Prolonged Impairment of Corneal Innervation After Exposure to Sulfur Mustard and Its Relation to the Development of Delayed Limbal Stem Cell Deficiency

Cell therapy in the cornea: The emerging role of microenvironment

The cornea is an ideal testing field for cell therapies. Its highly ordered structure, where specific cell populations are sequestered in different layers, together with its accessibility, has allowed the development of the first stem cell-based therapy approved by the European Medicine Agency. Today, different techniques have been proposed for autologous and allogeneic limbal and non-limbal cell transplantation. Cell replacement has also been attempted in cases of endothelial cell decompensation as it occurs in Fuchs dystrophy: injection of cultivated allogeneic endothelial cells is now in advanced phases of clinical development. Recently, stromal substitutes have been developed with excellent integration capability and transparency. Finally, cell-derived products, such as exosomes obtained from different sources, have been investigated for the treatment of severe corneal diseases with encouraging results. Optimization of the success rate of cell therapies obviously requires high-quality cultured cells/products, but the role of the surrounding microenvironment is equally important to allow engraftment of transplanted cells, to preserve their functions and, ultimately, lead to restoration of tissue integrity and transparency of the cornea.

Animal Models for Limbal Stem Cell Deficiency: A Critical Narrative Literature Review

This literature review will provide a critical narrative overview of the highlights and potential pitfalls of the reported animal models for limbal stem cell deficiency (LSCD) and will identify the neglected aspects of this research area. There exists significant heterogeneity in the literature regarding the methodology used to create the model and the predefined duration after the insult when the model is supposedly fully fit for evaluations and/or for testing various therapeutic interventions. The literature is also replete with examples wherein the implementation of a specific model varies significantly across different studies. For example, the concentration of the chemical, as well as its duration and technique of exposure in a chemically induced LSCD model, has a great impact not only on the validity of the model but also on the severity of the complications. Furthermore, while some models induce a full-blown clinical picture of total LSCD, some are hindered by their ability to yield only partial LSCD. Another aspect to consider is the nature of the damage induced by a specific method. As thermal methods cause more stromal scarring, they may be better suited for assessing the anti-fibrotic properties of a particular treatment. On the other hand, since chemical burns cause more neovascularisation, they provide the opportunity to tap into the potential treatments for anti-neovascularisation. The animal species (i.e., rats, mice, rabbits, etc.) is also a crucial factor in the validity of the model and its potential for clinical translation, with each animal having its unique set of advantages and disadvantages. This review will also elaborate on other overlooked aspects, such as the anaesthetic(s) used during experiments, the gender of the animals, care after LSCD induction, and model validation. The review will conclude by providing future perspectives and suggestions for further developments in this rather important area of research.

Mouse Model of Nitrogen Mustard Ocular Surface Injury Characterization and Sphingolipid Signaling

Vesicating chemicals like sulfur mustard (SM) or nitrogen mustard (NM) can cause devastating damage to the eyes, skin, and lungs. Eyes, being the most sensitive, have complicated pathologies that can manifest immediately after exposure (acute) and last for years (chronic). No FDA-approved drug is available to be used as medical counter measures (MCMs) against such injuries. Understanding the pathological mechanisms in acute and chronic response of the eye is essential for developing effective MCMs. Here, we report the clinical and histopathological characterization of a mouse model of NM-induced ocular surface injury (entire surface) developed by treating the eye with 2% (w/v) NM solution for 5 min. Unlike the existing models of specific injury, our model showed severe ocular inflammation, including the eyelids, structural deformity of the corneal epithelium and stroma, and diminished visual and retinal functions. We also observed alterations of the inflammatory markers and their expression at different phases of the injury, along with an activation of acidic sphingomyelinase (aSMase), causing an increase in bioactive sphingolipid ceramide and a reduction in sphingomyelin levels. This novel ocular surface mouse model recapitulated the injuries reported in human, rabbit, and murine SM or NM injury models. NM exposure of the entire ocular surface in mice, which is similar to accidental or deliberate exposure in humans, showed severe ocular inflammation and caused irreversible alterations to the corneal structure and significant vision loss. It also showed an intricate interplay between inflammatory markers over the injury period and alteration in sphingolipid homeostasis in the early acute phase.

Predicting clinical outcome of sulfur mustard induced ocular injury using machine learning model

The sight-threatening sulfur mustard (SM) induced ocular injury presents specific symptoms in each clinical stage. The acute injury develops in all exposed eyes and may heal or deteriorate into chronic late pathology. Early detection of eyes at risk of developing late pathology may assist in providing unique monitoring and specific treatments only to relevant cases. In this study, we evaluated a machine-learning (ML) model for predicting the development of SM-induced late pathology based on clinical data of the acute phase in the rabbit model. Clinical data from 166 rabbit eyes exposed to SM vapor was used retrospectively. The data included a comprehensive clinical evaluation of the cornea, eyelids and conjunctiva using a semi-quantitative clinical score. A random forest classifier ML model, was trained to predict the development of corneal neovascularization four weeks post-ocular exposure to SM vapor using clinical scores recorded three weeks earlier. The overall accuracy in predicting the clinical outcome of SM-induced ocular injury was 73%. The accuracy in identifying eyes at risk of developing corneal neovascularization and future healed eyes was 75% and 59%, respectively. The most important parameters for accurate prediction were conjunctival secretion and corneal opacity at 1w and corneal erosions at 72 h post-exposure. Predicting the clinical outcome of SM-induced ocular injury based on the acute injury parameters using ML is demonstrated for the first time. Although the prediction accuracy was limited, probably due to the small dataset, it pointed out towards various parameters during the acute injury that are important for predicting SM-induced late pathology and revealing possible pathological mechanisms.

Insights into mustard gas keratopathy- characterizing corneal layer-specific changes in mice exposed to nitrogen mustard

Exposure to mustard agents, such as sulfur mustard (SM) and nitrogen mustard (NM), often results in ocular surface damage. This can lead to the emergence of various corneal disorders that are collectively referred to as mustard gas keratopathy (MGK). In this study, we aimed to develop a mouse model of MGK by using ocular NM exposure, and describe the subsequent structural changes analyzed across the different layers of the cornea. A 3 μL solution of 0.25 mg/mL or 5 mg/mL NM was applied to the center of the cornea via a 2-mm filter paper for 5 min. Mice were evaluated prior to and after exposure on days 1, 3, 7, 14, and 28 for 4 weeks using slit lamp examination with fluorescein staining. Anterior segment optical coherence tomography (AS-OCT) and in vivo confocal microscopy (IVCM) tracked changes in the epithelium, stroma, and endothelium of the cornea. Histologic evaluation was used to examine corneal cross-sections collected at the completion of follow-up. Following exposure, mice experienced central corneal epithelial erosion and thinning, accompanied by a decreased number of nerve branches in the subbasal plexus and increased activated keratocytes in the stroma in both dosages. The epithelium was recovered by day 3 in the low dose group, followed by exacerbated punctuate erosions alongside persistent corneal edema that arose and continued onward to four weeks post-exposure. The high dose group showed persistent epitheliopathy throughout the study. The endothelial cell density was reduced, more prominent in the high dose group, early after NM exposure, which persisted until the end of follow-up, along with increased polymegethism and pleomorphism. Microstructural changes in the central cornea at 4 weeks post-exposure included dysmorphic basal epithelial cells and reduced epithelial thickness, and in the limbal cornea included decreased cellular layers. We present a mouse model of MGK using NM that successfully replicates ocular injury caused by SM in humans who have been exposed to mustard gas.

Mustard Gas-Induced Ocular Surface Disorders: An Update on the Pathogenesis, Clinical Manifestations, and Management

PURPOSE

Mustard gas (MG) is a potent blistering and alkylating agent that has been used for military and terrorism purposes. Ocular surface injuries are common after exposure to MG. This review provides an update on the pathophysiology, ocular surface complications, and treatment options for MG-related ocular injuries.

METHODS

Required information was obtained by reviewing various databases such as Cochrane Library, Google Scholar, and PubMed until March 2022. Data were collected by using keywords: "mustard gas" OR "sulfur mustard" AND "eye" OR "cornea" OR "ocular complication" OR "keratitis" OR "keratopathy" OR "limbal stem cell deficiency" OR "dry eye."

RESULTS

Chronic intracellular toxicity, inflammation, and ischemia have been shown to play an essential role in the pathogenesis of MG injury. Ocular surface injuries can have acute, chronic, and most distinctly a delayed-onset presentation leading to various degrees of limbal stem cell deficiency. To date, no treatment has been agreed on as the standard treatment for chronic/delayed-onset MG keratopathy. Based on the authors' experience, we propose a management algorithm for MG-related ocular surface injuries involving optimization of ocular health, anti-inflammatory therapy, and if needed surgical interventions. The management of chronic and delayed-onset presentation remains challenging.

CONCLUSIONS

MG keratopathy is a unique form of chemical injury which can lead to a range of ocular surface pathologies. Long-term anti-inflammatory therapy even in patients with seemingly mild disease may potentially reduce the likelihood of the development of more severe delayed-onset disease.

Insights into mustard gas keratopathy: Characterizing corneal layer-specific changes in mice exposed to nitrogen mustard

Exposure to mustard agents, such as sulfur mustard (SM) and nitrogen mustard (NM), often results in ocular surface damage. This can lead to the emergence of various corneal disorders that are collectively referred to as mustard gas keratopathy (MGK). In this study, we aimed to develop a mouse model of MGK by using ocular NM exposure, and describe the subsequent structural changes analyzed across the different layers of the cornea. A 3 μL solution of 0.25 mg/mL NM was applied to the center of the cornea via a 2-mm filter paper for 5 min. Mice were evaluated prior to and after exposure on days 1 and 3, and weekly for 4 weeks using slit lamp examination with fluorescein staining. Anterior segment optical coherence tomography (AS-OCT) and in vivo confocal microscopy (IVCM) tracked changes in the epithelium, stroma, and endothelium of the cornea. Histologic evaluation and immunostaining were used to examine corneal cross-sections collected at the completion of follow-up. A biphasic ocular injury was observed in mice exposed to NM, most prominent in the corneal epithelium and anterior stroma. Following exposure, mice experienced central corneal epithelial erosions and thinning, accompanied by a decreased number of nerve branches in the subbasal plexus and increased activated keratocytes in the stroma. The epithelium was recovered by day 3, followed by exacerbated punctuate erosions alongside persistent stromal edema that arose and continued onward to four weeks post-exposure. The endothelial cell density was reduced on the first day after NM exposure, which persisted until the end of follow-up, along with increased polymegethism and pleomorphism. Microstructural changes in the central cornea at this time included dysmorphic basal epithelial cells, and in the limbal cornea included decreased cellular layers and p63⁺ area, along with increased DNA oxidization. We present a mouse model of MGK using NM that successfully replicates ocular injury caused by SM in humans who have been exposed to mustard gas. Our research suggests DNA oxidation contributes to the long-term effects of nitrogen mustard on limbal stem cells.

Progress towards a standardized model of ocular sulfur mustard injury for therapeutic testing

Sulfur mustard (SM) remains a highly dangerous chemical weapon capable of producing mass casualties through liquid or vapor exposure. The cornea is highly sensitive to SM toxicity and exposure to low vapor doses can cause incapacitating acute injuries. At higher doses, corneas fail to fully heal and subsequently develop a constellation of symptoms known as mustard gas keratopathy (MGK) that causes reduced quality of life and impaired or lost vision. Despite a century of research, there are no specific treatments for acute or persistent ocular SM injuries. Here I summarize toxicological, clinical and pathophysiological mechanisms of SM vapor injury in the cornea, describe a preclinical model of ocular SM vapor exposure for reproducible therapeutic studies, and propose new approaches to improve evaluation of therapeutic effects. I also describe recent findings illustrating the delayed development of a transient but severe recurrent corneal lesion that, in turn, triggers the emergence of secondary keratopathies characteristic of the chronic form of MGK. Development of this recurrent lesion is SM dose-dependent, although the severity of the recurrent lesion appears SM dose-independent. Similar recurrent lesions have been reported in multiple species, including humans. Given the mechanistic relationship between the recurrent lesion and chronic, secondary keratopathies, I hypothesize that preventing the development of the recurrent lesion represents a novel and potentially valuable therapeutic approach for treatment of severe corneal SM injuries. Although ocular exposure to SM vapor continues to be a challenging therapeutic target, establishing consistent and reproducible models of corneal injury that enhance mechanistic and pathophysiological understanding will help satisfy regulatory requirements and accelerate the development of effective therapies.

Sulfur mustard corneal injury is associated with alterations in the epithelial basement membrane and stromal extracellular matrix

Sulfur mustard (SM; bis(2-chloroethyl) sulfide) is a highly reactive bifunctional alkylating agent synthesized for chemical warfare. The eyes are particularly sensitive to SM where it causes irritation, pain, photophobia, and blepharitis, depending on the dose and duration of exposure. In these studies, we examined the effects of SM vapor on the corneas of New Zealand white male rabbits. Edema and hazing of the cornea, signs of acute injury, were observed within one day of exposure to SM, followed by neovascularization, a sign of chronic or late phase pathology, which persisted for at least 28 days. Significant epithelial-stromal separation ranging from ~8-17% of the epithelial surface was observed. In the stroma, there was a marked increase in CD45+ leukocytes and a decrease of keratocytes, along with areas of disorganization of collagen fibers. SM also disrupted the corneal basement membrane and altered the expression of perlecan, a heparan sulfate proteoglycan, and cellular fibronectin, an extracellular matrix glycoprotein. This was associated with an increase in basement membrane matrix metalloproteinases including ADAM17, which is important in remodeling of the basement membrane during wound healing. Tenascin-C, an extracellular matrix glycoprotein, was also upregulated in the stroma 14-28 d post SM, a finding consistent with its role in organizing structural components of the stroma necessary for corneal transparency. These data demonstrate that SM vapor causes persistent alterations in structural components of the cornea. Further characterization of SM-induced injury in rabbit cornea will be useful for the identification of targets for the development of ocular countermeasures.

Research models of sulfur mustard- and nitrogen mustard-induced ocular injuries and potential therapeutics

Sulfur mustard (SM) is a notorious, bifunctional alkylating vesicant that was first used in warfare during World War I in 1917 and since then has been deployed in numerous skirmishes with its most recent documented use being during the Middle Eastern conflicts. Apart from its use in combat and terrorist activities, continual threat of accidental exposure from old stockpiles and improperly discarded munitions is ever present, especially to the innocent and unassuming civilian populations. SM can cause devastating injuries, depending on the dosage of SM exposure, route of exposure, as well as the physiological conditions of the individuals exposed. The most common routes of exposure are ocular, dermal, and exposure to the lungs and respiratory tissues through inhalation. Eyes are the most susceptible organ to SM-induced toxicities owing to their high moisture content and rapidly dividing cells. Additionally, ocular injury causes the most expeditious disablement of individuals even upon whole-body exposures. Therefore, it is imperative to understand the mechanisms underlying SM-induced ocular toxicity and design therapeutic interventions to prevent/mitigate ocular injuries. Ocular SM exposure may cause a wide range of symptoms such as inflammation, lacrimation, itching, dryness, photophobia, edema of the cornea/sclera/retina/iris, conjunctivitis, degradation of the corneal layer, fusion of two or more ocular layers, neovascularization, fibrosis, and temporary or permanent structural damage to one or more ocular layers. These symptoms may lead to vision impairments, resulting in partial or complete blindness that may be permanent. The highly toxic and exceedingly notorious nature of SM makes it a highly regulated chemical, requiring very expensive licensing, security, and safety requirements; thus, the more easily accessible analogue, nitrogen mustard (NM) that mimics SM-induced toxicity and injuries is employed in plethora of studies conducted in different animal models and culture systems. This review provides a comprehensive account of the injuries and symptoms that occur upon ocular SM exposures in human patients as well as studies in animal (in vivo, ex vivo) and cell (in vitro) models of SM and NM ocular exposures. Special emphasis has been laid on highlighting the strengths and lacunae in the research as well as the possible unexplored avenues of mechanisms underlying mustard-induced ocular injury that can be explored in future research endeavors. Furthermore, development of therapeutic interventions and targets of interest in the ocular system exposed to SM and NM, based on studies in human patients as well as in vivo, ex vivo, and in vitro models has been discussed in great depth, providing a valuable knowledge database to delineate pathways associated with vesicant-induced toxicity, and strategies/diagnostic tools against SM-induced toxicity.

Cell Morphology as an In Vivo Parameter for the Diagnosis of Limbal Stem Cell Deficiency

PURPOSE

The aim of this study was to investigate basal epithelial cell morphology (CM) in the central cornea and limbal areas of eyes with limbal stem cell deficiency (LSCD).

METHODS

This was a prospective, cross-sectional comparative study. We developed a CM scoring system based on basal epithelial cell phenotypes graded from 0 (normal) to 3 (severe morphologic alterations); this system was evaluated by 2 independent masked observers. The CM score was compared with the LSCD clinical score, mean best-corrected visual acuity, and in vivo laser scanning confocal microscopy parameters used to stage LSCD (ie, basal epithelial cell density, basal epithelial thickness, and subbasal corneal nerve fiber length density).

RESULTS

One hundred sixty-eight eyes with LSCD and 63 normal eyes were included. Compared with the control group, the LSCD group had significantly higher mean (±SD) CM scores in the central cornea (1.8 ± 0.7 vs. 0.5 ± 0.4, respectively; P = 0.01) and limbal areas (1.6 ± 0.2 vs. 1.3 ± 0.0, respectively; P < 0.05). The mean CM score in the central cornea was positively correlated with the clinical score (P < 0.01, r = 0.66) and negatively correlated with the best-corrected visual acuity (P < 0.01, r = 0.42). The CM scores were positively correlated with all other in vivo laser scanning confocal microscopy parameters in the central cornea and limbal areas (all P < 0.001).

CONCLUSIONS

Basal epithelial CM is altered in the central cornea and limbus of eyes with LSCD and thus can be used to stage the clinical severity of the disease.

Ocular toxicity of mustard gas: A concise review

Sulfur mustard (SM) is a chemical warfare agent that has been used throughout recent history and remains a threat today. Exposed soldiers and civilians experience a variety of symptoms primarily in the respiratory system, skin, and eyes. The ocular tissues are highly sensitive to damage by SM and undergo unique manifestations of acute, chronic, and delayed complications that can persist for months and years after exposure. The mechanisms of this unique mustard gas keratopathy are still not fully understood and animal models for the study of this disease are discussed. Recent advances in mechanisms of injury are included in this review. Ophthalmic manifestations of SM injury including persistent epithelial defects, limbal stem cell deficiency, corneal neovascularization, dry eye, and corneal opacification have been reported. A wide variety of medical and surgical therapies have been studied and are reviewed here along with potential future therapies.

Understanding cornea epithelial stem cells and stem cell deficiency: Lessons learned using vertebrate model systems

Animal models have contributed greatly to our understanding of human diseases. Here, we focus on cornea epithelial stem cell (CESC) deficiency (commonly called limbal stem cell deficiency, LSCD). Corneal development, homeostasis and wound healing are supported by specific stem cells, that include the CESCs. Damage to or loss of these cells results in blindness and other debilitating ocular conditions. Here we describe the contributions from several vertebrate models toward understanding CESCs and LSCD treatments. These include both mammalian models, as well as two aquatic models, Zebrafish and the amphibian, Xenopus. Pioneering developments have been made using stem cell transplants to restore normal vision in patients with LSCD, but questions still remain about the basic biology of CESCs, including their precise cell lineages and behavior in the cornea. We describe various cell lineage tracing studies to follow their patterns of division, and the fates of their progeny during development, homeostasis, and wound healing. In addition, we present some preliminary results using the Xenopus model system. Ultimately, a more thorough understanding of these cornea cells will advance our knowledge of stem cell biology and lead to better cornea disease therapeutics.

Therapeutic measures for sulfur mustard-induced ocular injury

The use of sulfur mustard (SM) in global terrorism is still a relevant threat to both civilian population and military personnel. Casualties exposed to SM may present mild, moderate or severe acute ocular lesions followed by a complete ocular resolution, chronic lesions or re-emerged ocular pathologies after a latent period. Current treatment for SM-induced ocular injury is based mainly on the clinical manifestation at the different stages of the injury and includes pharmaceutical and surgical interventions. These therapeutic measures are beneficial but not sufficient, and the ocular injury remains a continuous challenge for medical professionals. This review focuses on treatment experience carried out in humans and studied in animal models, for both SM-induced ocular acute injury and late pathology. In general, therapeutic measures are based on clinical features of the ocular injury or on the involvement of specific factors during the ocular injury that point out towards potential treatments. Anti-inflammatory treatments and limbal stem cell transplantation techniques were developed based on the clinical manifestation of the ocular injury. Optional therapies for impaired corneal innervation and endothelium are suggested for future research. Additionally, studies on potential treatments with anti-matrix metalloproteinase (MMP), anti-vascular endothelial growth factor (VEGF) and anti-IL-6 agents are discussed. Consequently, future studies may reveal the potential of additional pharmacological and biological treatments or advanced cellular and molecular biology methods to serve as novel therapeutic measures and techniques for this complicated ocular injury.

Phosgene oxime: a highly toxic urticant and emerging chemical threat

Highly toxic industrial chemicals that are widely accessible, and hazardous chemicals like phosgene oxime (CX) that can be easily synthesized, pose a serious threat as potential chemical weapons. In addition, their accidental release can lead to chemical emergencies and mass casualties. CX, an urticant, or nettle agent, grouped with vesicating agents, causes instant pain, injury and systemic effects, which can lead to mortality. With faster cutaneous penetration, corrosive properties, and more potent toxicity compared to other vesicating agents, CX causes instantaneous and severe tissue damage. CX, a potential chemical terrorism threat agent, could therefore be weaponized with other chemical warfare agents to enhance their harmful effects. CX is the least studied vesicant and its acute and long-term toxic effects as well as its mechanism of action are largely unknown. This has hampered the identification of therapeutic targets and the development of effective medical countermeasures. There are only protective measures, decontamination, and supportive treatments available for reducing the toxic effects from CX exposure. This review summarizes CX toxicity, its known mechanism of action, and our current studies exploring the role of mast cell activation and associated signaling pathways in CX cutaneous exposure under the National Institutes of Health Countermeasures Against Chemical Threats program. Potential treatment options and the development of effective targeted countermeasures against CX-induced morbidity and mortality is also discussed.

Advances in treatment of acute sulfur mustard poisoning - a critical review

Sulfur mustard (SM) is a blistering chemical warfare agent that was used during the World War I and in the Iraq-Iran conflict. The aim of this paper is to discuss and critically review the published results of experiments on the treatment of SM poisoning based on our clinical and research experience. The victims must remove from the contaminated zone immediately. The best solution for decontamination is large amounts of water, using neutral soap and 0.5% sodium hypochlorite. Severely intoxicated patients should be treated according to advanced life support protocols and intensive care therapy for respiratory disorders and the chemical burn. Sodium thiosulfate infusion (100-500 mg/kg/min) should be started up to 60 min after SM exposure. However, N-acetyle cysteine (NAC) is recommended, none of them acts as specific or effective antidote. The important protective and conservative treatment of SM-induced pulmonary injuries include humidified oxygen, bronchodilators, NAC as muculytic, rehydration, mechanical ventilation, appropriate antibiotics and respiratory physiotherapy as clinically indicated. Treatment of acute SM ocular lesions start with topical antibiotics; preferably sulfacetamide eye drop, continue with lubricants, and artificial tears. Treatment for cutaneous injuries include: moist dressing; preferably with silver sulfadiazine cream, analgesic, anti-pruritic, physically debridement, debridase, Laser debridement, followed by skin autologous split-thickness therapy as clinically indicated. The new suggested medications and therapeutic approaches include: anti-inflammatory agents, Niacinamide, Silibinin, Calmodulin antagonists, Clobetasol, full-thickness skin grafting for skin injuries; Doxycycline; Bevacizumab, and Colchicine for ocular injuries. Recommended compounds based on animal studies include Niacinamide, Aprotinin, des-aspartate-angiotensin-I, Gamma-glutamyltransferase, vitamin E, and vitamin D. In vitro studies revealed that Dimethylthiourea, L-nitroarginine, Methyl-ester, Sodium pyruvate, Butylated hydroxyanisole, ethacrynic acid, and macrolide antibiotics are effective. However, none of them, except macrolide antibiotics have been proved clinically. Avoidance of inappropriate polypharmacy is advisable.

Understanding cornea homeostasis and wound healing using a novel model of stem cell deficiency in Xenopus

Limbal Stem Cell Deficiency (LSCD) is a painful and debilitating disease that results from damage or loss of the Corneal Epithelial Stem Cells (CESCs). Therapies have been developed to treat LSCD by utilizing epithelial stem cell transplants. However, effective repair and recovery depends on many factors, such as the source and concentration of donor stem cells, and the proper conditions to support these transplanted cells. We do not yet fully understand how CESCs heal wounds or how transplanted CESCs are able to restore transparency in LSCD patients. A major hurdle has been the lack of vertebrate models to study CESCs. Here we utilized a short treatment with Psoralen AMT (a DNA cross-linker), immediately followed by UV treatment (PUV treatment), to establish a novel frog model that recapitulates the characteristics of cornea stem cell deficiency, such as pigment cell invasion from the periphery, corneal opacity, and neovascularization. These PUV treated whole corneas do not regain transparency. Moreover, PUV treatment leads to appearance of the Tcf7l2 labeled subset of apical skin cells in the cornea region. PUV treatment also results in increased cell death, immediately following treatment, with pyknosis as a primary mechanism. Furthermore, we show that PUV treatment causes depletion of p63 expressing basal epithelial cells, and can stimulate mitosis in the remaining cells in the cornea region. To study the response of CESCs, we created localized PUV damage by focusing the UV radiation on one half of the cornea. These cases initially develop localized stem cell deficiency characteristics on the treated side. The localized PUV treatment is also capable of stimulating some mitosis in the untreated (control) half of those corneas. Unlike the whole treated corneas, the treated half is ultimately able to recover and corneal transparency is restored. Our study provides insight into the response of cornea cells following stem cell depletion, and establishes Xenopus as a suitable model for studying CESCs, stem cell deficiency, and other cornea diseases. This model will also be valuable for understanding the nature of transplanted CESCs, which will lead to progress in the development of therapeutics for LSCD.

Acute corneal injury in rabbits following nitrogen mustard ocular exposure

Sulfur mustard (SM), a potent vesicating chemical warfare agent, and its analog nitrogen mustard (NM), are both strong bi-functional alkylating agents. Eyes, skin, and the respiratory system are the main targets of SM and NM exposure; however, ocular tissue is most sensitive, resulting in severe ocular injury. The mechanism of ocular injury from vesicating agents' exposure is not completely understood. To understand the injury mechanism from exposure to vesicating agents, NM has been previously employed in our toxicity studies on primary human corneal epithelial cells and ex vivo rabbit cornea organ culture model. In the current study, corneal toxicity from NM ocular exposure (1%) was analyzed for up to 28 days post-exposure in New Zealand White male rabbits to develop an acute corneal injury model. NM exposure led to conjunctival and eyelid swelling within a few hours after exposure, in addition to significant corneal opacity and ulceration. An increase in total corneal thickness and epithelial degradation was observed starting at day 3 post-NM exposure, which was maximal at day 14 post-exposure and did not resolve until 28 days post-exposure. There was an NM-induced increase in the number of blood vessels and inflammatory cells, and a decrease in keratocytes in the corneal stroma. NM exposure resulted in increased expression levels of cyclooxygenase-2, Interleukin-8, vascular endothelial growth factor and Matrix Metalloproteinase 9 indicating their involvement in NM-induced corneal injury. These clinical, biological, and molecular markers could be useful for the evaluation of acute corneal injury and to screen for therapies against NM- and SM-induced ocular injury.

Efficacy of anti-inflammatory, antibiotic and pleiotropic agents in reversing nitrogen mustard-induced injury in ex vivo cultured rabbit cornea

Vesicating agent, Sulfur mustard (SM), causes devastating eye injury; however, there are no effective antidotes available. Using nitrogen mustard (NM), a bi-functional analog of SM, we have earlier reported that NM-induced corneal injury in ex vivo rabbit cornea organ culture model parallels corneal injury reported with SM. Using this model, we have demonstrated the therapeutic efficacy of dexamethasone (DEX), doxycycline (DOX) and silibinin (SB) in reversing NM (2h exposure)-induced corneal injuries when added immediately after washing NM. In the present study, we further examined the efficacy of similar/higher doses of these agents when added immediately, 2, or 4h after washing NM following its 2h exposure. All three treatment agents caused a reversal in established NM-induced injury biomarkers when added immediately or 2h after washing NM following its 2h exposure; however, when treatments were carried out 4h after washing NM, there was no significant effect. Together, our results further show the beneficial effect of these agents in reversing NM-induced corneal injury and indicate the time window for effective treatment. This could be useful towards future development of targeted therapeutics against vesicant-induced ocular injury.

Ocular Surface Changes After Sulfur Mustard Exposure in Rabbits, Monitored by Impression Cytology

PURPOSE

Ocular injuries after exposure to sulfur mustard (SM) are characterized by acute corneal erosion and inflammation of the anterior segment that may be followed by delayed corneal neovascularization and epithelial defects, associated with limbal stem cell deficiency in part of the exposed eyes. This study aimed to further clarify the mechanism of the late injury by monitoring SM-induced cytological alterations in the ocular surface, in relation to the clinical symptoms, using impression cytology (IC).

METHODS

Rabbit eyes were exposed to SM vapor (n = 20) and were clinically observed up to 4 weeks. Samples for IC were collected simultaneously from the upper bulbar conjunctiva, limbus, and cornea and then fixed and stained with periodic acid-Schiff and hematoxylin. At 1 month, animals were killed and eyes dissected and processed for histology.

RESULTS

Concomitant with clinical symptoms of SM ocular toxicity, IC showed significant long-term loss of conjunctival goblet cells shortly after exposure, followed by abnormal differentiation toward squamous metaplasia. Simultaneously with corneal erosion, apoptotic bodies and cellular debris were seen in the corneal epithelium, followed by regeneration at 1 week. Migration of conjunctival goblet cells toward the cornea was noted in neovascularized eyes, as early as 1 week, indicating limbal stem cell deficiency. The IC findings were supported by histological evaluation.

CONCLUSIONS

Continuous monitoring of the ocular surface after SM exposure by IC enables earlier detection of pathology and therapeutic intervention, therefore, is recommended for routine follow-up of casualties. Prolonged loss of goblet cells may point toward the role of mucin in the pathogenesis.

Phosgene oxime: Injury and associated mechanisms compared to vesicating agents sulfur mustard and lewisite

Phosgene Oxime (CX, Cl₂CNOH), a halogenated oxime, is a potent chemical weapon that causes immediate acute injury and systemic effects. CX, grouped together with vesicating agents, is an urticant or nettle agent with highly volatile, reactive, corrosive, and irritating vapor, and has considerably different chemical properties and toxicity compared to other vesicants. CX is absorbed quickly through clothing with faster cutaneous penetration compared to other vesicating agents causing instantaneous and severe damage. For this reason, it could be produced as a weaponized mixture with other chemical warfare agents to enhance their deleterious effects. The immediate devastating effects of CX and easy synthesis makes it a dangerous chemical with both military and terrorist potentials. Although CX is the most potent vesicating agent, it is one of the least studied chemical warfare agents and the pathophysiology as well as long term effects are largely unknown. CX exposure results in immediate pain and inflammation, and it mainly affects skin, eye and respiratory system. There are no antidotes available against CX-induced injury and the treatment is only supportive. This review summarizes existing knowledge regarding exposure, toxicity and the probable underlying mechanisms of CX compared to other important vesicants' exposure.

Clinical Practice Guidelines for Prevention, Diagnosis and Management of Early and Delayed-onset Ocular Injuries Due to Mustard Gas Exposure

PURPOSE

To develop clinical practice guidelines (CPGs) for prevention, diagnosis, treatment and follow-up of ocular injuries caused by exposure to mustard gas.

METHODS

The clinical questions were designed by the guideline team. Websites and databases including National Guidelines Clearinghouse, National Institute for Clinical Excellence, Cochrane, and PubMed were searched to find related CPGs and explore possible answers to the clinical questions. Since there were no relevant CPGs in the literature, related articles in Persian and English languages were extracted. Each article along with its level of evidence was summarized. Additionally, hand search was performed by looking the reference list of each article. Consequently, recommendations were developed considering the clinical benefits and side effects of each therapeutic modality. The recommendations were re-evaluated in terms of customization criteria. All recommendations along with the related evidence were scored from 1 to 9 by experts from all medical universities of Iran. The level of agreement among the experts was evaluated by analyzing the given scores.

RESULTS

The agreement was achieved for all recommendations. The experts suggested a number of minor modifications which were applied to the recommendations. Finally, CPGs were developed with 98 recommendations under three major domains including prevention of injury, diagnosis and management of the acute and delayed-onset mustard gas ocular injuries.

CONCLUSION

Considering the lack of CPGs for the prevention, diagnosis, and management of mustard gas-induced keratitis, these recommendations would be useful to prevent the serious ocular complications of mustard gas and standardize eye care services to the affected individuals.

Contributions of tissue-specific pathologies to corneal injuries following exposure to SM vapor

Corneal injuries resulting from ocular exposure to sulfur mustard (SM) vapor are the most prevalent chemical warfare injury. Ocular exposures exhibit three distinct, dose-dependent clinical trajectories: complete injury resolution, immediate transition to a chronic injury, or apparent recovery followed by the subsequent development of persistent ocular manifestations. These latter two trajectories include a constellation of corneal symptoms that are collectively known as mustard gas keratopathy (MGK). The etiology of MGK is not understood. Here, we synthesize recent findings from in vivo rabbit SM vapor studies, suggesting that tissue-specific damage during the acute injury can decrement the regenerative capacities of corneal endothelium and limbal stem cells, thereby predisposing the cornea to the chronic or delayed forms of MGK. This hypothesis not only provides a mechanism to explain the acute and MGK injuries but also identifies novel therapeutic modalities to mitigate or eliminate the acute and long-term consequences of ocular exposure to SM vapor.

Clinical progression of ocular injury following arsenical vesicant lewisite exposure

Ocular injury by lewisite (LEW), a potential chemical warfare and terrorist agent, results in edema of eyelids, inflammation, massive corneal necrosis and blindness. To enable screening of effective therapeutics to treat ocular injury from LEW, useful clinically-relevant endpoints are essential. Hence, we designed an efficient exposure system capable of exposing up to six New-Zealand white rabbits at one time, and assessed LEW vapor-induced progression of clinical ocular lesions mainly in the cornea. The right eye of each rabbit was exposed to LEW (0.2 mg/L) vapor for 2.5, 5.0, 7.5 and 10.0 min and clinical progression of injury was observed for 28 days post-exposure (dose-response study), or exposed to same LEW dose for 2.5 and 7.5 min and clinical progression of injury was observed for up to 56 days post-exposure (time-response study); left eye served as an unexposed control. Increasing LEW exposure caused corneal opacity within 6 h post-exposure, which increased up to 3 days, slightly reduced thereafter till 3 weeks, and again increased thereafter. LEW-induced corneal ulceration peaked at 1 day post-exposure and its increase thereafter was observed in phases. LEW exposure induced neovascularization starting at 7 days which peaked at 22-35 days post-exposure, and remained persistent thereafter. In addition, LEW exposure caused corneal thickness, iris redness, and redness and swelling of the conjunctiva. Together, these findings provide clinical sequelae of ocular injury following LEW exposure and for the first time establish clinically-relevant quantitative endpoints, to enable the further identification of histopathological and molecular events involved in LEW-induced ocular injury.

The beneficial effects of doxycycline, an inhibitor of matrix metalloproteinases, on sulfur mustard-induced ocular pathologies depend on the injury stage

PURPOSE

Sulfur mustard (SM) induces acute ocular lesions, including erosions and inflammation that may be followed by delayed injuries expressed by epithelial defects and neovascularization (NV). Based on the matrix metalloproteinases (MMPs) activity, we evaluated the clinical and biochemical effects of topical treatment with doxycycline, an MMP inhibitor, targeted to the various injury stages.

METHODS

Rabbit eyes were exposed to SM vapor. A clinical follow-up was carried out up to 2 months. Tear fluid and cornea samples were collected at different time points for measurements of MMPs activity by zymography. Efficacy of a post-exposure topical doxycycline (2 mg/ml in phosphate buffer saline, ×4/d), targeted to the different phases of the clinical injury, was evaluated.

RESULTS

Elevated MMP-9 and MMP-2 activities were found in all corneas during the acute injury and in vascularized corneas during the delayed pathology. In the tear fluid, high MMP-9 activity and negligible MMP-2 activity were found in all the exposed eyes until after the appearance of the delayed pathology symptoms. Prolonged doxycycline treatment reduced MMP-9 activity in the tear fluid. During the acute phase, doxycycline treatment reduced corneal MMP-9 activity and the severity of the injury. Targeting the delayed pathology, doxycycline was clinically efficient only when treatment began before NV appearance.

CONCLUSIONS

This in vivo study showed the involvement of MMP-9 and MMP-2 during different phases of the SM-induced ocular injury, and the potential of doxycycline treatment as a post exposure measure for reducing the acute injury and as a preventive therapy for ameliorating the delayed pathology. The tear fluid provided a non-invasive method for continuous follow-up of MMPs activity and revealed additional beneficial aspects of injury and the treatment.