A yeast-expressed RBD-based SARS-CoV-2 vaccine formulated with 3M-052-alum adjuvant promotes protective efficacy in non-human primates

Abstract

Ongoing SARS-CoV-2 vaccine development is focused on identifying stable, cost-effective, and accessible candidates for global use, specifically in low and middle-income countries. Here, we report the efficacy of a rapidly scalable, novel yeast expressed SARS-CoV-2 specific receptor-binding domain (RBD) based vaccine in rhesus macaques. We formulated the RBD immunogen in alum, a licensed and an emerging alum adsorbed TLR-7/8 targeted, 3M-052-alum adjuvants. The RBD+3M-052-alum adjuvanted vaccine promoted better RBD binding and effector antibodies, higher CoV-2 neutralizing antibodies, improved Th1 biased CD4+T cell reactions, and increased CD8+ T cell responses when compared to the alum-alone adjuvanted vaccine. RBD+3M-052-alum induced a significant reduction of SARS-CoV-2 virus in respiratory tract upon challenge, accompanied by reduced lung inflammation when compared with unvaccinated controls. Anti-RBD antibody responses in vaccinated animals inversely correlated with viral load in nasal secretions and BAL. RBD+3M-052-alum blocked a post SARS-CoV-2 challenge increase in CD14+CD16++ intermediate blood monocytes, and Fractalkine, MCP-1, and TRAIL in the plasma. Decreased plasma analytes and intermediate monocyte frequencies correlated with reduced nasal and BAL viral loads. Lastly, RBD-specific plasma cells accumulated in the draining lymph nodes and not in the bone marrow, contrary to previous findings. Together, these data show that a yeast expressed, RBD-based vaccine+3M-052-alum provides robust immune responses and protection against SARS-CoV-2, making it a strong and scalable vaccine candidate.

A Stabilized, Monomeric, Receptor Binding Domain Elicits High-Titer Neutralizing Antibodies Against All SARS-CoV-2 Variants of Concern

“…Pichia expression technology is widely available in several developing countries including India, Bangladesh, Brazil, Cuba, and Indonesia ( 53 ). There have been other attempts to express RBD in Pichia , both as a monomeric protein ( 53 – 55 ) and displayed on the surface of hepatitis B VLPs ( 56 ); both are in clinical trials, but data from the trials are not currently available. In the case of the hepatitis B VLP-RBD, neutralization titers against B.1.1.7 were comparable to those against B.1; however, titers against the B.1.351 titer were ~25-fold lower than against B.1 ( 56 ).…”

“…The Y365F, V395I, and F392W mutations present in Rpk9 are in the vicinity of a linoleic binding cavity, different from the ones in the present study, although one site, Y365, is common. In another study, a Pichia -expressed RBD (residues 332-549, C538A) adjuvanted with 3M-052 + alum, when used in macaque immunizations, elicited neutralizing antibodies that showed an ~5.4-fold decrease in ID 50 against B.1.351, relative to B.1 in a pseudoviral neutralization assay ( 55 ). It is difficult to directly compare animal immunogenicity data since different studies have used different animal models, adjuvants, and neutralization assays.…”

Breadth of SARS-CoV-2 Neutralization and Protection Induced by a Nanoparticle Vaccine

“…Our studies with NTD-scNP immunization demonstrated neutralizing antibodies may not be the only mediators of protection. While the RBD subunit has been shown to protect against SARS-CoV-2 challenge in animal models (Dai et al, 2020; Francica et al, 2021; Pino et al, 2021; Saunders et al ., 2021; Yang et al, 2020), the NTD is also an immunodominant region for neutralizing antibodies (Cerutti et al, 2021; Chi et al, 2020; Li et al ., 2021a; Martinez et al, 2021b; McCallum et al, 2021; Voysey et al, 2021). However, NTD is the site of multiple mutations and NTD antibody neutralization is, in general, less potent than RBD antibodies.…”

“…Cell-surface expression of CD107a was used as a marker for NK cell degranulation, a prerequisite process for ADCC (Ferrari et al, 2011), was performed as previously described (Pino et al, 2021).…”

A Perspective on the Roles of Adjuvants in Developing Highly Potent COVID-19 Vaccines

“…The vaccine adjuvanted with alum alone was compared with the yeast-expressed trimeric form of the RBD protein adjuvanted with a TLR7/8 agonist and alum formulation, termed as alum-3M-052. The latter formulation induced 100-fold higher neutralizing antibodies, improved Th1-based CD4 + T cell responses, and increased CD8 + T cell reactions [23,24], thus making it a powerful and scalable COVID-19 vaccine candidate (Table 1).…”