COVID-19 vaccination with bacterial peptide conjugated to receptor-binding domain elicits potent immune response

By Bhavana KunkalikarJul 6 2022Reviewed by Aimee Molineux

In a recent study published in the iScience journal, researchers assessed the efficiency of immunization with a bacterial peptide conjugated to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) receptor-binding domain against SARS-CoV-2 infection.

The constant emergence of novel SARS-CoV-2 variants has adversely affected the efficacy and impact of coronavirus disease 2019 (COVID-19) vaccines against infections and disease severity. Hence, new vaccine platforms are needed to develop effective and safe vaccination regimens against SARS-CoV-2 infections and related hospitalization.   

About the study

In the present study, researchers employed immune Boost (iBoost) technology to improve the response of the immune system against the SARS-CoV-2 spike (S) RBD.

The team achieved immune recognition of SARS-CoV-2 RBD by conjugating the sequence to a chimeric designer peptide (CDP) engineered by the researchers. The CDP comprises clusters of amino acids having either charged side chains or bulky hydrophilic groups, derived from three different bacterial proteins, namely ZapB which is the cell division protein, IbpA which is a small heat shock protein, and TFP (type I fimbrial protein). The developed conjugate protein named CPD-RBD was expressed and purified.

The immunogenicity of the conjugated CPD-RBD in vivo was assessed and compared to the corresponding unconjugated RBD by immunizing BALB/c mice with each purified protein on day 0 by primary vaccination and on day 14 by a booster vaccination. The team selected Montanide ISA 720 along with a toll-like receptor 9 agonist called CpG 1826 oligonucleotide as a vaccine adjuvant. Serological samples were collected from the mice before immunization on day 0 and on days 13, 21, 28, and 35. The samples were further analyzed for the presence of anti-RBD antibodies using an enzyme-linked immunosorbent assay (ELISA).

Furthermore, the team assessed the production of IgG1 and IgG2b, which represented T-helper (Th)-2-skewed immune responses, and IgG2a and IgG3, which correlated with the stimulation of a Th1 response. The researchers also immunized mice with CPD-RBD or RBD alone along with the Sepivac (Sep), which was a vaccine adjuvant used instead of Montanide.

Results

The study results showed that adding the DCP component to the SARS-CoV-2 RBD increased the solubility as well as the protein yield of the resulting conjugate protein CPD-RBD. The in silico analysis of the sequence of the conjugate protein displayed the alpha-helix or beta-strand domains. Furthermore, the B-cell and T-cell epitopes observed in the in silico analysis indicated potent immune recognition. 

The team found no antibodies against SARS-CoV-2 RBD in the pre-immune serum samples collected on day 0. However, seroconversion was observed by day 13 in a few mice belonging to both the groups that were vaccinated by the conjugated and the unconjugated purified proteins. Moreover, by day 21, all the mice that received the CPD-RBD vaccine displayed robust immunoglobulin (Ig) levels against viral RBD. Also, by day 21, only 50% of the mice immunized by the RBD revealed a strong RBD-specific humoral response while no anti-RBD antibodies were found in two out of the 10 mice vaccinated with unconjugated RBD protein. This suggested that the RBD had moderate immunogenicity.

Furthermore, the difference in the total anti-RBD Ig levels between the two vaccinated cohorts diminished at subsequent time points. However, the antibody titers corresponding to the RBD-immunized mice were lower than the CPD-RBD vaccinees three weeks after the mice were vaccinated with the booster dose. This indicated that the iBoost platform elicited faster and more robust anti-RBD antibody responses as compared to RBD alone.

Analysis of the IgG subclasses showed that by day 21, the mice immunized with CDP-RBD had substantially higher concentrations of both anti-RBD IgG3 and IgG1 as compared to the RBD-vaccinated mice. Moreover, the team noted a trend of increasingly robust and consistent IgG2 response among the CDP-RBD-immunized mice. Also, vaccination with CDP-RBD, and not unconjugated with RBD, resulted in a higher capability of mounting complete IgG1, IgG2a, IgG2b, and IgG3 responses. Thus, the CPD-RBD vaccine showed a more comprehensive response against SARS-CoV-2 RBD, even seven days post-booster vaccination. 

Analysis with Sep as the vaccine adjuvant revealed that CDP-RBD/Sep induced higher levels of anti-RBD total IgG as compared to the RBD/Sep vaccine, which was also observed for CDP-RBD with Montanide. This indicated that CDP was the key component that boosted the humoral immune response. Moreover, a surrogate neutralization assay showed that serum obtained from the CPD-RBD-immunized vaccine resulted in higher levels of inhibition of RBD-angiotensin-converting enzyme-2 (ACE-2) binding from day 21 onwards as compared to serum obtained from mice that were immunized with RBD alone.

Conclusion

Overall, the study findings showed that the CPD-RBD vaccination elicited a potent immune response and also protected against the severe symptoms associated with COVID-19.