ANTI-LEUKAEMIA ACTIVITY AS A BYSTANDER EFFECT OF GRAFT-VERSUS-HOST REACTIONS

Summary.-The production of graft-versus-host (GVH) reactions in (PVGc x Wis-tar) F1 hybrids by the transfer of PVGc spleen cells resulted in significant resistance of these recipients to a subsequent challenge with the PVGC leukaemia. Protection was markedly dependent on dose and timing of allogeneic cell transfer and was abrogated by irradiation of the cells prior to transfer. GVH activity was shown to be a prerequisite for induction of the protective effect but was equally effective when produced by the transfer of Wistar spleen cells in place of PVGC cells. These points, plus the fact that in vitro investigations of possible immune mechan-isms failed to demonstrate cytotoxic immunity in treated rats, suggested a non-specific " bystander " effect as the mechanism of protection. The implications of such a mechanism are discussed. THE DEMONSTRATIONS of increased immune responsiveness following allo-geneic cell interactions (Hamilton, 1973; Katz, 1972) raised the question whethe