Response to Skin Allografts in Xenopus Laevis Following Thymectomy at Early Stages of Lymphoid Organ Maturation

SUMMARY Removal of the thymus at an age (8 days) when lymphoid differentiation is only just beginning is possible in Xenopus. Growth and development proceed normally following the operation. However, absence of the thymus from this early stage of development (stage 48 of Nieuwkoop and Faber) impairs the animal's subsequent alloimmune response capacity. Thus, the extensive lymphocytic invasion of skin allografts, apparent in control animals during the 2nd week post-transplantation, failed to occur in thymectomized larvae, although slight manifestations of an alloimmune response were apparent in a few animals. On the other hand, when application of the graft was postponed until early adult life, the graft was eventually heavily invaded by lymphocytes, and complete rejection of transplants on these early thymectomized animals usually took place. However, graft survival times in these toadlets were always prolonged. Thymectomy at 11 and 14 days of age (stages 49 and early 50), i.e., during the time when the peripheral lymphoid organs are still relatively undifferentiated, likewise resulted in impairment of the larval and toadlet alloimmune response. In contradistinction, removal of the thymus as soon as the lymphoid organs have completed their lymphoid histogenesis, i.e., during the 3rd week of larval life (stages 51 and 52), had no apparent effect on the lymphocytic response of larvae to skin allografts or on the graft rejection capacity of Xenopus toadlets. The role of the thymus in the establishment of the amphibian immune system is discussed in the light of these findings