SPONTANEOUS LONG-TERM ACCEPTANCE OF RT-1-INCOMPATIBLE LIVER ALLOGRAFTS IN INBRED RATS

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Abstract
In several combinations of inbred rats, liver allografts are spontaneously tolerated, and after a few weeks liver tolerant rats are in a state of donor-specific transplantation tolerance. In vivo and in vitro experiments were conducted to analyze the immunological status of LEW or BN rats with spontaneously tolerated (LEWxBN) F1 liver allografts several months after transplantation. Acute rejection of secondary donor-specific heart allografts retransplanted from liver-tolerant rats to normal syngeneic hosts suggests that the state of tolerance in liver-tolerant rats is related to an active modification of the immune system of the rat and not to a reduced immunogenicity of the graft. No cytotoxic antibodies or cells were found in liver-tolerant rats. Reactivity in mixed lymphocyte culture was normal or slightly reduced. Arguments for the presence of splenic suppressor cells were found in LEW tolerant rats using a local graft-versus-host assay, but these could not be found in BN rats, or when attempting to transfer or to break the tolerance state. A nonspecific humoral blocking factor was found in vitro in liver-tolerant rats but transfer of serum from liver-tolerant rats to normal syngeneic hosts did not permit a significant prolongation of donor-specific heart allografts. These results suggest that more than one mechanism may be involved at the maintenance phase of liver allograft tolerance.