Immunological enhancement of heterotopic cardiac allografts in rats was achieved by pretreatment of recipients with donor spleen and bone marrow cells (active enhancement) and by administration of noncytotoxic alloantiserum obtained from recipient strain rats immunized against donor spleen or bone marrow cells (passive enhancement). The prolongation of survival, though statistically significant, was not as great as the indefinite survival of renal allografts reported by others using similar treatment protocols. Specific and dose-dependent inhibition of recipient-donor mixed lymphocyte reactions was caused by alloantisera which "enhanced" graft survival in vivo. Mixed lymphocyte reactions also indicated that in actively enhanced recipients peripheral inhibition of rejection rather than central tolerance was the mechanism responsible for prolongation of allograft survival.
Souther SG, Gordon RO, Oppenheim JJ, Stinson EB. In Vivo and in Vitro Studies of Cardiac Allografts in the RatImmunological Enhancement. Arch Surg. 1972;105(5):756-760. doi:10.1001/archsurg.1972.04180110077019