Covalent binding of the nitroso metabolite of sulfamethoxazole leads to toxicity and major histocompatibility complex-restricted antigen presentation.

Treatment with sulfamethoxazole (SMX) can lead to hypersensitivity reactions. T cells from hypersensitive patients recognize either the parent drug and/or the reactive nitroso (SMX-NO) metabolite. In this study, using a novel in vitro rat splenocyte assay, we have investigated the toxicological and immunological consequences of cell surface haptenation by SMX-NO. SMX-NO was found to be unstable in solution; spontaneous transformation yielded appreciable amounts of SMX-hydroxylamine, nitro-SMX, and the previously unknown azoxy and azo dimers within 15 min. Irreversible binding of SMX-NO to cellular protein was demonstrated by flow cytometry, with haptenation being greater on the surface of antigen-presenting cells than on T cells. The consequences of irreversible binding of SMX-NO were examined in two ways. First, haptenation above a threshold level led to a proportionate increase in cell death (both apoptosis and necrosis). Indeed, the cells that became haptenated were the same as those that underwent necrotic cell death. Second, sensitized splenocytes proliferated in the presence of major histocompatibility complex (MHC)-restricted antigen derived from both viable and dead cells haptenated with low and high levels of SMX-NO, respectively. However, direct modification of MHC by SMX-NO was not the mechanism of antigen presentation. The antigenic threshold of SMX-NO for T-cell proliferation and toxicity was estimated to be between 0.5 and 1 microM and 5 to 10 microM, respectively. The potential of SMX-NO to generate a potent antigen and cause cytotoxicity may in combination provide the signals necessary to induce a hypersensitivity reaction to SMX.