Target Discovery and Validation Reviews and Protocols

Overview of Immune System 285
expression of the coreceptor proteins CD4 and CD8 (18). During this proliferative
phase, the expression of RAG-1 and RAG-2 genes is inhibited. Hence, no
rearrangement of the α-chain locus occurs until the proliferative phase ends.
The TCRα chain genes are comparable with Igκ and λ light chain genes in that
they do not have D gene segments and are rearranged only after their partner receptor
chain gene has been expressed. Subsequent to the expression of productive Vβ
gene rearrangement and few cycles of cell division, the developing thymocytes exit
the cell cycle, reexpress RAG-1 and RAG-2, and eventually start rearranging the
α-chain genes. Unlike TCRβ-chain, the expression of the TCRα-chain is not in
itself sufficient to shut off gene rearrangement (20). As a result, up to one-third of
mature T-cells harbor two productively rearranged TCRα alleles (21). Although
the exact function of the second TCR is unclear, recent data indicated that the
second TCRs are involved in extending TCR repertoires against microorganisms,
and they also are involved in autoimmunity (22). Noteworthy, up to this stage, the
development of the thymocytes has been independent of antigens.
2.2. TCR-Mediated Selection of Thymocytes: Central Tolerance
Productive Vα gene rearrangement allows the developing T-cell to assemble
a complete TCR molecule, which is expressed on thymocyte surface in association
with CD3 complex, forming active TCR. Most of the TCR-mediated selection
processes begin at this stage of development that occurs primarily in the
thymus. The selection process includes both positive and negative selection of
immature CD4+ and CD8+ cells (Fig. 3). Both processes are regulated through
the interaction between the T-cell receptor expressed by a given DP thymocyte
and peptide presented by thymic epithelial cells (8,11). Thymocytes with TCRs
that bind strongly to MHC-associated self-peptide are eliminated (negative
Fig. 4. (Opposite page) B-cell development and selection. Various stages of B-cell
development in the bone marrow and spleen are shown. In stem cells, Ig genes are in the
germline configuration as found in all nonlympoid cells. In pro-B-cells D to JH rearrangement
on both heavy chain alleles precedes V to DJH rearrangement. Cells that succeed to
rearrange a functional heavy chain (pre-B-cells) assemble the pre-B-cell antigen receptor
(BCR), which leads to heavy chain allelic exclusion and activation of Ig light chain (IgL)
rearrangement (κ and λ). After successfully rearrangement of the IgL, immature B-cells
express the BCR (IgM+). Immature B-cells that interact strongly with self-antigens are
deleted or undergo another rearrangement at the IgL locus (receptor editing) to alter their
specificity. Most of the autoreactive B-cells are deleted in the bone marrow. However, some
of these autoreactive B-cells reacting with self-antigens can be rescued by undergoing
receptor editing to generate new receptor specificity. Nonautoreactive B-cells, exit the bone
marrow via sinusoidal endothelium as IgM-, IgD-positive cells. Notably, immature B-cells
(IgM high IgD low ) progressively acquire more IgD as they develop into mature B-cells.