The differential effects of donor versus host Langerhans cells in the rejection of MHC-matched corneal allografts

Role of Immune Cell Diversity and Heterogeneity in Corneal Graft Survival: A Systematic Review and Meta-Analysis

Corneal transplantation is one of the most successful forms of solid organ transplantation; however, immune rejection is still a major cause of corneal graft failure. Both innate and adaptive immunity play a significant role in allograft tolerance. Therefore, immune cells, cytokines, and signal-transduction pathways are critical therapeutic targets. In this analysis, we aimed to review the current literature on various immunotherapeutic approaches for corneal-allograft rejection using the PubMed, EMBASE, Web of Science, Cochrane, and China National Knowledge Infrastructure. Retrievable data for meta-analysis were screened and assessed. The review, which evaluated multiple immunotherapeutic approaches to prevent corneal allograft rejection, showed extensive involvement of innate and adaptive immunity components. Understanding the contribution of this immune diversity to the ocular surface is critical for ensuring corneal allograft survival.

Animal models of high-risk corneal transplantation: A comprehensive review

Over the past century, corneal transplantation has become the most commonly performed allogeneic solid tissue transplantation. Although more than 80% of the corneal transplantations have favorable outcomes, immune-mediated rejection continues to be the major cause of failure in well over 50% of graft recipients that have inflamed and vascularized host beds. Over the past two decades, the progress in our understanding of the immunological pathways that mediate graft rejection has aided in the development of novel therapeutic strategies. In order to successfully test the efficacy of these interventions, it is essential to model the immunological processes occurring as a consequence of corneal transplantation. Herein, we have comprehensively reviewed the established animal models used for replicating the immunopathological processes causing graft rejection in high-risk corneal transplantation settings. We have also discussed the practical and technical differences, as well as biological and immunological variations in different animal models.

Ocular Immune Privilege and Transplantation

Allografts are afforded a level of protection from rejection within immune-privileged tissues. Immune-privileged tissues involve mechanisms that suppress inflammation and promote immune tolerance. There are anatomical features, soluble factors, membrane-associated proteins, and alternative antigen-presenting cells (APC) that contribute to allograft survival in the immune-privileged tissue. This review presents the current understanding of how the mechanism of ocular immune privilege promotes tolerogenic activity by APC, and T cells in response to the placement of foreign antigen within the ocular microenvironment. Discussed will be the unique anatomical, cellular, and molecular mechanisms that lessen the chance for graft destroying immune responses within the eye. As more is understood about the molecular mechanisms of ocular immune privilege greater is the potential for using these molecular mechanisms in therapies to prevent allograft rejection.

Matching for Human Leukocyte Antigens (HLA) in corneal transplantation - to do or not to do

As many patients with severe corneal disease are not even considered as candidates for a human graft due to their high risk of rejection, it is essential to find ways to reduce the chance of rejection. One of the options is proper matching of the cornea donor and recipient for the Human Leukocyte Antigens (HLA), a subject of much debate. Currently, patients receiving their first corneal allograft are hardly ever matched for HLA and even patients undergoing a regraft usually do not receive an HLA-matched graft. While anterior and posterior lamellar grafts are not immune to rejection, they are usually performed in low risk, non-vascularized cases. These are the cases in which the immune privilege due to the avascular status and active immune inhibition is still intact. Once broken due to infection, sensitization or trauma, rejection will occur. There is enough data to show that when proper DNA-based typing techniques are being used, even low risk perforating corneal transplantations benefit from matching for HLA Class I, and high risk cases from HLA Class I and probably Class II matching. Combining HLA class I and class II matching, or using the HLAMatchmaker could further improve the effect of HLA matching. However, new techniques could be applied to reduce the chance of rejection. Options are the local or systemic use of biologics, or gene therapy, aiming at preventing or suppressing immune responses. The goal of all these approaches should be to prevent a first rejection, as secondary grafts are usually at higher risk of complications including rejections than first grafts.

Immunology of Corneal Allografts: Insights from Animal Models

Corneal transplantation stands alone as the most common and successful form of solid organ transplantation. Even though HLA matching and systemic antirejection drugs are not routinely used, 90% of the first time corneal allografts will succeed. By contrast, all other major categories of organ transplantation require HLA matching and the use of systemically administered immunosuppressive drugs. This remarkable success of corneal transplants under these conditions is an example of "immune privilege" and is the primary reason for the extraordinary success of corneal transplantation. A number of dogmas have emerged over the past century to explain immune privilege and the immunobiology of corneal transplantation. Many of these dogmas have been based largely on inferences from clinical observations on keratoplasty patients. The past 30 years have witnessed a wealth of rodent studies on corneal transplantation that have tested hypotheses and dogmas that originated from clinical observations on penetrating keratoplasty patients. Rodent models allow the application of highly sophisticated genetic and immunological tools for testing these hypotheses in a controlled environment and with experiments designed prospectively. These studies have validated some of the widely held assumptions based on clinical observations and in other cases, previous dogmas have been replaced with new insights that could only come from prospective studies performed under highly controlled conditions. This review highlights some of the key dogmas and these widely held assumptions that have been scrutinized through the use of rodent models of penetrating keratoplasty. This review also makes note of new immunological principles of corneal immunology that have emerged from rodent studies on corneal transplantation that most likely would not have been revealed in studies on corneal transplantation patients.

Cornea: Window to Ocular Immunology

The ocular surface is continuously exposed to environmental agents such as allergens, pollutants, and microorganisms, which could provoke inflammation. However, an array of anatomical, physiological, and immunological features of the ocular surface conspire to limit corneal inflammation and endow the eye with immune privilege. A remarkable example of ocular immune privilege is the success of corneal allografts, which unlike all other forms of organ transplantation, survive without the use of systemic immunosuppressive drugs or MHC matching. This review describes the anatomical, physiological, and dynamic immunoregulatory processes that contribute to immune privilege.

[Corneal allograft rejection after Descemet stripping automated endothelial keratoplasty (DSAEK): report of three cases]

INTRODUCTION

Descemet stripping automated endothelial keratoplasty (DSAEK) is an exciting alternative to standard full-thickness penetrating keratoplasty (PK) for the treatment of patients suffering from endothelial failure of the cornea. Corneal transplant rejection is the leading cause of endothelial failure after PK. While DSAEK may reduce the risk of corneal allograft rejection, endothelial rejection can still occur.

CASE REPORTS

We report three cases of endothelial rejection after DSAEK. Subacute endothelial rejection occurred in all three cases after the topical steroid treatment regimen had been stopped.

CONCLUSION

A lower rejection rate in comparison with PK cannot be assumed and comparative studies are necessary. Extended topical steroid may be used. Subacute endothelial graft rejection implies that the patient needs to be fully informed of the functional signs and the need to seek medical advice immediately.

Function of indoleamine 2,3-dioxygenase in corneal allograft rejection and prolongation of allograft survival by over-expression

Indoleamine 2,3-dioxygenase (IDO) suppresses T cell responses by its action in catabolising tryptophan. It is important in maintenance of immune privilege in the placenta. We investigated the activity of IDO in the cornea, following corneal transplantation and the effect of IDO over-expression in donor corneal endothelium on the survival of corneal allografts. IDO expression was analysed and functional activity was quantified in normal murine cornea and in corneas following transplantation as allografts. Low levels of IDO, at both mRNA and protein levels, was detected in the normal cornea, up-regulated by IFN-gamma and TNF. Expression of IDO in cornea was significantly increased following corneal transplantation. However, inhibition of IDO activity in vivo had no effect on graft survival. Following IDO cDNA transfer, murine corneal endothelial cells expressed functional IDO, which was effective at inhibiting allogeneic T cell proliferation. Over-expression of IDO in donor corneal allografts resulted in prolonged graft survival. While, on one hand, our data indicate that IDO may augment corneal immune privilege, up-regulated IDO activity following cytokine stimulation may serve to inhibit inflammatory cellular responses. While increasing IDO mRNA expression was found in allogeneic corneas at rejection, over-expression in donor cornea was found to significantly extend survival of allografts.

Corneal transplantation: the forgotten graft

The most commonly performed transplant is that of the cornea, with 2292 corneal grafts performed in the UK in 2002-03, compared with 1775 renal transplants [1]. In the USA approximately 40 000 transplants are performed every year [2]. However this preponderance is not reflected in the amount of attention given to this transplanted tissue by the scientific community: for example up till now there have been no papers published in the American Journal of Transplantation that have cornea as a key or title word (as determined by a Medline search in December 2003). There are several reasons for this. The first is that corneal grafting is the province of ophthalmologists, who (with notable exceptions) are isolated from the transplant community. The second is that there is a widespread belief that, because of the existence of immune privilege, corneal grafts are not rejected and so there is no need for further research. As we will discuss later, this is incorrect. In this article we will seek to show that study of corneal transplantation is important in its own right, and also that it has lessons for those interested in other forms of allograft.

The immune privilege of corneal grafts

Keratoplasty is the oldest and one of the most successful forms of solid tissue transplantation. In the United States, over 33,000 corneal transplants are performed each year. Unlike other forms of tissue transplantation, keratoplasties are routinely performed without the aid of tissue typing or systemic immunosuppressive drugs. In spite of this, 90% of the first-time corneal transplants will succeed-a condition that demonstrates the immune privilege of keratoplasties. The avascular nature of the corneal allograft bed led many to suspect that corneal grafts were sequestered from the immune apparatus. Although pleasing in its simplicity, this explanation has given way to a more comprehensive hypothesis that embodies multiple, interdependent mechanisms, which promote the long-term survival of corneal allografts. These mechanisms conspire to interrupt the transmission of immunogenic stimuli to peripheral lymphoid tissues; induce the generation of a deviated immune response; and neutralize immune effector elements at the host-graft interface. This paradigm is analogous to a three-legged stool. Disassembly of any one of the three components results in the collapse of immune privilege. Strategies to re-establish corneal immune privilege may have clinical application for high-risk hosts who have rejected previous corneal allografts.

Recent developments in the pharmacological treatment and prevention of corneal graft rejection

At present, given the high initial success rate of corneal transplantation (although late survival is poor), immunosuppression is often reserved for 'high-risk' patients. Despite immune privilege, corneal graft rejection remains the leading cause of corneal allograft failure. Interpreting the limited and also restricted design of most trials, immunosuppressive therapy has not enjoyed the success seen in solid organ grafts. This review discusses the limited data available whilst proposing newer therapies that have developed as a result of our increased understanding of the immunobiology of corneal graft rejection.

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.

Twenty-five-year panorama of corneal immunology: emerging concepts in the immunopathogenesis of microbial keratitis, peripheral ulcerative keratitis, and corneal transplant rejection

PURPOSE

To describe the most recent advances in our understanding of the cellular and molecular mechanisms involved in the immunopathogenesis of corneal immunoinflammatory disorders including microbial keratitis, peripheral ulcerative keratitis. and allograft rejection.

METHODS

Review of the published peer-reviewed literature that has contributed significantly to our modern understanding of corneal immunology. In addition, the authors have summarized the information in conceptual diagrams that highlight the critical cellular and molecular pathways that lead to corneal immune responses in the two most thoroughly studied corneal immune disorders, herpes simplex keratitis (HSK) and transplant rejection.

RESULTS

In spite of the wide array of molecular and cellular factors that mediate corneal immunity, critical mechanistic facets are shared by the various corneal immunoinflammatory disorders. These include activation and migration of local antigen-presenting cells (APCs), including Langerhans cells (LCs), upregulation in pleiotropic proinflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor-alfa (TNF-alpha) that can mediate a wide array of immune functions in addition to up-regulating protease expression. and chemokines that play a critical role on the one hand in attracting nonantigen-specific inflammatory cells such as neutrophils and on the other in attracting CD4+ T helper type 1 (Th1) cells that mediate most of the destruction in the cornea.

CONCLUSIONS

In the last 25 years, we have seen our field develop from a descriptive stage into a new phase where the fundamental processes that mediate and effect corneal immunity are being accurately deciphered. It is anticipated that this new knowledge will allow development of specific molecular and genetic therapeutic strategies that could target critical steps in the immunopathogenesis of disease without the untoward side-effects of nonspecific generalized immune suppression that still remains the standard of care today.

The immune privilege of corneal allografts

BACKGROUND

Corneal transplantation is the oldest, most common, and arguably, the most successful form of tissue transplantation. In the United States alone, over 40,000 corneal transplantations are performed each year. Less than 10% of the uncomplicated, first-time corneal grafts will undergo immune rejection even though HLA matching is not routinely performed and the use of immunosuppressive drugs is limited to the topical application of corticosteroids. The success of corneal transplantations predates the use of corticosteroids and further emphasizes the remarkable privilege of corneal allografts.

METHODS

Several laboratories have used rat and mouse models of orthotopic corneal transplantation (keratoplasty) in an attempt to understand the basis for the immune privilege of corneal allografts.

RESULTS

The time-honored explanation for the immune privilege of corneal allografts was based on the conspicuous avascularity of the cornea, which was believed to sequester the graft from the immune apparatus. However, results from several laboratories indicate that at least three additional features of the corneal graft contribute to its immune privileged status: (a) absence of donor-derived, antigen-presenting passenger Langerhans cells in the corneal graft; (b) expression of Fas ligand on the epithelium and endothelium of the corneal allograft; and (c) capacity of the corneal allograft to induce immune deviation of the systemic immune response.

CONCLUSIONS

The immune privilege of corneal allografts is a product of at least three unique qualities of the corneal allograft that conspire to interfere with the induction and expression of allodestructive immune responses.

The effect of oral immunization on corneal allograft survival

The present study examined the potential of orally induced tolerance for preventing immunological rejection of corneal allografts. Orthotopic corneal allografts were transplanted from either C3H (MHC + multiple minor H-mismatched) or NZB (multiple minor H-mismatched only) donors to CB6F1 recipients on day 0. Tissue cultured corneal epithelial and endothelial cells from relevant donor strains were administered orally from day -14 to day -4 on a daily basis, The incidence of graft rejection, graft mean survival time (MST), and alloimmune responses, and the antigen specificity of induced tolerance were studied. Oral immunization induced a remarkable tolerance such that only 55% of the orally immunized hosts rejected their fully allogeneic corneal grafts (MST = 43 days) compared with 100% rejection (MST = 18 days) in normal controls. Likewise, rejection of MHC-matched, multiple minor H-mismatched corneal grafts fell from 80% in untreated controls to 36% in orally immunized hosts. Oral immunization was effective in desensitizing previously immunized hosts. Rejection of MHC-matched, multiple H minor-mismatched corneal allografts fell from 93% in preimmune, unfed hosts to 36% in preimmune, orally tolerized mice. Thus, oral immunization is a safe and effective method for desensitizing high-risk, preimmune hosts and promoting corneal allograft survival.

FK-506 delays corneal graft rejection in a model of corneal xenotransplantation

FK-506 is a relatively new immunosuppressant similar in action to cyclosporine A, but is much more potent. Its primary action is against T lymphocytes, the major cellular component in corneal allograft rejection. The purpose of this study was the evaluation of the ability of topical and systemic FK-506 in preventing corneal xenograft rejection in an experimental animal model. Cross-species xenotransplants were used as the most vigorous stimulus to induce corneal rejection. Corneas derived from Hartley guinea pigs were transplanted into the left eyes of 32 male Lewis rats. Topical treatment was administered by using FK-506 0.3 mg/ml in a cyclodextrin suspension or vehicle (cyclodextrin suspension) four times per day. For systemic treatment, 0.5 mg/kg/day of FK-506 or vehicle (saline) was administered intraperitoneally. Treatments were started 60 minutes after surgery and continued for 21 days. The grafts underwent a double-masked examination, and a score was given for clarity, edema, and vascularization. The animals were sacrificed 21 days after transplantation. The control groups had allograft rejection after 6.75 +/- 0.31 (topical vehicle) and after 7.37 +/- 0.32 (systemic vehicle) days. The FK-506-treated groups showed allograft rejection after 14 +/- 0.88 (topical FK-506) or after 16.25 +/- 1.23 (systemic FK-506) days. In addition, FK-506-treated rats manifested less corneal neovascularization than control animals. We conclude that systemic or topical FK-506 is effective in prolonging xenograft survival in the rat keratoplasty model.

Effect of cytokine-induced migration of Langerhans cells on corneal allograft survival

The unique paucity of Ia+ Langerhans cells (LCs) in the central cornea contributes to the immunological privilege of corneal allografts. A variety of stimuli can induce the centripetal migration of peripheral LCs. At least one of these stimuli (i.e. latex bead instillation) induces interleukin-1 (IL-1) secretion by corneal cells which acts as a potent chemoattractant for LCs. Within 30 minutes of intracorneal injection of IL-1, centripetal migration of LCs can be detected. The presence of donor-derived LCs in corneal allografts doubles the incidence of rejection of fully allogeneic corneal allografts as well as MHC matched, multiple minor H mismatched corneal allografts. Although the presence of donor-specific LCs greatly jeopardises corneal allograft survival, migration of host-derived LCs into corneal allografts does not appear to increase the risk of rejection.

Antigen presentation of herpes simplex virus by corneal epithelium--an in vitro and in vivo study

Ia antigen (class II antigen) is a histocompatibility antigen that foreign peptides associate with, before antigen presentation to T cells and subsequent triggering of the CD4 T cells. Although corneal epithelium is normally Ia negative it may become Ia positive under abnormal circumstances but the functional significance of this is uncertain. In this study the expression of Ia antigen on corneal epithelium of mice during in vivo primary and secondary herpes simplex keratitis and the in vitro accessory function of corneal epithelium in the presentation of herpes simplex virus (HSV) antigen to in vivo HSV primed T cells were evaluated. Whole mount preparations of corneal epithelium were found to express Ia antigen on days 3, 5, and 7 following corneal inoculation with live HSV. The intensity of the Ia expression was greater in non-immune mice on day 7 after corneal inoculation compared with immune mice. A cellular suspension of corneal epithelium induced HSV primed T cells to proliferate in the presence of HSV antigen. Induction of Ia antigen on corneal epithelium during herpes simplex keratitis may functionally expand the population of antigen presenting cells in the cornea and contribute to T cell activation.

Effect of donor Langerhans cells on corneal graft rejection

Unlike other cutaneous surfaces, the central portion of the corneal epithelium is typically devoid of Langerhans cells. The absence of Ia+ Langerhans cells in the central cornea is of more than casual interest and may explain the immunologic privilege that is characteristic of corneal allografts. The present communication summarizes previous studies that examined the role of corneal Langerhans cells in eliciting alloimmune responses and corneal graft rejection in rodents. Under normal circumstances, corneal allografts are poorly immunogenic when residing in the avascular ocular graft bed even though the graft displays large quantities of alloantigens. The afferent blockade of the immune response can be circumvented by donor-derived Langerhans cells that serve as potent immunogens for all categories of corneal allografts except grafts involving allodisparity only at class I major histocompatibility complex loci. Thus, the presence of donor-derived Langerhans cells exerts profound effects on the fate of corneal allografts.