The attenuated Bartha strain is worldwide the most commonly used vaccine strain against the porcine alphaherpesvirus pseudorabies virus (PRV). The Bartha strain has been generated in 1961 by serial passaging of PRV on cell culture (for a history of the Bartha strain, see https://www.mdpi.com/2076-0817/9/11/897 ). This serial passaging has resulted in a plethora of mutations in the Bartha genome and a concomittant lack of (neuro)virulence.

In the current paper, we performed a mass spectrometry-based comparison of the protein composition of virus particles of the Bartha strain versus those of three wild type PRV strains. Fascinatingly, we found that several viral proteins that are located inside the virus particle (tegument proteins) are less incorporated in Bartha virions compared to wild type virions. Most of these viral proteins with reduced incorporation are located in the outer part of the viral tegument, are not strictly essential for virus replication and have been associated with virus-host interactions, including viral immune evasion. Likely as a result of this reduced incorporation of several tegument proteins, we found that Bartha virions are also smaller and more icosahedral in shape compared to wild type virions.

We propose that the extensive serial cell culture passage of PRV that has been used to generate the Bartha strain has resulted in an in vitro micro-evolution to a more minimalistic version of PRV that has omitted immune evasion tools that may be critical for successful infection of the host, but possibly unnecessary ballast in cell culture. Intriguingly, such process may have contributed to the efficacy of Bartha as a vaccine strain, as it may contribute both to reduced virulence and increased immunogenicity.

Read the whole article here: https://journals.asm.org/doi/10.1128/jvi.01158-22

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