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

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

Study: Vaccination with bacterial peptide conjugated to SARS-CoV-2 RBD boosts immunity and protects against COVID-19. Image Credit: BaLL LunLa/Shutterstock

The constant emergence of new variants of SARS-CoV-2 has undermined the 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 associated hospitalizations.

About the study

In the current study, researchers used Immune Boost (iBoost) technology to enhance the immune system response against SARS-CoV-2 spike(S)RBD.

The team achieved immune identification of SARS-CoV-2 RBD by conjugating the sequence to a chimeric designer peptide (CDP) engineered by the researchers. 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 protein conjugate named CPD-RBD was expressed and purified.

The immunogenicity of the CPD-RBD conjugate live 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 with a toll type 9 receptor agonist called CpG oligonucleotide 1826 as the vaccine adjuvant. Serological samples were taken from the mice before immunization on day 0 and on days 13, 21, 28 and 35. The samples were then analyzed for the presence of anti-RBD antibodies using a enzyme immunoassay (ELISA).

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


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

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

Furthermore, the difference in total anti-RBD Ig levels between the two vaccinated cohorts decreased at later time points. However, the antibody titers corresponding to the RBD-immunized mice were lower than those of the CPD-RBD vaccinated 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.

IgG subclass analysis showed that at day 21, mice immunized with CDP-RBD had significantly higher concentrations of anti-RBD IgG3 and IgG1 compared to mice vaccinated with RBD. Additionally, the team noted a trend towards an increasingly robust and consistent IgG2 response among mice immunized against CDP-RBD. Additionally, vaccination with CDP-RBD, not unconjugated with RBD, resulted in a greater ability to mount full IgG1, IgG2a, IgG2b, and IgG3 responses. Thus, the CPD-RBD vaccine showed a more complete response against SARS-CoV-2 RBD, even seven days after the booster vaccination.

Analysis with Sep as the 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 key component that stimulated the humoral immune response. In addition, a substitution neutralization assay showed that serum obtained from the CPD-RBD immunized vaccine resulted in higher levels of inhibition of RBD-angiotensin-2 converting enzyme (ACE-) binding. 2) from day 21 compared to serum obtained from mice. who were immunized with RBD alone.


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

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