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Updated: May 26, 2026
Evaluation of Host-Pathogen Responses and Vaccine Efficacy in Mice
Published on: February 22, 2019
Immunization with the RrgB321 fusion protein protects mice against both high and low pilus-expressing Streptococcus
Monica Moschioni1, Gabriella De Angelis, Carole Harfouche
1Novartis Vaccines and Diagnostics, 53100 Siena, Italy. monica.moschioni@novartis.com
Abstract:
RrgB321, a fusion protein of the three Streptococcus pneumoniae pilus-1 backbone RrgB variants, is protective in vivo against pilus islet 1 (PI-1) positive pneumococci. In addition, antibodies to RrgB321 mediate a complement-dependent opsonophagocytosis of PI-1 positive strains at levels comparable to those obtained with antisera against glycoconjugate vaccines. In the pneumococcus, pilus-1 displays a biphasic expression pattern, with different proportions of two bacterial phenotypes, one expressing and one not expressing the pilus-1. These two populations can be stably separated in vitro giving rise to the enriched high (H) and low (L) pilus expressing populations. In this work we demonstrate that: (i) the opsonophagocytic killing mediated in vitro by RrgB321 antisera is strictly dependent on the pilus expression ratio of the strain used; (ii) during the opsonophagocytosis assay pilus-expressing pneumococci are selectively killed, and (iii) no switch towards the pilus non-expressing phenotype can be observed. Furthermore, in sepsis and pneumonia models, mice immunized with RrgB321 are significantly protected against challenge with either the H or the L pilus-expressing population of strains representative of the three RrgB variants. This suggests that the pilus-1 expression is not down-regulated, and also that the expression of the pilus-1 could be up-regulated in vivo. In conclusion, these data provide evidence that RrgB321 is protective against PI-1 positive strains regardless of their pilus expression level, and support the rationale for the inclusion of this fusion protein into a multi-component protein-based pneumococcal vaccine.
Insights
RrgB321 fusion protein protects against Streptococcus pneumoniae, even with varying pilus-1 expression levels. Antibodies against RrgB321 promote opsonophagocytosis, supporting its use in pneumococcal vaccines.
Area of Science:
- Microbiology
- Immunology
- Vaccine Development
Background:
- Streptococcus pneumoniae causes significant morbidity and mortality.
- Pilus islet 1 (PI-1) positive pneumococci express pilus-1, a virulence factor.
- RrgB321, a fusion protein of PI-1 backbone variants, shows protective potential.
Purpose of the Study:
- To evaluate the protective efficacy of RrgB321 against Streptococcus pneumoniae.
- To investigate the role of pilus-1 expression levels in RrgB321-mediated protection.
- To assess the opsonophagocytic activity of antibodies against RrgB321.
Main Methods:
- In vitro opsonophagocytosis assays using high (H) and low (L) pilus-expressing pneumococcal strains.
- In vivo protection studies in mouse models of sepsis and pneumonia.
- Analysis of pilus-1 expression and phenotype switching during opsonophagocytosis.
Main Results:
- RrgB321 antisera mediated complement-dependent opsonophagocytosis, dependent on pilus expression ratio.
- Pilus-expressing pneumococci were selectively killed in vitro; no phenotype switching occurred.
- Mice immunized with RrgB321 were protected against challenge with both H and L pilus-expressing strains in vivo.
Conclusions:
- RrgB321 provides protection against PI-1 positive pneumococci irrespective of pilus expression levels.
- Pilus-1 expression may be up-regulated in vivo, suggesting its importance during infection.
- RrgB321 is a promising candidate for inclusion in protein-based pneumococcal vaccines.
