The immunodominance of RBD antigen of delta variant as vaccine candidate against SARS‐CoV‐2 infection

Dear Editor,

Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) is a novel subset of coronavirus that causes coronavirus disease 2019 (COVID‐19), but vaccine development is hampered by the high mutation of virus. The spike protein (S protein) is a major virulence factor. The receptor‐binding domain (RBD) region is considered as the conserved region which was used for vaccine development. ¹ But some mutations were still found in RBD gene of different SARS‐CoV‐2 strains. To date, regardless of a high demand globally, no universal vaccine has been licensed against SARS‐CoV‐2 infections. To determine the immunodominance of RBD antigen of different virus strains, a series of studies were carried out.

Bagg Albino (BALB)/c mice were immunized with three dose of aluminum adjuvanted RBDs (20 µg each mouse, delta variant B.1.617.2 [RBD/617], beta variant B.1.351 [RBD/351] and original strain BetaCoV/BJ01 [RBD/131]) via intraperitoneal administration. To screen the immunodominance antigen, the IgG antibody titers were detected by ELISA. The antibody level of RBD/617 is higher than others (Figure 1A). Recent genome surveillance has recorded novel epidemic SARS‐CoV‐2 strains were potentially associated with virus transmission and pathogenesis in India and South Africa. ² Next, the cross immunity of RBD vaccine candidates against different virus strains should be evaluated. Immunization with RBD/617 induced the production of neutralizing antibodies with NT50 approached 21 strain pseudoviruses, including Omicron variants and variant B.1.640.2 which are the epidemic strains now (Figures 1C,F). The neutralizing antibody titers against more than 12 pseudovirus are much higher than other vaccine candidate induced (S1). We presume that the RBD/617 has the immunodominance against SARS‐CoV‐2 based on these data.

Figure 1.

The immunodominance of RBD antigen of virus strain B.1.617.2. (A) Female BALB/c mice were immunized with three dose of aluminum adjuvanted RBDs (20 μg each mouse) via intraperitoneal administration, and sera were collected for tests. The IgG titers induced by RBDs of different virus strains (n = 7 mice per group); (B) The cross immunity to different strain of authentic viruses in vitro were shown (n = 5 mice per group); (C) The cross immunity to six strain of pseudoviruses were shown (n = 10 mice per group); (D) Survival of ACE2 transgenic mice immunized with RBDs challenged with B.1.617.2 strain (n = 5 mice per group); (E) BALB/c mice were inoculated with 6 × 10³ plaque forming unit of MASCp6 and killed at 5 days after inoculation. Viral RNA copies were determined by means of quantitative reverse transcription polymerase chain reaction (RT‐PCR) (n = 7 or 8 mice per group). (F) The cross immunity to different single point mutation strain of pseudoviruses were shown (n = 10 mice per group); (G) H&E staining of lung sections from MASCp6‐infected mice (×10, n = 5 mice per group). Data are presented as means ± SD. (*<0.05, **<0.01, ***<0.001). RBD, receptor‐binding domain.

To confirm this conclusion, the neutralizing antibodies with NT50 approached 3 strain authentic viruses were evaluated (Figure 1B). As expected, sera from RBD/617 vaccinated mice showed the higher neutralizing capability against all three epidemic strains. In comparation with antibody titers induced by RBD/351 and RBD/131, the neutralizing antibody titers induced by RBD/617 have two to threefold increase.

Further, this study has tested if RBD/617 vaccination could increase the survival efficacy of mice challenged with mouse‐adapted SARS‐CoV‐2 strain MASCp6 (6 × 10³ plaque forming units [PFUs]/mouse) and SARS‐CoV‐2 strain B.1.617.2 (105 PFUs/mouse). ³ BALB/c mice and ACE2 transgenic mice were immunized with three dose of aluminum adjuvanted RBD/617, RBD/351 and RBD/131. All the aluminum‐treated mice could not block viral replication in lung. Most importantly, RBD/617 immunization was found to be significantly increased mice survival and reduced viral load in lung after challenge with B.1.617.2 variant of concern (VOC) strain (p = 0.0017) and MASCp6 adapted strain (p < 0.05). RBD alone were used as the antigen to avoid the antibody‐dependent enhancement, ⁴ but RBD is not as immunogenic as the whole S protein or the whole virus vaccine. The RBD protein could not induce cellular immunity (S2). Meanwhile, the disadvantage of this vaccine is that it is difficult to design a good structure of a recombinant protein. ⁵ For all that, RBD/351 could only provide moderate protective efficacy, and the virus loads increased in mice immunized with RBD/351 and RBD/131 vaccine candidate compared with RBD/617 immunization (Figures 1D,E). More importantly, mice vaccinated with placebo developed typical lung lesions, while no such pathological changes were seen in lung sections from all RBD/617 immunized and some RBD/351 immunized animals (Figure 1G). The SARS‐CoV‐2 B.1.1.529 VOC is now spread all over the world. Anti‐RBD/617 antibody in the study could still neutralize Omicron pseudoviruses. The neutralizing antibody titer against this VOC reached to 1708 or higher (Figure 1C).

B.1.617.2 VOC lacks mutations at amino acid positions 501 or 484 in its RBD, which commonly associated with escaping from neutralizing antibodies. Thus, RBD protein of B.1.617.2 VOC possesses broad‐spectrum antiviral functions because of this kind of mutations. ⁶ An ideal COVID‐19 vaccine is supposed to avoid the induction of non‐neutralizing antibody and induce high cross immunity to VOC. ⁷ Shioda's group reported that the antibodies against the N‐terminal domain of S protein enhanced the infectivity of SARS‐CoV‐2. ⁴ Thus, in comparation with the full‐length S protein, RBD antigen may induce fewer non‐neutralizing antibodies. The RBD of virus strain of B.1.617.2 can induce high levels of cross neutralizing antibodies and conferred protection against SARS‐CoV‐2. Importantly, this finding highlights RBD/617 has the immunodominance against SARS‐CoV‐2 infection.

AUTHOR CONTRIBUTIONS

Hui Wang and Zhongyu Hu designed the trial and study protocol. Deyan Luo, Tao Li, Xiaolan Yang, Nianzhi Ning, Liangyan Zhang, Hongjing Gu, Deyu Li, and Wenjing Yu were contributed to the laboratory testing and assay development. Deyan Luo, and Tao Li did the statistical analysis. Deyan Luo drafted the Article. Hui Wang and Deyan Luo contributed to critical review and revision of the Article.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

Supporting information

Supporting information.

DATA AVAILABILITY STATEMENT

All data are available within the article, Supporting Information, or available from the corresponding author upon reasonable request.