Evidence that a polyhexameric genome length is preferred, but not strictly required, for efficient mumps virus replication

Demonstration of bacterium-free very rapid reverse genetics system using mumps virus

The reverse genetics system is a very powerful tool for analyzing the molecular mechanisms of viral propagation and pathogenesis. However, full-length genome plasmid construction is highly time-consuming and laborious, and undesired mutations may be introduced by Escherichia coli. This study shows a very rapid E. coli-free method of full-genome construction using the mumps virus as an example. This method was able to reduce dramatically the time for full-genome construction, which was used very efficiently for virus rescue, from several days or more to ~2 days, with a similar accuracy and yield to the conventional method using E. coli/plasmid.

A highly efficacious live attenuated mumps virus-based SARS-CoV-2 vaccine candidate expressing a six-proline stabilized prefusion spike

With the rapid increase in SARS-CoV-2 cases in children, a safe and effective vaccine for this population is urgently needed. The MMR (measles/mumps/rubella) vaccine has been one of the safest and most effective human vaccines used in infants and children since the 1960s. Here, we developed live attenuated recombinant mumps virus (rMuV)-based SARS-CoV-2 vaccine candidates using the MuV Jeryl Lynn (JL2) vaccine strain backbone. The soluble prefusion SARS-CoV-2 spike protein (preS) gene, stablized by two prolines (preS-2P) or six prolines (preS-6P), was inserted into the MuV genome at the P-M or F-SH gene junctions in the MuV genome. preS-6P was more efficiently expressed than preS-2P, and preS-6P expression from the P-M gene junction was more efficient than from the F-SH gene junction. In mice, the rMuV-preS-6P vaccine was more immunogenic than the rMuV-preS-2P vaccine, eliciting stronger neutralizing antibodies and mucosal immunity. Sera raised in response to the rMuV-preS-6P vaccine neutralized SARS-CoV-2 variants of concern, including the Delta variant equivalently. Intranasal and/or subcutaneous immunization of IFNAR1-/- mice and golden Syrian hamsters with the rMuV-preS-6P vaccine induced high levels of neutralizing antibodies, mucosal immunoglobulin A antibody, and T cell immune responses, and were completely protected from challenge by both SARS-CoV-2 USA-WA1/2020 and Delta variants. Therefore, rMuV-preS-6P is a highly promising COVID-19 vaccine candidate, warranting further development as a tetravalent MMR vaccine, which may include protection against SARS-CoV-2.

Population Variability Generated during Rescue Process and Passaging of Recombinant Mumps Viruses

Recombinant mumps viruses (MuVs) based on established vaccine strains represent attractive vector candidates as they have known track records for high efficacy and the viral genome does not integrate in the host cells. We developed a rescue system based on the consensus sequence of the L-Zagreb vaccine and generated seven different recombinant MuVs by (a) insertion of one or two additional transcription units (ATUs), (b) lengthening of a noncoding region to the extent that the longest noncoding region in MuV genome is created, or (c) replacement of original L-Zagreb sequences with sequences rich in CG and AT dinucleotides. All viruses were successfully rescued and faithfully matched sequences of input plasmids. In primary rescued stocks, low percentages of heterogeneous positions were found (maximum 0.12%) and substitutions were predominantly obtained in minor variants, with maximally four substitutions seen in consensus. ATUs did not accumulate more mutations than the natural MuV genes. Six substitutions characteristic for recombinant viruses generated in our system were defined, as they repetitively occurred during rescue processes. In subsequent passaging of primary rescue stocks in Vero cells, different inconsistencies within quasispecies structures were observed. In order to assure that unwanted mutations did not emerge and accumulate, sub-consensus variability should be closely monitored. As we show for Pro408Leu mutation in L gene and a stop codon in one of ATUs, positively selected variants can rise to frequencies over 85% in only few passages.

Evolutionary history of cotranscriptional editing in the paramyxoviral phosphoprotein gene

The phosphoprotein gene of the paramyxoviruses encodes multiple protein products. The P, V, and W proteins are generated by transcriptional slippage. This process results in the insertion of non-templated guanosine nucleosides into the mRNA at a conserved edit site. The P protein is an essential component of the viral RNA polymerase and is encoded by a faithful copy of the gene in the majority of paramyxoviruses. However, in some cases, the non-essential V protein is encoded by default and guanosines must be inserted into the mRNA in order to encode P. The number of guanosines inserted into the P gene can be described by a probability distribution, which varies between viruses. In this article, we review the nature of these distributions, which can be inferred from mRNA sequencing data, and reconstruct the evolutionary history of cotranscriptional editing in the paramyxovirus family. Our model suggests that, throughout known history of the family, the system has switched from a P default to a V default mode four times; complete loss of the editing system has occurred twice, the canonical zinc finger domain of the V protein has been deleted or heavily mutated a further two times, and the W protein has independently evolved a novel function three times. Finally, we review the physical mechanisms of cotranscriptional editing via slippage of the viral RNA polymerase.