Panagiotis Tsapogas

EBF1 Is Essential for B-Lineage Priming and Establishment of a Transcription Factor Network in Common Lymphoid Progenitors

Development of B-lymphoid cells in the bone marrow is a process under strict control of a hierarchy of transcription factors. To understand the development of a B-lymphoid-restricted functional network of transcription factors, we have investigated the cell autonomous role of the transcription factor EBF1 in early B cell development. This revealed that even though transplanted EBF1-deficient fetal liver cells were able to generate common lymphoid progenitors (CLPs) as well as B220+CD43+AA4.1+ candidate precursor B cells, none of these populations showed signs of B lineage priming. The isolated CLPs were able to generate T lymphocytes in vitro supporting the idea that the phenotype of EBF1-deficient mice is restricted to the development of the B lineage. Furthermore, EBF deficient CLPs displayed a reduction in Ig H chain recombination as compared with their wild-type counterpart and essentially lacked transcription of B-lineage-associated genes. Among the genes displaying reduced expression in the EBF1 deficient CLPs were the transcription factors Pax5, Pou2af1 (OcaB), and FoxO1 that all appear to be direct genetic targets for EBF1 because their promoters contained functional binding sites for this factor. This leads us to suggest that EBF1 regulates a transcription factor network crucial for B lineage commitment.

Single-cell analysis of the common lymphoid progenitor compartment reveals functional and molecular heterogeneity

To investigate molecular events involved in the regulation of lymphoid lineage commitment, we crossed lambda5 reporter transgenic mice to Rag1-GFP knockin mice. This allowed us to subfractionate common lymphoid progenitors and pre-pro-B (fraction A) cells into lambda5(-)Rag1(low), lambda5(-)Rag1(high), and lambda5(+)Rag1(high) cells. Clonal in vitro differentiation analysis demonstrated that Rag1(low) cells gave rise to B/T and NK cells. Rag1(high) cells displayed reduced NK-cell potential with preserved capacity to generate B- and T-lineage cells, whereas the lambda5(+) cells were B-lineage restricted. Ebf1 and Pax5 expression was largely confined to the Rag1(high) populations. These cells also expressed a higher level of the surface protein LY6D, providing an additional tool for the analysis of early lymphoid development. These data suggest that the classic common lymphoid progenitor compartment composes a mixture of cells with relatively restricted lineage potentials, thus opening new possibilities to investigate early hematopoiesis.

Isolation and killing of candidate chronic myeloid leukemia stem cells by antibody targeting of IL-1 receptor accessory protein

Chronic myeloid leukemia (CML) is genetically characterized by the Philadelphia (Ph) chromosome, formed through a reciprocal translocation between chromosomes 9 and 22 and giving rise to the constitutively active tyrosine kinase P210 BCR/ABL1. Therapeutic strategies aiming for a cure of CML will require full eradication of Ph chromosome-positive (Ph+) CML stem cells. Here we used gene-expression profiling to identify IL-1 receptor accessory protein (IL1RAP) as up-regulated in CML CD34+ cells and also in cord blood CD34+ cells as a consequence of retroviral BCR/ABL1 expression. To test whether IL1RAP expression distinguishes normal (Ph−) and leukemic (Ph+) cells within the CML CD34+CD38− cell compartment, we established a unique protocol for conducting FISH on small numbers of sorted cells. By using this method, we sorted cells directly into drops on slides to investigate their Ph-chromosome status. Interestingly, we found that the CML CD34+CD38−IL1RAP+ cells were Ph+, whereas CML CD34+CD38−IL1RAP− cells were almost exclusively Ph−. By performing long-term culture-initiating cell assays on the two cell populations, we found that Ph+ and Ph− candidate CML stem cells could be prospectively separated. In addition, by generating an anti-IL1RAP antibody, we provide proof of concept that IL1RAP can be used as a target on CML CD34+CD38− cells to induce antibody-dependent cell-mediated cytotoxicity. This study thus identifies IL1RAP as a unique cell surface biomarker distinguishing Ph+ from Ph− candidate CML stem cells and opens up a previously unexplored avenue for therapy of CML.

IL-7 mediates Ebf-1-dependent lineage restriction in early lymphoid progenitors

Deficiencies in the IL-7 signaling pathway result in severe disruptions of lymphoid development in adult mice. To understand more about how IL-7 deficiency impacts early lymphoid development, we have investigated lineage restriction events within the common lymphoid progenitor (CLP) compartment in IL-7 knockout mice. This revealed that although IL-7 deficiency had a minor impact on the development of LY6D(-) multipotent CLPs, the formation of the lineage restricted LY6D(+) CLP population was dramatically reduced. This was reflected in a low-level transcription of B-lineage genes as well as in a loss of functional B-cell commitment. The few Ly6D(+) CLPs developed in the absence of IL-7 displayed increased lineage plasticity and low expression of Ebf-1. Absence of Ebf-1 could be linked to increased plasticity because even though Ly6D(+) cells develop in Ebf-1-deficient mice, these cells retain both natural killer and dendritic cell potential. This reveals that IL-7 is essential for normal development of Ly6D(+) CLPs and that Ebf-1 is crucial for lineage restriction in early lymphoid progenitors.

Single-cell analysis of early B-lymphocyte development suggests independent regulation of lineage specification and commitment in vivo

To better understand the process of B-lymphocyte lineage restriction, we have investigated molecular and functional properties in early B-lineage cells from Pax-5–deficient animals crossed to a B-lineage–restricted reporter mouse, allowing us to identify B-lineage–specified progenitors independently of conventional surface markers. Pax-5 deficiency resulted in a dramatic increase in the frequency of specified progenitor B-cells marked by expression of a λ5 (Igll1) promoter-controlled reporter gene. Gene expression analysis of ex vivo isolated progenitor cells revealed that Pax-5 deficiency has a minor impact on B-cell specification. However, single-cell in vitro differentiation analysis of ex vivo isolated cells revealed that specified B-lineage progenitors still displayed a high degree of plasticity for development into NK or T lineage cells. In contrast, we were unable to detect any major changes in myeloid lineage potential in specified Pax-5–deficient cells. By comparison of gene expression patterns in ex vivo isolated Pax-5– and Ebf-1–deficient progenitors, it was possible to identify a set of B-cell–restricted genes dependent on Ebf-1 but not Pax-5, supporting the idea that B-cell specification and commitment is controlled by distinct regulatory networks.

Interleukin-7-induced Stat-5 Acts in Synergy with Flt-3 Signaling to Stimulate Expansion of Hematopoietic Progenitor Cells

The development of lymphoid cells from bone marrow progenitors is dictated by interplay between internal cues such as transcription factors and external signals like the cytokines Flt-3 ligand and Il-7. These proteins are both of large importance for normal lymphoid development; however, it is unclear if they act in direct synergy to expand a transient Il-7R+Flt-3+ population or if the collaboration is created through sequential activities. We report here that Flt-3L and Il-7 synergistically stimulated the expansion of primary Il-7R+Flt-3+ progenitor cells and a hematopoietic progenitor cell line ectopically expressing the receptors. The stimulation resulted in a reduced expression of pro-apoptotic genes and also mediated survival of primary progenitor cells in vitro. However, functional analysis of single cells suggested that the anti-apoptotic effect was additive indicating that the synergy observed mainly depends on stimulation of proliferation. Analysis of downstream signaling events suggested that although Il-7 induced Stat-5 phosphorylation, Flt-3L caused activation of the ERK and AKT signaling pathways. Flt-3L could also drive proliferation in synergy with ectopically expressed constitutively active Stat-5. This synergy could be inhibited with either receptor tyrosine kinase or MAPK inhibitors suggesting that Flt-3L and Il-7 act in synergy by activation of independent signaling pathways to expand early hematopoietic progenitors.

RNA analysis of B cell lines arrested at defined stages of differentiation allows for an approximation of gene expression patterns during B cell development

The development of a mature B lymphocyte from a bone marrow stem cell is a highly ordered process involving stages with defined features and gene expression patterns. To obtain a deeper understanding of the molecular genetics of this process, we have performed RNA expression analysis of a set of mouse B lineage cell lines representing defined stages of B cell development using Affymetrix™ microarrays. The cells were grouped based on their previously defined phenotypic features, and a gene expression pattern for each group of cell lines was established. The data indicated that the cell lines representing a defined stage generally presented a high similarity in overall expression profiles. Numerous genes could be identified as expressed with a restricted pattern using dCHIP‐based, quantitative comparisons or presence/absence‐based, probabilistic state analysis. These experiments provide a model for gene expression during B cell development, and the correctly identified expression patterns of a number of control genes suggest that a series of cell lines can be useful tools in the elucidation of the molecular genetics of a complex differentiation process.