Immunogenicity of IpaC-Hybrid Proteins Expressed in the Shigella flexneri 2a Vaccine Candidate SC602 |
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| Authors: | Simona Barzu Josette Arondel Sophie Guillot Philippe J. Sansonetti Armelle Phalipon |
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| Affiliation: | Unité de Pathogénie Microbienne Moléculaire, U389 Institut National de la Santé et de la Recherche Médicale,1. and Laboratoire d’Epidémiologie Moléculaire des Entérovirus,2. Institut Pasteur, 75015 Paris, France |
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| Abstract: | We have investigated the capacity of live attenuated Shigella flexneri strains to act as vectors for the induction of local and systemic antibody responses against heterologous epitopes. The S. flexneri IpaC antigen was selected as a carrier protein into which the C3 neutralizing epitope of the poliovirus VP1 protein was inserted in eight sites distributed along IpaC. The resulting IpaC-C3 hybrid proteins were expressed from recombinant plasmids in the S. flexneri 2a vaccine candidate, SC602. Their production was similar to that of wild-type IpaC. All of the hybrid proteins but one were secreted as efficiently as wild-type IpaC. Immunization of mice with each of the recombinant SC602 derivatives reveals that one construct is able to induce serum and local anti-C3 antibodies, showing that at least one permissive site of insertion within IpaC can be defined. Furthermore, mouse-to-mouse variability in the anti-C3 response indicates that the amount of hybrid proteins produced in the host by SC602 should be improved for optimal use of S. flexneri live attenuated strains as mucosal vectors for foreign epitopes.Live attenuated vectors are one of the most efficient delivery systems for stimulation of the mucosa-associated immune system (for a review, see reference 10). They have therefore been extensively used to express foreign antigens and epitopes selected from pathogens against which the induction of a local immune response is required for protection (for a review, see reference 23). Usually, foreign epitopes are inserted within a carrier protein that is expressed in the live vector.Shigella flexneri live attenuated strains have been developed as candidates for vaccines against shigellosis, an invasive disease of the human colon (22). The capacity of such strains to act as mucosal vectors has been recently reported (16). Local and systemic antibody responses to fimbriae and CS3 fibrillae of enterotoxigenic Escherichia coli were generated in guinea pigs or mice following immunization with these antigens expressed in CVD1203, an S. flexneri 2a live attenuated strain that confers protection against Shigella keratoconjunctivitis in the guinea pig model (15).The purpose of the present study was to investigate whether S. flexneri vaccine strains could be used as immunization vectors to express heterologous epitopes of eukaryotic origin and, in turn, elicit local and systemic antibody responses to foreign sequences. The IpaC antigen, previously reported as a potential carrier protein (1), was selected for the insertion of the neutralizing C3 epitope of the VP1 protein of poliovirus (26). This 11-residue-long sequence has been previously used as a reporter epitope (5, 25). As immunogenicity of a B-cell epitope depends on its flanking sequences within the hybrid protein (25), the C3 epitope was inserted into eight different sites within the IpaC coding sequence. SC602, an S. flexneri 2a vaccine strain attenuated both in its capacity to move intra- and intercellularly and in its survival in tissues (1), was used as vector. This strain is safe and protective in the macaque monkey model (8) as well as in human volunteers (9). The IpaC-C3 hybrid proteins were expressed from recombinant plasmids within SC602 to retain the functionality of the wild-type (wt) IpaC, thus maintaining the invasiveness of the live attenuated vector and ensuring efficient stimulation of local immunity. Immunogenicity of the IpaC-C3 proteins expressed in SC602 was assessed following an immunization protocol that allows the induction of local and systemic anti-IpaC antibody responses in mice (1). |
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