Splenic involvement in amphibian transplantation immunity.

The spleen has been identified as a centre of alloimmune reactivity in control Xenopus. Levels of tritiated thymidine labelling and pyroninophilic cells are elevated in spleens of skin-allografted toadlets when compared with autografted and nongrafted animals. Second-set alloimmune reactivity can be transferred by implanting a spleen from a donor that has rejected one or two grafts into a nonsensitized host. Spleen donor and host in these experiments were mutually tolerant, following reciprocal transfer of embryonic tissue grafts. In contrast, studies on the uptake of tritiated thymidine and levels of pyroninophilia in animals thymectomized at 7 or 8 days of age suggest lack of splenic involvement in the chronic first-set allograft rejection that can still occur in the absence of the thymus. The lymphoid organ origin of "thymic-independent" alloimmunity still awaits clarification.