The role of the limbus in corneal allograft rejection

Current and emerging strategies for the manufacture, implantation, and clinical management of corneal tissue allografts

Approximately 40,000 Australians have received a donor corneal tissue transplant over the last 40 years, with the primary indications being keratoconus, Fuchs' endothelial dystrophy, bullous keratopathy, and failure of a prior corneal transplant. Although corneal cross-linking and rigid contact lenses have emerged as alternative strategies for the management of keratoconus, the demand for donor corneas is increasing in-line with the ageing population in Australia. Moreover, owing to the lack of tissue banking resources in less-developed countries, the global demand for donor corneas exceeds supply by 70-fold. These supply issues, combined with evolving tissue banking and surgical techniques, have led to the emergence of new strategies for the storage, processing and implantation of corneal cells and tissues. Organ culture techniques have been developed that support the storage of donor corneas for up to 30 days, facilitating improvements in tissue supply and surgery scheduling. Bespoke surgical methods have been developed that are tailored to the requirements of specific conditions, allowing reductions in both the volume of tissue required to be transplanted and the size of the necessary surgical incision. Further efficiencies and improvements in patient care may be achieved via exploitation of cell culture technologies as exemplified through use of cultured corneal epithelial cells for the treatment of limbal stem cell deficiency. Promising progress has also been made in developing a cultured corneal endothelial cell therapy for patients with corneal endothelial dysfunction. These evolving strategies are discussed with respect to their potential impact on the clinical presentation and management of patients who have received an implant of donor corneal tissue or cells.

[Impact of the ratio between graft and host corneal size on immune rejection, re-bubbling rate and postoperative endothelial cell loss in 457 eyes after Descemet membrane endothelial keratoplasty (DMEK)]

BACKGROUND

The aim of this study was to assess the impact of the ratio between the graft and host corneal size (RGH) on postoperative complications, such as immune reactions, re-bubbling rate and endothelial cell loss (ECL) after Descemet membrane endothelial keratoplasty (DMEK).

PATIENTS AND METHODS

Retrospectively, 457 patient eyes were included which had undergone surgery between 2016 and 2019 in the Department of Ophthalmology, Saarland University Medical Center in Homburg/Saar using DMEK or triple DMEK, diagnosed as Fuchs' endothelial dystrophy (n = 431), pseudophakic bullous keratopathy (n = 9) and others (n = 17). The follow-up period extended until the end of 2020. Main outcome measures included immune reaction (IR), re-bubbling rate and the postoperative endothelial cell loss (ECL) at 6 weeks, 6 months and 12 months and whether these measures depended on the RGH.

RESULTS

The RGH in this study ranged from 0.35 to 0.62 (0.46 ± 0.04). There were 33 (7.2%) postoperative IRs (DMEK n = 25; triple DMEK n = 8). The average RGH without IR (0.46 ± 0.04) was significantly (p = 0.038) smaller than in the group with IR (0.47 ± 0.05). Re-bubbling was necessary in 159 of 457 (34.8%) patient eyes. The RGH in patient eyes with re-bubbling (0.47 ± 0.04) was significantly (p = 0.014) higher than that in eyes without re-bubbling (0.45 ± 0.04). The mean preoperative endothelial cell count (ECD) was 2603 ± 251 cells/mm2 (min: 2161, max: 3500 cells/mm2). It was shown that a larger RGH had no positive influence on endothelial cell loss (r = 0.001; p = 0.974).

CONCLUSION

Our results suggest that a larger graft diameter compared to host corneal size is associated with an increased rate of immune reactions and a higher re-bubbling rate after DMEK. Otherwise, a larger RGH had no positive influence on endothelial cell loss after DMEK. Accordingly, the graft size for DMEK should not be unnecessarily large, especially in eyes with Fuchs' endothelial dystrophy.

Corneal Epithelial Stem Cells-Physiology, Pathophysiology and Therapeutic Options

In the human cornea, regeneration of the epithelium is regulated by the stem cell reservoir of the limbus, which is the marginal region of the cornea representing the anatomical and functional border between the corneal and conjunctival epithelium. In support of this concept, extensive limbal damage, e.g., by chemical or thermal injury, inflammation, or surgery, may induce limbal stem cell deficiency (LSCD) leading to vascularization and opacification of the cornea and eventually vision loss. These acquired forms of limbal stem cell deficiency may occur uni- or bilaterally, which is important for the choice of treatment. Moreover, a variety of inherited diseases, such as congenital aniridia or dyskeratosis congenita, are characterized by LSCD typically occurring bilaterally. Several techniques of autologous and allogenic stem cell transplantation have been established. The limbus can be restored by transplantation of whole limbal grafts, small limbal biopsies or by ex vivo-expanded limbal cells. In this review, the physiology of the corneal epithelium, the pathophysiology of LSCD, and the therapeutic options will be presented.

Molecular identity of human limbal heterogeneity involved in corneal homeostasis and privilege

PURPOSE

The corneal limbus maintains the homeostasis, immune and angiogenic privilege of cornea. This study aimed to depict the landscape of human limbal tissues by single-cell RNA sequencing (scRNA-seq).

METHODS

Single cells of human limbus collected from donor corneas were subjected to 10x scRNA-seq, followed by clustering cell types through the t-distributed stochastic neighbor embedding (t-SNE) and unbiased computational informatic analysis. Immunofluorescent staining was performed using human corneas to validate the analysis results.

RESULTS

47,627 cells acquired from six human limbal tissues were collected and subjected to scRNA-seq. 14 distinct clusters were identified and 8 cell types were annotated with representative markers. In-depth dissection revealed three limbal epithelial cell subtypes and refined the X-Y-Z hypothesis of corneal epithelial maintenance. We further unveiled two cell states with higher stemness (TP63⁺ and CCL20⁺ cells), and two other differentiated cell states (GPHA2⁺ and KRT6B ⁺ cells) in homeostatic limbal stem/progenitor cells (LSPCs) that differ in transcriptional profiles. Cell-cell communication analysis revealed the central role of LSPCs and their bidirectional regulation with various niche cells. Moreover, comparative analysis between limbus and skin deciphered the pivotal contribution of limbal immune cells, vascular and lymphatic endothelial cells to corneal immune and angiogenic privilege.

CONCLUSIONS

The human limbus atlas provided valuable resources and foundations for understanding corneal biology, disease and potential interventions.

A practical approach to interpretation of corneal specimens

Corneal specimens form only a small part of the routine practice for most histopathologists but their assessment often requires considerable effort and specialized knowledge. The most common corneal specimens, full-thickness corneal discs and corneal biopsies, are discussed in this review. Corneal discs removed at keratoplasty are non-urgent specimens, as definitive treatment has already been undertaken, and while the pathologic diagnosis may change the prognosis for the graft, it rarely affects immediate treatment. Accurate diagnosis is still important, and will affect counseling of the patient, but referral to a colleague with a special interest is possible if necessary. Conversely, small partial-thickness corneal biopsies, which are mostly undertaken for culture negative keratitis with underlying suspected infection, are very urgent. Infectious keratitis can follow an extremely aggressive course, resulting in destruction of the cornea within hours. Accurate diagnosis is imperative. Due to the urgency of such specimens and the importance of diagnosis for immediate treatment, referral is not usually possible. It is the role of the pathologist to make optimal use of a small specimen to reach the relevant diagnosis in the minimum space of time.

Management of the Late Ocular Sequelae of Stevens-Johnson Syndrome

The acute conjunctivitis seen initially in Stevens-Johnson syndrome is followed by a cicatricial phase, which often leads to severe ocular surface disease and visual morbidity. Manifestations include keratinization of the conjunctiva, lid margins, and lacrimal and meibomian ducts, resulting in an unstable tear film and mechanical trauma to the conjunctiva and cornea with blinking. Limbal stem cell deficiency is the most vision-threatening sequela of Stevens-Johnson syndrome, as it causes corneal neovascularization, chronic corneal inflammation, and an irregular corneal epithelium. Management of late sequelae often requires a multipronged approach, including strategies for ocular surface protection, ocular surface support, and ocular surface reconstruction. In this review, established therapies, as well as new experimental therapies, are discussed.

Living related conjunctival limbal allograft for the treatment of stem cell deficiency

PURPOSE

To evaluate the outcomes of living related conjunctival limbal allograft transplantation for the treatment of stem cell deficiency.

DESIGN

Retrospective, noncomparative case series.

PARTICIPANTS

Nine living related donors, eight recipients (10 eyes) with Stevens-Johnson syndrome (3 eyes), ectodermal dysplasia (3 eyes), chemical injury (2 eyes), ocular cicatricial pemphigoid (1 eye), and atopic keratoconjunctivitis (n = 1).

INTERVENTION

Four clock hours of limbal conjunctival tissue from the best matched human leukocyte antigen (HLA) relative donor were transplanted to the recipient eye superiorly and inferiorly after conjunctival peritomy and removal of conjunctival pannus. Systemic cyclosporine was administered to all recipients.

MAIN OUTCOME MEASURES

Restoration of corneal epithelium, reduction of vascularity and conjunctivalization, improved comfort, improved corneal clarity, and visual improvement.

RESULTS

Mean follow-up period was 26.2 months. Two highly inflamed eyes failed to initially epithelialize. The remainder all survived with restoration of corneal epithelium and reduction of vascularization. Corneal opacification was reduced (four of eight eyes) and visual improvement was achieved in seven eyes. All five eyes with pain had an improvement in symptoms. Allograft rejection occurred in two eyes (25%), and both were treated successfully. Both eyes had two class I HLA mismatches, and both had an underlying diagnosis of Stevens-Johnson syndrome. One eye developed a recurrent epithelial defect and perforated, requiring a penetrating keratoplasty that remained clear with an intact epithelial surface. The two initial failures also perforated and required penetrating keratoplasties that failed. None of the donor eyes had any complications.

CONCLUSIONS

Restoration of the ocular surface by HLA-matched conjunctival limbal allograft transplantation can be accomplished in selected recipients. Systemic cyclosporine, even at low doses, is useful in ensuring long-term survival.

Reconstruction of the corneal epithelium by limbal allograft transplantation for severe ocular surface disorders

PURPOSE

Although penetrating keratoplasty is an established surgical procedure, it often is ineffective for severe ocular surface diseases such as alkali burns or limbal deficiency. The authors have performed limbal allograft transplantation for the reconstruction of the corneal epithelium.

METHODS

A total of nine patients (3 with chemical injury, 3 with limbal deficiency with unknown etiology, 2 with moderate ocular pemphigoid [OCP], and 1 with traumatic limbal deficiency) were treated by limbal allograft transplantation. Penetrating keratoplasties were performed in all patients with the exception of one with OCP. Patients received cyclosporine both systemically (10 mg/kg) and topically (0.05%) as well as high-dose intravenous dexamethasone (8 mg).

RESULTS

The corneal epithelium was reconstructed in all patients, although two showed partial increased fluorescein permeability and two others required a second surgery. The other five epithelia remained clear at mean follow-up of 12.3 months, with two episodes of graft rejection which were controlled successfully by medication.

CONCLUSIONS

Limbal allograft transplantation with intensive immunosuppression by cyclosporine and high-dose steroids appears to be a promising surgical intervention for the reconstruction of corneas affected by severe ocular surface disease.

Systemic (serum) soluble interleukin-2 receptor levels in corneal transplant recipients

Elevated soluble interleukin-2 receptor levels (sIL-2R), a measure of T cell activation, have been used as a serum marker for early rejection in solid organ transplant patients. In this preliminary study, we measured sequential sIL-2R levels of 53 otherwise immunologically normal corneal transplant recipients. Eleven of these transplants rejected; sIL-2R was significantly elevated during the acute rejection episode, compared to pre-rejection and post-rejection levels (p = 0.01). Five patients' sIL-2R rose one to nine months prior to rejection. These data indicate that sIL-2R levels may correlate with corneal graft rejection and may be predictive of impending rejection. A larger prospective study with many immunologically normal patients and careful monitoring of sIL-2R levels prior to and during the acute rejection episode will be necessary to determine the value of sIL-2R monitoring as a predictive tool for corneal graft rejection.

Immunohistochemical characterization of dendritic cells and macrophages in the aqueous outflow pathways of the rat eye

Immunohistochemical studies were performed to determine the distribution, phenotype and ontogeny of macrophages and dendritic cells (DCs) in the aqueous humour outflow pathways of the rat eye. Optimal fixation and indirect immunoperoxidase techniques were employed in conjunction with a panel of mAbs on tangential frozen sections of ocular tissues from a total of 37 Wistar Furth rats aged 12-13 days (n = 8), 3 weeks (n = 12), 7 weeks (n = 5) and 15 weeks (n = 12). The density of immunopositive cells was scored qualitatively. A moderate to low density of Ia+ cells with a dendritic morphology were observed in the trabecular meshwork. DCs were also identified in the suprachoroidal space and in the connective tissues of nerves and vessels piercing the sclera, i.e. in association with non-conventional aqueous outflow pathways. The phenotypical and morphological characteristics of these cells would indicate that they may potentially act as antigen presenting cells (APCs). Non-dendritic pleomorphic cells with a macrophage phenotype were also identified in the trabecular meshwork, and bipolar or elongated cells with a macrophage phenotype were a noticeable feature in the perivascular region of collector channels and the limbal episcleral veins. Some macrophage and DC-like cells were observed in intimate association with limbal mast cells. Theories on the mechanisms of Anterior Chamber Associated Immune Deviation (ACAID) have assumed APCs are largely absent from the tissues lining the anterior chamber. Our findings of a low but moderate density of putative APCs in the conventional and non-conventional aqueous humour outflow pathways are discussed in relation to the various theories of ACAID.

Immunological aspects of corneal graft rejection

Immunological graft rejection is a major cause of corneal graft failure. HLA class I and II antigens are expressed by various cells within the cornea and during sensitisation of the recipient donor antigens appear to be presented by both donor and recipient antigen presenting cells. Certain donor and host factors have been shown to influence the incidence of corneal graft rejection, and the manipulation of these factors is discussed.

The Montgomery Lecture. Some factors which affect the visual outcome of corneal transplantation

Corneal transplantation, which began in Dublin early in the nineteenth century, is now widely practised. Most grafts are done for visual reasons, some to manage corneal destructive disease. The factors associated with allograft rejection are well known, but a clear graft does not mean that the transplantation procedure has been successful. Successful corneal grafts enable patients to improve their life-style. To achieve this, the graft must be transparent and free of optical aberrations, the eye must be capable of achieving good vision, and the patient must have a life-style which would benefit from a successful functioning graft. A study of patients who have had corneal grafts identified the presence of a functioning graft, the lack of need for a contact lens, and the achievement of visual acuity in the operated eye which exceeds that of the contralateral eye, as requirements for patient satisfaction.

The corneal endothelium

The endothelium is a monolayer of cells on the posterior corneal surface that transports water from the stroma into the anterior chamber. This movement of water counters a natural tendency for the stroma to swell and is necessary to maintain a transparent cornea. Embryologic studies, in particular the demonstration of the derivation of the endothelium from the neural crest, have provided insight into the factors that govern the response of this tissue to disease. In some species the endothelium can regenerate after injury, but in man cellular enlargement is the main mechanism of repair after cell loss. A clinical estimate of endothelial cell density and function is provided by specular microscopy, fluorophotometry and pachymetry. In this paper we review the development, structure and function of the corneal endothelium, and then consider the pathological processes that can affect this tissue.