The taming of the shrew? The immunology of corneal transplantation

Corneal transplantation, first reported a century ago, is the oldest and most frequent form of solid tissue transplantation. Although keratoplasty is also considered as the most successful transplant procedure, several studies indicate that the long term survival of corneal grafts is even lower than that of transplanted parenchymatous organs. Despite the immune privilege enjoyed by the cornea and anterior segment of the eye, immunologic graft rejection is a major limitation to corneal transplantation. This review gives an update on corneal immunobiology and the mechanisms of corneal graft rejection, focusing on antigen presentation, as well as on the molecular and cellular mediators of this particular immune response.     

Current concepts and techniques in corneal transplantation

The corneal endothelium is the most important single layer in corneal transplantation. In his Castroviejo Lecture, William Bourne, MD, summarizes his work on the corneal endothelium and its importance to corneal transplantation. Almost half the corneal transplants performed in the United States are done so because of malfunctioning, diseased, or absent endothelial cells. If just this layer could be transplanted, the long wait for better vision after keratoplasty (up to two years in some countries) can be eliminated, as well as the problems of epithelial and subepithelial graft rejection. The significant astigmatism after keratoplasty could also be reduced. Transplantation of the endothelium in deep lamellar keratoplasty is being done in limited fashion throughout the world and the first patients have now been done in the United States. In many countries where corneal tissue is difficult to obtain, keratoplasty is only performed on one eye, even though both eyes may need it. One article this year discusses binocular vision recovery in bilateral keratoplasty and the objective and subjective improvements after bilateral keratoplasty. Patients who are bilaterally blind from diseases such as Stevens-Johnson syndrome, and ocular pemphigoid have little hope of visual recovery from conventional corneal transplants. The use of a keratoprosthesis to bypass the totally abnormal conjunctival surface has helped many people in the past. The preoperative prognostic categories of patients who may benefit from keratoprosthesis has been carefully reviewed in a large number of keratoprosthesis patients and this information will help ophthalmologists decide who will benefit the most benefit from keratoprosthesis. This year, articles on corneal transplantation after conjunctival flaps, suture-related complications in keratoplasty, the implantation of an intraocular lens after penetrating keratoplasty, and long-term results of penetrating keratoplasty with glaucoma drainage tube implants are subjects that warrant in depth discussion and evaluation. Finally, eye bank considerations will be discussed concerning the long-term question of transmission of cancer through corneal transplantation.     

New thoughts on the immunology of corneal transplantation

Significant advances derived from rodent models of penetrating keratoplasty have transformed our understanding of the pathogenesis of rejection of orthotopic corneal transplants. The high rate of success of corneal allografts placed in low-risk eyes without cover of immunosuppression arises from immune privilege of the cornea graft itself, and of the anterior chamber where it forms the anterior wall. Immune privilege owes its existence in penetrating keratoplasty to an absence of blood and lymph vessels in the graft and its bed, the absence of MHC class II(+) antigen presenting cells in the graft, reduced expression of MHC-encoded alloantigens on graft cells, constitutive expression of T cell-deleting CD95 ligand on corneal graft endothelium, the existence of an immunosuppressive local microenvironment (aqueous humor), and the capacity of the graft to induce anterior chamber associated immune deviation (ACAID). The results of recent experiments provide answers to pertinent questions regarding cornea graft failure: How does the cornea as a graft suppress inflammation and angiogenesis locally? How does the graft promote ACAID to the alloantigens it expresses? and How do corneal cells reduce their vulnerability as targets of effector T cells? The answers offer the possibilities of novel strategies for preventing immune-based corneal allograft failure.     

Corneal transplantation after failed grafts: Options and outcomes

Corneal transplantation is the most commonly performed human tissue transplantation procedure worldwide. Because of the large number of transplants, corneal graft failure has become one of the most common indications for corneal transplantation. The relatively recently developed lamellar transplant techniques have brought about specific potential complications leading to graft failure that may require different approaches to repeat transplantation other than penetrating keratoplasty. On the other hand, these new lamellar techniques also provide novel ways of rescuing failed penetrating grafts, with potential advantages over successive penetrating keratoplasties, such as reduced intraoperative risks and faster visual rehabilitation. We summarize the incidence and risk factors of graft failure for penetrating and lamellar (stromal and endothelial) corneal transplants and discuss the various surgical alternatives currently available to rescue such failed grafts, with a focus on the reported outcomes and limitations.     

Penetrating keratoplasty in infants.

PURPOSE: To analyze graft survival and visual outcome after penetrating keratoplasty in infants with congenital corneal opacity. METHODS: We retrospectively reviewed the records of 11 patients with congenital corneal opacity who underwent penetrating keratoplasty as infants. Six patients had a diagnosis of Peter's anomaly, 1 of congenital hereditary endothelial dystrophy, 1 of posterior polymorphous dystrophy, and 2 of sclerocornea, and in the other patient, the cause of the opacity was unknown. RESULTS: In total, 26 penetrating keratoplasties were performed on 16 eyes of 11 infants. All patients initially underwent surgery before the age of 13 months. Five patients underwent bilateral penetrating keratoplasty, and 10 of the transplants were repeat operations on eyes that had already had at least one previously failed graft. The age of the infants at the time of first penetrating keratoplasty ranged from 2 to 56 weeks (median, 13 weeks). The graft survival time for all transplants ranged from 3 to 137 months (median, 16 months). Overall first graft survival at 12 months was 61% (95% CI, 33%-81%), with 10 of 16 eyes retaining a clear corneal graft. Peter's anomaly, lensectomy, and repeat penetrating keratoplasty were factors most highly associated with poor graft survival and a low final visual acuity. CONCLUSION: Early penetrating keratoplasty for congenital corneal opacity may prevent deprivation amblyopia. Although this procedure carries a high risk of failure, particularly in those patients with Peter's anomaly, careful case selection, optical correction, and management of postoperative amblyopia may result in a successful visual outcome.     

Automatized large diameter lamellar keratoplasty and stem cell transplantation for the treatment of ocular surface diseases with limbal insufficiency

PURPOSE: To report a new surgical procedure for the treatment of ocular surface diseases associated with severe limbal insufficiency. METHODS: A retrospective review of four patients with severe ocular surface disease who required stem cell transplantation and keratoplasty for the correction of limbal insufficiencies. They underwent large diameter lamellar keratoplasty with microkeratome. When limbal dysfunction was associated with limited alteration of the ocular surface and transparent deep corneal stroma only the anterior corneal stroma was transplanted. When the entire corneal thickness was compromised, both anterior and deep donor buttons were transplanted. RESULTS: Patients remained stable and improved their visual acuity after surgery. Best-corrected visual acuity ranged from 20/200 to 20/30. No corneal graft rejections were found. The main complication found in one of our patients was a central stromal opacity which required a secondary penetrating keratoplasty. CONCLUSIONS: Automatized large diameter lamellar keratoplasty provides a safe and successful alternative to limbal transplantation for limbal insufficiency associated with corneal opacity. This technique enables a single-stage surgical procedure and the use of a single donor which reduces the risk of rejection. In addition, better refractive results are achieved due to the quality of the interface and the absence of corneal sutures.     

Preliminary findings in corneal allograft rejection in patients with keratoconus

PURPOSE: Classically, corneal allograft rejection is thought to be a T(H)1-mediated phenomenon. However, T(H)2-mediated allograft rejection has been reported in other transplanted organ systems, including the heart and kidney. We previously reported a form of T(H)2-mediated corneal allograft rejection in a murine model with a T(H)2 immune bias. In this study we sought to determine if there was any evidence for this form of corneal allograft rejection in humans. DESIGN: Experimental study with an interventional case series. METHODS: The clinical records of all keratoconus patients undergoing penetrating keratoplasty at the University of Texas, Southwestern Medical Center from 1994 to 1999 were reviewed. Careful attention was paid to a clinical history of atopy. Atopic patients were selected, because these patients have been shown to have a "T(H)2 immune bias." The corneal graft rejection rate in these patients and the number of repeat corneal transplants performed was determined. The experimental group consisted of patients with a clinical history of atopy and keratoconus who had at least one repeat penetrating keratoplasty for an immunologically rejected corneal transplant. Any patient with evidence of primary allograft failure was excluded from this study. Tissue specimens from these patients were embedded in paraffin, serially sectioned, stained with Giemsa stains, and examined histologically. The control group consisted of patients without a clinical history of allergy (and therefore no T(H)2 immune bias) who underwent corneal transplantation for Fuch corneal endothelial dystrophy, or aphakic/pseudophakic bullous keratopathy. Failed grafts from these control patients were also paraffin embedded, serially sectioned, stained, and examined histologically. The human experimental and control corneal specimens were compared with data obtained in a murine model of T(H)2-mediated corneal allograft rejection. Briefly, full-thickness penetrating C57BL/6ByJ corneal allografts were transplanted onto Balb/cByJ and Balb/c-IFN-gamma(tm1Ts) (Balb/c-IFN-gamma knockout) mice. Additionally, full-thickness Balb/cByJ corneal allografts were transplanted onto C57BL/6ByJ and C57BL/6ByJ-IFN-gamma(tm1Ts) mice. Corneal allograft rejection rates and mean rejection times were calculated and compared between wild-type and interferon gamma (IFN-gamma) knockout hosts. The rejected allografts were examined histologically by the same methods used in the human tissue. RESULTS: There were 84 penetrating keratoplasties performed from 1994 to 1999 for keratoconus. Seven of these 84 patients rejected their corneal grafts. Of the 7 patients who rejected their corneal allografts, 4 had repeat penetrating keratoplasty. Of these 4 repeat corneal allografts, 3 showed eosinophilia when compared with rejected grafts in control patients. Atopic keratoconus patients had a mixed inflammatory cellular infiltrate in the rejected corneal tissue specimen with a significantly greater density of eosinophils (P =.001) compared with patients who did not have a pre-existing T(H)2 bias. The inflammatory infiltrate in these patients without a T(H)2 immune bias was mononuclear. In the murine model, corneal allograft rejection did occur in the absence of IFN-gamma, a critical T(H)1 cytokine in both fully allogeneic donor-host combinations. Histologically, rejection in these ("T(H)2 mice") was characterized by a predominant eosinophilic infiltrate in the rejected graft bed when compared with wild-type animals ("T(H)1 mice") that had a predominantly mononuclear infiltrate in the rejected corneal graft bed. CONCLUSIONS: Preliminary findings show that corneal allograft rejection in patients with a pre-existing T(H)2 phenotype is similar to what is seen in the murine model of T(H)2-mediated corneal allograft rejection. Based on this small sample, it appears that eosinophils may play a role in corneal allograft rejection in this group of patients. However, further study is necessary to determine the importance of these cells in allograft rejection.     

Retrospective evaluation of risk factors for graft rejection in patients after corneal transplantation performed at the Eye Clinic in the years 2001-2003

The aim of the study was retrospective evaluation of risk factors for graft rejection in patients after corneal transplantation performed in Department of Ophthalmology in years 2001-2003. 349 cases of penetrating keratoplasty, lamellar, or penetrating with limbal graft, were analyzed. Deep stromal vascularization, anterior synechiae, the use of topical glaucoma medications, previous inflammations in anterior segment of the eye and surgical procedures were evaluated. Period from transplantation to rejection, number of relapses of rejection and changes of visual acuity were also estimated. Corneal graft rejection occurred more frequently in patients with one or more risk factors, particularly deep stromal vascularization, the use of topical glaucoma medications and in young age.     

Immune mechanisms of corneal allograft rejection

Penetrating keratoplasty has been successfully performed on humans for over 100 years and remains the most common form of solid tissue transplantation. Although corneal allografts enjoy a remarkable degree of immune privilege, immune rejection remains the leading cause of keratoplasty failure. The immunologic basis for corneal allograft rejection was established in animal studies over 50 years ago, yet large gaps remain in our knowledge regarding the cellular and molecular mechanisms of corneal allograft rejection. The enormous redundancy in the mammalian immune system creates a condition that favors the development of multiple independent immune mechanisms that can produce corneal allograft rejection. Although there are few absolute principles, it is certain that the immune rejection of corneal allografts is (1) T cell-dependent, (1) heavily dependent upon CD4(+) T cells, (3) not restricted to either Th1 or Th2 T cell populations, and (4) dependent upon an intact repertoire of resident antigen presenting cells.     

Corneal graft rejection

Penetrating keratoplasty is the most widely practiced type of transplantation in humans. Irreversible immune rejection of the transplanted cornea is the major cause of human allograft failure in the intermediate and late postoperative period. This immunological process causes reversible or irreversible damage to the grafted cornea in several cases despite the use of intensive immunosuppressive therapy. Corneal graft rejection comprises a sequence of complex immune responses that involves the recognition of the foreign histocompatibility antigens of the corneal graft by the host's immune system, leading to the initiation of the immune response cascade. An efferent immune response is mounted by the host immune system against these foreign antigens culminating in rejection and graft decompensation in irreversible cases. A variety of donor- and host-related risk factors contribute to the corneal rejection episode. Epithelial rejection, chronic stromal rejection, hyperacute rejection, and endothelial rejection constitute the several different types of corneal graft rejection that might occur in isolation or in conjunction. Corneal graft failure subsequent to graft rejection remains an important cause of blindness and hence the need for developing new strategies for suppressing graft rejection is colossal. New systemic pharmacological interventions recommended in corneal transplantation need further evaluation and detailed guidelines. Two factors, prevention and management, are of significant importance among all aspects of immunological graft rejection. Preventive aspects begin with the recipient selection, spread through donor antigenic activity, and end with meticulous surgery. Prevention of corneal graft rejection lies with reduction of the donor antigenic tissue load, minimizing host and donor incompatibility by tissue matching and suppressing the host immune response. Management of corneal graft rejection consists of early detection and aggressive therapy with corticosteroids. Corticosteroid therapy, both topical and systemic, is the mainstay of management. Addition of immunosuppressive to the treatment regimen helps in quick and long term recovery. Knowledge of the immunopathogenesis of graft rejection may allow a better understanding of the immunological process thus helping in its prevention, early detection and management.     

重视角膜移植植片内皮细胞慢性失功的研究

随着配型技术及新型免疫抑制剂的临床应用,器官或组织移植术后早期急性免疫排斥反应发生率明显下降,慢性移植物失功则成为导致远期移植失败的主要原因之一。研究表明角膜移植术后和全身器官移植术后同样存在角膜植片慢性失功的过程,且角膜植片慢性失功已经成为再次角膜移植的主要原因之一。因此,重视角膜植片慢性失功对其临床防治及延长角膜植片的存活率有着重要的意义。     

铆钉形角膜移植应用于高危患者的疗效观察

目的 研究铆钉形角膜移植(rivet-shaped keratoplasty，RSK)对高危移植患者的治疗效果。方法 对26例(26只眼)高危移植患者随机分为两组，分别施行RSK和穿透性角膜移植联合环状板层角膜缘移植(penetrating keratoplasty combined with ring-shaped limbal transplantation，PKRL)。观察两组术后植片、植床的变化和角膜移植排斥反应发生率。结果 与PKRL相比，RSK可促进植片上皮愈合，减少术后散光，降低角膜移植排斥反应发生率。结论 RSK是一种有效的眼前段重建手术方法。     

Immunsuppression durch Kombinationstherapie mit Basiliximab und Ciclosporin bei Hochrisikokeratoplastik Eine Pilotstudie

BACKGROUND: The risk of postoperative immune reaction/transplant rejection is increased in the presence of distinct risk factors. In these situations, the use of established immunosuppressive agents (steroids, cyclosporin) is indicated. However, immune reactions can occur despite this therapy. Basiliximab is a monoclonal antibody with a high specific binding affinity to the IL-2 receptor of activated T-cells. After renal transplantation, the combination therapy with basiliximab and cyclosporin reduces the risk of acute rejection by 30% against placebo. The aim of this study was to investigate the effect of complementary immune suppression with the IL-2 synthesis inhibitor cyclosporin and the IL-2 receptor antagonist basiliximab on the incidence of transplant rejection after high-risk keratoplasty. PATIENTS AND METHODS: In an open pilot study, seven patients with high-risk keratoplasty were treated with basiliximab perioperatively and cyclosporin postoperatively. All patients had penetrating keratoplasty with a non-HLA-matched corneal graft. RESULTS: Seven patients were operated on from 12/98 through 12/99 with a corneal transplant diameter between 7.1 and 9.5 mm. Risk factors included extensive corneal neovascularization in all patients (indications for surgery: 4 cases with corneal scars after herpetic disease, 2 cases with conditions after alkali burns, 1 case with corneal ulceration after thermal burn). During the follow-up of 14-25 months, no immune reaction has occurred, and all transplants are clear. The mean visual acuity increased from 0.04 to 0.41 postoperatively. DISCUSSION: The preliminary data on a combination therapy with basiliximab and cyclosporin are promising. For further evaluation a prospective multicenter study is planned.     

穿透性角膜移植50例的临床研究

目的:探讨穿透性角膜移植的疗效和相关并发症。方法:对50例(50眼)穿透性角膜移植进行回顾性分析。结果:统计分析表明:角膜植片透明者45例(90%),视力≤0.04者6例(12%),0.05～0.2者27例(54%),≥0.3者17例(34%)。主要并发症:角膜植片免疫排斥反应6例(12%),继发性青光眼4例(8%)。结论:穿透性角膜移植是提高角膜盲视力的有效手段,应重视防治术后免疫排斥反应。     

穿透性角膜移植联合扇形角膜缘移植术的观察

目的　观察穿透性角膜移植联合扇形角膜缘移植术的临床疗效。方法　将 61例 (61眼 )患者分为两组 ,一组 3 0例 (3 0眼 )进行部分穿透性角膜移植 (PKP)术 ,另一组 3 1例 (3 1眼 )进行穿透性角膜移植联合扇形角膜缘移植 (PKP +LKP)术 ,比较其临床疗效。结果　PKP组中有 12例的植片发生了免疫排斥反应 ,其中 2例 (2眼 )因植片血管化而导致植片浑浊。PKP+FLT组穿透植片 4例发生排斥反应。两组进行 χ2 检验 ,P <0 0 5。结论　对于合并有局部新生血管的角膜白斑患者 ,PKP+FLT术较传统的PKP术有更好的临床效果。     

The historical development and an overview of contemporary keratoprostheses

Corneal blindness is a major cause of ocular morbidity that affects 4.5 million people worldwide. Though penetrating keratoplasty is an excellent option for most patients with corneal blindness, there are various conditions for which corneal transplantation carries a low likelihood of success. In cases of multiple failed transplants, the keratoprosthesis, an artificial cornea, is a well-documented surgical option, though few models are commercially available. Keratoprostheses also provide a solution to those in developing areas of the world who do not have access to penetrating keratoplasty owing to limitations in the supply of corneal donor tissue. We summarize the history of keratoprostheses, examine the various keratoprosthesis models used across the globe, and highlight efforts to improve the accessibility and biointegration of keratoprosthesis through novel technological developments.     

200例穿透性角膜移植临床研究

目的：观察穿透性角膜移植的疗效和并发症。方法：对200例（200眼）穿透性角膜移植进行回顾性总结。结果：统计分析表明：角膜植片透明者180例（90.0%）视力≤0.04者73例（36.5%）,0.05～0.2者91例（45.5%）≥0.3者36例（18.0%）。主要并发症：角膜植片免疫排斥反应22例（11.0%）,继发性青光眼7例（3.5%）。结论：穿透性角膜移植可使角膜盲患者复明,但防治术后免疫排斥反应是成功关键。     

Tacrolimus immunosuppression in high-risk corneal grafts

BACKGROUND: Unlike the immune privilege enjoyed by low-risk corneal grafts, high-risk corneal grafts experience rejection rates comparable to liver and kidney transplants. Systemic immunosuppression reduces the risk of rejection in high-risk corneal grafts. METHODS: Systemic tacrolimus, a specific T cell inhibitor, was used at a mean daily dose of 2.5 mg to immunosuppress 43 patients undergoing high-risk corneal transplantation. Immunosuppression was continued for a period of 18-24 months after the high-risk corneal graft. RESULTS: During a mean follow-up period of 33.7 months, clarity of the graft was maintained in 65% of patients. Eight patients experienced rejection episodes while on tacrolimus, and this led to graft failure in five patients. CONCLUSION: Tacrolimus is relatively safe and effective in reducing rejection and prolonging graft survival in patients with high-risk keratoplasty compared with other series where similar immunosuppression was not used.     

Corneal transduction to inhibit angiogenesis and graft failure

PURPOSE: To test whether lentivirus-mediated expression of an endostatin::kringle-5 (E::K-5) fusion gene has an inhibitory effect on neovascularization and failure of corneal transplants. METHODS: A lentiviral vector containing a fusion transgene comprising the human endostatin gene and the kringle-5 domain of the human plasminogen gene (E::K-5) was used for transduction of corneal buttons ex vivo. The corneal buttons were transplanted after overnight incubation in media containing either lentivirus or PBS. Sixteen rabbits underwent allogenic penetrating keratoplasty in one eye. The area of neovascularization from the limbus to within the graft was documented after surgery. RT-PCR was performed to demonstrate the presence of transgene mRNA within the graft. Histopathology was used to analyze neovascularization, inflammation, and rejection morphology. RESULTS: Less neovascularization was observed in corneas treated with the lentivirus E::K-5 fusion vector. Early onset and profound neovascularization was observed in control eyes. E::K-5-treated animals did not have graft failure, whereas five of the six control animals had graft failure, as classified by opacification of the graft. All E::K-5 transduced corneas tested were positive by RT-PCR for the unique fusion gene sequence. Histopathology corroborated a significant increase of blood vessel presence and inflammatory reaction in control compared with treated eyes. CONCLUSIONS: Corneas transduced with a lentivirus containing an endostatin::kringle-5 fusion gene demonstrated an inhibition of neovascularization and graft failure. E::K-5 gene transduction through a lentiviral vector system may be a useful adjunct to prevent graft neovascularization and corneal graft rejection in high-risk corneal transplants with antecedent rejection or neovascularization.     

Cellular changes in transplanted human corneas

PURPOSE: To measure endothelial cell and keratocyte densities in transplanted corneas and the changes in these densities with time. METHODS: The endothelia of 500 consecutive penetrating corneal transplants were studied longitudinally by specular microscopy for 10 to 20 years. The keratocytes of 36 corneal transplants that varied in postoperative times from 1 month to 20 years were studied cross-sectionally by clinical confocal microscopy. The keratocytes of five transplanted corneas were studied longitudinally by confocal microscopy at 1 day, 1 week, and 1 month postkeratoplasty. RESULTS: Endothelial cell density decreased progressively at an accelerated rate for 20 years after transplantation, with concurrent increases in the coefficient of variation of cell area and corneal thickness and decreases in the percentage of hexagonal cells. Grafts with insufficient endothelial cells developed late endothelial failure, which was the primary cause of graft failure after the first 5 postoperative years. The grafts with late endothelial failure did not lose endothelial cells faster than grafts that did not fail, but instead had fewer cells immediately after transplantation, diminishing to a critically low cell density earlier. The keratocyte density was also decreased in transplanted corneas. Keratocytes became "activated" during the first week after keratoplasty and in grafts with late endothelial failure. CONCLUSION: It should be possible to prevent or delay late endothelial failure, the primary cause of graft failure, by increasing the number of endothelial cells on transplanted corneas. The status of the keratocytes appears to affect corneal transparency and, thus, visual quality in the grafted eye.