Journal Logo

Advanced Search
Editorials and Perspectives: Overview

The Immunobiology of Corneal Transplantation

Williams, Keryn A.; Coster, Douglas J.

Author Information

Department of Ophthalmology, Flinders University, Adelaide, Australia.

Supported by the National Health & Medical Research Council of Australia, Canberra, Australia.

Address correspondence to: Keryn A. Williams, Ph.D., Department of Ophthalmology, Flinders Medical Centre, Bedford Park, SA 5042, Australia.

E-mail: [email protected]

Received 7 June 2007. Revision requested 16 July 2007.

Accepted 3 August 2007.

doi: 10.1097/01.tp.0000285489.91595.13
  • Free

Abstract

Just over 100 years ago, Zirm reported the first successful human corneal allograft (1). Corneal transplantation is now widely practiced and is sight-restorative in those with congenital or acquired corneal opacification, disease, or damage. Dozens of eye banks now preserve donor human corneas for periods of 1 to 4 weeks (2). The number of corneal transplants carried out worldwide is unknown, but anecdotal reports suggest that more than 60,000 procedures are performed per annum (at least 35,000 in North America, 16,000 in Europe, 10,000 in India, and 1,250 in Australasia). In most countries, the major indications for corneal transplantation include keratoconus (a thinning and warping of the cornea that results in distorted vision), bullous keratopathy (corneal edema that results in pain and poor vision), failed previous graft, corneal scarring, corneal dystrophy, and infection, with the relative proportions differing in different geographic locations and tending to fluctuate over time.

Most corneal transplantation is penetrating; that is, the full thickness of the cornea is replaced (Fig. 1A, B). Lamellar (partial thickness) grafts are also performed and this surgical technique, in particular, has evolved over recent years (3). Although lamellar grafts were once performed mostly for pterygium (a benign growth from the conjunctiva that may encroach on the cornea), modified procedures are now applied to cases that used to be treated by penetrating keratoplasty. Deep anterior lamellar keratoplasty (4), in which the recipient's own corneal endothelial cell monolayer (Fig. 1C) is retained, is increasingly being used for keratoconus and anterior corneal scarring. Endothelial cell transplantation (5), in which the anterior portion of the host cornea including the ocular surface is retained but the posterior portion including the corneal endothelium is replaced, is now being used for Fuchs' endothelial dystrophy (an adult-onset dystrophy caused by corneal endothelial cell dysfunction) and bullous keratopathy.

F1-2
FIGURE 1.:
The normal human cornea. (A) Diagrammatic representation of a human eye in cross-section. In corneal transplantation, a full or partial-thickness disc of central cornea is generally replaced. (B) Histology of human cornea, stained with H&E. The corneal endothelial monolayer (arrowed) pumps water from the corneal stroma, thereby maintaining corneal clarity. Image provided by Dr. S. Klebe. (C) Silver-stained flat-mount of the human corneal endothelial cell monolayer, shown en face. Human endothelial cells are postmitotic and have little, if any, replicative capacity.

Outcomes of Corneal Transplantation in Humans

Corneal transplants are considered to be immune-privileged (6) and are widely held to enjoy an enviably high success rate. Certainly the early outcomes of corneal allotransplantation are typically excellent, and graft survival figures of 90% at 1 year after surgery are not uncommon. This apparent success often stands in stark contrast to graft survival figures for vascularized organ grafts, which are seldom so high at 1 year. Closer inspection of corneal transplant registry data (7), however, tells a different story. Kaplan-Meier survival of penetrating corneal allografts at 15 years is 55% (Fig. 2A). The kinetics of corneal graft failure are different from those of many vascularized organ grafts, where many failures occur within the first year posttransplant. For corneal grafts, the attrition rate is typically slow but inexorable.

F2-2
FIGURE 2.:
Clinical corneal graft outcomes. (A) Kaplan-Meier plots of human clinical corneal allograft survival, showing Registry data for penetrating and lamellar corneal grafts separately. Numbers on the plots indicate the number of grafts initially at risk. (B) Desired outcomes of corneal transplantation, as specified by the surgeon at the time of transplantation. Most grafts are performed on an elective basis, to improve vision. Others are performed to relieve pain, to improve vision and to relieve pain, for structural repair (tectonic grafts), to improve cosmesis, and for various other combinations of these reasons. All data are from the Australian Corneal Graft Registry 2007, with permission.

Corneal transplantation is performed to improve vision, to reduce pain, for structural repair, and very occasionally to improve cosmesis (Fig. 2B). A satisfactory visual outcome after corneal transplantation clearly depends upon long-term graft survival, which in turn depends upon the indication for transplantation. For patients with keratoconus or a corneal dystrophy, where the cornea is noninflamed and avascular, penetrating graft survival rates of 90% at 10 years postoperatively are common (Fig. 3). For patients suffering the sequelae of inflammation of the anterior segment of the eye, graft survival rates are much poorer (7). For penetrating keratoplasty at least, minor differences in surgical technique do not significantly affect outcomes (8), although comorbidities including cataract, refractive errors, or retinal dysfunction may limit the visual potential of an eye with a transparent corneal graft. However, the majority of clinical studies have shown that irreversible rejection is the most important cause of corneal graft failure (9), despite the immune-privileged status of the cornea and anterior segment of the eye.

F3-2
FIGURE 3.:
Kaplan-Meier corneal allograft survival, stratified according to indication for transplantation. The plots show survival of first penetrating grafts for keratoconus, compared with survival of all other penetrating grafts. Numbers indicate the numbers of grafts initially at risk. Data from the Australian Corneal Graft Registry 2007, with permission.

Corneal Immune Privilege

The mechanisms that underlie the immune privilege of the normal cornea have been reviewed elsewhere (6, 10). These mechanisms include weak expression of major histocompatibility complex (MHC) antigens on corneal cells, the expression of Fas ligand by the cornea, the relative dearth of mature antigen-presenting cells within the cornea, and the presence of immunomodulating molecules in the anterior chamber fluid that bathes the posterior surface of the cornea. Immune privilege of the cornea is relative, however, and is readily subverted by inflammation, which predisposes a corneal graft to rejection.

Corneal Allograft Rejection

Much of our understanding of the immunobiology of corneal allograft rejection has been gained from studies of orthotopic corneal transplantation in experimental animals (Fig. 4). Two of the first species to be used were the rabbit (11) (Fig. 4A) and the domestic cat (12) (Fig. 4B). In both, corneal graft rejection looks similar to the corresponding process in humans (Fig. 4C–E); in particular so-called rejection lines, which are accumulations of leukocytes, are frequently observed in rabbits (Fig. 4A). However, neovascularization and inflammation must often be induced in the grafted eye, or the recipient must be systemically sensitized to the donor, before rejection will occur. These models, despite being outbred, are generally low-risk. More recently, the sheep has proved to be an excellent outbred model species, in which rejection reliably occurs without the need for additional manipulation (Fig. 4F), probably because the normal sheep cornea is slightly vascularized (13). The development of orthotopic corneal transplantation in small rodents such as the rat (14) and mouse (15) was a useful advance (Fig. 4G), but the anatomy and physiology of the rodent eye are not entirely comparable with those of the human eye. In particular, the lens occupies a proportionally much larger volume of the rodent eye (Fig. 4H), and the corneal endothelium in rodents is replicative (16), unlike human corneal endothelium which is postmitotic. Lymphatic drainage pathways may also be different. Nevertheless, the availability of inbred, transgenic, and gene-targeted strains of rodent has provided important information on the mechanisms of corneal graft rejection.

F4-2
FIGURE 4.:
Montage of orthotopic penetrating corneal transplants in different species. (A) Rejecting corneal graft in a rabbit: Rose Bengal has been instilled into the tear film to identify an epithelial “rejection line” (arrowed). (B) Transparent corneal graft in a cat. (C) Transparent human corneal graft. (D) Human corneal graft undergoing acute rejection: note the spreading edema, loss of clarity and presence of leukocytes on the corneal endothelium. (E) Human cornea graft that has failed from rejection. (F) Rejecting corneal graft in a sheep, showing neovascularization of the graft and corneal edema. (G) Transparent corneal isograft in an inbred rat: the vessels are present in the iris. (H) Enucleated rat eye showing the shallow anterior chamber and relatively large lens (arrowed). (I) Failed corneal concordant xenograft (guinea pig to rat) on day 3 after transplantation.

Sensitization to Cornea-Derived Antigen: Antigen-Presenting Cells in the Anterior Segment

There are several anatomically-distinct groups of antigen-presenting cell (APC) in the anterior segment of the eye. One group, located within the iris and the trabecular meshwork, consists of macrophages and MHC class II+ dendritic cells (17). These cells are ideally positioned to capture soluble antigen shed from the endothelium of a corneal graft. Macrophages in the iris and ciliary body have been shown to capture and internalize soluble antigen introduced into the anterior chamber, and iris macrophages are capable of stimulating primed T cells and play a role in local antigen presentation during secondary responses (18, 19). Cells in the suprachoroidal space and around limbal-episcleral vessels trap antigen leaving the eye through aqueous drainage pathways (20). However, substantial amounts of soluble antigen leave the eye in soluble form (20). Resident APCs lining the anterior chamber may process antigens in the immunomodulatory milieu of the anterior chamber fluid, resulting in suppression or anergy, rather than induction of antigen-specific immunity. These APCs do not appear to migrate to regional lymph nodes (21). Foreign antigen introduced into the anterior chamber elicits systemic, antigen-specific suppression of delayed-type hypersensitivity responses, a phenomenon called anterior chamber-associated immune deviation, or ACAID (22). However, classical ACAID is an active regulatory process that requires time to develop, and that appears to be insufficient to prevent active sensitization to corneal alloantigen (23).

Another group of MHC class II+ APCs is located in the conjunctiva and cornea. These APCs exhibit site-specific variation in surface phenotype and density (24–26), and tend to be CD1−(27). Resident CD45+, MHC class II-low APCs have also been discovered in the central murine cornea (28). When transgenic corneas expressing green fluorescent protein (GFP) were transplanted orthotopically into wild-type mice, GFP+ cells were detected in the superficial cervical lymph nodes within 6 hr, and were shown to possess some allostimulatory capacity after maturation (29). These APCs were considered to be immature dendritic cells. Independent confirmation of the presence of a network of CD45+, MHC class II-low APCs in the murine central cornea has been provided from another laboratory, but the phenotype of the cells was reported as being more consistent with an origin in the macrophage lineage (30). Conjunctival and corneal APCs do not encounter antigen in an immunomodulatory environment and are thus likely to direct T cell responses towards rejection.

Presentation of Cornea-Derived Alloantigen

Where does presentation of cornea-derived alloantigen occur? Options include within the cornea itself, in surrounding tissues including the conjunctiva and ocular environs, in locoregional lymph nodes or in more distant secondary lymphoid tissue. Given that APC-T cell interactions in general occur within organized secondary lymphoid tissue, the question may validly be raised as to why the issue even merits consideration. The reason is that immunologists and anatomists have long held that there is no lymphatic drainage of the cornea or the anterior segment. Indeed, the immune privilege ascribed to the cornea has often been attributed to its avascularity and lack of lymphatics (31, 32). However, anterior segment inflammation is a potent trigger for corneal neovascularization, and it has become clear that lymphatic vessels expressing LYVE-1 can also invade inflamed corneas (33). Whether these new capillaries and lymphatics provide a functional conduit for ingress of recipient APCs and other leukocytes into a cornea, or egress of soluble or cell-bound alloantigen from the cornea, is not clear.

As well as containing resident APCs, the conjunctiva also contains conjunctiva-associated lymphoid tissue, or CALT (34, 35). As early as 3 days after surgery, rats bearing corneal grafts develop organized lymphoid aggregates within the conjunctiva, in which antigen presentation and T cell proliferation may occur (36). Interestingly, there has been at least one report that foreign antigen applied directly to the conjunctiva leads to T cell anergy rather than sensitization (37). Some local antigen presentation within the ocular environs, if not within the conjunctiva, is suggested by the finding that expression of recombinant CTLA4-Ig and some cytokines including interleukin (IL)-10 by corneal endothelium can prolong corneal graft survival (38, 39). However, some antigen released from the cornea appears later in superficial cervical lymph nodes (40), and the number of T cells increases in these nodes after corneal transplantation (41). Other evidence implicates presentation in submandibular lymph nodes after antigen injection into the posterior chamber of the eye (42), or after corneal transplantation (43). The loco-regional lymph nodes that may be involved in antigen presentation in the human are unknown.

The preceding discussion has been predicated on the implicit assumption that cornea-derived alloantigens are either captured by APC within the anterior segment or leave the eye in soluble form, and are thereafter presented by the indirect pathway. Indirect presentation is certainly important in the immune response to a corneal graft. The presence of APC in an avascular recipient cornea prior to corneal transplantation is enough to erode immune privilege (44), presumably by triggering indirect alloantigen presentation. Furthermore, once the recipient's APC have migrated into the cornea, they tend to remain and thereafter exert a continuing negative influence on corneal graft survival (45). However, the direct pathway also operates: corneal grafts that contain many donor-derived APC as passenger cells exhibit a higher risk of rejection than grafts with fewer APC (46). Thus, depending upon the state of the graft bed and the degree of major and minor histocompatibility antigen disparity involved, both direct and indirect antigen presentation appear to be involved in the sensitization phase to foreign alloantigens displayed by a corneal graft (47).

Effector Mechanisms in Corneal Graft Rejection

Multiple and redundant mechanisms operate in the effector phase of corneal graft rejection. A substantial literature over several decades has implicated a cell-mediated response as being of primary importance (48, 49). The CD4+ T cell controls the response to a corneal allograft (50–52), but the inflammatory infiltrate in a rejecting graft is heterogeneous. Acutely-rejecting histoincompatible corneal grafts become infiltrated with CD4+ T cells, macrophages, CD8+ T cells, natural killer (NK) cells and neutrophils (53–57). No particular difference in the composition of the infiltrate is apparent among different species including the human, except that atopic corneal graft recipients are reported to show significantly more eosinophils than do patients without a history of atopy (58).

Corneal neovascularization and lymphangiogenesis are also implicated in rejection (59). Corneal allograft rejection in both humans (60) and experimental animals (61) is associated with local production of a similar array of chemokines and pro-inflammatory cytokines within the cornea and anterior chamber fluid, and some cell death probably results from the actions of tumor necrosis factor (TNF)-α and interferon (IFN)-γ, as well as from the local release of toxic products such as nitric oxide. Reactive nitrogen species have been shown to be associated with inflammation within the eye (62), and inhibition of inducible nitric oxide synthase can prolong corneal graft survival in mice (63).

Corneal cells express mRNA for the apoptosis-related molecules Fas ligand, Bax, Bcl-2, and ICE (64), and T cells infiltrating the anterior segment of the eye rapidly become apoptotic through ligation of their Fas receptors by constitutively-expressed FasL (65, 66). Whether or not the cornea expresses Fas is controversial (64, 66). The reality is that despite expression of Fas ligand, corneal allografts are still damaged by the rejection response in both humans (10) and in wild-type experimental animals (67). One possibility is that infiltrating cytotoxic T lymphocytes and NK cells can kill corneal cells through the cytotoxic granule-mediated pathway (68) before they themselves die. Corneal graft rejection occurs readily in beta 2-microglobulin (69) and perforin (53) knockout mice, suggesting that CD8+ T cells are not obligatory for rejection, but nevertheless CD8+ T cells are clearly capable of mediating corneal graft damage (70, 71).

Donor-specific alloantibodies and complement can also cause damage to a corneal allograft, but immunoglobulin (Ig) G1 alloantibodies are not always produced in response to a histoincompatible graft and are not essential for graft rejection (72–74). Clinically, cross-matching for corneal allotransplantation is seldom performed.

Role of Histocompatibility Antigen Matching in Corneal Transplantation

Human leukocyte antigen (HLA) matching for corneal transplantation is carried out routinely in relatively few centers, based predominantly in Europe. HLA determinants are weakly expressed on normal corneal tissue, but expression can be upregulated by the actions of pro-inflammatory cytokines, including those produced during a rejection episode. While some early studies (75–77) showed a beneficial effect of HLA matching in patients considered to be at high risk of corneal graft rejection, two large, prospective randomized trials found either no significant effect, or a slight adverse effect of matching (78, 79). More recent studies from Europe in which modern molecular methods of typing were used showed a clear beneficial effect of HLA matching in high-risk patients (80, 81). A randomized, multicenter trial of the influence of matching in corneal transplantation is now being performed in Bristol. All of the typing is being performed by molecular methods and an interim analysis on 800 transplants is expected in late 2007 (Armitage, personal communication).

There has been at least one report that ABO matching improves corneal graft survival in high-risk patients (78). More recently, matching for the H-Y minor histocompatibility antigen has also been shown to improve human corneal graft survival (82). The evidence from animal models certainly supports a role for minor transplantation antigens being important in the immune response to a corneal graft (83).

Immunosuppression for Corneal Transplantation

Developments in immunosuppressive regimens for clinical corneal transplantation have recently been reviewed elsewhere (84). Topical immunosuppression is the method of choice for delivery of immunosuppressive drugs to a corneal graft. Virtually all patients who receive a corneal graft are treated for periods of months or years with topical glucocorticosteroids, according to widely differing protocols (85–88). Corticosteroids with a beta-hydroxy group at position 11 of one carbon ring are readily absorbed across the ocular surface and high levels can be achieved in the anterior chamber. Prednisolone is the most commonly used of the available corticosteroids, but some ophthalmologists also use a “soft” steroid such as loteprednol etabonate (88). The calcineurin inhibitors and other hydrophobic immunosuppressive drugs are poorly absorbed through the cornea, and need to be carefully formulated for satisfactory ocular absorption. The evidence for their efficacy in prolonging corneal survival when delivered topically is weak in both experimental and clinical studies (89–91).

Systemic immunosuppression with corticosteroids has been used to reverse ongoing corneal graft rejection episodes in some high-risk patients (92). Other systemic immunosuppressants are occasionally administered on a prophylactic basis in very high-risk patients, especially those with salvageable vision in only one eye, but morbidities may outweigh any improvement in graft survival (93–97). A possible exception is rapamycin, which is showing some promise in early clinical studies (98).

Monoclonal antibodies and other biologics are not used routinely in corneal transplantation, but may be useful for both prophylaxis and for rescue after graft rejection (reviewed in [99, 84]). CAMPATH-1H, in particular, has been shown to be effective in rescuing corneal grafts undergoing acute rejection. Intact antibody molecules do not pass through the ocular surface when delivered topically, although antibody fragments will do so, provided they are appropriately formulated (100).

The evidence-base favoring the use of one immunosuppressive drug regimen over another for the prophylaxis and treatment of corneal allograft rejection is poor. This uncertainty over best practice is reflected in the great diversity of immunosuppressive protocols in widespread use for corneal transplantation.

Gene Therapy for Corneal Transplantation

The postmitotic corneal endothelium is the most important target for the allograft response because once damaged, it cannot be replaced by the host. Gene therapy directed at the corneal endothelium or the eyelids has been shown to prolong corneal allograft survival in mice, rats, rabbits and sheep (101, 102). Those studies that have demonstrated significant prolongation of corneal allograft survival thus far have been performed with replication- deficient adenoviral or lentiviral vectors. Nonviral vectors have improved markedly in efficiency (103) but do not provide long-term gene expression. Therapeutic transgenes that have been shown to prolong corneal allograft survival in animal models have included interleukin 10, the p40 subunit of interleukin 12, interleukin 4, the TNFα receptor, endostatin-kringle 5 fusion protein, soluble CTLA4 or CTLA4-Ig, indoleamine 2,3-dioxygenase and nerve growth factor (102, 104, 105).

CONCLUSIONS

The long-term outcomes of corneal transplantation are no better than those for vascularized organ transplantation, and irreversible rejection remains the most frequent cause of graft failure. New lamellar procedures such as deep anterior lamellar keratoplasty, designed to retain more of the recipient's own cornea and thereby decrease foreign antigen load, may help to reduce the incidence of rejection but long-term outcome data are not yet available. Other strategies to decrease the incidence of corneal graft rejection include matching for both major and minor histocompatibility antigens. Topical glucocorticosteroids remain the mainstay of immunosuppression for human corneal transplantation, but are used in a bewildering array of different protocols. The new biologics and novel approaches such as gene therapy show promise in experimental models, but have yet to be tested systematically in humans.

ACKNOWLEDGMENTS

The photographic expertise of Dr. E. Sherrard, Mr. L. Emerson, Mrs. A. Chappell, and Mr. D. Summerhayes is gratefully acknowledged. The authors thank Mr. D. Jones for the artwork.

REFERENCES

1. Zirm E. Eine erfolgreiche totale Keratoplastik. Albrecht von Graefes Archiv fur Ophthalmol 1906; 64: 580.
2. Chu W. The past twenty-five years in eye banking. Cornea 2000; 19: 754.
3. Alio JL, Shah S, Barraquer C, et al. New techniques in lamellar keratoplasty. Curr Opin Ophthalmol 2002; 13: 224.
4. Shimmura S, Tsubota K. Deep anterior lamellar keratoplasty. Curr Opin Ophthalmol 2006; 17: 349.
5. Terry MA. Endothelial keratoplasty: History, current state, and future directions. Cornea 2006; 25: 873.
6. Hori J, Niederkorn JY. Immunogenicity and immune privilege of corneal allografts. Chem Immunol Allergy 2007; 92: 290.
7. Williams KA, Esterman AJ, Bartlett C, et al. How effective is penetrating corneal transplantation? Factors influencing long-term outcome in multivariate analysis. Transplantation 2006; 81: 896.
8. Frost NA, Wu J, Lai TF, Coster DJ. A review of randomized controlled trials of penetrating keratoplasty techniques. Ophthalmology 2006; 113: 942.
9. Coster DJ, Williams KA. The impact of corneal allograft rejection on the long-term outcome of corneal transplantation. Am J Ophthalmol 2005; 140: 1112.
10. George AJ, Larkin DF. Corneal transplantation: The forgotten graft. Am J Transplant 2004; 4: 678.
11. Khodadoust AA. Penetrating keratoplasty in the rabbit. Am J Ophthalmol 1968; 66: 899.
12. Bahn CF, Meyer RF, MacCallum DK, et al. Penetrating keratoplasty in the cat. A clinically applicable model. Ophthalmology 1982; 89: 687.
13. Williams KA, Standfield SD, Mills RA, et al. A new model of orthotopic penetrating corneal transplantation in the sheep: Graft survival, phenotypes of graft-infiltrating cells and local cytokine production. Aust N Z J Ophthalmol 1999; 27: 127.
14. Williams KA, Coster DJ. Penetrating corneal transplantation in the inbred rat: a new model. Invest Ophthalmol Vis Sci 1985; 26: 23.
15. She SC, Steahly LP, Moticka EJ. A method for performing full-thickness, orthotopic, penetrating keratoplasty in the mouse. Ophthalmic Surg 1990; 21: 781.
16. Tuft SJ, Williams KA, Coster DJ. Endothelial repair in the rat cornea. Invest Ophthalmol Vis Sci 1986; 27: 1199.
17. McMenamin PG, Crewe J, Morrison S, Holt PG. Immunomorphologic studies of macrophages and MHC class II-positive dendritic cells in the iris and ciliary body of the rat, mouse, and human eye. Invest Ophthalmol Vis Sci 1994; 35: 3234.
18. Camelo S, Voon AS, Bunt S, McMenamin PG. Local retention of soluble antigen by potential antigen-presenting cells in the anterior segment of the eye. Invest Ophthalmol Vis Sci 2003; 44: 5212.
19. Steptoe RJ, McMenamin PG, Holt PG. Resident tissue macrophages within the normal rat iris lack immunosuppressive activity and are effective antigen-presenting cells. Ocul Immunol Inflamm 2000; 8: 177.
20. Camelo S, Shanley A, Voon AS, et al. The distribution of antigen in lymphoid tissues following its injection into the anterior chamber of the eye. J Immunol 2004; 172: 5388.
21. Dullforce PA, Garman KL, Seitz GW, et al. APCs in the anterior uveal tract do not migrate to draining lymph nodes. J Immunol 2004; 172: 6701.
22. Streilein JW. Unraveling immune privilege. Science 1995; 270: 1158.
23. Streilein JW. Immunological non-responsiveness and acquisition of tolerance in relation to immune privilege in the eye. Eye 1995; 9: 236.
24. Williams KA, Ash JK, Coster DJ. Histocompatibility antigen and passenger cell content of normal and diseased human cornea. Transplantation 1985; 39: 265.
25. Catry L, Van den Oord J, Foets B, et al. Morphologic and immunophenotypic heterogeneity of corneal dendritic cells. Graefes Arch Clin Exp Ophthalmol 1991; 229: 182.
26. Yamagami S, Ebihara N, Usui T, et al. Bone marrow-derived cells in normal human corneal stroma. Arch Ophthalmol 2006; 124: 62.
27. Baudouin C, Brignole F, Pisella PJ, et al. Immunophenotyping of human dendriform cells from the conjunctival epithelium. Curr Eye Res 1997; 16: 475.
28. Hamrah P, Zhang Q, Liu Y, Dana MR. Novel characterization of MHC class II-negative population of resident corneal Langerhans cell-type dendritic cells. Invest Ophthalmol Vis Sci 2002; 43: 639.
29. Liu Y, Hamrah P, Zhang Q, et al. Draining lymph nodes of corneal transplant hosts exhibit evidence for donor major histocompatibility complex (MHC) class II-positive cells derived from MHC class II-negative grafts. J Exp Med 2002; 195: 259.
30. Brissette-Storkus CS, Reynolds SM, Lepisto AJ, Hendricks RL. Identification of a novel macrophage population in the normal mouse corneal stroma. Invest Ophthalmol Vis Sci 2002; 43: 2264.
31. Medawar PB. Immunity to homologous grafted skin. III. Br J Exp Pathol 1948; 29: 58.
32. Barker CF, Billingham RE. Immunologically privileged sites. Adv Immunol 1977; 25: 1.
33. Cursiefen C, Schlötzer-Schrehardt U, Küchle M, et al. Lymphatic vessels in vascularized human corneas: Immunohistochemical investigation using LYVE-1 and podoplanin. Invest Ophthalmol Vis Sci 2002; 43: 2127.
34. Knop N, Knop E. Conjunctiva-associated lymphoid tissue in the human eye. Invest Ophthalmol Vis Sci 2000; 41: 1270.
35. Chen H, Hendricks RL. B7 costimulatory requirements of T cells at an inflammatory site. J Immunol 1998; 160: 5045.
36. Banerjee S, Figueiredo FC, Easty DL, et al. Development of organised conjunctival leukocyte aggregates after corneal transplantation in rats. Br J Ophthalmol 2003; 87: 1515.
37. Egan RM, Yorkey C, Black R, et al. In vivo behavior of peptide-specific T cells during mucosal tolerance induction: antigen introduced through the mucosa of the conjunctiva elicits prolonged antigen-specific T cell priming followed by anergy. J Immunol 2000; 164: 4543.
38. Comer RM, King WJ, Ardjomand N, et al. Effect of administration of CTLA4-Ig as protein or cDNA on corneal allograft survival. Invest Ophthalmol Vis Sci 2002; 43: 1095.
39. Klebe S, Sykes PJ, Coster DJ, et al. Prolongation of sheep corneal allograft survival by transfer of the gene encoding ovine interleukin 10. Transplantation 2001; 71: 1214.
40. Yamagami S, Dana MR. The critical role of lymph nodes in corneal alloimmunization and graft rejection. Invest Ophthalmol Vis Sci 2001; 42: 1293.
41. Okada K, Mishima HK, Kawano MM, et al. Involvement of CD8+ RT1.B+ and CD4+ RT1.B+ cells of cervical lymph nodes in the immune response after corneal transplantation in the rat. Jpn J Ophthalmol 1997; 41: 209.
42. Egan RM, Yorkey C, Black R, et al. Peptide-specific T cell clonal expansion in vivo following immunization in the eye, an immune-privileged site. J Immunol 1996; 157: 2262.
43. Hoffmann F, Zhang EP, Mueller A, et al. Contribution of lymphatic drainage system in corneal allograft rejection in mice. Graefes Arch Clin Exp Ophthalmol 2001; 239: 850.
44. Niederkorn JY. Effect of cytokine-induced migration of Langerhans cells on corneal allograft survival. Eye 1995; 9: 215.
45. Williams KA, White MA, Ash JK, Coster DJ. Leukocytes in the graft bed associated with corneal graft failure. Ophthalmology 1989; 96: 38.
46. He YG, Niederkorn JY. Depletion of donor-derived Langerhans cells promotes corneal allograft survival. Cornea 1996; 15: 82.
47. Boisgerault F, Liu Y, Anosova N, et al. Role of CD4(+) and CD8(+) T cells in allorecognition: Lessons from corneal transplantation. J Immunol 2001; 167: 1891.
48. Niederkorn JY. Mechanisms of corneal graft rejection: The sixth annual Thygeson Lecture. Cornea 2001; 20: 675.
49. Bertelmann E, Jaroszewski J, Pleyer U. Corneal allograft rejection: current understanding. 2. Clinical implications. Ophthalmologica 2002; 216: 2.
50. Ayliffe W, Alam Y, Bell EB, et al. Prolongation of rat corneal graft survival by treatment with anti-CD4 monoclonal antibody. Br J Ophthalmol 1992; 76: 602.
51. He YG, Ross J, Niederkorn JY. Promotion of murine orthotopic corneal allograft survival by systemic administration of anti-CD4 monoclonal antibody. Invest Ophthalmol Vis Sci 1991; 32: 2723.
52. Hegde S, Niederkorn JY. The role of cytotoxic T lymphocytes in corneal allograft rejection. Invest Ophthalmol Vis Sci 2000; 41: 3341.
53. Larkin DFP, Alexander RA, Cree IA. Infiltrating inflammatory cell phenotypes and apoptosis in rejected human corneal allografts. Eye 1997; 11: 68.
54. Williams KA, Standfield SD, Mills RA, et al. A new model of orthotopic penetrating corneal transplantation in the sheep: Graft survival, phenotypes of graft-infiltrating cells and local cytokine production. Aust N Z J Ophthalmol 1999; 27: 127.
55. Claerhout I, Kestelyn P, Debacker V, et al. Role of natural killer cells in the rejection process of corneal allografts in rats. Transplantation 2004; 77: 676.
56. Yamagami S, Hamrah P, Zhang Q, et al. Early ocular chemokine gene expression and leukocyte infiltration after high-risk corneal transplantation. Mol Vis 2005; 11: 632.
57. Nicholls SM, Banerjee S, Figueiredo FC, et al. Differences in leukocyte phenotype and interferon-gamma expression in stroma and endothelium during corneal graft rejection. Exp Eye Res 2006; 83: 339.
58. Hargrave S, Chu Y, Mendelblatt D, et al. Preliminary findings in corneal allograft rejection in patients with keratoconus. Am J Ophthalmol 2003; 135: 452.
59. Cursiefen C, Cao J, Chen L, et al. Inhibition of hemangiogenesis and lymphangiogenesis after normal-risk corneal transplantation by neutralizing VEGF promotes graft survival. Invest Ophthalmol Vis Sci 2004; 45: 2666.
60. Funding M, Hansen TK, Gjedsted J, Ehlers N. Simultaneous quantification of 17 immune mediators in aqueous humour from patients with corneal rejection. Acta Ophthalmol Scand 2006; 84: 759.
61. King WJ, Comer RM, Hudde T, et al. Cytokine and chemokine expression kinetics after corneal transplantation. Transplantation 2000; 70: 1225.
62. Hoey S, Grabowski PS, Ralston SH, et al. Nitric oxide accelerates the onset and increases the severity of experimental autoimmune uveoretinitis through an IFN-gamma-dependent mechanism. J Immunol 1997; 159: 5132.
63. Strestikova P, Plskova J, Filipec M, Farghali H. FK 506 and aminoguanidine suppress iNOS induction in orthotopic corneal allografts and prolong graft survival in mice. Nitric Oxide 2003; 9: 111.
64. Wilson SE, Li Q, Weng J, et al. The Fas-Fas ligand system and other modulators of apoptosis in the cornea. Invest Ophthalmol Vis Sci 1996; 37: 158.
65. Stuart PM, Griffith TS, Usui N, et al. CD95 ligand (FasL)-induced apoptosis is necessary for corneal allograft survival. J Clin Invest 1997; 99: 396.
66. Stuart PM, Yin X, Plambeck S, et al. The role of Fas ligand as an effector molecule in corneal graft rejection. Eur J Immunol 2005; 35: 2591.
67. Williams KA, Standfield SD, Smith JR, Coster DJ. Corneal graft rejection occurs despite Fas ligand expression and apoptosis of infiltrating cells. Br J Ophthalmol 2005; 89: 632.
68. Trapani JS, Sutton VR, Smyth MJ. Cytotoxic granules. Immunol Today 1999; 20: 351.
69. Yamada J, Ksander BR, Streilein JW. Cytotoxic T cells play no essential role in acute rejection of orthotopic corneal allografts in mice. Invest Ophthalmol Vis Sci 2001; 42: 386.
70. Niederkorn JY, Stevens C, Mellon J, Mayhew E. CD4+ T-cell- independent rejection of corneal allografts. Transplantation 2006; 81: 1171.
71. Niederkorn JY, Stevens C, Mellon J, Mayhew E. Differential roles of CD8+ and CD8- T lymphocytes in corneal allograft rejection in ‘high-risk' hosts. Am J Transplant 2006; 6: 705.
72. Hegde S, Mellon JK, Hargrave SL, Niederkorn JY. Effect of alloantibodies on corneal allograft survival. Invest Ophthalmol Vis Sci 2002; 43: 1012.
73. Hargrave SL, Mayhew E, Hegde S, Niederkorn J. Are corneal cells susceptible to antibody-mediated killing in corneal allograft rejection? Transpl Immunol 2003; 11: 79.
74. Holan V, Vitova A, Krulova M, et al. Susceptibility of corneal allografts and xenografts to antibody-mediated rejection. Immunol Lett 2005; 100: 211.
75. Batchelor JR, Casey TA, Werb A, et al. HLA matching and corneal grafting. Lancet 1976; 1: 551.
76. Völker-Dieben HJ, Kok-van Alphen CC, Lansbergen Q, Persijn GG. The effect of prospective HLA-A and -B matching on corneal graft survival. Acta Ophthalmol(Copenh) 1982; 60: 203.
77. Boisjoly HM, Tourigny R, Bazin R, et al. Risk factors of corneal graft failure. Ophthalmology 1993; 100: 1728.
78. The Collaborative Corneal Transplantation Studies Research Group. The collaborative corneal transplantation studies (CCTS). Effectiveness of histocompatibility matching in high-risk corneal transplantation. Arch Ophthalmol 1992; 110: 1392.
79. Vail A, Gore SM, Bradley BA, et al. Influence of donor and histocompatibility factors on corneal graft outcome. Transplantation 1994; 58: 1210.
80. Khaireddin R, Wachtlin J, Hopfenmüller W, Hoffmann F. HLA-A, HLA-B, and HLA-DR matching reduces the rate of corneal allograft rejection. Graefes Arch Clin Exp Ophthalmol 2003; 241: 1020.
81. Völker-Dieben HJ, Claas FHJ, Schreuder GMT, et al. Beneficial effect of HLA-DR matching on the survival of corneal allografts. Transplantation 2000; 70: 640.
82. Bohringer D, Spierings E, Enczmann J, et al. Matching of the minor histocompatibility antigen HLA-A1/H-Y may improve prognosis in corneal transplantation. Transplantation 2006; 82: 1037.
83. Katami M, Madden PW, White DJ, et al. The extent of immunological privilege of orthotopic corneal grafts in the inbred rat. Transplantation 1989; 48: 371.
84. Banerjee S, Dick AD. Recent developments in the pharmacological treatment and prevention of corneal graft rejection. Expert Opin Investig Drugs 2003; 12: 29.
85. Rinne JR, Stulting RD. Current practices in the prevention and treatment of corneal graft rejection. Cornea 1992; 11: 326.
86. Barker NH, Henderson TR, Ross CA, et al. Current Australian practice in the prevention and management of corneal allograft rejection. Clin Experiment Ophthalmol 2000; 28: 357.
87. Koay PY, Lee WH, Figueiredo FC. Opinions on risk factors and management of corneal graft rejection in the United kingdom. Cornea 2005; 24: 292.
88. Randleman JB, Stulting RD. Prevention and treatment of corneal graft rejection: current practice patterns (2004). Cornea 2006; 25: 286.
89. Williams KA, Grutzmacher RD, Roussel TJ, Coster DJ. A comparison of the effects of topical cyclosporine and topical steroid on rabbit corneal allograft rejection. Transplantation 1985; 39: 242.
90. Price MO, Price FW Jr. Efficacy of topical cyclosporine 0.05% for prevention of cornea transplant rejection episodes. Ophthalmology 2006; 113: 1785.
91. Tavandzi U, Prochazka R, Usvald D, et al. A new model of corneal transplantation in the miniature pig: Efficacy of immunosuppressive treatment. Transplantation 2007; 83: 1401.
92. Hill JC, Maske R, Watson PG. The use of a single pulse of intravenous methylprednisolone in the treatment of corneal graft rejection. A preliminary report. Eye 1991; 5: 420.
93. Hill JC. Systemic cyclosporine in high-risk keratoplasty. Short- versus long-term therapy. Ophthalmology 1994; 101: 128.
94. Sundmacher R, Reinhard T, Heering P. Six years' experience with systemic cyclosporin A prophylaxis in high-risk perforating keratoplasty patients. A retrospective study. Ger J Ophthalmol 1992; 1: 432.
95. Poon AC, Forbes JE, Dart JK, et al. Systemic cyclosporin A in high-risk penetrating keratoplasties: a case-control study. Br J Ophthalmol 2001; 85: 1464.
96. Inoue, Kimura C, Amano S, et al. Long-term outcome of systemic cyclosporine treatment following penetrating keratoplasty. Jpn J Ophthalmol 2001; 45: 378.
97. Rumelt S, Bersudsky V, Blum-Hareuveni T, Rehany U. Systemic cyclosporin A in high failure risk, repeated corneal transplantation. Br J Ophthalmol 2002; 86: 988.
98. Birnbaum F, Reis A, Bohringer D, et al. An open prospective pilot study on the use of rapamycin after penetrating high-risk keratoplasty. Transplantation 2006; 81: 767.
99. Thiel MA, Coster DJ, Williams KA. The potential of antibody-based immunosuppressive agents for corneal transplantation. Immunol Cell Biol 2003; 81: 93.
100. Thiel MA, Coster DJ, Standfield SD, et al. Penetration of engineered antibody fragments into the eye. Clin Exp Immunol 2002; 128: 67.
101. George AJ, Arancibia-Carcamo CV, Awad HM, et al. Gene delivery to the corneal endothelium. Am J Respir Crit Care Med 2000; 162: S194.
102. Williams KA, Jessup CF, Coster DJ. Gene therapy approaches to prolonging corneal allograft survival. Expert Opin Biol Ther 2004; 4: 1059.
103. Collins L, Fabre JW. A synthetic peptide vector system for optimal gene delivery to corneal endothelium. J Gene Med 2004; 6: 185.
104. Beutelspacher SC, Pillai R, Watson MP, et al. Function of indoleamine 2,3-dioxygenase in corneal allograft rejection and prolongation of allograft survival by over-expression. Eur J Immunol 2006; 36: 690.
105. Gong N, Pleyer U, Vogt K, et al. Local overexpression of nerve growth factor in rat corneal transplants improves allograft survival. Invest Ophthalmol Vis Sci 2007; 48: 1043.
Keywords:

Corneal transplantation; Rejection; Tissue matching; Immunosuppression

© 2007 Lippincott Williams & Wilkins, Inc.