Immunogenicity of Escherichia coli Outer Membrane Vesicles: Elucidation of Humoral Responses against OMV-Associated Antigens

Outer membrane vesicles (OMVs) produced by Gram-negative bacteria have emerged as a novel and flexible vaccine platform. OMVs can be decorated with foreign antigens and carry potent immunostimulatory components. Therefore, after their purification from the culture supernatant, they are ready to be formulated for vaccine use. It has been extensively demonstrated that immunization with engineered OMVs can elicit excellent antibody responses against the heterologous antigens. However, the definition of the conditions necessary to reach the optimal antibody titers still needs to be investigated. Here, we defined the protein concentrations required to induce antigen-specific antibodies, and the amount of antigen and OMVs necessary and sufficient to elicit saturating levels of antigen-specific antibodies. Since not all antigens can be expressed in OMVs, we also investigated the effectiveness of vaccines in which OMVs and purified antigens are mixed together without using any procedure for their physical association. Our data show that in most of the cases OMV-antigen mixtures are very effective in eliciting antigen-specific antibodies. This is probably due to the capacity of OMVs to "absorb" antigens, establishing sufficiently stable interactions that allow antigen-OMV co-presentation to the same antigen presenting cell. In those cases when antigen-OMV interaction is not sufficiently stable, the addition of alum to the formulation guarantees the elicitation of high titers of antigen-specific antibodies.

Engineered Bacterial Outer Membrane Vesicles with Lipidated Heterologous Antigen as an Adjuvant-Free Vaccine Platform for Streptococcus suis

Bacterial outer membrane vesicles (OMVs) are considered a promising vaccine platform for their high built-in adjuvanticity and ability to efficiently induce immune responses. OMVs can be engineered with heterologous antigens based on genetic engineering strategies. However, several critical issues should still be validated, including optimal exposure to the OMV surface, increased production of foreign antigens, nontoxicity, and induction of powerful immune protection. In this study, engineered OMVs with the lipoprotein transport machinery (Lpp) were designed to present SaoA antigen as a vaccine platform against Streptococcus suis. The results suggest that Lpp-SaoA fusions can be delivered on the OMV surface and do not have significant toxicity. Moreover, they can be engineered as lipoprotein and significantly accumulated in OMVs at high levels, thus accounting for nearly 10% of total OMV proteins. Immunization with OMVs containing Lpp-SaoA fusion antigen induced strong specific antibody responses and high levels of cytokines, as well as a balanced Th1/Th2 immune response. Furthermore, the decorated OMV vaccination significantly enhanced microbial clearance in a mouse infection model. It was found that antiserum against lipidated OMVs significantly promoted the opsonophagocytic uptake of S. suis in RAW246.7 macrophages. Lastly, OMVs engineered with Lpp-SaoA induced 100% protection against a challenge with 8× the 50% lethal dose (LD50) of S. suis serotype 2 and 80% protection against a challenge with 16× the LD50 in mice. Altogether, the results of this study provide a promising versatile strategy for the engineering of OMVs and suggest that Lpp-based OMVs may be a universal adjuvant-free vaccine platform for important pathogens. IMPORTANCE Bacterial outer membrane vesicles (OMVs) have become a promising vaccine platform due to their excellent built-in adjuvanticity properties. However, the location and amount of the expression of the heterologous antigen in the OMVs delivered by the genetic engineering strategies should be optimized. In this study, we exploited the lipoprotein transport pathway to engineer OMVs with heterologous antigen. Not only did lapidated heterologous antigen accumulate in the engineered OMV compartment at high levels, but also it was engineered to be delivered on the OMV surface, thus leading to the optimal activation of antigen-specific B cells and T cells. Immunization with engineered OMVs induced a strong antigen-specific antibodies in mice and conferred 100% protection against S. suis challenge. In general, the data of this study provide a versatile strategy for the engineering of OMVs and suggest that OMVs engineered with lipidated heterologous antigens may be a vaccine platform for significant pathogens.

Antibody-mediated immunity induced by engineered Escherichia coli OMVs carrying heterologous antigens in their lumen

Background

Outer membrane vesicles (OMVs) from Gram-negative bacteria are gaining increasing attention as vaccine platform for their built-in adjuvanticity and for their potential use as carriers of heterologous antigens. These 2 properties offer the opportunity to make highly effective, easy to produce multi-valent vaccines. OMVs can be loaded with foreign antigens by targeting protein expression either to the outer membrane or to the periplasm of the OMV-producing strain. Periplasmic expression is simple and relatively efficient but leads to the accumulation of recombinant antigens in the lumen of OMVs and the ability of OMVs carrying internalized antigens to induce antigen-specific antibody responses has been only marginally investigated and is considered to be sub-optimal.

Methods

We have systematically analyzed in qualitative and quantitative terms antibody responses induced by OMVs carrying different heterologous antigens in their lumen. Group A Streptococcus (GAS) Slo, SpyCEP, Spy0269 and Group B Streptococcus (GBS) SAM_1372 were fused to the OmpA leader sequence for secretion and expressed in Escherichia coli. OMVs from the recombinant strains were purified and tested for immunogenicity and protective activity.

Results

All proteins were incorporated into the OMVs lumen in their native conformation. Upon mice immunization, OMVs induced high functional antibody titers against the recombinant proteins. Furthermore, immunization with Slo-OMVs and SpyCEP-OMVs protected mice against GAS lethal challenge.

Conclusions

The efficiency of antigen delivery to the vesicular lumen via periplasmic expression, and the surprisingly high immunogenicity and protective activity of OMVs carrying internalized recombinant antigens further strengthens the potential of OMVs as vaccine platform.