Target Discovery and Validation Reviews and Protocols

286 Sioud
selection), whereas thymocytes with TCR that bind with lower affinity to
MHC-associated self-peptides receive prosurvival signals and are allowed to
progress to the simple positive (SP) stage (positive selection). Thymocytes
expressing TCRs that do not interact with the appropriate MHC die by neglect
(immunological ignorance).
Recent studies indicated that the positively selected thymocytes are subjected
to a maturation step, which increases the threshold for their activation by
self-selected peptides (23). As a consequence, peripheral mature T-cells are
properly activated by only foreign peptides of higher affinity (dominant epitopes).
In conclusion, T-cell development in the thymus starts with CD4–CD8– DN
thymocytes, which progress to CD4+CD8+ DP thymocytes, and finally to
CD4+ or CD8+ SP thymocytes. Subsequent to maturation, these positively
selected T-cells leave the thymus and eventually enter the circulation (Fig. 3).
3. B-Cell Development and Maturation
3.1. Pre-B-Cell Antigen Receptor (BCR) Functions as a Biological Sensor
Similar to T-cells, early B-cell development is ordered by successive rearrangements
of IgH and IgL gene segments during pro-B- and pre-B-cell development
(Fig. 4). This crucial developmental process leads to the generation of a diverse
antibody repertoire for humoral defense against all possible foreign antigens
(24,25). The first step is antigen independent and takes place in primary lymphoid
organs, the bone marrow, and the fetal liver. In developing B-cells (pro-B-cells),
assembly of the IgH gene occurs before that of the IgL genes. The pro-B-cells
express surrogate light chain encoded by VpreB and λ5 genes and the rearrangement
machinery encoded by the RAG-1, RAG-2, and TdT genes. In human,
only one VpreB gene has been found, whereas several λ5-like genes have been
identified. They are all located on chromosome 22, which also carries the λ
light chain genes.
Within the IgH locus, rearrangement begins on both alleles with joining of a
DH to a JH gene segment. Notably, most cells successfully rearrange two D-J
sequences. These pro-B-cells then go on to recombine a VH segment with one
of the partially assembled DJH alleles, leading to the generation of pre-B-cell
population. After productive VDJH rearrangement, the IgH allele is transcribed
and its message is translated, resulting in the production of the transmembrane
form of the heavy chain protein Igµ. Because there are two alleles for each
immunoglobulin locus in the diploid genome, each of which can be activated,
each cell must prevent both alleles from making productive chains. This is
accomplished by checking for productive chain as soon as an allele has
rearranged. Thus, µ heavy chain seems to be functionally equivalent to TCRβchain
in the early states of lymphocyte development.