Exhaustive differentiation of alloreactive CD8⁺ T cells : critical for determination of graft acceptance or rejection

Background. The precise role that CD8⁺ T cells play in the rejection and acceptance of different types of allograft is unclear and has been shown to vary between donor–recipient combinations.

Methods. The response of adoptively transferred CD8⁺ T cells reactive to the donor alloantigen H2K^b was examined after transplantation of H2K^b+ liver, kidney, and heart grafts in mice.

Results. After transfer of 6×10⁶ alloreactive CD8⁺ T cells to T-cell depleted syngeneic mice spontaneous long-term acceptance of liver grafts was observed, whereas kidney and heart grafts were acutely rejected. Within 5 days of liver transplantation, we found that the entire H2K^b-reactive T-cell pool was stimulated to proliferate and differentiate into memory or effector cells that were detectable within lymphoid tissues as well as the liver graft itself. However, despite the generation of effector or memory T cells, liver allografts were accepted, which correlated with the exhaustion or deletion of such cells. In contrast, although activation and proliferation of H2Kb-reactive CD8+ T cells was observed after transplantation of heart or kidney grafts, unactivated, H2K^b-reactive CD8⁺ T cells were still present in the spleen even long term. Interestingly, differences in the effector function of liver and kidney graft infiltrating donor-reactive CD8⁺ T cells were not detected after adoptive transfer into immunodeficient mice, despite a reduction in Th1-type cytokines within liver grafts.

Conclusions. The rapid and extensive initial activation and differentiation of donor-reactive CD8⁺ T cells that occurs after liver transplantation leads to clonal exhaustion or deletion of the alloreactive CD8⁺ T-cell repertoire resulting in spontaneous tolerance induction.