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

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, pre-dominantly 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)-alpha and IL-10 (but not for interferon (IFN)-gamma or IL-4) were found in autographed corneas. Proportionately, more allografts than autografts contained transcripts for IL-2 and TNF-alpha and IFN-gamma was detected in three of four allografts.Conclusions: Corneal graft rejection in the sheep is macroscopically and histologically similar to human corneal graft CD-8 positive T cells and local production of pro-inflammatory cytokines occurs during graft rejection.