Corneal allograft rejection. The role of the major histocompatibility complex

The greater success of corneal transplantation compared to other organ transplants has led to the concept that the cornea is a site of "immunological privilege." Corneal cells possess the antigens of the major histocompatibility complex responsible for allograft rejection in other tissues (i.e., HLA antigens). The avascularity of the cornea accounts for the relative protection of the donor cornea from the immunological surveillance of the recipient. As the roles and functions of the major histocompatibility complex are unravelled, the mechanisms responsible for host sensitization, lymphocyte activation and allograft rejection are becoming better understood. In particular, the HLA-DR antigen in humans is believed to play an integral part in allograft rejection. Langerhans cells in human corneal epithelium have been shown to bear this antigen. Evidence suggests that these cells or similar HLA-DR-bearing cells in the cornea play a major role in corneal allograft rejection. In light of these advances in transplantation immunobiology, new methods of suppressing and possibly preventing allograft rejection in corneal transplantation are presented.     

Corneal transplantation--immunological mechanisms of rejection episode

Organ transplantation including cornea is often only the one method of treatment in case of their irreversible destruction. The genetic difference between donor and graft recipient makes the immunological system recognizes foreign antigens and triggers off a rejection episode. This article reviews corneal graft rejection immunology and indicates possible future options for the clinical evaluation of more specific therapeutic agents that modulate the immunological mechanisms of allograft rejection. It discusses also the role of HLA matching in the survival of corneal grafts especially in "high risk" eyes.     

Rejection of corneal allografts

Corneal graft rejection is the major cause of penetrating keratoplasty failure. It is a complex immunological process that involves recognition of alloantigens from the corneal graft by the host's immune system, leading to an efferent immune response against the graft. Each layer of the cornea can undergo rejection, endothelial rejection being the most severe form. In some cases, rejection will lead to corneal graft failure. Many donor- and host-related risk factors contribute to corneal graft rejection. Corticosteroid therapy, topical or systemic, is the gold-standard in the preventive and curative treatment of rejection. Other immunosuppressive agents are promising but require further evaluation. Early detection of rejection is essential to establish an aggressive treatment and reduce the risk of graft failure. Prevention of rejection is also based on tissue matching between donor and recipient. In high-risk patients, ABO compatibility decreases the risk of rejection. HLA compatibility could positively influence corneal graft survival in some cases.     

新生淋巴管与角膜移植排斥的研究进展

角膜新生淋巴管构成了角膜免疫反应的传入弧,在免疫反应中发挥了重要作用。由于新生淋巴管的出现破坏了免疫机制使角膜移植术后排斥反应发生率升高。随着淋巴管内皮标记物的相继出现和对淋巴管生成因子的研究深入,众多学者对角膜淋巴管在角膜免疫排斥反应机制、治疗和预防等方面进行了大量研究,通过抑制新生淋巴管提高植片存活率。     

加强我国角膜移植免疫研究

通过角膜移植更换混浊或病变的角膜,是帮助角膜盲患者恢复视力的有效措施,但植片排斥反应的发生是导致角膜移植手术远期失败的主要原因,如何减少因植片排斥引起的再次致盲是角膜病研究的重点。本文分析了我国角膜移植免疫研究存在的问题：（1）没有建立标准的抗排斥治疗方案;（2）缺乏安全、有效的抗排斥反应治疗药物;（3）忽视围手术期的并发症处理;（4）角膜移植免疫基础研究薄弱。针对上述问题产生的原因,提出了相应的处理措施和建议,以增强角膜病专科医生对角膜移植免疫临床和基础研究的重视,促进我国角膜盲的防治工作取得更大的发展。     

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.     

Immune Privilege of Corneal Allografts

Corneal transplantation has been performed successfully for over 100 years. Normally, HLA typing and systemic immunosuppressive drugs are not utilized, yet 90% of corneal allografts survive. In rodents, corneal allografts representing maximal histoincompatibility enjoy >50% survival even without immunosuppressive drugs. By contrast, other categories of transplants are invariably rejected in such donor/host combinations. The acceptance of corneal allografts compared to other categories of allografts is called immune privilege. The cornea expresses factors that contribute to immune privilege by preventing the induction and expression of immune responses to histocompatibility antigens on the corneal allograft. Among these are soluble and cell membrane molecules that block immune effector elements and also apoptosis of T lymphocytes. However, some conditions rob the corneal allograft of its immune privilege and promote rejection, which remains the leading cause of corneal allograft failure. Recent studies have examined new strategies for restoring immune privilege to such high-risk hosts.     

Pharmacologic strategies in the prevention and treatment of corneal transplant rejection

Corneal transplantation remains one of the most successful organ transplantation procedures in humans. The unique structure of the cornea, with its absence of blood vessels and corneal lymphatic, allows the survival of corneal allograft. Recent advances in sutures, storage media, microsurgical instrumentation, and new pharmacological strategies have greatly improved the success of corneal transplantation and the prevention of corneal allograft rejection. Our strategies in the management and prevention of corneal graft rejection can modify and improve the survival of corneal allografts. Preoperative evaluation, understanding the risk factors, and management of ocular surface disorders may greatly improve the survival of the corneal transplant. Early recognition of corneal allograft rejection and aggressive treatment may improve the survival of the corneal graft. Furthermore, patients who undergo corneal transplantation should be maintained under close ophthalmic surveillance and patients should be informed to report immediately whenever symptoms of corneal graft rejection occur. The mainstay of therapy is topical corticosteroids. In severe cases, periocular, intravenous, and oral corticosteroids therapy can be rendered. New therapeutic modalities such as cyclosporine, tacrolimus, daclizumab, mycophenolate mofetil, leflunomide, rapamycin, and others may prove to be of help in the prevention and treatment of corneal graft rejection. Early recognition of corneal graft rejection and prompt treatment are mandatory for the successful survival of the corneal allograft.     

Gene Therapy in Corneal Transplantation

Abstract

Corneal transplantation is the most commonly performed organ transplantation. Immune privilege of the cornea is widely recognized, partly because of the relatively favorable outcome of corneal grafts. The first-time recipient of corneal allografts in an avascular, low-risk setting can expect a 90% success rate without systemic immunosuppressive agents and histocompatibility matching. However, immunologic rejection remains the major cause of graft failure, particularly in patients with a high risk for rejection. Corticosteroids remain the first-line therapy for the prevention and treatment of immune rejection. However, current pharmacological measures are limited in their side-effect profiles, repeated application, lack of targeted response, and short duration of action. Experimental ocular gene therapy may thus present new horizons in immunomodulation. From efficient viral vectors to sustainable alternative splicing, we discuss the progress of gene therapy in promoting graft survival and postulate further avenues for gene-mediated prevention of allogeneic graft rejection.     

Corneal allograft rejection: risk factors, diagnosis, prevention, and treatment

Recent advances in corneal graft technology, including donor tissue retrieval, storage and surgical techniques, have greatly improved the clinical outcome of corneal grafts. Despite these advances, immune mediated corneal graft rejection remains the single most important cause of corneal graft failure. Several host factors have been identified as conferring a "high risk" status to the host. These include: more than two quadrant vascularisation, with associated lymphatics, which augment the afferent and efferent arc of the immune response; herpes simplex keratitis; uveitis; silicone oil keratopathy; previous failed (rejected) grafts; "hot eyes"; young recipient age; and multiple surgical procedures at the time of grafting. Large grafts, by virtue of being closer to the host limbus, with its complement of vessels and antigen-presenting Langerhans cells, also are more susceptible to rejection. The diagnosis of graft rejection is entirely clinical and in its early stages the clinical signs could be subtle. Graft rejection is largely mediated by the major histocompatibility antigens, minor antigens and perhaps blood group ABO antigens and some cornea-specific antigens. Just as rejection is mediated by active immune mediated events, the lack of rejection (tolerance) is also sustained by active immune regulatory mechanisms. The anterior chamber associated immune deviation (ACAID) and probably, conjunctiva associated lymphoid tissue (CALT) induced mucosal tolerance, besides others, play an important role. Although graft rejection can lead to graft failure, most rejections can be readily controlled if appropriate management is commenced at the proper time. Topical steroids are the mainstay of graft rejection management. In the high-risk situations however, systemic steroids, and other immunosuppressive drugs such as cyclosporin and tacrolimus (FK506) are of proven benefit, both for treatment and prevention of rejection.     

前房相关性免疫偏离诱导及预防角膜移植排斥反应实验研究

目的研究前房相关性免疫偏离（ＡＣＡＩＤ）诱导及预防角膜移植排斥反应的作用。方法采用甘氨酸法制备可溶性角膜分类抗原,分组行兔前房注射诱导ＡＣＡＩＤ,并观察各组部分耳膨胀试验和穿透角膜移植术（ＰＫＰ）术后排斥反应。结果角膜上皮、内皮、基质及全角膜可溶性抗原皆可诱导 ＡＣＡＩＤ,诱导质量浓度为 ２． ０ｍｇ／ｍｌ。成功率分别为９０％,１００％,８０％和１００％。同期角膜移植免疫排斥率降低,植片存活时间延长。高危观察组仍可诱导出ＡＣＡＩＤ,并可推迟排斥反应的发生。结论角膜可溶性抗原可以诱导产生ＡＣＡＩＤ,无严重并发症,对兔角膜移植排斥反应具有一定的预防和治疗作用。     

Graft failure IV. Immunologic mechanisms of corneal transplant rejection

Corneal transplantation is the oldest and the most common form of solid tissue transplantation in humans. Immunologic graft rejection is one of the main causes of short and long-term graft failure. Rejection involves donor tissue recognition and destruction by allo-specific immune cells of the recipient. This review outlines (1) the immunobiology of transplantation, with reference to ocular immune privilege, (2) factors that confer “high-risk” status to a graft and (3) the pathophysiologic mechanisms of corneal transplant rejection.     

Angiogenesis and lymphangiogenesis in corneal transplantation–A review

Corneal transplantation has been proven effective for returning the gift of sight to those affected by corneal disorders such as opacity, injury, and infections that are a leading cause of blindness. Immune privilege plays an important role in the success of corneal transplantation procedures; however, immune rejection reactions do occur, and they, in conjunction with a shortage of corneal donor tissue, continue to pose major challenges. Corneal immune privilege is important to the success of corneal transplantation and closely related to the avascular nature of the cornea. Corneal avascularity may be disrupted by the processes of angiogenesis and lymphangiogenesis, and for this reason, these phenomena have been a focus of research in recent years. Through this research, therapies addressing certain rejection reactions related to angiogenesis have been developed and implemented. Corneal donor tissue shortages also have been addressed by the development of new materials to replace the human donor cornea. These advancements, along with other improvements in the corneal transplantation procedure, have contributed to an improved success rate for corneal transplantation. We summarize recent developments and improvements in corneal transplantation, including the current understanding of angiogenesis mechanisms, the anti-angiogenic and anti-lymphangiogenic factors identified to date, and the new materials being used. Additionally, we discuss future directions for research in corneal transplantation.     

Effect of local macrophage depletion on cellular immunity and tolerance evoked by corneal allografts

Corneal graft rejection can be prevented by local macrophage depletion, via subconjunctival injections with clodronate liposomes. To unravel the underlying immunological mechanism responsible for prolonged graft survival in this circumstance, the effect of this regimen on induction of donor-specific delayed type hypersensitivity (DTH) and anterior chamber associated immune deviation (ACAID) was determined. The study showed that although subconjunctival clodronate liposome-treatment failed to alter systemically induced DTH and ACAID, both types of immune response were absent in clodronate liposome-treated rats after corneal transplantation. Thus, elimination of macrophages from the corneal transplant site renders corneal allografts immunologically "invisible" to the recipient. PURPOSE: To determine whether local macrophage depletion, via administration of clodronate liposomes, alters delayed type hypersensitivity (DTH) responses and induction of anterior chamber associated immune deviation (ACAID) after corneal allotransplantation. METHODS: Clodronate liposome-treated and untreated rats received orthotopic corneal allografts and were tested for DTH responses and induction of ACAID towards donor antigens. Also in subconjunctivally treated and untreated rats, DTH responses were measured after subcutaneous immunization or induction of ACAID with allogeneic spleen cells. RESULTS: Subconjunctival injected clodronate liposomes prevented grafted rats from developing donor-specific DTH as well as ACAID. By contrast, subconjunctivally injected clodronate liposomes had no effect on donor-specific DTH responses after systemic immunization or on the induction of ACAID with allogeneic cells. CONCLUSIONS: Depletion of macrophages at the time of corneal allografting seems to render the grafts immunologically invisible to the recipients. This could explain why these grafts survive "indefinitely" without any other form of therapy.     

CD86在排斥反应和前房相关免疫偏离过程中的作用

目的检测大鼠角膜共刺激分子CD86（B7—2）的原位表达,探讨CD86分子与角膜移植排斥反应和前房相关免疫偏离（ACAID）过程的关系。方法制作穿透角膜移植排斥反应和同一供体前房内注射脾细胞诱导ACAID的大鼠模型;角膜移植组进行排斥反应指数（RI）评分;ACAID组进行迟发型超敏反应（DTH）评价;采用免疫组织化学的方法检测CD86在角膜中的原位表达（以脾脏的表达为阳性对照）。结果角膜移植组植片均出现不同程度的新生血管、角膜水肿、混浊、增厚;ACAID组角膜透明,房水清,DTH评价术后2周及4周诱导成功率100％;免疫组织化学检测CD86在正常大鼠角膜组织全层无阳性表达,在移植后出现急性排斥反应的大鼠角膜上皮层中有大量阳性细胞表达,在ACAID诱导成功的大鼠角膜中未见阳性细胞表达。结论共刺激分子CD86参与移植后的急性排斥反应,但可能不参与免疫赦免过程。     

同种异体角膜移植后角膜内皮细胞的特征表现及生物学意义

同种异体角膜移植目前仍是治疗角膜疾病最为有效的方法,但术后发生的免疫排斥所引起的角膜炎症反应和血管化,最终导致植片水肿,甚至功能丧失仍是造成手术失败的最主要原因.角膜内皮细胞作为免疫赦免及角膜正常生理功能维系的首要屏障,在眼部系统中占据着至关重要的位置.本文就同种异体角膜移植术后免疫的识别,应答和最终反应的各阶段角膜内皮所呈现的特征改变及生物学意义进行阐述.

Abstract：

Allograft corneal transplantation is currently the most effective and optimum method to treat corneal disease, however it is the immune rejection that the main reason for graft edema caused by vascularization and inflammation of the cornea and surgical failure after transplantation. Corneal endothelial cells occupy a crucial position as immune privilege and the primary barriers for maintaining normal physiological function of eye. In this paper, the various stages (recognition, response and reaction of immune system) of corneal endothelial changes in the morphological and biological characteristics had been presented after allograft corneal transplantation.     

ICOS共刺激通路参与角膜移植免疫排斥反应的研究

目的研究可诱导共刺激分子（ICOS）共刺激通路与角膜移植急性免疫排斥反应的关系。方法建立大鼠同种异体穿透角膜移植模型,分别于术后7d、14d取植片进行病理学观察,采用RT-PCR法检测植片组织ICOS mRNA的表达情况,免疫组织化学法检测植片组织淋巴细胞ICOS蛋白水平;同时采用流式细胞术检测外周血CD3^＋ICOS^＋T／CD3^＋T的表达情况。均以正常大鼠作为正常对照。结果正常大鼠角膜组织未检测到ICOS蛋白及ICOS mRNA的表达,移植术后植片组织可以检测到ICOS蛋白及ICOS mRNA的表达,且术后14d高于术后7d（P=0．000）;与正常大鼠外周血CD3^＋ICOS^＋T／CD3^＋T的表达相比术后表达皆升高（方差齐性,P=0．156）,且术后14d外周血CD3^＋ICOS^＋T／CD3^＋T的表达高于术后7d的表达（P=0．000）。结论共刺激分子ICOS与角膜移植急性免疫排斥反应密切相关。     

角膜移植免疫排斥反应防治的研究进展

角膜移植是众多器官和组织移植中成功率最高的,然而移植后的免疫排斥反应仍是导致角膜移植术失败的主要原因。本文综述角膜移植后免疫排斥反应发生的机制、排斥反应的预防及其治疗等几个方面的研究进展。     

The effect of tissue dose and site of grafting on immunity to corneal allografts

The purpose of this investigation was to determine the quantitative relationship between corneal alloantigens and host immunity. In addition, the effect of the site of introduction of the corneal antigens on the host response was determined. Two alloantigenic strains of rats were reciprocally grafted at three different locations in the body with carefully quantitated amounts of corneal tissue: (1) orthotopically in the cornea of the eye; (2) subcutaneously; and (3) intraperitoneally. Corneal tissue placed orthotopically into vascularized graft beds did not elicit a systemic immune response. Subcutaneous grafts elicited a weak systemic alloantigenic response, whereas corneal tissue placed in the peritoneal cavity consistently induced a vigorous cellular and humoral alloantigenic response. Eight or more full thickness corneal allografts grafted subcutaneously were required to elicit a systemic response. On the other hand, as few as four corneal allografts placed intraperitoneally invariably elicited a systemic alloimmune response. The results of this investigation demonstrate that both the amount and route of introduction of alloantigen affect the recipient's response to corneal tissue and that the rejection of a single orthotopic cornea graft is a site-limited response. Immune effector cells were not found in the spleens and alloantibodies were not present in the serum of animals that had rejected corneal allografts.     

重视角膜移植术后免疫排斥反应的防治

角膜移植术是角膜病盲人复明的最主要手段,但术后的免疫排斥反应仍是手术失败的最主要原因.随着我国医疗水平的不断提高,开展角膜移植手术的医院也不断增加.而如何减少术后的并发症和提高植片的长期透明率,是摆在众多从事角膜移植医师面前的一个重要问题.保持角膜移植术后植片长期透明的核心问题是重视术后随诊和免疫排斥反应的防治.本文主要从我国角膜移植的现状、存在的问题及如何减少免疫排斥反应的措施三个主要方面进行分析,并对目前角膜移植术后存在的一些普遍问题提出相应的处理措施和建议,以达到减少角膜移植术后由免疫排斥反应导致植片混浊的再次致盲问题.