Low-dose exposure and immunogenicity of transgenic maize expressing the Escherichia coli heat-labile toxin B subunit

BACKGROUND: Transgenic maize, which produces the nontoxic B subunit of the Escherichia coli heat-labile toxin (LT-B) in seed, has proven to be an effective oral immunogen in mice. Currently, there is considerable concern over accidental consumption of transgenic maize expressing LT-B by humans and domestic animals. We have yet to define nonimmunogenic levels of transgenic LT-B when ingested. OBJECTIVES: Our goal in this study was to determine the highest dose of LT-B orally administered in mice that does not result in a measurable immune response. We defined an immune response as specific serum or mucosal IgG or IgA significantly greater than background after three feedings (0.0002-20 mu g) or a priming response induced by the intermittent feeding. METHODS: We fed transgenic maize pellets on days 0, 7, 21, and 49 and collected serum and fecal samples weekly. Serum was analyzed for LT-B-specific IgG and IgA, and feces was analyzed for LT-B-specific IgA. RESULTS: We observed a dose-dependent anti-LT-B antibody response with high specific antibody concentrations in groups fed high doses (0.2, 2, 20 mu g) of LT-B maize. Mice fed 0.02 mu g LT-B demonstrated immune priming in 62.5% of the animals. Mice that were fed <= 0.002 mu g LT-B showed no increase in specific antibody nor did they demonstrate immune priming, indicating that 0.002 mu g LT-B was the highest nonimmunogenic dose tested. CONCLUSION: Our results demonstrate that LT-B derived from transgenic maize is immunogenic at nanogram levels when orally administered to mice.