Behaviorally conditioned suppression of a graft-versus-host response.

Cyclophosphamide (CY), previously used to condition suppression of humoral immune responses, was used to condition suppression of a graft-versus-host response (GvHR). Female (Lewis x Brown Norway) F1 rats were conditioned by pairing consumption of a saccharin solution with an intraperitoneal injection of CY at 50 mg/kg of body weight 48 days before immunization. On day 0, all animals were injected with a suspension of splenic leukocytes (2 x 10(7) cells per footpad) obtained from female Lewis donors. The regional GvHR was assessed on day 5 by weighing popliteal nodes. Conditioned animals given a single low-dose injection of CY and reexposed to conditioned stimuli had lymph node weights significantly lower than control groups and did not differ from animals given three injections of CY during the ongoing GvHR. The results suggest that conditioned immunosuppression, previously demonstrated in thymus-dependent and thymus-independent humoral immune responses, also affects the popliteal GvHR, a cellular immune response.