Safe and effective two-in-one replicon-and-VLP minispike vaccine for COVID-19

Abstract

The large SARS-CoV-2 spike (S) protein is the main target of current COVID-19 vaccine candidates but can induce non-neutralizing antibodies, which may cause vaccination-induced complications or enhancement of COVID-19 disease. Besides, encoding of a functional S in replication-competent virus vector vaccines may result in the emergence of viruses with altered or expanded tropism. Here, we have developed a safe single round rhabdovirus replicon vaccine platform for enhanced presentation of the S receptor-binding domain (RBD). Structure-guided design was employed to build a chimeric minispike comprising the globular RBD linked to a transmembrane stem-anchor sequence derived from rabies virus (RABV) glycoprotein (G). Vesicular stomatitis virus (VSV) and RABV replicons encoding the minispike not only allowed expression of the antigen at the cell surface but also incorporation into the envelope of secreted non-infectious particles, thus combining classic vector-driven antigen expression and particulate virus-like particle (VLP) presentation. A single dose of a prototype replicon vaccine, VSVΔG-minispike-eGFP (G), stimulated high titers of SARS-CoV-2 neutralizing antibodies in mice, equivalent to those found in COVID-19 patients. Boost immunization with the identical replicon further enhanced neutralizing activity. These results demonstrate that rhabdovirus minispike replicons represent effective and safe alternatives to vaccination approaches using replication-competent viruses and/or the entire S antigen.

Newcastle Disease Virus-Like Particles Displaying Prefusion-Stabilized SARS-CoV-2 Spikes Elicit Potent Neutralizing Responses

“…In agreement with the findings obtained with a prime immunization of S2P-NDVLP, sufficient SARS-CoV-2-neutralizing activity elicited after two or four weeks has been observed for some S2P spike-presented viral-vector-based or virus-derived replicon RNAbased vaccine candidates in mice [8,[29][30][31]. The general similarity in antigenicity between S2P-NDVLP and soluble S2P (Figure 2) suggests the S2P spike presented on S2P-NDVLP to be similar to that of the soluble S2P trimer.…”

“…Although multiple vaccines are proceeding through clinical trials, some of which are achieving high levels of efficacy against COVID-19 [ 3 , 4 , 5 , 6 ], it seems prudent to explore the development of second-generation vaccines for improved immunogenicity, manufacturability, and scalability to fill gaps in the global vaccine portfolio. Particularly for immunogenicity, improvements could be made in the speed, potency, and durability of neutralizing antibody responses and/or T-cell responses [ 5 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. The SARS-CoV-2 spike glycoprotein is the main target for neutralizing antibodies and has, therefore, been a focus of vaccine design efforts [ 16 , 17 ].…”

“…Protein nanoparticles displaying spike or RBD have been shown to elicit greater than 10-fold higher neutralizing titers than non-nanoparticle immunogens [ 10 , 11 ]. Current vector-based viral particle platforms to present SARS-CoV-2 antigens include the Moloney murine leukemia virus (MLV)-vectored COVID-19 vaccine [ 28 ], Newcastle disease virus (NDV)-vectored COVID-19 vaccine [ 9 ], rabies-virus-based COVID-19 vaccine [ 7 ], vesicular stomatitis virus (VSV)-vectored COVID-19 vaccines [ 29 , 30 ], virus-derived replicon RNA-based COVID-19 vaccines [ 8 , 31 ], and recombinant adenovirus-vectored COVID-19 vaccines [ 5 , 32 , 33 ]. Viral vector or virus-derived replicon RNA-based vaccine candidates should be able to produce molecules antigenically equivalent to those displayed on SARS-CoV-2 virions and should thus be able to induce humoral immune responses that are similar to those observed in people with SARS-CoV-2 [ 8 ].…”

“…Current vector-based viral particle platforms to present SARS-CoV-2 antigens include the Moloney murine leukemia virus (MLV)-vectored COVID-19 vaccine [ 28 ], Newcastle disease virus (NDV)-vectored COVID-19 vaccine [ 9 ], rabies-virus-based COVID-19 vaccine [ 7 ], vesicular stomatitis virus (VSV)-vectored COVID-19 vaccines [ 29 , 30 ], virus-derived replicon RNA-based COVID-19 vaccines [ 8 , 31 ], and recombinant adenovirus-vectored COVID-19 vaccines [ 5 , 32 , 33 ]. Viral vector or virus-derived replicon RNA-based vaccine candidates should be able to produce molecules antigenically equivalent to those displayed on SARS-CoV-2 virions and should thus be able to induce humoral immune responses that are similar to those observed in people with SARS-CoV-2 [ 8 ]. However, some viral vaccine candidates have been associated with biological safety concerns, including the potential for viral genome integration, as observed with human adenovirus- or measles virus-vectored vaccines, although these vaccines are considered quite safe [ 34 , 35 ].…”

Quantitative Assays Reveal Cell Fusion at Minimal Levels of SARS-CoV-2 Spike Protein and Fusion-from-Without

“…An expression plasmid for codon-optimized SARS-CoV-2 S with a C-terminal HA tag (pCG-SARS-CoV-2-S-HA) was kindly provided by Karl-Klaus Conzelmann (Hennrich et al , 2020). The Plasmid pCG-SARS-CoV-2-SΔ19 encoding the SΔ19 variant was generated by PCR amplification of the truncated S sequence, inserting PacI and SpeI restriction sites as well as a stop codon by PCR with primers 5’-TTATTAATTAAATGTTCGTGTTTCTGGTG-3’ and 5’-TATACTAGTTCTAGCAGCAGCTGCC-3’.…”