In vivo studies on allotolerance perimetamorphically induced in control and thymectomized Xenopus

Implantation of either major histocompatibility complex (MHC)-disparate thymus to 7-day thymectomized (Tx) Xenopus in late larval life, or allogeneic skin to perimetamorphic controls, routinely induces tolerance towards implant-strain skin grafts applied in adult life. To characterize this allotolerance further, additional in vivo approaches were attempted. Injection of gamma-irradiated (5000 rads) implant-strain splenocytes into frogs bearing tolerant skin grafts revealed (within 3 days) significantly elevated tritiated thymidine uptake by host spleen cells, compared to siblings injected with isogeneic cells. Although this in vivo 'mixed leucocyte reaction' proved to be thymus dependent, the identity of the cells involved awaits clarification. When non-restored Tx Xenopus are injected with live MHC-disparate splenocytes, graft-versus-host (GVH)-induced mortality ensued within 2 weeks. Such GVH disease also occurred (albeit more chronically) when Tx allothymus-implanted animals were given MHC-incompatible splenocytes, but only when these came from the thymus donor strain. Splenocytes from thymus-implanted animals failed to achieve GVH-induced splenomegaly when transferred to appropriate hosts (bearing MHC antigens of the thymus donor strain). Overall, the experiments indicate that alloreactivity against donor cells is impaired but not completely inhibited in Xenopus following perimetamorphic implantation.