Target Discovery and Validation Reviews and Protocols

Overview of Immune System 281
Fig. 2. Notch signaling is required for T-cell development. Notch-1 signaling promotes
T-cell development more likely by activating T-cell transcriptional program. In
absence of Notch-1 signaling, B-cell development is activated.
progenitors fail to develop into T-cells in the thymus and consequently develop
ectopically into B-cells (13). Notch-1 signaling also may affect pre-T-cell
receptor (TCR) expression and functions in immature thymocytes. Recent data
indicate that Notch-1 expression is under the control of microRNA-181 (see
Chapter 16, Volume 2).
In contrast to most genes, the genes encoding T- and B-cell antigen receptors
are unusual in that they are composed of a series of V (variable), D (diversity),
and J (joining) gene segments, which require assembly by a site-specific DNA
recombination reaction known as V (D) J recombination. The ability of V, D,
and J gene segments to combine randomly introduces a large element of combinatorial
diversity into the Ig and TCR repertoires. Also, during the rearrangement
process, additional nucleotides not encoded by either gene segments can
be added at the junction between the joined gene segments. This joining process
is random, and, on average, two out of three rearrangements will be nonproductive
because the joining has occurred in different translational reading frames.
In developing T-cells, rearrangement of TCRβ gene occurs before TCRα,
whereas in developing B-cells, assembly of the Ig heavy chain (IgH) gene
occurs before that of the Ig light chain (IgL) genes.
2.1. Pre-TCR Functions as a Biological Sensor
Subsequent to entry into the thymus, T-cell precursors go through a series of
well defined marker changes and selective events (Fig. 3). CLP and the immature
T-cell precursor do not express CD4 and CD8 molecules and are referred
to as CD4–CD8– (double negative [DN]) thymocytes (11). These thymocytes