Architectural and Biochemical Expressions of Mustard Gas Keratopathy: Preclinical Indicators and Pathogenic Mechanisms

Abstract

A subset of victims of ocular sulfur mustard (SM) exposure develops an irreversible, idiotypic keratitis with associated secondary pathologies, collectively referred to as mustard gas keratopathy (MGK). MGK involves a progressive corneal degeneration resulting in chronic ocular discomfort and impaired vision for which clinical interventions have typically had poor outcomes. Using a rabbit corneal vapor exposure model, we previously demonstrated a clinical progression with acute and chronic sequelae similar to that observed in human casualties. However, a better understanding of the temporal changes that occur during the biphasic SM injury is crucial to mechanistic understanding and therapeutic development. Here we evaluate the histopathologic, biochemical and ultrastructural expressions of pathogenesis of the chronic SM injury over eight weeks. We confirm that MGK onset exhibits a biphasic trajectory involving corneal surface regeneration over the first two weeks, followed by the rapid development and progressive degeneration of corneal structure. Preclinical markers of corneal dysfunction were identified, including destabilization of the basal corneal epithelium, basement membrane zone abnormalities and stromal deformation. Clinical sequelae of MGK appeared abruptly three weeks after exposure, and included profound anterior edema, recurring corneal erosions, basement membrane disorganization, basal cell necrosis and stromal degeneration. Unlike resolved corneas, MGK corneas exhibited frustrated corneal wound repair, with significantly elevated histopathology scores. Increased lacrimation, disruption of the basement membrane and accumulation of pro-inflammatory mediators in the aqueous humor provide several mechanisms for corneal degeneration. These data suggest that the chronic injury is fundamentally distinct from the acute lesion, involving injury mechanisms that operate on different time scales and in different corneal tissues. Corneal edema appears to be the principal pathology of MGK, in part resulting from persistent necrosis of the basal corneal epithelium and deterioration of the basement membrane. The findings also provide a potential explanation as to why administration of anti-inflammatories transiently delays, but does not prevent, the development of MGK sequelae.

Structural, Morphological, and Functional Correlates of Corneal Endothelial Toxicity Following Corneal Exposure to Sulfur Mustard Vapor

“…11,13 In contrast, MGK corneas develop a persistent edema as soon as 3 weeks with recurring epithelial toxicity, basement membrane zone degeneration, and redundant deposition of basement membrane components following cyclical attempts to regenerate the epithelium. 14 Secondary pathologies such as epithelial bullae, neovascularization, and limbal stem cell niche dysfunction subsequently appear, further interfering with stable repair of the ocular surface. 11,13 The distinct pathophysiology of MGK revealed in these studies suggests the involvement of injury mechanisms that operate on different time scales and in different corneal compartments than during the acute injury.…”

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

“…An O‐ring is used to restrict SM exposure to the area of the vapor cap, eliminating pericorneal lesions resulting from lateral diffusion. We have used this vapor exposure model to describe dose‐ and time‐dependent effects of SM on the structure, biochemistry, and function of corneal tissues . Briefly, within 24 h of exposure, corneas develop acute lesions characterized by vesication of the corneal epithelium, stromal keratocytosis, corneal edema, and corneal endothelial cell (CEC) loss.…”

“…Histopathological and structural markers of MGK are rapidly expressed from 2 to 4 weeks in these corneas and include recurring basal epithelial cell cytotoxicity, basement membrane zone degeneration, loss of limbal epithelial stem cells (LESCs), inflammatory cell infiltration, elevation of cytokines, persistent endothelial failure, delayed in‐migration of keratocytes, stromal degeneration, and redundant deposition of basement membrane components caused by cyclical attempts to regenerate the epithelium and endothelium (Fig. D) . Secondary clinical symptoms, such as epithelial bullae formation, recurring corneal erosions, neovascularization, and limbal stem cell disorder (LSCD), subsequently develop, preventing stable repair of the ocular surface .…”

“…We have used this vapor exposure model to describe dose-and time-dependent effects of SM on the structure, biochemistry, and function of corneal tissues. 35,[44][45][46] Briefly, within 24 h of exposure, corneas develop acute lesions characterized by vesication of the corneal epithelium, stromal keratocytosis, corneal edema, and corneal endothelial cell (CEC) loss. The corneal epithelium undergoes a robust healing response to regenerate an impermeable epithelial cap within 5 days ( Fig.…”

“…3D). 36,43,[45][46][47] Secondary clinical symptoms, such as epithelial bullae formation, recurring corneal erosions, neovascularization, and limbal stem cell disorder (LSCD), subsequently develop, preventing stable repair of the ocular surface. 48 Postexposure, steroid-based anti-inflammatory treatment protocols reduce and postpone the appearance of the late injury.…”