ABROGATION BY RAPAMYCIN OF ACCELERATED REJECTION IN SENSITIZED RATS BY INHIBITION OF ALLOANTIBODY RESPONSES AND SELECTIVE SUPPRESSION OF INTRAGRAFT MONONUCLEAR AND ENDOTHELIAL-CELL ACTIVATION, CYTOKINE PRODUCTION, AND CELL-ADHESION

This study evaluated the efficacy and mode of action of rapamycin (RPM ) in a model of accelerated (24-hr) rejection of LBNF1 cardiac allogra fts in specifically sensitized LEW rats. RPM treatment (0.25 mg/kg/day i.p.) between the day of sensitizing skin grafts (day -7) and subsequ ent heart (day 0) transplantation (Tx), abrogated fulminant rejection and prolonged cardiac allograft survival to 46+/-22 days (mean+/-SD, P <0.0001). The delayed introduction of RPM until day -2 or day -1 was e qually effective, whereas treatment initiated after cardiac Tx was ine ffectual. Untreated accelerated rejection was associated with strong p roduction of circulating IgM, whereas an IgG alloantibody response was not detected until after rejection was complete. RPM therapy (day -7 to -1) diminished this systemic IgM response and prevented the switch from IgM to IgG alloantibody production. Immunohistologic evaluation a t 24 hr after cardiac Tx showed that compared with untreated hosts RPM treatment largely abolished intragraft cellularity, and was associate d with decreased mononuclear and endothelial cell activation. Specific ally, Ia and ICAM-1 upregulation was abolished, and no cells elaborati ng IL-2 or IFN-gamma were detected. In addition, RPM treatment prevent ed intragraft production of the proinflammatory cytokines IL-1beta, IL -6, and IL-8. The effects of RPM therapy on recipient cellular respons es were evaluated in vitro by mixed lymphocyte reaction. Surprisingly, the donor-specific proliferative response of cells from RPM-treated h osts at 1 or 7 days after Tx was markedly increased, compared with cel ls from rejecting, untreated controls, and bioassay of IL-2 within sup ernatants of MLR cultures showed comparable levels of IL-2 in both gro ups. The effects of RPM upon adhesion properties of lymph node lymphoc ytes were also tested in an in vitro binding assay. The binding of nai ve cells to sections of cardiac allografts collected from RPM-treated hosts at 24 hr post-Tx was decreased compared with that in untreated r ecipients. Interestingly, the binding of mononuclear cells to high end othelial venules of peripheral lymph nodes in RPM-treated hosts remain ed relatively high. Thus, treatment with RPM prevents and/or erases th e sensitization, which otherwise leads to accelerated allograft reject ion. Abrogation of allograft injury by RPM was associated with profoun d and long-lasting depression of host IgM and IgG alloantibody respons es in the circulation, and selective downregulation of host cellular i mmunity and endothelial activation at the graft site. In contrast, ant igen alloreactivity and endothelial adhesivity in peripheral lymphoid tissues were spared, indicating novel and potent selective effects of RPM therapy in allograft recipients.