Page 538 - Veterinary Immunology, 10th Edition

optional. Pigs have at least six TRDD genes compared to three and
VetBooks.ir two in human and mouse respectively. These pig genes may form

transcripts with up to four joined TRDD domains.

γ Chain

The TRG locus contains 4 to 17 V genes, from 4 to 16 J genes, and
from two to eight C genes depending on species. In the dog, the
TRG locus is organized into eight cassettes, each containing a basic
V-J-J-C unit, except for a J-J-C cassette at the 3′ end. It contains a
total of 40 genes (16 V, 16 J, and 8 C). Eight of the 16 canine TRGV

genes, 7 of 16 TRGJ genes, and six of eight TRGC genes are
functional. The existence of these multiple TRGC genes suggests
that the TCRs they generate may have diverse biological properties.

Generation of T Cell Receptor V-Region

Diversity

There are three hypervariable regions (CDRs) in each TCR V region.
The first two, located within the V genes, have probably arisen

through selection. The third is by far the most variable and is
located in the region where V, D, and J genes recombine (Table
17.3). Neither somatic mutation nor gene conversion occurs in TCR
genes. The genes that are separate in the germline are brought
together by DNA rearrangement and are then modified by base

insertion or deletion as the T cells differentiate (Fig. 17.14).

TABLE 17.3
TCR Diversity Among Mammal Germlines*

TCRA TCRD TCRB TCRG
Species
V J C V J D C V D J C V J C
Horses 5 5 1 8 3 1 16 1 14 2 2
Bovine >300 52 1 >100 3 5 1 134 3 21 3 17 8 6
Sheep 4 28 120 3 18 3 13 13 5
Pigs 33 61 1 31 10 6 1 10 3 21 3 6
Dogs 34 37 1 6 1 16 16 8
Cats 4 8 6
Mice 100 61 1 10 2 2 1 52 2 13 2 7 4 4
Humans 54 61 1 8 4 3 1 88 2 14 2 15 5 2
* The numbers in this table have been drawn from multiple references. They represent the
number of germline genes reported at each locus. Not all these genes will be expressed,

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