Safe and effective two-in-one replicon-and-VLP minispike vaccine for COVID-19: Protection of mice after a single immunization

Vaccines of outstanding efficiency, safety, and public acceptance are needed to halt the current SARS-CoV-2 pandemic. Concerns include potential side effects caused by the antigen itself and safety of viral DNA and RNA delivery vectors. 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 might 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 complemented with VSV G, VSV Delta G-minispike-eGFP (G), stimulated high titers of SARS-CoV-2 neutralizing antibodies in mice, equivalent to those found in COVID-19 patients, and protected transgenic K18-hACE2 mice from COVID-19-like disease. Homologous boost immunization further enhanced virus neutralizing activity. The results demonstrate that non-spreading rhabdovirus RNA replicons expressing minispike proteins represent effective and safe alternatives to vaccination approaches using replication-competent viruses and/or the entire S antigen. Author summary Two critical problems are associated with replicating paramyxo- and rhabdovirus vaccines expressing SARS-CoV-2 spike (S) protein such as VSV Delta G(S). One is eliciting of potentially disease-enhancing non-neutralizing antibodies, the other the S-mediated spread in humans. In view of the multi-organ tropism of SARS-CoV-2 in humans, their pathogenic outcome is not predictable. Here, we address and resolve both issues. We describe an innovative VSV vaccine, which is safe both in terms of virus propagation and immune response, as it is a non-spreading single round replicon vector, and the immunogen is limited to the spike's receptor binding domain (RBD), which emerged as the antigen eliciting the desired virus-neutralizing antibodies in humans. An excellent protective immune response in animals is achieved by the design of a chimeric RBD-minispike, which allows a combined "2-in-1" approach, meaning that the optimized antigen is simultaneously presented on cells and on noninfectious virus-like particles (VLPs). With such enhanced RBD antigen presentation it is thus not necessary to use replication-competent virus or entire S antigen.

Ludwig-Maximilians-Universität München
Veterinärmedizinische Universität Wien
Paul Ehrlich Institut, Langen
Paul Ehrlich Institute