Postnatal Thymus Research Articles

Defects in early DN cell development and CD4 SP cell selection in Foxn1Lacz mice (82.7)

Abstract The Foxn1 gene is required for thymic epithelial cell (TEC) development and is expressed in the thymus from the fetal to adult stages. To track Foxn1 gene expression, we inserted an IRES-lacZ gene into the 3′UTR in exon 9 of the Foxn1 gene. We found that the postnatal thymus was progressively atrophic in Foxn1Lacz/LacZ mice and displayed a gradual disorganization. The total thymocyte cell number was reduced 10%, 56% and 80% at postnatal day 8, 14, and 28. The percent of CD4 SP cells in Foxn1Lacz/LacZ mice was significant reduced, but not CD8 SP cells compared to control Foxn1+/Lacz mice. The percent of DP cell was slightly increased, and the percent of DN cell was significant increased 2 to 3 fold in day 21 to 28 mice. The percent of DN1 cells was dramatically increased to more than 60% (around 20% in control mice), and more than 80% of these cells were CD19+ cells. The percent of DN2, DN3 cells were correspondingly reduced to 15% (around 60% in control mice), and the percent of DN4 cells was also reduced in Foxn1Lacz/LacZ mice during the period. Our preliminary data suggests that the expression of MHCII on surface of TSC and Foxn1 mRNA from the Foxn1Lacz allele was reduced; furthermore, the phenotype was more severe in Foxn1Lacz/nu mice, indicating that the defects depended on the dosage of the Foxn1 gene. Taken together, our results suggest that reduction of Foxn1expression in the postnatal thymus caused defects in TEC maintenance, resulting in defects in DN1 cells and in positive selection for CD4 SP cells, which ultimately greatly reduces production of T cells in the thymus. Furthermore, the resulting thymic microenvironment is favorable for B cells development in the postnatal thymus. Supported by grant AI055001 NIH.

EVA regulates thymic stromal organisation and early thymocyte development

Epithelial V-like antigen (EVA) is an immunoglobulin-like adhesion molecule identified in a screen for molecules developmentally regulated at the DN to DP progression in thymocyte development. We show that EVA is expressed during the early stages of thymus organogenesis in both fetal thymic epithelia and T cell precursors, and is progressively downregulated from day 16.5 of embryonic development. In the postnatal thymus, EVA expression is restricted to epithelial cells and is distributed throughout both cortical and medullary thymic regions. Transgenic overexpression of EVA in the thymus cortex resulted in a modified stromal environment, which elicited an increase in organ size and absolute cell number. Although peripheral T lymphocyte numbers are augmented throughout life, no imbalance either in the repertoire, or in the different T cell subsets was detected. Collectively, these data suggest a role for EVA in structural organisation of the thymus and early lymphocyte development.

Claudins provide a breath of fresh Aire

'Promiscuous' gene expression in medullary thymic epithelial cells positive for the autoimmune regulator Aire is critical for central T cell tolerance. Expression of the tight-junction components claudin-3 and claudin-4 'marks' Aire+ epithelial cells in the postnatal thymus and their direct precursors in the embryo.

Presence of CD4+ SP and DP (γδ, αβ) T-Cells Expressing BPDE-DNA Adducts in Progeny of Mouse Dams Exposed to Benzo(α) pyrene at Mid-Gestation

After one exposure of C3H/HeJ pregnant females (at mid-gestation) to B(α) P (at 150 μ g/g BW), their progeny evidenced suppression of both humoral and cell-mediated immunity, as well as quantitative deficiencies in the levels of Lyt1+ and Lyt2+cells, and CD4+ CD8+, Vγ 3+, Vγ δ+, and Vα β+T-cells. We hypothesized that these conditions could be a result, in part, of covalent binding of BPDE to DNA within these cells. To test this, antiserum to BPDE-DNA was generated in rabbits; after multiple purification steps, an anti-BPDE-DNA (rendered ≈99.5% specific for BPDE-DNA and did not react with free BPDE or DNA antigens at dilutions even of < 1:50) antiserum was isolated. Flow cytometry and immunofluorescence analyses showed the adduct was present in CD4+ cells of progeny fetal and in both post-natal thymus and spleen tissues. Using a [32P]-post-labelling method, adduct was also detected in samples of fetal liver during the period from Day 15 to 18 gestation. Surprisingly, it was found that thymus cells from B(α)P-exposed mice not exhibiting the adduct could severely suppress allogeneic mixed lymphocyte responses, while those in which the adduct was detected caused had a more pronounced suppression. We suspect from the findings here that the presence of BPDE-DNA adducts in T-cells contributes to, but is not necessarily the causa sola for, the immunosuppression that develops in the offspring of pregnant mothers who are exposed to B(α)P (among many other agents) via smoking, ingestion, or inhalation of environmental pollution.

Expression of Dll4 and CCL25 in Foxn1-negative epithelial cells in the post-natal thymus

Foxn1 transcription factor is known to be essential for development of the thymic organ. We analyzed whether Foxn1 expression in thymic epithelial cells is necessary for the expression of functional molecules such as Delta-like 4 (Dll4) and CCL25, and whether maintenance of these molecular expressions depends on the Foxn1 transcription factor. We show that almost all thymic epithelial cells in the early thymus anlagen express Foxn1, and Dll4 and CCL25 are limitedly expressed in Foxn1-positive epithelial cells. The results are consistent with previous reports suggesting the indispensability of Foxn1 for epithelial cell differentiation which enables these cells to induce the expressions of CCL25 (Bleul, C. C. and Boehm, T. 2000. Chemokines define distinct microenvironments in the developing thymus. Eur. J. Immunol. 30:3371), Dll1 and Dll4 (Tsukamoto, N., Itoi, M., Nishikawa, M. and Amagai, T. 2005. Lack of Delta like 1 and 4 expressions in nude thymus anlages. Cell. Immunol. 234:77). On the other hand, the expression of Foxn1 was not detectable in a large number of post-natal thymic epithelial cells. Both Foxn1-positive and -negative epithelial cells seem to express Dll4 and CCL25. Therefore, the expressions of Dll4 and CCL25 are independent of Foxn1 transcription factor in the post-natal thymus. These results indicate that in the post-natal thymus, epithelial cells may maintain the expressions of those functional molecules without the aid of Foxn1 transcription factor.

Notch1 and IL-7 Receptor Interplay Maintains Proliferation of Human Thymic Progenitors while Suppressing Non-T Cell Fates

Notch signaling is critical for T cell development of multipotent hemopoietic progenitors. Yet, how Notch regulates T cell fate specification during early thymopoiesis remains unclear. In this study, we have identified an early subset of CD34high c-kit+ flt3+ IL-7Ralpha+ cells in the human postnatal thymus, which includes primitive progenitors with combined lymphomyeloid potential. To assess the impact of Notch signaling in early T cell development, we expressed constitutively active Notch1 in such thymic lymphomyeloid precursors (TLMPs), or triggered their endogenous Notch pathway in the OP9-Delta-like1 stroma coculture. Our results show that proliferation vs differentiation is a critical decision influenced by Notch at the TLMP stage. We found that Notch signaling plays a prominent role in inhibiting non-T cell differentiation (i.e., macrophages, dendritic cells, and NK cells) of TLMPs, while sustaining the proliferation of undifferentiated thymocytes with T cell potential in response to unique IL-7 signals. However, Notch activation is not sufficient for inducing T-lineage progression of proliferating progenitors. Rather, stroma-derived signals are concurrently required. Moreover, while ectopic IL-7R expression cannot replace Notch for the maintenance and expansion of undifferentiated thymocytes, Notch signals sustain IL-7R expression in proliferating thymocytes and induce IL-7R up-regulation in a T cell line. Thus, IL-7R and Notch pathways cooperate to synchronize cell proliferation and suppression of non-T lineage choices in primitive intrathymic progenitors, which will be allowed to progress along the T cell pathway only upon interaction with an inductive stromal microenvironment. These data provide insight into a mechanism of Notch-regulated amplification of the intrathymic pool of early human T cell progenitors.

Delayed functional maturation of natural regulatory T cells in the medulla of postnatal thymus: role of TSLP.

BackgroundGeneration of functional CD4+CD8-CD25+ regulatory T cells (Treg) in the murine thymus depends on FoxP3. Removal of the thymus from neonatal mice has been shown to result in a multiple organ autoimmune disease phenotype that can be prevented by introducing the FoxP3+ Treg population to the animal. It has therefore, been proposed that functional FoxP3+ Treg cells are not made in the neonatal thymus; however, it remains unclear when and where functional FoxP3+CD4+CD8-CD25+ thymocytes are generated in postnatal thymus.ResultsWe report that neither FoxP3 mRNA nor protein is expressed in CD4+CD8-CD25+, or CD4+CD8-CD25- thymocytes until 3–4 days post birth, despite the presence of mature CD4+CD8-CD25+/- thymocytes in the thymus by 1–2 days after birth. FoxP3-CD4+CD8-CD25+ thymocytes from day 2 newborn mice show no Treg activity. Interestingly, we are able to detect low numbers of FoxP3+ thymocytes dispersed throughout the medullary region of the thymus as early as 3–4 days post birth. Expression of FoxP3 is induced in embryonic day 17 fetal thymus organ culture (FTOC) after 4–6 days of in vitro culture. Treatment of FTOCs with thymic stromal derived lymphopoietin (TSLP) enhanced expression of FoxP3, and blocking the TSLP receptor reduces FoxP3 expression in FTOC. Furthermore, TSLP stimulates FoxP3 expression in purified CD4+CD8- thymocytes, but not in CD4+CD8+, CD4-CD8+ and CD4-CD8- thymocytes.ConclusionExpression of FoxP3 or Treg maturation is ontogenically distinct and kinetically delayed from the generation of CD4+CD8-CD25+ or CD4+CD8-CD25- thymocytes in the postnatal thymus. TSLP produced from medullary thymic epithelia cells (mTEC) contributes to the expression of FoxP3 and the maturation of natural regulatory T cells. Overall, these results suggest that the development of Treg cells requires paracrine signaling during late stages of thymocyte maturation that is distinct from signaling during positive or negative selection.

CXCR4 and CCR5 mediate homing of primitive bone marrow–derived hematopoietic cells to the postnatal thymus

Factors governing the entry of cells into the postnatal thymus are poorly understood. We aimed to define molecular mechanisms mediating the homing of bone marrow cells to the thymus using a sublethally irradiated in vivo murine model. Entry of unfractionated and lineage-depleted bone marrow cells to the thymus, but not bone marrow, was a Galphai-mediated phenomenon. Lineage-depleted cells that had homed to the thymus expressed abundant CXCR4 and CCR5 mRNA, alone of 17 chemokine receptors evaluated by QPCR. Thymic-homed cells were distinct from cells that had homed to bone marrow in expression of CXCR4 and CCR5 by mRNA quantification and cell-surface expression of protein. Abrogation of CXCR4 and CCR5 function by genetic, antibody, or pharmacologic means impaired homing of lineage-depleted cells to the thymus, although not in a synergistic manner, implying interdependency of these receptors in the homing process. Competitive repopulation experiments demonstrated that inhibiting CXCR4-mediated homing adversely affected the double-negative cell pool at 2 weeks, suggesting that cells with prothymocytic activity may in part home via CXCR4. Overall, our data demonstrate differential homing mechanisms governing entry of unfractionated and lineage-depleted cells to irradiated bone marrow or thymus, with thymic homing of immature cells being pertussis-sensitive and mediated by the chemokine receptors CXCR4 and CCR5.

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.

Critical Role of flt3 in Fetal and Adult T Lymphopoiesis.

We recently demonstrated that signalling through the cytokine tyrosine kinase receptor flt3 and interleukin-7 receptor α (IL-7Rα ) is indispensable for fetal and adult B cell commitment and development (Sitnicka et al., J. Exp. Med . 198: 1495, 2003). We have also shown that flt3 ligand (FL)-deficient mice have reduced levels of the common lymphoid progenitor (CLP) but normal levels of mature T cells (Sitnicka et al., Immunity , 17:463, 2002). However, previous studies failed to demonstrate a role of flt3 and its ligand in T lymphopoiesis. Herein, although having normal numbers of thymocytes, FL-deficient mice were shown to have distinct reductions in the earliest double negative (DN) progenitors in the fetal, postnatal and adult thymus. A critical role of flt3 in thymocyte development was most evident in the absence of Interleukin-7 receptor a (IL-7Rα ) signalling. Strikingly, in adult FL−/−IL-7Rα −/− (double deficient) mice thymic cellularity was reduced 400 fold as compared to FL−/− and wild type (WT) controls and 30 fold as compared to IL-7Rα −/− mice. In agreement with previous studies, IL-7Rα −/− thymocytes revealed a partial block at the progression from the DN2 (CD4−CD8−CD44+CD25+) to DN3 (CD4−CD8−CD44−CD25+) stage, while in FL−/−IL-7Rα −/− mice DN1 (CD4−CD8−CD44+CD25−), DN2 and DN3 thymic progenitors were undetectable. The DP CD4+CD8+ thymocytes were reduced more than 1000 fold as compared to IL-7Rα −/− mice and low levels of peripheral T cells remaining in FL−/−IL-7Rα −/− mice had exclusively an activated/memory phenotype. Furthermore, FL−/−IL-7Rα −/− bone marrow cells lacked T cell reconstituting potential after transplantation into lethally irradiated recipients. Taken together these data demonstrate indispensable and complementary roles of flt3 and IL-7Rα in T lymphopoiesis.

Up-Regulation of IL-7, Stromal-Derived Factor-1α, Thymus-Expressed Chemokine, and Secondary Lymphoid Tissue Chemokine Gene Expression in the Stromal Cells in Response to Thymocyte Depletion: Implication for Thymus Reconstitution

Three in vivo adult mouse models were established to study which signals are required to restore the postnatal thymus. Single administration of dexamethasone, estradiol, or exposure to sublethal dose of gamma irradiation served as prototype thymus-ablating therapies. In all models, transient thymic atrophy was manifested due to the loss of the predominant portion of CD4- CD8- double negative and CD4+ CD8+ double positive thymocytes and was followed by a complete regeneration of the thymuses. Acute atrophy/regeneration was observed in the dexamethasone and irradiation models; in the estradiol-treated animals, slow kinetics of atrophy and regeneration was observed. Importantly, in both acute and chronic models, high levels of IL-7 mRNA were detected in the thymuses isolated from mice during maximum atrophy. In addition, chemokine gene array analysis of involuted thymuses revealed high levels of mRNA expression of stromal-derived factor-1alpha (SDF-1alpha), thymus-expressed chemokine (TECK), and secondary lymphoid tissue chemokine (SLC) but not of other chemokines. The levels of IL-7, SDF-1alpha, TECK, and SLC mRNA inversely correlated with the kinetics of regeneration. RT-PCR analysis of stromal cells purified from involuted thymuses confirmed increased IL-7, SDF-1alpha, and SLC gene expression in MHC class II+ CD45- epithelial cells and increased IL-7 and TECK gene expression in class II+ CD45+ CD11c+ dendritic cells. Thus, our data showed for the first time that expression of IL-7, SDF-1alpha, TECK, and SLC mRNA is induced in the thymic stroma during T cell depletion and may play an important role in the reconstitution of the adult thymus.

Thymic regulatory T cells

Several types of T regulatory (Treg) cells have been described in both mice and humans, including natural or professional (CD4+CD25+ T cells) and adaptive (Th3 and Tr1 cells) Treg cells. The former develops in the thymus and results in an endogeneous long-lived population of self-antigen-specific T cells in the periphery poised to prevent potentially autoimmune reactions. The second subset develops as a consequence of activation of mature T cells under particular conditions of sub-optimal antigen exposure and/or costimulation. Natural Treg cells are positively selected in the cortex through their TCR interactions with self-peptides presented by thymic stromal cells. It is likely that this high-affinity recognition results in signals rendering them anergic and able to produce anti-apoptoptic molecules which protect them from negative selection. Recently, small subsets of CD4+CD25+ and of CD8+CD25+ cells sharing similar characteristics have been detected in human fetal and post-natal thymuses. Both CD4+CD25+ and CD8+CD25+ human thymocytes express Foxp3 and GITR mRNA, as well as surface CCR8 and TNFR2 and cytoplasmic CTLA-4 proteins, which are common features of mature Treg cells. Following activation they do not proliferate or produce cytokines, but express surface CTLA-4 and TGF-β1. They suppress the proliferation of autologous CD4+CD25− thymocytes to allogeneic stimulation by a contact-dependent mechanism related to the combined action of surface CTLA-4 and TGF-β leading to the inhibition of the IL-2R α chain on target T cells. Lastly, both CD4+CD25+ and CD8+CD25+ Treg thymocytes exert strong suppressive activity on Th1, but much lower on Th2 cells, since these latter may escape from suppression via their ability to respond to growth factors other than IL-2. Treg cells that develop in, and emerge from, the thymus are certainly responsible for the maintenance of self-tolerance and prevention of autoimmune disorders. The result that Th1 cells are highly susceptible to the suppressive activity of Treg thymocytes is consistent with the important role of these cells in protecting against the Th1-mediated immune response to autoantigens.

Limited correlation between human thymus and blood NKT cell content revealed by an ontogeny study of paired tissue samples

NKT cells are a CD1d-restricted T cell subset with strong immunoregulatory properties. Human NKT deficiencies are associated with autoimmune diseases such as type 1 diabetes and several types of cancer, yet there is little understanding of how the human NKT cell pool develops or is maintained. In this study, we present the first detailed analysis of human NKT cells from donor-matched postnatal thymus and blood samples. In mice, NKT cells are a thymus-dependent population that migrates to the periphery at an immature stage. Our data show that human NKT cells also undergo early stages of development in the thymus, forming a CD4(+)CD161(-/low) population that predominates neonatal thymic and blood NKT cell pools. CD4(-) and CD161(+) NKT cells accumulate with age in the blood, but not thymus, to the point that they dominate the NKT cell compartment in adult blood. This is consistent with the post-thymic maturation of NKT cells exported from the thymus at the putatively immature CD4(+)CD161(-/low) stage. Interestingly, while thymus and peripheral NKT cell frequencies vary widely between patients and are relatively stable between age groups, there is no clear relationship between the NKT cell frequency in thymus and blood.

The effects of cytotoxic and immunosuppressive agents on thymic reconstitution

One key component of long-term outcome after stem cell transplant (SCT) is successful reconstitution of the immune system. In this regard, effective reconstitution of antigen-specific immunity requires de novo T cell generation. Bone marrow derived progenitors seed the thymus and undergo a complex process involving lineage commitment, proliferation and selection. Coordinated interaction of marrow-derived lymphoid progenitors with thymic stromal cells is required for successful T lymphopoiesis in the post-natal thymus.

Ontogeny and Regulation of IL-7-Expressing Thymic Epithelial Cells

Epithelial cells in the thymus produce IL-7, an essential cytokine that promotes the survival, differentiation, and proliferation of thymocytes. We identified IL-7-expressing thymic epithelial cells (TECs) throughout ontogeny and in the adult mouse thymus by in situ hybridization analysis. IL-7 expression is initiated in the thymic fated domain of the early primordium by embryonic day 11.5 and is expressed in a Foxn1-independent pathway. Marked changes occur in the localization and regulation of IL-7-expressing TECs during development. IL-7-expressing TECs are present throughout the early thymic rudiment. In contrast, a major population of IL-7-expressing TECs is localized to the medulla in the adult thymus. Using mouse strains in which thymocyte development is arrested at various stages, we show that fetal and postnatal thymi differ in the frequency and localization of IL-7-expressing TECs. Whereas IL-7 expression is initiated independently of hemopoietic-derived signals during thymic organogenesis, thymocyte-derived signals play an essential role in regulating IL-7 expression in the adult TEC compartment. Moreover, different thymocyte subsets regulate the expression of IL-7 and keratin 5 in adult cortical epithelium, suggesting that despite phenotypic similarities, the cortical TEC compartments of wild-type and RAG-1(-/-) mice are developmentally and functionally distinct.

Cytotoxic Injury to Thymic Stroma and Impaired Reconstitution.

A key component of long-term outcome after stem cell transplant (SCT) is successful reconstitution of the immune system. Effective reconstitution of antigen-specific T-cell immunity requires de novo T cell generation. Bone marrow derived progenitors seed the thymus and undergo a complex process involving lineage commitment, proliferation and selection. Coordinated interaction of marrow-derived lymphoid progenitors with thymic stromal cells is required for successful T lymphopoiesis in the post-natal thymus. Disruption of the microenvironment can result in disrupted T cell lymphopoiesis. One cause of prolonged defects in generating functional T lymphocytes after BMT is damage to the thymic microenvironment induced by radiation or cytotoxic therapy. However, the impact of individual agents, administered at myeloablative or non-myeloablative doses, on the thymic microenvironment has not been fully evaluated. In addition, mechanisms by which stromal injury modifies T cell production and maturation have only begun to be understood. We have developed a model system using immunodeficient mice as a platform on which to assess thymic reconstitution. The thymus of mice deficient for the alpha chain of the IL-7 receptor (IL7R−/−) is relatively depleted of lymphoid cells and can be reconstituted following transplant of wild type marrow administered without myeloablative or immunosuppressive treatment. Injection of low doses of wild type bone marrow into these mice results in low levels of marrow chimerism and a normally cellular thymus repopulated with donor-derived lymphocytes. The ability to achieve this reconstitution appears to depend on absolute numbers of early intra-thymic precursors, rather than on total thymic cellularity. We have exploited this model to differentially assess the effects of cytotoxic agents including radiation and immunosuppressive drugs, on the capacity of the thymic microenvironment to support the maturation of normal lympoid progenitors (Figure 1). We demonstrate that some agents do not affect the ability of the thymic microenvironment to support reconstitution (eg fludarabine), others nearly ablate it (cyclophosphamide). We are also able to show dose, schedule, and synergistic effects on the ability of the thymic microenvironment to support de novo T cell lymphopoeisis. Distinct morphologic and phenotypic effects can be demonstrated by different agents (eg busulfan versus thiotepa) with preliminary data suggesting that the effects are mediated by injury to different stromal subsets. It is anticipated that this information will lead to strategies to both minimize delayed immune reconstitution and to augment T cell lymphopoiesis post-transplant. In addition, further evaluation of impaired thymic reconstitution will augment the understanding of lymphostromal interactions crucial to normal T cell lymphopoiesis. [Display omitted]

In Vitro T Cell Differentiation from Human Hematopoietic Stem Cells (HSC) and Lymphoid Progenitors.

The study of human thymopoiesis has been hampered by our inability to provide all the signals of the normal thymic microenvironment to human stem and progenitor cells in vitro. Although T cell production from CD34+ progenitors can be achieved in vitro, purified populations of the immature CD34+CD38- cells (which include HSC) do not generate T cells reliably with the existing assays. The most commonly cited system for human T cell assay, Fetal thymic organ culture (FTOC) from immune deficient mice, is logistically very difficult and low seeding efficiency precludes its use for clonal assays. Recently, a murine bone marrow stromal line engineered to constitutively express Delta-1 ligand (OP9-DL1 stroma), has been shown to support production of mature murine T cells from HSC [Schmitt, 2002] and ES cells [Schmitt, 2004]. Here we report the use of modifications to the OP9-DL1 system that allow assay of human T lymphopoiesis from HSC and progenitors. CD34+lin-CD38- cells (HSC) from cord blood (CB) and bone marrow (BM)cultured on OP9 control stroma (that does not express DL) produced CD19+ B cells and CD56+ NK cells and maintained CD34+ cells for several weeks. However, T cell commitment was not evident in these control cultures. In contrast, culture of HSC on OP9-DL1 stroma (Dr. Zuniga-Pflucker) resulted in robust growth of T cell precursors (as shown by expression of CD7 and cytoplasmic (cy) CD3 in over 50% of cells) and mRNA for pre-Tα. NK cells were also increased on OP9-DL1 relative to control OP9 stroma but B cell production was lost. Although T cell commitment to the CD34+CD7+ and CD34-cyCD3+CD7+ cells stages was accomplished on OP9-DL1 stroma, markers of further T cell maturation such as surface CD3, CD4 and CD8, were absent on the cells produced. Multiple combinations of growth factors (IL-7, IL-2, IL-15, IL-3, ckit ligand (KL) and thrombopoietin (Tpo)) to the OP9-DL1 stromal cultures failed to achieve further T cell maturation, although the combination of KL, Tpo, IL-7 produced optimal cell growth. However, when conditioned medium collected from human thymic stromal cultures (thyCM) (medium collected from adherent cells derived from human postnatal thymus and filtered through 0.45u) was added to OP9-DL1 co-cultures, cyCD3+ cells were increased and differentiation of CD3+CD4+ cells was achieved. Thymic CD34+lin-CD7+ cells produced large numbers of CD3+CD4+CD8+, CD4+CD8- and CD4-CD8+ cells in this system. We have now used the OP9-DL1/thyCM system to demonstrate the T cell potential of CB and BM CD34+CD38-lin-CD7- (HSC) and CB CD34+CD38-lin-CD7+ (CLP). In addition, efficient cloning of single CB HSC and primitive thymic progenitors has been achieved using this culture system. The data suggests that, although DL-1 is required and sufficient for early T lymphoid commitment, other factor(s) specific to human thymic stroma are required for production of more mature T cells; these factors can now be identified using the OP9-DL1 system. The availability of a simple monolayer culture system that allows T lymphoid commitment from HSC and progenitors and further differentiation to mature T cells will be of great value in the study of the lineage potential of progenitor populations and the regulation of human thymopoiesis.

Postnatal thymus transplantation with immunosuppression as treatment for DiGeorge syndrome

AbstractComplete DiGeorge syndrome is a fatal congenital disorder characterized by athymia, hypoparathyroidism, and heart defects. Less than half of patients are 22q11 hemizygous. The goal of this study was to assess if immune suppression followed by postnatal thymus transplantation would lead to T-cell function in 6 infant patients who had host T cells at the time of transplantation. All infants had fewer than 50 recent thymic emigrants (CD3+CD45RA+CD62L+) per cubic millimeter (mm3) and all had some proliferative response to the mitogen phytohemagglutinin. Four infants had rash, lymphadenopathy, and oligoclonal populations of T cells in the periphery. Five of 6 patients are alive at the follow-up interval of 15 months to 30 months. The 5 surviving patients developed a mean of 983 host CD3+ T cells/mm3 (range, 536/mm3-1574/mm3), a mean of 437 recent thymic emigrants/mm3 (range, 196/mm3-785/mm3), and normal proliferative responses to phytohemaglutinin (follow-up from day 376 to day 873). The TCR repertoire became polyclonal in patients who presented with oligoclonal T cells. All patients had thymopoiesis on allograft biopsy. Postnatal thymus transplantation after treatment with Thymoglobulin shows promise as therapy for infants with complete DiGeorge syndrome who have significant proliferative responses to mitogens or who develop rash, lymphadenopathy, and oligoclonal T cells.

Endocrine activity and developmental toxicity of cosmetic UV filters--an update.

UV filters represent a new class of endocrine active chemicals. In vitro, 8/9 chemicals showed estrogenic (MCF-7 cells), and 2/9 antiandrogenic activity (MDA-kb2 cells). Six/nine filters (benzophenone (Bp)-1, Bp-2, Bp-3, 3-benzylidene camphor (3-BC), 4-methylbenzylidene camphor (4-MBC), octyl-methoxycinnamate (OMC)) increased uterine weight in immature rats. 3-Benzylidene camphor and 4-MBC displaced 16alpha125I-estradiol from human estrogen receptor (ER)beta, not ERalpha. Developmental toxicity of 4-MBC (0.7-47 mg/kg body weight/day) and 3-BC (0.24-7 mg/kg), administered in chow was investigated in Long Evans (LE) rats. Weight gain of pregnant rats was reduced only by 3-BC, early postnatal survival rate and thymus weight by both compounds at higher doses. 4-Methylbenzylidene camphor and 3-BC delayed male puberty, and dose-dependently affected reproductive organ weights of adult male and female F1 offspring, with partly different effect patterns. Thyroid weight was increased by higher 4-MBC doses. Tissue-specific changes in mRNA levels of estrogen-regulated genes in prostate, uterus and brain regions, determined by real-time PCR, and in their response to acute estradiol challenge in adult gonadectomized offspring were observed. Lowest effective doses were 0.24 mg/kg/day for 3-BC and 7 mg/kg/day for 4-MBC. Fat tissue levels at 7 mg/kg 4-MBC (GC-MS) approached the range of UV filters in fish (Nagtegaal et al., 1997; Balmer et al., 2004).

The TALE homeodomain protein Pbx2 is not essential for development and long-term survival.

Pbx2 is one of four mammalian genes that encode closely related TALE homeodomain proteins, which serve as DNA binding partners for a subset of Hox transcription factors. The expression and contributions of Pbx2 to mammalian development remain undefined, in contrast to the essential roles recently established for family members Pbx1 and Pbx3. Here we report that Pbx2 is widely expressed during embryonic development, particularly in neural and epithelial tissues during late gestation. Despite wide Pbx2 expression, mice homozygous mutant for Pbx2 are born at the expected Mendelian frequencies and exhibit no detectable abnormalities in development and organogenesis or reduction of long-term survival. The lack of an apparent phenotype in Pbx2(-)/(-) mice likely reflects functional redundancy, since the Pbx2 protein is present at considerably lower levels than comparable isoforms of Pbx1 and/or Pbx3 in embryonic tissues. In postnatal bone marrow and thymus, however, Pbx2 is the predominant high-molecular-weight (MW)-isoform Pbx protein detectable by immunoblotting. Nevertheless, the absence of Pbx2 has no measurable effect on steady-state hematopoiesis or immune function in adult mice, suggesting possible compensation by low-MW-isoform Pbx proteins present in these tissues. We conclude that the roles of Pbx2 in murine embryonic development, organogenesis, hematopoiesis, immune responses, and long-term survival are not essential.