Role of donor-specific cytotoxic T cells in rejection of corneal allografts in normal and high-risk eyes.

Clinical ophthalmologists are highly successful in grafting allogeneic corneas onto healthy eye graft beds--only a small percentage are rejected. By contrast, a high percentage of allografts are rejected when grafted into vascularized or '"high-risk' recipient eyes. The following experiments test the hypothesis that donor-specific cytotoxic T cells mediate graft rejection in high-risk, but not normal, eyes. MHC plus minor histoincompatible C57BL/6 corneas were grafted orthotopically onto BALB/c mice. Healthy recipient eyes were trephined and served as normal graft beds; corneal vascularization was induced by penetrating sutures and these eyes served as 'high-risk' graft beds. Cytotoxic T cells were assayed at 2 and 8 weeks postgrafting using either draining cervical lymph nodes or spleen cells restimulated for 3 days with irradiated allogeneic splenic stimulator cells. As a positive control, donor-specific cytotoxic T cells were induced in mice immunized by subcutaneous injection of allogeneic spleen cells. A low percentage (only 50%) of corneal allografts were rejected when placed orthotopically onto normal healthy eyes. Donor-specific cytotoxic cells were not detected in the draining lymph nodes or spleens of mice that either accepted, or rejected their corneal graft. The failure to detect cytotoxic T cells was not due to anergy or the deletion of allospecific precursors of cytotoxic T cells. By contrast, 97% of corneal allografts were rejected from high-risk recipient eyes (no immune privilege). Donor-specific cytotoxic T cells were routinely detected in the draining lymph nodes of these mice and their appearance coincided with graft rejection. We conclude that allografts placed onto normal healthy eyes fail to induce donor-specific cytotoxic T cells. In the absence of specific cytotoxic T cells, other alloimmune effectors are less successful at mediating rejection. By contrast, allografts placed onto high-risk eyes induce donor-specific cytotoxic T cells, and all grafts are universally rejected. These results imply that immune privilege can protect corneal allografts from most effector mechanisms, except cytotoxic T cells.