Purpose.: The pathway between the glucocorticoid-induced tumor necrosis factor receptor family-related protein (GITR) and GITR ligand (GITRL) has been shown to control the function of regulatory T cells (Treg). The present study was conducted to investigate the role of this pathway and Treg in establishing immune privilege status for corneal allografts.

Methods.: Corneas of C57BL/6 mice were orthotopically transplanted into the eyes of BALB/c mice, and graft survival was assessed. A separate set of BALB/c mice received an anterior chamber injection of C57BL/6 splenocytes, and induction of allo-specific anterior chamber-associated immune deviation was assessed. Recipients were intraperitoneally administrated anti-GITRL, anti-CD25 monoclonal antibodies (mAb), or control immunoglobulin (IgG). Expressions of GITRL, GITR, and Foxp3 in the allografts were assessed. In vitro, cornea pretreated with anti-GITRL mAb or control IgG was incubated with T cells, and destruction of corneal endothelial cells and the population of Foxp3+ CD25+ CD4+ T cells were assessed.

Results.: GITRL was expressed constitutively in the cornea and iris-ciliary body. GITRL-expressing allografts were infiltrated with Foxp3+ GITR+ CD25+ CD4+ T cells. Blockade of GITRL did not affect allo-specific ACAID but led to infiltration of Foxp3 (−) CD4+ T cells and allograft rejection. Depletion of CD25+ CD4+ Treg also accelerated allograft rejection. Destruction of corneal endothelial cells by T cells was significantly enhanced in GITRL-blocked cornea compared with control cornea. Foxp3+ CD25+ CD4+ T cells were increased after incubation with GITRL-expressing cornea, but not with GITRL-blocked cornea.

Conclusions.: Presence of Foxp3+ CD25+ CD4+ Treg in the allograft is necessary for allograft survival. GITRL-dependent expansion of Treg within the cornea is one mechanism underlying immune privilege in corneal allografts.