Localization of intravenously administered verocytotoxins (Shiga-like toxins) 1 and 2 in rabbits immunized with homologous and heterologous toxoids and toxin subunits

Rabbits challenged intravenously with Shiga toxin or with Escherichia coli verocytotoxin 1 or 2 (VT1 or VT2) are known to develop diarrhea, paralysis, and death, which can be prevented by immunization with a toroid. The pathological effects of VT1 in the central nervous system and the gastrointestinal tract of unimmunized rabbits correlate with the localization of I-125-VT1 in these tissues, whereas in immunized animals, localization of I-125-VT1 in target tissues is inhibited and labeled toxin is cleared by the liver and spleen. By using the approach described above in this study, rabbits immunized with VT1 toroid, VT2 toroid, or with the A or B subunit of each toxin were challenged with intravenous I-125-VT1 or I-125-VT2. After 2 h, the animals were sacrificed, and selected tissues were analyzed for uptake of labeled toxin. It was found that animals immunized with either VT1 toroid or VT2 toroid were protected from target tissue uptake of both I-125-VT1 and I-125-VT2. Rabbits immunized with either the VT1 A or VT2 A subunit were also protected from target tissue uptake of both the homologous and heterologous I-125-labeled holotoxins. In contrast, in animals immunized with the toxin B subunits, protection extended only against challenge by the homologous toxin. These results provide evidence of VT1 and VT2 cross-neutralization in vivo in the rabbit model and indicate that the in vivo cross-neutralization is a function, mainly, of antibodies directed to the VT A subunits. This suggests that the VT1 A or VT2 A subunit may be a suitable immunogen for immunizing humans against systemic VT-mediated disease.