As in other species ovine T cells are subdivided on the basis of their TCR and express
either a λδ or αß molecule. In the ovine, the majority of the λδ T cell subset are further
defined by the expression of a cell surface glycoprotein termed T19. The T19 molecule
is not expressed by ovine αß T cells which do express CD2, CD4 and CD8.
In this thesis a panel of monoclonal antibodies was utilised to investigate the prevalence
of λδ T cells in peripheral blood isolated from animals of different ages and exposed to
various antigenic burden. In addition these reagents enabled a characterisation of the cell
surface molecules expressed by ovine λδ T cells. These studies demonstrated that ovine
λδ T cells lack the expression of the co-receptor molecules CD4 and CD8, as do λδ T
cells in most species. CD4 and CD8 are associated with T cell activation pathways. This
suggested'that aspects of activation in y 8 T cells may differ to activation pathways in
CD4-positive and CD8-positive T cells.
An investigation of λδ T cell activation, as assessed by CD25 expression, demonstrated
two novel aspects of λδ T cell immunobiology. Firstly, the majority of λδ T cells within
afferent lymph draining epithelial sites, in the absence of any intentional in vivo antigen
challenge, were activated. This was in contrast to λδ T cells within peripheral blood and
efferent lymph which were primarily CD25-negative. Secondly, λδ T cells were activated
prior to αß T cells in vitro. Furthermore, ovine λδ T cells respond by proliferation to
antigens in vitro presented by antigen presenting cells within peripheral blood
mononuclear cells. Ovine λδ T cells appear to recognise their ligand as a complex of
antigen in association with MHC class I and MHC class II molecules.