Conditioned immune response associated with allogeneic skin grafts in mice.

Groups of CBA mice were given sham skin grafts (conditioned stimulus, CS) or alloantigen only (unconditioned stimulus, US; C57BL/6 lymphoid cells inoculated i.p.). A further group of animals received simultaneous US and CS (alloantigen (US), in this case actually given in the context of a C57BL/6 skin graft). The peripheral blood leukocyte (PBL) pool of all animals was analyzed in tissue culture for the frequency of cytotoxic T lymphocyte precursors (CTLp) specific for alloantigens on the grafted tissue. After three "conditioning" trials, spaced at intervals to allow recovery in all groups of the CTLp frequency to that of naive control mice, individuals in all groups were sham-grafted (given CS alone). PBL of these mice were tested 2 days after removal of the casts. Only group CS + US, and only some 50 to 60% of these, were able to show a "conditioned" increase in the CTLp frequency specific for alloantigens of the grafted tissue after sham-grafting. When these "responder" mice were subdivided into two groups that were subsequently given two trials of CS only or CS + US, followed by retesting with CS alone, either extinction or reinforcement of the previously conditioned response was observed.