Page 542 - Veterinary Immunology, 10th Edition

VetBooks.ir γ/δ T Cell Diversity

The function of γ/δ T cells differs among mammals. For example, in
“γ/δ-low” species such as humans and mice there are relatively few

V genes in the TRD and TRG loci, and the combinational repertoire
is therefore small. In addition, the cells bearing these receptors use
only a few V gene combinations. In contrast, human α/β T cells
show a much wider range of binding specificities. Thus there is a
marked difference between the size of the α/β and γ/δ TCR

repertoires. Human γ/δ T cells probably have a limited role in
adaptive defense but recognize conserved PAMPs.
The situation in “γ/δ-high” ruminants is very different. In these,

γ/δ T cells form a large proportion of total T cells. In young lambs
or calves, they account for up to 60% of T cells. In addition,
ruminant γ/δ T cells show a considerably greater receptor diversity.
In the sheep, γ/δ V-region diversity results from the use of 28 TRDV
genes and 13 TRGV genes that contain two distinct hypervariable

segments similar to the CDRs seen in immunoglobulin V genes. In
addition, there are multiple TCR γ/δ isoforms generated by the
association of a single Cδ chain with up to six or eight Cγ chains.

All this suggests that γ/δ T cells in domestic mammals recognize a
very wide diversity of antigens and mount adaptive rather than
innate responses.

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