Cornea–Associated B7–H1 Promotes T–Cell Apoptosis as a Potential Mechanism of Immune Privilege of Corneal Allografts

Abstract

Abstract: : Purpose: Programmed Death 1 (PD–1) is a new member of the CD28/CTLA–4 family, which has been implicated in the maintenance of peripheral tolerance. B7–H1 (PD–L1), a new member of the B7 family, has been identified as one of ligands for PD–1. We have demonstrated that cornea, iris–ciliary body, and neural retina of normal mouse eyes constitutively express B7–H1, and that PD–1/B7–H1 pathway has a role in corneal allograft survival. Purpose of the present study is to investigate a mechanism of immune privilege of corneal allografts – associated PD–1 / B7–H1 pathway. Methods: Normal corneas of C57BL/6 were transplanted orthotopically into normal eyes of BALB/c mice. Recipients were administrated intraperitoneally with 0.2 mg of anti–B7–H1 monoclonal antibody (mAb) or isotype control rat IgG, three times a week for 8 weeks after grafting. Graft survival was assessed clinically and histologically. Expression of B7–H1 on corneal allografts was assessed immunohistochemically at various time points. Phenotype and expression of PD–1 on inflammatory cells infiltrated in corneal allografts were also assessed under confocal microscopy. TUNEL (TdT–mediated dUTP nick and labeling) staining was performed to detect cells undergoing apoptosis in corneal allografts. Results: Inflammatory cell infiltration was found in both control allografts and allografts treated with anti–B7–H1mAb, at 2 weeks. TUNEL assay revealed that many of infiltrating cells were undergoing apoptosis in B7–H1 positive control allografts at 2 weeks. These allografts showed little inflammation at 4 weeks and survived more than 8 weeks. On the other hands, allografts treated with anti–B7–H1mAb were occupied with TUNEL negative inflammatory cells including PD–1 positive CD4 T cells at 2 weeks. At 4 weeks, majority of corneal cells – i.e. epithelial, stromal, and endothelial cells and infiltrating cells in these allografts were undergoing apoptosis, and the tissue was destructed. Conclusions: Expression of B7–H1 on cornea induces apoptosis of effector T cells via PD–1. It is indicated that PD–1/B7–H1 pathway plays a role on peripheral deletion of allo–reactive effector T cells within cornea to maintain immune privilege of corneal allografts.

Keywords: immune tolerance/privilege • apoptosis/cell death • immunomodulation/immunoregulation