RETRACTED: Expression of SARS-CoV-2 Spike Protein Receptor Binding Domain on Recombinant B. subtilis on Spore Surface: A Potential COVID-19 Oral Vaccine Candidate

Abstract

Various types of vaccines, such as mRNA, adenovirus, and inactivated virus by injection, have been developed to prevent SARS-CoV-2 infection. Although some of them have already been approved under the COVID-19 pandemic, various drawbacks, including severe side effects and the requirement for sub-zero temperature storage, may hinder their applications. Bacillus subtilis (B. subtilis) is generally recognized as a safe and endotoxin-free Gram-positive bacterium that has been extensively employed as a host for the expression of recombinant proteins. Its dormant spores are extraordinarily resistant to the harsh environment in the gastrointestinal tract. This feature makes it an ideal carrier for oral administration in resisting this acidic environment and for release in the intestine. In this study, an engineered B. subtilis spore expressing the SARS-CoV-2 spike protein receptor binding domain (sRBD) on the spore surface was developed. In a pilot test, no adverse health event was observed in either mice or healthy human volunteers after three oral courses of B. subtilis spores. Significant increases in neutralizing antibody against sRBD, in both mice and human volunteers, after oral administration were also found. These findings may enable the further clinical developments of B. subtilis spores as an oral vaccine candidate against COVID-19 in the future.

Safety and Immunogenicity of Inactivated Bacillus subtilis Spores as a Heterologous Antibody Booster for COVID-19 Vaccines

“…The oral dose vaccine booster candidate in this study is made by a recombinant technique, allowing expression of the spike protein receptor binding domain of SARS-CoV-2 on the surfaces of B. subtilis , a non-pathogenic bacterium [ 29 ]. The oral booster vaccine candidate was found to be immunogenic in all participants of the vaccine group, regardless of which primary vaccine regimen they had received, and the results were in general in agreement with previous results of the same oral vaccine candidate after the first dose [ 20 ]. Apart from enhancing the neutralizing antibody titer in the host, previous study of the vaccine candidate by incubation of B. subtilis spores with immune cells detected upregulation of in vitro cytokine levels [ 20 ]: significant increases in IgM, IgG and IgA antibodies against the spike protein receptor binding domain were detected in the mouse serum and intestinal tissue.…”

“…Although heterologous vaccination delivery strategies have been demonstrated successfully in many different infectious diseases [ 20 ], before the COVID-19 pandemic, they were usually inactivated virus vaccines and recombinant protein subunits only. This is the first safety and immunoreactivity report of using an oral dose of B. subtilis bacterial spores as a heterologous COVID-19 vaccine booster candidate for use in healthy adults who have previously received two doses of either mRNA or inactivated virus vaccines.…”

“…Sixteen (16) trial participants from 21–62 years old were randomly assigned into the active booster or placebo groups. The booster was prepared as previously described [ 20 ]. Briefly, B. subtilis expressing the spike protein receptor binding domain from SARS-CoV2 (Wuhan-Hu-1, Clade 19, a.a. 319–541, UniProt ID: P0DTC2 [ 23 ]), linked with the spore coat protein CotC, were cultured and induced into spores.…”

“…In response to the need for vaccine boosters, a coordinated program for an oral dose, recombinant B. subtilis -spore-based COVID-19 vaccine candidate was conducted in Hong Kong [ 20 ]. The program was designed to test the safety and immunogenicity of the vaccine candidate, where three courses of 5 × 10 7 spores/kg in healthy adults at 48 to 72 years of age within 28 days elicited an acceptable level of reactogenicity.…”

An Algae-Made RBD from SARS-CoV-2 Is Immunogenic in Mice

“…Therefore, intensive research on this direction is needed. Several groups are exploring mucosal vaccines based on adenoviral vectors, nanoparticles, live attenuated virus, and subunit vaccines, among other technologies [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ].…”