scholarly journals Hierarchy of Notch–Delta interactions promoting T cell lineage commitment and maturation

2007 ◽  
Vol 204 (2) ◽  
pp. 331-343 ◽  
Author(s):  
Valerie Besseyrias ◽  
Emma Fiorini ◽  
Lothar J. Strobl ◽  
Ursula Zimber-Strobl ◽  
Alexis Dumortier ◽  
...  
Keyword(s):  
T Cell ◽  
T Cell Development ◽  
Cell Lineage ◽  
Cell Development ◽  
Lineage Commitment ◽  
Binding Studies ◽  
Maturation In Vitro ◽  
Cell Commitment

Notch1 (N1) receptor signaling is essential and sufficient for T cell development, and recently developed in vitro culture systems point to members of the Delta family as being the physiological N1 ligands. We explored the ability of Delta1 (DL1) and DL4 to induce T cell lineage commitment and/or maturation in vitro and in vivo from bone marrow (BM) precursors conditionally gene targeted for N1 and/or N2. In vitro DL1 can trigger T cell lineage commitment via either N1 or N2. N1- or N2-mediated T cell lineage commitment can also occur in the spleen after short-term BM transplantation. However, N2–DL1–mediated signaling does not allow further T cell maturation beyond the CD25+ stage due to a lack of T cell receptor β expression. In contrast to DL1, DL4 induces and supports T cell commitment and maturation in vitro and in vivo exclusively via specific interaction with N1. Moreover, comparative binding studies show preferential interaction of DL4 with N1, whereas binding of DL1 to N1 is weak. Interestingly, preferential N1–DL4 binding reflects reduced dependence of this interaction on Lunatic fringe, a glycosyl transferase that generally enhances the avidity of Notch receptors for Delta ligands. Collectively, our results establish a hierarchy of Notch–Delta interactions in which N1–DL4 exhibits the greatest capacity to induce and support T cell development.

scholarly journals Delta-like 4 is the essential, nonredundant ligand for Notch1 during thymic T cell lineage commitment

2008 ◽  
Vol 205 (11) ◽  
pp. 2515-2523 ◽  
Author(s):  
Ute Koch ◽  
Emma Fiorini ◽  
Rui Benedito ◽  
Valerie Besseyrias ◽  
Karin Schuster-Gossler ◽  
...  
Keyword(s):  
T Cell ◽  
B Cells ◽  
T Cell Development ◽  
Cell Lineage ◽  
Cell Development ◽  
Lineage Commitment ◽  
Thymic Epithelial Cells ◽  
Hematopoietic Progenitors ◽  
Immature B Cells

Thymic T cell lineage commitment is dependent on Notch1 (N1) receptor–mediated signaling. Although the physiological ligands that interact with N1 expressed on thymic precursors are currently unknown, in vitro culture systems point to Delta-like 1 (DL1) and DL4 as prime candidates. Using DL1- and DL4-lacZ reporter knock-in mice and novel monoclonal antibodies to DL1 and DL4, we show that DL4 is expressed on thymic epithelial cells (TECs), whereas DL1 is not detected. The function of DL4 was further explored in vivo by generating mice in which DL4 could be specifically inactivated in TECs or in hematopoietic progenitors. Although loss of DL4 in hematopoietic progenitors did not perturb thymus development, inactivation of DL4 in TECs led to a complete block in T cell development coupled with the ectopic appearance of immature B cells in the thymus. These immature B cells were phenotypically indistinguishable from those developing in the thymus of conditional N1 mutant mice. Collectively, our results demonstrate that DL4 is the essential and nonredundant N1 ligand responsible for T cell lineage commitment. Moreover, they strongly suggest that N1-expressing thymic progenitors interact with DL4-expressing TECs to suppress B lineage potential and to induce the first steps of intrathymic T cell development.

Constitutive Expression of MEF2C, LYL1, or LMO2 in CD34+ HSCs Blocks the in Vitro Differentiation before the T-Cell Commitment Point and Could be Oncogenic in Early T-Cell Progenitor ALL

Blood ◽  
2015 ◽  
Vol 126 (23) ◽  
pp. 2423-2423
Author(s):  
Kirsten Canté-Barrett ◽  
Rui D Mendes ◽  
Wilco K Smits ◽  
Rob Pieters ◽  
Jules PP Meijerink
Keyword(s):  
Gene Expression ◽  
T Cells ◽  
Cell Differentiation ◽  
T Cell ◽  
T Cell Development ◽  
Cell Development ◽  
T Cell Differentiation ◽  
Cell Commitment

Abstract Background: T-cell development in the thymus is a complex process that depends on sequential transcriptional and epigenetic events that induce T-cell lineage commitment and simultaneously suppress alternative cell fates. In T-cell acute lymphoblastic leukemia (T-ALL), aberrantly expressed oncogenes result in the arrest of developing thymocytes, which can lead to the acquisition of secondary mutations, uncontrolled proliferation and disease progression. MEF2C is often expressed as a result of chromosomal rearrangements in immature, early T-cell progenitor ALL (ETP-ALL), but is also expressed in normal thymocyte progenitors before T-cell commitment (in the ETP stage). As the only hematopoietic lineage, thymocytes that have passed the T-cell commitment checkpoint (as well as mature T-cells) do no longer express MEF2C. Aims: We aimed to investigate the effect of constitutive MEF2C expression on early T-cell development. OP9-DL1 co-cultures have been most useful for mimicking in vitro T-cell development starting with hematopoietic stem cells (HSCs) derived from human cord blood or bone marrow. We also aimed to investigate the impact of MEF2C in comparison to LYL1 and LMO2; two T-ALL oncogenes also highly expressed at the ETP stage. Methods: We have utilized the OP9-DL1 in vitro co-culture system to gradually differentiate CD34+ HSCs from umbilical cord blood into the T-cell lineage. HSCs in this co-culture will recapitulate in vivo T-cell development as measured by incremental acquisition of surface markers CD7, CD5, CD1a, and reach the CD4, CD8 double-positive (DP) stage. We generated gene expression profiles of 11 subsequent in vitro stages of differentiation to help us match them to in vivo development stages. We investigated in vitro T-cell differentiation of HSCs after lentiviral transduction with MEF2C or control vectors, as well as with other transcriptional regulators LYL1 and LMO2 that are expressed at the ETP stage. Results: The major change in gene expression of subsequent early T-cell differentiation stages defines two distinct T-cell differentiation clusters that correlate with in vivo pre- and post-T-cell commitment profiles. We found that T-cell commitment occurs in CD7+ CD5+ cells before the acquisition of CD1a surface expression. Expression of control vectors in HSCs does not affect the in vitro T-cell differentiation, but MEF2C expression blocks differentiation into the direction of T-cells as measured by the failure of most cells to acquire CD7 as the first marker. Instead, with increased passage number cells gradually lose CD34 expression and eventually disappear from the co-culture. Similar effects were observed for the expression of LYL1 and LMO2; LYL1 expression arrests the cells at the most immature CD7+ ETP stage and prevents the transition towards CD7+ CD5+ cells, whereas LMO2 expressing cells reach the CD7+ CD5+ stage but fail to acquire CD1a as a marker of T-cell commitment. Summary/Conclusion: The gene expression profiles of 11 human in vitro T-cell differentiation subsets has enabled us to pinpoint T-cell commitment to a stage in which cells have acquired CD7 and CD5, just prior to the acquisition of CD1a. MEF2C, LYL1, and LMO2, expressed in ETP-ALL as well as in normal thymocyte progenitors, do not allow the transition to T-cell commitment when constitutively expressed. These proteins each result in the arrest of in vitro differentiating T-cells at different ETP stages, all before the T-cell commitment as marked by CD1a expression. Constitutive expression of MEF2C, LYL1, or LMO2 in very early thymocyte progenitors is incompatible with development into and beyond the T-cell commitment checkpoint and these proteins could therefore play important roles in the pathogenesis of ETP-ALL. Disclosures No relevant conflicts of interest to declare.

scholarly journals T-cell lineage commitment and cytokine responses of thymic progenitors

Blood ◽  
1995 ◽  
Vol 86 (5) ◽  
pp. 1850-1860 ◽  
Author(s):  
TA Moore ◽  
A Zlotnik
Keyword(s):  
T Cells ◽  
T Cell ◽  
Cell Function ◽  
Cultured Cells ◽  
Critical Role ◽  
Cell Lineage ◽  
Lineage Commitment ◽  
Organ Cultures ◽  
Cell Commitment

The earliest steps of intrathymic differentiation recently have been elucidated. It has been reported that both CD4lo (CD44+ CD25- c-kit+ CD3- CD4lo CD8-) and pro-T cells (CD44+ CD25+ c-kit+ CD3- CD4- CD8-, representing the next step in maturation) exhibit germline T-cell receptor beta and gamma loci, suggesting that neither population is exclusively committed to the T-cell lineage. Several groups have shown that CD4lo cells retain the capacity to generate multiple lymphoid lineages in vivo; however, the lineage commitment status of pro-T cells is unknown. To determine when T-cell lineage commitment occurs, we examined the ability of sorted CD4lo and pro-T cells to generate lymphoid lineage cells in vivo or in fetal thymic organ cultures (FTOCs). When intravenously injected into scid mice, CD4lo cells generated both T and B cells, whereas the progeny of pro-T cells contained T cells exclusively. Fetal thymic organ cultures repopulated with CD4lo cells contained both T and natural killer (NK) cells, whereas cultures repopulated with pro-T cells contained T cells almost exclusively. These observations strongly suggest that T-cell lineage commitment occurs during the transition of CD4lo to pro-T cells. Because it is likely that the thymic microenvironment plays a critical role in T-cell commitment, we compared the responses of CD4lo and pro-T cells to various cytokine combinations in vitro, as well as the ability of the cultured cells to repopulate organ cultures. Cytokine combinations that maintained T-cell repopulation potential for both CD4lo and pro-T cells were found. CD4lo cells proliferated best in response to the combination containing interleukin-1 (IL-1), IL-3, IL- 6, IL-7, and stem cell factor (SCF). Unlike CD4lo cells, pro-T cells were much more dependent upon IL-7 for proliferation and FTOC repopulation. However, combinations of cytokines lacking IL-7 were found that maintained the T-cell repopulating potential of pro-T cells, suggesting that, whereas this cytokine is clearly very important for normal pro-T cell function, it is not an absolute necessity during early T-cell expansion and differentiation.

The in Vitro and In Vivo Oncogenic Activity of Homodimeric Mutant of Interleukin-7 Receptor a Chain (IL7Rα) Highlights the Significance of the IL7Rα/Jak1 Pathway in T-Cell Acute Lymphoblastic Leukemia

Blood ◽  
2011 ◽  
Vol 118 (21) ◽  
pp. 73-73
Author(s):  
Kazuaki Yokoyama ◽  
Nozomi Yokoyama ◽  
Kiyoko Izawa ◽  
Ai Kotani ◽  
Ratanakanit Harnprasopwat ◽  
...  
Keyword(s):  
T Cell ◽  
Lymphoblastic Leukemia ◽  
T Cell Development ◽  
Cell Development ◽  
Gamma Delta ◽  
Interleukin 7 ◽  
Signaling Axis ◽  
A Chain

Abstract Abstract 73 Interleukin-7 (IL7) is essential for T cell development and homeostasis. Dysregulation of signals that control normal T-cell development has been implicated in the onset of T-cell acute lymphoblastic leukemia (T-ALL). By analogy to activating mutations in the Notch pathways, we hypothesized that any mutations in the IL7 signaling axis might also contribute to T-ALL. Direct sequencing of human IL7 receptor a chain (hIL7RA) gene in a panel of 16 T-ALL cell lines identified two types of mutations in two different cell lines. One was an insertion mutation of 4 amino acids (LSRC) in the transmembrane region (INS, Fig.1A) from DND-41, a gamma-delta TCR+ T-ALL cell line, and the other was a truncated, loss-of-function, mutation in the cytoplasmic region from MOLT-4. We demonstrated that hIL7RA-INS mutant spontaneously formed a homodimer and constitutively activated downstream signals including Stat family members (1, 3 and 5), Akt and Erk via Jak1, but not Jak3. Next, we investigated oncogenic activity of hIL7RA-INS in primary hematopoietic progenitor cells. To this aim, lin− E.14 Balb/c fetal liver (FL) cells were retrovirally transduced with hIL7RA-INS in parallel with hIL7RA-wild type (WT), and then tested for their cytokine dependence in vitro. As expected, only hIL7RA-INS-transduced lin−FL cells showed abrogation of cytokine dependence. hIL7RA-transduced lin−FL cells were also transplanted into lethally irradiated syngeneic mice. Within 7–9 weeks after transplantation of lin−FL cells transduced with hIL7RA-INS, but not with hIL7RA-WT, recipient mice developed well-tolerated myelo- and lymphoproliferative disorders, characterized by marked leukocytosis, systemic lymphadenopathy and splenomegaly (Fig.1B). Notably, concomitant increase in hIL7RA+gamma-delta TCR+ T cells and decrease in B cells were observed in peripheral blood (Fig.1C). Histological examination of bone marrow, spleen and liver specimens from diseased mice revealed moderate to severe myeloid hyperplasia, disrupted splenic architecture by disseminated mature myeloid cells and infiltration of both myeloid and mononuclear cells into hepatic parenchyma, respectively. In addition, recipient mice for hIL7RA-INS-transduced lin−FL cells frequently manifested ruffled fur as well as mononuclear cell infiltration into salivary gland and pericardium, suggesting an autoimmune-like disorder. However, during median follow-up of 11 weeks, these recipient mice did not develop either overt leukemia or lymphoma, indicating that additional transforming events are required for evolution to aggressive hematological malignancies. These in vivo findings highlighted the possibility that aberrant signals via IL7RA in hematopoietic stem/progenitor cells might preferentially stimulate myelopoiesis over lymphopoiesis, and also confirmed the essential role of IL7RA in gamma-delta TCR+ T cell development, previously shown by IL7RA-knockout mice. Taken together, we speculated that dysregulated IL7RA signaling axis might be involved in the onset of T-ALL, especially with gamma-delta TCR+ phenotype. Finally, the present study, together with the recent report (JEM 208:901, 2011), emphasizes the significance of the sequential Notch-IL7RA pathways in the pathogenesis of T-ALL as well as the dominant role of the IL7RA/Jak1 axis in IL7 proliferative signal. Disclosures: No relevant conflicts of interest to declare.

T-cell generation by lymph node resident progenitor cells

Blood ◽  
2005 ◽  
Vol 106 (1) ◽  
pp. 193-200 ◽  
Author(s):  
Rafik Terra ◽  
Isabelle Louis ◽  
Richard Le Blanc ◽  
Sophie Ouellet ◽  
Juan Carlos Zúñiga-Pflücker ◽  
...  
Keyword(s):  
T Cells ◽  
T Cell ◽  
Stromal Cells ◽  
Integration Site ◽  
T Cell Development ◽  
Cell Development ◽  
Lymphoid Organ ◽  
Oncostatin M ◽  
Cell Commitment

In the thymus, 2 types of Lin–Sca-1+ (lineage-negative stem cell antigen-1–positive) progenitors can generate T-lineage cells: c-Kithi interleukin-7 receptor α–negative (c-KithiIL-7Rα–) and c-KitloIL-7Rα+. While c-KithiIL-7Rα– progenitors are absent, c-KitloIL-7Rα+ progenitors are abundant in the lymph nodes (LNs). c-KitloIL-7Rα+ progenitors undergo abortive T-cell commitment in the LNs and become arrested in the G1 phase of the cell cycle because they fail both to up-regulate c-myb, c-myc, and cyclin D2 and to repress junB, p16INK4a, and p21Cip1/WAF. As a result, development of LN c-KitloIL-7Rα+ progenitors is blocked at an intermediate CD44+CD25lo development stage in vivo, and LN-derived progenitors fail to generate mature T cells when cultured with OP9-DL1 stromal cells. LN stroma can provide key signals for T-cell development including IL-7, Kit ligand, and Delta-like–1 but lacks Wnt4 and Wnt7b transcripts. LN c-KitloIL-7Rα+ progenitors are able to generate mature T cells when cultured with stromal cells producing wingless-related MMTV integration site 4 (Wnt4) or upon in vivo exposure to oncostatin M whose signaling pathway intersects with Wnt. Thus, supplying Wnt signals to c-KitloIL-7Rα+ progenitors may be sufficient to transform the LN into a primary T-lymphoid organ. These data provide unique insights into the essence of a primary T-lymphoid organ and into how a cryptic extrathymic T-cell development pathway can be amplified.

In Vitro Exposure to DL-4 Increases the in Vitro and In Vivo T-Lymphopoietic Potential of CB Derived CD34+ Progenitor Cells

Blood ◽  
2011 ◽  
Vol 118 (21) ◽  
pp. 2980-2980
Author(s):  
Christian Reimann ◽  
Liliane Liliane Dal-Cortivo ◽  
Emmanuelle M. Six ◽  
Andrea Schiavo ◽  
Marina Cavazzana-Calvo ◽  
...  
Keyword(s):  
T Cell ◽  
Developmental Stages ◽  
T Cell Development ◽  
Cell Development ◽  
T Cell Reconstitution ◽  
Cd34 Cells ◽  
Peripheral T Cells ◽  
Lymphoid Progenitors

Abstract Abstract 2980 Notchligand-based culture systems such as OP9-DL1 cells induce HSC to engage towards the T-cell developmental program and allow generation of T-lymphoid progenitors in vitro. In vitro generated murine T-lymphoid progenitors accelerated T-cell reconstitution in vivo. In consistency, human T-lymphoid progenitors generated in co-culture with OP9-DL1 cells enhanced thymic repopulation when injected into NOD/SCID/gc−/− mice (NSG). However, positive effects of human T-lymphoid progenitors on peripheral T-cell reconstitution have not been reported yet. Besides, Notchligand-based culture systems, consisting of genetically modified murine cells might raise safety concern for clinical use. It has been described that exposure of CD34+ cells to immobilized DL4 induces the T-cell developmental program even in absence of stromal cell support. Recently, we have made use of this system to generate T-lymphoid progenitors in vitro. In the present study we have further characterized their T-lymphoid potential in vitro and in vivo. Exposure of human CB-derived CD34+ cells to immobilized DL4 allowed generation of CD34+CD7+ and CD34−CD7++CD5+ progenitors displaying a similar phenotype as early thymic progenitors (ETP) and the prethymocytes (pre-T). Within the DL-4 derived ETP- and preT-like progenitors we observed subsequent up regulation of genes involved in T-cell development and silencing of genes implied in B-cell and myeloid differentiation. T-cell commitment of DL-4 progenitors could be further confirmed by early and intermediate rearrangement events within the TCR d/g/b genes. The pattern of gene expression profile and TCR-rearrangement events displayed a pattern similar to what we observed in corresponding intrathymic developmental stages. DL4-progenitors obtained after 7 days of culture displayed a 30-fold increased in vitro T-lymphoid potential as compared to untreated CD34+ CB progenitors. DL4 ETP-like and preT-like progenitors further completed T-cell differentiation in vitro (in OP9DL1 co-culture) faster than native CD34+ CB progenitors. When transferred into NSG, DL4 progenitors obtained after 7 days of culture were able to repopulate the recipients' thymus and to give rise to mature, polyclonal intrathymic and peripheral T-cells. Two months after transfer recipients of DL4 progenitors displayed advanced intrathymic T-cell development as compared to recipients of CD34+ CB cells. Furthermore, peripheral T-cells could be observed in a number of DL-4 progenitor recipients but not in control mice. Our experiments provide further evidence that DL4 allows in vitro induction of T-cell development and generation of early T-lymphoid progenitors in a system devoid of stromal cell support. These progenitors feature phenotypical and molecular characteristics of immature thymic developmental stages. Moreover, they are able to accelerate T-cell development in vitro and when transferred into NSG. This work provides further evidence of the ability of in vitro -generated human T-cell progenitors to accelerate T-cell reconstitution and simultaneously introduces a culture technique that could be rapidly transferred into a clinical setting. Disclosures: No relevant conflicts of interest to declare.

T-cell developmental blockage by tachykinin antagonists and the role of hemokinin 1 in T lymphopoiesis

Blood ◽  
2003 ◽  
Vol 102 (6) ◽  
pp. 2165-2172 ◽  
Author(s):  
Yu Zhang ◽  
Christopher J. Paige
Keyword(s):  
T Cell ◽  
B Cell ◽  
T Cell Development ◽  
Cell Development ◽  
Double Positive ◽  
Peptide Family ◽  
Tachykinin Antagonists ◽  
B Cell Precursors

Abstract Hemokinin 1 (HK-1) is a new member of the tachykinin peptide family that is expressed in hematopoietic cells. Recent reports studying mouse, rat, and human orthologs of HK-1 demonstrate a broader distribution than originally reported. Our previous studies demonstrated that HK-1, by promoting proliferation, survival, and possibly maturation of B-cell precursors, plays an important role in B lymphopoiesis. Here we present data showing that HK-1 also influences T-cell development at a similar stage of differentiation. This peptide enhanced the proliferation of T-cell precursors and increased the number of thymocytes in fetal thymus organ cultures (FTOCs). Tachykinin antagonists, on the other hand, greatly reduced the cellularity of thymi both in vivo and in vitro. The major reduction occurred in the CD4/CD8 double-positive (DP) cells and the CD44–CD25+ subset of the CD4/CD8 double-negative (DN) cells. Of note, these populations also express HK-1, raising the possibility of autocrine or paracrine pathways influencing T-cell development as we previously reported for B-cell development. Consistent with this, the detrimental effect of tachykinin antagonists could be partially overcome with exogenous HK-1 peptide.

scholarly journals T-Cell Development: T-cell lineage commitment revisited

1995 ◽  
Vol 5 (8) ◽  
pp. 829-831 ◽  
Author(s):  
Richard Boismenu ◽  
Wendy L. Havran
Keyword(s):  
T Cell ◽  
T Cell Development ◽  
Cell Lineage ◽  
Cell Development ◽  
Lineage Commitment

scholarly journals Normal T-Cell Development and Immune Functions in TRIM-Deficient Mice

2006 ◽  
Vol 26 (9) ◽  
pp. 3639-3648 ◽  
Author(s):  
Uwe Kölsch ◽  
Börge Arndt ◽  
Dirk Reinhold ◽  
Jonathan A. Lindquist ◽  
Nicole Jüling ◽  
...  
Keyword(s):  
Immune Response ◽  
T Cells ◽  
T Cell ◽  
T Cell Development ◽  
Cell Development ◽  
T Cell Subsets ◽  
Immune Functions ◽  
Deficient Mice

ABSTRACT The transmembrane adaptor molecule TRIM is strongly expressed within thymus and in peripheral CD4+ T cells. Previous studies suggested that TRIM is an integral component of the T-cell receptor (TCR)/CD3 complex and might be involved in regulating TCR cycling. To elucidate the in vivo function of TRIM, we generated TRIM-deficient mice by homologous recombination. TRIM−/− mice develop normally and are healthy and fertile. However, the animals show a mild reduction in body weight that appears to be due to a decrease in the size and/or cellularity of many organs. The morphology and anatomy of nonlymphoid as well as primary and secondary lymphoid organs is normal. The frequency of thymocyte and peripheral T-cell subsets does not differ from control littermates. In addition, a detailed analysis of lymphocyte development revealed that TRIM is not required for either positive or negative selection. Although TRIM−/− CD4+ T cells showed an augmented phosphorylation of the serine/threonine kinase Akt, the in vitro characterization of peripheral T cells indicated that proliferation, survival, activation-induced cell death, migration, adhesion, TCR internalization and recycling, TCR-mediated calcium fluxes, tyrosine phosphorylation, and mitogen-activated protein family kinase activation are not affected in the absence of TRIM. Similarly, the in vivo immune response to T-dependent and T-independent antigens as well as the clinical course of experimental autoimmune encephalomyelitis, a complex Th1-mediated autoimmune model, is comparable to that of wild-type animals. Collectively, these results demonstrate that TRIM is dispensable for T-cell development and peripheral immune functions. The lack of an evident phenotype could indicate that TRIM shares redundant functions with other transmembrane adaptors involved in regulating the immune response.

CBFB Is Required for Early T Cell Development in the Thymus.

Blood ◽  
2006 ◽  
Vol 108 (11) ◽  
pp. 648-648
Author(s):  
Ling Zhao ◽  
Jennifer L. Cannons ◽  
Stacie Anderson ◽  
Martha R. Kirby ◽  
Liping Xu ◽  
...  
Keyword(s):  
T Cell ◽  
Bone Formation ◽  
Fusion Gene ◽  
T Cell Development ◽  
Cell Lineage ◽  
Cell Development ◽  
Dominant Negative ◽  
Compound Heterozygous ◽  
Thymocyte Development

Abstract Runx1 and Runx3 play important roles in early T cell development. Runx1 is required in the development of double negative cells. Runx1 is also required to repress CD4 expression in DN cells while Runx 3 is essential for epigenetic silencing of CD4 expression in CD8 cells. Both Runx1 and Runx3 are required for CD8 cell development. Because Cbfβ heterodimerizes with both Runx1 and Runx3, we hypothesized that Cbfb is also important in T cell development. To address this issue we analyzed transgenic mouse models with three Cbfb alleles. The first is a null allele for Cbfb and embryos homozygous for this allele die in midgestation due to failure of definitive hematopoiesis and hemorrhage. The second one is a GFP knockin. This Cbfb-GFP allele is a hypomorphic one in that the fusion protein Cbfβ-GFP produced from the allele behaves similarly as the wildtype Cbfβ protein but RNA and protein production from the allele is lower than that of the wildtype allele. Interestingly, AGM hematopoiesis is relatively normal and there is no hemorrhage in the CbfbGFP/GFP embryos, which die at birth due to bone formation defects. The third model is our knock-in mouse model expressing Cbfb-MYH11, the fusion gene found in human AML MeEo with inv (16)(p13; q22). Heterozygous knock-in mice had a phenotype identical to that of the Cbfb and Runx1 null mice, suggesting that the fusion gene Cbfb-MYH11 functions in a dominant-negative manner. We conditionally expressed the Cbfb-MYH11 fusion gene in T cells by using Cre-lox recombination with a floxed Cbfb-MYH11 allele and a Lck-Cre transgene, which starts to express the Cre enzyme at the DN2 stage. By analyzing embryos compound-heterozygous for the null and the hypomorphic GFP knockin alleles (Cbfb−/GFP), we found that CD4 expression was derepressed and thymocyte development was blocked at DN1 and DN2 stages in E17.5 Cbfb−/GFP embryos, which also had much smaller thymi with reduced cellularity compared to their litter mate controls. Further studies on cell proliferation and apoptosis indicated that increased cell death might account for the reduced cellularity. The compound heterozygous Cbfb−/GFP mice died at birth with severe bone formation defects. The Tg(Lck-Cre)/conditional Cbfb-MYH11 mice were viable. In adult thymus, Cbfb-MYH11 expression led to a 10-fold reduction in thymocyte numbers, resulting from both impaired survival of CD4+CD8+ thymocytes (similar as in Cbfb−/GFP embryos) and a differentiation block at DN3 stage. The reduced cellularity could be rescued by over expression of Bcl2 through crossing with Tg(Lck-hBcl2) mice. Cbfb-MYH11 did not derepress CD4 expression in the thymus even though it did so in reporter assays in vitro, which could be due to incomplete Cre-lox reaction, or that Cbfb-MYH11 acts more than just a pure dominant negative. Our data suggest that Cbfβ is critical for several stages of T cell development and may help to explain why CBFB-MYH11+ cells cannot be detected in the T cell lineage in AML patients with this fusion gene.

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