Acute rejection of orthotopic corneal xenografts in mice depends on CD4⁺ T cells and self-antigen-presenting cells

PURPOSE. Guinea pig corneal xenografts have been reported to be rejected acutely in eyes of normal adult mice. Rejection of this type is independent of xenoreactive antibodies, and mice deficient in CD8⁺ and NK T cells are unable to reject guinea pig corneal grafts acutely. Therefore, a study was conducted to determine the extent and manner by which CD4⁺ T cells are responsible for rejection of orthotopic corneal xenografts. METHODS. Xenogeneic corneas were prepared from eyes of normal guinea pigs and grafted orthotopically into normal eyes of C57BL/6 mice, class II major histocompatibility complex (MHC) knockout (KO) mice, and class II MHC KO mice reconstituted with syngeneic (C57BL/6) CD4⁺ T cells and/or bone marrow cells. Graft survival was assessed clinically, and success of cellular reconstitution was assayed using flow cytometric analysis of peripheral blood leukocytes. T cells from rejector mice were analyzed for proliferative responses to guinea pig xenoantigens in vitro. RESULTS. Median survival times (MST) of corneal xenografts in MHC class II KO mice was significantly delayed (31 days) compared with grafts in wild-type C57BL/6 eyes (9 days). Acute rejection was restored almost completely when MHC class II KO mice were reconstituted simultaneously with C57BL/6 bone marrow and CD4⁺ T cells, but not when the KO mice were reconstituted with either CD4⁺ T cells or bone marrow cells alone. Mice that rejected guinea pig corneas possessed only CD4⁺ T cells capable of responding to guinea pig xenoantigens in vitro. CONCLUSIONS. Acute rejection of orthotopic corneal xenografts in mice is mediated by CD4⁺ T cells that detect guinea pig xenoantigens that are presented on MHC class II⁺ syngeneic antigen-presenting cells. These results strongly suggest that rejection occurs exclusively through the indirect pathway of T-cell activation.