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

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Abstract
Cyclophosphamide (CY), previously used to condition suppression of humoral immune responses, was used to condition suppression of a graft-vs.-host response (GvHR). Female (Lewis .times. Brown Norway) F1 rats were conditioned by pairing consumption of a saccharin solution with an i.p. 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 .times. 107 cells/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 3 injections of CY during the ongoing GvHR. Conditioned immunosuppression, previously demonstrated in thymus-dependent and thymus-independent humoral immune responses, appears to affect the popliteal GvHR, a cellular immune response.