Screening of Efficient Adjuvants for the RBD-Based Subunit Vaccine of SARS-CoV-2

Abstract

The variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are more transmissible, with a reduced sensitivity to vaccines targeting the original virus strain. Therefore, developing an effective vaccine against both the original SARS-CoV-2 strain and its variants is an urgent need. It is known that the receptor-binding domain (RBD) in the S protein of SARS-CoV-2 is an important vaccine target, but subunit vaccines usually have lower immunogenicity and efficacy. Thus, selecting appropriate adjuvants to enhance the immunogenicity of protein-based subunit vaccine antigens is necessary. Here, an RBD-Fc subunit vaccine of SARS-CoV-2 has been generated, followed by vaccination in B6 mice, and four adjuvant regimens were investigated, including aluminum salts (Alum) + 3-O-desacyl-4′-monophosphoryl lipid A (MPL), AddaVax, QS21 + MPL, and Imiquimod. The adjuvant potency was evaluated by comparing the elicited polyclonal antibodies titers with measuring binding to RBD and S protein in ELISA and Western blot analysis, and also the cross-neutralizing antibodies titers using a pseudovirus infection assay of hACE2-expressing 293T cells, with pseudoviruses expressing the S protein of the SARS-CoV-2 original strain and Delta strain. The presence of QS21 + MPL adjuvant induced stronger polyclonal antibody response and neutralization potency blocking the original strain and Delta strain, as compared with the non-adjuvant RBD-Fc group and other adjuvant groups. Meanwhile, Imiquimod even had a negative effect in inducing specific antibodies and cross-neutralizing antibody production as an adjuvant.

CHO-produced RBD-Fc subunit vaccines with alternative adjuvants generate immune responses against SARS-CoV-2

“…Combining these results with the result of primary structure analysis using MS/MS, it was confirmed that CHO-produced RBD-Fc could display native-like properties. This results also were consistent with previous studies where RBD-Fc vaccine obtained from mammalian cell expression system exhibited desired biochemical characteristics [19][20][21]24].…”

“…This implied that RBD-based vaccine could elicit sufficient neutralizing antibodies when compared to whole spike candidate. These results were also consistent with other reports of RBD-Fc-based vaccine development that RBD-Fc shows high potency as vaccine candidate against SARS-CoV-2 [17,[19][20][21][22][23][24].…”

“…Additionally, a similar combination of adjuvants such as MPLA/Quil-A 1 also showed great potential. Juan Shi et al presented MPLA combined with, a plant extract, QS21, formulated with HEK293F-expressed RBD-Fc subunit vaccine showed high antibody titer and neutralizing activities against SARS-CoV-2 [19]. This report supports the use of MPLA/Quil-A 1 as a candidate for effective adjuvant system.…”

“…It was shown that the vaccine candidate is effective in viral protection in non-human primates and is now under Phase 2 clinical trial [18]. Additionally, RBD fused with human IgG Fc fragment (RBD-Fc) has been studied for SARS-CoV-2 vaccine development since it could enhance immunogenicity of RBD and its half-life [19][20][21][22][23][24]. Currently, at least two RBD-Fc-based vaccine candidates have been investigated in clinical trial studies including AKS-452 and Betuvax-CoV-2 [24,25] and evidence suggests that RBD-based subunit vaccines are indeed promising candidates.…”

Flublok Quadrivalent Vaccine Adjuvanted with R-DOTAP Elicits a Robust and Multifunctional CD4 T Cell Response That Is of Greater Magnitude and Functional Diversity Than Conventional Adjuvant Systems

“…AddaVax was included as a key comparator because it is a commercially available squalene-in-water formulation analog of MF59 [ 71 ], which has been shown to promote antibody responses in humans [ 72 , 73 ]. AddaVax has been used with promising results in preclinical animal models of vaccination [ 30 , 74 , 75 , 76 , 77 ]. We also analyzed the impact of AddaVax with added CpG, which is shown to promote a vaccine response that potentiates the response via engagement of TLR-9 and, thus, can promote a Th1 CD4+ T cell response [ 78 ].…”