Premature Expression of Chemokine Receptor CCR9 Impairs T Cell Development

Bioactive components in the marsupial pouch and milk

Marsupials give birth to immunologically naïve young after a relatively short gestation period compared with eutherians. Consequently, the joey relies significantly on maternal protection, which is the focus of the present review. The milk and the pouch environment are essential contributors to maternal protection for the healthy development of joeys. In this review, we discuss bioactive components found in the marsupial pouch and milk that form cornerstones of maternal protection. These bioactive components include immune cells, immunoglobulins, the S100 family of calcium-binding proteins, lysozymes, whey proteins, antimicrobial peptides and other immune proteins. Furthermore, we investigated the possibility of the presence of plurifunctional components in milk and pouches that are potentially bioactive. These compounds include caseins, vitamins and minerals, oligosaccharides, lipids and microRNAs. Where applicable, this review addresses variability in bioactive components during different phases of lactation, designed to fulfil the immunological needs of the growing pouch young. Yet, there are numerous additional research opportunities to pursue, including uncovering novel bioactive components and investigating their modes of action, dynamics, stability and ability to penetrate the gut epithelium to facilitate systemic effects.

Pathways and mechanisms of CD4+CD8αα+ intraepithelial T cell development

The mammalian small intestine epithelium harbors a peculiar population of CD4+CD8αα+ T cells that are derived from mature CD4+ T cells through reprogramming of lineage-specific transcription factors. CD4+CD8αα+ T cells occupy a unique niche in T cell biology because they exhibit mixed phenotypes and functional characteristics of both CD4+ helper and CD8+ cytotoxic T cells. The molecular pathways driving their generation are not fully mapped. However, recent studies demonstrate the unique role of the commensal gut microbiota as well as distinct cytokine and chemokine requirements in the differentiation and survival of these cells. We review the established and newly identified factors involved in the generation of CD4+CD8αα+ intraepithelial lymphocytes (IELs) and place them in the context of the molecular machinery that drives their phenotypic and functional differentiation.

A three-stage developmental pathway for human Vγ9Vδ2 T cells within the postnatal thymus

Vγ9Vδ2 T cells are the largest population of γδ T cells in adults and can play important roles in providing effective immunity against cancer and infection. Many studies have suggested that peripheral Vγ9Vδ2 T cells are derived from the fetal liver and thymus and that the postnatal thymus plays little role in the development of these cells. More recent evidence suggested that these cells may also develop postnatally in the thymus. Here, we used high-dimensional flow cytometry, transcriptomic analysis, functional assays, and precursor-product experiments to define the development pathway of Vγ9Vδ2 T cells in the postnatal thymus. We identify three distinct stages of development for Vγ9Vδ2 T cells in the postnatal thymus that are defined by the progressive acquisition of functional potential and major changes in the expression of transcription factors, chemokines, and other surface markers. Furthermore, our analysis of donor-matched thymus and blood revealed that the molecular requirements for the development of functional Vγ9Vδ2 T cells are delivered predominantly by the postnatal thymus and not in the periphery. Tbet and Eomes, which are required for IFN-γ and TNFα expression, are up-regulated as Vγ9Vδ2 T cells mature in the thymus, and mature thymic Vγ9Vδ2 T cells rapidly express high levels of these cytokines after stimulation. Similarly, the postnatal thymus programs Vγ9Vδ2 T cells to express the cytolytic molecules, perforin, granzyme A, and granzyme K. This study provides a greater understanding of how Vγ9Vδ2 T cells develop in humans and may lead to opportunities to manipulate these cells to treat human diseases.

Detection of CCL25 and the correlation between CCL25, CCL28, IL-7, and TSLP in human breast milk

In this study, CCL25, a chemokine that contributes to the immunological function of the thymus and intestines, was detected in human breast milk (HBM) for the first time. We then focused on the correlations of CCL25 with CCL28, TSLP, and IL-7, which were predicted to interact with CCL25 in HBM. We also compared their levels between primiparous and multiparous women. A total of 53 parturient women were recruited. Their HBM was collected during 0-5 days and at 1 month after parturition and the CCL25, CCL28, IL-7, and TSLP levels in the HBM were analyzed using ELISA. The results showed that CCL25 and TSLP levels were significantly higher in colostrum than in mature milk. Moreover, CCL28 and IL-7 levels in colostrum showed a positive correlation. These results indicate that CCL28 and IL-7 in colostrum might interact positively with each other when produced in the mammary glands during lactation. The findings also suggest that the level of parity has no effect on their levels in HBM. In conclusion, our results clarify that CCL25 is present in HBM and that the concentrations of CCL25 and TSLP are higher in colostrum than in mature milk. Moreover, the production of CCL28 and IL-7 might be closely correlated in human colostrum.

A single-cell analysis of thymopoiesis and thymic iNKT cell development in pigs

Many aspects of the porcine immune system remain poorly characterized, which poses a barrier to improving swine health and utilizing pigs as preclinical models. Here, we employ single-cell RNA sequencing (scRNA-seq) to create a cell atlas of the early-adolescent pig thymus. Our data show conserved features as well as species-specific differences in cell states and cell types compared with human thymocytes. We also describe several unconventional T cell types with gene expression profiles associated with innate effector functions. This includes a cell census of more than 11,000 differentiating invariant natural killer T (iNKT) cells, which reveals that the functional diversity of pig iNKT cells differs substantially from the iNKT0/1/2/17 subset differentiation paradigm established in mice. Our data characterize key differentiation events in porcine thymopoiesis and iNKT cell maturation and provide important insights into pig T cell development.

Chemokine receptor CCR9 suppresses the differentiation of CD4+CD8αα+ intraepithelial T cells in the gut

The chemokine receptor CCR9 equips T cells with the ability to respond to CCL25, a chemokine that is highly expressed in the thymus and the small intestine (SI). Notably, CCR9 is mostly expressed on CD8 but not on CD4 lineage T cells, thus imposing distinct tissue tropism on CD4 and CD8 T cells. The molecular basis and the consequences for such a dichotomy, however, have not been fully examined and explained. Here, we demonstrate that the forced expression of CCR9 interferes with the tissue trafficking and differentiation of CD4 T cells in SI intraepithelial tissues. While CCR9 overexpression did not alter CD4 T cell generation in the thymus, the forced expression of CCR9 was detrimental for the proper tissue distribution of CD4 T cells in the periphery, and strikingly also for their terminal differentiation in the gut epithelium. Specifically, the differentiation of SI epithelial CD4 T cells into immunoregulatory CD4+CD8αα+ T cells was impaired by overexpression of CCR9 and conversely increased by the genetic deletion of CCR9. Collectively, our results reveal a previously unappreciated role for CCR9 in the tissue homeostasis and effector function of CD4 T cells in the gut.

Recent advances in iNKT cell development

Recent studies suggest that murine invariant natural killer T (iNKT) cell development culminates in three terminally differentiated iNKT cell subsets denoted as NKT1, 2, and 17 cells. Although these studies corroborate the significance of the subset division model, less is known about the factors driving subset commitment in iNKT cell progenitors. In this review, we discuss the latest findings in iNKT cell development, focusing in particular on how T-cell receptor signal strength steers iNKT cell progenitors toward specific subsets and how early progenitor cells can be identified. In addition, we will discuss the essential factors for their sustenance and functionality. A picture is emerging wherein the majority of thymic iNKT cells are mature effector cells retained in the organ rather than developing precursors.

Guiding T lymphopoiesis from pluripotent stem cells by defined transcription factors

Achievement of immunocompetent and therapeutic T lymphopoiesis from pluripotent stem cells (PSCs) is a central aim in T cell regenerative medicine. To date, preferentially reconstituting T lymphopoiesis in vivo from PSCs remains a practical challenge. Here we documented that synergistic and transient expression of Runx1 and Hoxa9 restricted in the time window of endothelial-to-hematopoietic transition and hematopoietic maturation stages in a PSC differentiation scheme (iR9-PSC) in vitro induced preferential generation of engraftable hematopoietic progenitors capable of homing to thymus and developing into mature T cells in primary and secondary immunodeficient recipients. Single-cell transcriptome and functional analyses illustrated the cellular trajectory of T lineage induction from PSCs, unveiling the T-lineage specification determined at as early as hemogenic endothelial cell stage and identifying the bona fide pre-thymic progenitors. The induced T cells distributed normally in central and peripheral lymphoid organs and exhibited abundant TCRαβ repertoire. The regenerative T lymphopoiesis restored immune surveillance in immunodeficient mice. Furthermore, gene-edited iR9-PSCs produced tumor-specific T cells in vivo that effectively eradicated tumor cells. This study provides insight into universal generation of functional and therapeutic T cells from the unlimited and editable PSC source.

Chemokine-Mediated Choreography of Thymocyte Development and Selection

As they differentiate, thymocytes encounter spatially restricted cues critical for differentiation and selection of a functional, self-tolerant T cell repertoire. Sequential migration of developing T cells through distinct thymic microenvironments is enforced by the ordered expression of chemokine receptors. Herein, we provide an updated perspective on T cell differentiation through the lens of recent advances that illuminate the dynamics of chemokine-driven thymocyte migration, localization, and interactions with stromal cells. We consider these findings in the context of earlier groundwork exploring the contribution of chemokines to T cell development, recent advances regarding the specificity of chemokine signaling, and novel techniques for evaluating the T cell repertoire. We suggest future research should amalgamate visualization of localized cellular interactions with downstream molecular signals.

CCR9 AND CCR7 are overexpressed in CD4- CD8- thymocytes of myasthenia gravis patients

INTRODUCTION

Chemokine CC motif receptors 9 and 7 (CCR9 and CCR7) play a major role in the migration of T-cell precursors to the thymus to initiate T thymopoiesis. However, their role in development of T-cells in myasthenia gravis (MG) patients has not been fully elucidated.

METHODS

Expression and distribution of CCR9⁺ and CCR7⁺ cells were detected by flow cytometry and immunofluorescence. Real-time polymerase chain reaction was used to check the adhesion molecules on CD4^- CD8^- double-negative (DN) thymocytes.

RESULTS

CCR9 and CCR7 expression by DN thymocytes increased in the MG thymus; the levels of CCR9, CCR7, interleukin-7R mRNA increased, and CXCR4 levels decreased compared with levels in the non-MG thymus. More CCR7 and CCR9 double-positive (DP) thymocytes were gathered near the subcapsular region in MG thymus.

CONCLUSIONS

Enhanced expression of CCR9 and CCR7 may complicate the differentiation of DP thymocytes from the DN stage in MG thymus. Muscle Nerve, 2016 Muscle Nerve 55: 84-90, 2017.

Innate-like functions of natural killer T cell subsets result from highly divergent gene programs

Natural killer T cells (NKT cells) have stimulatory or inhibitory effects on the immune response that can be attributed in part to the existence of functional subsets of NKT cells. These subsets have been characterized only on the basis of the differential expression of a few transcription factors and cell-surface molecules. Here we have analyzed purified populations of thymic NKT cell subsets at both the transcriptomic level and epigenomic level and by single-cell RNA sequencing. Our data indicated that despite their similar antigen specificity, the functional NKT cell subsets were highly divergent populations with many gene-expression and epigenetic differences. Therefore, the thymus 'imprints' distinct gene programs on subsets of innate-like NKT cells that probably impart differences in proliferative capacity, homing, and effector functions.

Construction of a functional thymic microenvironment from pluripotent stem cells for the induction of central tolerance

The thymus is required for generation of a self-tolerant, self-restricted T-cell repertoire. The capacity to manipulate or replace thymus function therapeutically would be beneficial in a variety of clinical settings, including for improving recovery following bone marrow transplantation, restoring immune system function in the elderly and promoting tolerance to transplanted organs or cells. An attractive strategy would be transplantation of thymus organoids generated from cells produced in vitro, for instance from pluripotent stem cells. Here, we review recent progress toward this goal, focusing on advances in directing differentiation of pluripotent stem cells to thymic epithelial cells, a key cell type of the thymic stroma, and related direct reprogramming strategies.

The Contribution of Chemokines and Migration to the Induction of Central Tolerance in the Thymus

As T cells develop, they migrate throughout the thymus where they undergo essential bi-directional signaling with stromal cells in distinct thymic microenvironments. Immature thymocyte progenitors are located in the thymic cortex. Following T cell receptor expression and positive selection, thymocytes undergo a dramatic transition: they become rapidly motile and relocate to the thymic medulla. Antigen-presenting cells (APCs) within the cortex and medulla display peptides derived from a wide array of self-proteins, which promote thymocyte self-tolerance. If a thymocyte is auto-reactive against such antigens, it undergoes either negative selection, via apoptosis, or differentiation into the regulatory T cell lineage. This induction of central tolerance is critical for prevention of autoimmunity. Chemokines and adhesion molecules play an essential role in tolerance induction, as they promote migration of developing thymocytes through the different thymic microenvironments and enhance interactions with APCs displaying self-antigens. Herein, we review the contribution of chemokines and other regulators of thymocyte localization and motility to T cell development, with a focus on their contribution to the induction of central tolerance.

Histone Deacetylase 3 Is Required for T Cell Maturation

Recent thymic emigrants are newly generated T cells that need to undergo postthymic maturation to gain functional competency and enter the long-lived naive T cell pool. The mechanism of T cell maturation remains incompletely understood. Previously, we demonstrated that the transcriptional repressor NKAP is required for T cell maturation. Because NKAP associates with histone deacetylase 3 (HDAC3), we examined whether HDAC3 is also required for T cell maturation. Although thymic populations are similar in CD4-cre HDAC3 conditional knockout mice compared with wild-type mice, the peripheral numbers of CD4(+) and CD8(+) T cells are dramatically decreased. In the periphery, the majority of HDAC3-deficient naive T cells are recent thymic emigrants, indicating a block in T cell maturation. CD55 upregulation during T cell maturation is substantially decreased in HDAC3-deficient T cells. Consistent with a block in functional maturation, HDAC3-deficient peripheral T cells have a defect in TNF licensing after TCR/CD28 stimulation. CD4-cre HDAC3 conditional knockout mice do not have a defect in intrathymic migration, thymic egress, T cell survival, or homeostasis. In the periphery, similar to immature NKAP-deficient peripheral T cells, HDAC3-deficient peripheral T cells were bound by IgM and complement proteins, leading to the elimination of these cells. In addition, HDAC3-deficient T cells display decreases in the sialic acid modifications on the cell surface that recruit natural IgM to initiate the classical complement pathway. Therefore, HDAC3 is required for T cell maturation.

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.

CCR9-CCL25 interaction suppresses apoptosis of lung cancer cells by activating the PI3K/Akt pathway

CC chemokine receptor-9 (CCR9) is highly expressed in non-small cell lung cancer (NSCLC) tissues and cell lines. However, the biological functions and the signals elicited by the interaction between CCR9 and its natural ligand CCL25 in NSCLC are unknown. Here, we selectively depleted CCR9 and inhibited CCR9-CCL25 interaction in NSCLC cells using small recombinant lentivirus-mediated miRNA, and investigated the tumorigenic effects in vitro and in vivo. Compromised CCR9-CCL25 interaction promoted apoptosis in NSCLC cells by activating phosphoinositide 3-kinase (PI3K)/Akt in vitro. In addition, we showed that CCR9-CCL25 interaction mediated the activation of the PI3K/Akt pathway in NSCLC cells, resulting in the up-regulation of anti-apoptotic proteins, as well as the down-regulation of apoptotic proteins in a PI3K-/Akt-dependent manner. These CCR9-CCL25-mediated effects were abrogated in the presence of a PI3K inhibitor (wortmannin 10 nM) or by inhibiting the CCR9-CCL25 interaction through CCR9 silencing, which also suggested that the biological function of CCR9-CCL25 was mainly regulated by PI3K. In vivo studies also demonstrated a significantly lower tumor burden in mice receiving CCR9-silence cells than those in mice receiving control cells. Together, these data suggested that CCR9-CCL25 interaction induced tumorigenesis of NSCLC cells and that this induction might be accomplished through the activation of the PI3K/Akt pathway. These findings may lead to a better understanding of the biological effects of CCR9-CCL25 interaction and provide clues for identifying novel therapeutic and preventive molecular markers for NSCLC.

Programmed downregulation of CCR6 is important for establishment of epidermal γδT cells by regulating their thymic egress and epidermal location

The skin as the outmost epithelial tissue is under frequent physical, chemical, and biological assaults. To counter the assaults and maintain the local tissue homeostasis, the skin is stationed with various innate or innate-like lymphocytes such as γδT cells. Increasing evidence suggests that an intrathymically programmed process is involved in coordinated expression of multiple homing molecules on specific γδT cell subsets to direct their localization in different regions of the skin for the protective functions. However, detailed molecular events underlying the programmed skin distribution of specific γδT cell subsets are not fully understood. We report in this study that the temporally and spatially regulated downregulation of chemokine receptor CCR6 on fetal thymic Vγ3(+) epidermal γδT precursors is involved in their thymic egress and proper localization in the epidermis. Failure of downregulation of CCR6 in the mature Vγ3(+) epidermal γδT precursor cells due to the constitutive expression of transgenic CCR6 resulted in their abnormal accumulation in the fetal thymus and reduced numbers of the epidermal γδT cells. In addition, the transgenic expression of CCR6 on the Vγ3(+) γδT cells also improperly increased their distribution in dermis of the skin. Those findings advanced our understanding of the molecular basis regulating the tissue specific distribution of various innate-like γδT cell lymphocytes in the skin.

Eph/Ephrin-mediated interactions in the thymus

In the present study, we review available information on the relevance of Eph and ephrins in numerous processes occurring in the thymus that regulate not only T cell differentiation but also thymic epithelial cell (TEC) development and organization. Eph/ephrins are a large family of receptors and ligands involved in organogenesis and homeostasis of adult tissues. They are extensively expressed in the thymus and seem to be involved in the colonization of lymphoid progenitor cells and their migration throughout the thymic parenchyma necessary to provide an adequate topological location of developing thymocytes in the epithelial network that ensures their correct differentiation. In addition, EphB2 and EphB3 play a cell-autonomous role in regulating the transitions of double-negative to double-positive cells and of double-positive to single-positive thymocytes and the lack of these molecules or their ligands ephrin B1 and ephrin B2 induces profound alterations of the TEC maturation and in the arrangement of epithelial network. We emphasize that these results are largely reflecting the role played by this family of molecules in controlling thymocyte-TEC interactions within the thymus.

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.

Monocytes/macrophages express chemokine receptor CCR9 in rheumatoid arthritis and CCL25 stimulates their differentiation

INTRODUCTION

Monocytes/macrophages accumulate in the rheumatoid (RA) synovium where they play a central role in inflammation and joint destruction. Identification of molecules involved in their accumulation and differentiation is important to inform therapeutic strategies. This study investigated the expression and function of chemokine receptor CCR9 in the peripheral blood (PB) and synovium of RA, non-RA patients and healthy volunteers.

METHODS

CCR9 expression on PB monocytes/macrophages was analysed by flow cytometry and in synovium by immunofluorescence. Chemokine receptor CCR9 mRNA expression was examined in RA and non-RA synovium, monocytes/macrophages from PB and synovial fluid (SF) of RA patients and PB of healthy donors using the reverse transcription polymerase chain reaction (RT-PCR). Monocyte differentiation and chemotaxis to chemokine ligand 25 (CCL25)/TECK were used to study CCR9 function.

RESULTS

CCR9 was expressed by PB monocytes/macrophages in RA and healthy donors, and increased in RA. In RA and non-RA synovia, CCR9 co-localised with cluster of differentiation 14+ (CD14+) and cluster of differentiation 68+ (CD68+) macrophages, and was more abundant in RA synovium. CCR9 mRNA was detected in the synovia of all RA patients and in some non-RA controls, and monocytes/macrophages from PB and SF of RA and healthy controls. CCL25 was detected in RA and non-RA synovia where it co-localised with CD14+ and CD68+ cells. Tumour necrosis factor alpha (TNFα) increased CCR9 expression on human acute monocytic leukemia cell line THP-1 monocytic cells. CCL25 induced a stronger monocyte differentiation in RA compared to healthy donors. CCL25 induced significant chemotaxis of PB monocytes but not consistently among individuals.

CONCLUSIONS

CCR9 expression by monocytes is increased in RA. CCL25 may be involved in the differentiation of monocytes to macrophages particularly in RA.

Role of CCL25/CCR9 in immune homeostasis and disease

Chemokines constitute a large family of low-molecular-weight proteins ( approximately 10 kDa in size), recognized primarily for their role in directing leukocyte migration under both homeostatic and inflammatory settings. The chemokine CCL25 displays a unique and highly restricted expression pattern compared with other chemokine family members. In the steady state, CCL25 is expressed at high levels primarily in the thymus and small intestine, while its sole functional receptor, CCR9, is expressed on subsets of developing thymocytes and intestinal lymphocytes. Mice that are deficient in CCR9 show relatively normal thymocyte development; however, in competitive transfer experiments, CCR9(-/-) bone-marrow cells are severely disadvantaged in their ability to generate mature T cells compared with wildtype cells. Indeed, expression data and analysis of genetically modified mice suggest that CCL25/CCR9 may be involved in multiple stages of thymocyte development. Recent in vivo studies have demonstrated a role for CCL25/CCR9 in mediating lymphocyte recruitment to the small intestine and in the development of the small intestinal T-cell receptor-gammadelta T-cell compartment. Finally, CCL25 is expressed in the small intestine of Crohn's disease patients and, in certain inflammatory conditions, outside the small intestine. Together, these results suggest an important role for CCL25/CCR9 in T-cell development and small intestinal immunity and suggest that targeting the CCL25/CCR9 pathway may provide a means to modulate small intestinal immune responses.

Differential contribution of chemotaxis and substrate restriction to segregation of immature and mature thymocytes

T cell development requires sequential localization of thymocyte subsets to distinct thymic microenvironments. To address mechanisms governing this segregation, we used two-photon microscopy to visualize migration of purified thymocyte subsets in defined microenvironments within thymic slices. Double-negative (CD4(-)8(-)) and double-positive (CD4(+)8(+)) thymocytes were confined to cortex where they moved slowly without directional bias. DP cells accumulated and migrated more rapidly in a specialized inner-cortical microenvironment, but were unable to migrate on medullary substrates. In contrast, CD4 single positive (SP) thymocytes migrated directionally toward the medulla, where they accumulated and moved very rapidly. Our results revealed a requisite two-step process governing CD4 SP cell medullary localization: the chemokine receptor CCR7 mediated chemotaxis of CD4 SP cells towards medulla, whereas a distinct pertussis-toxin sensitive pathway was required for medullary entry. These findings suggest that developmentally regulated responses to both chemotactic signals and specific migratory substrates guide thymocytes to specific locations in the thymus.

T and B lymphocyte differentiation from hematopoietic stem cell

Until the past few years, it has been thought that lymphoid and myeloid lineage segregation represents the first step of lineage restriction during hematopoiesis from hematopoietic stem cell. Recent investigation of the cell populations within multipotent progenitors in the bone marrow has led to new understanding of how hematopoietic stem cells diversify into different hematopoietic cell types. This review focuses on the recent advances in understanding the developmental events that occur during hematopoietic stem cell specification into the T and B lymphocyte lineages in adult mice.

Involvement of CCR9 at multiple stages of adult T lymphopoiesis

The chemokine CCL25 is constitutively expressed in the thymus, and its receptor CCR9 is expressed on subsets of developing thymocytes. Nevertheless, the function of CCL25/CCR9 in adult thymopoiesis remains unclear. Here, we demonstrate that purified CCR9(-/-) hematopoietic stem cells are deficient in their ability to generate all major thymocyte subsets including double-negative 1 (DN1) cells in competitive transfers. CCR9(-/-) bone marrow contained normal numbers of lineage(-) Sca-1+c-kit+, common lymphoid progenitors, and lymphoid-primed multipotent progenitors (LMPP), and CCR9(-/-) LMPP showed similar T cell potential as their wild-type (WT) counterparts when cultured on OP9-delta-like 1 stromal cells. In contrast, early thymic progenitor and DN2 thymocyte numbers were reduced in the thymus of adult CCR9(-/-) mice. In fetal thymic organ cultures (FTOC), CCR9(-/-) DN1 cells were as efficient as WT DN1 cells in generating double-positive (DP) thymocytes; however, under competitive FTOC, CCR9(-/-) DP cell numbers were reduced significantly. Similarly, following intrathymic injection into sublethally irradiated recipients, CCR9(-/-) DN cells were out-competed by WT DN cells in generating DP thymocytes. Finally, in competitive reaggregation thymic organ cultures, CCR9(-/-) preselection DP thymocytes were disadvantaged significantly in their ability to generate CD4 single-positive (SP) thymocytes, a finding that correlated with a reduced ability to form TCR-MHC-dependent conjugates with thymic epithelial cells. Together, these results highlight a role for CCR9 at several stages of adult thymopoiesis: in hematopoietic progenitor seeding of the thymus, in the DN-DP thymocyte transition, and in the generation of CD4 SP thymocytes.

Impaired accumulation of antigen-specific CD8 lymphocytes in chemokine CCL25-deficient intestinal epithelium and lamina propria

CCL25 and CCR9 constitute a chemokine/receptor pair involved in T cell development and in gut-associated immune responses. In this study, we generated CCL25(-/-) mice to answer questions that could not be addressed with existing CCR9(-/-) mice. Similar phenotypes were observed for both CCL25(-/-) and CCR9(-/-) mice, consistent with the notion that CCL25 and CCR9 interact with each other exclusively. We assessed the requirement for CCL25 in generating CCR9(high) CD8 intestinal memory-phenotype T cells and the subsequent accumulation of these cells within effector sites. TCR-transgenic naive CD8 T cells were transferred into wild-type or CCL25-deficient hosts. Oral sensitization with Ag allowed these naive donor cells to efficiently differentiate into CCR9(high) memory-phenotype cells within the mesenteric lymph nodes of wild-type hosts. This differentiation event occurred with equal efficiency in the MLN of CCL25-deficient hosts, demonstrating that CCL25 is not required to induce the CCR9(high) memory phenotype in vivo. However, we found that CCL25 deficiency severely impaired the Ag-dependent accumulation of donor-derived CD8 T cells within both lamina propria and epithelium of the small intestine. Thus, although CCL25 is not necessary for generating memory-phenotype CD8 T cells with "gut-homing" properties, this chemokine is indispensable for their trafficking to the small intestine.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.