Non-HLA antigens and HLA-DR matching in corneal transplantation

Experimental corneal allograft rejection

The major findings regarding corneal allograft rejection in experimental animals are reviewed. The principal anatomic and biological feature of the cornea that determines the immunologic privilege of this tissue is its avascularity. The surgical trauma of transplantation compromises the immunologic privilege, putting corneal allografts at risk for immune rejection. During the past 50 yr, rabbits, rats, and mice have been used extensively in the study of the process of immunologically mediated corneal allograft rejection. It is clear that the inflammation and neovascularization of the graft that occurs following transplantation predisposes a corneal allograft to the classic cell-mediated immune rejection response. The antigenicity of cornea cells has been studied and has been found to be significantly lower compared to other cells and tissues. Rejection of acorneal allograft is acell-mediated process directed against major histocompatibility complex antigens involving both CD4+ T helper cells and CD8+ cytotoxic cells. The prevention of corneal allograft rejection depends on the development of topically applied compounds that can prevent inflammation and vascularization and inhibit the activation of T lymphocytes. Considerable progress has been made using immunomodulators, including blocking antibodies and soluble coreceptor blocking agents such as CTLA4-Ig. Combinations of antiangiogenic agents and immunomodulators hold great promise for preventing corneal allograft rejection in patients.

MHC matching and mechanisms of alloactivation in corneal transplantation

BACKGROUND

In human corneal transplantation the value of matching, particularly for MHC class II, is unclear and controversial. The contribution of the direct pathway to T cell activation is also uncertain. We have determined the relative contribution of class I, II and non-MHC antigens to graft rejection and of the direct and indirect pathways to T cell activation in a rat model mimicking human incompatibilities.

METHODS

DA (RT1a) strain recipients received fully mismatched PVG (RT1c) strain grafts or grafts from one of three recombinant strains bearing DA MHC genes on a PVG background. Graft survival was assessed and the specificity of T cells generated in the draining lymph nodes was determined in mixed lymphocyte (MLR) proliferation assays. To assess the contribution of the direct pathway, fully mismatched graft were performed and allospecific proliferation was measured after depletion of recipient APC from the MLR reaction.

RESULTS

There was no significant difference in survival of grafts between the four grades of mismatch, which ranged from a full mismatch to non-MHC mismatches alone (median survival 12.5, 11, 13 and 12.5 days respectively). In conformity with clinical results, strong secondary responses were generated against targets matched for MHC with the recipient. Depletion of recipient APC from a fully allogeneic secondary MLR did not fully abrogate donor-specific proliferation.

CONCLUSIONS

Class II matching is of no benefit in this model. Strong indirect responses to non-MHC mismatches are sufficient to induce the rapid rejection, but the small numbers of class II+ cells in the donor appear sufficient to generate a direct response.

Effect of ABO blood group mismatching on corneal epithelial cells: an in vitro study

AIM

To determine, in vitro, the effects of blood group ABO mismatching on corneal epithelial cells.

METHODS

Corneal epithelial cell cultures were established from 32 human cadaver donor eyes. Epithelial cells (100 microl of 4 x 10(2) cells per microl) were incubated for 4 hours with antibodies against blood group antigens A, B, and AB, with and without complement. Cell lysis was assayed by a chemiluminescent assay using Cytolite reagent. Live cells, remaining after incubation, were counted in a scintillation counter. The blood group of the donors was determined retrospectively, in a blinded manner.

RESULTS

Retrospective tracing of donor blood groups was possible for 20 donors. In all cases the blood group corresponded with that suggested by the cell lysis assay. Significant cell lysis was observed when known A group cells were incubated with anti-A and anti-AB antibody, B group cells were incubated with anti-B and AB antibody, and AB group cells were incubated with anti-AB antibody. Lysis occurred only in the presence of complement. No lysis of O group cells was observed with any of the antibodies. In all cases, lysis was observed only with neat (serum) antibody concentrations.

CONCLUSIONS

Blood group ABO mismatching results in significant lysis of corneal epithelial cells. The antibody concentration required for lysis equals that found in serum. Such levels of antibody are unlikely to be achieved in tears and/or aqueous. This may offer an explanation for the conflicting reports of the studies on the effect of blood group matching on corneal grafts. The variability in the outcome may reflect the levels of antibodies gaining access to the corneal cells and not the mismatching alone.

Cell subpopulations in failed human corneal grafts

BACKGROUND/AIMS

Inflammatory cells and antigen presenting cells (APC) are not present under normal circumstances in the centre of the healthy cornea. The purpose of this study was to investigate and phenotype the inflammatory cell populations, particularly with reference to T cell subpopulations and macrophages, and to localise dendritic cells (DC) and other MHC class II positive cells in three groups of grafted corneas: rejected non-inflamed, rejected inflamed grafts, and control dystrophic explants.

METHODS

15 corneal buttons removed during keratoplasty from non-inflamed "quiet" previously grafted corneas, five inflamed corneas requiring urgent regrafting for "graft melting" (in "high risk" corneas), and 10 control dystrophic opaque corneas explanted during their first graft procedure were examined. Cryosections of corneas were immunostained with a panel of monoclonal antibodies (mAb) against CD3, CD4, CD8, CD14, CD25, CD68, HLA-DP, and HLA-DR molecules using the StreptABC method. DC were detected by dual immunostaining as CD1a+ and MHC class II+ and CD19-. Cell densities in immunostained tissue sections were evaluated using a scale from 0 to +4.

RESULTS

Immunostaining in control dystrophic corneas was negative for all antibodies. A moderate to high density of CD8+, CD14+, and CD68+ cells was observed in the majority of rejected non-inflamed as well as in rejected inflamed corneal buttons. Strong positivity for HLA-DP and HLA-DR molecules in the epithelium, stroma, and endothelium was also demonstrated. Weak positivity for CD4 and CD25 was observed in six of 15 and 11 of 15 rejected corneas, respectively. The presence of dendritic cells in the basal layer of the epithelium and in the stroma was observed in 50% of the grafts.

CONCLUSIONS

A high frequency of macrophages, the presence of DC in the explants, and strong expression of HLA-DP and HLA-DR molecules on resident cells are characteristics of rejected corneal allografts, whether actively inflamed or not. The presence of DC in the stroma of the grafted cornea suggests that they may be mainly responsible for T cell activation and graft rejection since DC are known to be a 100-fold more potent than macrophages as APC.