Origin and development of the γδ T-cell system in sheep: a critical role for the thymus in the generation of TcR diversity and tissue tropism

Ruminant γδ T cells are concentrated at epithelial surfaces and share many features in common with species such as mice and humans which contain relatively fewer γδ T cells than ruminants. To date no γδ T cells with invariant TcR have been found in sheep and the generation of γδ TcR diversity which is thymic dependent follows a developmentally regulated sequence. Analysis of thymic export of γδ T cells shows that emigration of γδ T-cell subsets increases markedly during fetal life and after birth suggesting intrathymic processes leading to mature γδ T cells may change during development. Skin homing γδ T cells acquire their tissue tropism inside the thymus and pathways of recirculation of γδ T cells to skin are laid down during fetal development independent of antigen and remain stable through into adult life.