Lineage commitment and developmental plasticity in early lymphoid progenitor subsets

Dissecting Epigenetic Dysregulation of Primary Antibody Deficiencies

Primary antibody deficiencies (PADs), the most prevalent inherited primary immunodeficiencies (PIDs), are associated with a wide range of genetic alterations (both monogenic or polygenic) in B cell-specific genes. However, correlations between the genotype and clinical manifestations are not evident in all cases indicating that genetic interactions, environmental and epigenetic factors may have a role in PAD pathogenesis. The recent identification of key defects in DNA methylation in common variable immunodeficiency as well as the multiple evidences on the role of epigenetic control during B cell differentiation, activation and during antibody formation highlight the importance of investing research efforts in dissecting the participation of epigenetic defects in this group of diseases. This review focuses on the role of epigenetic control in B cell biology which can provide clues for the study of potential novel pathogenic defects involved in PADs.

Lineage-related and particle size-dependent cytotoxicity of chitosan nanoparticles on mouse bone marrow-derived hematopoietic stem and progenitor cells

Chitosan nanoparticles (CSNPs) have potential applications in stem cell research. In this study, ex vivo cytotoxicity of CSNPs on mouse bone marrow-derived (MBMCs) hematopoietic stem and progenitor cells (HSPCs) was determined. MBMCs were exposed to CSNPs of different particle sizes at various concentrations for up to 72 h. Cytotoxicity effect of CSNPs on MBMCs was determined using MTT, Live/Dead Viability/Cytotoxicity assays and flow cytometry analysis of surface antigens on HSCs (Sca-1(+)), myeloid-committed progenitors (CD11b(+), Gr-1(+)), and lymphoid-committed progenitors (CD45(+), CD3e(+)). At 24 h incubation, MBMCs' viability was not affected by CSNPs. At 48 and 72 h, significant reduction was detected at higher CSNPs concentrations. Small CSNPs (200 nm) significantly reduced MBMCs' viability while medium-sized particle (∼400 nm) selectively promoted MBMCs growth. Surface antigen assessment demonstrated lineage-dependent effect. Significant decrease in Sca-1(+) cells percentage was observed for medium-sized particle at the lowest CSNPs concentration. Meanwhile, reduction of CD11b(+) and Gr-1(+) cells percentage was detected at high and intermediate concentrations of medium-sized and large CSNPs. Percentage of CD45(+) and CD3e(+) cells along with ROS levels were not significantly affected by CSNPs. In conclusion, medium-sized and large CSNPs were relatively non-toxic at lower concentrations. However, further investigations are necessary for therapeutic applications.

c-Myb is required for pro-B cell differentiation

The c-Myb transcription factor is required for normal adult hematopoiesis. However, the embryonic lethality of Myb-null mutations has been an impediment to identifying roles for c-Myb during lymphocyte development. We have used tissue-specific inactivation of the Myb locus in early progenitor cells to demonstrate that c-Myb is absolutely required for the differentiation of CD19(+) B-lineage cells and B cell differentiation is profoundly blocked beyond the pre-pro-B cell stage in Myb(f/f) Mb1-cre mice. We demonstrate that c-Myb is required for the intrinsic survival of CD19(+) pro-B cells as well as the proper expression of the alpha-chain of the IL-7 receptor (CD127) and Ebf1. However, survival of c-Myb-deficient CD19(+) pro-B cells cannot be rescued by transduction with CD127-producing retrovirus, suggesting that c-Myb controls a survival pathway independent of CD127. Furthermore, c-Myb-deficient progenitor cells inefficiently generate CD19(+) B-lineage cells during stromal cell culture but this process can be partially rescued with exogenous Ebf1. Thus, c-Myb does not appear to be required for commitment to B cell differentiation but is crucial for B cell differentiation to the CD19(+) pro-B cell stage as well as survival of CD19(+) pro-B cells. Surprisingly, forced c-Myb expression in lymphoid-primed multipotent progenitors favors differentiation toward the myeloid lineage, suggesting that proper c-Myb expression is crucial for B-lineage development.

Myeloid lineage commitment from the hematopoietic stem cell

Prospective isolation of hematopoietic stem and progenitor cells has identified the lineal relationships among all blood-cell types and has allowed their developmental mechanisms to be assayed at the single-cell level. These isolated cell populations are used to elucidate the molecular mechanism of lineage fate decision and of its plasticity directly by stage-specific enforcement or repression of lineage-instructive signaling in purified cells. With an emphasis on the myeloid lineage, this review summarizes current concepts and controversies regarding adult murine hematopoietic development and discusses the potential mechanisms, operated by single or by multiple transcription factors, of myeloid lineage fate decision.

The Transcriptional Regulation of B Cell Lineage Commitment

The expression of lineage-associated genes, as well as the survival and expansion of committed B cell progenitors, is controlled by multiple transcriptional regulators and growth-factor receptors. Whereas certain DNA-binding proteins, such as Ikaros and PU.1, are required primarily for the formation of more primitive lymphoid progenitors, other factors such as E2A and EBF1 have more direct roles in specifying the B cell-specific gene-expression program. Further, Pax5 functions to promote B cell commitment by repressing lineage-inappropriate gene expression and reinforcing B cell-specific gene expression. In this review, we focus on recent studies that have revealed that instead of a simple transcriptional hierarchy, efficient B cell commitment and differentiation requires the combinatorial activity of multiple transcription factors in a complex gene regulatory network.

Cartography of Hematopoietic Stem Cell Commitment Dependent upon a Reporter for Transcription Factor Activation

A hierarchical hematopoietic developmental tree has been proposed based on the result of prospective purification of lineage-restricted progenitors. For more detailed mapping for hematopoietic stem cell (HSC) commitment, we tracked the expression of PU.1, a major granulocyte/monocyte (GM)- and lymphoid-related transcription factor, from the HSC to the myelolymphoid progenitor stages by using a mouse line harboring a knockin reporter for PU.1. This approach enabled us to find a new progenitor population committed to GM and lymphoid lineages within the HSC fraction. This result suggests that there should be another developmental pathway independent of the conventional one with myeloid versus lymphoid bifurcation, represented by common myeloid progenitors and common lymphoid progenitors, respectively. The utilization of the transcription factor expression as a functional marker might be useful to obtain cartography of the hematopoietic development at a higher resolution.

Thymus Exclusivity: All the Right Conditions for T Cells

A fraction of primitive "lymphoid" precursors retain plasticity for myeloid differentiation. In this issue of Immunity, Laiosa et al. describe that Notch-Delta signals can protect thymic precursors from reprogramming into the myeloid lineage, antagonizing the enforced myeloid transcription factors such as PU.1 and C/EBPalpha.

Primitive lymphoid progenitors in bone marrow with T lineage reconstituting potential

Multiple subsets of the bone marrow contain T cell precursors, but it remains unclear which is most likely to replenish the adult thymus. Therefore, RAG-1+ early lymphoid progenitors (RAG-1+ ELP), and CD62L/L-selectin+ progenitors (LSP), as well as common lymphoid progenitors from C57BL6-Thy1.1-RAG-1/GFP mouse bone marrow were directly compared in transplantation assays. The two c-Kit(high) populations vigorously regenerated the thymus and were superior to common lymphoid progenitors in magnitude and frequency of thymic reconstitution. Regeneration was much faster than the 22 days described for transplanted stem cells, and RAG-1+ ELP produced small numbers of lymphocytes within 13 days. As previously reported, LSP were biased to a T cell fate, but this was not the case for RAG-1+ ELP. Although RAG-1+ ELP and LSP had reduced myeloid potential, they were both effective progenitors for T lymphocytes and NK cells. The LSP subset overlapped with and included most RAG-1+ ELP and many RAG-1- TdT+ ELP. LSP and RAG-1+ ELP were both present in the peripheral circulation, but RAG-1+ ELP had no exact counterpart among immature thymocytes. The most primitive of thymocytes were similar to Lin- c-Kit(high) L-selectin+ TdT+ RAG-1- progenitors present in the marrow, suggesting that this population is normally important for sustaining the adult thymus.

Constitutively Active β-Catenin Promotes Expansion of Multipotent Hematopoietic Progenitors in Culture

This study was designed to investigate one component of the Wnt/beta-catenin signaling pathway that has been implicated in stem cell self-renewal. Retroviral-mediated introduction of stable beta-catenin to primitive murine bone marrow cells allowed the expansion of multipotential c-Kit(low)Sca-1(low/-)CD19(-) CD11b/Mac-1(-)Flk-2(-)CD43(+)AA4.1(+)NK1.1(-)CD3(-)CD11c(-)Gr-1(-)CD45R/B220(+) cells in the presence of stromal cells and cytokines. They generated myeloid, T, and B lineage lymphoid cells in culture, but had no T lymphopoietic potential when transplanted. Stem cell factor and IL-6 were found to be minimal requirements for long-term, stromal-free propagation, and a beta-catenin-transduced cell line was maintained for 5 mo with these defined conditions. Although multipotential and responsive to many normal stimuli in culture, it was unable to engraft several types of irradiated recipients. These findings support previous studies that have implicated the canonical Wnt pathway signaling in regulation of multipotent progenitors. In addition, we demonstrate how it may be experimentally manipulated to generate valuable cell lines.

Life before the pre-B cell receptor checkpoint: specification and commitment of primitive lymphoid progenitors in adult bone marrow

The production of B cells is a complex process determined by well-timed combinations of intrinsic factors and environmental cues that guide the differentiation of primitive progenitors in the bone marrow. Expression of several key transcription factors and receptor-stromal cell ligand interactions are landmarks of the earliest events in B lymphopoiesis in adult bone marrow. We describe this as a gradual loss of options for other blood cell lineages coincident with gain of essential properties. Experimental, stress or infection-related deregulation may change B cell fate specification, commitment or population dynamics, and consequently the production rate of mature populations.

Lymphoid progenitors and primary routes to becoming cells of the immune system

Extraordinary progress has been made in charting the maturation of hematopoietic cells. However, these charted processes do not necessarily represent obligate pathways to specialized types of lymphocytes. In fact, there is a degree of plasticity associated with primitive progenitors. Moreover, all lymphocytes of a given kind are not necessarily produced through precisely the same sequence of events. Particularly contentious is the nature of cells that seed the thymus, because different progenitors can generate T cells under experimental circumstances. Non-renewing progenitors with a high density of c-Kit in bone marrow are likely to replenish the thymus under normal circumstances and most closely resemble canonical T cell progenitors.

Distinctive and indispensable roles of PU.1 in maintenance of hematopoietic stem cells and their differentiation

The PU.1 transcription factor is a key regulator of hematopoietic development, but its role at each hematopoietic stage remains unclear. In particular, the expression of PU.1 in hematopoietic stem cells (HSCs) could simply represent "priming" of genes related to downstream myelolymphoid lineages. By using a conditional PU.1 knock-out model, we here show that HSCs express PU.1, and its constitutive expression is necessary for maintenance of the HSC pool in the bone marrow. Bone marrow HSCs disrupted with PU.1 in situ could not maintain hematopoiesis and were outcompeted by normal HSCs. PU.1-deficient HSCs also failed to generate the earliest myeloid and lymphoid progenitors. PU.1 disruption in granulocyte/monocyte-committed progenitors blocked their maturation but not proliferation, resulting in myeloblast colony formation. PU.1 disruption in common lymphoid progenitors, however, did not prevent their B-cell maturation. In vivo disruption of PU.1 in mature B cells by the CD19-Cre locus did not affect B-cell maturation, and PU.1-deficient mature B cells displayed normal proliferation in response to mitogenic signals including the cross-linking of surface immunoglobulin M (IgM). Thus, PU.1 plays indispensable and distinct roles in hematopoietic development through supporting HSC self-renewal as well as commitment and maturation of myeloid and lymphoid lineages.

The Complex Cartography of Stem Cell Commitment

In this issue of Cell, a study by Adolfsson and coworkers (Adolfsson et al., 2005) provides insight into the early lineage commitment events of multipotent hematopoietic stem cells (HSCs). These studies demonstrate the importance of the Flt3 receptor tyrosine kinase as the earliest marker of hematopoietic cell fate commitment in that erythrocyte and megakaryocyte potentials are lost first as HSCs differentiate to lymphocyte progenitors.

Detailed analysis of gene expression during development of T cell lineages in the thymus

The genetic mechanisms that promote lineage commitment and eliminate autoreactive cells in the thymus are not well understood. To better understand this process, we have identified and quantitated transcripts in the two major thymocyte lineages by using serial analysis of gene expression. Approximately 25 genes displayed almost complete segregation to one or the other T cell lineage. Commitment to the CD4 lineage was marked by up-regulation of genes associated with increased survival and chaperone function followed by expression of genes that regulate nucleosome remodeling and T cell receptor signaling. Differentiation within the CD8 lineage, on the other hand, was marked by up-regulation of genes that regulate lymphocyte homing, followed by quenching of genes that inhibit apoptosis. Definition of differential gene expression during development of the two major thymocyte lineages will allow insight into mechanisms of T cell development after positive and negative selection.