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228 Part III Immunologic Basis of Hematology

signaling occurs, and the cell undergoes apoptosis. In other cells, a even those with MHC class I–restricted TCRs. In addition, mice with
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similar rearrangement process occurs in the γ and δ loci. Productive a spontaneous mutation in Th-POK lack virtually all CD4 T cells,
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rearrangements of these gene families create a functional, mature indicating that its expression is necessary for CD4 T-cell develop-
γδ TCR that also associates with the TCR signaling complex to ment. Another important factor is RUNX3, a member of the Runx
propagate signals to trigger further cellular development. transcription factor family. As DP thymocytes differentiate, RUNX3
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Although the determining factors that result in either γδ or αβ regulates CD8 T-cell differentiation by silencing CD4 transcription,
T-cell development have not been fully elucidated, several molecular promoting the initiation of CD8 gene transcription and downregulat-
events are thought to contribute. The expression of a TCR gene ing Th-POK expression. Additional studies have identified a network
rearrangement product likely plays a role in lineage determination of key transcription factors and signaling proteins important for
because there is evidence suggesting that developing thymocytes lineage choice in the thymus, underscoring the complexity of this
with a functional γδ TCR are often excluded from the αβ cell fate. stage of T-cell development.
However, TCR expression is not the only factor in determining
lineage fate; cytokine signals and TCR signal strength may also play
a role. Experiments have shown that DN2 thymocytes distinguished Negative Selection
according to IL-7 receptor expression differentiate into αβ or γδ
T cells, with DN2 cells expressing high IL-7 receptor levels pref- Although positive selection ensures that the random combinatorial
erentially developing into γδ T lymphocytes and those with lower rearrangement of gene segments results in a TCR that recognizes
expression more likely to differentiate into the αβ lineage. Other antigen presented by self-MHC proteins, until this point in T-cell
studies have suggested that the strong signals propagated by the γδ development, there is no guard against the emergence of T cells
TCR in comparison to those of the pre-TCR complex may promote that possess TCRs with high reactivity against self-peptides in the
γδ lineage commitment. MHC binding pockets. Thus, to prevent autoimmunity, there must
also be a mechanism to eliminate developing T cells with TCRs
expressing these potentially autoreactive specificities. This process is
Positive Selection called negative selection. Negative selection occurs primarily in the
thymic medulla, where thymocytes serially interact with medullary
Developing αβ T cells that have passed the first checkpoint thymic epithelial cells (mTECs) and other thymic APCs including
demonstrating functional β-chain rearrangement transition into DCs. At this stage, if thymocytes with TCRs engage peptide–MHC
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the double-positive (DP; CD4 CD8 ) stage and complete TCR-α complexes with high affinity, a strong TCR signal initiates apoptosis.
rearrangement to produce a mature αβ TCR heterodimer. The Whereas it is easy to see how this model allows for deletion of
stochastic nature of TCR gene rearrangements guarantees that a developing thymocytes with reactivity against self-antigens generated
significant proportion of cells expressing TCR-αβ complexes will within the thymus itself, it was difficult to imagine how cells with
not be able to interact with self-MHC proteins and hence would reactivity against antigens known to be expressed outside the thymus
not be stimulated by peptide–MHC complexes in the periphery. DP would also be deleted. An explanation for how this occurs came from
thymocytes therefore undergo a series of tests, collectively known as the discovery of the autoimmune regulator (AIRE) protein. Initially
positive and negative selection, to determine TCR fitness. If the TCR identified as the gene product mutated in a rare human autoimmune
is not stimulated via peptide–MHC complexes presented by thymic disorder, autoimmune polyendocrinopathy-candidiasis-ectoderm
APCs, the developing cell undergoes “death by neglect” through dystrophy syndrome (APECED), AIRE was later found to be essential
apoptosis. Approximately 90% of developing αβ DP thymocytes for the expression of peripheral tissue-specific antigens by mTECs.
express a TCR that cannot recognize self-peptide–MHC and thus Although AIRE does not regulate thymic expression of all peripheral
die by neglect. In contrast, those DP thymocytes that interact with antigens, its contribution to the elimination of autoreactive cells is
self-peptide–MHC complexes on thymic cortical epithelial cells with highlighted by the widespread, multiorgan autoimmunity seen in
sufficient strength pass this “positive selection” test and are protected patients with APECED. Identifying additional mechanisms respon-
from apoptosis. sible for thymic expression of tissue-specific genes is an area of active
investigation.
Negative selection is one mechanism for development of “toler-
CD4 and CD8 Lineage Commitment ance” or immune unresponsiveness to self-antigens; however, the
process is not perfect in eliminating all self-reactive T cells. Hence,
The MHC specificity of the TCR on a positively selected DP thy- other means exist to promote self-tolerance after T cells leave the
mocyte influences lineage fate. Cells signaled through a MHC class thymus. One such mechanism relies on development of regulatory
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I–restricted TCR develop into CD8 single-positive (SP; CD4 CD8 ) T cells (Tregs), which actively interfere with effector T-cell function.
cells, and those that receive signals via MHC class II–restricted TCRs Like conventional αβ T cells, a subset of Tregs (previously known
develop into CD4 SP T cells. The underlying molecular mechanisms as natural or nTregs and more recently designated thymic or tTregs)
governing CD4/CD8 lineage choice is much debated. Predicated also develops in the thymus. Tregs are characterized by the surface
on the thought that TCR signals during positive selection result expression of CD4 and CD25 (the α chain of the IL-2 receptor) and
in the termination of either CD4 or CD8 gene transcription, the depend on the transcription factor forkhead box protein 3 (FoxP3) for
two classical models of lineage fate are the stochastic selection and their lineage commitment. The gene encoding FoxP3 was originally
instructive models. In the stochastic selection model, TCR signals in identified as the causal mutation in a rare, and frequently fatal,
a positively selected DP thymocyte randomly terminate either CD4 human autoimmune disease called immunodysregulation, polydendo-
or CD8 expression. In the instructive model, certain TCR signal crinopathy, and enteropathy, X-linked (IPEX) syndrome. A mutation
qualities, such as strength or duration of signal, direct termination of in the mouse gene for FoxP3 causes a similar disease (scurfy mice).
mismatching coreceptor expression. More recently, a kinetic signaling These naturally occurring loss-of-function mutations demonstrate
model has emerged. It proposes that CD4 or CD8 lineage fate is the necessity for Tregs in maintaining self-tolerance. In the thymus,
determined by TCR signal duration. Experimental models continue development into a tTreg is enhanced in cells that have high-affinity
to be tested to fully elucidate the mechanisms underlying lineage fate. TCR-peptide-MHC interactions, suggesting that these cells develop
Among the many proteins that are involved in CD4 or CD8 specifically to counter autoreactive responses. The exact mechanism
lineage choice are key transcription factors. One such example is that drives these cells to adopt a Treg fate and avoid negative selection
T-helper-inducing POZ/Krüppel-like factor (Th-POK), a zinc finger during development is being investigated.
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protein that is expressed exclusively in CD4 T cells and not in CD8 The path of developing γδ thymocytes contrasts with that of
T cells. In transgenic mice, expression of this protein forces the αβ T-cell development, which is likely related to the function of
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majority of positively selected thymocytes to adopt CD4 T cell fate, mature γδ T cells. In the periphery, γδ T cells reside in secondary