Demonstration of cells possessing tolerance-inducing activity in Xenopus laevis rendered tolerant perimetamorphically.

J-strain (JJ) Xenopus laevis is easily made tolerant to semixenogeneic (X laevis x X borealis: JB) adult skin grafted onto immunologically competent larvae at stages 53-54. If the larvae are thymectomized shortly before skin grafting (late Txd), tolerance will never be induced. This suggests that suppression of the immune response to JB skin is mediated by a population of thymus-derived cells. In an attempt to confirm the presence of these cells, the spleen and tolerated JB skin were tested for their tolerance-inducing activity, by grafting either spleen or skin and a new JB skin piece simultaneously to late Txd JJ larva (secondary host). The results clearly indicated that both spleen and skin possessed the ability to induce tolerance. The injection of 1 x 10(5) splenocytes resulted in induction of tolerance in more than 80% of secondary hosts, and the rate of tolerance induction increased in relation to the injected cell number. Furthermore, tolerance was induced in about 90% of the animals by injecting as few as 100 cells isolated from the tolerated skin. Immunohistochemical observation of grafted skin showed that many host-derived T cells were distributed around the epidermal basal lamina. These results indicate that the cells with suppressive activity are a population of T cells that have differentiated in metamorphosing thymuses and have been released peripherally.