Rescuing eGFP-Tagged Canine Distemper Virus for 40 Serial Passages Separately in Ribavirin- and Non-Treated Cells: Comparative Analysis of Viral Mutation Profiles

An improved system to generate recombinant canine distemper virus

BACKGROUND

Canine distemper virus (CDV) is a pathogen with the capability of cross-species transmission. It has crossed the species barrier to infect many other species, and its host range is expanding. The reverse genetic platform, a useful tool for scientific research, allows the generation of recombinant viruses from genomic cDNA clones in vitro.

METHODS

To improve the reverse genetic system of CDV, a plasmid containing three independent expression cassettes was constructed for co-expression of the N, P, and L genes and then transfected with a full-length cDNA clone of CDV into Vero cells.

RESULTS

The results indicated that the established rescue system has the advantages of being more convenient, easy to control the transfection ratio, and high rescue efficiency compared with the conventional reverse genetics system.

CONCLUSION

This method not only reduces the number of transfection plasmids, but also improves the rescue efficiency of CDV, which could provide a reference for the recovery of other morbilliviruses.

Genomic profile of eGFP-expressing canine distemper virus that undergoes serial plaque-to-plaque transfers

Canine distemper virus (CDV) is classified into the genus Morbillivirus in the family Paramyxoviridae. This virus has a single-stranded genomic RNA with negative polarity. The wild-type CDV genome is generally composed of 15 690 nucleotides. We previously rescued an enhanced green fluorescence protein (eGFP)-tagged recombinant CDV (rCDV-eGFP) using reverse genetics. In this study, the rCDV-eGFP at passage-7 was subjected to 38 serial plaque-to-plaque transfers (or bottleneck passages) and two extra common passages in cells. In theory, the effect of Muller’s ratchet may fix deleterious mutations in a single viral population after consecutive plaque-to-plaque transfers. In order to uncover a mutated landscape of the rCDV-eGFP under the circumstances of bottleneck passages, the passage-47 progeny was collected for the in-depth analysis via next-generation sequencing. The result revealed a total of nine single-nucleotide mutations (SNMs) in the viral antigenome. Out of them, SNMs at nt 1832, 5022, 5536, 5580, 5746, 6913 and 8803 were identified as total single-nucleotide substitution, i.e., 100% of mutation frequency. The result suggested no notable formation of viral quasispecies in the rCDV-eGFP population after consecutive plaque-to-plaque transfers.

Recovery of Two Replication-Competent Canine Distemper Viruses That Separately Express Dabie Bandavirus Gn and Gc

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne zoonosis with a high mortality rate in humans. Additionally, dogs are frequently reported to be infected with this disease. There has been no commercially available vaccine for humans and animals as yet. The SFTS is caused by Dabie bandavirus (DBV), formerly known as SFTS virus. The DBV is now classified into the genus Bandavirus in the family Phenuiviridae. DBV Gn and Gc can induce specific immune responses in vivo. In this study, we used reverse genetics technique to construct two recombinant canine distemper viruses (rCDVs), rCDV-Gn and -Gc, which could express Gn and Gc in vitro, respectively. Both of the recombinants, derived from a common parental CDV, were independently subjected to twenty serial passages in cells for Sanger sequencing. Neither point mutation nor fragment deletion was found in the Gn open reading frame (ORF), whereas the rCDV-Gc showed a nonsynonymous mutation (A157C) in the Gc ORF, correspondingly resulting in a mutation of amino acid (T53P) in the Gc. Growth curve of the rCDV-Gc almost coincided with that of a wild-type CDV, but exhibited a significant difference from that of the rCDV-Gn. Much research remains to be performed to demonstrate whether both recombinants are able of inducing specific immune responses in vivo.

Recovery of Recombinant Canine Distemper Virus That Expresses CPV-2a VP2: Uncovering the Mutation Profile of Recombinant Undergoing 50 Serial Passages In Vitro

Canine distemper and canine parvoviral enteritis are infections caused by the canine distemper virus (CDV) and canine parvovirus type 2 (CPV-2), respectively. They are two common infectious diseases that cause high morbidity and mortality in affected dogs. Combination vaccines have been broadly used to protect dogs from infections of CDV, CPV-2, and other viruses. VP2 is the most abundant protein of the CPV-2 capsid. It elicits potent immunity in animals and, therefore, is widely used for designing subunit antigen-based vaccines. In this study, we rescued a recombinant CDV (QN vaccine strain) using reverse genetics. The recombinant CDV (rCDV-VP2) was demonstrated to express stably the VP2 in cells for at least 33 serial passages in vitro. Unfortunately, a nonsense mutation was initially identified in the VP2 open reading frame (ORF) at passage-34 (P34) and gradually became predominant in rCDV-VP2 quasispecies with passaging. Neither test strip detection nor indirect immunofluorescence assay demonstrated the expression of the VP2 at P50. The P50 rCDV-VP2 was subjected to next-generation sequencing, which totally identified 17 single-nucleotide variations (SNVs), consisting of 11 transitions and 6 transversions. Out of the 17 SNVs, 1 and 9 were identified as nonsense and missense mutations, respectively. Since the nonsense mutation arose in the VP2 ORF as early as P34, an earlier rCDV-VP2 progeny should be selected for the vaccination of animals in future experiments.