Dev Comp Immunol 1993 Jan 01;175:439-47.

Demonstration of cells involved in rejection of tolerogenic grafts in tolerant Xenopus.

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The J-strain (JJ) clawed frog, Xenopus laevis, can easily be made tolerant of semixenogeneic (X. laevis x X. borealis; JB) adult skin grafted onto larvae before stage 57 (larvally induced tolerance). To examine the cellular bases of this tolerance, we have established a simple and reliable method of quantifying the proliferating splenocytes in vivo by using BrdU and anti-BrdU antibody. When adult JJ frogs were injected with adult JB peripheral blood cells (PBCs), host splenic lymphocytes proliferated even in frogs that were tolerant of JB skin. Splenic lymphocytes from animals primed in vivo with JB PBCs were injected into early thymectomized (Txd) frogs that carried previously grafted JB skin. In Txd frogs injected with splenocytes from normal frogs, the rejection of JB skin grafts was initiated promptly and ended in about 10 days. Rejection was relatively delayed in the frogs injected with splenocytes depleted of proliferating cells, suggesting that the proliferating cells were actively involved in the rejection process. Early Txd frogs injected with splenocytes from undisturbed tolerant donors did not reject JB skin grafts. Quite unexpectedly, all five early Txd frogs, injected with undepleted splenocytes from tolerant donors previously stimulated with JB PBCs, rejected the JB skin grafts, four of them subacutely. In frogs injected with the splenocytes from PBC-injected tolerant donors that had been depleted of proliferating cells, the rejection was delayed. The most likely explanation is that cells actively involved in graft rejection exist in tolerant frogs and can be stimulated to proliferate, although cytotoxicity of the graft is usually suppressed or disabled by unknown mechanisms.

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