Replication-Deficient Particles: New Insights into the Next Generation of Bluetongue Virus Vaccines

Abstract

Bluetongue virus (BTV) is endemic in many parts of the world, often causing severe hemorrhagic disease in livestock. To date, at least 27 different serotypes have been recognized. Vaccination against all serotypes is necessary to protect susceptible animals and to prevent onward spread of the virus by insect vectors. In our previous studies, we generated replication-deficient (disabled infectious single-cycle [DISC]) virus strains for a number of serotypes and reported preliminary data on their protective efficacy in animals. In this report, to advance the DISC vaccines to the marketplace, we investigated different parameters of these DISC vaccines. First, we demonstrated the genetic stabilities of these vaccine strains and also the complementing cell line. Subsequently, the optimal storage conditions of vaccines, including additives, temperature, and desiccation, were determined and their protective efficacies in animals confirmed. Furthermore, to test if mixtures of different vaccine strains could be tolerated, we tested cocktails of DISC vaccines in combinations of three or six different serotypes in sheep and cattle, the two natural hosts of BTV. Groups of sheep vaccinated with a cocktail of six different vaccines were completely protected from challenge with individual virulent serotypes, both in early challenge and after 5 months of challenge without any clinical disease. There was no interference in protection between the different vaccines. Protection was also achieved in cattle with a mixture of three vaccine strains, albeit at a lesser level than in sheep. Our data support and validate the suitability of these virus strains as the next-generation vaccines for BTV.IMPORTANCE Bluetongue (BT) is a debilitating and in many cases lethal disease that affects ruminants of economic importance. Classical vaccines that afford protection against bluetongue virus, the etiological agent, are not free from secondary and undesirable effects. A surge in new approaches to produce highly attenuated, safer vaccines was evident after the development of the BTV reverse-genetics system that allows the introduction of targeted mutations in the virus genome. We targeted an essential gene to develop disabled virus strains as vaccine candidates. The results presented in this report further substantiate our previous evidence and support the suitability of these virus strains as the next-generation BTV vaccines.

Approaches and Perspectives for Development of African Swine Fever Virus Vaccines

“…Finally, some essential ASFV proteins have been characterized, including ASFV-Toposoimerase II [ 89 , 90 , 91 ], a histone-like protein [ 92 ], and the ASFV-E2 Ubiquitin conjugating enzyme, opening new avenues to generate effective single-cycle mutant virus vaccines using helper cell lines expressing these essential proteins. Similar approaches have been reported for Bluetongue disease [ 93 , 94 , 95 ] and African horse sickness [ 96 , 97 ]. Both gene deletion attenuated and replication deficient viruses have the advantage of presenting almost the entire virus repertoire via Major histocompatibility complex (MHC) class I and II to CD8 and CD4 T cells, respectively, thus stimulating both cellular and serological immunity.…”

Prospects of Next-Generation Vaccines for Bluetongue

“…The DISC vaccine platform has been applied for several serotypes by exchange of the serotype specific outer shell. Monovalent DISC vaccine and some DISC cocktail vaccines have been studied in sheep and cattle (150–152). A single DISC vaccination is protective in both sheep and cattle.…”

The Interaction of Bluetongue Virus VP6 and Genomic RNA Is Essential for Genome Packaging

“…Rescued BTV carrying a truncated VP6 protein was possible only in a VP6-complemented cell line, and although these rescued VP6-defective viruses could express BTV proteins upon infection, they assembled only a low level of particles, which lacked the viral genome, as visualized by electron microscopy (11). To confirm the correlation between a functional VP6 and RNA packaging, an available BTV strain with a truncated VP6 protein (triple stop codons introduced at residues 87 to 89) (16) was grown in a VP6-complemented BSR-derived cell line (BSR-VP6), and after 3 days, the recovered virion particles were used to infect both parental BSR cells and BSR-VP6 cells. Although these particles lacked the S9 RNA segment, which encodes VP6, they still contained VP6 protein incorporated from the BSR-VP6 cells used to recover the virus and thus were capable of synthesizing first-round ssRNAs, though not capable of completion of replication or second-round transcription following infection of wild-type (WT) BSR cells.…”