Immunological Memory Involving Weak Histocompatibility Barriers in Urodele Amphibians

SUMMARY The following three distinct rejection patterns were disclosed by consecutively transplanting five allografts to single urodele recipients: (1) second-set rejection resulted in minimal graft survival times; (2) both third- and second-set (but neither fourth- nor fifth-set) rejection times were accelerated, relative to the previous set; (3) rejection of third-sets, but not second-sets, was accelerated, relative to the previous set. The most rapid first-set rejection was associated with the highest incidence of both unaccelerated and maximally accelerated second-set rejection. These, as well as several other observations, point to significant differences among the weak histoincompatibility interactions leading to chronic allograft rejection in the Urodela. By varying the interval between first-set skin graft rejection and second-set skin transplantation and then noting the survival times of repeat grafts, the duration of transplantation immunity in the urodele was determined to be long lived (>3 months). Long lived memory, then, is not restricted to endothermic vertebrates or to strong histoincompatibilities. The occasional absence of an accelerated second-set rejection was unrelated to the interval between first-set rejection and second-set transplantation. Therefore, this phenomenon does not result from the gradual waning of immunological memory with time.