Oral mRNA Vaccines Against Infectious Diseases- A Bacterial Perspective [Invited]

The mRNA vaccines from Pfizer/BioNTech and Moderna were granted emergency approval in record time in the history of vaccinology and played an instrumental role in limiting the pandemic caused by SARS-CoV-2. The success of these vaccines resulted from over 3 decades of research from many scientists. However, the development of orally administrable mRNA vaccine development is surprisingly underexplored. Our group specializing in Salmonella-based vaccines explored the possibility of oral mRNA vaccine development. Oral delivery was made possible by the exploitation of the Semliki Forest viral replicon and Salmonella vehicle for transgene amplification and gene delivery, respectively. Herein we highlight the prospect of developing oral replicon-based mRNA vaccines against infectious diseases based on our recent primary studies on SARS-CoV-2. Further, we discuss the potential advantages and limitations of bacterial gene delivery.

Highly feasible immunoprotective multicistronic SARS-CoV-2 vaccine candidate blending novel eukaryotic expression and Salmonella bactofection

Introduction

The emergence of SARS-CoV-2 variants has raised concerns on future vaccine efficacy as most vaccines target only the spike protein. Hence, vaccines targeting multiple SARS-CoV-2 proteins will offer broader protection and improve our preparedness to combat the pandemic.

Objectives

The study aimed to develop a novel vaccine strategy by combining a eukaryotic vector expressing multiple SARS-CoV-2 genes and Salmonella-mediated in vivo DNA delivery.

Methods

The eukaryotic vector was designed to function as a DNA-launched RNA replicon in a self-replicating and self-amplifying mRNA mechanism. By exploiting the self-cleaving peptide, P2A, we fused four SARS-CoV-2 targets, including receptor-binding domain (RBD), heptad repeat domain (HR), membrane protein (M) and epitopes of nsp13, in a single open reading frame. Western blot and immunofluorescence assays were used to determine protein expression. In mice, the vaccine's safety and immunogenicity were investigated.

Results

Western blot analysis revealed co-expression all four proteins from the vaccine construct, confirming the efficiency of Salmonella-mediated gene delivery and protein expression. The vaccine candidate was safe and elicited robust antigen-specific antibody titers in mice, and a recall response from splenocytes revealed induction of strong cell-mediated immunity. Flow cytometry demonstrated an increase in sub-populations of CD4+ and CD8+ T cells with the highest CD4+ and CD8+ T cells recorded for HR and RBD, respectively. Overall, humoral and cellular immune response data suggested the induction of both Th1 and Th2 immunity with polarization towards an antiviral Th1 response. We recorded a potent SARS-CoV-2 neutralizing antibody titers in the immunized mice sera.

Conclusions

The Salmonella bactofection ensured optimum in vivo gene delivery, and through a P2A-enabled efficient multicistronic expression, the vaccine candidate elicited potent anti-SARS-CoV-2 immune responses. These findings provide important insight into development of an effective multivalent vaccine to combat SARS-CoV-2 and its variants.

The development of ghost vaccines trials

INTRODUCTION

Bacterial ghosts are intact bacterial cell envelopes that are emptied of their content by gentle biological or chemical poring methods. Ghost techniques increase the safety of the killed vaccines, while maintaining their antigenicity due to mild preparation procedures. Moreover, ghost-platforms may express and/or carry several antigens or plasmid-DNA encoding for protein epitopes.

AREAS COVERED

In this review, the development in ghost-vaccine production over the last 30 years is classified and discussed. The different applications of ghost-vaccines, how they trigger the immune system, their advantages and limitations are displayed. The phage-mediated lysis, molecular manipulation of the lysis-genes, and the biotechnological production of ghosts are described. The trials are classified according to the pattern of lysis and to the type of bacteria. Further subdivision includes chronological ordered application of the ghost as alternative-killed vaccine, recombinant antigen platform, plasmid DNA carrier, adjuvants, and dendritic cell inducer. Particular trials for specific pathogens or from distinct research schools are gathered.

EXPERT OPINION

Ghosts are highly qualified to act as immune-presenting platforms that express and/or carry several recombinant and DNA vaccines, as well as, being efficient alternative-killed vaccines. The coming years will show more molecular advances to develop ghost-production and to express more antigens.

A Salmonella Typhi ghost induced by the E gene of phage φX174 stimulates dendritic cells and efficiently activates the adaptive immune response

Previously, we genetically engineered a Salmonella Typhi bacterial ghost (STG) as a novel inactivated vaccine candidate against typhoid fever. The underlying mechanism employed by the ghost in stimulating the adaptive immune response remains to be investigated. In this study, we aimed to evaluate the immunostimulatory effect of STG on mouse bone marrow-derived dendritic cells (BMDCs) and its activation of the adaptive immune response in vitro. Immature BMDCs were stimulated with STG, which efficiently stimulated maturation events in BMDCs, as indicated by upregulated expressions of CD40, CD80, and major histocompatibility complex class II molecules on CD11⁺ BMDCs. Immature BMDCs responded to STG stimulation by significantly increasing the expression of interleukin (IL)-6, which might indicate the induction of dendritic cell maturation in vivo (p < 0.05). In addition, ghost-stimulated murine BMDCs showed significant expressions of interferon gamma and IL-4, which can drive the development of Th1 and Th2 cells, respectively, in co-cultured CD4⁺ T cells in vitro. These results suggest that STG can effectively stimulate maturation of BMDCs and facilitate subsequent immune responses via potent immunomodulatory cytokine responses.

ADP-ribosylating enterotoxins as vaccine adjuvants

Most infections are caused by pathogens that access the body at mucosal sites. Hence, development of mucosal vaccines to prevent local infection or invasion of pathogens appears highly warranted, especially since only mucosal immunization will stimulate strong local IgA responses and tissue resident memory CD4 and CD8 T cells. The most significant obstacle to developing such vaccines is the lack of approved adjuvants that can effectively and safely enhance relevant mucosal and systemic immune responses. The most potent mucosal adjuvants known today are the adenosine diphosphate (ADP)-ribosylating bacterial enterotoxins cholera toxin (CT) and Escherichia coli heat-labile toxins (LTs). Unfortunately, these molecules are also very toxic, which precludes their clinical use. However, much effort has been devoted to developing derivatives of these enterotoxins with low or no toxicity and retained adjuvant activity. Although it is fair to say that we know more about how these toxins affect the immune system than ever before, we still lack a detailed understanding of how and why these toxins are effective adjuvants. In the present review, we provide a state-of-the-art overview of the mechanism of action of the holotoxins and the strategies used for improving the toxin-based adjuvants.