In a new study, researchers engineered the bacterial strain, Salmonella, to express differently-sized antigens to serve as vaccine platforms. This platform presented low toxicity and high effective immunity with fewer inflammatory responses.
Study: Development of an Oral Salmonella-Based Vaccine Platform against SARS-CoV-2. Image Credit: iunewind/Shutterstock
The researchers demonstrated that this Salmonella-based vaccine strategy induces antigen-specific T-cell and cytotoxic T-lymphocytes (CTL) responses and robust antiviral response (interferon-gamma). Publishing their research in the journal Vaccines, the researchers proposed this using a Salmonella-based strategy to develop an oral anti-coronavirus vaccine.
Currently, vaccines are indispensable in managing the ongoing coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Despite the mitigation efforts and vaccine programs implemented worldwide, the virus has spread and mutated.
Additionally, the vaccines administered have caused a range of side effects that were not evaluated due to the short development period and the limitations in clinical assessment. The side-effects ranged from incidence of anaphylaxis, thrombosis with thrombocytopenia syndrome (TTS), Guillain–Barré syndrome, myocarditis, and pericarditis associated with the RNA- and virus-based vaccines. These occurred mainly due to the excessive activity of the immune system induced by the excessive inflammatory substances injected directly into the body.
The adjuvants used in these vaccines (lipid nanoparticles or adenoviruses) can also be painful, have low bioavailability and cause fever, nausea, headaches, or joint pains.
Therefore, there is a need for research to develop a new, rapid and safer vaccine technology. To this end, bacteria-based orally administered vaccines are an attractive option for a safer vaccine technology.
While the bacilli Calmette–Guerin and vaccinia virus are studied as alternatives, Salmonella, which can also promote cellular immune responses, is used in this study. Salmonella is an intracellular infectious bacterium; it presents antigens during intracellular parasitization by infecting mucosal cells.
Study design and findings
Of the four structural proteins in the coronavirus — S (spike), envelope (E), membrane glycoprotein (M), and nucleocapsid (N) proteins, the researchers constructed vectors to express the proteins E and ORF7a, along with the S protein, which is the major antigen currently used in the commercial vaccines. However, recent reports show the E and ORF7a are strongly related to the pathogenesis of COVID-19.
In the present study, the researchers engineered these viral proteins as potential neutralizing antigens; to trigger ‘SARS-CoV-2-specific antibody production and COVID-19-specific cellular immune responses.’
The researchers engineered the COVID-19 antigen proteins and cloned them into expression vectors. These vectors were transformed into the Salmonella cells. The researchers fused the viral antigens to the SipB signal peptide (SipB160 protein), which effectively expresses the antigens on the cells.
Because it is important for the stomach and the intestines to tolerate the orally ingested bacterial strain, the researchers used an attenuated Salmonella strain - with reduced virulence and toxicity for this study. The stability of the genetically engineered S. Typhimurium was reported to be 60% at 24 hours, further declining up to 72 hours.
The researchers highlighted that Salmonella promoted and activated B-cells and T-cells by phagocytosis or viral infection of macrophages that are resident in the intestine, with good bacterial cell viability.
After constructing the bacterial vaccine, the researchers characterized the Salmonella expressing COVID-19 antigen by evaluating the cytotoxicity in vitro and in vivo. The experiments showed no toxicity or accumulation in the body upon oral administration. It also did not affect the survival of the tested mice.
The antigen-specific antibodies and antigen-specific T-cell immunity were effectively activated in the treated mice, indicating robust humoral and cellular responses in vivo after oral administration of the Salmonella expressing COVID-19 antigen, with no significant toxicity effects.
While the desired results are observed in this study using the engineered Salmonella-based vaccine platform, the researchers caution against safety issues that need further investigation in these systems. On the positive front, the induced immunity may be long-lasting, and the production cost of the attenuated Salmonella-based vaccines is very low.
This study evaluates the possibility of an oral bacteria-based vaccine that can be safely used as a platform for large-scale, long-term immunization.
The researchers engineered recombinant Salmonella cells to express viral proteins related to COVID-19 pathogenesis and tested the formulation of the oral vaccine candidate strain in vitro and in vivo.
They demonstrated that the orally administered vaccine promoted antigen-specific antibody production and invoked antigen-specific cellular immunity with no significant toxicity.
These results suggest that Salmonella strains may represent a valuable platform for developing an oral vaccine for COVID-19 as an alternative to tackle the outbreak of various mutated coronavirus strains and new infectious diseases in the future, concluded the researchers.