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Target Discovery and Validation Reviews and Protocols

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280 Sioud<br />

Fig. 1. Schematic of hematopoietic cell generation. Hematopoietic stem cells (HSCs)<br />

have self-renewal ability <strong>and</strong> differentiate through several intermediates into all the lineage<br />

of the hematopoietic system. In the adult, the lymphoid lineage differentiates via the<br />

common lymphoid progenitor (CLP), resulting in the generation of B-, T- <strong>and</strong> natural<br />

killer (NK) cells.<br />

Activation of human CD34+ hematopoietic stem cells with R848, a specific<br />

lig<strong>and</strong> for TLR7 <strong>and</strong> TLR8, induced their differentiation into the myeloid cell<br />

lineage, including macrophages <strong>and</strong> dendritic cells (Johansen, L. F. <strong>and</strong> Sioud,<br />

M., unpublished data). This finding would indicate that microbial lig<strong>and</strong>s for<br />

TLRs might shape the homeostasis of innate immune cells. As shown in Fig.<br />

1, the adult lymphoid lineage, which includes NK, B-, <strong>and</strong> T-cells, is generated<br />

by the differentiation of HSCs via the common lymphoid progenitor (CLP),<br />

which is the earliest lineage-restricted lymphoid progenitor that has the potential<br />

to develop into these cells. Developing lymphocyte must go through several<br />

major developmental checkpoints before becoming a functional mature T-cell<br />

tolerant to self-antigens <strong>and</strong> having a huge potential to react with unlimited<br />

number of foreign antigens (11,12). The first checkpoint occurs when the CLP<br />

commits to the T-cell lineage. Recent studies indicated a crucial role for Notch-<br />

1 signaling in this process (Fig. 2). Notch proteins were first identified in<br />

Drosophila <strong>and</strong> Caenorhabditis elegans by their ability to direct cell fate decisions<br />

as well as control survival <strong>and</strong> proliferation. These proteins are widely<br />

expressed in different tissues, including the hematopoietic system. In the<br />

absence of Notch-1, bone marrow progenitors enter the thymus <strong>and</strong> develop<br />

into B-cells. Thus, Notch-1 “instructs” an early lymphoid progenitor to adopt a<br />

T- versus B-cell fate (13,14). Notably, transcriptional activation by Notch<br />

requires its binding to CSL/RBP-J, leading to the formation of an active transcriptional<br />

activation complex. In this respect CSL-deficient bone marrow

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