Evaluation of Immune Response to Mucosal Immunization with an Oral Probiotic-Based Vaccine in Mice: Potential for Prime-Boost Immunization against SARS-CoV-2

Following the conclusion of the COVID-19 pandemic, the persistent genetic variability in the virus and its ongoing circulation within the global population necessitate the enhancement of existing preventive vaccines and the development of novel ones. A while back, we engineered an orally administered probiotic-based vaccine, L3-SARS, by integrating a gene fragment that encodes the spike protein S of the SARS-CoV-2 virus into the genome of the probiotic strain E. faecium L3, inducing the expression of viral antigen on the surface of bacteria. Previous studies demonstrated the efficacy of this vaccine candidate in providing protection against the virus in Syrian hamsters. In this present study, utilizing laboratory mice, we assess the immune response subsequent to immunization via the gastrointestinal mucosa and discuss its potential as an initial phase in a two-stage vaccination strategy. Our findings indicate that the oral administration of L3-SARS elicits an adaptive immune response in mice. Pre-immunization with L3-SARS enhances and prolongs the humoral immune response following a single subcutaneous immunization with a recombinant S-protein analogous to the S-insert of the coronavirus in Enterococcus faecium L3.

SARS-CoV-2 Spike Protein-Expressing Enterococcus for Oral Vaccination: Immunogenicity and Protection

The declaration of the conclusion of the COVID-19 pandemic notwithstanding, coronavirus remains prevalent in circulation, and the potential emergence of novel variants of concern introduces the possibility of new outbreaks. Moreover, it is not clear how quickly and to what extent the effectiveness of vaccination will decline as the virus continues to mutate. One possible solution to combat the rapidly mutating coronavirus is the creation of safe vaccine platforms that can be rapidly adapted to deliver new, specific antigens in response to viral mutations. Recombinant probiotic microorganisms that can produce viral antigens by inserting specific viral DNA fragments into their genome show promise as a platform and vector for mucosal vaccine antigen delivery. The authors of this study have developed a convenient and universal technique for inserting the DNA sequences of pathogenic bacteria and viruses into the gene that encodes the pili protein of the probiotic strain E. faecium L3. The paper presents data on the immunogenic properties of two E. faecium L3 vaccine strains, which produce two different fragments of the coronavirus S1 protein, and provides an assessment of the protective efficacy of these oral vaccines against coronavirus infection in Syrian hamsters.

Construction of the Enterococcal Strain Expressing Immunogenic Fragment of SARS-Cov-2 Virus

Contemporary SARS-Cov-2 pandemic, besides its dramatic global influence on the human race including health care systems, economies, and political decisions, opened a window for the global experiment with human vaccination employing novel injectable vaccines providing predominantly specific IgG response with little knowledge of their impact on the mucosal immunity. However, it is widely accepted that protection against the pathogens at the gates of the infection - on mucosal surfaces—predominantly rely on an IgA response. Some genetically modified bacteria, including probiotics, represent attractive vehicles for oral or nasal mucosal delivery of therapeutic molecules. Probiotic-based vaccines for mucous membranes are easy to produce in large quantities; they have low cost, provide quite a long T-cell memory, and gut IgA response to oral vaccines is highly synchronized and strongly oligoclonal. Here we present a study demonstrating construction of the novel SARS-Cov-2 vaccine candidate employing the gene fragment of S1 SARS-Cov-2 gene. This DNA fragment was inserted in frame into major pili protein gene with d2 domain of enterococcal operon encoding for pili. The DNA sequencing proved the presence of the insert in enterococcal genome. RNA transcription, immunoprecipitation, and immune electron microscopy with human sera obtained from the SARS-Cov-2 patients demonstrated expression of SARS-Cov-2 antigens in bacteria. Taken together the data obtained allowed considering this genetically modified probiotic strain as an interesting candidate for vaccine against SARS-Cov-2.

Construction of the Enterococcal Strain Expressing Immunogenic Fragment of SARS-Cov-2 Virus

Contemporary SARS-Cov-2 pandemic, besides its dramatic global influence on the human race including health care systems, economies, and political decisions, opened a window for the global experiment with human vaccination employing novel injectable vaccines providing predominantly specific IgG response with little knowledge of their impact on the mucosal immunity. However, it is widely accepted that protection against the pathogens at the gates of the infection - on mucosal surfaces—predominantly rely on an IgA response. Some genetically modified bacteria, including probiotics, represent attractive vehicles for oral or nasal mucosal delivery of therapeutic molecules. Probiotic-based vaccines for mucous membranes are easy to produce in large quantities; they have low cost, provide quite a long T-cell memory, and gut IgA response to oral vaccines is highly synchronized and strongly oligoclonal. Here we present a study demonstrating construction of the novel SARS-Cov-2 vaccine candidate employing the gene fragment of S1 SARS-Cov-2 gene. This DNA fragment was inserted in frame into major pili protein gene with d2 domain of enterococcal operon encoding for pili. The DNA sequencing proved the presence of the insert in enterococcal genome. RNA transcription, immunoprecipitation, and immune electron microscopy with human sera obtained from the SARS-Cov-2 patients demonstrated expression of SARS-Cov-2 antigens in bacteria. Taken together the data obtained allowed considering this genetically modified probiotic strain as an interesting candidate for vaccine against SARS-Cov-2.

A Live Probiotic Vaccine Prototype Based on Conserved Influenza a Virus Antigens Protect Mice against Lethal Influenza Virus Infection

Background: Due to the highly variable nature of the antigenic properties of the influenza virus, many efforts have been made to develop broadly reactive influenza vaccines. Various vaccine platforms have been explored to deliver conserved viral antigens to the target cells to induce cross-reactive immune responses. Here, we assessed the feasibility of using Enterococcus faecium L3 as a bacterial vector for oral immunization against influenza virus. Methods: we generated two vaccine prototypes by inserting full-length HA2 (L3-HA2) protein or its long alpha helix (LAH) domain in combination with four M2e tandem repeats (L3-LAH+M2e) into genome of E.faecium L3 probiotic strain. The immunogenicity and protective potential of these oral vaccines were assessed in a lethal challenge model in BALB/c mice. Results: as expected, both vaccine prototypes induced HA stem-targeting antibodies, whereas only L3-LAH+4M2e vaccine induced M2e-specific antibody. The L3-HA2 vaccine partially protected mice against lethal challenge with two H1N1 heterologous viruses, while 100% of animals in the L3-LAH+4M2e vaccine group survived in both challenge experiments, and there was significant protection against weight loss in this group, compared to the L3 vector-immunized control mice. Conclusions: the recombinant enterococcal strain L3-LAH+4M2e can be considered as a promising live probiotic vaccine candidate for influenza prevention and warrants further evaluation in relevant pre-clinical models.

Development of experimental GBS vaccine for mucosal immunization

Streptococcus agalactiae, or group B streptococcus (GBS), is an important pathogen as it is the leading cause of neonatal deaths due to sepsis, meningitis or bacterial pneumonia. Although the development of an effective and safe GBS vaccine is on the agenda of many research labs, there is no GBS vaccine on the market yet. In the present study we attempted to engineer a live vaccine strain based on Bac, a surface protein of GBS, incorporated into a surface fimbrial protein of probiotic Enterococcus. The resulting strain induced specific systemic and local immune responses in mice and provided protection against GBS when administered via the intranasal, oral or intravaginal immunization routes.