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Toxoplasmosis is a major public health problem and the development of a human vaccine is of high priority. Efficient vaccination against Toxoplasma gondii requires both a mucosal and systemic Th1 immune response. Moreover, dendritic cells play a critical role in orchestrating the innate immune functions and driving specific adaptive immunity to T. gondii. In this study, we explore an original vaccination strategy that combines administration via mucosal and systemic routes of fusion proteins able to target the major T. gondii surface antigen SAG1 to DCs using an antibody fragment single-chain fragment variable (scFv) directed against DEC205 endocytic receptor. Our results show that SAG1 targeting to DCs by scFv via intranasal and subcutaneous administration improved protection against chronic T. gondii infection. A marked reduction in brain parasite burden is observed when compared with the intranasal or the subcutaneous route alone. DC targeting improved both local and systemic humoral and cellular immune responses and potentiated more specifically the Th1 response profile by more efficient production of IFN-γ, interleukin-2, IgG2a, and nasal IgA. This study provides evidence of the potential of DC targeting for the development of new vaccines against a range of Apicomplexa parasites.

In recent years, several molecular engineering methods of designing bispecific antibodies in various formats have been developed. Tandem-scFvs comprising two scFvs fused together via a peptide are 55-kDa molecules, and are one of the most promising and most straightforward approaches to bispecific antibody production. We report an attempt to design more effective antivenoms to the Androctonus australis scorpion using murine scFvs as building blocks to create a unique bispecific molecule that neutralizes the potent neurotoxins AahI and AahII. The tandem-scFv was produced in recombinant bacteria, purified by immobilized metal ion affinity chromatography, and analyzed by polyacrylamide gel electrophoresis, Western blot, gel filtration, mass spectrometry, and direct and competitive radioimmunoassay. In vivo, it neutralized the binding of the AahI and AahII toxins to their receptor, and protected mice against experimental envenomation. The findings reported here highlight the potential of recombinant antibody fragments for protecting against scorpion venom toxicity.

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