Antigenicity and immunogenicity of sulphamethoxazole:: demonstration of metabolism-dependent haptenation and T-cell proliferation in vivo

1 Sulphamethoxazole has been associated with the occurrence of hypersensitivity reactions. There is controversy as to whether the immune response is metabolism-dependent or -independent. We have therefore investigated the site of antigen formation and the nature of the drug signal presented to the immune system in vivo. 2 Male Wistar rats were dosed with sulphamethoxazole, sulphamethoxazole hydroxylamine or nitroso sulphamethoxazole. Antigen formation on cell surfaces was determined by flow cytometry using a specific anti-sulphamethoxazole antibody. Immunogenicity was determined by assessment of ex vivo T-cell proliferation. 3 Administration of nitroso sulphamethoxazole, but not sulphamethoxazole or sulphamethoxazole hydroxylamine, resulted in antigen formation on the surface of lymphocytes, splenocytes and epidermal keratinocytes, and a strong proliferative response of splenocytes on re-stimulation with nitroso sulphamethoxazole. Rats dosed with sulphamethoxazole or sulphamethoxazole hydroxylamine did not respond to any of the test compounds. 4 CD4(+) or CD8(+) depleted cells responded equally to nitroso sulphamethoxazole. The proliferative response to nitroso sulphamethoxazole was seen even after pulsing for only 5 min, and was not inhibited by glutathione. Responding cells produced IFN-gamma, but not IL-4. 5 Haptenation of cells by sulphamethoxazole hydroxylamine was seen after depletion of glutathione by pre-treating the rats with diethyl maleate. Splenocytes from the glutathione-depleted sulphamethoxazole hydroxylamine-treated rats responded weakly to nitroso sulphamethoxazole, but not to sulphamethoxazole or sulphamethoxazole hydroxylamine. 6 Dosing of rats with sulphamethoxazole produced a cellular response to nitroso sulphamethoxatole (but not to sulphamethoxazole or its hydroxylamine) when the animals were primed with complete Freund's adjuvant. 7 These studies demonstrate the antigenicity of nitroso sulphamethoxazole in vivo and provide evidence for the role of drug metabolism and cell surface haptenation in the induction of a cellular immune response in the rat.