New vaccine candidate with RBD and N protein targets protects against SARS-CoV-2 in preclinical trials

By Dr. Ramya Dwivedi, Ph.D.Jun 2 2021

Numerous vaccines based on different platforms are developed against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of the coronavirus disease 2019 (COVID-19) pandemic.

The SARS-CoV-2 genome encodes four structural proteins—spike (S), membrane (M), envelope (E), and nucleocapsid (N). The S protein binds to the angiotensin-converting enzyme (ACE) receptor on the host cell through its receptor-binding domain (RBD). The RBD is considered the main target antigen for vaccines against SARS-CoV-2.

Study: Immunization with RBD-P2 and N protects against SARS-CoV-2 in nonhuman primates. Image Credit: Numstocker / Shutterstock

A recent study published in the journal Science Advances, explores the immunization of RBD with the P2 epitope of tetanus toxoid, to enhance its immunogenicity, and also tests the benefit of N protein in the vaccine.

An interdisciplinary team from South Korea developed a previously unidentified SARS-CoV-2 subunit vaccine (RBD-P2) by fusing the P2 epitope of tetanus toxoid with the RBD of the spike protein. And they tested the efficacy of the RBD-P2 with or without the N protein in mice, rats, and nonhuman primates (NHPs).

Consistent with the previous finding, they found higher neutralizing antibody titers against SARS-CoV-2 in rats when immunized with the RBD-P2 than that with RBD.

Additionally, with the alum as an effective adjuvant, the (RBD-P2 + alum) induced a robust neutralizing antibody response in NHPs, protecting against the SARS-CoV-2 challenge. Thus, they conclude that immunization with RBD-P2 + alum can induce effective and sufficient virus-neutralizing activity and provide protection.

However, the researchers were unable to determine the long-term humoral immunity in NHPs post-RBD-P2 + alum boost, as the primates were challenged with SARS-CoV-2 6 weeks after boosting.

While many of the vaccine candidates currently under clinical or preclinical trials involve at least a portion of the spike protein, it is still unclear whether the N protein of SARS-CoV-2 in combination would be beneficial for immunization protection against the SARS-CoV-2.

Because in vivo protection conferred by vaccines containing the N protein has not yet been experimentally validated, the researchers find out here in the comparative study of the immune responses elicited by RBD-P2 and RBD-P2/N immunization in three different animals—mice, rats, and NHPs.

The researchers found enhanced T cell-mediated immune responses and increased CD69 expression in CD4+ T cells as well as IFN-γ and IL-4 production in mice.

In the study, the NHPs immunized with RBD-P2 or RBD-P2/N were protected from the SARS-CoV-2 challenge. The researchers found increased production of IFN- γ and the numbers of IFN- γ producing cells. However, they noted that the production of TH2-type cytokines (IL-4, IL-5, and IL-13) was very low. They also observed that the RBD-P2/N + alum–immunized NHPs showed faster SARS-CoV-2 clearance than that of RBD-P2 + alum immunized NHPs in throat swab samples.

Thus, increased T cell activation and TH1-prone responses may result in faster viral clearance and constrain viral spread,” observed the researchers.

A significant point discussed in the paper is the focus of the vaccine design on S protein vs. N protein. Because of the mutations in the spike protein, many variants of the SARS-CoV-2 have emerged. Relatively, the N protein is more conserved, and hence the researchers believe that vaccines based on the N protein may provide effective protection against SARS-CoV-2 variants too.

Thus, the researchers believed that the combination of an RBD-P2/N immunization might be advantageous in overcoming escape mechanisms of SARS-CoV-2 because of gene mutations encoding the spike protein.

The researchers discussed the limitations of this study. Due to the concentrations of the RBD-P2 and the N protein used to immunize the NHPs, the effectiveness of the N protein could vary depending on the dose of RBD-P2. While they called for the need for further studies to determine the optimal dose or ratio involving the N protein, they informed that they are evaluating the RBD-P2–based subunit vaccine in clinical trials (phase 1, no. NBP2001) - because RBD-P2 and RBP-P2/N showed similar protective efficacy in NHPs.

In summary, this study demonstrates two highlights: 1) RBD-P2 + alum immunization elicits a robust neutralizing antibody response and provides complete or near-complete elimination of live SARS-CoV-2 virus particles; and 2) the potential use of the N protein provides insight into the development of SARS-CoV-2 vaccines.