DISTINCT SENSITIZATION PATHWAYS IN LIVER TRANSPLANT RECIPIENTS

Abstract

441 Host sensitization continues to represent one of the most challenging problems in clinical organ transplantation. For the last several years we have been concentrating at dissecting the immune cascade leading to accelerated rejection of vascularized organ allografts (Tx) in sensitized rat recipients. Accordingly, LEW rats challenged with BN skin Tx (day -7) reject LBNF1 cardiacor renal Tx (transplanted at day 0) in <36 h (acute Tx rejection occurs at 7-8 days). Both cellular and humoral immunity contribute to this brisk host rejection response. Our present study was designed to establish a model of accelerated rejection of orthotopic liver transplant (OLT), and then to analyze host immune events triggered by transplantation of liver Tx in sensitized rat recipients. First, we have determined that normal otherwise untreated LEW rats survived>100 days following transplantation of LBNF1 liver Tx (Gr. I; n=6), similarly to isograft controls (n=3). Sensitization with BN skin Tx at day -7(1 wk prior to LBNF1 liver engraftment) prompted rejection of liver Tx and animal death within 7.8±2.3 days (Gr. II, mean±SD; n=5). Based on our previous studies in cardiac/renal Tx models, we then asked whether repeated sensitization accelerates host rejection response to liver Tx. Accordingly, LEW rats were challenged with BN skin Tx at day -14and day -7. Surprisingly, however, instead of augmenting host sensitization, and triggering “classic” accelerated rejection, all animals (n=5) survived >100 days despite repeated sensitization (Gr. III). Unlike in isografts, a moderate degree of necrosis, cellular infiltration, and bile duct damage were noted histologically by day 4 in all Tx recipient groups. In Gr. II, Tx infiltrating lymphocytes were scattered in sinusoids and portal tracts, whereas those in Gr. I and III were sequestered selectively in portal tracts. By day 8, liver Tx in Gr. II but not in Gr. I or III, were severely necrotic, and densely infiltrated throughout with neutrophils and lymphocytes. Immunohistological evaluation in Gr. I-III hosts has revealed high (+++) infiltration of liver Tx with ED-1+ macrophages. Interestingly, however, Tx infiltrating R73+ T cells were diminished in Gr. III (<100 cells/10 HPF), as compared to Gr. I or Gr. II liver Tx (ca. 175 cells/10 HPF; p<0.05). Finally, we have analyzed host systemic IgM and IgG allo-Ab responses by flow cytometry. By day 8, IgM and IgG allo-Ab levels were equally elevated in Gr. I-III rats. Strikingly, long-term (>100 day) Tx acceptance in untreated Tx recipients (Gr. I), as well as in those which were sensitized repeatedly with skin Tx (Gr. III), was accompanied by markedly augmented anti-donor IgM and IgG allo-Ab responses. This is the first report, which documents distinct sensitization pathways in the OLT rat model. Unlike in the cardiac or renal Tx model, repeated host sensitization leads to the acceptance of liver Tx. Such an unexpected tolerogenic mechanism may result from APC modifications, hematopoietic microchimerism, preferential activation of Th2-like cells, and/or production of enhancing allo-Abs.