Abstract
Abstract: :
Purpose: In solid organ transplantation, sensitization of host T cells to donor antigens is known to occur via two major routes: the direct pathway mediated by graft-derived cells including ‘passenger leukocytes' that migrate out of the graft shortly after transplantation, and the indirect pathway in which foreign antigens are processed and presented by host antigen-presenting cells (APC). Previously, the indirect pathway was thought to be essentially the sole route of allosensitization in corneal transplantation. Methods: Orthotopic keratoplasty was performed on BALB/c mice bearing either normal (avascular; low risk [LR]) or high risk (HR) (vascularized) beds using fully mismatched C57BL/6 donors. Lymphocytes were harvested from recipient animals' cervical lymph nodes of hosts at 72 hours and at two weeks post surgery. Host T cell responses to donor APC (direct pathway) were assessed using the ELISPOT assay. The frequencies of activated T cells producing IL-2 and IFN-g (T-helper 1 [Th1] cytokines) were measured. Graft survival was assessed in HR transplant recipients where the direct pathway was inactivated by using class II major histocompatibility antigen (MHC) knock out animals as donors. Results: Directly sensitized T cells were detected in HR transplant recipients as early as 72 hours post transplantation (determined by IL-2 production in the ELISPOT assay). At two weeks post transplantation, T-cells obtained from HR graft recipients continued to produce a strong Th1 direct response by generating both IL-2 and IFN-g. In the LR transplantation setting, no directly primed T-cells were detected. Finally, eliminating the direct pathway in HR transplantation significantly slowed both the tempo and rate of rejection. Conclusions: In a HR graft bed, directly sensitized T cells are generated as early as 72 hours. At two weeks, directly sensitized T cells continue to orchestrate Th1 type activity by generating IFN-g production. Elimination of the direct response in the HR transplantation setting ameliorates the tempo of graft rejection and improves graft survival. This, taken with the finding that almost no directly primed T-cells are generated in LR graft recipients suggests that the fulminant rejection noted in high risk transplantation is, at least in part, due to the activation of the direct pathway.
Keywords: antigen presentation/processing • cornea: basic science • transplantation