A small number of signalling pathways are used iteratively to regulate cell fates, cell proliferation and cell death in development. Notch is the receptor in one such pathway, and is unusual in that most of its ligands are also transmembrane proteins; therefore signalling is restricted to neighbouring cells. Although the intracellular transduction of the Notch signal is remarkably simple, with no secondary messengers, this pathway functions in an enormous diversity of developmental processes and its dysfunction is implicated in many cancers.

Notch signalling: a simple pathway becomes complex

A functional immune system requires the selection of T lymphocytes expressing receptors that are major histocompatibility complex restricted but tolerant to self-antigens. This selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a surface receptor. In this review we summarize the current state of the field regarding the natural ligands and molecular factors required for positive and negative selection and discuss a model for how these disparate outcomes can be signaled via the same receptor. We also discuss emerging data on the selection of regulatory T cells. Such cells require a high-affinity interaction with self-antigens, yet differentiate into regulatory cells instead of being eliminated.

Positive and Negative Selection of T Cells

Interferon-gamma (IFN-gamma) is crucial for immunity against intracellular pathogens and for tumor control. However, aberrant IFN-gamma expression has been associated with a number of autoinflammatory and autoimmune diseases. This cytokine is produced predominantly by natural killer (NK) and natural killer T (NKT) cells as part of the innate immune response, and by Th1 CD4 and CD8 cytotoxic T lymphocyte (CTL) effector T cells once antigen-specific immunity develops. Herein, we briefly review the functions of IFN-gamma, the cells that produce it, the cell extrinsic signals that induce its production and influence the differentiation of naive T cells into IFN-gamma-producing effector T cells, and the signaling pathways and transcription factors that facilitate, induce, or repress production of this cytokine. We then review and discuss recent insights regarding the molecular regulation of IFN-gamma, focusing on work that has led to the identification and characterization of distal regulatory elements and epigenetic modifications with the IFN-gamma locus (Ifng) that govern its expression. The epigenetic modifications and three-dimensional structure of the Ifng locus in naive CD4 T cells, and the modifications they undergo as these cells differentiate into effector T cells, suggest a model whereby the chromatin architecture of Ifng is poised to facilitate either rapid opening or silencing during Th1 or Th2 differentiation, respectively.

Regulation of interferon-gamma during innate and adaptive immune responses.

CD4+T cells are crucial in achieving a regulated effective immune response to pathogens. Naive CD4+T cells are activated after interaction with antigen-MHC complex and differentiate into specific subtypes depending mainly on the cytokine milieu of the microenvironment. Besides the classical T-helper 1 and T-helper 2, other subsets have been identified, including T-helper 17, regulatory T cell, follicular helper T cell, and T-helper 9, each with a characteristic cytokine profile. For a particular phenotype to be differentiated, a set of cytokine signaling pathways coupled with activation of lineage-specific transcription factors and epigenetic modifications at appropriate genes are required. The effector functions of these cells are mediated by the cytokines secreted by the differentiated cells. This paper will focus on the cytokine-signaling and the network of transcription factors responsible for the differentiation of naive CD4+T cells.

/pdf/cd4-t-cells-differentiation-and-functions-48b9x2bguo.pdf

CD4⁺T cells: differentiation and functions

The Notch Ligands Dll4 and Jagged1 Have Opposing Effects on Angiogenesis

Notch receptors and their ligands contribute to many developmental systems, but it is not apparent how they function after birth, as their null mutants develop severe defects during embryogenesis. Here we used the Cre-loxP system to delete the Delta-like 1 gene (Dll1) after birth and demonstrated the complete disappearance of splenic marginal zone B cells in Dll1-null mice. In contrast, T cell development was unaffected. These results demonstrated that Dll1 was dispensable as a ligand for Notch1 at the branch point of T cell-B cell development but was essential for the generation of marginal zone B cells. Thus, Notch signaling is essential for lymphocyte development in vivo, but there is a redundancy of Notch-Notch ligand signaling that can drive T cell development within the thymus.

Delta-like 1 is necessary for the generation of marginal zone B cells but not T cells in vivo.

The thymic microenvironment is required for T cell development in vivo. However, in vitro studies have shown that when hematopoietic progenitors acquire Notch signaling via Delta-like (Dll)1 or Dll4, they differentiate into the T cell lineage in the absence of a thymic microenvironment. It is not clear, however, whether the thymus supports T cell development specifically by providing Notch signaling. To address this issue, we generated mice with a loxP-flanked allele of Dll4 and induced gene deletion specifically in thymic epithelial cells (TECs). In the thymus of mutant mice, the expression of Dll4 was abrogated on the epithelium, and the proportion of hematopoietic cells bearing the intracellular fragment of Notch1 (ICN1) was markedly decreased. Corresponding to this, CD4 CD8 double-positive or single-positive T cells were not detected in the thymus. Further analysis showed that the double-negative cell fraction was lacking T cell progenitors. The enforced expression of ICN1 in hematopoietic progenitors restored thymic T cell differentiation, even when the TECs were deficient in Dll4. These results indicate that the thymus-specific environment for determining T cell fate indispensably requires Dll4 expression to induce Notch signaling in the thymic immigrant cells.

/pdf/delta-like-4-is-indispensable-in-thymic-environment-specific-c8toob7rzd.pdf

Delta-like 4 is indispensable in thymic environment specific for T cell development

Each of the sensory patches in the epithelium of the inner ear is a mosaic
of hair cells and supporting cells. Notch signalling is thought to govern this
pattern of differentiation through lateral inhibition. Recent experiments in
the chick suggest, however, that Notch signalling also has a prior function -
inductive rather than inhibitory - in defining the prosensory patches from
which the differentiated cells arise. Several Notch ligands are expressed in
each patch, but their individual roles in relation to the two functions of
Notch signalling are unclear. We have used a Cre-LoxP approach to knock out
two of these ligands, Delta1 (Dll1) and Jagged1 (Jag1), in the mouse ear. In
the absence of Dll1, auditory hair cells develop early and in excess,
in agreement with the lateral inhibition hypothesis. In the absence of
 Jag1 , by contrast, the total number of these cells is strongly
reduced, with complete loss of cochlear outer hair cells and some groups of
vestibular hair cells, indicating that Jag1 is required for the
prosensory inductive function of Notch. The number of cochlear inner hair
cells, however, is almost doubled. This correlates with loss of expression of
the cell cycle inhibitor p27 Kip1 (Cdkn1b), suggesting that
signalling by Jag1 is also needed to limit proliferation of prosensory cells,
and that there is a core part of this population whose prosensory character is
established independently of Jag1-Notch signalling. Our findings confirm that
Notch signalling in the ear has distinct prosensory and lateral-inhibitory
functions, for which different ligands are primarily responsible.

http://depts.washington.edu/audneuro/Brooker_etal2006.pdf

Notch ligands with contrasting functions: Jagged1 and Delta1 in the mouse inner ear

The Wnt and Notch signalling pathways regulate hair follicle maintenance, but how they intersect is unknown. We show that Notch signalling is active in the hair follicle pre-cortex, a region of high Wnt activity, where commitment to hair lineages occurs. Deletion of jagged 1 (Jag1) results in inhibition of the hair growth cycle and conversion of hair follicles into cysts of cells undergoing interfollicular epidermal differentiation. Conversely, activation of Notch in adult epidermis triggers expansion of the base of the hair follicle, sebaceous gland enlargement and abnormal clumping of the follicles. In adult epidermis, the induction of new hair follicle formation by beta-catenin is prevented by blocking Notch signalling pharmacologically or through Jag1 deletion. Conversely, activation of both pathways accelerates growth and differentiation of ectopic follicles. beta-catenin stimulates Notch signalling by inducing Jag1 transcription. We conclude that the Notch pathway acts downstream of the Wnt/beta-catenin pathway to determine epidermal cell fate.

Jagged 1 is a β-catenin target gene required for ectopic hair follicle formation in adult epidermis

Differentiation of naive CD4+ T cells into helper T (Th) cells is controlled by a combination of several transcriptional factors. In this study, we examined the functional role of the Runx1 transcription factor in Th cell differentiation. Naive T cells from transgenic mice expressing a dominant interfering form of Runx1 exhibited enhanced interleukin 4 production and efficient Th2 differentiation. In contrast, transduction of Runx1 into wild-type T cells caused a complete attenuation of Th2 differentiation and was accompanied by the cessation of GATA3 expression. Furthermore, endogenous expression of Runx1 in naive T cells declined after T cell receptor stimulation, at the same time that expression of GATA3 increased. We conclude that Runx1 plays a novel role as a negative regulator of GATA3 expression, thereby inhibiting the Th2 cell differentiation.

/pdf/the-runx1-transcription-factor-inhibits-the-differentiation-2a2n6nc8fq.pdf

Katsuto Hozumi

Papers

Delta-like 1 is necessary for the generation of marginal zone B cells but not T cells in vivo.

Delta-like 4 is indispensable in thymic environment specific for T cell development

Notch ligands with contrasting functions: Jagged1 and Delta1 in the mouse inner ear

Jagged 1 is a β-catenin target gene required for ectopic hair follicle formation in adult epidermis

The Runx1 Transcription Factor Inhibits the Differentiation of Naive CD4+ T Cells into the Th2 Lineage by Repressing GATA3 Expression