Characterization of alloimmune responses and the generation of transplantation tolerance in thymectomized, thymus-implanted xenopus
This Thesis investigates the role of the Xenopus thymus in educating T cells to destroy minor histocompatibility (H) antigen-disparate skin grafts and probes the extent to which allotolerance is established to major histocompatibility complex (MHC) antigens first encountered at metamorphosis. The work began with intrastrain skin grafting and mixed leucocyte culture (MLC) (Chapter 2) which confirmed that only minor H antigen differences exist between J strain Xenopus laevis individuals, thus making them suitable as an additional animal model to the X.laevis/X.gilli (LG) isogeneic hybrids available. Chapter 3 revealed that rejection of minor H disparate grafts was completely thymus-dependent, while 7-day thymectomy (Tx) severely impaired rejection of MHC disparate grafts. Implantation (at 4-6 weeks of age) of larval or adult thymus from MHC-compatible or MHC-disparate donors restored the ability of Tx Xenopus to reject 3rd-party MHC antigen-disparate grafts, while tolerance to skin of the thymus donor type always ensued. Restoration of minor H graft rejection was impaired when thymus donor and TX host were MHC- or minor H antigen-mismatched. The use of LG hybrids revealed that minor H graft rejection was restored in Tx hosts only by implantation of a fully identical thymus. In Chapter 4, skin graft tolerance to donor MHC antigens was demonstrated following perimetamorphic allografting of skin to control J and LG recipients; the skin graft tolerance induced in certain donor/host combinations was shown to be not entirely specific for minor H antigens. In Chapter 5, in vitro, 1-way MLC reactivity of splenocytes from thymus-implanted Tx animals to thymus donor-type cells was shown to be variable, but occasionally positive; control Xenopus, made allotolerant of skin grafts by prior skin implantation retained a splenic MLC towards skin donor strain splenocytes. In vivo MLR assays in Chapter 6 also detected proliferation towards alloantigens of the skin or thymus donor, but the nature of these alloreactive (T-dependent) cells remains uncertain. Preliminary graft-versus-host experiments indicated that the tolerance induced by allothymus restoration of Tx hosts was more complete than following skin alloimplantation to metamorphosing controls; these studies also suggested that it is the cytotoxic effector component of tolerant animals that is defective.