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

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

The constant emergence of new SARS-CoV-2 variants has effectiveness and impact of coronavirus disease 2019 (COVID-19) vaccines against infections and disease severity. Therefore, new vaccine platforms are needed to develop effective and safe vaccination regimens against SARS-CoV-2 infections and related hospitalizations.

About the study

In the current study, researchers used immune Boost (iBoost) technology to enhance the immune system’s response to 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) developed by the researchers. The CDP comprises clusters of amino acids with either charged side chains or bulky hydrophilic groups, derived from three different bacterial proteins, namely ZapB, the cell division protein, IbpA, a small heat shock protein, and TFP (type I fimbrial protein). The developed conjugated protein called CPD-RBD was expressed and purified.

The immunogenicity of the conjugated CPD-RBD in vivo was evaluated 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 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 Sepivac (Sep), a vaccine adjuvant used in place of Montanide.

Results

The study results showed that adding the DCP component to the SARS-CoV-2 RBD increased both the solubility and the protein yield of the resulting conjugated protein CPD-RBD. The in silico sequence analysis of the conjugated protein showed 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 to SARS-CoV-2 RBD in the pre-immune serum samples collected on day 0. However, on day 13, seroconversion was observed in some mice belonging to both the groups vaccinated by the conjugated and unconjugated purified proteins. In addition, by day 21, all mice receiving the CPD-RBD vaccine showed robust immunoglobulin (Ig) levels against viral RBD. Also, by day 21, only 50% of the mice immunized by the RBD showed a strong RBD-specific humoral response, while no anti-RBD antibodies were found in two of the 10 mice vaccinated with unconjugated RBD protein. This suggested that the RBD had moderate immunogenicity.

In addition, the difference in the total anti-RBD Ig levels between the two vaccinated cohorts decreased 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 compared to RBD alone.

Analysis of the IgG subclasses showed that by day 21 the CDP-RBD immunized mice had significantly higher concentrations of both anti-RBD IgG3 and IgG1 compared to the RBD vaccinated mice. In addition, the team noted a trend of an increasingly robust and consistent IgG2 response in the CDP-RBD-immunized mice. Also, vaccination with CDP-RBD, and not RBD unconjugated, resulted in a higher ability to mount complete IgG1, IgG2a, IgG2b and IgG3 responses. Thus, the CPD-RBD vaccine showed a more extensive response against SARS-CoV-2 RBD, even seven days after the booster vaccination.

Analysis with Sep as vaccine adjuvant revealed that CDP-RBD/Sep induced higher levels of anti-RBD total IgG compared to the RBD/Sep vaccine, which was also observed for CDP-RBD with Montanide. This indicated that CDP was the major component enhancing the humoral immune response. In addition, a surrogate neutralization test 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 compared to serum obtained from mice given only with RBD were immunized.

Conclusion

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