Molecular Regulation of Early T-Cell Development in the Thymus

The human body is under constant siege of pathogens - bacteria, viruses, fungi and parasites. We can only survive because these attackers are continuously fought off by our immune system. Important tasks within the immune system of vertebrates are performed by T lymphocytes, the executors of speci.c, cellular immunity. The speci.city of T lymphocytes lies in their T-cell receptor (TCR), through which they sense the presence of antigens in their environment. Each T cell expresses a TCR with a unique antigen-recognition site, so all T cells together can respond to an enormous variety of antigens. The highly diverse T-cell repertoire is generated by random recombination of discrete TCR gene segments. Via the TCR, T cells recognize peptide antigens that are displayed by antigen presenting cells (APCs), in the context of major histocompatibility complex (MHC) class I or class II molecules. Mature T cells carry out their function in cellular immunity as either CD8+ cytotoxic T cells or as CD4+ helper T cells. The humoral part of the speci.c immune system is supplied by B lymphocytes, which can secrete their antigen receptors in the form of antibodies. T cells develop from multipotent precursors via a highly ordered, but complex differentiation pathway. A number of critical events occur during this T-cell development process. Cells proliferate, adopt a T-cell fate, and produce a TCR molecule via a strictly ordered process of gene rearrangements. Stringent selection processes make sure that the produced TCR molecule is self-MHC restricted but not reactive to self-antigens. Finally, the selected T cells are allowed to mature into functional effector T cells. To become highly differentiated and thoroughly ‘educated’ cells, T cells need a specialized microenvironment for their development. The organ which has evolved to ful.ll this task and which also gave T cells their name, is the Thymus. The hematopoietic cells that undergo T-cell development in the thymus are called thymocytes. The importance of the thymus as essential microenvironment for T-cell development is illustrated by children af.icted by the DiGeorge syndrome, who sometimes completely lack a thymus. These children have severely reduced T cell numbers and suffer from recurrent life-threatening infections. DiGeorge syndrome also illustrates the fact that T cells are the only hematopoietic cell type that absolutely require the thymus for their development. Although human T-cell development is the main theme of this thesis, most of the published knowledge about T-cell differentiation has been acquired by studies in the mouse

Wnt target genes identified by DNA microarrays in immature CD34+ thymocytes regulate proliferation and cell adhesion

The thymus is seeded by very small numbers of progenitor cells that undergo massive proliferation before differentiation and rearrangement of TCR genes occurs. Various signals mediate proliferation and differentiation of these cells, including Wnt signals. Wnt signals induce the interaction of the cytoplasmic cofactor beta-catenin with nuclear T cell factor (TCF) transcription factors. We identified target genes of the Wnt/beta-catenin/TCF pathway in the most immature (CD4-CD8-CD34+) thymocytes using Affymetrix DNA microarrays in combination with three different functional assays for in vitro induction of Wnt signaling. A relatively small number (approximately 30) of genes changed expression, including several proliferation-inducing transcription factors such as c-fos and c-jun, protein phosphatases, and adhesion molecules, but no genes involved in differentiation to mature T cell stages. The adhesion molecules likely confine the proliferating immature thymocytes to the appropriate anatomical sites in the thymus. For several of these target genes, we validated that they are true Wnt/beta-catenin/TCF target genes using real-time quantitative PCR and reporter gene assays. The same core set of genes was repressed in Tcf-1-null mice, explaining the block in early thymocyte development in these mice. In conclusion, Wnt signals mediate proliferation and cell adhesion, but not differentiation of the immature thymic progenitor pool

Notch signaling during human T cell development

Notch signaling is critical during multiple stages of T cell development in both mouse and human. Evidence has emerged in recent years that this pathway might regulate T-lineage differentiation differently between both species. Here, we review our current understanding of how Notch signaling is activated and used during human T cell development. First, we set the stage by describing the developmental steps that make up human T cell development before describing the expression profiles of Notch receptors, ligands, and target genes during this process. To delineate stage-specific roles for Notch signaling during human T cell development, we subsequently try to interpret the functional Notch studies that have been performed in light of these expression profiles and compare this to its suggested role in the mouse

Radiolysis of NaCl at high and low temperatures: development of size distribution of bubbles and colloids

New experimental results are presented on low temperature irradiation (18 °C) of rock-salt samples which had been exposed to initial doses up to 320 GRad at 100 °C. Differential scanning calorimetry (DSC) shows that the latent heat of melting (LHM) of sodium colloids decreases during subsequent low-temperature irradiation, whereas the stored energy (SE) increases slowly, indicating that the process of radiolysis continues. The decrease of the LHM is due to dissolution of large colloids, because the intensities of the melting peaks decrease during the second stage irradiation at low temperature. The model is formulated to describe the nucleation kinetics and the evolution of the size distribution of chlorine precipitates and sodium colloids in NaCl under high dose irradiation. It is shown that the mechanism of dissolution of large Na colloids during low temperature irradiation can be related to melting of sodium colloids.

Wnt3a deficiency irreversibly impairs hematopoietic stem cell self-renewal and leads to defects in progenitor cell differentiation

Canonical Wnt signaling has been implicated in various aspects of hematopoiesis. Its role is controversial due to different outcomes between various inducible Wnt-signaling loss-of-function models and also compared with gain-of-function systems. We therefore studied a mouse deficient for a Wnt gene that seemed to play a nonredundant role in hematopoiesis. Mice lacking Wnt3a die prenatally around embryonic day (E) 12.5, allowing fetal hematopoiesis to be studied using in vitro assays and transplantation into irradiated recipient mice. Here we show that Wnt3a deficiency leads to a reduction in the numbers of hematopoietic stem cells (HSCs) and progenitor cells in the fetal liver (FL) and to severely reduced reconstitution capacity as measured in secondary transplantation assays. This deficiency is irreversible and cannot be restored by transplantation into Wnt3a competent mice. The impaired long-term repopulation capacity of Wnt3a-/- HSCs could not be explained by altered cell cycle or survival of primitive progenitors. Moreover, Wnt3a deficiency affected myeloid but not B-lymphoid development at the progenitor level, and affected immature thymocyte differentiation. Our results show that Wnt3a signaling not only provides proliferative stimuli, such as for immature thymocytes, but also regulates cell fate decisions of HSC during hematopoiesis

Cooperation between Wnt and Notch signalling in human breast cancer

The Wnt and Notch signalling pathways play major roles in mammary gland development and tumourigenesis. During development, these pathways have opposing effects. However, in a recent paper Ayyanan and coworkers show that expression of Wnt1 is sufficient to transform primary human mammary epithelial cells, and that this is in part due to activation of the Notch pathway. This indicates that during tumourigenesis the two pathways cooperate. Here we ask why activation of Wnt signalling alone is sufficient to cause transformation; whether there is evidence for inhibitory crosstalk between the pathways during tumourigenesis; and whether cooperation between these pathways occurs in other forms of cancer

Transition from fresh frozen plasma to solvent/detergent plasma in the Netherlands: comparing clinical use and transfusion reaction risks

Plasma transfusion is indicated for replenishment of coagulative pro- teins to stop or prevent bleeding. In 2014, the Netherlands switched from using similar to 300mL fresh frozen plasma (FFP) units to using 200mL Omniplasma, a solvent/detergent treated pooled plasma (SD plasma), units. We evaluated the effect of the introduction of SD plasma on clinical plasma use, associated bleeding, and transfusion reaction incidences. Using diagnostic data from six Dutch hospitals, national blood bank data, and national hemovigilance data for 2011 to 2017, we compared the plasma/red blood cell (RBC) units ratio (f) and the mean number of plasma and RBC units transfused for FFP (similar to 300mL) and SD plasma (200mL) for various patient groups, and calculated odds ratios comparing their associated transfusion reaction risks. Analyzing 13,910 transfusion episodes, the difference (Delta f = f(SD) (-) f(FFP)) in mean plasma/RBC ratio (f) was negligible (Delta f(entire-cohort) = 0.01 [95% confidence interval (CI): -0.02 - 0.05]; P=0.48). SD plasma was associated with fewer RBC units transfused per episode in gynecological (difference of mean number of units -1.66 [95% CI: -2.72, -0.61]) and aneurysm (-0.97 [-1.59, -0.35]) patients. SD plasma was further associated with fewer anaphylactic reactions than FFP (odds ratio 0.37 [0.18, 0.77; P<0.01]) while the differences for most transfusion reactions were not statistically significant. SD plasma units, despite being one third smaller in volume than FFP units, are not associated with a higher plasma/RBC ratio. SD plasma is associated with fewer anaphylactic reactions than FFP plasma/RBC units ratio.Clinical epidemiolog