A new model of orthotopic penetrating corneal transplantation in the sheep: Graft survival, phenotypes of graft‐infiltrating cells and local cytokine production

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Abstract
Background: Orthotopic penetrating keratoplasty in the sheep was developed as an outbred preclinical model to allow correlation of the cellular infiltrate during graft rejection with local production of cytokine mRNA. Methods: Penetrating corneal autografts and allografts were performed in Merino sheep. Graft outcome was followed at the slit‐lamp. Corneal infiltrates were examined by immunoperoxidase staining on postmortem specimens. Cytokine mRNA was detected by polymerase chain reaction. Results: Corneal autografts survived indefinitely. Allografts became vascularized and underwent rejection at a median of 20 days postgraft. Both endothelial and epithelial rejection lines were observed. Immunohistochemical staining of rejecting grafts showed up‐regulation of major histocompatibility complex class I molecules on corneal graft epithelium, damaged or absent graft endothelium and a marked, predominantly mononuclear cell infiltrate. CD4‐positive T cells were observed in the graft within 2 days of the onset of rejection, followed several days later by CD8‐positive T cells. Messenger RNA transcripts for interleukin (IL)‐2, tumour necrosis factor (TNF)‐α and IL‐10 (but not for interferon (IFN)‐γ or IL‐4) were found in autografted corneas. Proportionately, more allografts than autografts contained transcripts for IL‐2 and TNF‐α, and IFN‐γ was detected in three of four allografts. Conclusions: Corneal graft rejection in the sheep is macroscopically and histologically similar to human corneal graft rejection. Allografts become infiltrated by both CD4‐ and CD8‐positive T cells and local production of pro‐inflammatory cytokines occurs during graft rejection.