use surface immunoglobulins as antigen receptors and differentiate into plasma cells to secrete antibodies. Three classes of antibodies are produced: IgM, IgG (also called IgY), and IgA. Successful vaccinal response in a flock is often monitored by demonstrating a rise in antibody titer within a few days of vaccination. ELISA is used most commonly for serologic monitoring. T cells are the principal effector cells of specific cellular immunity. T cells differentiate into alpha beta and gamma delta cells. In adult birds, gamma delta cells may constitute up to 50% of the circulating T cells. Functionally, CD4+ cells serve as helper cells and CD8+ cells as cytotoxic/suppressor cells.

T-cell and natural killer cell development in thymectomized Xenopus

The Xenopus early-thymectomy model system is used to investigate the extent to which the thymus controls T-cell development and to probe the evolution of natural killer (NK) cells. Loss of T-cell function following thymectomy, together with the paucity of cells expressing monoclonal antibody-defined T-cell surface markers, and greatly reduced expression of T-cell receptor beta transcripts in spleen, liver and intestine, indicate that T-cell development in minimal in the absence of the thymus. Our findings therefore mitigate against the idea that a substantial extrathymic pathway of T-cell development exists in early vertebrate evolution. Rather, they suggest that in this amphibian representative T cells are predominately thymus dependent. In vitro studies with control and thymectomized Xenopus splenocytes reveal that a non-T/non-B population and also two T-cell subsets all display natural cytotoxicity towards allogeneic thymus lymphoid tumour cells (which are deficient in MHC antigen expression). Since Xenopus thymectomized early in larval development are permanently deficient in T cells, they may provide a useful phylogenetic model for the study of NK cells.

Renewal of thymocyte progenitors and emigration of thymocytes during avian development

The avian thymus is colonized by three waves of hemopoietic progenitors during embryogenesis. An in vivo thymus reconstitution assay based on intrathymic injection of irradiated chicks showed that cells of para-aortic foci were able to differentiate into T lymphocytes, confirming their putative role in the first wave of thymus colonization. This assay was also used to detect and to characterize T cell progenitors from the bone marrow which are involved in the second and third wave of thymus colonization. In the bone marrow, progenitors that differentiated into T cells were found in a subpopulation that expressed the molecules HEMCAM, c-kit and c128. Engraftment of thymus lobes into thymectomized young chick recipients showed that T cell progenitors are replaced in the thymus by subsequent waves of progenitors after hatching. Finally, analysis of thymocyte differentiation suggested that gamma delta and alpha beta T cells migrate from the thymus to the periphery in alternating waves.

Thymic function can be accurately monitored by the level of recent T cell emigrants in the circulation

Expression of the avian chT1 thymocyte antigen persists on a subpopulation of peripheral T cells enriched in the DNA deletion circles created by alphabeta and gammadelta TCR gene rearrangements. The chT1+ cells are evenly distributed among all of the peripheral T lymphocyte compartments. The levels of chT1+ T cells in the periphery gradually decline in parallel with age-related thymic involution, and these cells disappear following early thymectomy. Experiments in which variable numbers of the 14 thymic lobes are removed in young chicks indicate a direct correlation between the levels of circulating chT1+ cells and residual thymic mass. Measurement of recent thymic emigrants in the periphery thus provides an accurate indication of thymic function.

Sequential development of intraepithelial gamma delta and alpha beta T lymphocytes expressing CD8 alpha beta in neonatal rat intestine: requirement for the thymus

Previous studies in congenitally athymic nude rats have suggested that the thymus is important for the development of intestinal T cells. Here we have examined the effect of the nude mutation on intraepithelial lymphocyte (IEL) development from the perinatal period. By immunohistochemistry it was shown that CD3(-)CD8 alpha alpha + putative IEL precursors colonized the epithelium of both normal and athymic neonatal rats. Mature T cells, however, did not develop in athymic neonates. In normal rats, gamma delta T cells were present at birth and alpha beta T cells appeared within 8 days of postnatal life. At this age, the composition and relative number of intraepithelial T cells were similar to that in normal adult rats, with the exception that most neonatal T-cell receptor-gamma delta + and -alpha beta + IEL expressed CD8 beta. By contrast, extrathymic T-cell maturation in the gut of congenitally athymic rats occurred slowly, as CD3+ IEL did not appear until 4-6 months of age. These intraepithelial T cells displayed variable phenotypes and appeared to be induced by environmental antigens as they were not found in isolator-kept old nudes. In conclusion, the present results indicate that the major colonization of the gut epithelium with gamma delta and alpha beta T cells expressing CD8 alpha beta takes place perinatally and requires the presence of the thymus. The developmental relationship between these neonatal T cells and more immature CD3- CD8 alpha alpha +/- IEL remains elusive.