Background: Tolerance induction after allogeneic rat liver transplantation (LTx) is well described, however despite years of extensive research the underlying immune mechanisms remain unclear. Since the liver graft appears to be immunologically privileged among vascularized organs, we focused our study to analyze the cytokine production of intragraft CD4+ T cells at different times after LTx.
Methods: Orthotopic arterialized LTx was performed in two allogeneic rat strain combinations with remaining rejection (DA-LEW = REJ) and tolerance (LEW-DA = TOL). Intrahepatic leukocytes were isolated on days 3, 5, 7, 14, 30, 60 and 100 after transplantation and CD4+ T cells were purified by an immunomagnetic method. Their mRNA cytokine expression was analyzed by semiquantitative RT-PCR before and after in vitro restimulation with mAbs against abT cell receptor and CD28.
Results: Allografts in animals from REJ-model were rejected within 12 days, whereas liver grafts from animals in TOL-model survived long-term (>100 days) without immunosuppression despite a complete MHC mismatch (spontaneous liver tolerance). In the early phase after LTx (until day 7) liver grafts of both models were strongly infiltrated. Compared to normal levels (1.5x106 cells/g liver) the amount of intrahepatic leukocytes was up to 10-fold increased after transplantation. Intrahepatic activated (CD45RClow) recipient CD4+ T cells produced high levels of Th1-like cytokines (IL-2, IFN-g). However, these primed T cells were phenotypically unstable and switched back to the CD45RChigh (naive) state due to a reduced IL-2 production in the REJ-model between day 5 and 7. In the TOL-model the persistence of activated T cells was prolonged. Reverted to the CD45RChigh phenotype they were now able to produce IL-13 in response to a further alloantigen stimulation.
Conclusions: Recipient T cells are engaged in acute rejection as well as in tolerance induction after liver transplantation. The early phase after transplantation was clearly dominated by the Th1-type cytokins IL-2 and IFN-g in both models. CD4+ T cells from tolerized animals expressed after restimulation the Th2 cytokin IL-13 that shares a variety of biological functions with IL-4. This indicate that already in the early phase after LTx Th2-like CD4+ T cells have a putative regulatory function in tolerance induction.