In an attempt to induce the active enhancement of the cardiac allografts, recipient rats were injected intravenously with 1 x 107 donor-specific spleen cells or an equal number of their T or B cell subpopulations on each of 7 days before transplantation. Mean survival times (MST) in the group pretreated with donor-specific spleen cells (MST 27.5 ± 7.8 days) and the group pretreated with donor-specific B cells (MST 37.5 ± 15.5 days) were significantly prolonged (p<0.01) compared with the untreated control (MST 11.6 ± 2.0 days) and the group pretreated with donor-specific T cells. To investigate the mechanisms of the beneficial effect of donor-specific B cells on rat cardiac allografts, inhibition assay of mixed lymphocyte reaction (MLR) were carried out by the addition of recipient rat sera or spleen cells harvested 7 days following the intravenous administration of donor-specific spleen cells or their T and B cell subpopulations. Recipient rat sera harvested 7 days following the intravenous administration of donor-specific B cells showed significant inhibition of MLR; this inhibition was correlated to the prolonged survival of histoincompatible rat cardiac allografts. In contrast, MLR suppressor cells could not be detected in any experimental group. Thus, donor-specific B cell given 7 days before transplantation may possibly have beneficial effect on rat cardiac allografts and MLR inhibition induced by the intravenous administration of donor-specific B cells may be essential for prolonged allograft survival
