SM was extensively used during World War I and more recently during the Iran-Iraq war of 1980s [29
]. In addition to its military use, it is a potential weapon of mass destruction against civilian targets and causes both acute and chronic health effects. Respiratory, ocular, and cutaneous injuries are well-documented acute effects of SM exposure. Diverse chronic effects have also been reported, and respiratory complications are the major cause of disability and mortality in SM victims [29
]. The clinical manifestations of SM exposure result from multiple mechanisms, although the exact pathways are not yet elucidated. The recurrent exacerbation of the injuries suggests an autoimmune-like etiology; however, so far experimental evidence to support this possibility has been lacking.
SM is a potent alkylating agent [1
] and some of the highly reactive chemicals are known to cause immunologic sensitization and activation of T cells in vitro and in vivo. Thus natural and synthetic compounds in food, medicines, and cosmetic products can cause skin inflammation, allergic/asthma symptoms, and autoimmunity [33
]. Chemicals such as dinitrofluorobenzene, D-penicillamine, and diphenylhydantoin induce autoimmune responses that resemble graft-versus-host like disease [36
]. Multiple reports suggest that the T-cell responses, including the DTH response to many chemical sensitizers reflect the alloantigen-like response to modified “self” [38
] and may involve chemical modification of the major histocompatibility complex (MHC) antigens [38
]. SM being a highly reactive alkylating agent, it was likely that it modified self cellular antigens to yield a specific DTH-like response. Indeed, our data clearly shows that dermal application of subclinical doses of SM (i.e., the concentrations that did not cause overt blistering in hairless guinea pigs) induced a significant DTH response. SM sensitization is also associated with lymphoproliferation, leading to splenomegaly and lymphadenopathy of the draining lymph nodes. While it is likely that the DTH represents the T-cell response against SM-modified self MHC antigens, at this time we have no direct experiments (e.g., autologous mixed lymphocyte reaction, adoptive transfer of DTH by sensitized T cells) to prove this possibility.
At this time it is unclear how the development of SM-induced DTH would produce the delayed SM-induced sequelae. Drug-induced hypersensitivity has been correlated with the development of autoimmunity, sequential activation of herpesviruses, and loss of Treg function [40
], the latter may promote a hyperimmune response to various pathological agents. Infiltration of both CD4+
T cells was seen in the skin; however, their individual contribution to potential disease is not known. DTH is primarily contributed by CD4+
T cells [38
]; however, there is evidence that in humans intraepidermal CD8+
T cells may persist after drug-induced hypersensitivity, even in the absence of antigenic stimuli, for more than 4 years [39
]. Thus, there are a number of potential immunological pathways through which chemical hypersensitivity might lead to autoimmune-like responses and chronic tissue injury.
The clinical and histopathological aspects of SM cutaneous exposure have been well studied, and a variety of in vivo and in vitro experiments show induction of inflammatory mediators after exposure to SM. TNF-α, a prototype proinflammatory cytokine, is produced by a number of cell types, predominantly by macrophages and monocytes [39
], and is strongly upregulated in hairless guinea pigs by SM after dermal exposure. TNF-α stimulates the production of the neutrophil chemokine IL-8 by alveolar macrophages [39
] and, along with IFN-γ, it plays a critical role in the experimental models of lymphoproliferation [38
]. Thus, it is conceivable that increased expression of TNF-α and IFN-γ contribute to the lymphoproliferation observed in SM-treated animals. IFN-γ also promotes Th1 polarization [39
]; however, we have no evidence that the CD8+
T cells, seen at the site of inflammation, develop into cytotoxic T cells. To a lesser extent, SM also increased the expression of IL-8 in the lymph nodes and might contribute to the neutrophilic infiltration in the skin of SM-exposed animals. Qabar et al. [42
] have shown that the expression of TNF-α altered keratinocyte sensitivity to SM-induced cell death, and it is likely that SM stimulates the expression of other proinflammatory cytokine/chemokines, but the lack of guinea pig-specific reagents prevented us to determine their expression. However, the manners through which inflammatory factors or other hitherto unidentified factors produced in response to SM exposure, lead to tissue injury is not clear at present and remain to be delineated.
Increasing evidence suggests that noninhalation exposures of SM might produce pulmonary pathology, which is the major cause of morbidity and mortality among human SM victims [30
]; however, the mechanism of pulmonary toxicity under these conditions is unclear. Interestingly, guinea pigs exposed to SM through the cutaneous route also exhibit increased expression of TNF-α and IFN-γ in the lung. SM is a highly reactive compound and preliminary results suggest that very little if any enters circulation after the dermal exposure (Benson et al., unpublished observation). Whether SM promotes the emigration of activated leukocytes, particularly T cells, is not known, and might be critical for understanding the mechanism of lung injury following dermal exposure to SM.