Page 541 - Veterinary Immunology, 10th Edition

Where Does This Happen?
VetBooks.ir TCR genes are rearranged and expressed in the thymus. Although

both types of T cell arise from a common precursor stem cell, the

TCR class switch depends on the signals received by the developing
T cell within the thymus. Immature thymocytes begin to rearrange
TRB, TRG, and TRD. If TRB is productively rearranged, TRG is
silenced and TRD is deleted so that the cell commits to using TRA

and expressing α/β TCR. Because of the geometry of the TRA/D
locus, joining of a TRAV gene to TRAJ inevitably deletes the TRD
genes on that allele. Thus α-chain rearrangements eliminate any
possibility of δ-chain expression. Alternatively, the cell may

successfully rearrange TRG and TRD and so express γ/δ TCR.

T Cell Receptor Diversity

In the human TRA locus there are at least 61 TRAJ genes and 54
TRAV genes, giving more than 3000 possible combinations. N-

region addition and base deletion also occur, resulting in great
junctional diversity. After correction for codon redundancy and
correct reading frame, the number of potentially different TCR α
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chains is about 10 . In the human TRB locus, there are at least 88
TRBV genes, 2 TRBD genes, and 14 TRBJ genes giving 88 × 2 × 14 =
2464 possible combinations. In addition, there is junctional diversity
and the use of many TRBD combinations. After corrections there
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are about 5 × 10 possible VDJ β sequences. Thus, the number of
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possible TCR α/β combinations is about 5 × 10 .

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