Sulfur Mustard-Induced Skin Injury in Hairless Guinea Pigs

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.

A vapor exposure model for neonatal mice

Sulfur mustard (HD) is a vesicant compound that was first used as a chemical warfare agent in World War I. (Papirmeister et al. 1991). Numerous animal models have been used to study HD-induced vesication. In this article, we describe modifications of the vapor cup model of Mershon and colleagues (1990) to establish a new vapor cup model for use in neonatal mice. The need to develop this model resulted from the development of gene-targeted knockout mice that can be used to evaluate the function of specific genes and their contribution to HD-induced pathology. However, the knockouts are haired mice; therefore, it is necessary to perform vapor exposures on the pups prior to their growing hair. Neonatal mice were anesthetized with isofluorane inhalation and placed in sternal recumbency on a 37 degrees C isothermal pad to maintain body heat during exposure. The vapor cup consisted of a 1.5-mL microfuge tube cap (8 mm inside diameter) modified using a Dremel tool to contour its rim to better fit the curve of a mouse pups back. The inside of the cap was fitted with an 8-mm disk of Whatman #2 filter paper, and the rim of the cap was coated with a thin bead of Thomas Lubriseal grease. Ten muL of neat HD was placed on the filter paper disk, and the cup was immediately inverted and placed onto the back of an anesthetized mouse pup. Exposure times varied from 10 to 30 min. At 24 h postexposure, the mice were euthanized; the HD-exposed skin was removed and fixed in 10% neutral buffered formalin. Following a minimum of 24 h of formalin fixation, the skin sections were bisected across the exposed area. The sections were embedded in paraffin with the central straight-cut surfaces being the focus of histological evaluation. The amount of damage associated with the HD vapor cup exposure varied with time in a dose response fashion. Typical damage consisted of varying amounts of epidermal necrosis at the basal cell level, with occasional separation of epidermis from dermis (microvesication). In severe cases there was complete coagulation of the epidermis and no microvesication. This model should prove useful in identifying the biochemical mechanism of action of HD and ultimately aid in the evaluation of treatment compounds. It may also provide a relevant exposure model for other compounds for which the assessment of vapor-induced damage is necessitated.

Characterization of acute and long-term sulfur mustard-induced skin injuries in hairless guinea-pigs using non-invasive methods

BACKGROUND/PURPOSE

Skin exposure to sulfur mustard (HD) results in erythema, edema and severe injury, which take long time to heal and might impose a heavy burden on the health system. Despite many years of research, there is no treatment that prevents the development of the cytotoxic effects of HD causing acute and prolonged damage to the skin. Therefore, it is of great importance to develop treatments that will ameliorate the extent of injury and improve as well as shorten the healing process. The aim of the present study was to establish a small animal model for a long-term HD-induced skin injury using the hairless guinea-pig (HGP) and to further test the efficacy of anti-inflammatories in ameliorating the pathology.

METHODS

HGPs were exposed to HD vapor on four sites for various time durations (1, 5, 10, 15 and 30 min). Clinical evaluation was conducted using reflectance colorimetry, transepidermal water loss and wound-area measurements. Biochemical [prostaglandin (PGE) content and metalloproteinase-9 (MMP-9) activity] and histopathological evaluations were conducted up to 2 weeks post-exposure.

RESULTS

Typical symptoms of HD skin injury developed including erythema and edema and the extent of injury was closely related to the exposure duration. Histological evaluation revealed severe edema, infiltration of inflammatory cells, damage to basal cells and vesication. By 2 weeks, healing was not completed, impaired basement membrane and epithelial hyperplasia were observed. PGE content and MMP-9 activity increased at 2 h post-exposure; however, while PGE returned to baseline levels within 24 h, MMP-9 remained elevated at least up to 48 h. Furthermore, a short-term, topical, anti-inflammatory post-exposure treatment was effective in reducing the extent of the acute injury.

CONCLUSION

These results indicate that the effects of HD on HGP skin are similar to previously shown effects in the pig model and in humans and therefore support the use of the HGP as an animal model for long-term effects of HD on skin injury and for studying the efficacy of anti-inflammatory treatments.

Detection and measurement of sulfur mustard offgassing from the weanling pig following exposure to saturated sulfur mustard vapor

Animal models are employed to investigate mechanisms of injury and to evaluate protective measures against sulfur mustard (HD) exposure. The ability to detect and quantify HD enables the researcher to follow safe procedures in handling skin samples. We designed an experimental procedure to measure HD offgassing from animal models. A Minicams--a portable gas chromatograph equipped with a flame photometric detector and on-line sorbent collection and desorption--was used to monitor the HD concentration. Confirming measurements were made using a two-step process that trapped HD on a Tenax sorbent off-line and then transferred the sample by means of an ACEM 900 to a gas chromatograph equipped with either a flame photometric detector or a mass spectrometer. Sulfur mustard offgassing data are presented from three experiments in which weanling pigs were exposed to saturated HD vapor via vapor caps containing 10 microl of HD. The HD concentration was measured in time-weighted-average (TWA) units at a specific HD application site. The current 8-h maximum exposure limit for HD is 3-ng l(-1), (1 TWA unit). The largest TWA value measured near a 3 h time point was a Minicams measurement of 0.48 TWA at 2 h and 53 min after removal of a vapor cap containing HD from a single exposure site on an animal that had 24 concurrent dorsal exposure sites. Gas chromatography/flame photometric detection and gas chromatography/mass spectrometry were used to confirm the Minicams data and to provide greater sensitivity and selectivity down to 0.1 TWA. The gas chromatography/mass spectrometry data confirmed that HD concentrations fell below 0.1 TWA in <5 h for a specific site. These measurements of HD concentrations provided information on the expeditious and safe handling of HD-exposed tissue.

Therapeutic approaches to dermatotoxicity by sulfur mustard. I. Modulaton of sulfur mustard-induced cutaneous injury in the mouse ear vesicant model

The mouse ear edema model is recognized for its usefulness in studying skin responses and damage following exposure to chemical irritants, and for evaluating pharmacological agents against chemically induced skin injury. We recently modified the mouse ear edema model for use with sulfur mustard (HD) and used this model to study the protective effect of 33 topically applied compounds comprising five pharmaceutical strategies (anti-inflammatories, protease inhibitors, scavengers/chelators, poly(ADP-ribose) polymerase (PARP) inhibitors, calcium modulators/chelators) against HD-induced dermatotoxicity. Pharmacological modulation of HD injury in mouse ears was established by a reduction in edema or histopathology (epidermal necrosis and epidermal-dermal separation) at 24 h following topical liquid HD exposure. Ten of the 33 compounds administered as single topical pretreatments up to 2 h prior to HD challenge produced significant reductions in edema. Five of these ten also produced significant reductions in histological endpoints. Three candidates (olvanil, indomethacin, hydrocortisone) showing protection at 24 h were evaluated further for 'extended protection' at 48 and 72 h after HD challenge and showed significant modulation of edema at 48 h but not at 72 h. Olvanil also showed significant reductions in histology at 48 and 72 h. Olvanil and indomethacin were shown to reduce significantly the edema at 24 h post-exposure when administered topically 10 min after HD challenge, with olvanil additionally protecting against epidermal necrosis. These results demonstrate prophylactic and treatment effects of pharmacological agents against HD-induced skin injury in an in vivo model and support the continued use of the mouse ear vesicant model (MEVM) for evaluating medical countermeasures against HD.

Immunohistochemical characterization of the basement membrane epitopes in bis(2-chloroethyl) sulfide-induced toxicity in mouse ear skin

Sulfur mustard (bis(2-chloroethyl) sulfide (HD)), a potent cutaneous vesicant and bifunctional alkylating agent, produces significant time-dependent histopathological changes in the skin of the mouse. The right ears of male CD1 mice were exposed topically to 5.0 microl of 195 mM (0.16 mg) HD in dichloromethane and harvested at 6, 12, 18 and 24 h. The left ear control was dosed with 5.0 microl of dichloromethane. In all controls and HD-treated mouse ear, moderate immunofluorescence staining was seen at the epidermal-dermal junction with bullous pemphigoid (BP), epidermolysis bullosa acquisita (EBA) and laminin (Lam), and light staining was observed with bullous pemphigoid 180 (BP180), fibronectin (Fn) and type IV collagen (Coll IV). Mouse anti-human monoclonal antibodies for GB3, L3d and 19-DEJ-1 (Uncein) did not cross-react. In microvesicles, BP, BP180 and Fn showed areas of light focal epidermal staining and homogeneous dermal staining, and EBA, Lam and Coll IV showed moderate dermal staining. Both BP and Fn exhibited weak, inconsistent staining with time. Immunoelectron microscopy (IEM) revealed similar results, with an increase in cell damage from 6 to 24 h, which corresponded to a decrease in staining intensity. Cell proliferation, expressed as the growth fraction of proliferating cell nuclear antigen (PCNA), showed an increase in cell damage. The growth fraction was lower in the inner ear and showed time-dependent differences. The immunofluorescence and IEM results indicate that HD causes an undulating inconsistent separation in the uppermost lamina lucida with focal cleavage into the lower portion of the basal keratinocytes just above the plasma membrane. Although this pattern of separation differs from other in vivo models in which the split occurs exclusively within the lamina lucida, this should not preclude its role as a screening model to study the effects and development of specific prophylactic and therapeutic strategies.

Cutaneous uptake of 14C-HD vapor by the hairless guinea pig

The hairless guinea pig (HGP) is used by our laboratory to model the human cutaneous response to sulfur mustard (HD), bis(2-chloroethylsulfide), exposure. We determined the HD content in the skin of HGP after a 7-min exposure to vapors saturated with a mixture of HD and 14C-HD. Concentration/time (CT) values in the range of 2 micrograms/cm2/min were determined by counting skin 14C disintegrations per min (dpm) in animals euthanized immediately after exposure. These values are similar to human penetration rates obtained by other investigators. A rate curve monitoring the reduction in skin 14C dpm was developed for animals euthanized between 0 and 24 hr post- exposure. This curve showed the greatest change after 1 hr. The epidermal (62%) to dermal (38%) ratio of 14C at 24 hr was measured for two animals. We saw no site preference for HD penetration among the 8 sites used. The 14C content of template adhesive tape was determined to follow HD distribution. These results contribute to a better understanding of the cutaneous response to HD in the HGP model.

Cutaneous exposure to bis-(2-chloroethyl)sulfide results in neutrophil infiltration and increased solubility of 180,000 Mr subepidermal collagens

Exposure to bis-(2-chloroethyl)sulfide (BCES; "sulfur mustard") causes delayed formation of slowly healing skin blisters. Although the histopathology of BCES injury is well characterized [reviewed in Smith et al., J Am Acad Dermatol 32: 767-776, 1995], little is known of the cutaneous toxicity at the molecular level. To identify biological markers of exposure, epidermal and subepidermal extracts were prepared from 48 individual hairless guinea pigs (HGP) at successive 3-hr intervals following exposure to BCES vapor, and compared using gel electrophoresis, and lectin- and antisera-binding. Inflammation was assessed by measuring edema and myeloperoxidase activity. Edema reached peak levels at 15-18 hr and remained elevated above controls at 24 hr. Recruitment of neutrophils, deduced from increased myeloperoxidase, occurred as early as 3 hr after BCES exposure with maximum infiltration at 6-12 hr. Binding of concanavalin-A lectin revealed increased amounts, relative to contralateral control sites, of two approximately 180,000 Mr polypeptides in subepidermal protein extracts from the BCES-exposed skin obtained > or = 12 hr after exposure. This alteration was not found in epidermal protein extracts prepared from the same animals. Based upon the determined amino acid compositions, both polypeptides had significant collagenous triple helical content (>75%). They could be distinguished immunologically from collagen types I, III, and IV by using polyclonal antisera. We conclude that exposure of HGP skin to BCES results in an early neutrophil infiltration that precedes epidermal-dermal separation and selective alterations of the subepidermal extracellular matrix.

Ultrastructural characterization of sulfur mustard-induced vesication in isolated perfused porcine skin

The isolated perfused porcine skin flap (IPPSF) is a novel alternative, humane in vitro model consisting of a viable epidermis and dermis with a functional microvasculature. For this study, 200 microliters of either 10.0, 5.0, 2.5, 1.25, 0.50, or 0.20 mg/ml of bis (2-chloroethyl) sulfide (HD) in ethanol or ethanol control was topically applied to a 5.0 cm2 dosing area of the IPPSF and perfused for 8 h with recirculating media. HD dermatotoxicity was assessed in the flap by cumulative glucose utilization (CGU), vascular resistance (VR), light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). HD produced a statistically significant dose relationship for gross blisters and microvesicles. The HD-treated IPPSFs were also characterized by a decrease in CGU and an increase in VR. Light microscopic changes included mild intracellular and slight intracellular epidermal edema, multifocal epidermal-dermal separation, and dark basal cells. Ultrastructural alterations consisted of cytoplasmic vacuoles, pyknotic basal cells, nucleolar segregation, and epidermal-dermal separation occurring between the lamina lucida and lamina densa of the basement membrane. The severity of these changes increased in a dose-dependent manner. Morphologically, the IPPSF appeared similar to human skin exposed to HD with the formation of macroscopic blisters and microscopic vesicles. In conclusion, the IPPSF appears to be an appropriate in vitro model with which to study the pathogenesis of vesicant-induced toxicity.

Cutaneous histopathologic features in weanling pigs after exposure to three different doses of liquid sulfur mustard

Sulfur mustard (2,2' dichlorodiethyl sulfide, HD) is a chemical warfare agent that is easily produced, and may be used against civilian populations as well as against military troops. However, good therapeutic and prophylactic measures await a better understanding of the pathophysiology of lesions produced by this agent. Because the skin remains is one of the principal routes for HD exposure and damage, the study of HD-induced skin lesions is of major interest. Blister formation is a characteristic of HD-induced skin lesions in humans. Attempts have been made to find an animal model that produces cutaneous microblisters after exposure to the naturally occurring liquid as well as vaporized HD. Weanling pigs were exposed to three different doses of liquid HD. Histopathologic findings showed microblister formation as well as variable apoptosis and/or necrosis of epidermal keratinocytes and vascular endothelium. Pig skin is morphologically similar to human skin. In the pig, the epidermal lipids, the density of hair follicles, the presence of sweat glands, the proliferation kinetics, and the antigenicity are all closer to human skin than are rodent models. All these features may be important in lesions induced by HD, and may mean that the pig is a superior model for studying the pathophysiology of HD-induced cutaneous lesions.

Hematological profile of the euthymic hairless guinea pig following sulfur mustard vesicant exposure

Sulfur mustard (HD) is a potent vesicating agent of military importance, with known radiomimetic properties. The euthymic hairless guinea pig (EHGP) (Cavia porcellus) is emerging as the animal model of choice for cutaneous HD study. With elucidation of the systemic effects, we may better utilize this animal for all HD toxicity work. To this end, studies were conducted to determine the definitive median lethal dose (MLD) of subcutaneously applied sulfur mustard (HD) in the EHGP, and to correlate the induced hematological changes. Eight groups of two animals each were dosed at 0.3 log intervals from an extrapolated expected dose, deriving a tentative mean around which five groups of six animals each were dosed at 0.1 log intervals, resulting in a definitive MLD of 48.17 mg kg(-1). Sulfur mustard was then administered to seven groups of six animals each at a dose of 30 mg kg(-1) and hematology performed. Significant leukocyte count suppression was found to occur on days 4, 5 and 6, following a leukocyte elevation on day 1 after exposure. Serum potassium levels were found to be elevated all 7 days after HD exposure. Establishing the MLD for subcutaneously applied HD and the pattern of induced leukocyte suppression allows for more definitive evaluation of successful toxicity counter-measures.

Histopathologic features seen in sulfur mustard induced cutaneous lesions in hairless guinea pigs

Sulfur mustard (SM), a chemical warfare agent first used early in the 20th century, has re-emerged in the past decade as a major threat around the world. At present, there are no effective therapeutic measures for SM exposure. Because the skin as well as other interface epithelial surfaces are the first tissues effected as this agent is absorbed, reactions within the skin are an area of active research into the mechanism of action of this alkylating agent. The euthymic hairless guinea pig has been used as the animal model for the study of SM induced injuries because of morphologic similarity of its skin to human skin, with a multiple layer epidermis, and because this animal has a normal immune system. We reviewed 102 biopsy specimens from 51 animals exposed to three different dose times of saturated SM vapor. Histopathologic evidence exists for increased programmed cell death as well as cellular necrosis, subepidermal blister formation, and delayed re-epithelialization secondary to problems with adhesion. Information obtained from this study adds to the body of information important in the investigation of the mechanisms of action of SM.

Indirect immunohistochemistry and immunoelectron microscopy distribution of eight epidermal-dermal junction epitopes in the pig and in isolated perfused skin treated with bis (2-chloroethyl) sulfide

Sulfur mustard (bis [2-chloroethyl] sulfide, HD) is a potent cutaneous vesicant that causes gross blisters by separation of the epidermal-dermal junction (EDJ). The EDJ of the skin is a highly specialized and complex structure composed of several components and plays a major role in the integrity of the skin. The isolated perfused porcine skin flap (IPPSF) was dosed with 0.2 mg/ml (n = 4), 5.0 mg/ml (n = 4), and 10.0 mg/ml (n = 5) HD or ethanol (n = 4) for 8 hr (dose-response study) and 10.0 mg/ml HD or ethanol for 1, 3, 5, and 8 hr (n = 4/treatment) (time-response study). Successful EDJ mapping was carried out in normal pig skin (NPS), ethanol-treated IPPSFs, and HD-treated IPPSFs using the following antibodies: laminin, type IV collagen, fibronectin, GB3 (Nicein), bullous pemphigoid (BP), and epidermolysis bullosa acquisita (EBA). Two mouse anti-human monoclonal antibodies, L3d and 19-DEJ-1 (Uncein), did not cross-react with the EDJ of the pig. Antibody staining in NPS, ranging from very intense for laminin and type IV collagen to weak for fibronectin, was generally more discrete than in the IPPSF. No differences in staining were noted between the ethanol and nonblistered areas of the HD-treated IPPSFs. In HD-blistered areas, BP stained only the epidermal hemidesmosomes, and laminin, fibronectin, and GB3 stained primarily the dermis with fragments attached to the basal pole of the stratum base cells, while type IV collagen and EBA stained only the dermis. Mapping of these epitopes determined that the precise plane of EDJ separation in the HD-treated skin occurred beneath the hemidesmosomes within the upper portion of the lamina lucida. The conservation of human epitopes in the EDJ of the pig further emphasizes the similarities between human skin and pig skin. Therefore, pig skin and the IPPSF may be used to study HD-induced vesication and blistering diseases.

Changes in connective tissue macromolecular components of Yucatan mini-pig skin following application of sulphur mustard vapour

1. The aim of this study was to determine the nature of the macromolecular alterations in Yucatan mini-pig skin which occur following application of sulphur mustard vapour, with particular reference to laminin and type IV collagen. 2. The immunostaining of transfer blots from skin extracts run on SDS-PAGE gels revealed no evidence of cross-linking of type IV collagen or laminin. Laminin was, however, found to be partially degraded as determined by the resolution of 132 and 143 kDa fragments, possibly by the activation of proteases, following the application of sulphur mustard to pig skin. Type IV collagen was not subject to this form of degradation in the skin samples exposed to sulphur mustard. 3. Yucatan mini-pig skin was found to develop microblisters after exposure to sulphur mustard vapour. The immunohistochemical studies of sulphur mustard exposed skin revealed that separation of the epidermis from the dermis was found to occur within the lamina lucida of the subepidermal basement membrane, supporting the contention that cleavage of laminin networks occurs following mustard challenge. Immunohistochemical staining with anti-type IV collagen antibodies was restricted to the floor of the micro-blister lesions. 4. The results suggest that laminin may be a target for protease activation at the dermo-epidermal junction. This may account for the tendency of certain skin models to develop sulphur mustard-induced blistering. The Yucatan mini-pig may be valuable as a model to determine the efficacy of prophylactic and therapeutic regimes.

Hypochlorite solution as a decontaminant in sulfur mustard contaminated skin defects in the euthymic hairless guinea pig

Hypochlorite solutions are thought to be efficacious when used to topically decontaminate intact skin. However, few studies have examined the efficacy of decontamination of chemically contaminated wounds. Therefore, we compared the decontamination efficacy of sodium hypochlorite (0.5% and 2.5% solutions), calcium hypochlorite (0.5% and 2.5% solutions) and sterile water to untreated controls in wounds exposed to sulfur mustard (HD). Anesthetized euthymic hairless guinea pigs (EHGP) (n = 6) were exposed to 20 mg/kg (approximately 0.4 LD50) HD in a full-thickness 8 mm surgical biopsy skin defect (i.e., wound). Each animal was subsequently decontaminated, after a two-minute intra-wound exposure to liquid HD, with nothing or one of the decontamination solutions. Decontamination efficacy was determined by the visual grading of the HD-traumatized wound lesion and by comparison of the expected HD-induced leukocyte suppression. Leukocyte suppression was inconsistent in all animals; therefore, the visual grading was the only viable evaluation method. No significant differences were observed among wounds decontaminated with any of the solutions. However, the skin surrounding non-decontaminated (but exposed) control animals showed the least visual pathology. The lesions induced following decontamination are presumed to be due to the mechanical flushing of HD onto the peri-lesional skin, or by chemical damage induced by the solution, or HD-solution interaction. Further studies are required to best delineate the optimal decontamination process for HD contaminated wounds.