To take advantage of live recombinant vesicular stomatitis viruses (rVSVs) as vaccine vectors for their high yield and for their induction of strong and long-lasting immune responses, it is necessary to make live vaccine vectors safe for use without losing their immunogenicity. We have generated safer and highly efficient recombinant VSV vaccine vectors by combining the M51R mutation in the M gene of serotype VSV-Indiana (VSV Ind ) with a temperature-sensitive mutation (tsO23) of the VSV Ind Orsay strain. In addition, we have generated two new serotype VSV-New Jersey (VSV NJ ) vaccine vectors by combining M48R and M51R mutations with G22E and L110F mutations in the M gene, rVSV NJ (G22E M48R M51R) [rVSV NJ (GMM)] and VSV NJ (G22E M48R M51R L110F) [rVSV NJ (GMML)]. The combined mutations G21E, M51R, and L111F in the M protein of VSV Ind significantly reduced the burst size of the virus by up to 10,000-fold at 37°C without affecting the level of protein expression. BHK 21 cells and SH-SY5Y human neuroblastoma cells infected with rVSV Ind (GML), rVSV NJ (GMM), and rVSV NJ (GMML) showed significantly reduced cytopathic effects in vitro at 37°C, and mice injected with 1 million infectious virus particles of these mutants into the brain showed no neurological dysfunctions or any other adverse effects. In order to increase the stability of the temperaturesensitive mutant, we have replaced the phenylalanine with alanine. This will change all three nucleotides from UUG (leucine) to GCA (alanine). The resulting L111A mutant showed the temperature-sensitive phenotype of rVSV Ind (GML) and increased stability. Twenty consecutive passages of rVSV Ind (GML) with an L111A mutation did not convert back to leucine (UUG) at position 111 in the M protein gene. V esicular stomatitis virus (VSV) is a rapidly replicating virus.Eventually humoral and cellular immune responses against VSV are elicited in the animal host, like any other viral vectors (1-3). Animals infected with VSV develop immune responses within 2 weeks and start to neutralize the VSV (1). This hinders the efficacy of boost immunizations for vaccination with the same vector. Serotype-specific antibodies against the viral surface glycoprotein G neutralize VSV. Two different serotypes of VSV, VSV-Indiana (VSV Ind ) and VSV-New Jersey (VSV NJ ), show 50% amino acid homologies in glycoprotein G (4). Antibodies raised against one serotype of VSV do not neutralize the other serotype of VSV (5). Accordingly, other investigators have used VSV Ind with its own surface glycoprotein G and VSV Ind carrying the G gene from other serotypes as a vaccine vector (6, 7).VSV causes self-limiting disease in animals such as pigs, cows, and horses, whereby vesicular lesions on the mouth, nose, teats, and hooves are cleared in a couple of weeks after the onset (8).Although it is very rare, human infections with VSV have been reported mostly in animal care workers, veterinarians, and laboratory personnel who were in close contact with the diseased animals or with the viruses (9-11)....

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The development of safe and effective vaccines to prevent SARS-CoV-2 infections remains an urgent priority worldwide. We have used a recombinant vesicular stomatitis virus (rVSV)-based prime-boost immunization strategy to develop an effective COVID-19 vaccine candidate. We have constructed VSV genomes carrying exogenous genes resulting in the production of avirulent rVSV carrying the full-length spike protein (SF), the S1 subunit, or the receptor-binding domain (RBD) plus envelope (E) protein of SARS-CoV-2. Adding the honeybee melittin signal peptide (msp) to the N-terminus enhanced the protein expression, and adding the VSV G protein transmembrane domain and the cytoplasmic tail (Gtc) enhanced protein incorporation into pseudotype VSV. All rVSVs expressed three different forms of SARS-CoV-2 spike proteins, but chimeras with VSV-Gtc demonstrated the highest rVSV-associated expression. In immunized mice, rVSV with chimeric S protein-Gtc derivatives induced the highest level of potent neutralizing antibodies and T cell responses, and rVSV harboring the full-length msp-SF-Gtc proved to be the superior immunogen. More importantly, rVSV-msp-SF-Gtc vaccinated animals were completely protected from a subsequent SARS-CoV-2 challenge. Overall, we have developed an efficient strategy to induce a protective response in SARS-CoV-2 challenged immunized mice. Vaccination with our rVSV-based vector may be an effective solution in the global fight against COVID-19.

The Vero cell line is the most used continuous cell line in viral vaccine manufacturing. This adherent cell culture platform requires the use of surfaces to support cell growth, typically roller bottles, or microcarriers. We have recently compared the production of rVSV-ZEBOV on Vero cells between microcarrier and fixed-bed bioreactors. However, suspension cultures are considered superior with regard to process scalability. Therefore, we further explore the Vero suspension system for recombinant vesicular stomatitis virus (rVSV)-vectored vaccine production. Previously, this suspension cell line was only able to be cultivated in a proprietary medium. Here, we expand the adaptation and bioreactor cultivation to a serum-free commercial medium. Following small-scale optimization and screening studies, we demonstrate bioreactor productions of highly relevant vaccines and vaccine candidates against Ebola virus disease, HIV, and coronavirus disease 2019 in the Vero suspension system. rVSV-ZEBOV, rVSV-HIV, and rVSV Ind -msp-S F -Gtc can replicate to high titers in the bioreactor, reaching 3.87 × 10 7 TCID 50 /ml, 2.12 × 10 7 TCID 50 /ml, and 3.59 × 10 9 TCID 50 /ml, respectively. Furthermore, we compare cell-specific productivities, and the quality of the produced viruses by determining the ratio of total viral particles to infectious viral particles.

The matrix (M) protein of vesicular stomatitis virus (VSV) plays significant roles in the replication of VSV through its involvement in the assembly of virus particles as well as by facilitating the evasion of innate host cell defense mechanisms. The presence of methionine at position 51 (M51) of the matrix (M) protein of the VSV Indiana serotype (VSV(Ind)) has been proven to be crucial for cell rounding and inhibition of host cell gene expression. The M protein of VSV(Ind) with the substitution of M51 with arginine (R:M51R) results in the loss of inhibitory effects on host cell gene expression. The VSV(Ind) expressing the M(M51R) protein became the attractive oncolytic virus which is safer and more tumor-specific because the normal cells can clear the mutant VSV(Ind) easily but tumor cells are susceptible to the virus because a variety of tumor cells lack innate antiviral activities. We have studied the role of the methionines at positions 48 and 51 of the M protein of the New Jersey serotype of VSV (VSV(NJ)) in the induction of cytopathic effects (CPE) and host cell gene expression. We have generated human embryonic kidney 293 cell lines inducibly expressing M proteins with M to R mutations at positions 48 and 51, either separately or together as a double mutant, and examined expression of heat shock protein 70 (HSP70) as an indicator of host cell gene expression. We have also generated recombinant VSV(NJ) encoding the mutant M proteins M(M48R) or M(M48R+M51R) for the first time and tested for the expression of HSP70 in infected cells. Our results demonstrated that the M51 of VSV(NJ) M proteins has a major role in cell rounding and in suppressing the host cell gene expression either when the M protein was expressed alone in inducible cell lines or when expressed together with other VSV proteins by the recombinant VSV(NJ). Amino acid residue M48 may also have some role in cell rounding and in the inhibitory effects of VSV(NJ) M, which was demonstrated by the fact that the cell line expressing the double substitution mutant M(M48R+M51R) exhibited the least cytopathic effects and the least inhibitory effect on host cell gene expression.