Role of natural killer cells in the rejection process of corneal allografts in rats

Background. The exact mechanism of human corneal allograft rejection, which is the major cause of corneal transplant failure, remains unclear. We investigated the role of natural killer (NK) cells in rat corneal allograft rejection by examining the aqueous humor (AH) cell infiltrate on different postoperative days. Methods. Flow cytometric analysis was performed on the AH and submandibular draining lymph node (DLN) cells before transplantation and at different time points thereafter. In addition, we performed functional cytotoxicity assays with cells present in the AH during corneal rejection. Results. We demonstrated a gradual increase in the absolute cell number of different hematopoietic sub-populations in the AH after allogeneic cornea transplantation. CD3(-)CD4(+) cells, mainly monocytes and macrophages, were the predominant subpopulation 2 days after transplantation, followed by a successive relative increase of CD4(+) T cells, CD8(+) T cells, CD161(+) T cells, and NK cells. NK and CD161(+) T cells were present at a 10- to 15-fold higher percentage than in the DLN, suggestive of local expansion of these cells. A higher percentage of NK cells were CD8-negative compared with DLN NK cells. AH cells specifically lysed allogeneic cells, and this cytotoxicity was mainly attributable to NK cells but not to CD4(+) or CD8(+) T lymphocytes. Conclusion. These results confirm the crucial role of CD4(+) cells in the allogeneic corneal graft rejection process and implicate NK cells as possible mediators of the rejection.