Page 537 - Veterinary Immunology, 10th Edition

the α-chain genes to form a single locus.
VetBooks.ir Each of the three TCR loci contains two or more C genes. In the

TRA/D locus, one C gene codes for TRAC and the other for TRDC.
The TRB and TRG loci, in contrast, may contain multiple C genes.
For example, there are eight TRGC genes in dogs.
Cells with α/β TCRs rearrange and express TRA and TRB V
genes, whereas γ/δ T cells express TRG and TRD genes. α/β T cells

and γ/δ T cells arise from a common precursor, and the TCR class
switch is mediated by signals within the thymus. Developing T cells
committed to the α/β TCR lineage delete their TRD genes by

looping out and switch to using their TRA genes. Some of the V
genes in the TRA/D locus may be used by either α or δ TCR chains.

α Chain

The number of TRAV genes varies ranging from four and five in the
horse and sheep, 34 in the dog, and 33 in the pig to more than 300 in

cattle. Likewise, TRAJ gene numbers ranges from five in the horse
to 61 in pig, mouse, and human. Only one TRAC gene has been
identified in the mammals investigated so far.

β Chain

The TRB locus contains a cluster of V genes located upstream of
two D-J-C cassettes, each containing several functional J genes. The
D genes are all similar in sequence and length and their use is
optional. Any of the TRBV genes may be joined to either of the two
D-J-C cassettes, and a V gene may join to either a D or a J gene.

Dogs have about 21 TRBV genes, but about one-third of these make
up 90% of the T cell repertoire. TRBV gene use may in fact be
restricted to a single V gene family in the dog. In the pig, 10 TRBV

genes and three Dβ-Jβ-Cβ cassettes have been identified. Other
mammals may have very diversified TRBV genes, ranging from 16
in the horse to 134 in cattle.

δ Chain

The TRD locus contains between 8 and 100 V genes depending on
species, 2 to 10 J genes, two to six D genes, and only one C gene in
all species examined. As mentioned previously, D gene use is

537