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

Immunopathology of Corneal Allograft Rejection and Donor-Specific Antibodies (DSAs) as Immunological Predictors of Corneal Transplant Failure

Despite tremendous developments in the field of laboratory testing in transplantation, the rules of eligibility for corneal transplantation still do not include typing of human leukocyte antigens (HLAs) in the donor and recipient or detection of donor-specific antibodies (DSAs) in the patient. The standard use of diagnostic algorithms is due to the cornea belonging to immunologically privileged tissues, which usually determines the success of transplantation of this tissue. A medical problem is posed by patients at high risk of transplant rejection, in whom the immune privilege of the eye is abolished and the risk of transplant failure increases. Critical to the success of transplantation in patients at high risk of corneal rejection may be the selection of an HLA-matched donor and recipient, and the detection of existing and/or de novo emerging DSAs in the patient. Incorporating the assessment of these parameters into routine diagnostics may contribute to establishing immune risk stratification for transplant rejection and effective personalized therapy for patients.

Splenectomy as a Risk Factor for Graft Rejection Following Endothelial Transplantation: Retrospective Study

BACKGROUND

Anterior chamber-associated immune deviation (ACAID) is an active immunotolerance mechanism, which is induced by placing antigen into the anterior eye chamber as long as a major surgical trauma is avoided. For this reason, ACAID may be a major contributor to the favorable immunologic outcomes in Descemet membrane endothelial keratoplasty (DMEK). Rodent models have demonstrated the importance of a functional spleen for the development of an ACAID.

OBJECTIVE

This study aimed to investigate whether splenectomy leads to increased rejection rates after DMEK in humans.

METHODS

A retrospective evaluation was conducted on the course following DMEK at the Eye Center, Medical Center, University of Freiburg, for patients with a self-reported history of splenectomy compared to patients without this condition. Potential study patients were contacted by mail. A questionnaire to self-report splenectomy and the time thereof was sent out. The medical records of all consenting patients at the Eye Center were reviewed for graft survival and immune reactions.

RESULTS

We asked 1818 patients after DMEK to report their history of splenectomy. A total of 1340 patients responded and were included in the study. Of these 1340 patients, 16 (1.2%) reported a history of splenectomy (ie, 26 DMEKs, with 10 patients being transplanted in both eyes and 6 patients being transplanted in 1 eye; median age at surgery 73.7, range 66.7-76.1 y). The remaining patients (1324 patients, ie, 1941 eyes) served as controls, with 1941 DMEKs (median age at surgery 71.5, range 64.1-77.2 y). Five (19%) out of the 26 eyes from the splenectomy group required a second transplant due to dislocation (n=2.8%), failure (n=2.8%), and rejection (n=1.4%). Kaplan-Meier analysis revealed no relevant difference compared with controls.

CONCLUSIONS

Our results suggest that splenectomy has no major effect on the outcome following DMEK. Subsequent, ACAID may not be the main reason for the favorable immunological outcomes in DMEK, or the camero-splenic axis may be subordinate in humans. However, we only included 16 patients who underwent splenectomy, so it might be possible that we missed a minor effect.

The influence of HLA genetic variation on plasma protein expression

Genetic variation in the human leukocyte antigen (HLA) loci is associated with risk of immune-mediated diseases, but the molecular effects of HLA polymorphism are unclear. Here we examined the effects of HLA genetic variation on the expression of 2940 plasma proteins across 45,330 Europeans in the UK Biobank, with replication analyses across multiple ancestry groups. We detected 504 proteins affected by HLA variants (HLA-pQTL), including widespread trans effects by autoimmune disease risk alleles. More than 80% of the HLA-pQTL fine-mapped to amino acid positions in the peptide binding groove. HLA-I and II affected proteins expressed in similar cell types but in different pathways of both adaptive and innate immunity. Finally, we investigated potential HLA-pQTL effects on disease by integrating HLA-pQTL with fine-mapped HLA-disease signals in the UK Biobank. Our data reveal the diverse effects of HLA genetic variation and aid the interpretation of associations between HLA alleles and immune-mediated diseases.

Serum Extracellular Vesicle Protein Profiling for Prediction of Corneal Transplant Rejection

BACKGROUND

Corneal transplantation is the most common transplant procedure worldwide. Despite immune and angiogenic privilege of the cornea, 50% to 70% of corneal transplants fail in high-risk recipients, primarily because of immune rejection. Therefore, it is crucial to identify predictive biomarkers of rejection to improve transplant survival.

METHODS

In search for predictive biomarkers, we performed proteomics analysis of serum extracellular vesicles (EVs) in a fully major histocompatibility complex-mismatched (C57BL/6-to-BALB/c) murine corneal transplantation model, wherein 50% of transplants undergo rejection by day 28 following transplantation.

RESULTS

Our time course study revealed a decrease in the number of serum EVs on day 1, followed by a gradual increase by day 7. A comparative analysis of proteomics profiles of EVs from transplant recipients with rejection (rejectors) and without rejection (nonrejectors) found a distinct enrichment of histocompatibility 2, Q region locus 2, which is a part of major histocompatibility complex-class I of donor C57BL/6 mice, in day 7 EVs of rejectors, compared with nonrejectors, syngeneic controls, or naïve mice. In contrast, serum amyloid A2, a protein induced in response to injury, was increased in day 7 EVs of nonrejectors.

CONCLUSIONS

Our findings offer noninvasive EV-based potential biomarkers for predicting corneal allograft rejection or tolerance.

[Keratoplasty - an Overview of the Methods and their Perioperative Management]

Corneal transplantation can be divided into two groups: penetrating and lamellar keratoplasty. Newer minimally invasive procedures have emerged over the years, to improve the visual outcome and reduce complications. This article summarizes the different procedures, their indications and complications, and outlines the pre-, peri- and postoperative management in a clinical setting.Corneal transplantation is the most commonly performed transplantation of donor tissue in modern medicine. In the last years a shift away from penetrating keratoplasty (PK) towards minimally invasive lamellar operative techniques, associated with less complications, can be observed. The Descemet membrane endothelial keratoplasty (DMEK) is used to treat endothelial corneal pathologies and has overtaken the PK to become the most commonly performed form of keratoplasty. Preparation and identification of possible risk-factors are essential preoperative steps to reduce peri- and postoperative complications of keratoplasties. If corneal graft rejection occurs, early and maximum therapy is crucial for graft survival. Laser-assisted techniques offer different advantages in lamellar and penetrating keratoplasty but are not very cost-efficient.

Impact of culture media on primary human corneal endothelial cells derived from old donors

Corneal endothelial dysfunction is a major indication for corneal transplantation. However, a global shortage of donor corneal tissues and risks associated with corneal surgeries have prompted exploration of alternative options, including tissue-engineered grafts or cell injection therapy. Nonetheless, these approaches require a controlled culture of primary human corneal endothelial cells (HCEnCs). Although HCEnCs established from young donors are generally more proliferative and maintain a better phenotype, corneas from old donors are more frequently accessible from eye banks due to a lower corneal endothelial cell count than the necessary threshold required for transplantation. In this study, we investigated various culture media to evaluate which one is the most appropriate for stimulating the proliferation while maintaining cell morphology and function of HCEnCs derived from old donors (age >65 years). All experiments were performed on paired research-grade donor corneas, divided for the conditions under investigation in order to minimize the inter-donor variability. Cell morphology as well as expression of specific markers were assessed at both mRNA (CD166, SLC4A11, ATP1A1, COL8A1, α-SMA, CD44, COL1A1, CDKN2A, LAP2A and LAP2B) and protein (ZO-1, α-SMA, Ki67 and LAP2) levels. Results obtained showed how the Dual Media formulation maintained the hexagonal phenotype more efficiently than Single Medium, but cell size gradually increased with passages. In contrast, the Single Medium provided a higher proliferation rate and a prolonged in vitro expansion but acquired an elongated morphology. To summarize, Single medium and Dual media preserve morphology and functional phenotype of HCEnCs from old donor corneas at low passages while maintenance of the same cell features at high passages remains an active area of research. The new insights revealed within this work become particularly relevant considering that the elderly population a) is the main target of corneal endothelial therapy, b) represents the majority of corneal donors. Therefore, the proper expansion of HCEnCs from old donors is essential to develop novel personalised therapeutic strategies and reduce requirement of human corneal tissues globally.

Management and prevention of corneal graft rejection

The management of an episode of corneal graft rejection (CGR) is primarily by corticosteroids. Immunomodulators are useful for long-term immunosuppression and in dealing with cases of high-risk (HR) corneal grafts. The classical signs of CGR following penetrating keratoplasty (PKP) include rejection line, anterior chamber (AC) reaction, and graft edema. However, these signs may be absent or subtle in cases of endothelial keratoplasty (EK). Prevention of an episode of graft rejection is of utmost importance as it can reduce the need for donor cornea significantly. In our previous article (IJO_2866_22), we had discussed about the immunopathogenesis of CGR. In this review article, we aim to discuss the various clinical aspects and management of CGR.

Risk factors of rejection after penetrating keratoplasty: a retrospective monocentric study

PURPOSE

To assess risk factors of rejection after penetrating keratoplasty (PKP).

METHODS

This retrospective monocentric study assessed risk factors for rejection in patients who underwent PKP at Montpellier University Hospital between June 2005 and September 2018. Graft and donor data were obtained from our tissue bank in Montpellier. Clinical data of recipients were recorded from medical files. Survival was estimated by the Kaplan-Meir method. Potential risk factors of rejection were assessed by multivariate Cox proportional hazards analysis, estimating hazard ratios (HR) and 95% confidence intervals (CI).

RESULTS

Among the 316 consecutive patients (59% male, mean SD] age 52 [17]), 360 eyes underwent PKP. Indications for PKP were bullous keratopathy (27%), infectious keratitis (20%), and keratoconus (15%). The median follow-up was 44 months (IQR 22-73). The overall graft survival and irreversible rejection rate at 5 years were 70% and 29%, respectively. Factors associated with risk of rejection were prior indication for graft rejection (SHR [CI 95%] = 7.8 [2.6-23.1]), trauma (SHR [CI 95%] = 3.6 [1.1-11.7]), and infectious keratitis (SHR [CI 95%] = 2.7 [1.2-11.1]), history of corneal neovascularization (SHR [CI 95%] = 2.1 [1.2-3.8]), hypertonia (SHR [CI 95%] = 2.8 [1.8-4.3]), and mixed sex matching (SHR [CI 95%] = 2.0 [1.01-4.0]).

CONCLUSION

The significant risk factors of graft rejection after PKP found in this study agree with those from major international cohorts: prior indication for graft rejection, history of neovascularization and high intraocular pressure. Sex matching donor-recipient is a most recent parameter in the literature confirmed by the present analysis.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT04791696.

Donor and Recipient Sex Matching and Corneal Graft Failure in High-Risk and Non-High-Risk Patients

Purpose

It is controversial whether donor-recipient sex mismatch is a risk factor associated with corneal graft failure. The purpose of this study was to investigate the effect of sex mismatch on corneal graft failure in high-risk and non-high-risk patients.

Design

A retrospective study.

Methods

The medical charts of patients who underwent corneal transplantations by one surgeon between 2012 and 2017 were reviewed. Patients were defined as high-risk for failure if they had glaucoma, ocular surface disease, or corneal vascularization. Graft failure rates were compared using the Kaplan–Meier survival curves between sex matched and mismatched subjects and between male-to-female grafting and other patients.

Results

One hundred and thirteen patients with a minimum follow-up of 18 months were included. In 62 non-high-risk patients, graft failure rates were similar between the sex mismatched and the sex matched recipients (p=0.645, log-rank) and in male donor to female recipient transplantations and in the other transplantations (p=0.496, log-rank). Analysis of fifty-one eyes of 51 high-risk graft recipients (mean age of 73.4 ± 12.7 years, N = 26 females) showed that graft failure rates were significantly higher in the sex mismatched than sex matched recipients (p=0.022, log-rank) and in male donor to female recipient transplantations than in the other transplantations (p=0.002, log-rank).

Conclusions

Sex matching for every patient bares logistic difficulties; however, in patients who are at high-risk for graft failure, it may be a simple way to improve outcomes and better utilize corneal grafts.

Presence of Panel-reactive Antibodies after Penetrating Keratoplasty

PURPOSE

To evaluate the relationship between penetrating keratoplasty (PK) and postoperative PRA level and number of unacceptable antigens.

METHODS

A cross-sectionalstudy was performed on patients with history of PK. Patients with prior solid organ transplantation, pregnancy, or blood transfusion were excluded. These findings were combined with a retrospective review. Patients were grouped by single or multiple PKs. The primary outcome was postoperative PRA level.

RESULTS

Incidence of postoperative PRA elevation and mean peak PRA was higher in the multiple PK group (p = .08 and p = .010, respectively). Mean number of unacceptable antigens was elevated in the multiple PK group (p = .024). There was a moderately positive correlation between number of PK grafts and PRA level (r = 0.629, p = .0002).

CONCLUSIONS

PRA level may be influenced by PKs, with higher PRA associated with increased prior PKs. Further studies are necessary to determine the potential prognostic value.Abbreviations: PK: penetrating keratoplasty; PRA: panel reactive antibodies; OSST: ocular surface stem cell transplantation; LSCD: limbal stem cell deficiency.

Tissue engineered scaffolds for corneal endothelial regeneration: a material's perspective

Currently, the treatment of corneal diseases caused by damage to the corneal endothelium requires a donor cornea. Because of their limited availability (1 donor cornea for 70 patients in need), researchers are investigating alternative approaches that are independent of donor tissue. One of them includes the development of a tissue engineered scaffold onto which corneal endothelial cells are seeded. In order to function as a suitable substrate, some of its essential properties including thickness, permeability, transparency and mechanical strength should meet certain demands. Additionally, the membrane should be biocompatible and allow the formation of a functional endothelium on the surface. Many materials have already been investigated in this regard including natural, semi-synthetic and synthetic polymers. In the current review, we present an overview of their characteristics and provide a critical view on the methods exploited for material characterization. Next, also the suitability of scaffolds to serve their purpose is discussed along with an overview of natural tissues (e.g. amniotic membrane and lens capsule) previously investigated for this application. Eventually, we propose a consistent approach to be exploited ideally for membrane characterization in future research. This will allow a scientifically sound comparison of materials and membranes investigated by different research groups, hence benefitting research towards the creation of a suitable/optimal tissue engineered endothelial graft.

Genetic Modification of Limbal Stem Cells to Decrease Allogeneic Immune Responses

Limbal stem cell (LSC) transplantation is the only efficient treatment for patients affected by LSC deficiency (LSCD). Allogeneic LSC transplantation is one of the most successful alternative for patients with bilateral LSCD. Nevertheless, the high variability of the human leukocyte antigens (HLA) remains a relevant obstacle to long-term allogeneic graft survival. This study characterized the immunologic properties of LSCs and proposed a genetic engineering strategy to reduce the immunogenicity of LSCs and of their derivatives. Hence, LSC HLA expression was silenced using lentiviral vectors encoding for short hairpin (sh) RNAs targeting β2-microglobulin (β2M) or class II major histocompatibility complex transactivator (CIITA) to silence HLA class I and II respectively. Beside the constitutive expression of HLA class I, LSCs showed the capability to upregulate HLA class II expression under inflammatory conditions. Furthermore, LSCs demonstrated the capability to induce T-cell mediated immune responses. LSCs phenotypical and functional characteristics are not disturbed after genetic modification. However, HLA silenced LSC showed to prevent T cell activation, proliferation and cytotoxicity in comparison to fully HLA-expressing LSCs. Additionally; HLA-silenced LSCs were protected against antibody-mediated cellular-dependent cytotoxicity. Our data is a proof-of-concept of the feasibility to generate low immunogenic human LSCs without affecting their typical features. The use of low immunogenic LSCs may support for long-term survival of LSCs and their derivatives after allogeneic transplantation.

New developments in corneal endothelial cell replacement

Corneal transplantation is currently the most effective treatment to restore corneal clarity in patients with endothelial disorders. Endothelial transplantation, either by Descemet membrane endothelial keratoplasty (DMEK) or by Descemet stripping (automated) endothelial keratoplasty (DS(A)EK), is a surgical approach that replaces diseased Descemet membrane and endothelium with tissue from a healthy donor eye. Its application, however, is limited by the availability of healthy donor tissue. To increase the pool of endothelial grafts, research has focused on developing new treatment options as alternatives to conventional corneal transplantation. These treatment options can be considered as either 'surgery-based', that is tissue-efficient modifications of the current techniques (e.g. Descemet stripping only (DSO)/Descemetorhexis without endothelial keratoplasty (DWEK) and Quarter-DMEK), or 'cell-based' approaches, which rely on in vitro expansion of human corneal endothelial cells (hCEC) (i.e. cultured corneal endothelial cell sheet transplantation and cell injection). In this review, we will focus on the most recent developments in the field of the 'cell-based' approaches. Starting with the description of aspects involved in the isolation of hCEC from donor tissue, we then describe the different natural and bioengineered carriers currently used in endothelial cell sheet transplantation, and finally, we discuss the current 'state of the art' in novel therapeutic approaches such as endothelial cell injection.

Corneal Allografts: Factors for and against Acceptance

Cornea is one of the most commonly transplanted tissues worldwide. However, it is usually omitted in the field of transplantology. Transplantation of the cornea is performed to treat many ocular diseases. It restores eyesight significantly improving the quality of life. Advancements in banking of explanted corneas and progressive surgical techniques increased availability and outcomes of transplantation. Despite the vast growth in the field of transplantation laboratory testing, standards for corneal transplantation still do not include HLA typing or alloantibody detection. This standard practice is based on immune privilege dogma that accounts for high success rates of corneal transplantation. However, the increasing need for retransplantation in high-risk patients with markedly higher risk of rejection causes ophthalmology transplantation centers to reevaluate their standard algorithms. In this review we discuss immune privilege mechanisms influencing the allograft acceptance and factors disrupting the natural immunosuppressive environment of the eye. Current developments in testing and immunosuppressive treatments (including cell therapies), when applied in corneal transplantation, may give very good results, decrease the possibility of rejection, and reduce the need for retransplantation, which is fairly frequent nowadays.

Immunogenetics of the Ocular Anterior Segment: Lessons from Inherited Disorders

Autoimmune and autoinflammatory diseases cause morbidity in multiple organ systems including the ocular anterior segment. Genetic disorders of the innate and adaptive immune system present an avenue to study more common inflammatory disorders and host-pathogen interactions. Many of these Mendelian disorders have ophthalmic manifestations. In this review, we highlight the ophthalmic and molecular features of disorders of the innate immune system. A comprehensive literature review was performed using PubMed and the Online Mendelian Inheritance in Man databases spanning 1973-2020 with a focus on three specific categories of genetic disorders: RIG-I-like receptors and downstream signaling, inflammasomes, and RNA processing disorders. Tissue expression, clinical associations, and animal and functional studies were reviewed for each of these genes. These genes have broad roles in cellular physiology and may be implicated in more common conditions with interferon upregulation including systemic lupus erythematosus and type 1 diabetes. This review contributes to our understanding of rare inherited conditions with ocular involvement and has implications for further characterizing the effect of perturbations in integral molecular pathways.

Effectiveness of matching human leukocyte antigens (HLA) in corneal transplantation: a systematic review protocol

BACKGROUND

Corneal transplantation is the most frequently performed transplantation in the UK. Despite this, the therapeutic value of matching human leukocyte antigen (HLA) subtypes for transplanted corneas remains controversial. Ocular immune privilege was originally deemed to render matching unnecessary; however, more recently, matching has demonstrated improved outcomes including graft success, amongst others. This systematic review aims to evaluate the effectiveness of major and minor antigen matching on graft outcomes in corneal transplantation.

METHODS

Standard systematic review methodology will be used to identify, select and extract data from observational studies and clinical trials assessing the effects of HLA matching on corneal graft outcomes. Bibliographic databases (Cochrane Library, EMBASE, MEDLINE, Web of Science, Scopus), clinical trial registers, abstract and conference proceedings, in addition to dissertation, thesis and grey literature will be searched. Neither date of publication nor language will be restricted, and non-English articles will be translated where necessary. The primary outcome will be to assess corneal graft success for different degrees of HLA matching/mismatching. The precise end outcome measure varies amongst studies and includes graft rejection, immunoreaction, failure and survival. Therefore, data will be extracted across all relevant outcome parameters and grouped for subsequent statistical tests. Risk of bias assessment will be completed, appropriate to each study design. Study selection, data extraction and risk of bias assessment will be independently completed by two reviewers. Data will be tabulated, and a narrative synthesis presented. Meta-analysis will be performed where there is sufficient homogeneity between studies to warrant its effective completion. Subgroup and sensitivity analysis will be undertaken if appropriate.

DISCUSSION

Many studies have investigated the effectiveness of HLA matching for corneal transplantation. A systematic review is needed to collate and analyse this evidence. Findings of this systematic review may form the basis of evidence-based recommendations on pre-operative HLA typing and matching of corneal grafts for transplantation.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO reference CRD42020198882.

Corneal xenotransplantation: Where are we standing?

The search for alternatives to allotransplants is driven by the shortage of corneal donors and is demanding because of the limitations of the alternatives. Indeed, current progress in genetically engineered (GE) pigs, the introduction of gene-editing technology by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9, and advanced immunosuppressants have made xenotransplantation a possible option for a human trial. Porcine corneal xenotransplantation is considered applicable because the eye is regarded as an immune-privileged site. Furthermore, recent non-human primate studies have shown long-term survival of porcine xenotransplants in keratoplasty. Herein, corneal immune privilege is briefly introduced, and xenogeneic reactions are compared with allogeneic reactions in corneal transplantation. This review describes the current knowledge on special issues of xenotransplantation, xenogeneic rejection mechanisms, current immunosuppressive regimens of corneal xenotransplantation, preclinical efficacy and safety data of corneal xenotransplantation, and updates of the regulatory framework to conduct a clinical trial on corneal xenotransplantation. We also discuss barriers that might prevent xenotransplantation from becoming common practice, such as ethical dilemmas, public concerns on xenotransplantation, and the possible risk of xenozoonosis. Given that the legal definition of decellularized porcine cornea (DPC) lies somewhere between a medical device and a xenotransplant, the preclinical efficacy and clinical trial data using DPC are included. The review finally provides perspectives on the current standpoint of corneal xenotransplantation in the fields of regenerative medicine.

Corneal Transplant Follow-up Study II: a randomised trial to determine whether HLA class II matching reduces the risk of allograft rejection in penetrating keratoplasty

PURPOSE

A randomised trial to test the hypothesis that human leucocyte antigen (HLA) class II matching reduces the risk of allograft rejection in high-risk penetrating keratoplasty (PK).

METHODS

All transplants were matched for HLA class I antigens (≤2 mismatches at the A and B loci) and corneas were allocated to patients by cohort minimisation to achieve 0, 1 or 2 HLA class II antigen mismatches. The corneal transplants (n=1133) were followed for 5 years. The primary outcome measure was time to first rejection episode.

RESULTS

Cox regression analysis found no influence of HLA class II mismatching on risk of immunological rejection (HR 1.13; 95% CI 0.79 to 1.63; p=0.51). The risk of rejection in recipients older than 60 years was halved compared with recipients ≤40 years (HR 0.51; 95% CI 0.36 to 0.73; p=0.0003). Rejection was also more likely where cataract surgery had been performed after PK (HR 3.68; 95% CI 1.95 to 6.93; p<0.0001). In univariate analyses, preoperative factors including chronic glaucoma (p=0.02), vascularisation (p=0.01), inflammation (p=0.03), ocular surface disease (p=0.0007) and regrafts (p<0.001) all increased the risk of rejection. In the Cox model, however, none of these factors was individually significant but rejection was more likely where≥2 preoperative risk factors were present (HR 2.11; 95% CI 1.26 to 3.47; p<0.003).

CONCLUSIONS

HLA class II matching, against a background of HLA class I matching, did not reduce the risk of allograft rejection. Younger recipient age, the presence of ≥2 preoperative risk factors and cataract surgery after PK all markedly increased the risk of allograft rejection.

TRIAL REGISTRATION NUMBER

ISRCTN25094892.

Descemet Membrane Endothelial Keratoplasty in Vascularized Eyes: Outcome and Effect on Corneal Neovascularization

PURPOSE

To report the outcomes after Descemet membrane endothelial keratoplasty (DMEK) in vascularized eyes.

METHODS

Consecutive cases of DMEK in vascularized eyes (involving ≥2 vascularized quadrants) were selected from a prospective database. Best corrected visual acuity, endothelial cell density (ECD), central corneal thickness, corneal transplant rejection episode, graft survival, and area of neovascularization (quantified using image analysis software) were evaluated.

RESULTS

In this study, 24 eyes of 24 patients were selected [mean age, 65.0 years; mean follow-up duration, 14.8 months (6-36 months)], which consists of 14 vascularized eyes after failed penetrating keratoplasty and 10 vascularized eyes with bullous keratopathy. Best corrected visual acuity improved from 1.60 ± 1.02 LogMAR preoperatively to 0.47 ± 0.37 LogMAR 12 months postoperatively (P < 0.001). Central corneal thickness decreased from 824 ± 193 μm preoperatively to 544 ± 48 μm 12 months postoperatively (P = 0.001). The donor ECD decreased from 2272 ± 723 cells/mm2 preoperatively to 1570 ± 279 cells/mm2 12 months postoperatively. The total loss of ECD at the last visit was 40.7% ± 13.0%. Eight of 24 eyes (33.3%) required rebubbling, which resulted in final attachment. The corneal neovascularization area significantly regressed from 4.68% ± 3.26% preoperatively to 2.28% ± 1.58% (n = 18, P = 0.021). Corneal transplant rejection episodes occurred in 1 eye of 24 patients (4.2%). There was no primary graft failure.

CONCLUSIONS

DMEK is a feasible option to treat endothelial dysfunction in vascularized eyes.

Corneal endothelium tissue engineering: An evolution of signaling molecules, cells, and scaffolds toward 3D bioprinting and cell sheets

Cornea is an avascular and transparent tissue that focuses light on retina. Cornea is supported by the corneal-endothelial layer through regulation of hydration homeostasis. Restoring vision in patients afflicted with corneal endothelium dysfunction-mediated blindness most often requires corneal transplantation (CT), which faces considerable constrictions due to donor limitations. An emerging alternative to CT is corneal endothelium tissue engineering (CETE), which involves utilizing scaffold-based methods and scaffold-free strategies. The innovative scaffold-free method is cell sheet engineering, which typically generates cell layers surrounded by an intact extracellular matrix, exhibiting tunable release from the stimuli-responsive surface. In some studies, scaffold-based or scaffold-free technologies have been reported to achieve promising outcomes. However, yet some issues exist in translating CETE from bench to clinical practice. In this review, we compare different corneal endothelium regeneration methods and elaborate on the application of multiple cell types (stem cells, corneal endothelial cells, and endothelial precursors), signaling molecules (growth factors, cytokines, chemical compounds, and small RNAs), and natural and synthetic scaffolds for CETE. Furthermore, we discuss the importance of three-dimensional bioprinting strategies and simulation of Descemet's membrane by biomimetic topography. Finally, we dissected the recent advances, applications, and prospects of cell sheet engineering for CETE.

Gender medicine in corneal transplantation: influence of sex mismatch on rejection episodes and graft survival in a prospective cohort of patients

To evaluate the effect of donor-to-recipient sex mismatched (male donor corneas to female recipients) on the incidence of rejection episodes and failures up to 1 year after corneal transplantation. Prospective observational cohort study, with donor corneas randomly assigned and surgeons blind to the sex of donor. A unique eye bank retrieved and selected the donor corneas transplanted in 4 ophthalmic units in patients with clinical indication for primary or repeated keratoplasty for optical reasons, perforating or lamellar, either anterior or posterior. Rejection episode defined as any reversible or irreversible endothelial, epithelial or stromal sign, with or without development of corneal edema, and graft failure as a permanently cloudy graft or a regraft for any reason detected or acknowledged during a postoperative ophthalmic visit at any time up to 1 year after surgery were recorded.156 (28.6%) patients resulted donor-to-recipient gender mismatched for H-Y antigen (male donor to female recipient). During the 12 months follow-up, 83 (14.7%, 95% CI 12.0-17.9) grafts showed at least 1 rejection episode and 17 (3.2%, 95% CI 2.0-5.0) failed after immune rejection, among 54 (9.6%, 95% CI 7.4-12.3) grafts failed for all causes. No significant differences between matched and mismatched patients were found for cumulative incidence of both rejection episodes (15.2% and 13.5%) and graft failures following rejection (3.2% and 2.6%), respectively. Multivariable analyses showed that H-Y matching either is not a predictive factor for rejection or graft failure nor seems to influence incidence of failures on respect to patient's risk category. The lack of influence of donor-to-recipient mismatched on the rate of rejections and graft failures resulting from this study do not support the adoption of donor-recipient matching in the allocation of corneas for transplantation.

Corneal graft failure: an update

Corneal graft surgery is one of the most successful forms of human solid-tissue transplantation, and nowadays, there is a worldwide expansion of the surgical volume of corneal grafts. This surgery is continuously evolving, with new surgical techniques and postoperative treatments that have considerably increased the chance of survival for the grafts. Despite the high rate of success, corneal transplantation is still complicated by a relevant risk of graft failure. This study investigates the causes that lead to the failure of the different corneal graft surgical techniques and provides an updated synthesis on this topic. A comprehensive review of the main pathological pathways that determine the failure of corneal grafts is provided, analysing the main risk factors and disclosing the survival rates of the principal form of corneal grafts. Our results revealed that penetrating keratoplasty has higher failure rates than lamellar keratoplasty, with immunological rejection being the leading cause of graft failure, followed by late endothelial failure (LEF) and ocular surface disorders. Postoperative glaucoma and dehiscence of the surgical wound represent other important causes of failure. Endothelial keratoplasty showed the lowest rates of failure in the mid-term, with LEF, detachment of the graft and primary graft failure representing the most common pathological reasons for failure.

Topical VEGF-C/D Inhibition Prevents Lymphatic Vessel Ingrowth into Cornea but Does Not Improve Corneal Graft Survival

Vascular endothelial growth factor-C/D (VEGF-C/D) regulates lymphangiogenesis. Ingrowth of lymphatic vessels is negatively associated with corneal transplantation success. In this study, we therefore analyzed the effect local blockade of VEGF-C/D has on inflamed corneas. We used the murine model of suture-induced neovascularization and subsequent high-risk corneal transplantation. Mice were treated with a VEGF-C/D trap prior to transplantation. Topical inhibition of VEGF-C/D significantly reduced lymphatic vessel ingrowth, but increased Macrophage numbers in the cornea. Furthermore, corneal transplantation success was not improved by the topical application of the compound. This study demonstrates that local VEGF-C/D inhibition is insufficient to increases corneal transplantation success, likely due to interaction with immune cells.

The genetic contribution of HLA-E*01:03 and HLA-E*01:03-G*01:01 to Posner-Schlossman syndrome in southern Chinese

Background

The polymorphisms of classical HLA-Ia and HLA-II loci have been associated with Posner-Schlossman syndrome (PSS) in the southern Chinese population. However, the associations of non-classical HLA-Ib (e.g., HLA-E and HLA-G) loci with PSS have not been reported for in the southern Chinese population. This study aimed to evaluate the associations of the HLA-E and HLA-G loci with PSS in a southern Chinese Han population group.

Methods

Ninety-seven unrelated patients with PSS and 90 ethnically matched control subjects were recruited from the Shenzhen Eye Hospital in China. The full-length sequences of HLA-E and HLA-G genes were amplified by long-range high-fidelity PCR, and the third exon of the HLA-E gene and the coding region of the HLA-G gene were sequenced.

Results

The allele frequency of HLA-E*01:03 in patients with PSS was significantly higher than that in the control group (P=0.017, corrected P=0.034, OR =1.66). The genotype frequencies of HLA-E*01:01/01:03 and HLA-E*01:03/01:03 in the PSS group were significantly higher than that in the control group (P=0.027, OR =2.62; P=0.011, OR =3.05; respectively). There were no significant differences in the frequency of HLA-G alleles and genotypes between the two groups (all P>0.05). The haplotype frequency of HLA-E*01:03-G*01:01 in the PSS group was significantly higher than that in the control group (P=0.019, OR =1.63), although this association did not survive the Bonferroni correction (corrected P=0.13).

Conclusions

This study proved for the first time that HLA-E*01:03 and HLA-E*01:03-G*01:01 might be risk factors for PSS.

Rejection Prophylaxis in Corneal Transplant

BACKGROUND

Graft rejection. Twenty to thirty percent of patients with corneal transplants experience at least one rejection episode in the first 5 years after transplantation. Prophylaxis through matching for human leukocyte antigens (HLA) is controversial. We herein report the results of the Functional ANtigen matChing in keratoplastY (FANCY) trial.

METHODS

FANCY was a randomized, double-blind, multicenter clinical trial. The primary objective was to evaluate superiority of HLA matching versus random graft assignment. The primary endpoint was rejection-free graft survival. We included both normal-risk and high-risk indications. The study is registered with ClinicalTrials. gov (NCT00810472).

RESULTS

721 patients were included, 639 patients were randomized. 474 patients underwent keratoplasty within the study; 165 patients received grafts outside the trial. One patient died and one patient was lost to follow up. We observed 33 graft rejections in the HLA matching arm (n = 224). The corresponding estimated cumulative incidence rate of immune reactions after two years was 15.7%. We observed 40 rejections in the control arm (n = 249). After two years this yields an estimated cumulative incidence rate of 17%.

CONCLUSION

In our heterogenous study group, HLA matching did not show a significant advantage compared to random graft assignment. The rejection rate in our sample was lower than expected. Therefore no definite conclusions can be drawn as to whether HLA matching is beneficial in corneal transplantation.

Immune reactions after modern lamellar (DALK, DSAEK, DMEK) versus conventional penetrating corneal transplantation

In the past decade, novel lamellar keratoplasty techniques such as Deep Anterior Lamellar Keratoplasty (DALK) for anterior keratoplasty and Descemet stripping automated endothelial keratoplasty (DSAEK)/Descemet membrane endothelial keratoplasty (DMEK) for posterior keratoplasty have been developed. DALK eliminates the possibility of endothelial allograft rejection, which is the main reason for graft failure after penetrating keratoplasty (PK). Compared to PK, the risk of endothelial graft rejection is significantly reduced after DSAEK/DMEK. Thus, with modern lamellar techniques, the clinical problem of endothelial graft rejection seems to be nearly solved in the low-risk situation. However, even with lamellar grafts there are epithelial, subepithelial and stromal immune reactions in DALK and endothelial immune reactions in DSAEK/DMEK, and not all keratoplasties can be performed in a lamellar fashion. Therefore, endothelial graft rejection in PK is still highly relevant, especially in the "high-risk" setting, where the cornea's (lymph)angiogenic and immune privilege is lost due to severe inflammation and pathological neovascularization. For these eyes, currently available treatment options are still unsatisfactory. In this review, we will describe currently used keratoplasty techniques, namely PK, DALK, DSAEK, and DMEK. We will summarize their indications, provide surgical descriptions, and comment on their complications and outcomes. Furthermore, we will give an overview on corneal transplant immunology. A specific focus will be placed on endothelial graft rejection and we will report on its incidence, clinical presentation, and current/future treatment and prevention options. Finally, we will speculate how the field of keratoplasty and prevention of corneal allograft rejection will develop in the future.

In Vivo Confocal Microscopy Demonstrates Increased Immune Cell Densities in Corneal Graft Rejection Correlating With Signs and Symptoms

PURPOSE

Diagnosis of graft rejection is based on patient symptoms and on clinical signs detected by slit-lamp biomicroscopy. This study investigated whether laser in vivo confocal microscopy (IVCM) can aid in the diagnosis of corneal graft rejection by detecting cellular corneal changes that take place after transplantation.

DESIGN

Prospective case-control study.

SUBJECTS

Thirty-eight eyes of 38 patients with penetrating keratoplasty (15 eyes with corneal graft rejection, 23 eyes without rejection) and 9 age-matched normal controls.

METHODS

Laser IVCM was performed in the corneal grafts centrally. The density of immune cells (IC) was assessed for epithelial, sub-epithelial, stromal, and endothelial layers by 2 masked observers. IC density was compared among different groups and correlated to clinical signs and symptoms of corneal graft rejection.

MAIN OUTCOME MEASUREMENTS

Outcome measurement was the IC density in the corneal layers and its associations with the presence of clinical signs and symptoms of corneal graft rejection.

RESULTS

The IC density was significantly different between rejected and non-rejected grafts (P = 0.004) and different from that of normal controls (P = 0.001). Among corneal layers, IC density was significantly higher in rejected grafts than in non-rejected grafts in only the sub-basal (611.54 ± 573.74 vs. 340.61 ± 268.60 cells/mm², respectively; P = 0.049) and endothelial layers (250.62 ± 267.13 vs. 103.47 ± 81.91 cells/mm², respectively; P = 0.001). Patients with decreased best corrected visual acuity, Khodadoust line, and anterior chamber cells demonstrated a significant increase in total IC density (P < 0.05), whereas patients with symptoms of irritation, light sensitivity, and pain revealed a specific increase in IC density in the sub-basal layer (P < 0.05). Patients with ocular pain had higher IC density in the epithelial layer than those without pain (P = 0.03).

CONCLUSIONS

Patients with corneal graft rejection demonstrate a significant increase in corneal immune cells, particularly, in the sub-basal and endothelial layers compared to patients with non-rejected grafts and controls. Although symptoms associated with endothelial rejection demonstrate a general increase in IC, pain, irritation, and light sensitivity are associated with increased IC in the sub-basal layer. Assessment of patients with corneal graft rejection by IVCM may serve as an adjunctive tool in the diagnosis and management of corneal graft rejection.

Immune privilege in corneal transplantation

Corneal transplantation is the most successful solid organ transplantation performed in humans. The extraordinary success of orthotopic corneal allografts, in both humans and experimental animals, is related to the phenomenon of "immune privilege". Inflammation is self-regulated to preserve ocular functions because the eye has immune privilege. At present, three major mechanisms are considered to provide immune privilege in corneal transplantation: 1) anatomical, cellular, and molecular barriers in the cornea; 2) tolerance related to anterior chamber-associated immune deviation and regulatory T cells; and 3) an immunosuppressive intraocular microenvironment. This review describes the mechanisms of immune privilege that have been elucidated from animal models of ocular inflammation, especially those involving corneal transplantation, and its relevance for the clinic. An update on molecular, cellular, and neural interactions in local and systemic immune regulation is provided. Therapeutic strategies for restoring immune privilege are also discussed.

Electro-spun Membranes as Scaffolds for Human Corneal Endothelial Cells

BACKGROUND

Corneal endothelial dysfunction remains the most frequent indication for corneal transplantation, limited by donor material shortage, poor long-term graft survival, or allogeneic graft rejection. Therefore, tissue-engineered endothelial grafts (TEEG) represent a promising alternative to human donor tissue. In this study, we generated electro-spun scaffolds and tested these for their suitability for human corneal endothelial cell (hCEC) cultivation.

METHODS

The polymers poly(methyl-methacrylate) (PMMA), poly(lactic-co-glycolic acid) (PLGA), and polycaprolactone (PCL) were spun with equal parameters. HCEC-12 was cultured on the scaffolds for 3 to 7 days. Scaffolds were evaluated by light microscopy, porometry, light transmission, scanning electron microscopy (SEM), live/dead staining and cell viability assay.

RESULTS

Electro-spun fibers from PMMA (2.99 ± 0.24 µm) showed significantly higher diameters than PCL (2.29 ± 0.11 µm; p = 0.003) and PLGA (1.84 ± 0.21 µm; p < 0.001), while fibers from PCL also showed larger diameters than those from PLGA (p = 0.002). PMMA scaffolds (26.77 ± 17.48 µm) had significantly larger interstitial spaces than those from PCL (13.30 ± 5.47 µm; p = 0.04) and PLGA (10.42 ± 6.15 µm; p = 0.002), while PCL and PLGA did not differ significantly (p = 0.26). SEM analysis revealed that only PLGA fibers preserved a normal HCEC-12 morphology. PLGA and PCL did not differ in cell number, death, or viability after 7 days of HCEC-12 cultivation. PMMA showed significantly higher cytotoxicity (p < 0.001; PLGA: 1626.2 ± 183.8 RLU; PMMA: 841.9 ± 92.7 RLU; PCL: 1580.2 ± 171.02 RLU).

CONCLUSIONS

The biodegradable PLGA and PCL electro-spun scaffolds resulted in equal biocompatibility, while PMMA showed cytotoxicity. Only PLGA preserved hCEC morphology and consequently seems to be a promising candidate for TEEG construction.

Universal Corneal Epithelial-Like Cells Derived from Human Embryonic Stem Cells for Cellularization of a Corneal Scaffold

Purpose

We generated universal corneal epithelial cells (CEC) from human embryonic stem cells (hESC) by genetically removing human leukocyte antigens (HLA) class I from the cell surface.

Methods

The serum-free, growth factor-free, and defined medium E6 was used to differentiate hESC to CEC. Decellularized murine corneas were recellularized with hESC-derived CEC. Using CRISPR/Cas9, β-2-microglobulin (B2M) was deleted in hESC to block the assembly of HLA class-I antigens on the cell surface to generate B2M^−/− CEC.

Results

E6 alone was sufficient to allow hESC differentiation to CEC. A time-course analysis of the global gene expression of the differentiating cells indicates that the differentiation closely resembles the corneal development in vivo. The hESC-CEC were highly proliferative, and could form multilayer epithelium in decellularized murine cornea, retain its transparency, and form intact tight junctions on its surface. As reported before, B2M knockout led to the absence of HLA class-I on the cell surface of hESC and subsequently derived CEC following stimulation with inflammatory factors. Moreover, B2M^−/− CEC, following transplantation into mouse eyes, caused less T-cell infiltration in the limbal region of the eye than the wild-type control.

Conclusions

CEC can be derived from hESC via a novel and simple protocol free of any proteins, hESC-CEC seeded on decellularized animal cornea form tight junctions and allow light transmittance, and B2M^−/− CEC are hypoimmunogenic both in vitro and in vivo.

Translational Relevance

B2M^−/− hESC-CEC can be an unlimited and universal therapy for corneal repair in patients of any HLA type.

Effect of improved preservation solution with methoxy polyethylene glycol succinimidyl propionate on rat cornea

To observe the effect of DMEM/F12 pegylated with methoxy polyethylene glycol succinimidyl propionate (mPEG-SPA) on the biophysical and immune characteristics of the rat cornea preserved in it. Corneal grafts were harvested from Wistar rat and preserved in the DMEM/F12 plus mPEG-SPA, DMEM/F12 without mPEG-SPA, and standard Optisol-GS solution at 4 °C for 14 days, referred as plus-PEG, minus-PEG, and Optisol grafts, respectively. The biophysical properties of those grafts, including transmittance, thickness, water content, and biomechanics were investigated. The survival of those grafts was observed in the high-risk corneal transplantation model. Transmittance and biomechanics did not show any differences among those grafts. Thickness and water content of plus-PEG grafts were slightly improved. Proliferation and activation of lymphocytes were lower while they were incubated with plus-PEG grafts, compared with minus-PEG grafts and Optisol grafts. The mean survival time was significantly prolonged in plus-PEG grafts. DMEM/F12 solution plus mPEG-SPA improved the survival of corneal grafts and maintained the comparative biophysical characteristics of them, compared with the standard preservation solution.

Mesenchymal stem cells derived from human iPS cells via mesoderm and neuroepithelium have different features and therapeutic potentials

Mesenchymal stem cells (MSCs) isolated from adult human tissues are capable of proliferating in vitro and maintaining their multipotency, making them attractive cell sources for regenerative medicine. However, the availability and capability of self-renewal under current preparation regimes are limited. Induced pluripotent stem cells (iPSCs) now offer an alternative, similar cell source to MSCs. Herein, we established new methods for differentiating hiPSCs into MSCs via mesoderm-like and neuroepithelium-like cells. Both derived MSC populations exhibited self-renewal and multipotency, as well as therapeutic potential in mouse models of skin wounds, pressure ulcers, and osteoarthritis. Interestingly, the therapeutic effects differ between the two types of MSCs in the disease models, suggesting that the therapeutic effect depends on the cell origin. Our results provide valuable basic insights for the clinical application of such cells.

A study protocol for a multicentre randomised clinical trial evaluating the safety and feasibility of a bioengineered human allogeneic nanostructured anterior cornea in patients with advanced corneal trophic ulcers refractory to conventional treatment

INTRODUCTION

There is a need to find alternatives to the use of human donor corneas in transplants because of the limited availability of donor organs, the incidence of graft complications, as well as the inability to successfully perform corneal transplant in patients presenting limbal deficiency, neo-vascularized or thin corneas, etc. We have designed a clinical trial to test a nanostructured fibrin-agarose corneal substitute combining allogeneic cells that mimics the anterior human native cornea in terms of optical, mechanical and biological behaviour.

METHODS AND ANALYSIS

This is a phase I-II, randomised, controlled, open-label clinical trial, currently ongoing in ten Spanish hospitals, to evaluate the safety and feasibility, as well as clinical efficacy evidence, of this bioengineered human corneal substitute in adults with severe trophic corneal ulcers refractory to conventional treatment, or with sequelae of previous ulcers. In the initial phase of the trial (n=5), patients were sequentially recruited, with a safety period of 45 days, receiving the bioengineered corneal graft. In the second phase of the trial (currently ongoing), subjects are block randomised (2:1) to receive either the corneal graft (n=10), or amniotic membrane (n=5), as the control treatment. Adverse events, implant status, infection signs and induced neovascularization are evaluated as determinants of safety and feasibility of the bioengineered graft (main outcomes). Study endpoints are measured along a follow-up period of 24 months, including 27 post-implant assessment visits according to a decreasing frequency. Intention to treat, and per protocol, and safety analysis will be performed.

ETHICS AND DISSEMINATION

The trial protocol received written approval by the corresponding Ethics Committee and the Spanish Regulatory Authority and is currently recruiting subjects. On completion of the trial, manuscripts with the results of phases I and II of the study will be published in a peer-reviewed journal.

TRIAL REGISTRATION

CT.gov identifier: NCT01765244 (Jan2013). EudraCT number: 2010-024290-40 (Dec2012).

Alloimmunity and Tolerance in Corneal Transplantation

Corneal transplantation is one of the most prevalent and successful forms of solid tissue transplantation. Despite favorable outcomes, immune-mediated graft rejection remains the major cause of corneal allograft failure. Although low-risk graft recipients with uninflamed graft beds enjoy a success rate ∼90%, the rejection rates in inflamed graft beds or high-risk recipients often exceed 50%, despite maximal immune suppression. In this review, we discuss the critical facets of corneal alloimmunity, including immune and angiogenic privilege, mechanisms of allosensitization, cellular and molecular mediators of graft rejection, and allotolerance induction.

Update on the Management of High-Risk Penetrating Keratoplasty

PURPOSE OF REVIEW

In this article, we review the indications and latest management of high-risk penetrating keratoplasty.

RECENT FINDINGS

Despite the immune-privilege status of the cornea, immune-mediated graft rejection still remains the leading cause of corneal graft failure. This is particularly a problem in the high-risk graft recipients, namely patients with previous graft failure due to rejection and those with inflamed and vascularized corneal beds. A number of strategies including both local and systemic immunosuppression are currently used to increase the success rate of high-risk corneal grafts. Moreover, in cases of limbal stem cell deficiency, limbal stem cells transplantation is employed.

SUMMARY

Corticosteroids are still the top medication for prevention and treatment in cases of corneal graft rejection. Single and combined administration of immunosuppressive agents e.g. tacrolimus, cyclosporine and mycophenolate are promising adjunctive therapies for prolonging graft survival. In the future, cellular and molecular therapies should allow us to achieve immunologic tolerance even in high-risk grafts.

Sequential Bilateral Corneal Transplantation and Graft Survival

PURPOSE

To investigate graft survival and rejection following sequential bilateral corneal transplantation.

DESIGN

Retrospective cohort study.

METHODS

The study included patients with Fuchs endothelial dystrophy (FED), pseudophakic bullous keratopathy (PBK), or keratoconus who had undergone a penetrating keratoplasty (PK), endothelial keratoplasty (EK), or deep anterior lamellar keratoplasty (DALK) between 1999 and 2012. The main cohort included patients who had received a first transplant in both eyes for the same indication and a control cohort patients who had undergone a unilateral first corneal transplant. Main outcome measures were graft rejection or failure at 5 years.

RESULTS

A total of 11 822 patients were included, of whom 9335 had a unilateral and 2487 bilateral corneal transplantation. For patients with FED (P < .005) and KC (P = .03) but not PBK (P = .19), a transplant in the second eye was associated with a 50% reduction in risk of graft failure within 5 years in the first eye (FED: hazard ratio [HR] 0.47, 95% confidence interval [CI]: 0.34-0.64; KC: HR 0.50, 95% CI: 0.24-1.02). For FED this was dependent on the type of transplant (EK: HR 0.30, 95% CI: 0.17-0.52; PK: HR 0.61, 95% CI: 0.42-0.88). We found no association between a transplant in the second eye and a rejection episode in the first eye (KC P = .19, FED P = .39, PBK P = .19).

CONCLUSION

For FED and KC, a transplant in the second eye was associated with a reduced risk of graft failure in the first eye, independent of inter-transplant time. For FED this effect was pronounced following an EK in the first eye, where the risk of failure was reduced by 70%.

High-risk corneal allografts: A therapeutic challenge

Corneal transplantation is the most common surgical procedure amongst solid organ transplants with a high survival rate of 86% at 1-year post-grafting. This high success rate has been attributed to the immune privilege of the eye. However, mechanisms originally thought to promote immune privilege, such as the lack of antigen presenting cells and vessels in the cornea, are challenged by recent studies. Nevertheless, the immunological and physiological features of the cornea promoting a relatively weak alloimmune response is likely responsible for the high survival rate in “low-risk” settings. Furthermore, although corneal graft survival in “low-risk” recipients is favourable, the prognosis in “high-risk” recipients for corneal graft is poor. In “high-risk” grafts, the process of indirect allorecognition is accelerated by the enhanced innate and adaptive immune responses due to pre-existing inflammation and neovascularization of the host bed. This leads to the irreversible rejection of the allograft and ultimately graft failure. Many therapeutic measures are being tested in pre-clinical and clinical studies to counter the immunological challenge of “high-risk” recipients. Despite the prevailing dogma, recent data suggest that tissue matching together with use of systemic immunosuppression may increase the likelihood of graft acceptance in “high-risk” recipients. However, immunosuppressive drugs are accompanied with intolerance/side effects and toxicity, and therefore, novel cell-based therapies are in development which target host immune cells and restore immune homeostasis without significant side effect of treatment. In addition, developments in regenerative medicine may be able to solve both important short comings of allotransplantation: (1) graft rejection and ultimate graft failure; and (2) the lack of suitable donor corneas. The advances in technology and research indicate that wider therapeutic choices for patients may be available to address the worldwide problem of corneal blindness in both “low-risk” and “high-risk” hosts.

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.