Specific in vivo and nonspecific in vitro alloreactivities of adult frogs (Xenopus laevis) that were thymectomized during early larval life.

Thymectomy of very young Xenopus larvae abrogated in vitro proliferative responses to phytohemagglutinin (PHA) and allogeneic leukocytes. Thymectomized (Txd) frogs, however, still rejected first-set skin allografts chronically and second-set grafts (but not third-party grafts) more rapidly. The specific second-set rejection reaction by Txd frogs was transferable in vivo to secondary Txd hosts (with major histocompatibility complex identical to the cell donor) by the subcutaneous injection of a mixture of splenic and peripheral blood leukocytes. Spleen cells used in this adoptive transfer experiment (i.e., immune cells) were responsive to allogeneic cells from the original donor strain as well as from unrelated donors in mixed leukocyte culture in vitro but were still unreactive to PHA. The results are consistent with an extrathymic pathway of alloreactive cell differentiation in this species, although the physiological significance of such a pathway is not clear.