Involvement of CCR9 at multiple stages of adult T lymphopoiesis

Thymocyte migration and emigration

Hematopoietic precursors (HPCs) entering into the thymus undergo a sequential process leading to the generation of a variety of T cell subsets. This developmental odyssey unfolds in distinct stages within the thymic cortex and medulla, shaping the landscape of T cell receptor (TCR) expression and guiding thymocytes through positive and negative selection. Initially, early thymic progenitors (ETPs) take residence in the thymic cortex, where thymocytes begin to express their TCR and undergo positive selection. Subsequently, thymocytes transition to the thymic medulla, where they undergo negative selection. Both murine and human thymocyte development can be broadly classified into distinct stages based on the expression of CD4 and CD8 coreceptors, resulting in categorizations as double negative (DN), double positive (DP) or single positive (SP) cells. Thymocyte migration to the appropriate thymic microenvironment at the right differentiation stage is pivotal for the development and the proper functioning of T cells, which is critical for adaptive immune responses. The journey of lymphoid progenitor cells into the T cell developmental pathway hinges on an ongoing dialogue between the differentiating cell and the signals emanating from the thymus niche. Herein, we review the contribution of the key factors mentioned above for the localization, migration and emigration of thymocytes.

T-Cell Progenitors As A New Immunotherapy to Bypass Hurdles of Allogeneic Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is the treatment of preference for numerous malignant and non-malignant hemopathies. The outcome of this approach is significantly hampered by not only graft-versus-host disease (GvHD), but also infections and relapses that may occur because of persistent T-cell immunodeficiency following transplantation. Reconstitution of a functional T-cell repertoire can take more than 1 year. Thus, the major challenge in the management of allogeneic HSCT relies on the possibility of shortening the window of immune deficiency through the acceleration of T-cell recovery, with diverse, self-tolerant, and naïve T cells resulting from de novo thymopoiesis from the donor cells. In this context, adoptive transfer of cell populations that can give rise to mature T cells faster than HSCs while maintaining a safety profile compatible with clinical use is of major interest. In this review, we summarize current advances in the characterization of thymus seeding progenitors, and their ex vivo generated counterparts, T-cell progenitors. Transplantation of the latter has been identified as a worthwhile approach to shorten the period of immune deficiency in patients following allogeneic HSCT, and to fulfill the clinical objective of reducing morbimortality due to infections and relapses. We further discuss current opportunities for T-cell progenitor-based therapy manufacturing, including iPSC cell sources and off-the-shelf strategies. These opportunities will be analyzed in the light of results from ongoing clinical studies involving T-cell progenitors.

Making Thymus Visible: Understanding T-Cell Development from a New Perspective

T-cell development is coupled with a highly ordered migratory pattern. Lymphoid progenitors must follow a precise journey; starting from the hematopoietic tissue, they move toward the thymus and then migrate into and out of distinct thymic microenvironments, where they receive signals and cues required for their differentiation into naïve T-cells. Knowing where, when, and how these cells make directional “decisions” is key to understanding T-cell development. Such insights can be gained by directly observing developing T-cells within their environment under various conditions and following specific experimental manipulations. In the last decade, several model systems have been developed to address temporal and spatial aspects of T-cell development using imaging approaches. In this perspective article, we discuss the advantages and limitations of these systems and highlight a particularly powerful in vivo model that has been recently established. This model system enables the migratory behavior of all thymocytes to be studied simultaneously in a noninvasive and quantitative manner, making it possible to perform systems-level studies that reveal fundamental principles governing T-cell dynamics during development and in disease.

Thymus Colonization: Who, How, How Many?

De novo generation of T cells depends on continual colonization of the thymus by bone marrow-derived progenitors. Thymus seeding progenitors (TSPs) constitute a heterogeneous population comprising multipotent and lineage-restricted cell types. Entry into the thymic microenvironment is tightly controlled and recent quantitative studies have revealed that the adult murine thymus only contains approximately 160 niches to accommodate TSPs. Of these niches only about 6% are open for seeding on average at steady-state. Here, I review the state of understanding of colonization of the adult murine thymus with a particular focus on past and current controversies in the field. Improving thymus colonization and/or maintaining intact TSP niches during the course of pre-conditioning regimens are likely to be critical for efficient T-cell regeneration after hematopoietic stem cell transplantation.

Progressive Changes in CXCR4 Expression That Define Thymocyte Positive Selection Are Dispensable For Both Innate and Conventional αβT-cell Development

The ordered migration of immature thymocytes through thymic microenvironments generates both adaptive MHC restricted αβT-cells and innate CD1d-restricted iNKT-cells. While several chemokine receptors and ligands control multiple stages of this process, their involvement during early thymocyte development often precludes direct analysis of potential roles during later developmental stages. For example, because of early lethality of CXCR4-/- mice, and stage-specific requirements for CXCR4 in thymus colonisation and pre-TCR mediated selection, its role in thymic positive selection is unclear. Here we have examined CXCR4-CXCL12 interactions during the maturation of CD4+CD8+ thymocytes, including downstream stages of iNKT and αβT-cell development. We show CXCL12 expression is a common feature of cortical thymic epithelial cells, indicating widespread availability throughout the cortex. Moreover, CXCR4 expression by CD4+CD8+ pre-selection thymocytes is progressively downregulated following both MHC and CD1d-restricted thymic selection events. However, using CD4Cre-mediated deletion to bypass its involvement in CD4-CD8- thymocyte development, we show CXCR4 is dispensable for the maintenance and intrathymic positioning of CD4+CD8+ thymocytes, and their ability to generate mature αβT-cells and CD1d-restricted iNKT-cells. Collectively, our data define dynamic changes in CXCR4 expression as a marker for intrathymic selection events, and show its role in T-cell development is restricted to pre-CD4+CD8+ stages.

T Cell Development by the Numbers

T cells are continually generated in the thymus in a highly dynamic process comprising discrete steps of lineage commitment, T cell receptor (TCR) gene rearrangement, and selection. These steps are linked to distinct rates of proliferation, survival, and cell death, but a quantitative picture of T cell development is only beginning to emerge. Here we summarize recent technical advances, including genetic fate mapping, barcoding, and molecular timers, that have allowed the implementation of computational models to quantify developmental dynamics in the thymus. Coupling new techniques with mathematical models has recently resulted in the emergence of new paradigms in early hematopoiesis and might similarly open new perspectives on T cell development.

Paracoccidioides brasiliensis infection promotes thymic disarrangement and premature egress of mature lymphocytes expressing prohibitive TCRs

BACKGROUND

Paracoccidioidomycosis, a chronic granulomatous fungal disease caused by Paracoccidioides brasiliensis yeast cells affects mainly rural workers, albeit recently cases in immunosuppressed individuals has been reported. Protective immune response against P. brasiliensis is dependent on the activity of helper T cells especially IFN-γ-producing Th1 cells. It has been proposed that Paracoccidioides brasiliensis is able to modulate the immune response towards a permissive state and that the thymus plays a major role in it.

METHODS

In this paper, we show that acute infection of BALB/c mice with P. brasiliensis virulent isolate (Pb18) might cause alterations in the thymic environment as well as the prohibitive TCR-expressing T cells in the spleens.

RESULTS

After seven days of infection, we found yeast cells on the thymic stroma, the thymic epithelial cells (TEC) were altered regarding their spatial-orientation and inflammatory mediators gene expression was increased. Likewise, thymocytes (differentiating T cells) presented higher migratory ability in ex vivo experiments. Notwithstanding, P. brasiliensis-infected mice showed an increased frequency of prohibitive TCR-expressing T cells in the spleens, suggesting that the selection processes that occur in the thymus may be compromised during the acute infection.

CONCLUSION

In this paper, for the first time, we show that acute infection with Paracoccidioides brasiliensis yeast cells promotes thymic alterations leading to a defective repertoire of peripheral T cells. The data presented here may represent new mechanisms by which P. brasiliensis subverts the immune response towards the chronic infection observed in humans.

Repression of Ccr9 transcription in mouse T lymphocyte progenitors by the Notch signaling pathway

The chemokine receptor CCR9 controls the immigration of multipotent hematopoietic progenitor cells into the thymus to sustain T cell development. Postimmigration, thymocytes downregulate CCR9 and migrate toward the subcapsular zone where they recombine their TCR β-chain and γ-chain gene loci. CCR9 is subsequently upregulated and participates in the localization of thymocytes during their selection for self-tolerant receptor specificities. Although the dynamic regulation of CCR9 is essential for early T cell development, the mechanisms controlling CCR9 expression have not been determined. In this article, we show that key regulators of T cell development, Notch1 and the E protein transcription factors E2A and HEB, coordinately control the expression of Ccr9. E2A and HEB bind at two putative enhancers upstream of Ccr9 and positively regulate CCR9 expression at multiple stages of T cell development. In contrast, the canonical Notch signaling pathway prevents the recruitment of p300 to the putative Ccr9 enhancers, resulting in decreased acetylation of histone H3 and a failure to recruit RNA polymerase II to the Ccr9 promoter. Although Notch signaling modestly modulates the binding of E proteins to one of the two Ccr9 enhancers, we found that Notch signaling represses Ccr9 in T cell lymphoma lines in which Ccr9 transcription is independent of E protein function. Our data support the hypothesis that activation of Notch1 has a dominant-negative effect on Ccr9 transcription and that Notch1 and E proteins control the dynamic expression of Ccr9 during T cell development.

Thymus medulla fosters generation of natural Treg cells, invariant γδ T cells, and invariant NKT cells: what we learn from intrathymic migration

The organization of the thymus into distinct cortical and medullary regions enables it to control the step-wise migration and development of immature T-cell precursors. Such a process provides access to specialized cortical and medullary thymic epithelial cells at defined stages of maturation, ensuring the generation of self-tolerant and MHC-restricted conventional CD4⁺ and CD8⁺ αβ T cells. The migratory cues and stromal cell requirements that regulate the development of conventional αβ T cells have been well studied. However, the thymus also fosters the generation of several immunoregulatory T-cell populations that form key components of both innate and adaptive immune responses. These include Foxp3⁺ natural regulatory T cells, invariant γδ T cells, and CD1d-restricted invariant natural killer T cells (iNKT cells). While less is known about the intrathymic requirements of these nonconventional T cells, recent studies have highlighted the importance of the thymus medulla in their development. Here, we review recent findings on the mechanisms controlling the intrathymic migration of distinct T-cell subsets, and relate this to knowledge of the microenvironmental requirements of these cells.

Heterogeneity across the murine small and large intestine

The small and large intestine of the gastrointestinal tract (GIT) have evolved to have discrete functions with distinct anatomies and immune cell composition. The importance of these differences is underlined when considering that different pathogens have uniquely adapted to live in each region of the gut. Furthermore, different regions of the GIT are also associated with differences in susceptibility to diseases such as cancer and chronic inflammation. The large and small intestine, given their anatomical and functional differences, should be seen as two separate immunological sites. However, this distinction is often ignored with findings from one area of the GIT being inappropriately extrapolated to the other. Focussing largely on the murine small and large intestine, this review addresses the literature relating to the immunology and biology of the two sites, drawing comparisons between them and clarifying similarities and differences. We also highlight the gaps in our understanding and where further research is needed.

Snail transcription factors in hematopoietic cell development: a model of functional redundancy

Coordinated gene expression is crucial in facilitating proper lymphoid cell development and function. The precise patterns of gene expression during B- and T-cell development are regulated through a complex interplay between a multitude of transcriptional regulators, both activators and repressors. We have recently identified the Snail family of transcription factors as playing significant and overlapping roles in lymphoid cell development, in that deletion of both SNAI2 and SNAI3 was required to fully impact the generation of mature T and B cells. Analyses using compound heterozygote animals further demonstrated that SNAI2 and SNAI3 were partially haplosufficient and relatively equivalent in their ability to preserve B-cell generation in the bone marrow. In this review, we summarize studies elucidating the role of the Snail family in hematopoiesis, with a focus on lymphoid cell development. Using the Snail family as an example, we discuss the concepts of functional redundancy and strategies employed to assay transcription factor families for intramember compensation.

Paracrine Activation of Chemokine Receptor CCR9 Enhances The Invasiveness of Pancreatic Cancer Cells

Chemokine receptors mediate cancer progression and metastasis. We have previously examined chemokine receptor CCR9 expression in pancreatic cancer. Here, our objective was to evaluate pancreatic stellate cells (PSCs) as a source of CCL25, the CCR9 ligand, and as an activator of CCL25-CCR9 signaling in pancreatic cancer cells. CCL25 and CCR9 expression levels in human pancreatic cancer tissues and normal human pancreas were assessed by immunohistochemsitry. In vitro secretion of CCL25 in PSCs and PANC-1 cells was verified by enzyme-linked immunosorbent assay. Pancreatic cancer cell invasion was measured using a modified Boyden chamber assay with CCL25, PSC secreted proteins, and PANC-1 secreted proteins as the chemoattractant. There was immunostaining for CCR9 expression in human pancreatic tumor tissues, but not in normal pancreatic tissue. CCL25 expression was absent in the normal pancreatic tissue sample, but was observed in cancer cells and in the stromal cells surrounding the tumor. In vitro, both PANC-1 cells and PSCs secreted CCL25. In an invasion assay, exposure to CCL25, PSC- and PANC-1-conditioned media significantly increased the invasiveness of PANC-1 cells. Inclusion of a CCR9-neutralizing antibody in the invasion assay blocked the increase in invading cells elicited by the chemoattractants. Our studies show that pancreatic cancer invasiveness is enhanced by autocrine and paracrine stimulation of CCR9. PSCs in the tumor microenvironment appear to contribute to paracrine activation of CCR9. Investigations into CCR9 as a potential therapeutic target in pancreatic cancer must consider cancer cell autocrine signaling and also paracrine signaling from interactions in the tumor microenvironment.

The chemokine superfamily revisited

The chemokine superfamily consists of a large number of ligands and receptors. At first glance, this family appears redundant and their ligand-receptor relationships promiscuous, making its study challenging. However, analyzing this family from the evolutionary perspective greatly simplifies understanding both the organization and function of this apparently complex system. In particular, the functions of a subgroup of chemokines (designated homeostatic chemokines) have played pivotal roles in advancing our understanding of the organization and function of the cellular networks that shape the immune system. Here, we update the full scope of the human and mouse chemokine superfamilies and their relationships and summarize several important roles that homeostatic chemokines play in the immune system.

Thymic function is maintained during Salmonella-induced atrophy and recovery

Thymic atrophy is a frequent consequence of infection with bacteria, viruses, and parasites and is considered a common virulence trait between pathogens. Multiple reasons have been proposed to explain this atrophy, including premature egress of immature thymocytes, increased apoptosis, or thymic shutdown to prevent tolerance to the pathogen from developing. The severe loss in thymic cell number can reflect an equally dramatic reduction in thymic output, potentially reducing peripheral T cell numbers. In this study, we examine the relationship between systemic Salmonella infection and thymic function. During infection, naive T cell numbers in peripheral lymphoid organs increase. Nevertheless, this occurs despite a pronounced thymic atrophy caused by viable bacteria, with a peak 50-fold reduction in thymocyte numbers. Thymic atrophy is not dependent upon homeostatic feedback from peripheral T cells or on regulation of endogenous glucocorticoids, as demonstrated by infection of genetically altered mice. Once bacterial numbers fall, thymocyte numbers recover, and this is associated with increases in the proportion and proliferation of early thymic progenitors. During atrophy, thymic T cell maturation is maintained, and single-joint TCR rearrangement excision circle analysis reveals there is only a modest fall in recent CD4(+) thymic emigrants in secondary lymphoid tissues. Thus, thymic atrophy does not necessarily result in a matching dysfunctional T cell output, and thymic homeostasis can constantly adjust to systemic infection to ensure that naive T cell output is maintained.

Transcription factor zinc finger and BTB domain 1 is essential for lymphocyte development

Absent T lymphocytes were unexpectedly found in homozygotes of a transgenic mouse from an unrelated project. T cell development did not progress beyond double-negative stage 1 thymocytes, resulting in a hypocellular, vestigial thymus. B cells were present, but NK cell number and B cell isotype switching were reduced. Transplantation of wild-type hematopoietic cells corrected the defect, which was traced to a deletion involving five contiguous genes at the transgene insertion site on chromosome 12C3. Complementation using bacterial artificial chromosome transgenesis implicated zinc finger BTB-POZ domain protein 1 (Zbtb1) in the immunodeficiency, confirming its role in T cell development and suggesting involvement in B and NK cell differentiation. Targeted disruption of Zbtb1 recapitulated the T(-)B(+)NK(-) SCID phenotype of the original transgenic animal. Knockouts for Zbtb1 had expanded populations of bone marrow hematopoietic stem cells and also multipotent and early lymphoid lineages, suggesting a differentiation bottleneck for common lymphoid progenitors. Expression of mRNA encoding Zbtb1, a predicted transcription repressor, was greatest in hematopoietic stem cells, thymocytes, and pre-B cells, highlighting its essential role in lymphoid development.

T cell lineage commitment: identity and renunciation

Precursors undertaking T cell development shed their access to other pathways in a sequential process that begins before entry into the thymus and continues through many cell cycles afterward. This process involves three levels of regulatory change, in which the cells' intrinsic transcriptional regulatory factors, expression of signaling receptors (e.g., Notch1), and expression of distinct homing receptors separately contribute to confirmation of T cell identity. Each alternative potential has a different underlying molecular basis that is neutralized and then permanently silenced through different mechanisms in early T cell precursors. This regulatory mosaic has notable implications for the hierarchy of relationships linking T lymphocytes to other hematopoietic fates.

Clonal analysis reveals uniformity in the molecular profile and lineage potential of CCR9(+) and CCR9(-) thymus-settling progenitors

The entry of T cell progenitors to the thymus marks the beginning of a multistage developmental process that culminates in the generation of self-MHC-restricted CD4(+) and CD8(+) T cells. Although multiple factors including the chemokine receptors CCR7 and CCR9 are now defined as important mediators of progenitor recruitment and colonization in both the fetal and adult thymi, the heterogeneity of thymus-colonizing cells that contribute to development of the T cell pool is complex and poorly understood. In this study, in conjunction with lineage potential assays, we perform phenotypic and genetic analyses on thymus-settling progenitors (TSP) isolated from the embryonic mouse thymus anlagen and surrounding perithymic mesenchyme, including simultaneous gene expression analysis of 14 hemopoietic regulators using single-cell multiplex RT-PCR. We show that, despite the known importance of CCL25-CCR9 mediated thymic recruitment of T cell progenitors, embryonic PIR(+)c-Kit(+) TSP can be subdivided into CCR9(+) and CCR9(-) subsets that differ in their requirements for a functional thymic microenvironment for thymus homing. Despite these differences, lineage potential studies of purified CCR9(+) and CCR9(-) TSP reveal a common bias toward T cell-committed progenitors, and clonal gene expression analysis reveals a genetic consensus that is evident between and within single CCR9(+) and CCR9(-) TSP. Collectively, our data suggest that although the earliest T cell progenitors may display heterogeneity with regard to their requirements for thymus colonization, they represent a developmentally homogeneous progenitor pool that ensures the efficient generation of the first cohorts of T cells during thymus development.

The autophagy protein Atg7 is essential for hematopoietic stem cell maintenance

Adult mouse LSK cells unable to undergo autophagy contain fewer HSCs, accumulate mitochondria, and fail to reconstitute lethally irradiated mice.

The role of autophagy, a lysosomal degradation pathway which prevents cellular damage, in the maintenance of adult mouse hematopoietic stem cells (HSCs) remains unknown. Although normal HSCs sustain life-long hematopoiesis, malignant transformation of HSCs leads to leukemia. Therefore, mechanisms protecting HSCs from cellular damage are essential to prevent hematopoietic malignancies. In this study, we crippled autophagy in HSCs by conditionally deleting the essential autophagy gene Atg7 in the hematopoietic system. This resulted in the loss of normal HSC functions, a severe myeloproliferation, and death of the mice within weeks. The hematopoietic stem and progenitor cell compartment displayed an accumulation of mitochondria and reactive oxygen species, as well as increased proliferation and DNA damage. HSCs within the Lin⁻Sca-1⁺c-Kit⁺ (LSK) compartment were significantly reduced. Although the overall LSK compartment was expanded, Atg7-deficient LSK cells failed to reconstitute the hematopoietic system of lethally irradiated mice. Consistent with loss of HSC functions, the production of both lymphoid and myeloid progenitors was impaired in the absence of Atg7. Collectively, these data show that Atg7 is an essential regulator of adult HSC maintenance.

Finding their niche: chemokines directing cell migration in the thymus

T lymphocytes are generated throughout life, arising from bone marrow-derived progenitors that complete an essential developmental process in the thymus. Thymic T cell education leads to the generation of a self-restricted and largely self-tolerant peripheral T-cell pool and is facilitated by interactions with thymic stromal cells residing in distinct supportive niches. The signals governing thymocyte precursor migration into the thymus, directing thymocyte navigation through thymic microenvironments and mature T-cell egress into circulation were, until recently, largely unknown, but presumed to be mediated to a large extent by chemokine signalling. Recent studies have now uncovered various specific functions for members of the chemokine superfamily in the thymus. These studies have not only revealed distinct but also in some cases overlapping roles for several chemokine family members in various thymocyte migration events and have also shown that homing and positioning of other cells in the thymus, such as dendritic cells and natural killer T cells is also chemokine-dependent. Here, we discuss current understanding of the role of chemokines in the thymus and highlight key future avenues for investigation in this field.

The thymus microenvironment in regulating thymocyte differentiation

The thymus plays a crucial role in the development of T lymphocytes by providing an inductive microenvironment in which committed progenitors undergo proliferation, T-cell receptor gene rearrangements and thymocyte differentiate into mature T cells. The thymus microenvironment forms a complex network of interaction that comprises non lymphoid cells (e.g., thymic epithelial cells, TEC), cytokines, chemokines, extracellular matrix elements (ECM), matrix metalloproteinases and other soluble proteins. The thymic epithelial meshwork is the major component of the thymic microenvironment, both morphologically and phenotypically limiting heterogeneous regions in thymic lobules and fulfilling an important role during specific stages of T-cell maturation. The process starts when bone marrow-derived lymphocyte precursors arrive at the outer cortical region of the thymic gland and begin to mature into functional T lymphocytes that will finally exit the thymus and populate the peripheral lymphoid organs. During their journey inside the thymus, thymocytes must interact with stromal cells (and their soluble products) and extracellular matrix proteins to receive appropriate signals for survival, proliferation and differentiation. The crucial components of the thymus microenvironment, and their complex interactions during the T-cell maturation process are summarized here with the objective of contributing to a better understanding of the function of the thymus, as well as assisting in the search for new therapeutic approaches to improve the immune response in various pathological conditions.

CC chemokine receptor 7 and 9 double-deficient hematopoietic progenitors are severely impaired in seeding the adult thymus

T-cell development depends on recruitment of bone marrow-derived precursor cells to the thymus via a multistep adhesion cascade involving the chemokine receptor CCR9. However, CCR9 deficiency does not result in complete abrogation of progenitor entry into the adult thymus. Therefore, we tested the hypothesis that additional chemokine/chemokine receptor systems might play a role in this process. To this end, we generated mice deficient in both CCR9 and CCR7. Deficiency in both chemokine receptors resulted in severely reduced numbers of early T-cell progenitors and in near-complete abrogation of thymus reconstitution. Progenitors in bone marrow and peripheral blood remained largely unaffected in CCR7(-/-)CCR9(-/-) mice, and direct intrathymic transfer of precursors from CCR7(-/-)CCR9(-/-) mice as well as single-mutant mice showed that intrathymic differentiation of these precursors remained functional. Thus, our data reveal a previously unrecognized role of CCR7 in progenitor seeding of the adult thymus, which is largely masked by compensatory effects of CCR9 signals. In turn, CCR7 signals can partially compensate for CCR9 signals, thus explaining the rather mild phenotype of CCR9(-/-) mice with respect to progenitor seeding.

GATA-3 is required for early T lineage progenitor development

Most T lymphocytes appear to arise from very rare early T lineage progenitors (ETPs) in the thymus, but the transcriptional programs that specify ETP generation are not completely known. The transcription factor GATA-3 is required for the development of T lymphocytes at multiple late differentiation steps as well as for the development of thymic natural killer cells. However, a role for GATA-3 before the double-negative (DN) 3 stage of T cell development has to date been obscured both by the developmental heterogeneity of DN1 thymocytes and the paucity of ETPs. We provide multiple lines of in vivo evidence through the analysis of T cell development in Gata3 hypomorphic mutant embryos, in irradiated mice reconstituted with Gata3 mutant hematopoietic cells, and in mice conditionally ablated for the Gata3 gene to show that GATA-3 is required for ETP generation. We further show that Gata3 loss does not affect hematopoietic stem cells or multipotent hematopoietic progenitors. Finally, we demonstrate that Gata3 mutant lymphoid progenitors exhibit neither increased apoptosis nor diminished cell-cycle progression. Thus, GATA-3 is required for the cell-autonomous development of the earliest characterized thymic T cell progenitors.

T-cell recruitment to the intestinal mucosa

The intestinal epithelium and underlying lamina propria contains large numbers of T cells that play an important role in maintaining intestinal homeostasis and defense against intestinal pathogens. Recent years have seen several significant advances in our understanding of the mechanisms regulating T-cell localization to the intestinal mucosa. For instance, we now know that the small intestine 'imprints' gut homing properties on T cells by inducing the expression of specific integrins and chemokine receptors. Further studies have identified distinct subsets of intestinal dendritic cells that use retinoic acid to generate both gut-tropic and regulatory T cells. As our understanding of the mechanisms regulating the generation of gut tropic T-cell populations evolves, the possibility of targeting these processes for mucosal vaccine development and treatment of intestinal immune pathology become more apparent.

Exogenous insulin-like growth factor 1 enhances thymopoiesis predominantly through thymic epithelial cell expansion

Insulin-like growth factor 1 (IGF-1) enhances thymopoiesis but given the broad distribution of IGF-1 receptors (IGF-1Rs), its mechanism of action has remained unclear. To identify points of thymic regulation by IGF-1, we examined its effects on T-cell precursors, thymocytes, and thymic epithelial cells (TECs) in normal and genetically altered mice. In thymus-intact but not thymectomized mice, IGF-1 administration increased peripheral naive and recent thymic emigrant (RTE) populations, demonstrating its effect on T-cell production, not peripheral expansion. IGF-1 administration increased bone marrow LSK (lineage(-), Sca-1(+), c-kit(+)) precursor proliferation and peripheral LSK populations, increased thymocyte populations in a sequential wave of expansion, and proportionately expanded TEC subpopulations and enhanced their chemokine expression. To separate IGF-1's effects on thymocytes and TECs, we generated mice lacking IGF-1R on thymocytes and T cells. Thymocyte and RTE numbers were decreased in these mice, but IGF-1 treatment produced comparable thymocyte numbers to similarly treated wild-type mice. We additionally separated thymic- from LSK-specific effects by demonstrating that IGF-1 increased thymocyte numbers despite impaired early thymic progenitor (ETP) importation in PSGL-1KO mice. These results indicate the critical point thymic function regulation by IGF-1 involves TEC expansion regulating thymocyte precursor entry and facilitating thymocyte development.