Histopathology of rejected orthotopic corneal grafts in the rat

We have used an orthotopic graft model in the rat to study the histologic characteristics of corneal allograft rejection. Unrejected allogeneic grafts could not be distinguished from clear syngeneic grafts. Although donor Langerhans cells are necessary for the development of delayed-type hypersensitivity (DTH), the histopathological characteristics of rejecting corneal allografts in immunologically naive hosts were identical regardless of the presence or absence of donor Langerhans cells. By contrast, preimmunization had a dramatic effect on the histology of graft rejection. Untreated allografts placed onto pre-immunized recipients underwent a marked cellular necrosis accompanied by minimal inflammation that easily distinguished these grafts from the previous groups. These results suggest that neither the presence nor absence of DTH responsiveness correlates with the histopathological events that accompany corneal graft rejection. However, preimmunization leads to a different histologic pattern of rejection that is characterized by an intense cellular necrosis.