Transcription factor Zbtb1 interacts with bridging factor Lmo2 and maintains the T-lineage differentiation capacity of lymphoid progenitor cells

Hematopoietic stem and progenitor cells can differentiate into all types of blood cells. Regulatory mechanisms underlying pluripotency in progenitors, such as the ability of lymphoid progenitor cells to differentiate into T-lineage, remain unclear. We have previously reported that LIM domain only 2 (Lmo2), a bridging factor in large transcriptional complexes, is essential to retain the ability of lymphoid progenitors to differentiate into T-lineage. However, biochemical characterization of Lmo2 protein complexes in physiological hematopoietic progenitors remains obscure. Here, we identified approximately 600 Lmo2-interacting molecules in a lymphoid progenitor cell line by two-step affinity purification with LC-MS/MS analysis. Zinc finger and BTB domain containing 1 (Zbtb1) and CBFA2/RUNX1 partner transcriptional corepressor 3 (Cbfa2t3) were found to be the functionally important binding partners of Lmo2. We determined CRISPR/Cas9-mediated acute disruption of Zbtb1 or Cbfa2t3 in the lymphoid progenitor or bone marrow–derived primary hematopoietic progenitor cells causes significant defects in the initiation of T-cell development when Notch signaling is activated. Our transcriptome analysis of Zbtb1- or Cbfa2t3-deficient lymphoid progenitors revealed that Tcf7 was a common target for both factors. Additionally, ChIP-seq analysis showed that Lmo2, Zbtb1, and Cbfa2t3 cobind to the Tcf7 upstream enhancer region, which is occupied by the Notch intracellular domain/RBPJ transcriptional complex after Notch stimulation, in lymphoid progenitors. Moreover, transduction with Tcf7 restored the defect in the T-lineage potential of Zbtb1-deficient lymphoid progenitors. Thus, in lymphoid progenitors, the Lmo2/Zbtb1/Cbfa2t3 complex directly binds to the Tcf7 locus and maintains responsiveness to the Notch-mediated inductive signaling to facilitate T-lineage differentiation.

Long-term lymphoid progenitors independently sustain naïve T and NK cell production in humans

Our mathematical model of integration site data in clinical gene therapy supported the existence of long-term lymphoid progenitors capable of surviving independently from hematopoietic stem cells. To date, no experimental setting has been available to validate this prediction. We here report evidence of a population of lymphoid progenitors capable of independently maintaining T and NK cell production for 15 years in humans. The gene therapy patients of this study lack vector-positive myeloid/B cells indicating absence of engineered stem cells but retain gene marking in both T and NK. Decades after treatment, we can still detect and analyse transduced naïve T cells whose production is likely maintained by a population of long-term lymphoid progenitors. By tracking insertional clonal markers overtime, we suggest that these progenitors can support both T and NK cell production. Identification of these long-term lymphoid progenitors could be utilised for the development of next generation gene- and cancer-immunotherapies.

Sublethal Total Body Irradiation Causes Long-Term Deficits in Thymus Function by Reducing Lymphoid Progenitors

Total body irradiation (TBI) damages hematopoietic cells in the bone marrow and thymus; however, the long-term effects of irradiation with aging remain unclear. In this study, we found that the impact of radiation on thymopoiesis in mice varied by sex and dose but, overall, thymopoiesis remained suppressed for ≥12 mo after a single exposure. Male and female mice showed a long-term dose-dependent reduction in thymic cKit⁺ lymphoid progenitors that was maintained throughout life. Damage to hematopoietic stem cells (HSCs) in the bone marrow was dose dependent, with as little as 0.5 Gy causing a significant long-term reduction. In addition, the potential for T lineage commitment was radiation sensitive with aging. Overall, the impact of irradiation on the hematopoietic lineage was more severe in females. In contrast, the rate of decline in thymic epithelial cell numbers with age was radiation-sensitive only in males, and other characteristics including Ccl25 transcription were unaffected. Taken together, these data suggest that long-term suppression of thymopoiesis after sublethal irradiation was primarily due to fewer progenitors in the BM combined with reduced potential for T lineage commitment. A single irradiation dose also caused synchronization of thymopoiesis, with a periodic thymocyte differentiation profile persisting for at least 12 mo postirradiation. This study suggests that the number and capability of HSCs for T cell production can be dramatically and permanently damaged after a single relatively low TBI dose, accelerating aging-associated thymic involution. Our findings may impact evaluation and therapeutic intervention of human TBI events.

Pioneering Activity of the C-Terminal Domain of EBF1 Shapes the Chromatin Landscape for B Cell Programming

Lymphopoiesis requires the activation of lineage-specific genes embedded in naive, inaccessible chromatin or in primed, accessible chromatin. The mechanisms responsible for de novo gain of chromatin accessibility, known as "pioneer" function, remain poorly defined. Here, we showed that the EBF1 C-terminal domain (CTD) is required for the regulation of a specific gene set involved in B cell fate decision and differentiation, independently of activation and repression functions. Using genome-wide analysis of DNaseI hypersensitivity and DNA methylation in multipotent Ebf1(-/-) progenitors and derivative EBF1wt- or EBF1ΔC-expressing cells, we found that the CTD promoted chromatin accessibility and DNA demethylation in previously naive chromatin. The CTD allowed EBF1 to bind at inaccessible genomic regions that offer limited co-occupancy by other transcription factors, whereas the CTD was dispensable for EBF1 binding at regions that are occupied by multiple transcription factors. Thus, the CTD enables EBF1 to confer permissive lineage-specific changes in progenitor chromatin landscape.

Progress in the use of swine in developmental immunology of B and T lymphocytes

The adaptive immune system of higher vertebrates is believed to have evolved to counter the ability of pathogens to avoid expulsion because their high rate of germline mutations. Vertebrates developed this adaptive immune response through the evolution of lymphocytes capable of somatic generation of a diverse repertoire of their antigenic receptors without the need to increase the frequency of germline mutation. The focus of our research and this article is on the ontogenetic development of the lymphocytes, and the repertoires they generate in swine. Several features are discussed including (a) the "closed" porcine placenta means that de novo fetal development can be studied for 114 days without passive influence from the mother, (b) newborn piglets are precocial permitting them to be reared without their mothers in germ-free isolators, (c) swine are members of the γδ-high group of mammals and thus provides a greater opportunity to characterize the role of γδ T cells and (d) because swine have a simplified variable heavy and light chain genome they offer a convenient system to study antibody repertoire development.

Development and maturation of natural killer cells

Natural killer (NK) cells are innate lymphocytes that are critical for host protection against pathogens and cancer due to their ability to rapidly release inflammatory cytokines and kill infected or transformed cells. In the 40 years since their initial discovery, much has been learned about how this important cellular lineage develops and functions. We now know that NK cells are the founding members of an expanded family of lymphocyte known as innate lymphoid cells (ILC). Furthermore, we have recently discovered that NK cells can possess features of adaptive immunity such as antigen specificity and long-lived memory responses. Here we will review our current understanding of the molecular mechanisms driving development of NK cells from the common lymphoid progenitor (CLP) to mature NK cells, and from activated effectors to long-lived memory NK cells.

Functionally Distinct Subsets of Lineage-Biased Multipotent Progenitors Control Blood Production in Normal and Regenerative Conditions

Despite great advances in understanding the mechanisms underlying blood production, lineage specification at the level of multipotent progenitors (MPPs) remains poorly understood. Here, we show that MPP2 and MPP3 are distinct myeloid-biased MPP subsets that work together with lymphoid-primed MPP4 cells to control blood production. We find that all MPPs are produced in parallel by hematopoietic stem cells (HSCs), but with different kinetics and at variable levels depending on hematopoietic demands. We also show that the normally rare myeloid-biased MPPs are transiently overproduced by HSCs in regenerating conditions, hence supporting myeloid amplification to rebuild the hematopoietic system. This shift is accompanied by a reduction in self-renewal activity in regenerating HSCs and reprogramming of MPP4 fate toward the myeloid lineage. Our results support a dynamic model of blood development in which HSCs convey lineage specification through independent production of distinct lineage-biased MPP subsets that, in turn, support lineage expansion and differentiation.

Differential regulation of microRNA expression in irradiated and bystander cells

The ionizing radiation (IR) induces a variety of biological effects in irradiated cells. Additionally, the irradiated cells communicate with unirradiated cells and induce changes in them through a phenomenon termed as the bystander effect. The nature of the bystander effect signal and how it impacts unirradiated cells remains to be discovered. Examination of molecular changes in bystander cells due to signals from irradiated cells could lead to the identification of the pathways underlying the bystander effect. To gain insight into the molecular pathways affected by the transmission of signal from irradiated cells to bystander cells, we monitored the microRNA (miRNA) transcriptional changes. miRNAs control gene expression at the posttranscriptional level. In previous studies from our laboratory the modulation of miRNA in irradiated human cells were identified. In the present work human lymphoblasts TK6 cells in a medium exchanged bystander effect model system were used to analyze miRNA expression alterations by employing the real time RT-PCR technology. The relative expression of several miRNAs involved in RAS, c-MYC and BCL2 gene regulation were examined. The let-7 family of miRNAs was upregulated in irradiated cells but most of these miRNAs remained repressed in bystander cells. The miR-17-3p, miR-19b, and miR-18a were upregulated in irradiated cells but were repressed in the bystander cells. The miR-17-5p, miR-142-3p, miR-142-5p, and miR-19a were induced only for a short time in bystander cells. The miR-15a, miR-16, miR-143, miR-145, miR-155, and miR21 were upregulated in irradiated TK6 cells. While the expression of miR-15a, miR-16, miR-155, and miR-21 was repressed, the miR-143 and miR-145 expression was induced in bystander cells. These results indicate the involvement of miRNA modulation in irradiated and bystander cells.

Interleukin-22-producing innate immune cells: new players in mucosal immunity and tissue repair?

Mucosal tissues, lying at the interface with the external environment, are constantly challenged by microbial, physical and chemical assaults. To provide the necessary immune defence to such challenges, lymph nodes and Peyer's patches are formed in utero in response to inductive signals from lymphoid-tissue inducer (LTi) cells. As discussed in this Progress article, a series of recent reports has identified a population of interleukin-22-producing mucosal cells in the gut and tonsils that share features with both LTi cells (by expressing RORgammat) and natural killer cells (by expressing NKp46) and that might be involved in immunity and homeostasis in mucosal tissues.