CD4+ T-cell-mediated mechanisms of corneal allograft rejection: Role of Fas-induced apoptosis

The role of CD4(+) T cells as effector cells in corneal allograft rejection is poorly understood. We investigated the role of CD4(+) T cells as helper cells in the generation of allospecific effector macrophages in corneal graft rejection and the role of CD4(+) T cells as apoptosis-inducing effector cells. Corneal allografts were transplanted to CD4 knockout, FasL-deficient, and macrophage-depleted hosts. An Annexin-V binding assay was used to evaluate the susceptibility of corneal cells to both Fas-dependent and CD4 T-cell-mediated apoptosis in vitro. Macrophages were essential for graft rejection, but not as effector cells. Anti-BALB/c CD4(+) T cells from immunized C57BL/6 mice induced apoptosis of BALB/c corneal epithelial and endothelial cells. However, anti-BALB/c CD4(+) T cells from FasL-deficient gld/gld mice did not induce apoptosis of BALB/c corneal endothelial cells. Moreover, gld/gld mice had a reduced capacity to reject BALB/c corneal allografts. Although the initial results suggested a role for Fas-induced apoptosis in corneal graft rejection, additional experiments indicated otherwise. The incidence and tempo of immune rejection of Fas-deficient lpr/lpr corneal allografts were no different than those for corneal grafts from Fas-bearing C57BL/6 donors. Moreover, CD4(+) T-cell-mediated apoptosis of corneal cells could not be blocked with either Fas-Fc fusion protein or anti-FasL blocking antibody. The results suggest that CD4(+) T cells function directly as effector cells and not as helper cells in the rejection of corneal allografts. Although the corneal endothelium is highly susceptible to Fas-induced apoptosis, this is apparently not the primary mechanism of CD4(+) T-cell-dependent rejection.