Target Discovery and Validation Reviews and Protocols

Overview of Immune System 283
then express the recombinases (RAG-1 and RAG-2) and enter a major checkpoint
in T-cell development, where the DN thymocyte commits to either
the α/β or γ/δ T-cell lineage (11). Developmental control checkpoints seem to
ensure that T-cells do not complete their intrathymic differentiation program in
the absence of productive TCR gene rearrangements. As a result, T-cells do not
mature with TCRαβ having inappropriate specificities. If the γ and δ genes are
rearranged productively, the cells express the TCR γδ receptor at the surface and
subsequently follow the γδ lineage (15). In these cells, the CD4 and CD8 expression
remains off (Fig. 3). Although successful rearrangement of either β or γδ
genes may influence the lineage decision, the mechanism by which thymocytes
choose between the αβ and γδ lineage remain unknown (16). However, the successful
rearrangement of a β-chain gene must occur before successful rearrangement
of both γ and δ. Notably, most thymocytes develop into the αβ lineage. In
this case, the checkpoint transition is induced by the rearrangement of the TCR.
Productive rearrangement of TCRβ gene leads to the formation of the pre-
TCR signaling complex, which contains a TCRβ chain, a nonpolymorphic surrogate
α-chain (pTα), and the multisubunit CD3 signaling complex. Assembly
of the pre-TCR of the cell surface of DN thymocytes promotes their survival,
informs them to progress to the CD4+CD8+ (double-positive [DP]) stage of
development and causes them to undergo significant clonal expansion through
an Lck/Ras-dependent signaling pathway (17,18). The assembly of the pre-TCR
establishes a key checkpoint in early thymocyte differentiation at which developing
T-cells undergo “β-selection.” Pre-TCR signaling triggers the phosphorylation
and degradation of RAG-2, halting β-chain gene rearrangement, thereby
enforcing strict allelic exclusion at the β-locus. This process is mainly controlled
by the protein kinase C (PKC)α signaling pathway (16–18). We recently have
found that PKCα also is involved in the differentiation of HSC into the myeloid
lineage (19). Pre-TCR also induces rapid cell proliferation and eventually the
Fig. 3. (Opposite page) T-cell development. Various stages of T-cell development
in the thymus are shown. Productive V to DJβ rearrangement in double negative (DN)
pro-T-cells results in the assembly of the pre-TCR. These pre-T-cells become CD4/CD8
double positive (DP) thymocytes. After TCRβ locus allelic exclusion and activation of
V to Jα rearrangement, DP thymocytes express mature αβ TCR. T-cell receptors that
recognize self-peptide/major histocompatability complex (MHC) on thymic epithelial
cells (TECs) with high affinity are deleted by apoptosis (negative selection), whereas
T-cells reacting with self-peptide/MHC complexes with low affinity are positively
selected, resulting in the generation of CD4 or CD8 single positive thymocytes. Also,
the selection results in the progression into regulatory T-cells (Treg) cells and natural
killer T-cells (NKT). Some specific markers are indicated. DN pro-T-cells that succeed
to rearrange the T-cell receptor (TCR) γδ will lead to the generation of γδ T-cells.