Immunotoxicity of 2-Acetyl-4-tetrahydroxybutylimidazole in BALB/c mice with different vitamin B6 nutritional statuses

Caramel color is a widely used food pigment, and 2-Acetyl-4-tetrahydroxybutylimidazole (THI) is a by-products of Class III caramel color. Some studies have shown that THI can reduce the number of peripheral blood lymphocytes. However, the comprehensive mechanism of THI immunotoxicity requires further study. In this study, the effects of THI on lymphocyte count, humoral immunity, cellular immunity and nonspecific immunity were determined and the effect of the nutritional status of VB6 on THI immunotoxicity was evaluated. Female BALB/c mice were divided into 3 groups and fed chow containing different doses of VB6: VB6-normal (6 mg/kg VB6), VB6-deprived (0.5 mg/kg VB6) or VB6-enhanced (12 mg/kg VB6) feed. Each group was further divided into 4 subgroups and treated with THI (0.5, 2.5 or 12.5 mg/kg bw) or the solvent control by gavage for 30 days. The thymic cortical thickness was measured with ViewPoint; the proportions of major immune cells and T cells in peripheral blood and tissues were detected via flow cytometry; the transformation and proliferation abilities of T and B cells were detected via T and B lymphocyte proliferation assays; NK cell activity was assessed via lactate dehydrogenase assays; humoral immune function was assessed via plaque-forming cell assays; and the immune function of T lymphocytes was assessed via delayed type hypersensitivity assays. The results showed that compared with those in the corresponding control group, the white blood cell count and lymphocyte count decreased significantly in all the VB6-deprived groups, in the 2.5 and 12.5 mg/kg VB6 groups, and in the 12.5 mg/kg VB6-enhanced group. With increasing THI dose, the thymic cortical layer became thinner. In the thymus, THI increased the proportions of CD3+ T cells and mature CD8+ T cells and decreased the proportions of immature double-positive, double-negative T cells and CD69-expressing lymphocytes. The proportions of naïve T cells and Tcm (central memory T) cells related to homing decreased. The proportion of mature T cells in the spleen decreased significantly. The proliferation of T cells stimulated by ConA decreased after THI exposure. VB6-deficient mice were more sensitive to THI immunotoxicity, and supplementation with VB6 had a certain protective effect on these mice. The results of the PFC and NK cell activity assays indicated that THI exposure might not affect humoral immune or innate immune function.