Intragraft events preceding chronic renal allograft rejection in a modified tolerance protocol

BACKGROUND: Inbred miniature swine treated for 12 days with high-dose cyclosporine A develop tolerance to histocompatibility complex (MHC) class I-mismatched renal allografts. When this protocol was modified by adding thymectomy before transplant, all animals developed acute rejection. Thereafter, by day 100, one half developed chronic rejection (progression group) and the other half recovered (recovery group). This provides an excellent experimental model to identify the mechanisms of chronic rejection as well as the early changes that may predict chronic rejection. METHODS: We assessed the cellular infiltration, immune activation, humoral immunity, and cell- and antibody-mediated graft injury in the progression and the recovery groups. In addition, we also examined circulating donor reactive cytotoxic T lymphocyte (CTL) and antidonor antibody in both groups. RESULTS: From days 8 to 18 after transplantation, the two groups were indistinguishable. Both showed acute rejection with endarteritis (type II); had IgG and IgM deposition in glomeruli and small vessels; had an infiltrate with similar numbers of T cells, proliferating (PCNA+) and activated (interleukin-2 receptor+) cells; and had a similar degree of parenchymal cell apoptosis [in situ DNA nick-end labeling (TUNEL)+]. However, by days 30 to 60, the two groups could be distinguished by several intragraft features. The recovery group became tolerant and had diminished T-cell infiltration, activation and proliferation, and no detectable antibody deposition. The number of TUNEL+-injured parenchymal cells decreased. In contrast, the progression group showed persistent cell infiltration with activation and proliferation. Significantly prominent TUNEL+ apoptotic parenchymal cells in tubules, glomeruli, peritubular capillaries and arteries were seen from day 30 to day 100. Circulating donor reactive CTL and antidonor class I IgG were detected in the progression group at higher levels than in the recovery group from days 30 to 60. CONCLUSION: In tolerance-induction protocols, unstable tolerance induction is associated with the persistent immunologic activation that mediates immunologic destruction of graft parenchymal cells and chronic rejection. Certain of the described immunopathologic findings (activation, proliferation, apoptosis, and antibody deposition) may be useful in distinguishing the type of rejection, that is, whether the allograft will progress to chronic rejection or recovery.