Enhancement of long-lasting immunoprotective effect against Androctonus australis hector envenomation using safe antigens: Comparative role of MF59 and Alum adjuvants.

Envenomation is a public health problem in many regions of the world. The only available treatment is the serotherapy that has limited efficiency due to the delay of its administration. The goal of this study is to provide a new and more efficient alternative to this treatment. A comparative study of the effects of two adjuvants in their ability to enhance the efficiency of the detoxified and safe antigens to produce a long lasting immunoprotection is undertaken using Aluminum Hydroxide adjuvant (Alum) or the water-in-oil MF59 adjuvant mixed with Androctonus australis hector (Aah) detoxified venom, and compare their effects on the immune system. Immunization schedule was performed with two groups of rabbits, which were injected with attenuated venom and Alum or MF59 adjuvant preparations, once a month during three months. Blood samples were collected each week for cell count, evaluation of myeloperoxidase (MPO) and eosinoperoxydase (EPO) activities and antibody titer. After four months from the last immunization, rabbits were challenged with increased doses of native Aah venom. Results showed that MF59 effect was immediate in the first 24h post-immunization by activating the recruitment of lymphocytes, monocytes and neutrophils, while Alum adjuvant effect seems to be delayed, and appeared in the second week after immunization. An important cell infiltration was observed with Alum preparation, due to its specific local depot effect. However, immunized animals with MF59 preparation challenged with the native venom showed a protective effect against its toxicity until 6 LD50 compared to those immunized with Alum preparation which are only protected at 4 LD50. One week after challenge, only immunized animals with Alum preparation present an increase in cell infiltration, MPO and EPO activities. These results are correlated with the ability of MF59 adjuvant to induce a potent immunoprotective effect against Aah venom compared to Alum adjuvant.