Print this page

MEM INST OSWALDO CRUZ, RIO DE JANEIRO, 99(4) June 2004
PAGES: 381-388 DOI: Full paper
Differential expression of notch signaling-related transcripts accompanies Pro-thymocyte proliferation and phenotype transition Induced by Epidermal Growth Factor plus Insulin in fetal thymus Organ Cultures

Claudia Sondermann FreitasII +, Sergio Ranto DalmauI, Eliana AbdelhayII

Programa de Medicina Experimental, Instituto Nacional de Câncer, Rio de Janeiro, RJ, Brasil
IDepartamento de Bioquímica, Universidade do Estado do Rio de Janeiro, Rio de Janeiro, RJ, Brasil
IILaboratório de Biologia Molecular Maury Miranda, Instituto de Biofísica, Universidade Federal do Rio de Janeiro, Ilha do Fundão, CCS, Bloco G, sala G1059, 21949-900, Rio de Janeiro, RJ, Brasil

617 DOWNLOADS 1407 VIEWS
Abstract

Thymus regression upon stressing stimuli, such as infectious diseases, is followed by organ reconstitution, paralleling its development in ontogeny. A narrow window of thymus development was here studied, encompassing the pro-T lymphoid precursor expansion during specification stages, by the use of epidermal growth factor plus insulin (INS) in murine fetal thymus organ cultures. Aiming to disclose signaling pathways related to these stages, cultured thymus lobes had their RNA extracted, for the search of transcripts differentially expressed using RNAse protection assays and reverse transcriptase-polymerase chain reactions. We found no difference that could explain INS-driven thymocyte growth, in the pattern of transcripts for death/proliferation mediators, or for a series of growth factor receptors and transcriptional regulators known as essential for thymus development. Thymocyte suspensions from cultured lobes, stained for phenotype analysis by fluorescence activated cell sorting, showed a decreased staining for Notch1 protein at cell surfaces upon INS addition. We analyzed the expression of Notch-related elements, and observed the recruitment of a specific set of transcripts simultaneous and compatible with INS-driven thymocyte growth, namely, transcripts for Notch3, for its ligand Jagged2, and for Deltex1, a mediator of a poorly characterized alternative pathway downstream of the Notch receptor.

key words:

Thymus plays a central role in the development of immunity. As a target organ in infectious diseases, it displays a plastic behavior, its regression in response to infectious or stressing agents being followed, as soon as these stimuli are discontinued, by a prompt reconstitution that in many aspects mimics ontogeny (Savino et al. 1992). The organ development involves an ordered sequence of bi-directional signaling events between two cellular compartments, a complex microenvironment that eventually turns permissive to seeding by pluripotent hematopoietic precursors, and the progeny of the last (Anderson 2000). These lead, finally, to the exportation of mature abTCR-bearing T-lymphocytes to periphery, besides minority subsets as products of divergent pathways (Rodewald 1995). Therefore, under microenvironmental influence, pluripotent precursors undergo a progressive specification, narrowing their potential to generate non-T descendants, and acquiring essential T-lymphoid characteristics (Rothenberg 2002). While intrathymic double-negative (CD4-CD8-) pro-T cells (DN1 to 3) can still diverge to other lymphoid (B, NK) and, under defined conditions, even to myeloid fates, the pre-T subset (DN4) represents cells committed to the mainstream abTCR fate. They show differentiated requirement for growth factors, down-regulating c-kit (stem cell receptor), expressing interleukin-7 receptor (IL7R) only at low levels (Hattori et al. 1996a, DiSanto et al. 2000), and expanding upon pre-TCR signaling. Once thymocytes acquire a DP (double-positive, CD4+CD8+) phenotype, they must endure a series of TCR-mediated checkpoints, encompassing the survival to selective events that result in massive death, and the expansion of positively selected subsets, finally leaving thymus as mature CD4+ or CD8+ T- lymphocytes (Von Boehmer et al. 1997).

Precursor growth in the mouse thymus occurs at many points of the journey. While DN1 cells (CD44+25-) show limited expansion, CD25 acquisition is followed by 2 to 3 cell cycles, at the DN2 (CD44+25+) and DN3 (CD44-25+) stages (Pénit et al. 1995). Two more cell cycles, corresponding to the transition from a DN3 pro-T to a DN4 (CD44-25-) pre-T phenotype, result from the productive TCRb-chain gene rearrangement and CD3-driven, p56lck-mediated signaling (Pénit et al. 1995). While most DN3 cells express intracellular (IC) CD3e, only 10-30% of them can productively rearrange the gene for TCRb- chain and express IC TCRb as well, otherwise they undergo apoptosis at DN4 (Falk & Eichman 2002). At this point, cell death signal involves the adaptor protein Fas-associated-with-a-death-domain (FADD) that would, conversely, promote proliferation in cells able to express a pre-TCR, in the so-called beta-selection (Newton et al. 2000). Thus, in normal physiology, cells cycling in DN3 stage are likely to be a mixture of cells finishing the cell cycles induced at the DN2 step and those starting the b-chain-, lck- mediated proliferation at DN4 (Pénit et al. 1995).

While studying the effect of growth factors upon thymus development in vitro, using fetal thymus organ cultures (FTOC), we observed that exogenously added epidermal growth factor (EGF) could, complete but reversibly, block both growth and differentiation of the abTCR, but not of gdTCR lymphocytes (Freitas et al. 1998a), while provoking extensive microenvironment disarrangement. Cells remnant from EGF-FTOC showed a DN1/2 surface phenotype. The simultaneous addition of insulin (INS) prompted pro-T cells to acquire a majority DN3 phenotype (Freitas et al. 1998b), besides inducing microenvironment changes (unpublished results). These observations provided us with a narrow window of analysis of growth and differentiation in the thymus during T-cell specification stages.

Aiming to disclose signaling pathways related to this particular window of thymus development, we searched for transcripts differentially expressed by EGF- and EGF+INS-FTOC, starting with those coding for factors implied in early thymus development and/or T-lymphoproliferative diseases. These included death-related mediators, growth factor receptors and transcription regulators essential during T-cell specification stages, as well as members of Notch family and their ligands, known to intermediate lympho-stromal relationships.

Notch family, in vertebrates, comprises a group of cell surface proteins homologous to Drosophila Notch that, upon binding to ligands such as Delta-like and Jagged (respective homologous of Drosophila Delta and Serrate), mediate interactions between contiguous cells (Osborne & Miele 1999). Being essential for embryonic development, Notch action resumes further in many organs and systems, included the immunological system, mediating the microenvironmental inductive events that lead to growth, death and differentiation decisions (Osborne & Miele 1999). For example, Notch and its ligands are essential for the Common Lymphoid Precursors decision between a B- or T- lymphoid fate (Radtke et al. 1999, Schmitt & Zúñiga-Pflücker 2002, Hozumi et al. 2003).

We here report the sudden expression of a specific set of Notch-related transcripts, simultaneous and compatible with the lymphoid precursor growth and the one-step-limited thymocyte differentiation here analyzed.

 

MATERIALS AND METHODS

Animals - C57BL6/J mice, originated from the animal facilities of the National Cancer Institute of Rio de Janeiro, were bred in the Laboratory of Transgenic Animals, Federal University of Rio de Janeiro. Females bred overnight were separated from males in the morning (day 0). At day 14, pregnant females were killed by cervical dislodgement, and fetuses were harvested under sterile conditions.

FTOC - The cultures were performed as described in the literature, with some modifications. Fourteen-day-fetal thymuses, containing hematopoieitc precursors up to the pro-T stage, were used. They were excised by the use of watchmaker forceps, cleaned from contaminant tissues with the aid of surgical needles, and assembled (5-10 per dish) on a 0.22 µm Millipore membrane sustained by a stainless steel grid inside a delta Nunc plate (Nunclon, Roskilde, Denmark), containing 1.5 ml of culture medium (DMEM 310 mOsm) plus 10% fetal calf serum (FCS) (defined serum, Hyclone, Logan, UT, US), glutamine, non-essential amino acids (Life Technologies, Gaithersburg, MD, US), 60 mg/l penicillin and 100 mg/l streptomycin. Medium was changed after 3 days, and lobes were harvested after 7 days of culture, when were pooled and stored at _70oC before processing. A series of cultures were performed, and two different pools of lobes cultured in each condition were used for RNA extraction.

Cytokines - Natural EGF from mouse submaxillary glands (Sigma Chem. Co., St. Louis, MO, US) was added to a 100 ng/ml final concentration. Purified porcine and bovine insulin was obtained from CEME (Brazil), and added to a 20 nM final concentration.

Reverse transcriptase-polymerase chain reactions (RT-PCR) - Cultured lobes from FTOC-C, FTOC-EGF, and FTOC-EGF+INS were harvested, pooled, and their total RNA extracted by the use of TRIZOL (Life Technologies). The cDNA was synthesized from 2 µg each RNA by the use of Superscript TMII RT and oligo-dT (Life Technologies). PCR amplification used Taq DNA polymerase (Promega, Madison, WI, US), and specific primers (Invitrogen, SP, Brazil) as described in Table I. PCR assays started with 3 min at 94oC, followed by 35 cycles of 94oC for 45 s, variable annealing temperatures for 30 s, and 72oC for 1.5 min, with a final extension for 10 min at 72oC. Reaction products were analyzed in 1.5% agarose gels in TAE containing ethidium bromide.

RNAse protection assays (RPA) - Five µg of total RNA extracted from pools of intact cultured lobes were subjected to RPA, using [a-32P] UTP radiolabelled mAPO-2 and mAPO-3 probe sets as specified by the manufacturer (BD Pharmingen, CA, US). The resulting protected RNAs were resolved on 5% denaturing polyacrylamide gels and exposed to x-ray films.

Fluorescence activated cell sorting (FACS) analyses - Thymus lobes from FTOCs were smashed under a glass coverslip, and the lymphocytes recovered were surface- stained with anti-mouse NK1.1 monoclonal antibodies (moAbs) (clone PK136, PE-conjugated, Southern Biotechnology Associates, AL, US), or with anti-Notch1 moAbs (clone 18G, kindly sent by Dr David Flowers, Seattle, WA, US) plus PE-conjugated goat anti-rat IgG (Gibco/BRL, Life Technologies). Unspecific labeling was prevented by pre-incubation of the cells with Fcg-blocker (BD Pharmingen). Dead cells were excluded from analyses by the addition of propidium iodide (2 µg/ml final). Intracellular TCRb staining followed fixation of the cells with 1% paraformaldehyde and permeabilization with 0.01% Tween-20; anti- mouse TCRb antibodies (rat Ig, clone H57-597, biotin-conjugated, Gibco) and Streptavidin-Tricolor (Caltag, CA, US) were used. Cytofluorometric acquisition was performed in a FACScan apparatus (Becton Dickinson, San Jose, CA, US) equipped with a 15-mW air-cooled 488-nm argon-ion laser. Data acquisition was carried out using the LYSIS II software program (BDIS).

 

RESULTS

RPA assessment of differential expression of death- related transcripts in EGF- and EGF+INS-FTOCs -Proliferation and rescue from death are hardly unraveled processes in the thymus, sometimes sharing signal transducers, as is the case of FADD (Newton et al. 2000). We thus investigated the expression of death-related transcripts, some of which are developmentally regulated in this organ (Chao & Korsmeyer 1998, Tomayko et al. 1999, Newton et al. 2000). We used RPA with probe sets that cover both pro- and anti-apoptotic factors associated with Bcl-family- and TNFR-family-related signaling. If normalized for loading on the basis of constitutively expressed transcripts (G3PDH and L32), similar patterns of expression were seen in both EGF- and EGF+INS- transcripts, including those for FADD (Fig. 1). Low levels of signal for Fas were present in these RNAs, when compared to Control-FTOC-derived RNAs (Fig. 1A), consistent with Fas expression being restricted, among immature subsets, to DP cells (Newton et al. 2000). The overall ratio of Bcl-family death antagonists to agonists, which determines the susceptibility to death stimuli (Chao & Korsmeyer 1998), was also maintained in EGF- and EGF+INS- derived transcripts (Fig. 1B).

Availability of transcripts coding for factors critical during T-cell specification stages in the thymus Fig. 2shows results from RT-PCR assays, comparing EGF- and EGF+INS-FTOCs levels of expression of transcripts for factors known to be essential during early thymus development. No difference that could explain the proliferation blockade and its reversal by INS was observed. Transcripts for the growth factor receptors IL7Ra and c-kit were present in EGF-FTOCs, and thus seemingly were not growth-limiting, although their contents in EGF+INS-FTOC seemed to vary in keeping with thymocyte developmental stages (Hattori et al. 1996a, DiSanto et al. 2000).

Transcripts for gene regulators such as GATA 3, that here was not correlated with cellular proliferation as was reported in literature (Hattori et al. 1996b, Hendriks et al. 1999), for TCF1a (Hattori et al. 1996b), PU.1 (Anderson et al. 2002), c-MYB (Rothenberg 2002), c-MYC (Douglas et al. 2001), SCL and HEB (Herblot et al. 2000), E47 (E2A) and its inhibitor Id2 (Anderson et al. 2002), as for the T-cell receptor gene-rearranging enzymes RAG-1 and RAG-2, were seen in both conditions (Fig. 2A). Transcripts for the two pT-a isoforms were also observed in both situations (Fig. 2B). The smallest (pTaa) was reported as preferentially represented in early thymocyte subsets, besides T-lymphoid malignancies, while the expression of the largest transcript (pTab) would represent a delayed event in development, preparing beta-selection (Ramiro et al. 2001).

Cytofluorometric assays - TCR beta-chains, however, necessary for the assembly of pre-TCR complexes and for beta-selection to occur, seemed absent, since FACS analyses revealed a negative IC staining with anti-TCRb chain-Abs in both EGF- and EGF+INS-FTOC (Fig. 3A). Unselected DN thymocytes could still diverge into alternative routes, such as for an NK fate. Surface staining for the NK1.1 marker, however, showed this was not the case (Fig. 3B). Analyses of surface characteristics of the lymphoid remnants from FTOCs revealed a high Notch1 surface staining for EGF-FTOC, with a shift of the fluorescence peak to an intermediate pattern, for EGF+INS-FTOCs (Fig. 3C). Thus, Notch1 down-modulation from thymocyte surfaces suggested an involvement of Notch- mediated signaling in INS-driven effects.

Differential expression of transcripts involved with Notch signaling - RT-PCR assays revealed that INS addition to EGF-FTOC resulted in a clear-cut change in the expression of transcripts involved in Notch signaling. Besides transcripts for Notch1, its ligands Jagged1, Preadipocyte factor-1 (Pref-1; delta-like; dlk), Delta-like1 (Dll1), and Delta-like4 (Dll4), and for the Notch target genes "Hairy and Enhancer of Split" 1 (HES1), the EGF+INS-FTOC- derived RNA contained those for Notch3, as well as for Jagged2, and for the downstream modulator of Notch action, Deltex1 (Fig. 4).

 

DISCUSSION

The interactions between lymphoid and stromal elements in early thymus still remain partially obscure (Allman et al. 2002, Harman et al. 2003, Hozumi et al. 2003), as is the role of intermediate messengers of transduction pathways in developmental processes of the thymus (Rothenberg 2002). We here intended to depict, among the many parameters reported as taking part of thymus development, those which expression could be associated with the induction of thymocyte growth and one-step-limited phenotype transition in our model. Briefly, after a 7-day culture, typically around 1-4 x 104 thymocytes per lobe could be recovered from EGF-FTOC (DN1/2, surface phenotype: CD44+25-/int, CD3-, 4-, 8-, 2-, 16/32+, abTCR-), while 1-2 x 105 thymocytes per lobe from EGF+INS-FTOC (DN3, CD44-25+, CD3-, 4-, 8-, 2-, 16/32+, abTCR-) (Freitas et al. 1998b). Since lymphoid cells recovered from EGF+INS-FTOC in their majority have not missed CD16/32 staining nor acquired CD2 (Freitas et al. 1998b), an impairment to progress to a DN4 stage (CD44-25-, CD3-, 4-, 8-, 2+, 16/32, abTCR-) was suggested, or alternatively, their putative death at this stage. Being surface-negative for NK1.1 (Fig. 3B), their expansion seemingly did not represent a diversion of precursors into this alternative route. They seemed not yet definitively committed to a T-cell fate, as judged by negative staining for both surface (Freitas et al. 1998b) and IC TCRb-chain protein (Fig. 3A). The INS-driven thymocyte growth could not be abolished by tyrphostin PP2, a specific lck inhibitor (Freitas et al. 1998b), showing that it indeed represents the "first wave of thymocyte growth" (Pénit et al. 1995), not mediated by pre-TCR-lck. This expansion of intra-thymic precursors occurred even in the absence of EGF, and accounted for an accelerated thymus recovery after EGF subtraction from EGF-FTOCs, when INS was added, compared to lobes in INS absence (unpublished observations). Thus, despite a putative multi-factorial and indirect action, the introduction of the exogenous growth factors to FTOC allowed us to focus specifically at the changes that result in thymocyte proliferation during CD25 acquisition, in the transition from DN1/2 to DN3 T-cell specification stages, taking advantage of an ex-vivo model that does not rely on transformed cells or constitutively active Notch constructs.

Assessment of the expression of death-related mediators in RPA assays, comparing EGF- to EGF+INS-derived RNA, did not provide evidence for any important difference in the availability of transcripts for "death/ proliferation mediators" that could account for INS-induced growth, unless if upon post-translational mechanisms, or if related to the expression of alternative death receptors/ligands here not tested (Chao & Korsmeyer 1998). The higher levels of transcripts for BclxL/S than for Bcl-2 observed were unexpected, in view of literature. DN thymocytes were reported to express Bcl-2 but not Bcl-xL, in a reciprocal feedback mechanism of regulation (Chao & Korsmeyer 1998). It is possible that here a cell type other than lymphoid contributed to Bcl-xL transcript levels, since intact lobes, containing also stromal components, were used for RNA extraction. Anyway, if an imbalance of the relative levels of transcripts for BclxL/S and Bcl-2 were to be accounted for the inability of DN3 cells to evolve to a DN4 phenotype in our model, what was not tested, it would not account for the INS-driven growth, since the levels were similar in both culture conditions.

Considering that the transition to the DN3 phenotype was not a 100% event, it would not be surprising that transcripts seen in the DN1 and DN2 stages could still be observed among RNAs from EGF+INS-FTOC. Newly expressed messages, however, were considered as representative of the new condition. We found a clear-cut differential expression of Notch signaling- related transcripts, comprising thymocyte/stromal ligands and downstream effectors, simultaneous and compatible with proliferation and DN1/2 to DN3 phenotype transition. This differential expression of Notch transcripts seemed in keeping with that reported for early thymus development. Notch3 expression, for example, was seen at the end of the "HES1-dependent CD25+ proliferation" (Tomita et al. 1999). Notch3-related unregulation of thymocyte development specifically affected late DN cells, while Notch1 impairment would be critical at earlier points (Wolfer et al. 2002). Here, compared to EGF-FTOC-derived cells, lowered levels of Notch1 protein were observed at the surface of cells from EGF+INS-FTOC (Fig. 3C), although transcripts for Notch1 were still present. Inversely related to Notch1 surface levels, newly transcribed messages for Notch3 were observed, simultaneously with those for Deltex1 (Fig. 4). Although Izon et al. (2002) could detect transcripts for Deltex1 from the DN1 stage, contrasting with our EGF-FTOC-derived- DN1/2, they found an up-regulation in the transition from DN2 to DN3. In thymus physiology, Deltex up-regulation would succeed the specifying action of Notch1, its enforced expression in hematopoietic progenitors resulting in B cell development at the expense of T-cells in FTOC (Izon et al. 2002). Besides acting as a modulator of Notch action through the "classical CSL-mediated transduction pathway", Deltex1 was pointed as a mediator of a still poorly characterized pathway, acting as a transcriptional regulator downstream of the Notch receptor (Yamamoto et al. 2001, Allman et al. 2002).

Concerning the "classical downstream effectors" of Notch signaling, HES1 and HES5, we could not, again, observe differences that could explain growth blockade or its reversal upon INS addition (Fig. 4). In our model, HES1 transcripts were seen in EGF-FTOCs, when lymphoid growth was blocked. Unless if dose-dependent (not tested), HES1 transcripts would not be growth-limiting. Neither would be the HES5 transcripts, absent even during INS-driven growth. Active intracellular Notch was reported to augment also a HES-1-independent thymocyte proliferation, giving support to an alternative pathway (Ordentlich et al. 1998, Huang et al. 2003) that could putatively be associated with Deltex1 expression.

Notch signaling is not functional in Common Lymphoid Precursors until they enter the thymic rudiment, where, upon contact with ligands expressed by epithelial stromal cells, its activity turns essential (Harman et al. 2003). T- lineage commitment would require a high Notch expression within a thymic microenvironment, but such specific stromal ligands were not yet completely identified (Allman et al. 2002, Schmitt & Zúñiga-Pflücker 2002, Harman et al. 2003). Transcripts for the Notch ligands Jagged1 and Jagged2 have been described in the thymus, the first expressed by thymic stromal components, and the last by both stromal cells and thymocytes (Felli et al.1999, Kaneta et al. 2000). Here, when assessed by RT-PCR, Jagged2 was co-expressed with Notch3, upon INS action, differently from Jagged1, which expression was seen in both culture conditions. Transcripts for other ligands belonging to the Delta-like- family did not differ among the culture conditions here tested (Fig. 4). Were they: Pref1, reported to regulate the levels of HES1 expression and so influence the cellularity of developing thymus (Kaneta et al. 2000); Dll1, able to direct hematopoietic progenitors to a T/NK fate (Schmitt & Zúñiga-Pflücker 2002); and Dll4, implied in lymphoproliferative disease and T-cell lymphoma in mice (Dorsch et al. 2002).

Thus, while the redundant action of the different members of Notch family, their ligands and downstream targets, is under discussion (Felli et al. 1999, Osborne & Miele 1999, Anderson et al. 2000, Wolfer et al. 2002, Hozumi et al. 2003), our results point to the recruitment of a specific set of Notch signaling-related transcripts, if not causally related, at least simultaneous and compatible with thymocyte proliferation and DN1/2 to DN3 transition, as summarized in Fig. 5.

 

ACKNOWLEDGEMENTS

To Drs João PB Viola and Lilian DS Carvalho, for helping with the RPA assay. To Teresa G Correa and Terezinha de C Silva, for technical assistance with the animals.

 

REFERENCES

Allman D, Punt JÁ, Izon DJ, Áster JC, Pear WS 2002. An invitation to T and more: signaling in lymphopoiesis.Cell 109: S1-S11.

Anderson G, Harman BC, Hare KJ, Jenkinson EJ 2000. Microenvironmental regulation of T cell development in the thymus. Semin Immunol 12: 457-464.

Anderson MK, Hernandez-Hoyos G, Dionne CJ, Arias AM, Chen D, Rothenberg EV 2002. Definition of regulatory network elements for T cell development by perturbation analysis with PU.1 and GATA-3. Dev Biol 246: 103-121.

Bettenhausen B, de Angelis MH, Simon D, Guénet J-L, Gossler A 1995. Transient and restricted expression during mouse embryogenesis of Dll1, a murine gene closely related to Drosophila Delta. Development 121: 2407-2418.

Chao DT, Korsmeyer SJ 1998. BCL-2 family: regulators of cell death. Annu Rev Immunol 16: 395-419.

Dai M-S, Ge Y, Xia Z-B, Broxmeyer HE, Lu l 2000. Introduction of human erythropoietin receptor complementary DNA by retrovirus-mediated gene transfer into murine embryonic stem cells enhances erythropoiesis in developing embryoid bodies. Biol Blood Marrow Transpl 6: 395-407.

DiSanto J, Radtke F, Rodewald H-R 2000. To be or not to be a pro-T? Curr Op Immunol 12: 159-165.

Dorsch M, Zheng G, Yowe D, Rao P, Wang Y, Shen Q, Murphy C, Xiong X, Shi Q, Gutierrez-Ramos J-C, Fraser C, Villeval J-L 2002. Ectopic expression of Delta4 impairs hematopoietic development and leads to lymphoproliferative disease. Blood 100: 2046-2055.

Douglas NC, Jacobs H, Bothwell ALM, Hayday AC 2001. Defining the specific physiological requirements for c-myc in T-cell development. Nat Immunol 2: 307-315.

Falk I, Eichman K 2002. Heterogeneity of the DN4 (CD44-CD25-) subset of CD4-CD8- double negative thymocytes; dependence on CD3 signaling. Immunol Lett 82: 123-130.

Felli MP, Maroder M, Mitsiadis TA, Campese AF, Bellavia D, Vacca A, Mann RS, Frati L, Lendahl U, Gulino A, Screpanti I 1999. Expression pattern of notch1, 2 and 3 and jagged1 and 2 in lymphoid and stromal thymus components: distinct ligand- receptor interactions in intrathymic T cell development. Int Immunol 11: 1017-1025.

Freitas CS, Dalmau SR, Kovary K, Savino W 1998a. Epidermal growth factor modulates fetal thymocyte growth and differentiation. Dev Immunol 5: 169-82.

Freitas CS, Dalmau SR, Savino W 1998b. Epidermal growth factor modulates fetal thymocyte growth and differentiation: partial reversal by insulin, mimicking by specific inhibitors of EGF receptor tyrosine kinase activity, and differential expression of CD45 phosphatase isotypes. J Immunol 161: 3384-3392.

Harman BC, Jenkinson EJ, Anderson G 2003. Microenvironmental regulation of notch signaling in T cell development. Semin Immunol 15: 91-97.

Hattori N, Kawamoto H, Katsura Y 1996a. Isolation of the most immature population of murine fetal thymocytes that includes progenitors capable of generating T, B, and myeloid cells. J Exp Med 184: 1901-1908.

Hattori N, Kawamoto H, Shinji F, Kuno K, Katsura Y 1996b. Involvement of transcription factors TCF-1 and GATA-3 in the initiation of the earliest step of T cell development in the thymus. J Exp Med 184: 1137-1147.

Herblot S, Steff AM, Hugo P, Aplan PD, Hoang T 2000. SCL and LMO1 alter thymocyte differentiation: inhibition of E2A-HEB function and pre-T alpha chain expression. Nat Immunol 1: 138-144.

Hendriks RW, Nawjin MC, Engel JD, van Doornick H, Grosveld F, Karis A 1999. Expression of the transcription factor GATA3 is required for the development of the earliest T cell progenitors and correlates with stages of cellular proliferation in the thymus. Eur J Immunol 29: 1912-1918.

Hozumi K, Kobori A, Sato T, Nishimura T, Habu S 1996. Transcription and demethylation of TCR b gene initiated prior to the gene rearrangement in c-kit+ thymocytes with CD3 expression: evidence of T-cell commitment in the thymus. Int immunol 8: 1473-1481.

Hozumi K, Abe N, Chiba S, Hirai H, Habu S 2003. Active form of notch members can enforce T lymphopoiesis on lymphoid progenitors in the monolayer culture specific for B cell development. J Immunol 170: 4973-4979.

Huang EY, Gallegos AM, Richards RM, Lehar SM, Bevan MJ 2003. Surface expression of Notch1 on thymocytes: correlation with the Double-negative to Double-positive transition. J Immunol 171: 2296-2304.

Izon DJ,Aster JC, He Y, Weng A, Kamell FG, Patriub V,Xu L, Bakkour S, Rodriguez C, Allman D, Pear WS 2002. Deltex 1 redirects lymphoid progenitors to the B cell lineage by antagonizing notch 1. Immunity 16: 231-243.

Kaneta M, Osawa M, Osawa M, Sudo K, Nakauchi H, Farr Ag, Takahama Y 2000. A role for pref-1 and HES-1 in thymocyte development. J Immunol 164: 256-264.

Newton K, Harris AW, Strasser A 2000. FADD/MORT1 regulates the pre-TCR checkpoint and can function as a tumour suppressor. EMBO J 19: 931-941.

Ordentlich P, Lin A, Shen CP, Blaumueller K, Matsuno K, Artavanis-Tsakonas S, Kadesch T 1998. Notch inhibition of E47 supports the existence of a novel signaling pathway. Mol Cell Biol 18: 2230-2239.

Osborne B, Miele L 1999. Notch and the immune system. Immunity 11: 653-663.

Pénit C, Lucas B, Vasseur F 1995. Cell expansion and growth arrest during the transition from precursor (CD4-8-) to immature (CD4+8+) thymocytes in normal and genetically modified mice. J Immunol 154: 5103-5113.

Radtke F, Wilson A, Stark G, Bauer M, vanMeerwijk J, MacDonald HR, Aguet M 1999. Deficient T cell specification in mice with an induced inactivation of Notch1. Immunity 10: 547-558.

Ramiro AR, Navarro MN, Carreira A, Carrasco YR, de Yébenes VG, Carrillo G, Millán JLS, Rubin B, Toribio ML 2001. Differential developmental regulation and functional effects on pre-TCR surface expression of human pTaa and pTab spliced isoforms. J Immunol 167: 5106-5114.

Rodewald H-R 1995. Pathways from hematopoietic stem cells to thymocytes. Curr Op Immunol 7:176-187.

Rothenberg EV 2002. T-lineage specification and commitment: a gene regulation perspective. Semin Immunol 14: 431-440.

Savino W, de Moraes M do C, Barbosa SD, Da Fonseca EC, De Almeida VC, Hontebeyrie-Joscowicz M 1992. Is the thymus a target organ in infectious diseases? Mem Inst Oswaldo Cruz 87 (Suppl. 5): 73-78.

Schmitt TM, Zúñiga-Pflücker JC 2002. Induction of T cell development from hematopoietic progenitor cells by delta-like-1 in vitro. Immunity 17: 749-756.

Sestan N, Artavanis-Tsakonas S, Rakic P 2001. Contact-dependent inhibition of cortical neurite growth mediated by notch signaling. Science 286: 741-746.

Smas CM, Sul HS, 1993. Pref-1, a protein containing EGF-like repeats, inhibits adipocyte differentiation. Cell 73: 725-734.

Taubenberger JK, Reid AH, Izon D, Boehme SA 1996. Development and characterization of v-myc/v-raf- transformed murine fetal thymocyte cell lines. Cell Immunol 171: 41-47.

Tomayko MM, Punt JÁ, Bolcavage JM, Levy SL, Allman DM, Cancro MP 1999. Expression of the Bcl-2 family member A1 is developmentally regulated in T cells. Int Immunol 11: 1753-1761.

Tomita K, Hattori M, Nakamura E, Nakanishi S, Minato N, Kageyama R 1999. The bHLH gene Hes1 is essential for expansion of early T cell precursors. Genes & Dev 13: 1203-1210.

Von Boehmer H, Sarukhan A, Buer J 1997. Induction of and rescue from programmed cell death. A recurrent theme in lymphocyte physiology. The Immunologist 5: 185-191.

Wolfer A, Wilson A, Nemir M, MacDonald HR, Radtke F 2002. Inactivation of notch1 impairs VDJb rearrangement and allows pre-TCR-independent survival of early ab lineage thymocytes. Immunity 16: 869-879.

Yamamoto N, Yamamoto S, Inagaki F, Kawaichi M, Fukamizu A, Kishi N, Matsuno K, Nakamura K, Weinmaster G, Okano H, Nakafuku M 2001. Role of Deltex-1 as a transcriptional regulator downstream of the Notch receptor. J Biol Chem 276: 45031-45040.

Yoneya T, Tahara T, Nagao K, Yamada Y, Yamamoto T, Osawa M, Miyatani S, Nishikawa M 2001. Molecular cloning of Delta-4, a new mouse and human notch ligand. J Biochem 129: 27-39.