Service Communautaire d'Information sur la Recherche et le Développement - CORDIS

Expression of Meningoccal antigens in Streptococcus gordonii strains and assessment of their immunogenicity

The Gram positive commensal bacterium Streptococcus gordonii was employed as a live delivery system for mucosal immunization. The genetic system that allows the expression of foreign proteins on the bacterial surface is based on the integration of the heterologous gene into the bacterial chromosome downstream of a strong resident promoter.

The streptococcal M6 surface protein is employed as a fusion partner to allow the surface expression of heterologous proteins. By using this system it is also possible to simultaneously express two different foreign proteins on the same bacterial surface. This vector is therefore able to co-express a vaccine antigen with an adjuvant molecule.

Recombinant strains of S. gordonii developed within the MUCIMM project expressed the meningococcal antigens NadA (NMB1984) and Orf40 (NMB 0992) as single heterologous proteins or co-expressed NadA with the B subunit of LT (LTB). Meningococcal antigens had previously been identified by Chiron as novel and potential vaccine candidates. The expression of the proteins, tested by flow cytometric analysis, was very efficient.

Recombinant bacterial strains were employed in an immunization experiment of mice by the intranasal route. BALB/c mice were inoculated at 0, 3, and 6 weeks with 109 CFU of different bacterial strains. The recombinant strains GP1349 and GP1369 were administered alone or mixed with the soluble mucosal adjuvant LTR72, a genetically modified LT-derived molecule (developed by Chiron). This adjuvant was chosen as it was shown to be an effective mucosal adjuvant in previous studies.

Recombinant bacteria administered by the mucosal route were extremely efficient in inducing serum antibody response both against NadA and Orf40. The mean value (calculated among 7 mice) of serum NadA-specific IgG concentration was 21.3 mg/ml while anti Orf40 was 6.3 mg/ml. When recombinant bacteria were intranasally administered with the LTR72 mucosal adjuvant, a further increase in serum antigen-specific IgG was detected (320 mg/ml of anti-NadA and 25 mg/ml of anti-Orf40 serum IgG).

A significant serum bactericidal activity against N. meningitidis 2996, measured in collaboration with partner CR3, was detected in most mice immunized with recombinant bacteria expressing NadA. A correlation between antigen-specific antibodies titre and bactericidal titre was observed in 57% of mice immunized with GP1349 (4 out of 7 mice) and in 85% of mice immunized with GP1349 mixed with LTR72 (6 out of 7 mice). NadA-specific immune response against was also observed at the local level in nasal washes (NW) and bronchoalveolar lavages (BAL).

Antigen-specific IgA production was statistically significant (P = 0.015 and P = 0.045 in NW and BAL respectively) compared to mice immunized with wild type bacteria (with or without LTR72). Local IgA response against Orf40, assessed in both NW and BAL, was not significant. Intranasal immunization was also performed with recombinant bacteria co-expressing NadA and LTB on the bacterial surface (GP1351). The lower result observed compared to mice immunized with GP1349 (both with and without LTR72), is probably due to the lower expression efficiency of the NadA vaccine antigen on the bacterial vector. The expected results were all reached.

We have indeed constructed recombinant S. gordonii strains efficiently expressing the meningococcal vaccine candidates NadA and Orf40. Immunization experiments in mice have shown the immunogenicity of the meningococcal antigens expressed on the bacterial surface and delivered by the mucosal route. A high antibody response was detected both locally and systemically.

A significant bactericidal activity against N. meningitidis 2996 was assayed by partner CR3 in sera of mice immunized with bacteria expressing NadA and a correlation between NadA-specific IgG titre and bactericidal titre was assayed in a high percentage of animals. As a potential vaccine candidate carried by recombinant S. gordonii, NadA has shown to be more immunogenic (both at the systemic and at the local level) than Orf40. The use of the soluble mucosal adjuvant LTR72 was able to increase the serum antibody response against NadA, even if a high immune response was induced also by recombinant bacteria alone, specially at the mucosal level.

More information on the MUCIMM project can be found at:

Informations connexes

Reported by

University of Siena
Via Laterina, 8
53100 Siena
See on map