Early human T cell development: analysis of the human thymus at the time of initial entry of hematopoietic stem cells into the fetal thymic microenvironment

Conserved and divergent aspects of human T-cell development and migration in humanized mice

Humanized mice represent an important model to study the development and function of the human immune system. While it is known that mouse thymic stromal cells can support human T-cell development, the extent of interspecies cross-talk and the degree to which these systems recapitulate normal human T-cell development remain unclear. To address these questions, we compared conventional and non-conventional T-cell development in a neonatal chimera humanized mouse model with that seen in human fetal and neonatal thymus samples, and also examined the impact of a human HLA-A2 transgene expressed by the mouse stroma. Given that dynamic migration and cell–cell interactions are essential for T-cell differentiation, we also studied the intrathymic migration pattern of human thymocytes developing in a murine thymic environment. We found that both conventional T-cell development and intra-thymic migration patterns in humanized mice closely resemble human thymopoiesis. Additionally, we show that developing human thymocytes engage in short, serial interactions with other human hematopoietic-derived cells. However, non-conventional T-cell differentiation in humanized mice differed from both fetal and neonatal human thymopoiesis, including a marked deficiency of Foxp3⁺ T-cell development. These data suggest that although the murine thymic microenvironment can support a number of aspects of human T-cell development, important differences remain, and additional human-specific factors may be required.

Immunity to HIV in Early Life

The developing immune system is adapted to the exposure to a plethora of pathogenic and non-pathogenic antigens encountered in utero and after birth, requiring a fine balance between protective immunity and immune tolerance. In early stages of life, this tolerogenic state of the innate and adaptive immune system and the lack of immunological memory render the host more susceptible to infectious pathogens like HIV. HIV pathogenesis is different in children, compared to adults, with more rapid disease progression and a substantial lack of control of viremia compared to adults. Plasma viral load remains high during infancy and only declines gradually over several years in line with immune maturation, even in rare cases where children maintain normal CD4 T-lymphocyte counts for several years without antiretroviral therapy (ART). These pediatric slow progressors also typically show low levels of immune activation despite persistently high viremia, resembling the phenotype of natural hosts of SIV infection. The lack of immunological memory places the fetus and the newborn at higher risk of infections; however, it may also provide an opportunity for unique interventions. Frequencies of central memory CD4+ T-lymphocytes, one of the main cellular reservoirs of HIV, are very low in the newborn child, so immediate ART could prevent the establishment of persistent viral reservoirs and result in "functional cure." However, as recently demonstrated in the case report of the "Mississippi child" who experienced viral rebound after more than 2 years off ART, additional immunomodulatory strategies might be required for sustained viral suppression after ART cessation. In this review, we discuss the interactions between HIV and the developing immune system in children and the potential implications for therapeutic and prophylactic interventions.

Tracking migration during human T cell development

Specialized microenvironments within the thymus are comprised of unique cell types with distinct roles in directing the development of a diverse, functional, and self-tolerant T cell repertoire. As they differentiate, thymocytes transit through a number of developmental intermediates that are associated with unique localization and migration patterns. For example, during one particular developmental transition, immature thymocytes more than double in speed as they become mature T cells that are among the fastest cells in the body. This transition is associated with dramatic changes in the expression of chemokine receptors and their antagonists, cell adhesion molecules, and cytoskeletal components to direct the maturing thymocyte population from the cortex to medulla. Here we discuss the dynamic changes in behavior that occur throughout thymocyte development, and provide an overview of the cell-intrinsic and extrinsic mechanisms that regulate human thymocyte migration.

Broad T-cell receptor repertoire in T-lymphocytes derived from human induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSCs) have enormous potential for the treatment of inherited and acquired disorders. Recently, antigen-specific T lymphocytes derived from hiPSCs have been reported. However, T lymphocyte populations with broad T cell receptor (TCR) diversity have not been generated. We report that hiPSCs derived from skin biopsy are capable of producing T lymphocyte populations with a broad TCR repertoire. In vitro T cell differentiation follows a similar developmental program as observed in vivo, indicated by sequential expression of CD7, intracellular CD3 and surface CD3. The γδ TCR locus is rearranged first and is followed by rearrangement of the αβ locus. Both γδ and αβ T cells display a diverse TCR repertoire. Upon activation, the cells express CD25, CD69, cytokines (TNF-α, IFN-γ, IL-2) and cytolytic proteins (Perforin and Granzyme-B). These results suggest that most, if not all, mechanisms required to generate functional T cells with a broad TCR repertoire are intact in our in vitro differentiation protocol. These data provide a foundation for production of patient-specific T cells for the treatment of acquired or inherited immune disorders and for cancer immunotherapy.

Immunology of pediatric HIV infection

Most infants born to human immunodeficiency virus (HIV)-infected women escape HIV infection. Infants evade infection despite an immature immune system and, in the case of breastfeeding, prolonged repetitive exposure. If infants become infected, the course of their infection and response to treatment differs dramatically depending upon the timing (in utero, intrapartum, or during breastfeeding) and potentially the route of their infection. Perinatally acquired HIV infection occurs during a critical window of immune development. HIV's perturbation of this dynamic process may account for the striking age-dependent differences in HIV disease progression. HIV infection also profoundly disrupts the maternal immune system upon which infants rely for protection and immune instruction. Therefore, it is not surprising that infants who escape HIV infection still suffer adverse effects. In this review, we highlight the unique aspects of pediatric HIV transmission and pathogenesis with a focus on mechanisms by which HIV infection during immune ontogeny may allow discovery of key elements for protection and control from HIV.

Immunological features of 22q11 deletion syndrome

PURPOSE OF REVIEW

22q11 deletion syndrome is the most common genetic abnormality. More patients are surviving cardiac surgery, and many do not have cardiac anomalies. Adult patients are now being described. It is important for paediatricians, and increasingly adult physicians, to be aware of the optimum management of these patients.

RECENT FINDINGS

Three main immunological patterns are recognized, namely, athymic and incomplete 22q11 deletion syndrome and autoimmunity. Newborn screening for severe combined immunodeficiency detects athymic patients, although diagnosis may be complicated, and instructive cases are described. Incomplete 22q11 deletion syndrome is the most common presentation; new findings predict which patients are likely to experience significant infection. B lymphocyte deficiencies are often overlooked. Data regarding autoimmunity in adult patients is reported, as well as newly reported immunological findings. Finally, management guidelines are now published, and these are highlighted.

SUMMARY

Newborn screening detects patients with athymic 22q11 deletion syndrome, but significant illness may complicate the picture, and dual diagnoses can confound treatment. Treatment options for these patients are becoming clearer. Hypoparathyroidism is associated with more severe infection, and immunoglobulin abnormalities are more common than previously recognized. Adult patients are symptomatic and management guidelines will help general physicians in managing these patients.

CD6 as a therapeutic target in autoimmune diseases: successes and challenges

The transmembrane surface glycoprotein CD6 was one of the first antigens identified on T lymphocytes. The recognition of its involvement in T-cell signaling processes heralds the potential of CD6 as a target for therapy in a number of pathologies associated with imbalances in T-cell function. Its tissue distribution, cellular expression, and overall molecular structure are well described, and the interaction with its physiological ligand CD166 has been determined to the amino-acid level. Nevertheless, the involvement of CD6 in signaling pathways remains poorly characterized and its biological function is controversial; still unresolved are whether CD6 is a co-stimulatory molecule in T-cell activation or, similar to the related CD5 antigen, a modulator of intracellular signaling. Here we revisit the earliest attempts of modulating immune function using CD6 monoclonal antibodies, and review the current thinking behind the recent developments in immunotherapy targeting CD6. Notwithstanding the promises and hopes brought by monoclonals already in clinical trials, the fact is that very little is known about the mechanism of action of these reagents, whether they enhance the physiological role of the receptor or whether they may induce a completely novel biochemical response that might, nevertheless, be beneficially used to treat human immune pathology.

Immunodeficiency in DiGeorge Syndrome and Options for Treating Cases with Complete Athymia

The commonest association of thymic stromal deficiency resulting in T-cell immunodeficiency is the DiGeorge syndrome (DGS). This results from abnormal development of the third and fourth pharyngeal arches and is most commonly associated with a microdeletion at chromosome 22q11 though other genetic and non-genetic causes have been described. The immunological competence of affected individuals is highly variable, ranging from normal to a severe combined immunodeficiency when there is complete athymia. In the most severe group, correction of the immunodeficiency can be achieved using thymus allografts which can support thymopoiesis even in the absence of donor-recipient matching at the major histocompatibility loci. This review focuses on the causes of DGS, the immunological features of the disorder, and the approaches to correction of the immunodeficiency including the use of thymus transplantation.

FOXN1: A Master Regulator Gene of Thymic Epithelial Development Program

T cell ontogeny is a sophisticated process, which takes place within the thymus through a series of well-defined discrete stages. The process requires a proper lympho-stromal interaction. In particular, cortical and medullary thymic epithelial cells (cTECs, mTECs) drive T cell differentiation, education, and selection processes, while the thymocyte-dependent signals allow thymic epithelial cells (TECs) to maturate and provide an appropriate thymic microenvironment. Alterations in genes implicated in thymus organogenesis, including Tbx1, Pax1, Pax3, Pax9, Hoxa3, Eya1, and Six1, affect this well-orchestrated process, leading to disruption of thymic architecture. Of note, in both human and mice, the primordial TECs are yet unable to fully support T cell development and only after the transcriptional activation of the Forkhead-box n1 (FOXN1) gene in the thymic epithelium this essential function is acquired. FOXN1 is a master regulator in the TEC lineage specification in that it down-stream promotes transcription of genes, which, in turn, regulate TECs differentiation. In particular, FOXN1 mainly regulates TEC patterning in the fetal stage and TEC homeostasis in the post-natal thymus. An inborn null mutation in FOXN1 leads to Nude/severe combined immunodeficiency (SCID) phenotype in mouse, rat, and humans. In Foxn1^−/− nude animals, initial formation of the primordial organ is arrested and the primordium is not colonized by hematopoietic precursors, causing a severe primary T cell immunodeficiency. In humans, the Nude/SCID phenotype is characterized by congenital alopecia of the scalp, eyebrows, and eyelashes, nail dystrophy, and a severe T cell immunodeficiency, inherited as an autosomal recessive disorder. Aim of this review is to summarize all the scientific information so far available to better characterize the pivotal role of the master regulator FOXN1 transcription factor in the TEC lineage specifications and functionality.

Cell-autonomous defects in thymic epithelial cells disrupt endothelial-perivascular cell interactions in the mouse thymus

The thymus is composed of multiple stromal elements comprising specialized stromal microenvironments responsible for the development of self-tolerant and self-restricted T cells. Here, we investigated the ontogeny and maturation of the thymic vasculature. We show that endothelial cells initially enter the thymus at E13.5, with PDGFR-β⁺ mesenchymal cells following at E14.5. Using an allelic series of the thymic epithelial cell (TEC) specific transcription factor Foxn1, we showed that these events are delayed by 1–2 days in Foxn1^Δ/Δ mice, and this phenotype was exacerbated with reduced Foxn1 dosage. At subsequent stages there were fewer capillaries, leaky blood vessels, disrupted endothelium - perivascular cell interactions, endothelial cell vacuolization, and an overall failure of vascular organization. The expression of both VEGF-A and PDGF-B, which are both primarily expressed in vasculature-associated mesenchyme or endothelium in the thymus, were reduced at E13.5 and E15.5 in Foxn1^Δ/Δ mice compared with controls. These data suggest that Foxn1 is required in TECs both to recruit endothelial cells and for endothelial cells to communicate with thymic mesenchyme, and for the differentiation of vascular-associated mesenchymal cells. These data show that Foxn1 function in TECs is required for normal thymus size and to generate the cellular and molecular environment needed for normal thymic vascularization. These data further demonstrate a novel TEC-mesenchyme-endothelial interaction required for proper fetal thymus organogenesis.

Activation of islet autoreactive naïve T cells in infants is influenced by homeostatic mechanisms and antigen-presenting capacity

Islet autoimmunity precedes type 1 diabetes onset. We previously found that islet autoimmunity rarely starts before 6 months of age but reaches its highest incidence already at ∼1 year of age. We now examine whether homeostatic expansion and immune competence changes seen in a maturating immune system may account for this marked variation in islet autoimmunity risk in the first year of life. We found naïve proinsulin- and GAD65-responsive T cells in cord blood (CB) of healthy newborns, with highest responses observed in children with type 1 diabetes-susceptible HLA-DRB1/DQB1 genotypes. Homeostatic expansion characteristics with increased IL-7 concentrations and enhanced T-cell responsiveness to IL-7 were observed throughout the first year of life. However, the ability of antigen-presenting cells to activate naïve T cells was compromised at birth, and CB monocytes had low surface expression of CD40 and HLA class II. In contrast, antigen presentation and expression of these molecules had reached competent adult levels by the high incidence age of 8 months. We propose that temporal changes in islet autoimmunity seroconversion in infants are a consequence of the changing balance between homeostatic drive and antigen presentation competence. These findings are relevant for early prevention of type 1 diabetes.

Dynamics of thymus organogenesis and colonization in early human development

The thymus is the central site of T-cell development and thus is of fundamental importance to the immune system, but little information exists regarding molecular regulation of thymus development in humans. Here we demonstrate, via spatial and temporal expression analyses, that the genetic mechanisms known to regulate mouse thymus organogenesis are conserved in humans. In addition, we provide molecular evidence that the human thymic epithelium derives solely from the third pharyngeal pouch, as in the mouse, in contrast to previous suggestions. Finally, we define the timing of onset of hematopoietic cell colonization and epithelial cell differentiation in the human thymic primordium, showing, unexpectedly, that the first colonizing hematopoietic cells are CD45(+)CD34(int/-). Collectively, our data provide essential information for translation of principles established in the mouse to the human, and are of particular relevance to development of improved strategies for enhancing immune reconstitution in patients.

Immunogenicity of in vitro maintained and matured populations: potential barriers to engraftment of human pluripotent stem cell derivatives

The potential to develop into any cell type makes human pluripotent stem cells (hPSCs) one of the most promising sources for regenerative treatments. Hurdles to their clinical applications include (1) formation of heterogeneously differentiated cultures, (2) the risk of teratoma formation from residual undifferentiated cells, and (3) immune rejection of engrafted cells. The recent production of human isogenic (genetically identical) induced PSCs (hiPSCs) has been proposed as a "solution" to the histocompatibility barrier. In theory, differentiated cells derived from patient-specific hiPSC lines should be histocompatible to their donor/recipient. However, propagation, maintenance, and non-physiologic differentiation of hPSCs in vitro may produce other, likely less powerful, immune responses. In light of recent progress towards the clinical application of hPSCs, this review focuses on two antigen presentation phenomena that may lead to rejection of isogenic hPSC derivates: namely, the expression of aberrant antigens as a result of long-term in vitro maintenance conditions or incomplete somatic cell reprogramming, and the unbalanced presentation of receptors and ligands involved in immune recognition due to accelerated differentiation. Finally, we discuss immunosuppressive approaches that could potentially address these immunological concerns.

The impact of differential antiviral immunity in children and adults

The course of immune maturation has evolved to favour survival at each stage of development in early life. Fetal and neonatal immune adaptations facilitate intrauterine survival and provide early postnatal protection against extracellular pathogens, but they leave infants susceptible to intracellular pathogens such as viruses that are acquired perinatally. This Review focuses on three such pathogens--HIV, hepatitis B virus and cytomegalovirus--and relates the differential impact of these infections in infants and adults to the antiviral immunity that is generated at different ages. A better understanding of age-specific antiviral immunity may inform the development of integrated prevention, treatment and vaccine strategies to minimize the global disease burden resulting from these infections.

Pathogenesis of hepatitis C during pregnancy and childhood

The worldwide prevalence of HCV infection is between 1% and 8% in pregnant women and between 0.05% and 5% in children. Yet the pathogenesis of hepatitis C during pregnancy and in the neonatal period remains poorly understood. Mother-to-child transmission (MTCT), a leading cause of pediatric HCV infection, takes place at a rate of <10%. Factors that increase the risk of MTCT include high maternal HCV viral load and coinfection with HIV-1 but, intriguingly, not breastfeeding and mode of delivery. Pharmacological prevention of MTCT is not possible at the present time because both pegylated interferon alfa and ribavirin are contraindicated for use in pregnancy and during the neonatal period. However, this may change with the recent introduction of direct acting antiviral agents. This review summarizes what is currently known about HCV infection during pregnancy and childhood. Particular emphasis is placed on how pregnancy-associated immune modulation may influence the progression of HCV disease and impact MTCT, and on the differential evolution of perinatally acquired HCV infection in children. Taken together, these developments provide insights into the pathogenesis of hepatitis C and may inform strategies to prevent the transmission of HCV from mother to child.

Placental expression of CD100, CD72 and CD45 is dysregulated in human miscarriage

Context and Objective

The etiology of miscarriage is often multifactorial. One major cause, immunological rejection of the fetus, has not been clearly elucidated. Our aim was to establish whether the semaphorin CD100, its natural receptor CD72, and the glycoprotein CD45, implicated in immune mechanisms, are involved in pregnancy loss by examining their placental expression with real-time PCR, immunohistochemistry and western blotting techniques.

Patients

Placenta tissue from 72 Caucasian women undergoing surgical uterine evacuation due to early spontaneous pregnancy loss between the 8^th and 12^th week of gestation was divided into four groups based on miscarriage number. Gestational age-matched placentas from 18 healthy women without a history of miscarriage undergoing voluntary pregnancy termination were the control group. Placenta from 6 Caesarean deliveries performed at 38–40 weeks of gestation was also studied.

Results

CD100, CD72 and CD45 were expressed in placenta and exhibited different mRNA and protein levels in normal pregnancy and miscarriage. In particular, protein levels were highly dysregulated around 10 weeks of gestation in first and second miscarriage placentas. The CD100 soluble form was produced and immediately shed from placental tissue in all samples.

Conclusions

Fetal CD100, CD72 and CD45 seem to play a role in miscarriage. The present data support the involvement of the fetal immune system in pregnancy maintenance as well as failure.

From murine to human nude/SCID: the thymus, T-cell development and the missing link

Primary immunodeficiencies (PIDs) are disorders of the immune system, which lead to increased susceptibility to infections. T-cell defects, which may affect T-cell development/function, are approximately 11% of reported PIDs. The pathogenic mechanisms are related to molecular alterations not only of genes selectively expressed in hematopoietic cells but also of the stromal component of the thymus that represents the primary lymphoid organ for T-cell differentiation. With this regard, the prototype of athymic disorders due to abnormal stroma is the Nude/SCID syndrome, first described in mice in 1966. In man, the DiGeorge Syndrome (DGS) has long been considered the human prototype of a severe T-cell differentiation defect. More recently, the human equivalent of the murine Nude/SCID has been described, contributing to unravel important issues of the T-cell ontogeny in humans. Both mice and human diseases are due to alterations of the FOXN1, a developmentally regulated transcription factor selectively expressed in skin and thymic epithelia.

Cellular therapies supplement: the peritoneum as an ectopic site of hematopoiesis following in utero transplantation

BACKGROUND

In utero transplantation (IUT) has the potential to treat birth defects early before full development of the immune system. Relatively small grafts, which are not matched for major histocompatibility antigens, can be delivered even before onset of disease symptoms. IUT of hematopoietic stem cells is usually performed via intraperitoneal injection, yet the fate of donor cells in the peritoneal cavity is not fully understood. We review our recent work and present new data demonstrating that the peritoneum can be a site of ectopic hematopoiesis with implications for IUT and immune tolerance induction.

STUDY DESIGN AND METHODS

Haplogeneic and allogeneic fetal transplants were performed in mice and engraftment tracked by flow cytometry. Immune tolerance was studied by mixed lymphocyte reactions and skin transplantation. Adult syngeneic murine transplants and xenogeneic human into immunodeficient mouse transplants were performed to follow hematopoietic retention in the peritoneum and engraftment of the marrow.

RESULTS

Although most transplanted cells rapidly clear the peritoneum, hematopoietic cells and cells with the phenotype of hematopoietic precursors can remain in the peritoneal cavity for months after transplant. The presence of donor cells in the peritoneum can contribute to donor-specific tolerance, but sufficient peripheral blood chimerism is required to ensure acceptance of donor skin grafts.

CONCLUSION

Ectopic hematopoiesis and the survival of stem cells in the peritoneum offer the possibility of better using the peritoneal cavity to delivery stem cells and foster the development of immune tolerance to alloantigens or other foreign antigens.

Immunological aspects of 22q11.2 deletion syndrome

Chromosome 22q11 deletion is the most common chromosomal deletion syndrome and is found in the majority of patients with DiGeorge syndrome and velo-cardio-facial syndrome. Patients with CHARGE syndrome may share similar features. Cardiac malformations, speech delay, and immunodeficiency are the most common manifestations. The immunological phenotype may vary widely between patients. Severe T lymphocyte immunodeficiency is rare-thymic transplantation offers a new approach to treatment, as well as insights into thymic physiology and central tolerance. Combined partial immunodeficiency is more common, leading to recurrent sinopulmonary infection in early childhood. Autoimmunity is an increasingly recognized complication. New insights into pathophysiology are reviewed.

How to find your way through the thymus: a practical guide for aspiring T cells

Thymocytes must complete an elaborate developmental program in the thymus to ultimately generate T cells that express functional but neither harmful nor useless TCRs. Each developmental step coincides with dynamic relocation of the thymocytes between anatomically discrete thymic microenvironments, suggesting that thymocytes' migration is tightly regulated by their developmental status. Chemokines produced by thymic stromal cells and chemokine receptors on the thymocytes play an indispensable role in guiding developing thymocytes into the different microenvironments. In addition to long-range migration, chemokines increase the thymocytes' motility, enhancing their interaction with stromal cells. During the past several years, much progress has been made to determine the various signals that guide thymocytes on their journey within the thymus. In this review, we summarize the progress in identifying chemokines and other chemoattractant signals that direct intrathymic migration. Furthermore, we discuss the recent advances of two-photon microscopy in determining dynamic motility and interaction behavior of thymocytes within distinct compartments to provide a better understanding of the relationship between thymocyte motility and development.

Gimme shelter: the immune system during pregnancy

Antigen-presenting molecules vary between individuals of the same species, making it more difficult for pathogens to evade immune recognition and spread through the whole population. As a result of this genetic diversity, transplants between individuals are recognized as foreign and are rejected. This alloreactivity turns placental viviparity into a major immunological challenge. The maternal immune system has to balance the opposing needs of maintaining robust immune reactivity to protect both mother and fetus from invading pathogens, while at the same time tolerating highly immunogenic paternal alloantigens in order to sustain fetal integrity. Regulatory T cells are responsible for the establishment of tolerance by modulating the immune response, and uterine natural killer cells direct placentation by controlling trophoblast invasion. A variety of other cell types, including decidual stromal cells, dendritic cells, and immunomodulatory multipotent mesenchymal stromal cells, are found at the fetal-maternal interface. These cells conspire to establish a suitable environment for fetal development without compromising systemic immunity. Defects in any of these components can lead to gestational failure despite successful fertilization.

Relationship between diarrhea and peripheral leukocyte population in neonatal Japanese black calves

Neonatal Japanese Black (JB) calves show a high incidence of diarrhea. The objective of this study was to analyze the immune cell populations of neonatal JB calves in detail and examine its correlation with the incidence of diarrhea immediately after birth. Understanding the immune cell populations is helpful in clinics in order to determine the condition of the immune system for prevention of diseases. Blood samples were obtained from JB calves on the day of birth. The peripheral leukocyte populations were analyzed separately for calves that had diarrhea within 2 weeks after birth (diarrhea group; n = 26) and for calves without diarrhea (control group; n = 74). The numbers of the peripheral blood CD3(+)TcR1-N12(+) and CD8(+) T cells were significantly lower in the diarrhea group compared with the control group. These findings suggest that the congenital lower peripheral γδ and CD8(+) T cells results in a high risk of diarrhea in neonatal JB calves.

Potential barriers to therapeutics utilizing pluripotent cell derivatives: intrinsic immunogenicity of in vitro maintained and matured populations

The potential to develop into any tissue makes pluripotent stem cells (PSCs) one of the most promising sources for cellular therapeutics. However, numerous hurdles exist to their clinical applications, three of the most concerning include the inability to separate therapeutic population from heterogeneously differentiated cultures, the risk of teratoma formation from residual pluripotent cells, and immunologic rejection of engrafted cells. The recent development of induced PSCs has been proposed as a solution to the histocompatibility barrier. Theoretically, creation of patient-specific induced PSC lines would exhibit a complete histocompatibility antigen match. However, regardless of the PSC source, in vitro propagation and nonphysiologic differentiation may result in other, likely less powerful, mechanisms of immune rejection. In light of recent progress towards clinical application, this review focuses on two such potential immunologic mechanisms applicable to isogenic PSC derivates: namely, the immunogenicity of aberrant antigens resulting from long-term in vitro maintenance and alterations in immunologic properties due to rapid in vitro differentiation. These issues will be considered with attention to their relation to effector cells in the adult immune system. In addition, we highlight immunosuppressive approaches that could potentially address the immunogenicity of these proposed mechanisms.

Polyclonal B cell differentiation and loss of gastrointestinal tract germinal centers in the earliest stages of HIV-1 infection

BACKGROUND

The antibody response to HIV-1 does not appear in the plasma until approximately 2-5 weeks after transmission, and neutralizing antibodies to autologous HIV-1 generally do not become detectable until 12 weeks or more after transmission. Moreover, levels of HIV-1-specific antibodies decline on antiretroviral treatment. The mechanisms of this delay in the appearance of anti-HIV-1 antibodies and of their subsequent rapid decline are not known. While the effect of HIV-1 on depletion of gut CD4(+) T cells in acute HIV-1 infection is well described, we studied blood and tissue B cells soon after infection to determine the effect of early HIV-1 on these cells.

METHODS AND FINDINGS

In human participants, we analyzed B cells in blood as early as 17 days after HIV-1 infection, and in terminal ileum inductive and effector microenvironments beginning at 47 days after infection. We found that HIV-1 infection rapidly induced polyclonal activation and terminal differentiation of B cells in blood and in gut-associated lymphoid tissue (GALT) B cells. The specificities of antibodies produced by GALT memory B cells in acute HIV-1 infection (AHI) included not only HIV-1-specific antibodies, but also influenza-specific and autoreactive antibodies, indicating very early onset of HIV-1-induced polyclonal B cell activation. Follicular damage or germinal center loss in terminal ileum Peyer's patches was seen with 88% of follicles exhibiting B or T cell apoptosis and follicular lysis.

CONCLUSIONS

Early induction of polyclonal B cell differentiation, coupled with follicular damage and germinal center loss soon after HIV-1 infection, may explain both the high rate of decline in HIV-1-induced antibody responses and the delay in plasma antibody responses to HIV-1. Please see later in the article for Editors' Summary.

Characterization in vitro and engraftment potential in vivo of human progenitor T cells generated from hematopoietic stem cells

T-cell development follows a defined set of stage-specific differentiation steps. However, molecular and cellular events occurring at early stages of human T-cell development remain to be fully elucidated. To address this, human umbilical cord blood (UCB) hematopoietic stem cells (HSCs) were induced to differentiate to the T lineage in OP9-DL1 cocultures. A developmental program involving a sequential and temporally discrete expression of key differentiation markers was revealed. Quantitative clonal analyses demonstrated that CD34(+)CD38(-) and CD34(+)CD38(lo) subsets of UCB contain a similarly high T-lineage progenitor frequency, whereas the frequency in CD34(+)CD38(+/hi) cells was 5-fold lower. Delta-like/Notch-induced signals increased the T-cell progenitor frequency of CD34(+)CD38(-/lo) cells differentiated on OP9-DL1, and 2 distinct progenitor subsets, CD34(+)CD45RA(+)CD7(++)CD5(-)CD1a(-) (proT1) and CD34(+)CD45RA(+)CD7(++)CD5(+)CD1a(-) (proT2), were identified and their thymus engrafting capacity was examined, with proT2 cells showing a 3-fold enhanced reconstituting capacity compared with the proT1 subset. Furthermore, in vitro-generated CD34(+)CD7(++) progenitors effectively engrafted the thymus of immunodeficient mice, which was enhanced by the addition of an IL-7/IL-7 antibody complex. Taken together, the identification of T-progenitor subsets readily generated in vitro may offer important avenues to improve cellular-based immune-reconstitution approaches.

Sonographic measurements of the thymus in male and female fetuses

OBJECTIVE

The purpose of this study was to determine whether the size of the thymus is different in male and female fetuses.

METHODS

In this prospective study, the transverse diameter and perimeter of the thymus were measured in healthy fetuses between 24 and 37 weeks' gestation. The means of the study variables from male and female fetuses were compared by the Student t test, and the relationships between the transverse diameter and perimeter of the thymus and gestational age and other common fetal biometric parameters were determined by linear regression modeling.

RESULTS

No differences were noted between male and female fetuses for the means of the study variables. After the relationship between the transverse diameter and perimeter of the thymus and gestational age was confirmed (R(2) = 0.8 and 0.75, respectively; both P < .01), the 95% confidence interval-predicted changes were calculated, and the scatterplots of the measurements suggested that sex did not affect the size of the thymus. Bland-Altman plots were used to analyze intraobserver variability and showed good agreement for both of these thymic measurements for male and female fetuses.

CONCLUSIONS

These results suggest that fetal sex does not affect the size of the thymus and, together with previous reports, support the hypothesis that a quantitative reduction in fetal thymus size could serve as an indirect marker of abnormal thymopoiesis and congenital thymic insufficiency.

Maternal alloantigens promote the development of tolerogenic fetal regulatory T cells in utero

As the immune system develops, T cells are selected or regulated to become tolerant of self antigens and reactive against foreign antigens. In mice, the induction of such tolerance is thought to be attributable to the deletion of self-reactive cells. Here, we show that the human fetal immune system takes advantage of an additional mechanism: the generation of regulatory T cells (Tregs) that suppress fetal immune responses. We find that substantial numbers of maternal cells cross the placenta to reside in fetal lymph nodes, inducing the development of CD4+CD25highFoxP3+ Tregs that suppress fetal antimaternal immunity and persist at least until early adulthood. These findings reveal a form of antigen-specific tolerance in humans, induced in utero and probably active in regulating immune responses after birth.

Longitudinal analysis of immune function in the first 3 years of life in thymectomized neonates during cardiac surgery

The purpose of this study is to evaluate the effects of neonatal thymectomy in the functional capacity of the immune system. We selected a group of 23 subjects, who had undergone thymectomy in their first 30 days of life, during an intervention for congenital heart disease. Several parameters of the immune system were evaluated during their first 3 years of life. Lymphocyte populations and subpopulations (including naive, memory and effector subpopulations), T cell receptor (TCR) Vbeta repertoire, response of T cells following in vitro stimulation by mitogen, quantification of immunoglobulins, TCR excision circles (TRECS) and interleukin (IL)-7 were measured. We found that neonatal thymectomy produces long-term diminution in total lymphocyte counts, especially in naive CD4+ and CD8+ T cells. Additionally, TRECS were decreased, and plasma IL-7 levels increased. A statistically significant negative correlation was found between absolute CD4+ T cells and IL-7 (r = -0.470, P = 0.02). The patients did not suffer more infectious events than healthy control children, but thymectomy in neonates resulted in a significant decrease in T lymphocyte levels and TRECS, consistent with cessation of thymopoiesis. This could produce a compromise in immune function later in life, especially if the patients suffer T cell depletion and need a reconstitution of immune function.

Coordination between CCR7- and CCR9-mediated chemokine signals in prevascular fetal thymus colonization

Thymus seeding by T-lymphoid progenitor cells is a prerequisite for T-cell development. However, molecules guiding thymus colonization and their roles before and after thymus vascularization are unclear. Here we show that mice doubly deficient for chemokine receptors CCR7 and CCR9 were defective specifically in fetal thymus colonization before, but not after, thymus vascularization. The defective prevascular fetal thymus colonization was followed by selective loss of the first wave of T-cell development generating epidermal Vγ3+ γδ T cells. Unexpectedly, CCL21, a CCR7 ligand, was expressed not by Foxn1-dependent thymic primordium but by Gcm2-dependent parathyroid primordium, whereas CCL25, a CCR9 ligand, was predominantly expressed by Foxn1-dependent thymic primordium, revealing the role of the adjacent parathyroid in guiding fetal thymus colonization. These results indicate coordination between Gcm2-dependent parathyroid and Foxn1-dependent thymic primordia in establishing CCL21/CCR7- and CCL25/CCR9-mediated chemokine guidance essential for prevascular fetal thymus colonization.

CD3 delta immunodeficiency

PURPOSE OF REVIEW

The review describes advances in understanding the role of the CD3 delta subunit in human T-cell development as deduced from a recently described human immunodeficiency. The review also compares CD3 delta deficiency with other human CD3 subunit deficiencies and with corresponding animal models.

RECENT FINDINGS

In describing CD3 delta deficiency in humans this review shows that patients with profound T-cell depletion, who present at 2-3 months with severe viral infection, lack CD3 delta as a result of a mutation in the extracellular domain of this gene. The genetic aberration was discovered by comparing patients' and normal thymocytes, using mass gene screening with the microarray technique. In humans the absence of CD3 delta results in a complete arrest in thymocyte development at the stage of double negative to double positive transition and the development of gamma delta T-cell receptor-positive T cells is also impaired.

SUMMARY

Unlike patients with CD3 gamma or CD3 epsilon deficiency who have a milder condition, patients with CD3 delta deficiency present with severe lethal susceptibility to infections during early infancy. As expected, this profound immunodeficiency was cured with an allogenic bone marrow transplantation. In contrast to murine CD3 (-/) delta, which retains a normal gamma delta T-cell receptor-positive T-cell population and only partly affects the developmental transition of double positive to single positive thymocytes, CD3 delta in humans appears to be more critically required for the development of both alpha beta and gamma delta T-cell receptor-positive T-cell lineages. The studies also show for the first time that comparing relevant patients' with normal tissue using microarray technology can aid in the discovery of the genetic basis of inherited disorders.

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.

Dynamics of Thymus-Colonizing Cells during Human Development

Here, we identify fetal bone marrow (BM)-derived CD34hiCD45RAhiCD7+ hematopoietic progenitors as thymus-colonizing cells. This population, virtually absent from the fetal liver (FL), emerges in the BM by development weeks 8-9, where it accumulates throughout the second trimester, to finally decline around birth. Based on phenotypic, molecular, and functional criteria, we demonstrate that CD34hiCD45RAhiCD7+ cells represent the direct precursors of the most immature CD34hiCD1a- fetal thymocytes that follow a similar dynamics pattern during fetal and early postnatal development. Histological analysis of fetal thymuses further reveals that early immigrants predominantly localize in the perivascular areas of the cortex, where they form a lymphostromal complex with thymic epithelial cells (TECs) driving their rapid specification toward the T lineage. Finally, using an ex vivo xenogeneic thymus-colonization assay, we show that BM-derived CD34hiCD45RAhiCD7+ progenitors are selectively recruited into the thymus parenchyma in the absence of exogenous cytokines, where they adopt a definitive T cell fate.

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.

Chemokine control of lymphocyte trafficking: a general overview

Chemokines are a large family of small, generally secreted polypeptides which guide lymphocyte movement throughout the body by controlling integrin avidity and inducing migration. Here, we look at recent, exciting findings on chemokine function throughout lymphocyte development and co-ordinated T and B cell migration during immune responses. Finally, we will review data on the regional control of immunity by tissue-specific chemokine receptors on effector/memory lymphocytes.

Studies of patients' thymi aid in the discovery and characterization of immunodeficiency in humans

Studying the molecular and genetic bases of primary immunodeficiency is valuable at several levels. First, such information directly benefits patients in both short- and long-term management. Sophisticated diagnostic tools based on these studies can be used early and lead to appropriate treatment before potentially fatal infections and complications arise. Genotyping is also critical for future development and implementation of gene therapy. Secondly, investigating primary immunodeficiency helps understand the normal immune system in humans. As described in this report, the roles of zeta-associated protein of 70 kDa (Zap-70), CD25, and CD3delta are substantially different in humans when compared with the roles of homologous molecules in other species. Last, information obtained from these studies can be applied to other fields of investigation. Prominent examples for such applications include the intensive effort to design and produce specific inhibitors of Zap-70 and Janus kinase 3 as specific immunosuppressive agents. Most types of primary immunodeficiency in general and severe combined immunodeficiency in particular are rare and therefore cannot be easily studied by using traditional genetic methodology. Instead, biochemical methods were used to explore for candidate genes as was the case in the discovery of Zap-70 deficiency. Critical to the success of these discoveries was the careful analysis of patients' thymus glands. Detection of abnormalities in the thymus in these patients, which preceded identification of the genetic defect, aided in the assessment of the severity and nature of the immune disorder (primary versus secondary). Such assessment is critical before high-risk bone marrow transplantation. Equally important was the contribution of studies of the thymus to the description of novel phenotype of immunodeficiency as clearly demonstrated in defining CD8 lymphocytopenia, Zap-70 deficiency, and CD25 deficiency. Indeed, analysis of the thymus directly pointed to CD25 as candidate gene. Recently, the study of thymocyte-derived transcripts using DNA microarrays was key to discovering CD3delta deficiency. Finally, immunohistochemical analysis of the thymus was critical in pinpointing the roles of Zap-70, CD25, and CD3delta in the development of human T cells.

Isoforms of terminal deoxynucleotidyltransferase: developmental aspects and function

The immune system develops in a series of programmed developmental stages. Although recombination-activating gene (RAG) and nonhomologous end-joining (NHEJ) proteins are indispensable in the generation of immunoglobulins and T-cell receptors (TCRs), most CDR3 diversity is contributed by nontemplated addition of nucleotides catalyzed by the nuclear enzyme terminal deoxynucleotidyltransferase (TdT) and most nucleotide deletion is performed by exonucleases at V(D)J joins. Increasing TdT expression continuing into adult life results in N region addition and diversification of the T and B cell repertoires. In several species including mice and humans, there are multiple isoforms of TdT resulting from alternative mRNA splicing. The short form (TdTS) produces N additions during TCR and B-cell receptor (BCR) gene rearrangements. Other long isoforms, TdTL1 and TdTL2, have 3' --> 5' exonuclease activity. The two forms of TdT therefore have distinct and opposite functions in lymphocyte development. The enzymatic activities of the splice variants of TdT play an essential role in the diversification of lymphocyte repertoires by modifying the composition and length of the gene segments involved in the production of antibodies and T-cell receptors.

The role of CCL21 in recruitment of T-precursor cells to fetal thymi

During embryonic development, T-lymphoid precursor cells colonize the thymus. Chemoattraction by the fetal thymus is thought to mediate T-precursor cell colonization. However, the molecules that attract T-precursor cells to the thymus remain unclear. By devising time-lapse visualization in culture, the present results show that alymphoid fetal thymus lobes attract T-precursor cells from fetal liver or fetal blood. CD4(-)CD8(-)CD25(-)CD44+ fetal thymocytes retained the activity to specifically re-enter the thymus. The attraction was predominantly due to I-A-expressing thymic epithelial cells and was mediated by pertussis toxin-sensitive G-protein signals. Among the chemokines produced by the fetal thymus, CCL21, CCL25, and CXCL12 could attract CD4(-)CD8(-)CD25(-)CD44+ fetal thymocytes. However, fetal thymus colonization was markedly diminished by neutralizing antibodies specific for CCL21 and CCL25, but not affected by anti-CXCL12 antibody. Fetal thymus colonization was partially defective in CCL21-deficient plt/plt mice and was further diminished by anti-CCL25 antibody. These results indicate that CCL21 is involved in the recruitment of T-cell precursors to the fetal thymus and suggest that the combination of CCL21 and CCL25 plays a major role in fetal thymus colonization.

Improved thymic function in exclusively breastfed infants is associated with higher interleukin 7 concentrations in their mothers' breast milk

BACKGROUND

In rural Gambians, the season of birth strongly predicts adult mortality. Those born during the harvest season have longer life spans than do those born during the hungry season, and the deaths associated with infectious diseases suggest permanent early-life influences on immunity. Thymic measurements showed significantly smaller thymuses in infants born during the hungry season than in those born during the harvest season. The differences were greatest at 8 wk of age, a time when all infants were exclusively breastfed, which suggests the involvement of breast milk factors.

OBJECTIVE

This study tested whether thymic size differences reflect thymic output and ascertained whether thymic output is associated with breast milk interleukin 7 (IL-7) concentrations.

DESIGN

We studied thymic size and output in a prospective cohort of 138 Gambian infants born in either the hungry or the harvest season by measuring signal-joint T cell receptor-rearrangement excision circles (sjTRECs) at birth and at 8 wk of age. IL-7 concentrations in breast milk were measured by using an enzyme-linked immunosorbent assay.

RESULTS

By age 8 wk, those born in the hungry season had significantly lower sjTREC counts than did those born in the harvest season (0.97 and 2.12 sjTRECs/100 T cells, respectively; P = 0.006). At 1 wk postpartum, the breast milk of mothers of infants born in the hungry season had significantly lower IL-7 than did that of mothers of infants born in the harvest season (79 and 100 pg/mL, respectively; P = 0.02). The findings were similar at 8 wk postpartum.

CONCLUSION

These data show a plausible pathway linking external seasonal insults to mothers with thymic development in their infants, which suggests possible implications for long-term programming of immunity.

2B4 Acts As a Non–Major Histocompatibility Complex Binding Inhibitory Receptor on Mouse Natural Killer Cells

Natural killer (NK) cells are critical in the immune response to tumor cells, virally infected cells, and bone marrow allografts. 2B4 (CD244) is expressed on all NK cells and the ligand for 2B4, CD48, is expressed on hematopoietic cells. Cross-linking 2B4 on NK cells with anti-2B4 monoclonal antibody leads to NK cell activation in vitro. Therefore, 2B4 is considered to be an activating receptor. Surprisingly, we have found, using antibody-blocking and 2B4-deficient NK cells, that NK lysis of CD48⁺ tumor and allogeneic targets is inhibited by 2B4 ligation. Interferon γ production by NK cells is also inhibited. Using a peritoneal tumor clearance assay, it was found that 2B4^−/− mice have increased clearance of CD48⁺ tumor cells in vivo. Retroviral transduction of 2B4 was sufficient to restore inhibition in 2B4^−/− primary NK cells. It was found that although mature NK cells express SH2D1A, in vitro–derived NK cells do not. However, both populations are inhibited by 2B4 ligation. This indicates that 2B4 inhibitory signaling occurs regardless of the presence of SH2D1A. These findings reveal a novel role for 2B4 as a non–major histocompatibility complex binding negative regulator of NK cells.

Abnormal development of thymic dendritic and epithelial cells in human X-linked severe combined immunodeficiency

The X-linked form of severe combined immunodeficiency (X-SCID) is caused by mutations in the common cytokine receptor gamma chain and results in lack of T and NK cells and defective B cells. Without immune reconstitution, X-SCID patients typically die from infection during infancy. This report describes thymic epithelial (TE), lymphocyte, and dendritic cell (DC) differentiation in the thymic microenvironment of seven X-SCID patients who died before or after treatment for their immunodeficiency. X-SCID thymus consisted predominately of TE cells without grossly evident corticomedullary distinction. CD3+ and CD1a+ developing T cells and CD83+ thymic DC were reduced >50-fold when compared to age- and gender-matched control thymus (P < 0.001). TE expression of epithelial differentiation markers CK14, involucrin, and high molecular weight cytokeratins also differed in X-SCID versus normal thymus. These histopathologic findings indicate that in addition to T cells, thymic DC development and differentiation of TE cells are also abnormal in X-SCID.

Age-dependent requirement for gammadelta T cells in the primary but not secondary protective immune response against an intestinal parasite

Between weaning (3 wk of age) and adulthood (7 wk of age), mice develop increased resistance to infection with Eimeria vermiformis, an abundant intestinal parasite that causes coccidiosis. This development of resistance was perturbed in T cell receptor (TCR)δ^−/− mice, which at 4 wk of age remained largely susceptible to infection and prone to infection-associated dehydration. These phenotypes were rescued by the repopulation of γδ cells after adoptive transfer of lymphoid progenitors into newborn recipients. Because αβ T cells are necessary and sufficient for the protection of adult mice against E. vermiformis, the requirement for γδ cells in young mice shows a qualitative difference between the cellular immune responses operating at different ages. An important contribution toward primary immune protection in young hosts may have provided a strong selective pressure for the evolutionary conservation of γδ cells. This notwithstanding, the development of effective, pathogen-specific immunity in young mice requires αβ T cells, just as it does in adult mice.

Apoptosis in primary lymphoid organs with aging

Age-associated changes in the immune system are responsible for an increased likelihood of infection, autoimmune diseases, and cancer in the elderly. Immunosenescence is characterized by reduced levels of the peripheral naive T cell pool derived from thymus and the loss of immature B lineage cells in the bone marrow. Primary lymphoid organs, i.e., bone marrow and thymus, exhibit a loss of cellularity with age, which is especially dramatic in the thymus. A summary of major changes associated with aging in primary lymphoid organs is described in this article. The participation of apoptosis in cell loss in the immune system, a change associated with age, as well as a description of molecular machinery involved, is presented. Finally, the involvement of different hormonal and non-hormonal agents in counteracting apoptosis in thymus and bone marrow during aging is explained. Here, we underlie the important role of glucocorticoids as immunodepressors and melatonin as an immunostimulatory agent.

Homeostatic control of T-cell generation in neonates

T cells are produced through 2 mechanisms, thymopoiesis and proliferative expansion of postthymic T cells. Thymic output generates diversity of the pool, and proliferation achieves optimal clonal size of each individual T cell. To determine the contribution of these 2 mechanisms to the formation of the initial T-cell repertoire, we examined neonates of 30 to 40 weeks' gestation. Peripheral T cells were in a state of high proliferative turnover. In premature infants, 10% of T cells were dividing; the proliferation rates then declined but were still elevated in mature newborns. Throughout the third trimester, concentrations of T-cell-receptor excision circles (TRECs) were 10 per 100 T cells. Stability of TREC frequencies throughout the period of repertoire generation suggested strict regulation of clonal size to approximately 10 to 20 cells. Neonatal naive CD4+ and CD8+ T cells were explicitly responsive to IL-7; growth-promoting properties of IL-15 were selective for newborn CD8+ T cells. Neonatal T cells expressed telomerase and, in spite of the high turnover, built up a telomeric reserve. Thus, proliferative expansion, facilitated by increased cytokine responsiveness, and thymopoiesis complement each other as mechanisms of T-cell production in neonates. Maintaining optimal clonal size instead of filling the space in a lymphopenic host appears to regulate homeostatic T-cell proliferation during fetal development.

A developmental look at thymus organogenesis: where do the non-hematopoietic cells in the thymus come from?

The origins of the non-hematopoietic cell types that comprise the thymic stroma remain a topic of considerable controversy. Three recent studies, using lineage analysis and other methods to determine the developmental potential of specific cell types within the thymus, have provided strong evidence of a single endodermal origin for all thymic epithelial cells. Together with other investigations that merge immunological and developmental biology approaches, these studies have suggested a new model of thymus organogenesis, and have begun to uncover the molecular pathways that control this process.

Having it all? Stem cells, haematopoiesis and lymphopoiesis in adult human liver

Because of its location and function, the liver is continuously exposed to large antigenic loads that include pathogens, toxins and tumour cells, as well as harmless dietary and commensal proteins and peptides. Therefore, the liver must be actively immunocompetent and, at the same time, control inappropriate inflammatory responses to dietary and other harmless antigens encountered in the portal circulation. In addition to conventional CD4+ and CD8+ T lymphocytes from the circulation, several specialized lymphoid populations are found in the liver to meet these diverse immunological challenges. These populations display the functional and phenotypic properties of innate cells as well as conventional CD4+ or CD8+ helper and cytotoxic T lymphocytes and B cells. The innate lymphoid cells include gammadeltaTCR+ T cells, B1-B cells and NKT cells as well as large numbers of NK cells. The origin of these cells is unknown, but their murine counterparts have been shown to be capable of differentiation in situ in adult liver. Because haematopoietic stem cells have been found in adult human liver as well as molecular evidence of T-cell maturation, we hypothesize that some resident human hepatic lymphoid cells, particularly those expressing innate phenotypes, also differentiate locally. In particular, it is likely that the adult human liver is an important site of NK cell maturation. In this review, we explore the evidence for an active lymphopoietic role for the normal adult human liver.

Comparison of thymocyte development and cytokine production in CD7-deficient, CD28-deficient and CD7/CD28 double-deficient mice

CD7 and CD28 are Ig superfamily molecules expressed on thymocytes and mature T cells that share common signaling 0mechanisms and are co-mitogens for T cell activation. CD7-deficient mice are resistant to lipopolysaccharide (LPS)-induced shock syndrome, and have diminished in vivo LPS-triggered IFN-gamma and tumor necrosis factor (TNF)-alpha production. CD28-deficient mice have decreased serum Ig levels, defective IgG isotype switching, decreased T cell IL-2 production and are resistant to Staphylococcus aureus enterotoxin-induced shock. To determine synergistic roles CD7 and CD28 might play in thymocyte development and function, we have generated and characterized CD7/CD28 double-deficient mice. CD7/CD28-deficient mice were healthy, reproduced normally, had normal numbers of thymocyte subsets and had normal thymus histology. Anti-CD3 mAb induced similar levels of apoptosis in CD7-deficient, CD28-deficient and CD7/CD28 double-deficient thymocytes as in control C57BL/6 mice (P = NS). Similarly, thymocyte viability, apoptosis and necrosis following ionomycin or dexamethasone treatment were the same in control, CD7-deficient, CD28-deficient and CD7/CD28-deficient mice. CD28-deficient and CD7/CD28-deficient thymocytes had decreased [3H]thymidine incorporation responses to concanavalin A (Con A) stimulation compared to control mice (P < or = 0.01 and P < or = 0.05 respectively). CD7/CD28 double-deficient mice had significantly reduced numbers of B7-1/B7-2 double-positive cells compared to freshly isolated wild-type, CD7-deficient and CD28-deficient thymocytes. Con A-stimulated CD4/CD8 double-negative (DN) thymocytes from CD7/CD28 double-deficient mice expressed significantly lower levels of CD25 when compared to CD4/CD8 DN thymocytes from wild-type, CD7-deficient and CD28-deficient mice (P < 0.05). Anti-CD3-triggered CD7/CD28-deficient thymocytes also had decreased IFN-gamma and TNF-alpha production compared to C57BL/6 control, CD7-deficient and CD28-deficient mice (P < or = 0.05). Thus, CD7 and CD28 deficiencies combined to produce abnormalities in the absolute number of B7-1/B7-2-expressing cells in the thymus, thymocyte IL-2 receptor expression and CD3-triggered cytokine production.

The role of the thymus in immune reconstitution in aging, bone marrow transplantation, and HIV-1 infection

The human thymus is a complex chimeric organ comprised of central (thymic epithelial space) and peripheral (perivascular space) components that functions well into adult life to produce naive T lymphocytes. Recent advances in identifying thymic emigrants and development of safe methods to study thymic function in vivo in adults have provided new opportunities to understand the role that the human thymus plays in immune reconstitution in aging, in bone marrow transplantation, and in HIV-1 infection. The emerging concept is that there are age-dependent contributions of thymic emigrants and proliferation of postthymic T cells to maintain the peripheral T cell pool and to contribute to T cell regeneration, with the thymus contributing more at younger ages and peripheral T cell expansion contributing more in older subjects. New studies have revealed a dynamic interplay between postnatal thymus output and peripheral T cell pool proliferation, which play important roles in determining the nature of immune reconstitution in congenital immunodeficiency diseases, in bone marrow transplantation, and in HIV-1 infection. In this paper, we review recent data on human postnatal thymus function that, taken together, support the notion that the human thymus is functional well into the sixth decade and plays a role throughout life to optimize human immune system function.

Cellular immune responses in neonates

Reduced numbers of lymphocytes and antigen presenting cells have been described as some of the main factors responsible for antigenic tolerance or low responsiveness in neonates. However, by changing the parameters of immunization, such as dose of antigen and frequency of antigen presenting cells we and others have shown that neonates have the option of developing the same variety of immune responses seen in adults. Several aspects of the development of cellular immunity in human and murine neonates are reviewed in this article, with a special focus on the development of T cell mediated responses, from ontogeny to effector function.