Descemet's Membrane Modulation of Posterior Corneal Fibrosis

Corneal fibrosis: From in vitro models to current and upcoming drug and gene medicines

Fibrotic diseases are characterised by myofibroblast differentiation, uncontrolled pathological extracellular matrix accumulation, tissue contraction, scar formation and, ultimately tissue / organ dysfunction. The cornea, the transparent tissue located on the anterior chamber of the eye, is extremely susceptible to fibrotic diseases, which cause loss of corneal transparency and are often associated with blindness. Although topical corticosteroids and antimetabolites are extensively used in the management of corneal fibrosis, they are associated with glaucoma, cataract formation, corneoscleral melting and infection, imposing the need of far more effective therapies. Herein, we summarise and discuss shortfalls and recent advances in in vitro models (e.g. transforming growth factor-β (TGF-β) / ascorbic acid / interleukin (IL) induced) and drug (e.g. TGF-β inhibitors, epigenetic modulators) and gene (e.g. gene editing, gene silencing) therapeutic strategies in the corneal fibrosis context. Emerging therapeutical agents (e.g. neutralising antibodies, ligand traps, receptor kinase inhibitors, antisense oligonucleotides) that have shown promise in clinical setting but have not yet assessed in corneal fibrosis context are also discussed.

Evaluation of biophysical alterations in the epithelial and endothelial layer of patients with Bullous Keratopathy

The cornea is a fundamental ocular tissue for the sense of sight. Thanks to it, the refraction of two-thirds of light manages to participate in the visual process and protect against mechanical damage. Because it is transparent, avascular, and innervated, the cornea comprises five main layers: Epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. Each layer plays a key role in the functionality and maintenance of ocular tissue, providing unique ultrastructural and biomechanical properties. Bullous Keratopathy (BK) is an endothelial dysfunction that leads to corneal edema, loss of visual acuity, epithelial blisters, and severe pain, among other symptoms. The corneal layers are subject to changes in their biophysical properties promoted by Keratopathy. In this context, the Atomic Force Microscopy (AFM) technique in air was used to investigate the anterior epithelial surface and the posterior endothelial surface, healthy and with BK, using a triangular silicone tip with a nominal spring constant of 0.4 N/m. Six human corneas (n = 6) samples were used for each analyzed group. Roughness data, calculated by third-order polynomial adjustment, adhesion, and Young's modulus, were obtained to serve as a comparison and identification of morphological and biomechanical changes possibly associated with the pathology, such as craters and in the epithelial layer and exposure of a fibrotic layer due to loss of the endothelial cell wall. Endothelial cell membrane area and volume data were calculated, obtaining a relevant comparison between the control and patient. Such results may provide new data on the physical properties of the ocular tissue to understand the physiology of the cornea when it has pathology.

Safety and efficacy of human ESC-derived corneal endothelial cells for corneal endothelial dysfunction

BACKGROUND

Research on human pluripotent stem cells (hPSCs) has shown tremendous progress in cell-based regenerative medicine. Corneal endothelial dysfunction is associated with the loss and degeneration of corneal endothelial cells (CECs), rendering cell replacement a promising therapeutic strategy. However, comprehensive preclinical assessments of hPSC-derived CECs for this cell therapy remain a challenge.

RESULTS

Here we defined an adapted differentiation protocol to generate induced corneal endothelial cells (iCECs) consistently and efficiently from clinical-grade human embryonic stem cells (hESCs) with xeno-free medium and manufactured cryopreserved iCECs. Cells express high levels of typical CECs markers and exhibit transendothelial potential properties in vitro typical of iCECs. After rigorous quality control measures, cells meeting all release criteria were available for in vivo studies. We found that there was no overgrowth or tumorigenicity of grafts in immunodeficient mice. After grafting into rabbit models, the surviving iCECs ameliorated edema and recovered corneal opacity.

CONCLUSIONS

Our work provides an efficient approach for generating iCECs and demonstrates the safety and efficacy of iCECs in disease modeling. Therefore, clinical-grade iCECs are a reliable source for future clinical treatment of corneal endothelial dysfunction.

The corneal fibroblast: The Dr. Jekyll underappreciated overseer of the responses to stromal injury

PURPOSE

To review the functions of corneal fibroblasts in wound healing.

METHODS

Literature review.

RESULTS

Corneal fibroblasts arise in the corneal stroma after anterior, posterior or limbal injuries and are derived from keratocytes. Transforming growth factor (TGF) β1 and TGFβ2, along with platelet-derived growth factor (PDGF), are the major modulators of the keratocyte to corneal fibroblast transition, while fibroblast growth factor (FGF)-2, TGFβ3, and retinoic acid are thought to regulate the transition of corneal fibroblasts back to keratocytes. Adequate and sustained levels of TGFβ1 and/or TGFβ2, primarily from epithelium, tears, aqueous humor, and corneal endothelium, drive the development of corneal fibroblasts into myofibroblasts. Myofibroblasts have been shown in vitro to transition back to corneal fibroblasts, although apoptosis of myofibroblasts has been documented as a major contributor to the resolution of fibrosis in several in situ corneal injury models. Corneal fibroblasts, aside from their role as a major progenitor to myofibroblasts, also perform many critical functions in the injured cornea, including the production of critical basement membrane (BM) components during regeneration of the epithelial BM and Descemet's membrane, production of non-basement membrane-associated stromal collagen type IV to control and downregulate TGFβ effects on stromal cells, release of chemotactic chemokines that attract bone marrow-derived cells to the injured stroma, production of growth factors that modulate regeneration and maturation of the overlying epithelium, and production of collagens and other ECM components that contribute to stromal integrity after injury.

CONCLUSIONS

Corneal fibroblasts are major contributors to and overseers of the corneal response to injuries.

Basement membrane regeneration and TGF-β1 expression in rabbits with corneal perforating injury

Purpose

To evaluate the relationship between basement membrane (BM) regeneration and the spatiotemporal expression of TGF-β1 during wound healing in rabbits with corneal perforating injury.

Methods

Forty-two rabbits were randomly allocated into 7 experimental groups, with 6 rabbits per group at each time point. The central cornea of the left eye was injured with 2.0 mm trephine to establish the perforating injury model. Six rabbits that received no treatment were used as controls. The cornea was evaluated at 3 days, 1-3 weeks, and 1-3 months after injury with a slit lamp for haze levels. Real-time quantitative polymerase chain reaction (qRT-PCR) was performed to quantify the relative expression of TGF-β1 and α-SMA mRNA. Immunofluorescence (IF) was used to assess TGF-β1 and alpha-smooth actin (α-SMA) expression and localization. BM regeneration was assessed using transmission electron microscopy (TEM).

Results

After injury, dense haze appeared at 1 month and then gradually faded. The relative expression of TGF-β1 mRNA peaked at 1 week and then decreased until 2 months. The relative α-SMA mRNA expression reached its peak at 1 week, then reached a small peak again at 1 month. IF results showed that TGF-β1 was initially detected in the fibrin clot at 3 days and then in the entire repairing stroma at 1 week. TGF-β1 localization gradually diminished from the anterior region to the posterior region at 2 weeks to 1 month, and it was nearly absent at 2 months. The myofibroblast marker α-SMA was observed in the entire healing stroma at 2 weeks. Localization of α-SMA gradually disappeared from the anterior region at 3 weeks to 1 month, remaining only in the posterior region at 2 months and disappearing at 3 months. Defective epithelial basement membrane (EBM) was first detected at 3 weeks after injury, then gradually repaired, and was nearly regenerated at 3 months. A thin and uneven Descemet's membrane (DM) was initially detected at 2 months after injury, then gradually regenerated to some extent, but remained abnormal at 3 months.

Conclusions

In the rabbit corneal perforating injury model, EBM regeneration was observed earlier than DM. At 3 months, complete EBM regeneration was observed, while the regenerated DM was still defective. TGF-β1 was distributed throughout the entire wound area in the early stages and then decreased from the anterior to the posterior region. α-SMA exhibited a similar temporospatial expression to TGF-β1. EBM regeneration may play a key role in low expression of TGF-β1 and α-SMA in the anterior stroma. Meanwhile, incomplete DM regeneration may contribute to the sustained expression of TGF-β1 and α-SMA in the posterior stroma.

Topical Losartan: Practical Guidance for Clinical Trials in the Prevention and Treatment of Corneal Scarring Fibrosis and Other Eye Diseases and Disorders

Losartan is an angiotensin II receptor blocker (ARB) that impedes transforming growth factor (TGF) beta signaling by inhibiting activation of signal transduction molecule extracellular signal-regulated kinase (ERK). Studies supported the efficacy of topical losartan in decreasing scarring fibrosis after rabbit Descemetorhexis, alkali burn, and photorefractive keratectomy injuries, and in case reports of humans with scarring fibrosis after surgical complications. Clinical studies are needed to explore the efficacy and safety of topical losartan in the prevention and treatment of corneal scarring fibrosis, and other eye diseases and disorders where TGF beta has a role in pathophysiology. These include scarring fibrosis associated with corneal trauma, chemical burns, infections, surgical complications, and persistent epithelial defects, as well as conjunctival fibrotic diseases, such as ocular cicatricial pemphigoid and Stevens-Johnson syndrome. Research is also needed to explore the efficacy and safety of topical losartan for hypothesized treatment of transforming growth factor beta-induced (TGFBI)-related corneal dystrophies (Reis-Bu¨cklers corneal dystrophy, lattice corneal dystrophy type 1, and granular corneal dystrophies type 1 and type 2) where deposited mutant protein expression is modulated by TGF beta. Investigations could also explore the efficacy and safety of topical losartan treatments to reduce conjunctival bleb scarring and shunt encapsulation following glaucoma surgical procedures. Losartan and sustained release drug delivery devices could be efficacious in treating intraocular fibrotic diseases. Dosing suggestions and precautions that should be considered in trials of losartan are detailed. Losartan, as an adjuvant to current treatments, has the potential to augment pharmacological therapeutics for many ocular diseases and disorders where TGF beta plays a central role in pathophysiology.

Standardization of corneal alkali burn methodology in rabbits

Alkali burns are one of the most common injuries used in corneal wound healing studies. Investigators have used different conditions to produce corneal alkali injuries that have varied in sodium hydroxide concentration, application methods, and duration of exposure. A critical factor in the subsequent corneal healing responses, including myofibroblast generation and fibrosis localization, is whether, or not, Descemet's membrane and the endothelium are injured during the initial exposure. After exposures that produce injuries confined to the epithelium and stroma, anterior stromal myofibroblasts and fibrosis are typical, with sparing of the posterior stroma. However, if there is also injury to Descemet's membrane and the endothelium, then myofibroblast generation and fibrosis is noted full corneal thickness, with predilection to the most anterior and most posterior stroma and a tendency for relative sparring of the central stroma that is likely related to the availability of TGF beta from the tears, epithelium, and the aqueous humor. A method is described where a 5 mm diameter circle of Whatman #1 filter paper wetted with only 30 μL of alkali solution is applied for 15 s prior to profuse irrigation in rabbit corneas. When 0.6N, or lower, NaOH is used, then the injury, myofibroblasts, and fibrosis generation are limited to the epithelium and stroma. Use of 0.75N NaOH triggers injury to Descemet's membrane and the corneal endothelium with fibrosis throughout the stroma, but rare corneal neovascularization (CNV) and persistent epithelial defects (PED). Use of 1N NaOH with this method produces greater stromal fibrosis and increased likelihood that CNV and PED will occur in individual corneas.

The Yin and Yang of Mesenchymal Cells in the Corneal Stromal Fibrosis Response to Injury: The Cornea as a Model of Fibrosis in Other Organs

Mesenchymal cells (keratocytes, corneal fibroblasts, and myofibroblasts), as well as mesenchymal progenitor bone marrow-derived fibrocytes, are the major cellular contributors to stromal fibrosis after injury to the cornea. Corneal fibroblasts, in addition to being major progenitors to myofibroblasts, also have anti-fibrotic functions in (1) the production of non-basement membrane collagen type IV that binds activated transforming growth factor (TGF) beta-1 and TGF beta-2 to downregulate TGF beta effects on cells in the injured stroma, (2) the production of chemokines that modulate the entry of bone marrow-derived cells into the stroma, (3) the production of hepatocyte growth factor and keratinocyte growth factor to regulate corneal epithelial healing, (4) the cooperation with the epithelium or corneal endothelium in the regeneration of the epithelial basement membrane and Descemet's membrane, and other functions. Fibrocytes also serve as major progenitors to myofibroblasts in the corneal stroma. Thus, mesenchymal cells and mesenchymal cell progenitors serve Yin and Yang functions to inhibit and promote tissue fibrosis depending on the overall regulatory milieu within the injured stroma.

Losartan Inhibition of Myofibroblast Generation and Late Haze (Scarring Fibrosis) After PRK in Rabbits

PURPOSE

To study the effect of topical losartan compared to vehicle on the generation of myofibroblasts and development of late haze scarring fibrosis after photorefractive keratectomy (PRK) in rabbits.

METHODS

Twelve rabbits had -9.00 diopter (D) PRK in one eye followed by 50 µL of topical 0.2 mg/mL losartan or 50 µL of vehicle six times per day for 1 month. Standardized slit-lamp photographs were obtained prior to death. Duplex immunohistochemistry was performed on cryofixed corneas for myofibroblast marker alpha-smooth muscle actin (α-SMA) and keratocyte marker keratocan or collagen type IV and transforming growth factor (TGF)-β1. ImageJ software (National Institutes of Health) was used for quantitation.

RESULTS

Topical losartan compared to vehicle significantly decreased corneal opacity (P = .04) and anterior stromal myofibroblast generation (P = .01) at 1 month after PRK. Topical losartan compared to vehicle also decreased anterior stromal non-basement membrane collagen type IV at 1 month after PRK (P = .004).

CONCLUSIONS

Topical angiotensin converting enzyme II receptor inhibitor losartan, a known inhibitor of TGF-β signaling, decreased late haze scarring fibrosis and myofibroblast generation after -9.00 D PRK in rabbits compared to vehicle. It also decreases TGF-β-modulated, corneal fibroblast-produced, non-basement membrane stromal collagen type IV-likely also through inhibition of TGF-β signaling. [J Refract Surg. 2022;38(12):820-829.].

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.

Defective perlecan-associated basement membrane regeneration and altered modulation of transforming growth factor beta in corneal fibrosis

In the cornea, the epithelial basement membrane (EBM) and corneal endothelial Descemet's basement membrane (DBM) critically regulate the localization, availability and, therefore, the functions of transforming growth factor (TGF)β1, TGFβ2, and platelet-derived growth factors (PDGF) that modulate myofibroblast development. Defective regeneration of the EBM, and notably diminished perlecan incorporation, occurs via several mechanisms and results in excessive and prolonged penetration of pro-fibrotic growth factors into the stroma. These growth factors drive mature myofibroblast development from both corneal fibroblasts and bone marrow-derived fibrocytes, and then the persistence of these myofibroblasts and the disordered collagens and other matrix materials they produce to generate stromal scarring fibrosis. Corneal stromal fibrosis often resolves completely if the inciting factor is removed and the BM regenerates. Similar defects in BM regeneration are likely associated with the development of fibrosis in other organs where perlecan has a critical role in the modulation of signaling by TGFβ1 and TGFβ2. Other BM components, such as collagen type IV and collagen type XIII, are also critical regulators of TGF beta (and other growth factors) in the cornea and other organs. After injury, BM components are dynamically secreted and assembled through the cooperation of neighboring cells-for example, the epithelial cells and keratocytes for the corneal EBM and corneal endothelial cells and keratocytes for the corneal DBM. One of the most critical functions of these reassembled BMs in all organs is to modulate the pro-fibrotic effects of TGFβs, PDGFs and other growth factors between tissues that comprise the organ.

Fibrosis Is a Basement Membrane-Related Disease in the Cornea: Injury and Defective Regeneration of Basement Membranes May Underlie Fibrosis in Other Organs

Every organ develops fibrosis that compromises functions in response to infections, injuries, or diseases. The cornea is a relatively simple, avascular organ that offers an exceptional model to better understand the pathophysiology of the fibrosis response. Injury and defective regeneration of the epithelial basement membrane (EBM) or the endothelial Descemet's basement membrane (DBM) triggers the development of myofibroblasts from resident corneal fibroblasts and bone marrow-derived blood borne fibrocytes due to the increased entry of TGF beta-1/-2 into the stroma from the epithelium and tears or residual corneal endothelium and aqueous humor. The myofibroblasts, and disordered extracellular matrix these cells produce, persist until the source of injury is removed, the EBM and/or DBM are regenerated, or replaced surgically, resulting in decreased stromal TGF beta requisite for myofibroblast survival. A similar BM injury-related pathophysiology can underly the development of fibrosis in other organs such as skin and lung. The normal liver does not contain traditional BMs but develops sinusoidal endothelial BMs in many fibrotic diseases and models. However, normal hepatic stellate cells produce collagen type IV and perlecan that can modulate TGF beta localization and cognate receptor binding in the space of Dissé. BM-related fibrosis is deserving of more investigation in all organs.

Corneal Opacity: Cell Biological Determinants of the Transition From Transparency to Transient Haze to Scarring Fibrosis, and Resolution, After Injury

Purpose

To highlight the cellular, matrix, and hydration changes associated with opacity that occurs in the corneal stroma after injury.

Methods

Review of the literature.

Results

The regulated transition of keratocytes to corneal fibroblasts and myofibroblasts, and of bone marrow-derived fibrocytes to myofibroblasts, is in large part modulated by transforming growth factor beta (TGFβ) entry into the stroma after injury to the epithelial basement membrane (EBM) and/or Descemet's membrane. The composition, stoichiometry, and organization of the stromal extracellular matrix components and water is altered by corneal fibroblast and myofibroblast production of large amounts of collagen type I and other extracellular matrix components-resulting in varying levels of stromal opacity, depending on the intensity of the healing response. Regeneration of EBM and/or Descemet's membrane, and stromal cell production of non-EBM collagen type IV, reestablishes control of TGFβ entry and activity, and triggers TGFβ-dependent myofibroblast apoptosis. Eventually, corneal fibroblasts also disappear, and repopulating keratocytes reorganize the disordered extracellular matrix to reestablish transparency.

Conclusions

Injuries to the cornea produce varying amounts of corneal opacity depending on the magnitude of cellular and molecular responses to injury. The EBM and Descemet's membrane are key regulators of stromal cellularity through their modulation of TGFβ. After injury to the cornea, depending on the severity of the insult, and possibly genetic factors, trace opacity to severe scarring fibrosis develops. Stromal cellularity, and the functions of different cell types, are the major determinants of the level of the stromal opacity.

Descemet's membrane injury and regeneration, and posterior corneal fibrosis, in rabbits

The purpose of this investigation was to study Descemet's membrane and corneal endothelial regeneration, myofibroblast generation and disappearance, and TGF beta-1 localization after Descemet's membrane-endothelial excision (Descemetorhexis) in rabbits. Thirty-six rabbits had 8 mm Descemetorhexis and standardized slit lamp photos at 1, 2 and 4 days, 1, 2 and 4 weeks, and 2, 4 and 6 months, as well as multiplex IHC for stromal cell markers keratocan, vimentin, and alpha-smooth muscle actin (SMA); basement membrane (BM) components perlecan, nidogen-1, laminin alpha-5, and collagen type IV; and corneal endothelial marker Na,K-ATPase β1, and TGF beta-1, with ImageJ quantitation. Stromal transparency increased from the periphery beginning at two months after injury and progressed into the central cornea by six months. At six months, central transparency was primarily limited by persistent mid-stromal neovascularization. Stromal myofibroblast zone thickness in the posterior stroma peaked at one month after injury, and then progressively decreased until to six months when few myofibroblasts remained. The regeneration of a laminin alpha-5 and nidogen-1 Descemet's membrane "railroad track" structure was accompanied by corneal endothelial closure and stromal cell production of BM components in corneas from four to six months after injury. TGF beta-1 deposition at the posterior corneal surface from the aqueous humor peaked at one day after Descemetorhexis and diminished even before regeneration of the endothelium and Descemet's membrane. This decrease was associated with collagen type IV protein production by corneal fibroblasts, and possibly myofibroblasts, in the posterior stroma. Descemet's membrane and the corneal endothelium regenerated in the rabbit cornea by six months after eight mm Descemetorhexis. Real-time quantitative RT-PCR experiments in vitro with marker-verified rabbit corneal cells found that 5 ng/ml or 10 ng/ml TGF beta-1 upregulated col4a1 or col4a2 mRNA expression after 6 h or 12 h of exposure in corneal fibroblasts, but not in myofibroblasts. Stromal cells produced large amounts of collagen type IV that likely decreased TGF beta-1 penetration into the stroma and facilitated the resolution of myofibroblast-generated fibrosis.

Human platelet lysate as a replacement for fetal bovine serum in human corneal stromal keratocyte and fibroblast culture

The isolation and propagation of primary human corneal stromal keratocytes (CSK) are crucial for cellular research and corneal tissue engineering. However, this delicate cell type easily transforms into stromal fibroblasts (SF) and scar inducing myofibroblasts (Myo‐SF). Current protocols mainly rely on xenogeneic fetal bovine serum (FBS). Human platelet lysate (hPL) could be a viable, potentially autologous, alternative. We found high cell survival with both supplements in CSK and SF. Cell numbers and Ki67+ ratios increased with higher fractions of hPL and FBS in CSK and SF. We detected a loss in CSK marker expression (Col8A2, ALDH3A1 and LUM) with increasing fractions of FBS and hPL in CSK and SF. The expression of the Myo‐SF marker SMA increased with higher amounts of FBS but decreased with incremental hPL substitution in both cell types, implying an antifibrotic effect of hPL. Immunohistochemistry confirmed the RT‐PCR findings. bFGF and HGF were only found in hPL and could be responsible for suppressing the Myo‐SF conversion. Considering all findings, we propose 0.5% hPL as a suitable substitution in CSK culture, as this xeno‐free component efficiently preserved CSK characteristics, with non‐inferiority in terms of cell viability, cell number and proliferation in comparison to the established 0.5% FBS protocol.

Foundational concepts in the biology of bacterial keratitis

Bacterial infections of the cornea, or bacterial keratitis (BK), are notorious for causing rapidly fulminant disease and permanent vision loss, even among treated patients. In the last sixty years, dramatic upward trajectories in the frequency of BK have been observed internationally, driven in large part by the commercialization of hydrogel contact lenses in the late 1960s. Despite this worsening burden of disease, current evidence-based therapies for BK - including broad-spectrum topical antibiotics and, if indicated, topical corticosteroids - fail to salvage vision in a substantial proportion of affected patients. Amid growing concerns of rapidly diminishing antibiotic utility, there has been renewed interest in urgently needed novel treatments that may improve clinical outcomes on an individual and public health level. Bridging the translational gap in the care of BK requires the identification of new therapeutic targets and rational treatment design, but neither of these aims can be achieved without understanding the complex biological processes that determine how bacterial corneal infections arise, progress, and resolve. In this chapter, we synthesize the current wealth of human and animal experimental data that now inform our understanding of basic BK pathophysiology, in context with modern concepts in ocular immunology and microbiology. By identifying the key molecular determinants of clinical disease, we explore how novel treatments can be developed and translated into routine patient care.

[Corneal wound healing-Pathophysiology and principles]

The cornea forms the anterior border of the eye and significantly contributes to a sharp optical image quality on the retina through its transparency, avascular nature and curvature. Because of its anatomical structure and as a barrier to the environment, the cornea is particularly exposed to various external factors, such as injuries and pathogens. A correct wound healing without the formation of light diverging scarring is therefore essential to preserve the integrity and function of the cornea. Misguided wound healing is of outstanding clinical relevance and can lead to corneal fibrogenesis. Corneal fibrosis results in scarring with a loss of optical transparency, which significantly reduces eyesight and can lead to blindness. Understanding the pathophysiological mechanisms of wound healing and fibrogenesis is of great importance for the diagnostics, treatment and evaluation of the subsequent healing process in order to prevent permanent damage as far as possible.

TGF beta -1, -2 and -3 in the modulation of fibrosis in the cornea and other organs

The TGF beta-1, -2 and -3 isoforms are transcribed from different genes but bind to the same receptors and signal through the same canonical and non-canonical signal transduction pathways. There are numerous regulatory mechanisms controlling the action of each isoform that include the organ-specific cells producing latent TGF beta growth factors, multiple effectors that activate the isoforms, ECM-associated SLRPs and basement membrane components that modulate the activity and localization of the isoforms, other interactive cytokine-growth factor receptor systems, such as PDGF and CTGF, TGF beta receptor expression on target cells, including myofibroblast precursors, receptor binding competition, positive and negative signal transduction effectors, and transcription and translational regulatory mechanisms. While there has long been the view that TGF beta-1and TGF beta-2 are pro-fibrotic, while TGF beta-3 is anti-fibrotic, this review suggests that view is too simplistic, at least in adult tissues, since TGF beta-3 shares far more similarities in its modulation of fibrotic gene expression with TGF beta-1 and TGF beta-2, than it does differences, and often the differences are subtle. Rather, TGF beta-3 should be seen as a fibro-modulatory partner to the other two isoforms that modulates a nuanced and better controlled response to injury. The complex interplay between the three isoforms and numerous interactive proteins, in the context of the cellular milieu, controls regenerative non-fibrotic vs. fibrotic healing in a response to injury in a particular organ, as well as the resolution of fibrosis, when that occurs.

Interleukin-1 and Transforming Growth Factor Beta: Commonly Opposing, but Sometimes Supporting, Master Regulators of the Corneal Wound Healing Response to Injury

Purpose

Interleukin (IL)-1α/IL-1β and transforming growth factor (TGF)β1/TGFβ2 have both been promoted as “master regulators” of the corneal wound healing response due to the large number of processes each regulates after injury or infection. The purpose of this review is to highlight the interactions between these systems in regulating corneal wound healing.

Methods

We conducted a systematic review of the literature.

Results

Both regulator pairs bind to receptors expressed on keratocytes, corneal fibroblasts, and myofibroblasts, as well as bone marrow-derived cells that include fibrocytes. IL-1α and IL-1β modulate healing functions, such as keratocyte apoptosis, chemokine production by corneal fibroblasts, hepatocyte growth factor (HGF), and keratinocyte growth factor (KGF) production by keratocytes and corneal fibroblasts, expression of metalloproteinases and collagenases by corneal fibroblasts, and myofibroblast apoptosis. TGFβ1 and TGFβ2 stimulate the development of myofibroblasts from keratocyte and fibrocyte progenitor cells, and adequate stromal levels are requisite for the persistence of myofibroblasts. Conversely, TGFβ3, although it functions via the same TGF beta I and II receptors, may, at least in some circumstances, play a more antifibrotic role—although it also upregulates the expression of many profibrotic genes.

Conclusions

The overall effects of these two growth factor-cytokine-receptor systems in controlling the corneal wound healing response must be coordinated during the wound healing response to injury or infection. The activities of both systems must be downregulated in coordinated fashion to terminate the response to injury and eliminate fibrosis.

Translational Relevance

A better standing of the IL-1 and TGFβ systems will likely lead to better approaches to control the excessive healing response to infections and injuries leading to scarring corneal fibrosis.

Immunology and Pathology in Ocular Drug Development

Clear vision is dependent on features that protect the anatomical integrity of the eye (cornea and sclera) and those that contribute to internal ocular homeostasis by conferring hemangiogenic (avascular tissues and antiangiogenic factors), lymphangiogenic (lack of draining lymphatics), and immunologic (tight junctions that form blood-ocular barriers, immunosuppressive cells, and modulators) privileges. The later examples are necessary components that enable the eye to maintain an immunosuppressive environment that responds to foreign invaders in a deviated manner, minimizing destructive inflammation that would impair vision. These conditions allowed for the observations made by Medawar, in 1948, of delayed rejection of allogenic tissue grafts in the anterior chamber of mouse eye and permit the sequestration of foreign invaders (eg, Toxoplasma gondii) within the retina of healthy individuals. Yet successful development of intraocular drugs (biologics and delivery devices) has been stymied by adverse ocular pathology, much of which is driven by immune pathways. The eye can be intolerant of foreign protein irrespective of delivery route, and endogenous ocular cells have remarkable plasticity when recruited to preserve visual function. This article provides a review of current understanding of ocular immunology and the potential role of immune mechanisms in pathology observed with intraocular drug delivery.

TGFβ1 and TGFβ2 proteins in corneas with and without stromal fibrosis: Delayed regeneration of apical epithelial growth factor barrier and the epithelial basement membrane in corneas with stromal fibrosis

The purpose of this study was to investigate the expression and localization of transforming growth factor (TGF) β1 and TGFβ2 in rabbit corneas that healed with and without stromal fibrosis, and to further study defective perlecan incorporation in the epithelial basement membrane (EBM) in corneas with scarring fibrosis. A total of 120 female rabbits had no surgery, -4.5D PRK, or -9D PRK. Immunohistochemistry (IHC) was performed at time points from unwounded to eight weeks after surgery, with four corneas at each time point in each group. Multiplex IHC was performed for TGFβ1 or TGFβ2, with Image-J quantitation, and keratocan, vimentin, alpha-smooth muscle actin (SMA), perlecan, laminin-alpha 5, nidogen-1 or CD11b. Corneas at the four-week peak for myofibroblast and fibrosis development were evaluated using Imaris 3D analysis. Delayed regeneration of both an apical epithelial growth factor barrier and EBM barrier function, including defective EBM perlecan incorporation, was greater in high injury -9D PRK corneas compared to -4.5D PRK corneas without fibrosis. Defective apical epithelial growth factor barrier and EBM allowed epithelial and tear TGFβ1 and tear TGFβ2 to enter the corneal stroma to drive myofibroblast generation in the anterior stroma from vimentin-positive corneal fibroblasts, and likely fibrocytes. Vimentin-positive cells and unidentified vimentin-negative, CD11b-negative cells also produce TGFβ1 and/or TGFβ2 in the stroma in some corneas. TGFβ1 and TGFβ2 were at higher levels in the anterior stroma in the weeks preceding myofibroblast development in the -9D group. All -9D corneas (beginning two to three weeks after surgery), and four -4.5D PRK corneas developed significant SMA + myofibroblasts and stromal fibrosis. Both the apical epithelial growth factor barrier and/or EBM barrier functions tended to regenerate weeks earlier in -4.5D PRK corneas without fibrosis, compared to -4.5D or -9D PRK corneas with fibrosis. SMA-positive myofibroblasts were markedly reduced in most corneas by eight weeks after surgery. The apical epithelial growth factor barrier and EBM barrier limit TGFβ1 and TGFβ2 entry into the corneal stroma to modulate corneal fibroblast and myofibroblast development associated with scarring stromal fibrosis. Delayed regeneration of these barriers in corneas with more severe injuries promotes myofibroblast development, prolongs myofibroblast viability and triggers stromal scarring fibrosis.

Fibroblastic and bone marrow-derived cellularity in the corneal stroma

The unwounded, normal corneal stroma is a relatively simple, avascular tissue populated with quiescent keratocytes, along with corneal nerves and a few resident dendritic and monocyte/macrophage cells. In the past, the resting keratocytes were thought of as a homogenous cellular population, but recent work has shown local variations in vimentin and nestin expression, and responsiveness to transforming growth factor (TGF)-β1. Studies have also supported there being "stromal stem cells" in localized areas. After corneal wounding, depending on the site and severity of injury, profound changes in stromal cellularity occur. Anterior or posterior injuries to the epithelium or endothelium, respectively, trigger apoptosis of adjacent keratocytes. Many contiguous keratocytes transition to keratocan-negative corneal fibroblasts that are proliferative and produce limited amounts of disorganized extracellular matrix components. Simultaneously, large numbers of bone marrow-derived cells, including monocytes, neutrophils, fibrocytes and lymphocytes, invade the stroma from the limbal blood vessels. Ongoing adequate levels of TGFβ1, TGFβ2 and platelet-derived growth factor (PDGF) from epithelium, tears, endothelium and aqueous humor that penetrate defective or absent epithelial barrier function (EBF) and epithelial basement membrane (EBM) and/or Descemet's basement membrane (DBM) drive corneal fibroblasts and fibrocytes to differentiate into alpha-smooth muscle actin (SMA)-positive myofibroblasts. If the EBF, EBM and/or DBM are repaired or replaced in a timely manner, typically measured in weeks, then corneal fibroblast and fibrocyte progeny, deprived of requisite levels of TGFβ1 and TGFβ2, undergo apoptosis or revert to their precursor cell-types. If the EBF, EBM and/or DBM are not repaired or replaced, stromal levels of TGFβ1 and TGFβ2 remain elevated, and mature myofibroblasts are generated from corneal fibroblasts and fibrocyte precursors that produce prodigious amounts of disordered extracellular matrix materials associated with scarring fibrosis. This fibrotic stromal matrix persists, at least until the EBF, EBM and/or DBM are regenerated or replaced, and keratocytes remove and reorganize the affected stromal matrix.

Corneal myofibroblasts and fibrosis

Myofibroblasts are alpha-smooth muscle actin (SMA)+ cells that have a critical role in the corneal stromal response to infections, injuries, and surgeries, and which produce corneal scarring fibrosis when they develop in excess. These contractile and opaque cells-produce large amounts of disordered extracellular matrix (ECM)-and develop from keratocyte-derived corneal fibroblasts or bone marrow-derived fibrocytes, and possibly other cell types, in response to TGFβ1, TGFβ2 and PDGF from the epithelium, tears, endothelium, and other stromal cells. Recent proteomic analyses have revealed that the myofibroblasts that develop from different progenitors aren't interchangeable, but have major differences in protein expression and functions. Absence or defective regeneration of the epithelial basement membrane (EBM) and/or Descemet's basement membrane (DBM) results in development and persistence of myofibroblasts in the corneal stroma. The functions of myofibroblasts in the cornea include production of volume-additive ECM, tissue contraction, production of various growth factors, cytokines and chemokines that regulate stromal cells, including other myofibroblasts, production of collagenases and metalloproteinases involved in tissue remodeling, and the expression of toll-like receptors that likely have critical roles in the clearance of bacteria and viruses causing corneal infections.

Corneal wound healing

The corneal wound healing response is typically initiated by injuries to the epithelium and/or endothelium that may also involve the stroma. However, it can also be triggered by immune or infectious processes that enter the stroma via the limbal blood vessels. For mild injuries or infections, such as epithelial abrasions or mild controlled microbial infections, limited keratocyte apoptosis occurs and the epithelium or endothelium regenerates, the epithelial basement membrane (EBM) and/or Descemet's basement membrane (DBM) is repaired, and keratocyte- or fibrocyte-derived myofibroblast precursors either undergo apoptosis or revert to the parent cell types. For more severe injuries with extensive damage to EBM and/or DBM, delayed regeneration of the basement membranes leads to ongoing penetration of the pro-fibrotic cytokines transforming growth factor (TGF) β1, TGFβ2 and platelet-derived growth factor (PDGF) that drive the development of mature alpha-smooth muscle actin (SMA)+ myofibroblasts that secrete large amounts of disordered extracellular matrix (ECM) components to produce scarring stromal fibrosis. Fibrosis is dynamic with ongoing mitosis and development of SMA + myofibroblasts and continued autocrine-or paracrine interleukin (IL)-1-mediated apoptosis of myofibroblasts and their precursors. Eventual repair of the EBM and/or DBM can lead to at least partial resolution of scarring fibrosis.

Descemet's membrane development, structure, function and regeneration

Basement membranes are layers of extracellular matrix which anchor the epithelium or endothelium to connective tissues in most organs. Descemet's membrane- which is the basement membrane for the corneal endothelium- is a dense, thick, relatively transparent and cell-free matrix that separates the posterior corneal stroma from the underlying endothelium. It was historically named Descemet's membrane after Jean Descemet, a French physician, but it is also known as the posterior limiting elastic lamina, lamina elastica posterior, and membrane of Demours. Normal Descemet's membrane ultrastructure in humans has been shown to consist of an interfacial matrix that attaches to the overlying corneal stroma, an anterior banded layer and a posterior non-banded layer-upon which corneal endothelial cells attach. These layers have been shown to have unique composition and morphology, and to contribute to corneal homeostasis and clarity, participate in the control of corneal hydration and to modulate TGF-β-induced posterior corneal fibrosis. Pathophysiological alterations of Descemet's membrane are noted in ocular diseases such as Fuchs' dystrophy, bullous keratopathy, keratoconus, primary congenital glaucoma (Haab's striae), as well as in systemic conditions. Unrepaired extensive damage to Descemet's membrane results in severe corneal opacity and vision loss due to stromal fibrosis, which may require penetrating keratoplasty to restore corneal transparency. The purpose of this article is to highlight the current understanding of Descemet's membrane structure, function and potential for regeneration.

Fibrocytes, Wound Healing, and Corneal Fibrosis

Purpose

This review highlights the roles of fibrocytes—their origin, markers, regulation and functions—including contributions to corneal wound healing and fibrosis.

Methods

Literature review.

Results

Peripheral blood fibroblast-like cells, called fibrocytes, are primarily generated as mature collagen-producing cells in the bone marrow. They are likely derived from the myeloid lineage, although the exact precursor remains unknown. Fibrocytes are identified by a combination of expressed markers, such as simultaneous expression of CD34 or CD45 or CD11b and collagen type I or collagen type III. Fibrocytes migrate into the wound from the blood where they participate in pathogen clearance, tissue regeneration, wound closure and angiogenesis. Transforming growth factor beta 1 (TGF-β₁) and adiponectin induce expression of α-smooth muscle actin and extracellular matrix proteins through activation of Smad3 and adenosine monophosphate-activated protein kinase pathways, respectively. Fibrocytes are important contributors to the cornea wound healing response and there are several mechanisms through which fibrocytes contribute to fibrosis in the cornea and other organs, such as their differentiation into myofibroblasts, production of matrix metalloproteinase, secretion of tissue inhibitor of metalloproteinase, and release of TGF-β₁. In some tissues, fibrocytes may also contribute to the basement membrane regeneration and to the resolution of fibrosis.

Conclusions

New methods that block fibrocyte generation, fibrocyte migration, and their differentiation into myofibroblasts, as well as their production of matrix metalloproteinases, tissue inhibitor of metalloproteinase, and TGF-β₁, have therapeutic potential to reduce the accumulation of collagens, maintain tissue integrity and retard or prevent the development of fibrosis.

Coordinated Modulation of Corneal Scarring by the Epithelial Basement Membrane and Descemet's Basement Membrane

PURPOSE

To provide an overview of the importance of the coordinated role of the epithelial basement membrane (EBM) and Descemet's basement membrane (DBM) in modulating scarring (fibrosis) in the cornea after injuries, infections, surgeries, and diseases of the cornea.

METHODS

Literature review.

RESULTS

Despite their molecular and ultrastructural differences, the EBM and DBM act in a coordinated fashion to modulate the entry of transforming growth factor beta (TGF-β) and other growth factors from the epithelium/tear film and aqueous humor, respectively, into the corneal stroma where persistent levels of these modulators trigger the development and persistence of myofibroblasts that produced disordered, opaque extracellular matrix not usually present in the corneal stroma. The development of these myofibroblasts and the extracellular matrix they produce is often detrimental to visual function of the cornea after penetrating keratoplasty, LASIK buttonhole flaps, persistent epithelial defects, microbial keratitis, Descemet stripping automated endothelial keratoplasty, or Descemet membrane endothelial keratoplasty, while being beneficial in other situations such as the scarred edge of LASIK flaps and donor-recipient interface in penetrating keratoplasty. Efforts to modulate the repair or replacement of the EBM and DBM, and thereby the development or disappearance of myofibroblasts, should be a major emphasis of treatments provided by refractive and corneal surgeries, infections, trauma, or diseases of the cornea.

CONCLUSIONS

The EBM and DBM are critical modulators of the localization of profibrotic growth factors, such as TGF-β, that modulate the development and persistence of myofibroblasts that produce corneal scars (stromal fibrosis). Therapeutic efforts to regenerate or repair EBM and/or DBM, and interfere with the development of myofibroblasts or facilitate their disappearance are often the key to clinical outcomes. [J Refract Surg. 2019;35(8):506-516.].