T-cells and transplantation tolerance in thymectomised Xenopus implanted with foreign thymus
This thesis investigates the expression of a T-cell differentiation antigen, (XTLA- 1), in various strains and species of Xenopus, and demonstrates the effect of early-thymectomy, (by microcautery), on XTLA-1 expression. It further examines restoration of the T-cell dependent immune system, (particularly with respect to transplantation responses), and the extent to which tolerance to donor antigens is achieved by implantation of xenogeneic, as well as allogeneic, thymi into early- thymectomised Xenopus larvae. The means by which transplantation tolerance is maintained in intact, control Xenopus, following perimetamorphic skin grafting, is also addressed. Initial work, reported in Chapter 2, showed that XTLA-1 is expressed by the majority of thymocytes and by a proportion of splenocytes from all X.laevis, X.borealis, and hybrid clonal Xenopus,(X.laevis x X.gilli and hybrid X.laevis x X.muelleri),examined. X.tropicalis lymphocytes, however, do not express XTLA- 1. Early-thymectomy by microcautery effectively removes T-cells, as detected by XTLA-1 expression. In Chapter 3, normal adult and larval tissue distribution of XTLA-1 positive cells is described, and the XTLA-1 and X.borealis,(quinacrine fluorescence), markers are employed to demonstrate the differentiation of T-cells derived from early- thymectomised hosts within xenogeneic, (X.tropicalis), thymus implants. The effects of implantation of allogeneic and xenogeneic larval thymi into early-thymectomised hosts, in terms of T-cell responses and of induction of tolerance to thymus donor antigens, is explored in Chapter 4; X.borealis xenogeneic thymus implants are apparently as effective in these regards as are allogeneic implants, but X.tropicalis xenogeneic thymus implants do not fully restore thymus-dependent immune responses. Preliminary investigations of skin graft rejection, mixed leukocyte culture and T-cell mitogen responses of X.tropicalis, in comparison to those of other Xenopus species, are reported in Chapter 5; the results of these experiments raise the possibility that X.tropicalis splenocytes are less responsive, in mixed leukocyte culture, to xenogeneic stimulators than are splenocytes of other Xenopus species. In Chapter 6, histological examination of skin grafts, accepted by virtue of the tolerance induced by prior implantation of a thymus gland from the skin graft donor into the early-thymectomised hosts, reveals some rapid alteration in the composition of these skin grafts; infiltration of the tolerated skin grafts by host-derived lymphocytes suggests that tolerance induced by thymus implantation does not abrogate recognition of thymus donor antigens. Finally, also in Chapter 6, tolerance induced in control, intact Xenopus by perimetamorphic skin grafting is shown to be susceptible to cyclophosphamide injection, suggesting that the maintenance of this tolerance is mediated by suppressor cells.