Sulfur mustard cutaneous injury characterization based on SKH-1 mouse model: relevance of non-invasive methods in terms of wound healing process analyses

Dermal Exposure to Vesicating Nettle Agent Phosgene Oxime: Clinically Relevant Biomarkers and Skin Injury Progression in Murine Models

Phosgene oxime (CX), categorized as a vesicating chemical threat agent, causes effects that resemble an urticant or nettle agent. CX is an emerging potential threat agent that can be deployed alone or with other chemical threat agents to enhance their toxic effects. Studies on CX-induced skin toxicity, injury progression, and related biomarkers are largely unknown. To study the physiologic changes, skin clinical lesions and their progression, skin exposure of SKH-1 and C57BL/6 mice was carried out with vapor from 10 μl CX for 0.5-minute or 1.0-minute durations using a designed exposure system for consistent CX vapor exposure. One-minute exposure caused sharp (SKH-1) or sustained (C57BL/6) decrease in respiratory and heart rate, leading to mortality in both mouse strains. Both exposures caused immediate blanching, erythema with erythematous ring (wheel) and edema, and an increase in skin bifold thickness. Necrosis was also observed in the 0.5-minute CX exposure group. Both mouse strains showed comparative skin clinical lesions upon CX exposure; however, skin bifold thickness and erythema remained elevated up to 14 days postexposure in SKH-1 mice but not in C57BL/6 mice. Our data suggest that CX causes immediate changes in the physiologic parameters and gross skin lesions resembling urticaria, which could involve mast cell activation and intense systemic toxicity. This novel study recorded and compared the progression of skin injury to establish clinical biomarkers of CX dermal exposure in both the sexes of two murine strains relevant for skin and systemic injury studies and therapeutic target identification. SIGNIFICANCE STATEMENT: Phosgene oxime (CX), categorized as a vesicating agent, is considered as a potent chemical weapon and is of high military and terrorist threat interest since it produces rapid onset of severe injury as an urticant. However, biomarkers of clinical relevance related to its toxicity and injury progression are not studied. Data from this study provide useful clinical markers of CX skin toxicity in mouse models using a reliable CX exposure system for future mechanistic and efficacy studies.

Skin Models Used to Define Mechanisms of Action of Sulfur Mustard

Sulfur mustard (SM) is a threat to both civilian and military populations. Human skin is highly sensitive to SM, causing delayed erythema, edema, and inflammatory cell infiltration, followed by the appearance of large fluid-filled blisters. Skin wound repair is prolonged following blistering, which can result in impaired barrier function. Key to understanding the action of SM in the skin is the development of animal models that have a pathophysiology comparable to humans such that quantitative assessments of therapeutic drugs efficacy can be assessed. Two animal models, hairless guinea pigs and swine, are preferred to evaluate dermal products because their skin is morphologically similar to human skin. In these animal models, SM induces degradation of epidermal and dermal tissues but does not induce overt blistering, only microblistering. Mechanisms of wound healing are distinct in these animal models. Whereas a guinea pig heals by contraction, swine skin, like humans, heals by re-epithelialization. Mice, rats, and rabbits are also used for SM mechanistic studies. However, healing is also mediated by contraction; moreover, only microblistering is observed. Improvements in animal models are essential for the development of therapeutics to mitigate toxicity resulting from dermal exposure to SM.

Antioxidant/stress response in mouse epidermis following exposure to nitrogen mustard

Nitrogen mustard (NM) is a highly reactive bifunctional alkylating agent that induces inflammation, edema and blistering in skin. An important mechanism mediating the action of NM and related mustards is oxidative stress. In these studies a modified murine patch-test model was used to analyze DNA damage and the antioxidant/stress response following NM exposure in isolated epidermis. NM (20 μmol) was applied to glass microfiber filters affixed to a shaved dorsal region of skin of CD-1 mice. NM caused structural damage to the stratum corneum as reflected by increases in transepidermal water loss and skin hydration. This was coordinate with edema, mast cell degranulation and epidermal hyperplasia. Within 3 h of NM exposure, a 4-fold increase in phosphorylated histone H2AX, a marker of DNA double-stranded breaks, and a 25-fold increase in phosphorylated p53, a DNA damage marker, were observed in the epidermis. This was associated with a 40% increase in 8-oxo-2'-deoxyguanosine modified DNA in the epidermis and a 4-fold increase in 4-hydroxynonenal modified epidermal proteins. At 12 h post NM, there was a 3-75 fold increase in epidermal expression of antioxidant/stress proteins including heme oxygenase-1, thioredoxin reductase, superoxide dismutase, glutathione reductase, heat shock protein 27 and cyclooxygenase 2. These data indicate that NM induces early oxidative epidermal injury in mouse skin leading to an antioxidant/stress response. Agents that enhance this response may be useful in mitigating mustard-induced skin injury.

Early indicators of survival following exposure to mustard gas: Protective role of 25(OH)D

The use of sulfur mustard (SM) as a chemical weapon for warfare has once again assumed center stage, endangering civilian and the military safety. SM causes rapid local skin vesication and late-onset systemic toxicity. Most studies on SM rely on obtaining tissue and blood for characterizing burn pathogenesis and assessment of systemic pathology, respectively. However the present study focuses on developing a non-invasive method to predict mortality from high dose skin SM exposure. We demonstrate that exposure to SM leads to a dose dependent increase in wound area size on the dorsal surface of mice that is accompanied by a progressive loss in body weight loss, blood cytopenia, bone marrow destruction, and death. Thus our model utilizes local skin destruction and systemic outcome measures as variables to predict mortality in a novel skin-based model of tissue injury. Based on our recent work using vitamin D (25(OH)D) as an intervention to treat toxicity from SM-related compounds, we explored the use of 25 (OH)D in mitigating the toxic effects of SM. Here we show that 25(OH)D offers protection against SM and is the first known demonstration of an intervention that prevents SM-induced mortality. Furthermore, 25 (OH)D represents a safe, novel, and readily translatable potential countermeasure following mass toxic exposure.

Time course of lewisite-induced skin lesions and inflammatory response in the SKH-1 hairless mouse model

Data on the toxicity of lewisite (L), a vesicant chemical warfare agent, are scarce and conflicting, and the use of the specific antidote is not without drawbacks. This study was designed to evaluate if the SKH-1 hairless mouse model was suitable to study the L-induced skin injuries. We studied the progression of lesions following exposure to L vapors for 21 days using paraclinical parameters (color, transepidermal water loss (TEWL), and biomechanical measurements), histological assessments, and biochemical indexes of inflammation. Some data were also obtained over 27 weeks. The development of lesions was similar to that reported in other models. The TEWL parameter appeared to be the most appropriate index to follow their progression. Histological analysis showed inflammatory cell infiltration and microvesications at day 1 and a complete wound closure by day 21. Biochemical studies indicated a deregulation of the levels of several cytokines and receptors involved in inflammation. An increase in the quantity of pro-matrix metalloproteinases 2 and 9 was shown as observed in other models. This suggests that the SKH-1 mouse model is relevant for the investigation of the physiopathological process of skin lesions induced by L and to screen new treatment candidates.

Time course of skin features and inflammatory biomarkers after liquid sulfur mustard exposure in SKH-1 hairless mice

Sulfur mustard (SM) is a strong bifunctional alkylating agent that produces severe tissue injuries characterized by erythema, edema, subepidermal blisters and a delayed inflammatory response after cutaneous exposure. However, despite its long history, SM remains a threat because of the lack of effective medical countermeasures as the molecular mechanisms of these events remain unclear. This limited number of therapeutic options results in part of an absence of appropriate animal models. We propose here to use SKH-1 hairless mouse as the appropriate model for the design of therapeutic strategies against SM-induced skin toxicity. In the present study particular emphasis was placed on histopathological changes associated with inflammatory responses after topical exposure of dorsal skin to three different doses of SM (0.6, 6 and 60mg/kg) corresponding to a superficial, a second-degree and a third-degree burn. Firstly, clinical evaluation of SM-induced skin lesions using non invasive bioengineering methods showed that erythema and impairment of skin barrier increased in a dose-dependent manner. Histological evaluation of skin sections exposed to SM revealed that the time to onset and the severity of symptoms including disorganization of epidermal basal cells, number of pyknotic nuclei, activation of mast cells and neutrophils dermal invasion were dose-dependent. These histopathological changes were associated with a dose- and time-dependent increase in expression of specific mRNA for inflammatory mediators such as interleukins (IL1β and IL6), tumor necrosis factor (TNF)-α, cycloxygenase-2 (COX-2), macrophage inflammatory proteins (MIP-1α, MIP-2 and MIP-1αR) and keratinocyte chemoattractant (KC also called CXCL1) as well as adhesion molecules (L-selectin and vascular cell adhesion molecule (VCAM)) and growth factor (granulocyte colony-stimulating factor (Csf3)). A dose-dependent increase was also noted after SM exposure for mRNA of matrix metalloproteinases (MMP9) and laminin-γ2 which are associated with SM-induced blisters formation. Taken together, our results show that SM-induced skin histopathological changes related to inflammation is similar in SKH-1 hairless mice and humans. SKH-1 mouse is thus a reliable animal model for investigating the SM-induced skin toxicity and to develop efficient treatment against SM-induced inflammatory skin lesions.

Temporal and spatial features of the formation of DNA adducts in sulfur mustard-exposed skin

Sulfur mustard (SM) is a chemical warfare agent that targets skin where it induces large blisters. DNA alkylation is a critical step to explain SM-induced cutaneous symptoms. We determined the kinetics of formation of main SM-DNA adducts and compare it with the development of the SM-induced pathogenesis in skin. SKH-1 mice were exposed to 2, 6 and 60 mg/kg of SM and treated skin was biopsied between 6h and 21 days. Formation of SM DNA adducts was dose-dependent with a maximum immediately after exposure. However, adducts were persistent and still detectable 21 days post-exposure. The time-dependent formation of DNA adducts was also found to be correlated with the appearance of apoptotic cells. This temporal correlation suggests that these two early events are responsible for the severity of the damage to the skin. Besides, SM-DNA adducts were also detected in areas located next to contaminated zone, thus suggesting that SM diffuses in skin. Altogether, this work provides for the first time a clear picture of SM-induced genotoxicity using DNA adducts as a marker.

Topical efficacy of dimercapto-chelating agents against lewisite-induced skin lesions in SKH-1 hairless mice

Lewisite is a potent chemical warfare arsenical vesicant that can cause severe skin lesions. Today, lewisite exposure remains possible during demilitarization of old ammunitions and as a result of deliberate use. Although its cutaneous toxicity is not fully elucidated, a specific antidote exists, the British anti-lewisite (BAL, dimercaprol) but it is not without untoward effects. Analogs of BAL, less toxic, have been developed such as meso-2,3-dimercaptosuccinic acid (DMSA) and have been employed for the treatment of heavy metal poisoning. However, efficacy of DMSA against lewisite-induced skin lesions remains to be determined in comparison with BAL. We have thus evaluated in this study the therapeutic efficacy of BAL and DMSA in two administration modes against skin lesions induced by lewisite vapor on SKH-1 hairless mice. Our data demonstrate a strong protective efficacy of topical application of dimercapto-chelating agents in contrast to a subcutaneous administration 1h after lewisite exposure, with attenuation of wound size, necrosis and impairment of skin barrier function. The histological evaluation also confirms the efficacy of topical application by showing that treatments were effective in reversing lewisite-induced neutrophil infiltration. This protective effect was associated with an epidermal hyperplasia. However, for all the parameters studied, BAL was more effective than DMSA in reducing lewisite-induced skin injury. Together, these findings support the use of a topical form of dimercaprol-chelating agent against lewisite-induced skin lesion within the first hour after exposure to increase the therapeutic management and that BAL, despite its side-effects, should not be abandoned.

Clinically-relevant cutaneous lesions by nitrogen mustard: useful biomarkers of vesicants skin injury in SKH-1 hairless and C57BL/6 mice

A paucity of clinically applicable biomarkers to screen therapies in laboratory is a limitation in the development of countermeasures against cutaneous injuries by chemical weapon, sulfur mustard (SM), and its analog nitrogen mustard (NM). Consequently, we assessed NM-caused progression of clinical cutaneous lesions; notably, skin injury with NM is comparable to SM. Exposure of SKH-1 hairless and C57BL/6 (haired) mice to NM (3.2 mg) for 12-120 h caused clinical sequelae of toxicity, including microblister formation, edema, erythema, altered pigmentation, wounding, xerosis and scaly dry skin. These toxic effects of NM were similar in both mouse strains, except that wounding and altered pigmentation at 12-24 h and appearance of dry skin at 24 and 72 h post-NM exposure were more pronounced in C57BL/6 compared to SKH-1 mice. Conversely, edema, erythema and microblister formation were more prominent in SKH-1 than C57BL/6 mice at 24-72 h after NM exposure. In addition, 40-60% mortality was observed following 120 h of NM exposure in the both mouse strains. Overall, these toxic effects of NM are comparable to those reported in humans and other animal species with SM, and thus represent clinically-relevant cutaneous injury endpoints in screening and optimization of therapies for skin injuries by vesicating agents.